JP6778621B2 - Austenitic stainless steel sheet for exhaust parts and its manufacturing method, and exhaust parts and their manufacturing method - Google Patents

Description

The present invention relates to an austenitic stainless steel sheet for exhaust parts and a method for manufacturing the same, and an exhaust part and a method for manufacturing the same.

Exhaust manifolds, front pipes, center pipes, mufflers, and environmentally friendly parts for exhaust gas purification of automobiles need to provide stable ventilation of high-temperature exhaust gas. Therefore, a material having excellent heat resistance such as oxidation resistance, high temperature strength, and thermal fatigue characteristics is used, but it is also required to have excellent corrosion resistance because it is also a condensed water corrosive environment.

Stainless steel is often used for these parts from the viewpoints of tightening exhaust gas regulations, improving engine performance, and reducing the weight of the vehicle body. Further, in recent years, exhaust gas regulations have been tightened further, fuel efficiency has been improved, downsizing and the like have led to an increase in the temperature of exhaust gas that ventilates the exhaust manifold directly under the engine.

In addition, there are many cases where a supercharger such as a turbocharger is mounted, and the exhaust manifold or the stainless steel used for the turbocharger is required to have further improved heat resistance. With regard to the rise in the exhaust gas temperature, it is expected that the exhaust gas temperature, which was about 900 ° C. in the past, will rise to about 1000 ° C.

On the other hand, the internal structure of the turbocharger is complicated, and it is important to improve supercharging efficiency and ensure heat resistance, and the use of heat-resistant austenitic stainless steel is mainly disclosed.

In addition to SUS310S (25% Cr-20% Ni), which is a typical heat-resistant austenitic stainless steel, and Ni-based alloys, Patent Document 1 discloses high Cr and high Ni steel. Patent Documents 2 and 3 disclose Si-containing high Cr-high Ni-Mo steel containing (C + N) = 0.25 to 0.35% by mass as an austenitic stainless steel having excellent resistance at 1100 ° C. There is.

Further, Patent Document 4 discloses an exhaust guide component of a nozzle vane type turbocharger using austenitic stainless steel containing 2 to 4% by mass of Si.

JP-A-2002-332862 International Publication 2014/157655 Japanese Unexamined Patent Publication No. 2016-89200 Japanese Patent No. 4937277

Among the turbochargers, various precision parts are used inside the nozzle vane type turbocharger. Specifically, among the internal precision parts, parts called back plates and oil deflectors are located between the turbine part and the compressor part and the center core. These parts are parts that stably rotate the turbine and compressor wheel while maintaining the sealing performance of each part. Therefore, in addition to heat resistance (oxidation resistance and high temperature strength), surface smoothness, that is, surface roughness is required to be small.

Further, there are nozzle parts composed of precision parts such as a nozzle mount, a nozzle plate, a nozzle ring, a nozzle vane, a drive ring, and a drive lever in order to adjust the flow velocity and the flow rate of the exhaust gas. Since these parts are in contact with high-temperature exhaust gas, heat-resistant properties such as high-temperature strength, creep characteristics, and oxidation resistance are required. In addition, since the exhaust gas flow velocity and the flow rate are adjusted by opening and closing the vane, high temperature slidability is also required.

The high-temperature slidability is a friction / wear characteristic at a high temperature, and refers to a surface smoothness after repeated wear at a high temperature. When the high temperature slidability is excellent, the surface smoothness is good and the surface roughness is small even after repeated wear at a high temperature.

Further, in the manufacturing process of the precision parts, holes are often drilled or holes are expanded, and extremely high dimensional accuracy and plate thickness accuracy are required, so workability is also important. Conventionally, since a material having excellent heat resistance is hard, a casting may be used. Further, even when a steel plate is used, it has been used after being cut and ground after processing due to problems of cracks, cracks, and processing accuracy during processing of parts. However, there is a drawback that the cost of processing materials and parts increases by going through the above manufacturing process.

In Patent Document 1, an exhaust guide assembly in a turbocharger is manufactured by a precision casting method and a metal injection molding method, and processing molding is not required. Therefore, Patent Document 1 does not examine hot workability. Further, if the above-mentioned manufacturing method is selected, the manufacturing cost of the parts increases.

In Patent Document 2, high temperature strength, oxidation resistance, and processability are studied. However, the friction and wear characteristics at high temperatures such as workability and high-temperature slidability, which are required for precision parts inside the turbocharger, such as more detailed hole expansion, have not been sufficiently studied.

Similar to Patent Document 2, Patent Document 3 examines high-temperature strength, oxidation resistance, and processability. However, the workability such as more detailed hole expansion and the friction / wear characteristics at high temperature such as high temperature slidability, which are required for precision parts inside the turbocharger, have not been sufficiently studied. Further, since the rare earth element (REM) is contained as an essential element, there is a problem that the manufacturing cost increases.

In the composition of the high Si component as shown in Patent Document 4, the amount of scale generated at high temperature is small and the scale adhesion is excellent. Therefore, when each component slides in a high temperature environment, scale peeling and wear are small, and excellent high temperature slidability can be obtained. However, when the Si content is high, problems such as cracks are likely to occur during plate material processing.

Therefore, Patent Document 4 defines the composition in consideration of hot workability. However, in the high temperature tensile test conducted to evaluate the high temperature strength in the examples, the adaptation temperature is set to 800 ° C., and the high temperature characteristics in a higher temperature range are unknown. Further, in Patent Document 4, it cannot be said that the hole expandability in the shape of the part, which is in a state where more strain is applied when used in a higher temperature range, is not sufficiently examined.

Therefore, both the above workability and the high-temperature slidability that can be obtained with a high Si component have not been sufficiently obtained by the conventional techniques. For this reason, measures have been taken such as ensuring high temperature slidability by subjecting low Si steel such as SUS310S to a surface treatment such as chromizing treatment. However, the above-mentioned treatment has a problem that the manufacturing cost increases.

The present invention solves the above problems, and is particularly suitable for automobile exhaust parts for turbocharger parts. Austenitic stainless steel sheets for exhaust parts, which are excellent in heat resistance, workability, and high-temperature slidability, and The purpose is to provide exhaust components.

In order to solve the above problems, the present inventors have detailed the austenitic stainless steel sheet for exhaust parts and its manufacturing method, and the exhaust parts and their manufacturing method from the viewpoint of steel composition, processing characteristics, surface properties, and high temperature characteristics. I did a study. As a result, the following findings (A) to (D) were obtained.

(A) An important characteristic of austenitic stainless steel sheets used in environments where heat resistance is required is heat resistance such as high temperature strength. In addition to high temperature strength, if the processing accuracy, that is, the workability, is poor when processing precision parts inside the turbocharger, in addition to contact and seizure between parts, gas airtightness or poor gas flow may occur. It ends up. As a result, the turbocharger is damaged or the thermal efficiency is lowered, which leads to a decrease in the reliability of component performance.

Among the internal parts of the turbocharger, specifically, the parts constituting the nozzle vane and the wastegate valve operate in a high temperature non-lubricating environment. For this reason, if the high temperature slidability is poor, phenomena such as seizure, adhesion, and wear occur, and as a result, the entire turbocharger becomes immobile.

Therefore, in addition to heat resistance such as high temperature strength, workability, surface smoothness, and high temperature slidability are also extremely important characteristics for austenitic stainless steel sheets used for turbocharger parts as described above. ..

(B) By appropriately controlling the steel composition, parts having excellent workability including heat resistance such as high temperature strength and hole expanding property can be obtained.

(C) It was found that excellent surface smoothness can be ensured by surface treatment (molten alkali salt treatment and pickling treatment). Further, in order to obtain excellent surface smoothness, it is usual to perform a polishing treatment after a cutting treatment, a grinding treatment, or cold rolling annealing of a cast product. However, in the present invention, excellent surface smoothness can be obtained by performing surface treatment after cold rolling annealing. Therefore, in the present invention, it is possible to omit the cutting and grinding steps or reduce the load.

(D) In order to obtain good high-temperature slidability, it is effective to form a (Si + Al) concentrated layer on the surface layer by surface treatment. Specifically, the following findings were obtained regarding the (Si + Al) concentrated layer and high-temperature slidability. FIG. 1 shows the surface coating profile of 19% Cr-13% Ni-3.5% Si steel.

Here, the surface film profile is obtained by measuring the concentration of each element from the surface using an Auger electron spectrometer and showing it as a cation fraction. In the high-temperature slidability test, a base sample having a thickness of 10 × 20 × 4.3 mm and a sliding sample were heated to 900 ° C., and a load of 0.4 MPa was applied in the direction perpendicular to the sliding plate. It was slid at a speed of 3.3 mm / s.

After that, the surface roughness (maximum roughness) of the sliding portion was used as a guideline for high-temperature slidability. The larger the maximum roughness, the worse the high temperature slidability. For example, if the maximum roughness exceeds 100 μm, the high temperature slidability required for the exhaust component cannot be obtained.

In FIG. 1A, the maximum (Si + Al) concentration of the surface layer is as high as 33.9% by mass, the roughness after the high temperature sliding test is small, and the high temperature sliding property is excellent. Further, in (b), although the maximum concentration of (Si + Al) on the surface layer is 9.8% by mass, which is lower than that of (a), the high temperature slidability is satisfied. On the other hand, in (c), the maximum concentration of (Si + Al) on the surface layer is as low as 7.9% by mass, and the high temperature slidability is poor.

This difference is caused by the presence or absence of heat treatment for enriching Si and Al, in addition to the difference in the pickling method including the molten alkali salt treatment of the cold rolled plate described later. Si and Al are concentrated in the oxide scale formed in the heat treatment assumed in the present invention to suppress wear resistance or scale peeling property. Therefore, if (Si + Al) is not concentrated on the surface layer, high temperature slidability cannot be obtained.

The present invention has been made to solve the above problems, and the gist of the present invention is the following austenitic stainless steel sheet and parts using the same.

(1) By mass%
C: 0.005-0.2%,
Si: 1.5-5.0%,
Mn: 0.1 to 5.0%,
P: 0.01-0.05%,
S: 0.0001 to 0.01%,
Ni: 5.0 to 15.0%,
Cr: 15.0 to 25.0%,
N: 0.02-0.4%,
Al: 0.003 to 1.0%,
Cu: 0.05-3.0%
Mo: 0.01-2.0%,
V: 0.05 to 1.0%,
Ti: 0-0.3%,
Nb: 0-0.3%,
B: 0 to 0.0050%,
Ca: 0-0.01%,
W: 0-3.0%,
Zr: 0-0.3%,
Sn: 0-0.5%,
Co: 0-0.3%,
Mg: 0-0.01%,
Sb: 0-0.5%,
REM: 0-0.2%,
Ga: 0-0.3%, and Ta: 0-1.0%,
Contains,
The rest consists of Fe and unavoidable impurities
An austenitic stainless steel sheet for exhaust parts, in which the maximum concentration of (Si + Al) on the surface layer is 8.0% or more in mass%.

(2) By mass%,
Ti: 0.005-0.3%,
Nb: 0.005-0.3%,
B: 0.0002 to 0.0050%,
Ca: 0.0005-0.01%,
W: 0.1 to 3.0%,
Zr: 0.05-0.3%,
Sn: 0.01-0.5%,
Co: 0.03 to 0.3%,
Mg: 0.0002 to 0.01%,
Sb: 0.005-0.5%,
REM: 0.001-0.2%,
Ga: 0.0002 to 0.3%, and Ta: 0.001 to 1.0%,
The austenitic stainless steel sheet for exhaust parts according to (1), which contains at least one selected from.

(3) The contents of C and N are
The austenitic stainless steel sheet for exhaust parts according to (1) or (2), which satisfies the relationship of C / N <1.0.

(4) A step of annealing a cold-rolled steel sheet having the chemical composition according to any one of (1) to (3), a step of immersing in a molten alkali salt of 450 ° C. or lower for 30 seconds or less, and a step of 10 ° C./sec or more. The method for producing an austenite-based stainless steel sheet for exhaust parts according to any one of (1) to (3), which comprises a step of cooling at a speed and a step of pickling.

(5) An exhaust component using the austenitic stainless steel plate according to any one of (1) to (3).

(6) The exhaust component according to (5), which is a component for a turbocharger.

(7) The exhaust component according to (5) or (6), wherein the maximum concentration of (Si + Al) on the surface layer of the exhaust component is 20.0% or more in mass%.

(8) A method for manufacturing an exhaust component, comprising a molding step of molding the austenitic stainless steel plate according to (1) to (3) into an exhaust component shape and a heat treatment step of 900 ° C. or higher for 5 seconds or longer. hand,
The heat treatment step is performed after the molding step or in the middle of the molding step.
The method for manufacturing an exhaust component according to any one of (5) to (7).

According to the present invention, it is possible to provide an austenitic stainless steel sheet having excellent heat resistance, workability, and high-temperature slidability, and an exhaust component. Since this steel sheet has good workability, particularly hole expansion property, grinding process and polishing process can be reduced. Further, since the high temperature slidability is excellent, the gas flow is good, and as a result, it is possible to provide an automobile exhaust component having excellent thermal efficiency.

FIG. 1 is a diagram showing the elemental concentration of the surface and the maximum roughness after the high temperature sliding test.

Hereinafter, each requirement of the present invention will be described in detail.

1. 1. Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content of each element means "mass%".

C: 0.005-0.2%
C is necessary for the formation of austenite structure and ensuring high temperature strength. Therefore, the C content is set to 0.005% or more. Further, considering the manufacturing cost and hot workability, the C content is preferably 0.008% or more. On the other hand, if it is excessively contained, work hardening becomes excessively large, and corrosion resistance, particularly intergranular corrosiveness of the welded portion, deteriorates due to Cr carbide formation. In addition, the surface roughness becomes rough due to deterioration of high-temperature slidability due to carbides and formation of intergranular erosion grooves during cold-rolled annealing plate pickling. Therefore, the C content is preferably 0.2% or less, preferably 0.1% or less.

Si: 1.5-5.0%
Si may be contained as a deoxidizing element, and internal oxidation of Si improves scale peeling property, high temperature slidability, and high temperature strength. Therefore, the Si content is set to 1.5% or more. Further, considering the high temperature slidability, the Si content is preferably 2.2% or more. On the other hand, Si is hardened by containing more than 5.0%, and coarse Si-based oxides are generated, which significantly reduces the processing accuracy of parts. Therefore, the Si content is set to 5.0% or less. The Si content is preferably 4.0% or less in consideration of the manufacturing cost, pickling property during steel sheet manufacturing, and solidification cracking property during welding.

Mn: 0.1 to 5.0%
Mn is an element that is used as a deoxidizing element and also ensures austenite structure formation and scale adhesion. Therefore, the Mn content is set to 0.1% or more. Further, in consideration of the manufacturing cost and the pickling property at the time of manufacturing the steel sheet, the Mn content is preferably 0.2% or more. On the other hand, when the content exceeds 5.0%, the change in work hardening becomes large, the cleanliness of inclusions is remarkably deteriorated, and the hole expandability is lowered. In addition, the pickling property is significantly deteriorated and the product surface becomes rough. Therefore, the Mn content is set to 5.0% or less. From the viewpoint of softening, the Mn content is preferably 1.5% or less, and more preferably less than 0.8%.

P: 0.01-0.05%
P is an element that promotes hot workability and solidification cracking during manufacturing, and is also an element that hardens the steel sheet. Therefore, the P content is preferably 0.05% or less, preferably 0.04% or less. The smaller the content of P, the better, but on the other hand, in consideration of the refining cost, the P content is preferably 0.01% or more, preferably 0.02% or more.

S: 0.0001 to 0.01%
S is an element that lowers the hot workability during manufacturing and also deteriorates the corrosion resistance. Further, when coarse sulfide (MnS) is formed, the cleanliness is remarkably deteriorated, and the hole expandability is deteriorated. Therefore, the S content is preferably 0.01% or less, preferably 0.0050% or less.
On the other hand, since excessive reduction leads to an increase in refining cost, the S content is set to 0.0001% or more. Further, considering the production cost and oxidation resistance, the S content is preferably 0.0005% or more.

Ni: 5.0 to 15.0%
Ni is an element that forms an austenite structure and also ensures corrosion resistance and oxidation resistance. Further, if it is less than 5.0%, coarsening of crystal grains occurs remarkably, so the Ni content is set to 5.0% or more. Further, considering the manufacturability, high temperature strength, and corrosion resistance, the Ni content is preferably 10.0% or more. On the other hand, the Ni content is set to 15.0% or less because an excessive content causes an increase in cost and hardening.

Cr: 15.0 to 25.0%
Cr is an element that improves corrosion resistance, oxidation resistance, and high-temperature slidability. Furthermore, it suppresses abnormal oxidation in the exhaust component environment. Therefore, the Cr content is set to 15.0% or more. In addition, the Cr content is preferably 17.0% or more in consideration of the manufacturing cost, the steel sheet manufacturability, and the processability. On the other hand, excessive content leads to hardness, deterioration of moldability, and cost increase. Therefore, the Cr content is preferably 25.0% or less, preferably 24.5% or less.

N: 0.02-0.4%
Like C, N is an element effective for forming an austenite structure and ensuring high-temperature strength and high-temperature slidability, and is known as a solid solution strengthening element with respect to high-temperature strength. In addition, in the present invention, in addition to the effect of N alone, the N content is set to 0.02% or more in consideration of the improvement of high temperature strength due to the formation of clusters with Cr. Further, from the viewpoint of high temperature strength and slidability, the N content is preferably 0.04% or more. On the other hand, when the content exceeds 0.4%, the material at room temperature becomes remarkably hard and the cold workability at the steel sheet manufacturing stage deteriorates. For this reason, the moldability at the time of processing the part and the accuracy of the part are deteriorated. Therefore, the N content is set to 0.4% or less. From the viewpoint of softening, suppressing pinholes during welding, and suppressing intergranular corrosion of the welded portion, it is preferably 0.3% or less.

Al: 0.003 to 1.0%
Al is used as a deoxidizing element and improves the hole expandability by improving the cleanliness of inclusions. In addition, it has the effect of contributing to the improvement of high-temperature slidability by suppressing the peeling of the oxidation scale and the trace internal oxidation, and the action is exhibited from 0.003% or more. Therefore, the Al content is set to 0.003% or more. Further, since it is a ferrite-forming element, a content of more than 1.0% lowers the stability of the austenite structure, lowers the pickling property, and causes an increase in surface roughness. Therefore, the Al content is set to 1.0% or less. Considering the refining cost and surface defects, the Al content is preferably 0.005% or more, and preferably 0.5% or less. Further, in consideration of hot workability, the Al content is preferably 0.02% or more, and more preferably 0.1% or less.

Cu: 0.05-3.0%
Cu is an effective element for stabilizing and softening the austenite phase. Therefore, the Cu content is set to 0.05% or more. On the other hand, the Cu content is set to 3.0% or less because excessive content leads to deterioration of oxidation resistance and manufacturability. Further, in consideration of corrosion resistance and manufacturability, the Cu content is preferably 0.1% or more, and preferably 1.0% or less.

Mo: 0.01-2.0%
Mo is an element that improves corrosion resistance and contributes to the improvement of high-temperature strength. In the present invention, in addition to solid solution strengthening, it contributes to precipitation strengthening by Mo carbide and improvement of wear resistance. Therefore, the Mo content is set to 0.01% or more. The Mo content is 2.0% or less. Further, considering that Mo is an expensive element, the strengthening stability due to the precipitate and the cleanliness of inclusions, the Mo content is preferably 0.2% or more, and preferably 1.7% or less. Is preferable.

V: 0.05 to 1.0%
V is an element that improves corrosion resistance and forms V carbides to improve high-temperature strength and high-temperature wear resistance. Therefore, the V content is set to 0.05% or more. On the other hand, excessive content leads to an increase in alloy cost or a decrease in abnormal oxidation limit temperature. Therefore, the V content is set to 1.0% or less. Further, in consideration of manufacturability and cleanliness of inclusions, the V content is preferably 0.1% or more, and preferably 0.5% or less.

The above are essential elements, but one or more of the following elements may be contained as a substitute for a part of Fe.

Ti: 0-0.3%
Ti is added as necessary in order to combine with C and N to improve corrosion resistance and intergranular corrosion resistance. However, a content of more than 0.3% tends to cause nozzle clogging at the casting stage and significantly deteriorates manufacturability. In addition, the formation of coarse Ti carbonitrides deteriorates the hole expansion workability. Therefore, the Ti content is set to 0.3% or less. Further, considering the high temperature strength, the intergranular corrosiveness of the welded portion, and the alloy cost, the Ti content is preferably 0.2% or less. In order to obtain the above effect, the Ti content is preferably 0.005% or more, and more preferably 0.01% or more.

Nb: 0 to 0.3%
Like Ti, Nb combines with C and N to improve corrosion resistance and intergranular corrosion resistance. In addition, since it is an element that improves high-temperature strength, it is contained as necessary. However, if the content exceeds 0.3%, the hot workability at the steel sheet manufacturing stage is significantly deteriorated, and the hole expandability is deteriorated due to the coarse Nb carbonitride. Therefore, the Nb content is set to 0.3% or less. On the other hand, the fixing action of binding to C and N is expressed when Nb is contained in an amount of 0.005% or more. Therefore, the Nb content is preferably 0.005% or more. Further, considering the high temperature strength, the intergranular corrosiveness of the welded portion, and the alloy cost, the Nb content is preferably 0.01% or more, and preferably less than 0.15%.

B: 0 to 0.0050%
B is an element that improves hot workability at the steel sheet manufacturing stage and has an effect of suppressing work hardening at room temperature. Therefore, it is contained as needed. However, excessive content results in deterioration of cleanliness, hole expandability, and intergranular corrosiveness due to the formation of boring carbides. Therefore, the B content is set to 0.0050% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.0002% or more. Further, considering the refining cost and the decrease in ductility, the B content is more preferably 0.0003% or more, and preferably 0.002% or less.

Ca: 0-0.01%
Ca is added as needed for desulfurization. However, when it is contained in excess of 0.01%, water-soluble inclusions CaS are formed. As a result, cleanliness and corrosion resistance are deteriorated. Therefore, the Ca content is set to 0.01% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.0005% or more. Further, from the viewpoint of manufacturability and surface quality, the Ca content is preferably 0.0010% or more, and preferably 0.0030% or less.

W: 0-3.0%
W is contained as necessary because it contributes to the improvement of corrosion resistance and high temperature strength. However, a content of more than 3.0% leads to hardening, deterioration of toughness during steel sheet production, and cost increase. Therefore, the W content is set to 3.0% or less. On the other hand, in order to obtain the above effect, the W content is preferably 0.1% or more. Further, in consideration of refining cost and manufacturability, the W content is preferably 0.1% or more, and preferably 2.0% or less.

Zr: 0-0.3%
Zr is added as necessary in order to combine with C and N to improve intergranular corrosion resistance and oxidation resistance of the welded portion. However, the content of more than 0.3% increases the manufacturing cost and significantly deteriorates the manufacturability and the hole expandability. Therefore, the Zr content is set to 0.3% or less. Further, in consideration of refining cost and manufacturability, the Zr content is preferably 0.1% or less. On the other hand, in order to obtain the above effect, the Zr content is preferably 0.05% or more.

Sn: 0-0.5%
Sn is contained as necessary because it contributes to the improvement of corrosion resistance and high temperature strength. However, if the content exceeds 0.5%, slab cracking may occur during steel sheet production, so the Sn content is set to 0.5% or less. Further, considering the refining cost and manufacturability, it is preferably 0.3% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably 0.01% or more, more preferably 0.03% or more, and further preferably 0.05% or more. ..

Co: 0-0.3%
Co is contained as necessary because it contributes to the improvement of high temperature strength. However, a content of more than 0.3% leads to hardening, deterioration of toughness during steel sheet manufacturing, and increase in manufacturing cost. Therefore, the Co content is set to 0.3% or less. Further, in consideration of refining cost and manufacturability, the Co content is preferably 0.1% or less. On the other hand, in order to obtain the above effect, the Co content is preferably 0.03% or more.

Mg: 0-0.01%
In addition to being used as a deoxidizing element, Mg is an element that finely disperses oxides in the slab structure and contributes to the improvement of inclusion cleanliness and the miniaturization of the metal structure. Therefore, it is contained as needed. However, excessive content leads to deterioration of weldability and corrosion resistance, and deterioration of hole expansion due to coarse inclusions. Therefore, the Mg content is set to 0.01% or less. On the other hand, in order to obtain the above effect, the Mg content is preferably 0.0002% or more. Further, in consideration of the refining cost, the Mg content is preferably 0.0003% or more, and preferably 0.005% or less.

Sb: 0-0.5%
Since Sb is an element that segregates at grain boundaries to increase high-temperature strength, it is contained as necessary. However, if the content exceeds 0.5%, Sb segregates and cracks occur during welding. Therefore, the Sb content is set to 0.5% or less. On the other hand, in order to obtain the above effect, the Sb content is preferably 0.005% or more. Considering the high temperature characteristics, the manufacturing cost, and the toughness, the Sb content is preferably 0.03% or more, and preferably 0.3% or less. Further, the Sb content is more preferably 0.05% or more, and more preferably 0.2% or less.

REM: 0-0.2%
REM (rare earth element) is effective in improving oxidation resistance or high-temperature slidability, and is contained as necessary. Further, even if it is contained in excess of 0.2%, the effect is saturated and the corrosion resistance due to the sulfide of REM is lowered. Therefore, the REM content is set to 0.2% or less. On the other hand, in order to obtain the above effect, the REM content is preferably 0.001% or more. Further, considering the processability of the product and the manufacturing cost, the REM content is preferably 0.002% or more, and preferably 0.10% or less.

REM (rare earth element) follows a general definition. It is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). It may be contained alone or as a mixture.

Ga: 0-0.3%
Ga is contained as necessary in order to improve corrosion resistance or suppress hydrogen embrittlement. If the content exceeds 0.3%, coarse sulfide is produced and the r value deteriorates. Therefore, the Ga content is set to 0.3% or less. On the other hand, from the viewpoint of sulfide or hydride formation, the Ga content is preferably 0.0002% or more. Further, from the viewpoint of manufacturability and manufacturing cost, the Ga content is more preferably 0.002% or more.

Ta: 0-1.0%
Ta is added as needed to improve high temperature strength. The Ta content is 1.0% or less. On the other hand, in order to obtain the above effect, the Ta content is preferably 0.001% or more. Further, in order to obtain high strength, the Ta content is more preferably 0.01% or more. Further, for the same reason as Ta, Hf is contained in an amount of 0.001 to 1.0% as needed. Bi is also contained in an amount of 0.001 to 0.02% as required. It is desirable to reduce generally harmful elements such as As and Pb and impurity elements as much as possible.

In the present invention, impurities are mixed from ore, scrap, manufacturing environment, etc. as a raw material when a steel material is industrially manufactured, and are within a range that does not adversely affect the steel material for nitrided metal of the present invention. Means what is acceptable.

2. Maximum Concentration of Surface Layer (Si + Al) In the present invention, the maximum concentration of surface layer (Si + Al) is specified in order to satisfy wear resistance and slidability at high temperatures. Specifically, the (Si + Al) concentration of the surface layer of the stainless steel sheet is set to a maximum of 8.0% or more in mass%. The surface layer shown here refers to a depth of 1.0 μm from the surface.

Si and Al are concentrated in the oxide scale formed in the heat treatment assumed in the present invention to suppress wear resistance or scale peeling property. Therefore, if (Si + Al) is not concentrated on the surface layer, high temperature slidability cannot be obtained. In the present invention, the maximum concentration of (Si + Al) on the surface layer is defined as 8.0% or more in order to satisfy the high temperature slidability, but 9.0% or more is more preferable. In addition, in order to improve reliability in a high temperature environment after processing the shape of the part, it is more preferable that the maximum concentration of (Si + Al) on the surface layer is 20.0% or more.

3. 3. C / N content In the present invention, the balance between C and N of high Si-containing austenitic stainless steel sheets is precisely examined for the drilling property, hole expandability, and high-temperature strength characteristics of the nozzle plate and nozzle mount inside the turbo. did. As a result, C / N, which is the ratio of the C content (mass%) to the N content (mass%), satisfies C / N <1.0, so that the workability is ensured while ensuring high temperature strength. It was found that it is possible to combine the above. Therefore, it is preferable to satisfy C / N <1.0. Further, it is more preferable to satisfy C / N <0.5 from the viewpoint of high temperature strength.

Specifically, both C, N, and both elements increase the work hardening characteristics at room temperature and deteriorate the drilling or hole expanding property. Both elements are elements that improve high-temperature strength, but N is more effective for high-temperature strengthening.

Table 1 shows the effect of C / N on the normal temperature and high temperature strength of Si-containing austenitic stainless steel.

The smaller the C / N, the larger the 0.2% proof stress at room temperature and high temperature, and the higher the high temperature strength when C / N <0.5.

4. Manufacturing method Next, the manufacturing method will be described. The method for producing a steel sheet of the present invention is not limited, but may consist of, for example, steelmaking-hot rolling-annealing / pickling, or steelmaking-hot rolling-annealing / pickling-cold rolling-annealing / pickling. Is preferable.

In steelmaking, a method is preferable in which steel containing the essential element or an optional element contained as needed is melted in an electric furnace or a converter, followed by secondary refining. The molten steel is slabized according to a known casting method (continuous casting or the like).

The slab is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness by continuous rolling. As described above, for the parts to which the present invention is intended, manufacturing conditions for ensuring predetermined crystal grain size, cross-sectional hardness, and surface roughness are set in the steps after hot rolling. The steel sheet after hot rolling is generally subjected to hot-rolled sheet annealing and pickling treatment, but hot-rolled sheet annealing may be omitted. The conditions for annealing the steel sheet after hot rolling and pickling are not particularly limited, and general conditions may be used. After that, it is cold-rolled to a predetermined plate thickness, and is subjected to cold-rolled plate annealing and pickling treatment.

Usually, annealing is to obtain a recrystallized structure by keeping the temperature below 1000 to 1100 ° C., but in the present invention, the annealing temperature is set to 1100 ° C. or higher. This is to ensure a crystal grain size number of 8.5 or less (standard name JIS G 0551) by annealing at a temperature higher than usual, and to ensure softness, drilling property, hole expanding property, and various processing accuracy. Is preferable.

4-1. Surface treatment conditions In the present invention, in order to ensure high-temperature slidability of exhaust parts, a cold-rolled annealed plate is immersed in a molten alkaline salt as a pretreatment for pickling treatment. In this molten alkali salt treatment, Cr in the scale generated by annealing is modified and dissolved in the molten alkali salt to obtain a Fe-based scale. The composition of the molten alkali salt is NaNO ₃ (10 to 30%) + NaOH solution. Then, as the pickling treatment, a chemical descale treatment such as nitric acid pickling or nitric acid electrolysis is performed.

On the other hand, in the case of an austenitic stainless steel sheet containing 1.5% or more of Si and 0.003% or more of Al, internal oxides of Si and Al form needles in the inner layer of the scale mainly composed of Fe and Cr. Generate. Although this internal oxide is difficult to dissolve during the pickling treatment, it dissolves when the reaction with the molten alkali salt proceeds. Here, it was found in the present invention that the internal oxides of Si and Al improve the high temperature slidability. Furthermore, it was clarified that the maximum concentration of (Si + Al) in the surface layer can be set to 8.0% or more by optimizing the molten alkali salt treatment conditions.

If the salt bath temperature of the molten alkali salt treatment exceeds 450 ° C., or if the immersion time exceeds 30 seconds, the Si oxide melts. Therefore, the salt bath temperature is set to 450 ° C. or less and the immersion time is set to 30 seconds or less as the temperature at which the Si oxide is difficult to melt during the molten alkali salt treatment. The salt bath temperature is preferably 440 ° C. or lower, and the immersion time is preferably 28 seconds or less.

On the other hand, when the salt bath temperature is 400 ° C. or lower, Cr oxide remains and scale residue is generated. Therefore, the salt bath temperature is preferably more than 400 ° C, more preferably 420 ° C or higher. Further, even if the immersion is performed for less than 5 seconds, scale remains, so that the immersion time is preferably 5 seconds or longer, and more preferably 10 seconds or longer.

In addition, in the present invention, the cooling rate from the molten alkali salt treatment to the pickling treatment is set to 10 ° C./sec or more. The cooling rate referred to here is an average cooling rate in a temperature range of 450 to 200 ° C. Further, cooling is preferably performed up to a temperature of 100 ° C. or lower. This is because at a cooling rate of 10 ° C./sec or less, scale is regenerated after the molten alkali salt treatment, and scale remains or surface roughness is likely to occur. As a result, the high temperature slidability deteriorates. A more preferable cooling rate is 15 ° C./sec or higher. Therefore, it is more preferable that the molten alkali salt temperature is 420 to 440 ° C., the molten alkali salt immersion time is 10 to 28 seconds, and the cooling rate is 15 ° C./sec or more.

4-2. Heat treatment for parts In the present invention, a (Si + Al) concentrated layer is formed on a stainless steel sheet product to obtain high-temperature slidability. However, Si and Al are applied to the surface layer after processing into turbocharger parts or during processing. It is even more effective to apply a heat treatment to thicken it. Specifically, for parts such as nozzle plates and nozzle mounts, the product plate is cut into a disk shape, and then holes are drilled and holes are expanded. After that, a hole is drilled to insert the precision pin, and heat treatment is performed at 900 ° C. or higher for 5 seconds or longer during or after the drilling. As a result, Si and Al are added to the surface layer to be concentrated by 20.0% or more. That is, the slidability of the sliding surface is ensured by setting the maximum concentration of (Si + Al) on the surface layer to 20.0% or more.

Both Si and Al are formed as internal oxides in the inner layer of the scale mainly composed of Fe and Cr of the stainless steel sheet, and the internal oxides are dense. Therefore, the peel resistance and the wear resistance are improved. Further heat treatment increases the (Si + Al) concentrated layer and improves the high temperature slidability. This heat treatment may be performed even after the part is processed, or may be applied during the processing to cut or grind the portion that does not require slidability.

The heat treatment atmosphere is not particularly specified, but N ₂ , Ar, N ₂ + H ₂ atmosphere is desirable. Further, if the heat treatment temperature is excessively high, the crystal grains become excessively coarse. As a result, roughened processed skin and the like occur, and conversely, high-temperature slidability deteriorates and exhaust gas flowability deteriorates. Therefore, the heat treatment temperature is preferably 1150 ° C. or lower.

If the heat treatment is performed for a long time, the crystal grains become excessively coarse, so the heat treatment time is preferably 10 minutes or less. The (Si + Al) enrichment does not have to be on the entire surface of the turbocharger component, but may be performed on a surface that requires high temperature sliding. For other parts, mechanical surface treatment may be performed as appropriate in consideration of dimensions or surface accuracy. Further, the heat treatment for (Si + Al) concentration may be carried out after the component is completed and before it is bonded to another component, or after it is bonded to another component.

4-3. Other conditions Other conditions in the manufacturing process may be appropriately selected. For example, the slab thickness, the hot-rolled plate thickness, and the like may be appropriately designed. In cold rolling, roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature and the like may be appropriately selected. Intermediate annealing may be inserted during cold rolling, and batch annealing or continuous annealing may be used.

Further, in the pickling step, in addition to nitric acid and nitric acid electrolytic pickling, a treatment using sulfuric acid or hydrochloric acid may be performed. After annealing and pickling of the cold-rolled sheet, the shape and material may be adjusted by temper rolling, tension leveler, or the like. Further, if the surface film of the present invention can be obtained, cold rolling and cold rolling plate annealing may be omitted. In addition, a lubricating film can be applied to the product plate for the purpose of improving press molding. Further, the heat resistance may be further improved by performing a special surface treatment such as nitriding treatment or carburizing treatment after processing the parts.

After melting the steels with the composition shown in Tables 2 and 3, hot-rolled to a thickness of 8.0 mm at 1250 ° C, hot-rolled sheet annealing, pickling, cold rolling at 1150 ° C, and final annealing and melting at 1150 ° C. Alkaline salt treatment and pickling were performed to obtain a steel sheet having a thickness of 4.3 mm.

The steel sheet of each composition is manufactured under the molten alkali salt treatment conditions shown in Tables 4 and 5, and measures the maximum concentration of (Si + Al) on the surface layer, a hole expansion test, and a high-temperature tensile test at 900 ° C. according to JIS G0567. , And a gas flow test was carried out.

The maximum concentration of (Si + Al) on the surface layer was measured by using an Auger electron spectrometer to measure the element concentration to a depth of about 80 μm from the surface. The depth analysis conditions were a sputtering rate of 100 Å / min and an integration number of 10 times. Only in Example 21 of the present invention in Table 4 and Comparative Example 29 in Table 5, the maximum concentration of (Si + Al) on the surface layer was measured at the stage of performing heat treatment after molding into a part shape.

In the hole expansion test, after expanding a punched hole having a diameter of 25 mm to an inner diameter of 32 mm, the cracking condition at the end of the rising portion having a height of 6 mm was confirmed. If the crack depth is 0.4 mm or less, the cutting load at the end is not applied and the performance of turbo parts is not hindered. Therefore, the case of 0.4 mm or less is passed (○), and cracks exceeding 0.4 mm are found. The case where it occurred was regarded as a failure (x).

Regarding the high-temperature tensile test, a steel having a proof stress of 40 MPa or more at 900 ° C. was passed (⊚), a case of 20 MPa or more and less than 40 MPa was generally passed (◯), and a case of less than 20 MPa was rejected (×). The strain rate was 2 × 10 ^-3 / sec.

In the test to confirm the gas flowability, the steel plate manufactured under the conditions shown in Tables 4 and 5 was processed into parts called nozzle mounts and nozzle plates, mounted on a nozzle vane type turbocharger, and the nozzles were opened and closed. Exhaust gas at a high temperature (900 ° C.) was flowed repeatedly, and the gas flowability (high temperature sliding property of the test material) was examined. The shape of the nozzle plate and nozzle mount is 4.5 mm in thickness, 100 mm in diameter, and 50 mm in center hole diameter. Steels with a gas flow velocity or flow rate change rate (before and after passing through the nozzle) of 95% or more during the test passed (◎), steels with 90% or more and less than 95% generally passed (○), steels with less than 90% Was rejected (x).

If one or more of the maximum concentration of (Si + Al) on the surface layer and the C / N ratio is out of the range of the present invention, the turbocharger performance such as high temperature strength or high temperature slidability becomes poor and a problem occurs. Therefore, those that do not satisfy these characteristics are described as rejected (x), those that satisfy one or more are generally accepted (○), and those that satisfy all are described as acceptable (◎).

The results are summarized in Table 4 below.

The results are summarized in Table 5 below.

It was confirmed that the steel of the example of the present invention is excellent in heat resistance, workability, and high temperature slidability, and satisfies the performance as a turbocharger component.

According to the present invention, it is possible to provide an austenitic stainless steel sheet having excellent heat resistance, workability, and high-temperature slidability. In particular, by using it as a part of a turbocharger of an automobile, it is possible to lead to exhaust gas regulation, weight reduction, and improvement of fuel efficiency. In addition, it is possible to omit cutting and grinding of parts and surface processing, which greatly contributes to cost reduction. Further, the present invention can be applied not only to exhaust parts for automobiles and motorcycles but also to parts used in high temperature environments such as various boilers and fuel cell systems, and the present invention is extremely useful in industry.

Claims (8)

By mass%
C: 0.005-0.2%,
Si: 1.87 ~5.0%,
Mn: 0.1 to 5.0%,
P: 0.01-0.05%,
S: 0.0001 to 0.01%,
Ni: 5.0 to 15.0%,
Cr: 15.0 to 25.0%,
N: 0.02 to 0.225 %,
Al: 0.003 to 1.0%,
Cu: 0.05-3.0%,
Mo: 0.01-2.0%,
V: 0.05 to 1.0%,
Ti: 0-0.3%,
Nb: 0-0.3%,
B: 0 to 0.0050%,
Ca: 0-0.01%,
W: 0-3.0%,
Zr: 0-0.3%,
Sn: 0-0.5%,
Co: 0-0.3%,
Mg: 0-0.01%,
Sb: 0-0.5%,
REM: 0-0.2%,
Ga: 0-0.3%, and Ta: 0-1.0%,
Contains,
The rest consists of Fe and unavoidable impurities
An austenitic stainless steel sheet for exhaust parts , wherein the maximum concentration of (Si + Al) in the surface layer is 8.0% or more in mass% when the surface layer is from the surface to a depth of 1.0 μm . By mass%,
Ti: 0.005-0.3%,
Nb: 0.005-0.3%,
B: 0.0002 to 0.0050%,
Ca: 0.0005-0.01%,
W: 0.1 to 3.0%,
Zr: 0.05-0.3%,
Sn: 0.01-0.5%,
Co: 0.03 to 0.3%,
Mg: 0.0002 to 0.01%,
Sb: 0.005-0.5%,
REM: 0.001-0.2%,
Ga: 0.0002 to 0.3%, and Ta: 0.001 to 1.0%,
The austenitic stainless steel sheet for exhaust parts according to claim 1, which contains at least one selected from. The content of C and N is
The austenitic stainless steel sheet for exhaust parts according to claim 1 or 2, which satisfies the relationship of C / N <1.0. A step of annealing a cold-rolled steel sheet having the chemical composition according to any one of claims 1 to 3, a step of immersing in a molten alkali salt at 450 ° C. or lower for 30 seconds or less, and cooling at a rate of 10 ° C./sec or higher. The method for producing an austenite-based stainless steel sheet for exhaust parts according to any one of claims 1 to 3, further comprising a step of rolling out and a step of pickling. An exhaust component using the austenitic stainless steel plate according to any one of claims 1 to 3. The exhaust component according to claim 5, which is a component for a turbocharger. The maximum concentration of (Si + Al) on the surface layer of the exhaust component is 20.0% or more in mass%.
The exhaust component according to claim 5 or 6. A method for manufacturing an exhaust component, comprising a molding step of molding the austenitic stainless steel plate according to claims 1 to 3 into an exhaust component shape, and a heat treatment step of 900 ° C. or higher for 5 seconds or longer.
The heat treatment step is performed after the molding step or in the middle of the molding step.
The method for manufacturing an exhaust component according to any one of claims 5 to 7.