JP7019482B2 - Ferritic stainless steel sheets with excellent high-temperature salt damage resistance and automobile exhaust system parts - Google Patents

Description

INDUSTRIAL APPLICABILITY The present invention is for heat-resistant stainless steel, which is most suitable for use in automobile exhaust system members that require high-temperature strength and oxidation resistance, and is for automobile exhaust systems having a structure that promotes salt adhesion, which is particularly excellent in high-temperature salt damage resistance. It relates to ferritic stainless steel plates and automobile exhaust system parts.

Exhaust system members such as automobile exhaust manifolds, front pipes and center pipes allow high-temperature exhaust gas discharged from the engine to pass through, so the materials that make up the exhaust system members are diverse, such as oxidation resistance, high-temperature strength, and thermal fatigue characteristics. Characteristics are required.

Conventionally, cast iron was generally used for automobile exhaust system members, but stainless steel exhaust manifolds are used from the viewpoints of tightening exhaust gas regulations, improving engine performance, and reducing the weight of the vehicle body. It became so. The exhaust gas temperature varies depending on the vehicle model, and in recent years, it is often about 750 to 850 ° C., but it may reach a higher temperature. There is a demand for a material having high high temperature strength and oxidation resistance in an environment where it is used for a long time in such a temperature range.

Among stainless steels, austenitic stainless steel has excellent heat resistance and workability, but because of its large coefficient of thermal expansion, it suffers from thermal fatigue failure when applied to members that are repeatedly heated and cooled, such as exhaust manifolds. Is likely to occur.

Ferritic stainless steel has an excellent thermal fatigue characteristic because it has a smaller thermal expansion coefficient than austenitic stainless steel. In addition, compared to austenitic stainless steel, it contains almost no expensive Ni, so the material cost is low and it is used for general purposes. However, since ferritic stainless steel has a lower high-temperature strength than austenitic stainless steel, a technique for improving the high-temperature strength has been developed. Furthermore, various technologies have been developed from the viewpoints of oxidation resistance, moldability, manufacturability, and further cost reduction.

For example, there are SUS430J1L (Nb-added steel), Nb-Si-added steel, and SUS444 (Nb-Mo-added steel), and the high-temperature strength is improved by adding Si and Mo based on the addition of Nb.

On the other hand, members such as the center pipe located at the bottom of the vehicle body have a slightly lower temperature than other exhaust system members, but snowmelt salt sprayed to prevent road surface freezing and salt derived from seawater easily adhere to them, causing high-temperature salt damage. I am concerned. High-temperature salt damage is a phenomenon in which high-temperature corrosion is promoted by the adhesion of salt in a high-temperature oxidizing environment. For these parts located in the lower part of the vehicle body, technologies that emphasize high temperature salt damage resistance have been developed.

However, in recent years, due to changes in the installation structure of the exhaust system and the addition of new parts, salt may adhere to members of the exhaust system such as exhaust manifolds and front pipes that are located at high temperatures. Here are two specific examples. The first is the case where the exhaust system is forcibly air-cooled by installing a fan or improving the air permeability in response to the high temperature of the exhaust gas. An example is shown in FIG. 2 (A). The forced cooling mechanism 3 for forcibly air-cooling the automobile exhaust system member 1 is a factor that promotes the adhesion of salt to the exhaust system member. The forced cooling mechanism may be applied mainly to turbocharged vehicles, which have been accelerating in recent years. The second is a case where an exhaust gas purification component such as a converter and a heat insulating structure for covering the periphery of the exhaust system in front of the converter with a heat insulating material are provided to promote the catalytic reaction by raising the exhaust gas temperature. An example is shown in FIG. 2 (B). Wool-like ceramics or the like is used for the heat insulating material 7 that covers the automobile exhaust system member 1, and it becomes easy to retain water. Therefore, covering with a heat insulating material makes it easier to retain salt and promotes salt adhesion to the exhaust system. The application of the heat insulating material may be mainly applied to the exhaust system of an engine that burns by HCCI (premixed compression automatic ignition), which is expected to be widely used in the future.

Furthermore, by using these forced cooling mechanisms or heat insulating structures, the environment becomes higher in humidity than the environment in which ordinary members are exposed. In the forced cooling mechanism, moisture is always sprayed in an environment where moisture is swirled around the parts, and when a heat insulating structure is used, the heat insulating material is covered with a cover, which creates an environment in which water vapor is trapped. Therefore, the high-temperature salt-damaged environment becomes a different environment from the conventional one in which water vapor oxidation accompanies the oxidation environment with salt adhesion.

In other words, when a forced cooling mechanism or a heat insulating structure is applied, not only is the upstream member such as the exhaust manifold and front pipe required to be newly resistant to high-temperature salt damage, but also an environment with steam oxidation different from the conventional one. High temperature salt damage resistance was required. That is, the automobile exhaust system member to which the forced cooling mechanism and the heat insulating structure are applied needs to be developed as a new application different from the conventional automobile exhaust system member.

As a technique for dealing with high temperature salt damage, Patent Document 1 discloses a technique for improving high temperature salt damage resistance by adding Mo and W. However, Mo and W are expensive elements, which causes an increase in material cost. Moreover, the environment accompanied by steam oxidation is not considered for the high temperature salt damage resistance.

Patent Document 2 discloses a technique for adjusting the amount of Al added to improve high temperature salt damage resistance. However, the Al concentration is more than 0.5 to 7.0% by mass, which is extremely higher than that of ordinary ferritic stainless steel, and may impair manufacturability and workability. Moreover, the environment accompanied by steam oxidation is not considered for the high temperature salt damage resistance.

Patent Document 3 discloses a technique for adjusting the addition amounts of N, V, and Al to improve high-temperature salt damage resistance. However, the V concentration is 0.30 to 0.60% in mass%, which is extremely higher than that of ordinary ferritic stainless steel, and may impair manufacturability and the like. Moreover, the environment accompanied by steam oxidation is not considered for the high temperature salt damage resistance.

Japanese Unexamined Patent Publication No. 6-1364888 Japanese Patent No. 3903853 Japanese Unexamined Patent Publication No. 2010-53421

When applying a forced cooling mechanism or a heat insulating structure to an automobile exhaust system as described above, it is not only necessary to newly impart high-temperature salt damage resistance to upstream members such as exhaust manifolds and front pipes, but also to the conventional case. Needed high temperature salt damage resistance in the environment with different steam oxidation. However, in the conventional high temperature salt damage resistance improving technique, it is necessary to add expensive Mo, W, or to add Al, V, etc. extremely higher than those of ordinary ferritic stainless steel. In addition, high-temperature salt damage accompanied by steam oxidation was not investigated.

That is, an object of the present invention is to provide a ferritic stainless steel sheet having excellent high temperature salt damage resistance and an automobile exhaust system component as an exhaust manifold without relying on Mo, W addition or extreme Al, V addition.

In order to solve the above problems, the present inventors have diligently investigated the influence of various components on the high temperature salt damage resistance of ferritic stainless steel. As a result, they have invented a ferritic stainless steel having excellent high temperature salt damage resistance. In addition, high-temperature salt damage also corresponds to the environment accompanied by steam oxidation.

That is, the gist of the present invention for solving the above problems is as follows.
(1) By mass%,
C: 0.0200% or less,
N: 0.0200% or less,
Si: 0.30% or more and 4.00% or less,
Mn: 0.01% or more, 0.40% or less, or 0.80% or more, 2.00% or less,
P: 0.040% or less,
S: 0.0014% or less,
Cr: 13.0% or more, 23.0% or less,
Ni: 0.01% or more, 0.40% or less,
Cu: 0.01% or more, less than 0.10%,
Nb: 0.20% or more, less than 0.60%,
Ti: 0.001% or more, 0.270% or less,
Al: 0.002% or more, 0.200% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0001% or more, 0.0050% or less,
O: 0.0050% or less,
A ferrite-based stainless steel sheet having excellent high-temperature salt damage resistance, characterized in that the balance is composed of Fe and unavoidable impurities and has a composition satisfying the following formulas (i) to (iv).
Nb + Ti + 0.4Si ≧ 0.630 ・ ・ ・ Equation (i)
Cr + 10Si ≧ 17.0 ・ ・ ・ Equation (ii)
0.0110 ≤ C + N ≤ 0.0270 ... Equation (iii)
Al / O ≧ 1.2 ・ ・ ・ Equation (iv)
However, the element symbol in the formula means the content (mass%) of the element.
(2) In mass%, Y: 0.001% or more, 0.20% or less,
REM: 0.001% or more, 0.20% or less,
Ca: 0.0002% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Hf: 0.001% or more, 1.0% or less,
Sn: 0.002% or more, 1.0% or less,
Mg: 0.0002% or more, 0.0030% or less,
Co: 0.01% or more, 0.30% or less,
Sb: 0.005% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0002% or more, 0.30% or less,
The ferrite-based stainless steel sheet having excellent high-temperature salt damage resistance according to (1), which contains one or more of the above.
(3) The ferrite-based stainless steel sheet having excellent high-temperature salt damage resistance according to (1) or (2), which is used for automobile exhaust system members in an environment where salt adhesion is promoted by a forced cooling mechanism.
(4) The resistance according to (1) or (2), which is used for an automobile exhaust system member in an environment where salt adhesion is promoted by applying a heat insulating structure that covers the periphery of the automobile exhaust system member with a heat insulating material. Ferritic stainless steel sheet with excellent high-temperature salt damage.

(5) An automobile exhaust system component provided with an automobile exhaust system member using the ferrite stainless steel plate having excellent high temperature salt damage resistance according to (1) or (2) and a forced cooling mechanism for forcibly cooling the member.
(6) An automobile exhaust system member having an automobile exhaust system member using the ferritic stainless steel plate having excellent high temperature salt damage resistance according to (1) or (2) and a heat insulating structure covering the periphery thereof with a heat insulating material. ..

Further, in the present invention, those in which the lower limit is not specified are shown to include unavoidable impurity levels.

According to the present invention, it is possible to provide a ferritic stainless steel sheet having excellent high temperature salt damage resistance, which is used as an automobile exhaust system member to which a forced cooling mechanism and a heat insulating structure are applied to promote salt adhesion.

For Examples A to T of the present invention in Table 1 and Comparative Examples e to i in Table 2, Si and Nb + Ti exert on the corrosion reduction after the high-temperature salt damage test in an environment including heating at 750 ° C. and immersion in a saturated NaCl aqueous solution and also accompanied by steam oxidation. It is a figure which showed the influence of. It is a figure which shows an example of an automobile exhaust system component, (A) is provided with a forced cooling mechanism, (B) is provided with a heat insulating structure.

Hereinafter, the present invention will be described in detail.

First, the reason for limiting the steel composition of the ferritic stainless steel of the present invention will be described. Here, "%" for the steel composition means mass%.

(C: 0.0200% or less)
C is an element that deteriorates moldability and corrosion resistance and causes a decrease in high-temperature strength, and is 0.0200% or less. Further, considering the decrease in oxidation resistance and intergranular corrosion resistance due to excessive addition, it is desirable that the upper limit is 0.0150%. However, since excessive reduction leads to an increase in refining cost, it is desirable to set the lower limit to 0.0010%.

(N: 0.0200% or less)
Like C, N is an element that deteriorates moldability and corrosion resistance and causes a decrease in high-temperature strength, and is 0.0200% or less. Further, considering the decrease in oxidation resistance and intergranular corrosion resistance due to excessive addition, it is desirable that the upper limit is 0.0150%. However, since excessive reduction leads to an increase in refining cost, it is desirable to set the lower limit to 0.0030%.

(Si: 0.30% or more and 4.00% or less)
Si is an element added as a deoxidizing agent and an element that improves oxidation resistance. In addition, Si is an important element for improving high temperature salt damage resistance. By adding Si, Si oxide is formed under the oxide of Fe and Cr, which acts as an auxiliary film and improves high temperature salt damage resistance. In the present invention containing no Mo, it is necessary to add 0.30% or more of Si in order to exhibit high temperature salt damage resistance. However, excessive addition causes deterioration of processability, so the content should be 4.00% or less. Further, in consideration of refining cost and manufacturability, the lower limit is preferably 0.50% and the upper limit is preferably 3.50%. More preferably, it is in the range of 0.80 to 2.90%.

(Mn: 0.01% or more, 0.40% or less, or 0.80% or more, 2.00% or less)
Mn is an element added as a deoxidizing agent, and is added in an amount of 0.01% or more. In addition, Mn affects the oxidation rate and scale peeling property, and from that point, it also affects the high temperature salt damage resistance. As the amount added increases, the protection of the scale decreases and the oxidation rate increases. This reduces high temperature salt damage. However, when the addition amount exceeds a certain level, the scale peeling resistance is improved. This also improves high temperature salt damage resistance. That is, there is a range in which the high temperature salt damage resistance is lowered, and in order to avoid this, it is necessary to set it to 0.40% or less or 0.80% or more. However, excessive addition causes a decrease in uniform elongation, so the content should be 2.00% or less. Further, in consideration of refining cost, hot workability and corrosion resistance, a range of 0.10 to 0.40% or 0.85 to 1.50% is desirable. More preferably, it is in the range of 0.15 to 0.35% or 0.90 to 1.20%.

(P: 0.040% or less)
P is an impurity mainly mixed from the raw material during steelmaking refining, and when the content is high, the toughness and weldability are lowered. Therefore, the smaller the content, the better, so the content is 0.040% or less. Further, in consideration of manufacturability, it is desirable that the upper limit is 0.035%. However, since excessive reduction leads to an increase in refining cost, it is desirable to set the lower limit to 0.01%.

(S: 0.0014% or less)
S is an impurity mainly mixed from the raw material during steelmaking refining and deteriorates corrosion resistance. Further, by lowering the scale peeling resistance, the high temperature salt damage resistance is also lowered. Therefore, it should be 0.0014% or less. Further, in consideration of manufacturability, it is desirable that the upper limit is 0.0010%. However, since excessive reduction leads to an increase in refining cost, it is desirable to set the lower limit to 0.0003%.

(Cr: 13.0% or more and 23.0% or less)
Cr is an element that improves corrosion resistance and oxidation resistance, and is also an element that improves high-temperature salt damage resistance, and is added in an amount of 13.0% or more. However, excessive addition causes deterioration of workability and toughness, so the content should be 23.0% or less. In consideration of high temperature strength, high temperature fatigue characteristics and raw material cost, the lower limit is preferably 13.5% and the upper limit is preferably 20.0%. More preferably, it is in the range of 14.0 to 18.5%.

(Ni: 0.01% or more, 0.40% or less)
Ni is an element that improves corrosion resistance and also has the effect of improving high-temperature strength and toughness. However, excessive addition causes a decrease in moldability. Therefore, the range is set to 0.01 to 0.40%. Further, considering the raw material cost, it is desirable that the upper limit is 0.30%. More preferably, it is in the range of 0.05 to 0.20%.

(Cu: 0.01% or more, less than 0.10%)
Cu is an element effective for improving corrosion resistance and high temperature strength. However, since it causes a decrease in oxidation resistance, it is an element that should be avoided as much as possible if high temperature strength is ensured by adding Nb. Therefore, the range is set to 0.01 to less than 0.10%. Desirably, it is in the range of 0.03 to 0.07%.

(Nb: 0.20% or more, less than 0.60%)
Nb is an element that improves high-temperature strength by strengthening solid solution and refining precipitates, and also fixes C and N as carbonitrides to improve corrosion resistance and oxidation resistance, and is added in an amount of 0.20% or more. .. However, excessive addition causes a decrease in uniform elongation and a deterioration in hole-spreading property, so the content is set to less than 0.60%. Further, considering the intergranular corrosion property, manufacturability, and raw material cost of the welded portion, the lower limit is preferably 0.26%, and the upper limit is preferably 0.55%. More preferably, it is in the range of 0.30 to 0.52%.

(Ti: 0.001% or more, 0.270% or less)
Ti is an element that binds to C, N, and S to improve the r value, which is an index of corrosion resistance, intergranular corrosion resistance, and deep drawing property, and is added in an amount of 0.001% or more. However, excessive addition causes a decrease in uniform elongation and a decrease in hole widening workability due to the formation of coarse Ti-based precipitates, so the content should be 0.270% or less. Further, considering the occurrence of surface defects and toughness, the lower limit is preferably 0.003%, and the upper limit is preferably 0.220%. More preferably, it is in the range of 0.005 to 0.160%.

(Al: 0.002% or more, 0.200% or less)
Al is an element that is added as a deoxidizing element and improves oxidation resistance, and is added in an amount of 0.002% or more. However, excessive addition causes a decrease in uniform elongation and a decrease in toughness, so the content should be 0.200% or less. Further, considering the refining cost, the occurrence of surface defects and weldability, the lower limit is preferably 0.010% and the upper limit is preferably 0.150%. More preferably, it is in the range of 0.015 to 0.130%.

(V: 0.01% or more, 0.20% or less)
V is an element that improves high temperature strength. However, excessive addition causes a decrease in high-temperature strength and a decrease in thermal fatigue life due to the coarsening of precipitates. Therefore, the range is set to 0.01 to 0.20%. Further, in consideration of manufacturability, it is desirable that the upper limit is 0.15%. More preferably, it is in the range of 0.02 to 0.10%.

(B: 0.0001% or more, 0.0050% or less)
B is an element that improves high temperature strength and thermal fatigue characteristics. However, excessive addition causes deterioration of hot workability and deterioration of the surface texture of the steel surface. Therefore, the range is 0.0001 to 0.0050%. Further, in consideration of manufacturability and moldability, it is desirable that the upper limit is 0.0030%. More preferably, it is in the range of 0.0003 to 0.0015%.

(O: 0.0050% or less)
O is an impurity that is inevitably contained and causes surface defects due to bubbles and inclusions. Therefore, it should be 0.0050% or less. Further, in consideration of manufacturability, it is desirable that the upper limit is 0.0040%. However, since excessive reduction leads to an increase in refining cost, it is desirable to set the lower limit to 0.0003%. Here, O is T.I. Means O.

Next, equations (i) to (iv) will be described.

The formation of Si oxide is the most effective for improving the resistance to high-temperature salt damage, and the balance with Cr, Nb, and Ti forming each oxide is important in order to utilize the effect. The Cr oxide formed as a scale layer promotes the formation of Si oxide directly beneath it. The Nb oxide is formed directly under the Cr oxide, and the Ti oxide is oxidized inside the base metal, and both exhibit an auxiliary effect when the Si oxide is insufficient. The present inventors have found these effects and obtained formulas (i) and (ii).
Nb + Ti + 0.4Si ≧ 0.630 ・ ・ ・ Equation (i)
Cr + 10Si ≧ 17.0 ・ ・ ・ Equation (ii)

Further, the balance of the addition amounts of C, N, Al and O affects the high temperature salt damage resistance. C and N form carbonitrides with Ti, Nb and Cr to reduce high temperature salt damage resistance. The present inventors have found this effect and set the upper limit of the formula (iii) to 0.0270. However, excessive reduction causes not only an increase in refining cost but also a coarsening of crystal grains and a decrease in strength. Taking this into consideration, the lower limit of equation (iii) was set to 0.0110. Further, it was found that when Al is more than a certain amount with respect to O in the steel, scale peeling is suppressed to improve the high temperature salt damage resistance, and the formula (iv) is obtained.
0.0110 ≤ C + N ≤ 0.0270 ... Equation (iii)
Al / O ≧ 1.2 ・ ・ ・ Equation (iv)
However, the element symbol in the formula means the content (mass%) of the element.

In addition, in the present invention, by selectively adding one or more of Y, REM, Ca, Zr, Hf, Sn, Mg, Co, Sb, Bi, Ta, and Ga as necessary. , The characteristics can be further improved.

(Y: 0.001% or more, 0.20% or less)
Y is an element that improves the cleanliness of steel, rust resistance and hot workability, as well as oxidation resistance and high temperature salt damage resistance, and is added in an amount of 0.001% or more as necessary. .. However, excessive addition causes an increase in alloy cost and a decrease in manufacturability, so the upper limit is set to 0.20%.

(REM: 0.001% or more, 0.20% or less)
REM (rare earth element) is an element that improves the cleanliness of steel, rust resistance and hot workability, as well as oxidation resistance and high temperature salt damage resistance, and is 0.001 if necessary. % Or more is added. However, excessive addition causes an increase in alloy cost and a decrease in manufacturability, so the upper limit is set to 0.20%. REM is a general term for 15 elements (lanthanoids) from scandium (Sc) and lanthanum (La) to lutetium (Lu). It may be added alone or as a mixture.

(Ca: 0.0002% or more, 0.0030% or less)
Ca is an element that promotes desulfurization, and 0.0002% or more is added as needed. However, excessive addition causes a decrease in corrosion resistance due to the formation of CaS, which is a water-soluble inclusion, so the upper limit is set to 0.0030%.

(Zr: 0.01% or more, 0.30% or less)
Zr is an element that improves corrosion resistance, intergranular corrosion resistance, high temperature strength, and oxidation resistance, and is added in an amount of 0.01% or more as necessary. However, excessive addition causes deterioration of processability and manufacturability, so the upper limit is set to 0.30%.

(Hf: 0.001% or more, 1.0% or less)
Hf is an element that improves corrosion resistance, intergranular corrosion resistance, high temperature strength, and oxidation resistance, and is added in an amount of 0.001% or more as necessary. However, excessive addition causes deterioration of processability and manufacturability, so the upper limit is set to 1.0%.

(Sn: 0.002% or more, 1.0% or less)
Sn is an element that improves corrosion resistance and high-temperature strength, and is added in an amount of 0.002% or more as necessary. However, excessive addition causes deterioration of toughness and manufacturability, so the upper limit is set to 1.0%.

(Mg: 0.0002% or more, 0.0030% or less)
Mg may be added as a deoxidizing element, and is an element that refines the structure of the slab and improves moldability, and is added in an amount of 0.0002% or more as necessary. However, excessive addition causes deterioration of corrosion resistance, weldability, and surface quality, so the upper limit is set to 0.0030%.

(Co: 0.01% or more, 0.30% or less)
Co is an element that improves high-temperature strength, and is added in an amount of 0.01% or more as necessary. However, excessive addition causes deterioration of toughness and manufacturability, so the upper limit is set to 0.30%.

(Sb: 0.005% or more, 0.50% or less)
Sb is an element that improves high-temperature strength, and 0.005% or more is added as needed. However, excessive addition causes deterioration of weldability and toughness, so the upper limit is set to 0.50%.

(Bi: 0.001% or more, 1.0% or less)
Bi is an element that suppresses roping that occurs during cold rolling and improves manufacturability, and is added in an amount of 0.001% or more as necessary. However, excessive addition causes a decrease in hot workability, so the upper limit is set to 1.0%.

(Ta: 0.001% or more, 1.0% or less)
Ta is an element that improves high-temperature strength, and 0.001% or more is added as necessary. However, excessive addition causes deterioration of toughness and manufacturability, so the upper limit is set to 1.0%.

(Ga: 0.0002% or more, 0.30% or less)
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance, and is added in an amount of 0.0002% or more as necessary. However, since excessive addition causes deterioration of processability, the upper limit is set to 0.30%.

Next, a method for manufacturing a ferrite-based stainless steel sheet having excellent high-temperature salt damage resistance in the present invention will be described.

As the method for producing a steel sheet of the present invention, a general process for producing a ferritic stainless steel can be adopted. Generally, molten steel is made in a converter or an electric furnace, refined in an AOD furnace or a VOD furnace to make steel pieces by a continuous casting method or an ingot method, and then hot-rolled-annealing of hot-rolled plates-pickling-cooling. Manufactured through the steps of inter-rolling-finish annealing-pickling. If necessary, annealing of the hot-rolled plate may be omitted, or cold rolling-finish annealing-pickling may be repeated. The conditions of each of these steps may be general conditions, for example, a hot-rolled plate annealing temperature of 1000 to 1300 ° C., a hot-rolled plate annealing temperature of 900 to 1200 ° C., a cold-rolled plate annealing temperature of 800 to 1200 ° C., and the like. However, the present invention is not characterized by manufacturing conditions, and the manufacturing conditions are not limited. Therefore, the hot-rolled conditions, the hot-rolled plate thickness, the presence or absence of hot-rolled plate annealing, the cold-rolled conditions, the hot-rolled and cold-rolled plate annealing temperatures, the atmosphere, and the like can be appropriately selected. In addition, the treatment before finishing pickling may be a general treatment, for example, a mechanical treatment such as shot blasting or a grinding brush, or a chemical treatment such as a melt salt treatment or a neutral salt electrolysis treatment. can. Further, temper rolling or tension leveler may be applied after cold rolling and annealing. Further, the product plate thickness may be selected according to the required member thickness. Further, this steel plate may be used as a welded pipe by a normal method for manufacturing a stainless steel pipe for an exhaust system member such as electric resistance welding, TIG welding, and laser welding.

As shown in FIG. 2A, the automobile exhaust system component of the present invention includes an automobile exhaust system member 1 using the ferritic stainless steel plate of the present invention and a forced cooling mechanism 3 for forcibly cooling the member. There is. In FIG. 2A, the cooling air flow 11 from the air outlet 5 of the cooling fan 4 forcibly air-cools the automobile exhaust system member 1 (exhaust manifold 2). Such a forced cooling mechanism 3 for forcibly air-cooling an automobile exhaust system member 1 located at a high temperature in an exhaust system such as an exhaust manifold 2 or a front pipe is a factor for promoting salt adhesion to the exhaust system member. By using the ferrite-based stainless steel sheet of the present invention as the automobile exhaust system member 1 in such an automobile exhaust system component, sufficient high-temperature salt damage resistance is imparted.

As shown in FIG. 2B, the automobile exhaust system component of the present invention also includes an automobile exhaust system member 1 using the ferritic stainless steel plate of the present invention, and a heat insulating structure 6 having a heat insulating material 7 covering the periphery thereof. It is equipped with. In the heat insulating structure 6 shown in FIG. 2B, the outer periphery of the automobile exhaust system member 1 (exhaust manifold 2) is covered with the heat insulating material 7 and protected by the heat insulating material cover 8. Wool-like ceramics or the like is used for the heat insulating material 7, which facilitates water retention. Therefore, covering with a heat insulating material makes it easier to retain salt and promotes salt adhesion to the exhaust system. By using the ferrite-based stainless steel sheet of the present invention as the automobile exhaust system member 1 in such an automobile exhaust system component, sufficient high-temperature salt damage resistance is imparted.

Hereinafter, the effects of the present invention will be further clarified by examples. The present invention is not limited to the following examples, and can be appropriately modified and implemented without changing the gist thereof.

Test materials having the component compositions shown in Tables 1 and 2 (Examples A to T of the present invention, Comparative Examples a to o) were melted in a vacuum melting furnace and cast into a 30 kg ingot. The obtained ingot was a hot-rolled steel plate having a thickness of 4.5 mm. The heating conditions for hot rolling were 1200 ° C. The hot-rolled sheet was annealed at 1000 ° C. After descaling with alumina blasting, a plate having a thickness of 1.5 mm was obtained by cold rolling, and finish annealing was carried out at 1100 ° C. From the cold-rolled annealed plate thus obtained, a test piece having a thickness of 1.5 mm, a width of 20 mm, and a length of 50 mm and having a P600 wet polishing finish on the entire surface was produced for a high-temperature salt damage test.

(High temperature salt damage test)
The following high-temperature salt damage test was carried out using Examples A to T of the present invention in Table 1 and Comparative Examples a to o in Table 2 as test materials. As a high-temperature salt damage test, the corrosion weight loss after 20 cycles of heating, cooling, salt water immersion, and drying of the test piece was evaluated. The heating conditions were a temperature of 750 ° C. and a holding time of 130 minutes. The cooling conditions were a temperature of room temperature and a holding time of 30 minutes. The salt water immersion conditions were a saturated NaCl aqueous solution for salt water, a temperature of room temperature, and an immersion time of 30 minutes. The drying conditions were a temperature of 50 ° C. and a holding time of 30 minutes. The atmosphere for heating, cooling, and drying was in air with a dew point of 40 to 50 ° C. The weight difference between the test pieces before the high-temperature salt damage test and after removing the corrosion products generated in the high-temperature salt damage test was measured, and the value per surface area of the test piece before the high-temperature salt damage test was used as the corrosion weight loss. To remove the corrosion products on the surface of the test piece after the high-temperature salt damage test, the test piece was immersed in a boiling 15% by mass aqueous solution of ammonium dihydrogen dihydrogen citrate for 20 minutes, washed with water, and then brushed several times. did. The corrosion resistance of the high temperature salt damage test thus obtained was used to evaluate the high temperature salt damage resistance. When the corrosion weight loss was 150 mg / cm ² or less, the high temperature salt damage resistance was considered to be good.

Table 3 shows the values of the formulas (i) to (iv) and the measurement results of the corrosion weight loss in the above-mentioned high-temperature salt damage test. Further, FIG. 1 shows the evaluation results of the high-temperature salt damage test corrosion reduction with the Si content in the steel as the horizontal axis and the “Nb + Ti” content as the vertical axis. The broken line in the figure indicates a straight line in the equal sign of the equation (i).

Examples A to T of the present invention have a component composition within an appropriate range, further satisfy the formulas (i) to (iv), and have good high temperature salt damage resistance.

In Comparative Example a, Si is outside the lower limit of the appropriate range, in Comparative Example b, Mn is within the inappropriate range, in Comparative Example c, S is outside the upper limit of the appropriate range, and in Comparative Example d, Cr is the lower limit of the appropriate range. Detachment, high temperature salt damage resistance is insufficient.

In Comparative Examples e to o, the individual component compositions are within the appropriate range, but e to i do not satisfy the formula (i), j and k do not satisfy the formula (ii), and l and m are the formulas (l and m). iii) is not satisfied, n and o do not satisfy the formula (iv), and the high temperature salt damage resistance is insufficient.

The high temperature salt damage resistance of the steel of the present invention is good even when the heating temperature is 650 ° C, 700 ° C, 800 ° C, etc., when the salt water is a saturated CaCl ₂ aqueous solution, or when the atmosphere is a dry atmosphere. Met. From this, it is considered that the examples of the present invention exhibit excellent high-temperature salt damage resistance in various high-temperature salt damage environments.

As is clear from these, it can be seen that the steels having the individual component compositions specified in the present invention and satisfying the formulas (i) to (iv) are excellent in high temperature salt damage resistance.

According to the present invention, it is possible to provide a ferrite stainless steel sheet having excellent high temperature salt damage resistance for applications such as exhaust manifolds and front pipes that require high temperature salt damage resistance. Specific applications include automobile exhaust system parts where salt adhesion is promoted by a forced cooling mechanism, and automobile exhaust where salt adhesion is promoted by applying a heat insulating structure that covers the periphery of the automobile exhaust system with a heat insulating material. It is a system part. By making these parts possible, it is possible to promote the spread of turbo-equipped vehicles to which these parts are applied and engine vehicles that burn HCCI (premixed compression automatic ignition), and contribute to improving the fuel efficiency of automobiles and reducing the environmental load. ..

1 Automobile exhaust system member 2 Exhaust manifold 3 Forced cooling mechanism 4 Cooling fan 5 Blower 6 Insulation structure 7 Insulation material 8 Insulation material cover 10 Exhaust gas 11 Cooling air flow

Claims (6)

By mass%,
C: 0.0200% or less,
N: 0.0200% or less,
Si: 0.30% or more and 4.00% or less,
Mn: 0.01% or more, 0.40% or less, or 0.80% or more, 2.00% or less,
P: 0.040% or less,
S: 0.0014% or less,
Cr: 13.0% or more, 23.0% or less,
Ni: 0.01% or more, 0.40% or less,
Cu: 0.01% or more, less than 0.10%,
Nb: 0.20% or more, less than 0.60%,
Ti: 0.001% or more, 0.270% or less,
Al: 0.002% or more, 0.200% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0001% or more, 0.0050% or less,
O: 0.0050% or less,
A ferrite-based stainless steel sheet having excellent high-temperature salt damage resistance, characterized in that the balance is composed of Fe and unavoidable impurities and has a composition satisfying the following formulas (i) to (iv).
Nb + Ti + 0.4Si ≧ 0.630 ・ ・ ・ Equation (i)
Cr + 10Si ≧ 17.0 ・ ・ ・ Equation (ii)
0.0110 ≤ C + N ≤ 0.0270 ... Equation (iii)
Al / O ≧ 1.2 ・ ・ ・ Equation (iv)
However, the element symbol in the formula means the content (mass%) of the element. By mass%, Y: 0.001% or more, 0.20% or less,
REM: 0.001% or more, 0.20% or less,
Ca: 0.0002% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Hf: 0.001% or more, 1.0% or less,
Sn: 0.002% or more, 1.0% or less,
Mg: 0.0002% or more, 0.0030% or less,
Co: 0.01% or more, 0.30% or less,
Sb: 0.005% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0002% or more, 0.30% or less,
The ferrite-based stainless steel sheet having excellent high-temperature salt damage resistance according to claim 1, which contains one or more of the above-mentioned. The ferritic stainless steel sheet having excellent high temperature salt damage resistance according to claim 1 or 2, which is used for an automobile exhaust system member in an environment where salt adhesion is promoted by a forced cooling mechanism. The high temperature salt damage resistance according to claim 1 or 2, which is used for an automobile exhaust system member in an environment where salt adhesion is promoted by applying a heat insulating structure that covers the periphery of the automobile exhaust system member with a heat insulating material. Excellent ferritic stainless steel plate. An automobile exhaust system component provided with an automobile exhaust system member using the ferritic stainless steel plate having excellent high temperature salt damage resistance according to claim 1 or 2, and a forced cooling mechanism for forcibly cooling the member. An automobile exhaust system component provided with an automobile exhaust system member using the ferritic stainless steel plate having excellent high temperature salt damage resistance according to claim 1 or 2, and a heat insulating structure covering the periphery thereof with a heat insulating material.

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