JP7050520B2 - Manufacturing method of austenitic stainless steel sheet for exhaust parts and austenitic stainless steel sheet for exhaust parts and exhaust parts - Google Patents

Description

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

Among automobile parts, exhaust parts need to stably ventilate high-temperature exhaust gas. In addition, the exhaust components are used in an environment where the exhaust gas is severely corroded by the condensed water generated by cooling. For this reason, materials having excellent heat resistance and corrosion resistance such as oxidation resistance, high temperature strength, and thermal fatigue characteristics are used for exhaust parts.

In recent years, from the viewpoint of reducing the environmental load, environmentally friendly product design has been promoted in automobiles. For example, in the above product design, it is required to purify exhaust gas, improve fuel efficiency by reducing the weight of the vehicle body, and improve engine performance such as downsizing.

By the way, there are various types of exhaust parts. Specific examples include an exhaust manifold, a front pipe, a center pipe, a muffler, and an exhaust gas purification component. Further, in recent years, from the viewpoint of the above-mentioned downsizing, the number of cases where a supercharger such as a turbocharger is mounted on an automobile is increasing, and such a turbocharger is also included in the exhaust parts.

It has been pointed out that when the above turbocharger is installed, the temperature of the exhaust gas to be ventilated rises. Specifically, it is expected that the exhaust gas temperature, which was about 900 ° C. in the past, will rise to about 1000 ° C. In this case, the temperature of the material applied to the component rises from about 800 ° C. to about 900 ° C. Under these circumstances, exhaust parts such as turbochargers and exhaust manifolds installed directly under the engine are required to have further improved heat resistance.

The above-mentioned turbocharger mainly uses heat-resistant austenitic stainless steel from the viewpoint of internal structure, heat-resistant reliability and supercharging efficiency. For example, Patent Document 1 discloses a heat-resistant austenitic stainless steel having high strength and excellent oxidation resistance, which is applicable to turbocharger parts. Further, Patent Document 2 discloses an austenitic stainless steel which is suitable for a back plate and has excellent heat resistance and surface smoothness among turbocharger parts.

Among turbocharger parts, parts that repeatedly rub under high temperature and no lubrication may wear and adhere to each other in contact with each other. In this case, there may be a problem in opening and closing the vane for adjusting the flow rate of the exhaust gas in the turbocharger. As a result, it is conceivable that a gas flow failure occurs in the turbocharger, causing damage to parts and the like. Therefore, the above-mentioned parts are also required to have high-temperature friction resistance and high-temperature slidability, which are wear resistance characteristics.

From the viewpoint of ensuring the above-mentioned high-temperature slidability, the turbocharger component may be subjected to surface treatment for hardening the surface.

Patent Document 3 discloses a surface-modified material in which the surface of the base material of SUS310S (25% Cr-20% Ni) used as a material for a turbocharger is subjected to carburizing treatment or nitrification treatment to reduce the amount of wear. Has been done.

Further, Patent Document 4 describes a surface treatment method called an aluminase method in which a high Cr-high Ni steel is used as a base material and the base material is embedded in a mixed powder mainly composed of Al powder and Si powder to perform heat treatment. Is disclosed.

Further, Patent Document 5 discloses a heat-resistant austenitic stainless steel for turbocharger nozzle parts, which has good hole expandability, which is an index of workability, and excellent high-temperature slidability.

Patent Document 6 discloses Fe—Cr—Co—Si based martensite cast steel that does not require surface treatment.

International Publication No. 2014/157655 Japanese Unexamined Patent Publication No. 2017-14538 Japanese Unexamined Patent Publication No. 2008-15706 Japanese Patent Application Laid-Open No. 2003-129217 Japanese Unexamined Patent Publication No. 2017-88928 Special Table 2005-359138

As described above, the parts used in the turbocharger are expected to be repeatedly rubbed under high temperature and no lubrication as well as heat resistance, so it is necessary to suppress wear and adhesion between the parts. Therefore, the above-mentioned parts are required to have high-temperature slidability.

In the steel disclosed in Patent Documents 1 and 2, the high temperature slidability required for the turbocharger component has not been examined. Further, the surface treatment methods disclosed in Patent Documents 3 and 4 require special equipment such as the use of halogen gas, so that there is a problem that the manufacturing cost increases.

Further, although Patent Document 5 studies high-temperature slidability, it cannot be said that, for example, a scale formed in a usage environment, which affects high-temperature slidability, has been sufficiently studied.

Further, in the cast steel disclosed in Patent Document 6, self-lubrication occurs due to slight oxidation of the surface in the exhaust gas environment. Therefore, the cast steel disclosed in Patent Document 6 can suppress adhesion or clogging between parts. However, if the scale having a self-lubricating function is peeled off, wear may progress until the scale is formed again, and the high temperature slidability may decrease. Although the cast steel disclosed in Patent Document 6 has been examined for its scale at the time of use, it has a problem that the manufacturing cost increases because Co having a high content of 2% or more is used as an essential element. Further, the cast steel disclosed in Patent Document 6 has a martensite structure and is hard. Therefore, there is also a problem that the workability of parts is poor.

INDUSTRIAL APPLICABILITY The present invention solves the above problems and manufactures an austenitic stainless steel sheet having heat resistance and high-temperature slidability suitable for automobile exhaust parts, particularly parts for turbochargers, and austenitic stainless steel sheets for exhaust parts and exhaust parts. The purpose is to provide.

The present invention mainly targets precision parts inside a nozzle vane type turbocharger in turbocharger parts. Then, as the above-mentioned internal precision parts, for example, a nozzle composed of precision parts such as a nozzle mount, a nozzle plate, a nozzle ring, a nozzle vane, a drive ring, a drive lever, and a shroud for adjusting the flow velocity and the flow rate of the exhaust gas. Parts are mentioned.

The present invention has been made to solve the above problems, and the gist of the present invention is the following method for manufacturing an austenitic stainless steel sheet for exhaust parts, an austenitic stainless steel sheet for exhaust parts, and an austenitic stainless steel sheet for exhaust parts.

(1) The chemical composition is mass%.
C: 0.005 to 0.3%,
Si: 0.1-5.0%,
Mn: 0.1 to 12.0%,
P: 0.01-0.05%,
S: 0.0001 to 0.01%,
Ni: 5.0-15.0%,
Cr: 15.0 to 25.0%,
N: 0.005 to 0.4%,
Al: 0.001-0.5%,
Cu: 0.05-4.0%,
Mo: 0.01-2.0%,
V: 0.01-1.0%,
Ti: 0-0.3%,
Nb: 0 to 0.3%,
B: 0 to 0.0050%,
Ca: 0-0.01%,
W: 0-3.0%,
Zr: 0-0.3%,
Sn: 0 to 0.5%,
Co: 0-0.3%,
Mg: 0-0.01%,
Sb: 0 to 0.5%,
REM: 0 to 0.20%,
Ga: 0-0.3%,
Ta: 0-1.0%,
Remaining: Fe and unavoidable impurities,
An austenitic stainless steel sheet for exhaust parts with a surface roughness Rz of 0.1 μm or less.

(2) The chemical composition is mass%.
Ti: 0.005 to 0.3%,
Nb: 0.005 to 0.3%,
B: 0.0002 to 0.0050%, and Ca: 0.0005 to 0.01%,
Contains one or more selected from
The austenitic stainless steel plate for exhaust parts according to (1) above.

(3) The chemical composition is mass%.
W: 0.1-3.0%,
Zr: 0.05-0.3%,
Sn: 0.01-0.5%,
Co: 0.03 to 0.3%,
Mg: 0.0002-0.01%,
Sb: 0.005 to 0.5%,
REM: 0.001 to 0.20%,
Ga: 0.0002 to 0.3%, and Ta: 0.001 to 1.0%,
Contains one or more selected from
The austenitic stainless steel sheet for exhaust parts according to (1) or (2) above.

(4) The depth of wear marks after the pin-on-disk method frictional wear test at 850 ° C. in the atmosphere with a load of 0.5 N and a distance of 20 m is 7.0 μm or less.
The austenitic stainless steel sheet for exhaust parts according to any one of (1) to (3) above.

(5) The surface roughness Rz of the scale formed on the surface of the steel sheet after being held at a temperature of 850 ° C. for 60 minutes in an atmospheric atmosphere is 0.5 μm or more.
The austenitic stainless steel sheet for exhaust parts according to any one of (1) to (4) above.

(6) The surface hardness of the steel sheet after being held at a temperature of 850 ° C. for 60 minutes in an atmospheric atmosphere is 160 or more in Vickers hardness.
The austenitic stainless steel sheet for exhaust parts according to any one of (1) to (5) above.

(7) The austenitic stainless steel sheet for exhaust parts according to any one of (1) to (6) above, which satisfies the following formula (i).
HV _aa / HV _ba ≧ 1.01 ... (i)
However, each symbol in the above equation (i) means the following.
HV _aa : Vickers hardness of the surface after aging at 850 ° C. for 60 minutes in the atmosphere HV _ba : Vickers hardness of the surface before aging

(8) The austenitic stainless steel sheet for exhaust parts according to any one of (1) to (7) above, which is used for turbochargers.

(9) Exhaust parts using the stainless steel plate according to any one of (1) to (7) above.

(10) The method for manufacturing an austenitic stainless steel sheet for exhaust parts according to any one of (1) to (7) above.
(A) A steel having the chemical composition according to any one of (1) to (3) above is melted, then a slab is produced by casting, and the produced slab is hot-rolled to obtain a hot-rolled plate. And the process to do
(B) The hot-rolled plate is cold-rolled to obtain a cold-rolled plate.
A method for manufacturing an austenitic stainless steel sheet for exhaust parts, wherein the roughness of the rolling roll used for the final pass of cold rolling in the step (b) is # 60 or more.

(11) (c) After the step of annealing and pickling the cold-rolled plate of (b) and (d) the step of (c), a tempering rolling step or # 60 with a strain amount of 1 to 3%. Further having the above-mentioned polishing process,
The method for manufacturing an austenitic stainless steel sheet for exhaust parts according to (10) above.

According to the present invention, it becomes possible to obtain an austenitic stainless steel sheet having heat resistance and high temperature slidability suitable for automobile exhaust parts, particularly parts for turbochargers.

The present inventors investigated the high-temperature slidability required for a steel plate for turbocharger parts, and obtained the following findings (a) to (e).

(A) Since turbocharger parts are used at high temperatures, scales are formed on the surface of the parts. Depending on the scale to be formed, when the parts rub against each other, they are transferred to the sliding surface of the mating surface, and a so-called self-lubricating action that lowers the coefficient of friction works. As a result, high-temperature slidability can be improved, so it is preferable to appropriately control the composition of the steel sheet to form a scale having a self-lubricating action.

(B) Even if the scale has a self-lubricating action, the scale formed on a part of the surface of the component may be peeled off due to factors such as a long usage time. As a result, the high temperature slidability may decrease in the portion where the scale is peeled off. Therefore, it is desirable to form a scale with good adhesion at the start of use, that is, at the initial stage of scale formation, so that the scale does not peel off as much as possible. In order to form a scale with good adhesion, it is necessary to appropriately control the surface texture of the steel sheet. In particular, when the maximum height roughness Rz (hereinafter referred to as "surface roughness Rz"), which is one of the indexes of the two-dimensional surface roughness of the steel sheet, is 0.1 μm or less, the adhesion is good. It becomes a scale.

(C) Further, even if a portion where the formed scale is peeled off occurs, if the scale can be formed again in a short time, a decrease in high temperature slidability can be avoided.

(D) Mn and Cu have a high effect of re-forming the scale at the portion where the scale is peeled off from the base material (hereinafter, referred to as “self-repairing function”). It is considered that this is because the speed at which Mn and Cu diffuse from the base material to the scale (also referred to as “outer diffusion”) is high.

(E) In order to enhance the self-healing function of the scale caused by Mn and Cu, it is effective to apply an appropriate amount of strain to the steel sheet at the manufacturing stage. This is because the strain introduced into the steel sheet promotes the outward diffusion of Mn and Cu that exerts the self-repairing function of the steel.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

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

C: 0.005 to 0.3%
C is an element necessary for forming an austenite structure and ensuring high-temperature strength and high-temperature hardness that affect high-temperature slidability. Further, since it is necessary for improving high temperature slidability, the C content is set to 0.005% or more. However, if C is excessively contained, work hardening becomes excessive. In addition, excess C forms Cr carbides, reducing corrosion resistance and intergranular corrosion resistance. As a result, intergranular corrosion grooves are formed during pickling of the cold-rolled annealed plate, and the surface roughness Rz becomes rough. In addition, the intergranular corrosion property of the welded portion is reduced. Therefore, the C content is set to 0.3% or less.

From the viewpoint of hot workability, high-temperature sliding property, and manufacturing cost, the C content is preferably 0.03% or more, and preferably 0.2% or less. The C content is more preferably 0.05% or more, and more preferably 0.19% or less.

Si: 0.1-5.0%
Si acts as a deoxidizing element. In addition, internal oxidation of Si improves scale peelability and high-temperature slidability. In addition, internal oxidation of Si results in improved high temperature strength and high temperature hardness. Therefore, the Si content is set to 0.1% or more. When it is desired to further increase the high-temperature hardness and high-temperature slidability, the Si content is preferably 1.5% or more.

However, if Si is contained in an amount of more than 5.0%, the steel sheet becomes excessively hard, and the workability and manufacturability of parts are deteriorated. Therefore, the Si content is 5.0% or less. From the viewpoint of manufacturing cost, pickling property, and solidification cracking property at the time of welding, the Si content is preferably 3.8% or less.

Mn: 0.1 to 12.0%
Mn is used as a deoxidizing element. In addition, Mn has the effect of stabilizing austenite and improving scale adhesion. In addition, the inclusion of Mn imparts a self-lubricating action and a self-repairing function to the scale, which leads to improvement in high-temperature slidability. Therefore, the Mn content is set to 0.1% or more.

However, if Mn is contained in an amount of more than 12.0%, work hardening becomes large and the workability of parts deteriorates. In addition, inclusion cleanliness and pickling properties are significantly reduced and the product surface becomes rough. Therefore, the Mn content is set to 12.0% or less. The Mn content is preferably 0.2% or more from the viewpoint of further improving the high-temperature slidability by utilizing the scale having good adhesion, the self-lubrication of the scale, and the self-repairing function. It is more preferably 0% or more. On the other hand, from the viewpoint of pickling property and manufacturing cost, the Mn content is preferably 10.0% or less, more preferably 4.0% or less.

P: 0.01-0.05%
P is an element that promotes hot workability and solidification cracking during manufacturing. Therefore, the P content is set to 0.05% or less. A low P content is preferable, but an excessive reduction in P increases the refining cost. Therefore, the P content is set to 0.01% or more. Further, from the viewpoint of reducing the production cost, the P content is preferably 0.02% or more, and preferably 0.04% or less.

S: 0.0001 to 0.01%
S is an element that reduces hot workability and corrosion resistance. Further, when coarse sulfide (MnS) is formed, the cleanliness of inclusions in the steel sheet is significantly deteriorated. Therefore, the S content is set to 0.01% or less. However, excessive reduction of S leads to an increase in refining cost. Therefore, the S content is set to 0.0001% or more. From the viewpoint of oxidation resistance and manufacturing cost, the S content is preferably 0.0005% or more, and more preferably 0.0050% or less.

Ni: 5.0-15.0%
Ni is an austenite stabilizing element. It is also an element that improves corrosion resistance and oxidation resistance. In addition, Ni also has the effect of finely granulating the structure, but if the Ni content is less than 5.0%, the crystal grains are significantly coarsened. Therefore, the Ni content is set to 5.0% or more. From the viewpoint of high temperature strength, corrosion resistance, and manufacturability, the Ni content is preferably 10.0% or more. However, excessive inclusion of Ni increases manufacturing costs and hardens the material. Therefore, the Ni content is preferably 15.0% or less, preferably 14.0% or less.

Cr: 15.0 to 25.0%
Cr is an element that improves corrosion resistance, oxidation resistance, and high-temperature slidability. The Cr content is set to 15.0% or more from the viewpoint of suppressing abnormal oxidation in the exhaust component environment. However, excessive inclusion of Cr not only hardens the material but also increases the manufacturing cost. Therefore, the Cr content is set to 25.0% or less. Further, from the viewpoint of processability, manufacturability, and manufacturing cost, the Cr content is preferably 17.0% or more, and preferably 24.0% or less.

N: 0.005 to 0.4%
N stabilizes austenite as well as C. It is also an effective element for improving high temperature strength, high temperature hardness and high temperature slidability. In particular, by acting as a solid solution strengthening element, N improves high temperature strength. Therefore, the N content is set to 0.005% or more.

However, when N is contained in an amount of more than 0.4%, the room temperature material becomes remarkably hardened. As a result, the cold workability at the time of manufacturing the steel sheet deteriorates, and the workability of the parts deteriorates. Therefore, the N content is set to 0.4% or less. In order to improve the high temperature slidability, the N content is preferably 0.01% or more, and more preferably more than 0.05%. On the other hand, the N content is preferably 0.3% or less from the viewpoint of suppressing intergranular corrosion of pinholes and welded portions during welding.

Al: 0.001 to 0.5%
Al acts as a deoxidizing element and improves the cleanliness of inclusions. In addition, Al suppresses exfoliation of the oxide scale and improves high temperature slidability by a small amount of internal oxidation. Therefore, the Al content is set to 0.001% or more. However, Al is a ferrite-forming element, and if it is contained in an amount of more than 0.5%, the stability of the austenite structure is lowered. Further, the excessive content of Al not only lowers the pickling property, but also increases the amount of inclusions, thereby increasing the surface roughness. Therefore, the Al content is set to 0.5% or less.

From the viewpoint of refining cost and surface texture, the Al content is preferably 0.01% or more, and preferably 0.3% or less. Further, from the viewpoint of hot workability, the Al content is preferably 0.04% or more, and preferably 0.1% or less.

Cu: 0.05-4.0%
Cu is an element that stabilizes austenite and is also effective in improving oxidation resistance. In addition, Cu imparts a self-lubricating action and a self-repairing function to the scale, and improves high-temperature slidability. Therefore, the Cu content is set to 0.05% or more. However, excessive inclusion of Cu conversely lowers oxidation resistance and, in addition, lowers manufacturability. Therefore, the Cu content is set to 4.0% or less. From the viewpoint of corrosion resistance and manufacturability, the Cu content is preferably 0.1% or more, and preferably 3.0% or less. The Cu content is more preferably 0.2% or more, and more preferably 2.8% or less.

Mo: 0.01-2.0%
Mo is an element that improves corrosion resistance. In addition, Mo has the effect of improving high-temperature strength and high-temperature hardness. When Mo is contained, in addition to solid solution strengthening, precipitation strengthening is caused by precipitating Mo carbide. Further, the wear resistance can be improved by forming the precipitate. Therefore, the Mo content is 0.01% or more. However, Mo is an expensive element, and if it is contained in an excessive amount, the production cost increases. Therefore, the Mo content is set to 2.0% or less. From the viewpoint of enhanced stability due to the precipitate and cleanliness of inclusions, the Mo content is preferably 0.2% or more, and preferably 1.7% or less.

V: 0.01-1.0%
V is an element that improves corrosion resistance. In addition, V forms carbides and improves high-temperature strength and wear resistance at high temperatures. Therefore, the V content is set to 0.01% or more. However, excessive content of V lowers the abnormal oxidation limit temperature in addition to increasing the manufacturing cost. Therefore, the V content is set to 1.0% or less. From the viewpoint of manufacturability and cleanliness of inclusions, the V content is preferably 0.05% or more, and preferably 0.8% or less.

In addition to the above components, the austenitic stainless steel sheet according to the present invention may contain one or more elements selected from Ti, Nb, B, and Ca.

Ti: 0-0.3%
Ti combines with C and N to improve corrosion resistance and intergranular corrosion resistance. Therefore, it may be contained as needed. However, if Ti is contained in an amount of more than 0.3%, nozzle clogging at the casting stage is likely to occur, and the manufacturability is significantly reduced. Further, when coarse Ti carbonitride is formed, the workability of parts is deteriorated. Therefore, the Ti content is set to 0.3% or less. On the other hand, in order to obtain the above effect, the Ti content is preferably 0.005% or more. From the viewpoint of high temperature strength, intergranular corrosion resistance of the welded portion, and manufacturing cost, the Ti content is more preferably 0.01% or more, and more preferably 0.2% or less.

Nb: 0 to 0.3%
Nb, like Ti, binds to C and N to improve corrosion resistance and intergranular corrosion resistance. In addition, Nb is also an element that improves high temperature strength. Therefore, it may be contained as needed. However, if Nb is contained in an amount of more than 0.3%, the hot workability is significantly deteriorated. Further, when coarse Nb carbonitride is formed, the workability of parts is lowered. Therefore, the Nb content is set to 0.3% or less. On the other hand, in order to obtain the above effect, the Nb content is preferably 0.005% or more. From the viewpoint of high temperature strength, intergranular corrosion of welds, and manufacturing cost, the Nb content is more preferably 0.01% or more, and more preferably less than 0.15%.

B: 0 to 0.0050%
B is an element that improves hot workability and may be contained as necessary in order to suppress work hardening at room temperature. However, excessive inclusion of B results in the formation of borocarbides. This reduces the cleanliness of inclusions and the intergranular corrosion resistance of the steel sheet. 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. From the viewpoint of refining cost and ductility, the B content is more preferably 0.0003% or more, and preferably 0.002% or less.

Ca: 0-0.01%
Ca may be contained as needed for desulfurization. However, when Ca is contained in excess of 0.01%, water-soluble inclusions CaS are formed. As a result, the cleanliness and corrosion resistance of the inclusions in the steel sheet are significantly reduced. 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 texture, the Ca content is more preferably 0.0010% or more, and preferably 0.0030% or less.

Further, in the austenitic stainless steel according to the present invention, in addition to the above components, one or more elements selected from W, Zr, Sn, Co, Mg, Sb, REM, Ga, Ta, Hf and Bi. May be contained.

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

Zr: 0-0.3%
Zr combines with C or N to improve intergranular corrosion and oxidation resistance of the weld. Therefore, it may be contained as needed. However, if Zr is contained in an amount of more than 0.3%, the manufacturability is significantly reduced and the manufacturing cost is increased. Therefore, the Zr content is set to 0.3% or less. From the viewpoint of manufacturability and refining cost, 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 to 0.5%
Sn may be contained as necessary because it contributes to the improvement of corrosion resistance and high temperature strength. However, if Sn is contained in an amount of more than 0.5%, slab cracking may occur during production. Therefore, the Sn content is set to 0.5% or less. Further, from the viewpoint of manufacturability and refining cost, the Sn content 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 may be contained as needed because it contributes to the improvement of high temperature strength. However, if Co is contained in an amount of more than 0.3%, it leads to hardening of the steel sheet, a decrease in toughness during manufacturing, and an increase in manufacturing cost. Therefore, the Co content is set to 0.3% or less. Considering the manufacturability and the refining cost, 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%
Mg may be added as a deoxidizing element. In addition, Mg contributes to the improvement of inclusion cleanliness by making the structure finer by making the slab structure finer or more dispersed by the oxide. Therefore, it may be contained as needed. However, if Mg is excessively contained, weldability and corrosion resistance are deteriorated. In addition, the formation of coarse inclusions in Mg reduces the workability of parts. 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. Considering the refining cost, the Mg content is preferably 0.0003% or more, and preferably 0.005% or less.

Sb: 0 to 0.5%
Sb may be contained as necessary in order to segregate at the grain boundaries and improve the high temperature strength. However, if Sb is contained in an amount of more than 0.5%, segregation occurs 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, toughness and manufacturing cost, the Sb content is more preferably 0.03% or more, and more 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 to 0.20%
Since REM (rare earth element) is effective in improving oxidation resistance and high-temperature slidability, it may be contained if necessary. However, even if REM is contained in an amount of more than 0.20%, the effect is saturated. In addition, sulfides containing REM are formed, which reduces corrosion resistance. Therefore, the REM content is set to 0.20% or less. On the other hand, in order to obtain the above effect, the REM content is preferably 0.001% or more. From the viewpoint of component workability and manufacturing cost, the REM content is more preferably 0.002% or more, and preferably 0.10% or less.

REM (rare earth element) refers to a total of 17 elements, including two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). The content of the REM means the total content of these elements, and may be added alone or as a mixture.

Ga: 0-0.3%
Ga may be contained as necessary in order to improve corrosion resistance and suppress hydrogen embrittlement. However, if Ga is contained in an amount of more than 0.3%, coarse sulfide is generated and the workability of parts is deteriorated. Therefore, the Ga content is set to 0.3% or less. On the other hand, in order to obtain the above effect, 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%
Hf: 0 to 1.0%
Bi: 0-0.02%
Ta may be contained as needed in order to improve the high temperature strength. However, since the excessive content of Ta increases the manufacturing cost, the Ta content is set to 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 further increase the strength, the Ta content is more preferably 0.01% or more. For the same reason, the Hf content is also set to 1.0% or less. Further, for the same reason, the Hf content is preferably 0.001% or more, and more preferably 0.01% or more.

Bi may be contained as needed for the same reason. However, since the excessive content of Bi increases the manufacturing cost, the Bi content is set to 0.02% or less. On the other hand, in order to obtain the same effect as described above, the Bi content is preferably 0.001% or more. It is desirable to reduce general harmful elements such as As and Pb and impurity elements as much as possible.

In the chemical composition of the present invention, the balance is Fe and unavoidable impurities. Here, the "unavoidable impurity" is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when steel is industrially manufactured, and is within a range that does not adversely affect the present invention. Means what is acceptable.

2. 2. Surface properties of steel sheet 2-1. Surface roughness The internal parts of the turbocharger, such as the nozzle plate, nozzle mount, nozzle vane, and wastegate valve, rub against each other under high temperature and no lubrication. When the above-mentioned parts rub against each other, if the friction surface has irregularities, the uneven portions may be caught, and friction defects or wear are likely to occur. Therefore, it is desirable that the surface texture is good and the surface is smooth. Further, in the present invention, by adjusting the surface texture extremely strictly, a scale having good adhesion is formed on the surface of the steel sheet (part) at the initial stage of scale formation. Therefore, the surface roughness Rz of the steel sheet surface is set to 0.1 μm or less. The smaller the surface roughness of the steel sheet, the more preferable.

When the surface roughness Rz of the steel sheet is more than 0.1 μm, it is difficult to form a scale having good adhesion. By setting the two-dimensional surface roughness of the steel sheet within the above range, it is possible to improve the high-temperature slidability in a high-temperature usage environment.

The surface roughness Rz specified in the present application indicates the maximum height roughness described in JIS B 0601 (2013).

In the present application, the roughness of the roll used for the final pass of cold rolling is specified as described later, and the cold rolled material is annealed and pickled, and then tempered rolling or polishing is performed. As a result, good surface texture of the steel sheet can be ensured. This also has the effect of omitting the grinding process for smoothing the friction surface, which has been conventionally performed, and can contribute to the reduction of manufacturing cost.

2-2. Surface scale formed in a high temperature use environment In the present invention, Mn and Cu, which are additive elements, diffuse to the scale in a high temperature use environment, so that self-lubricating action is generated and desired high temperature slidability can be obtained. .. Therefore, it is preferable to form an appropriate scale on the steel sheet. Specifically, the scale roughness Rz of the surface of the steel sheet after being held in the air at 850 ° C. for 60 minutes is preferably 0.5 μm or more, and more preferably 0.1 μm or more.

In the present invention, a scale having good adhesion is formed at the initial stage of formation, and then when the scale grows, the surface roughness of the scale is adjusted so that it can be easily transferred to the mating sliding surface, that is, it can be easily self-lubricated. The preferred range is the above range. In the present invention, in addition to the self-lubricating action of the scale, the self-repairing function is exhibited, so that the surface roughness of the scale tends to increase as the sliding is repeated.

3. 3. In order to ensure high temperature slidability better than the surface hardness, it is preferable to specify the ratio of the surface hardness after 850 ° C. aging or the surface hardness before and after 850 ° C. aging.

3-1. Surface hardness of steel sheet after aging at 850 ° C. In the present invention, by controlling the chemical composition to the above-mentioned composition, M ₂₃ C ₆ (M is Cr, Si, Mo, M is Cr, Si, Mo, in a high temperature usage environment. Precipitates called Ni) etc. are deposited. As a result, the steel sheet is strengthened by precipitation and the surface hardness is increased, so that it is considered that good high temperature slidability can be obtained. Therefore, the surface hardness after holding (aging) at 850 ° C. for 60 minutes in an air atmosphere is preferably 160 or more, preferably 170 or more, in terms of Vickers hardness (hereinafter referred to as “HV hardness”). More preferably, it is more preferably 180 or more. Further, the surface hardness after holding is 300 or less in HV hardness because the precipitates fall off during high-temperature sliding and the surface roughness becomes rough when the amount of precipitates increases and the hardness becomes excessive. Is preferable.

The surface hardness in the present invention is a hardness within 500 μm from the surface, and the surface was wet-polished with a # 1000 finish and measured with respect to the polished surface. After aging, the surface of the sample was polished to a depth of several hundred μm to remove scale, and then wet polishing with # 1000 finish was performed for measurement.

In the present invention, if the surface hardness after aging treatment at 850 ° C. for 60 minutes is 160 or more in HV hardness using a material actually used or a material whose heat history is unknown before. The requirement of 3-1 of the present invention is satisfied.

3-2. Ratio of surface hardness before and after aging at 850 ° C In the present invention, in order to ensure workability at room temperature and high-temperature slidability at high temperature use, the surface hardness after aging at 850 ° C for 60 minutes It is preferable to satisfy the following formula (i), which is the ratio to the surface hardness before aging. The aging condition was set to be maintained at 850 ° C. for 60 minutes in the air atmosphere as described above.

HV _aa / HV _ba ≧ 1.01 ... (i)
However, each symbol in the above equation (i) means the following.
HV _aa : Vickers hardness of the surface after aging at 850 ° C. for 60 minutes in an air atmosphere HV _ba : Vickers hardness of the surface before aging When the lvalue of equation (i) is less than 1.01, workability and high temperature Sufficient slidability cannot be ensured. The lvalue of the equation (i) is more preferably 1.03 or more, and further preferably 1.05 or more. On the other hand, the lvalue in equation (i) is preferably 2.00 or less.

In the present invention, if the material actually used or the material having an unknown thermal history is used and the aging treatment is performed at 850 ° C. for 60 minutes and the formula (i) is satisfied, the present invention 3 -The requirement of -2 will be satisfied.

4. Evaluation of high-temperature slidability In the present invention, high-temperature slidability is evaluated in the pin-on-disk method friction and wear test. In the present invention, when the wear mark depth measured by the above test method is 7.0 μm or less, it is determined that the high temperature slidability is good. When the wear mark depth measured by the above test method is 3.0 μm or less, it can be said that the high temperature slidability is further excellent. Table 1 shows the values of each numerical value when the steel of the present invention and the steel of the present invention are compared.

5. Manufacturing Method The manufacturing process of the steel sheet of the present invention comprises steelmaking-hot rolling-baking / pickling-cold rolling-baking / pickling-conditioning rolling (polishing).

5-1. Hot rolling process The method for manufacturing a steel sheet of the present invention will be specifically described below. First, a method of melting steel having the above chemical composition in an electric furnace or a converter and then performing secondary refining is preferable. The molten steel is made into a slab according to a known casting method (continuous casting or the like). Usually, the slab is heated at 1200 to 1300 ° C. and hot-rolled to a predetermined plate thickness by continuous rolling to become a hot-rolled plate. The thickness of the slab and the rolling reduction in hot rolling may be appropriately selected. 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. When annealing a hot-rolled plate, the temperature condition is in the range of 1100 to 1200 ° C., and the annealing time is in the range of 10 to 60 seconds. For the parts to which the present invention is intended, manufacturing conditions for ensuring predetermined surface hardness and surface roughness are set in the steps after hot rolling.

5-2. Cold rolling step Subsequently, cold rolling is performed to a predetermined plate thickness. The rolling reduction in cold rolling is in the range of 50 to 95%. Since the roll surface is transferred to the steel sheet during cold rolling, the surface roughness of the roll used affects the surface roughness of the rolled steel sheet. In order to obtain a steel sheet having a surface roughness Rz of 0.1 μm or less specified in the present application, the surface roughness of the roll used for the final pass of cold rolling is # 6.
A rolling roll having a value of 0 or more, that is, ground with # 60 or more may be used .

After cold rolling, first, cold rolled sheet annealing and pickling treatment are performed. Normally, the annealing temperature is set to less than 1000 to 1100 ° C. to obtain a recrystallized structure. The annealing time is in the range of 10 to 60 seconds. The annealing of the cold-rolled plate may be performed during the cold rolling pass, and may be batch annealing or continuous annealing. Subsequent pickling treatment is performed to remove the scale formed on the steel surface by annealing. The pickling method may be any chemical descale method such as sulfuric acid, nitric acid, or nitric acid electrolysis, and a molten alkali salt dip may be performed as a pretreatment thereof. The temperature and time conditions for immersion in the molten alkali salt can be appropriately changed depending on the characteristics of the desired steel sheet.

Next, in the present invention, it is preferable to have a temper rolling step or a polishing step after cold rolling annealing blunt pickling. Tempered rolling refers to rolling that applies a strain of 1 to 3%, and has the effect of correcting the shape of the steel sheet, reducing the surface roughness of the steel sheet, and promoting the self-healing function of the scale.

In order to reduce the surface roughness of the steel sheet, the surface roughness of the roll used for temper rolling may be equal to or smaller than the roll used for the final pass of cold rolling, and # 100 or more is desirable. That is, it is desirable to use a rolling roll ground by # 100 or higher . The polishing process can smooth the surface of the steel sheet and apply surface strain. The polishing cloth or polishing paper used in the polishing process shall be # 60 or more. As a result, the steel plate surface roughness Rz of the product can be stably obtained at 0.1 μm or less.

As mentioned above, in order to promote the self-healing function of the scale, it is advisable to increase the diffusion rate of the elements forming the scale. It is considered that the strain introduced into the steel sheet after the temper rolling step or the polishing step promotes so-called outward diffusion in which elements such as Mn or Cu diffuse to the scale from the inside of the steel sheet. As a result, it is possible to manufacture an austenitic stainless steel sheet having a small surface roughness and an excellent scale self-healing function.

When a turbocharger part is manufactured from the above-mentioned austenitic stainless steel plate, the shape of the turbocharger part is formed from an austenitic stainless steel plate having an appropriate size by precision processing such as fine blanking. After processing the parts, special surface treatments such as nitriding treatment or carburizing treatment may be applied to improve high temperature slidability.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

After melting the steels having the chemical compositions shown in Tables 2 and 3, the following steps were carried out to obtain steel sheets. Specifically, it was hot-rolled at 1250 ° C., annealed on a hot-rolled plate at 1150 ° C. for 60 seconds, and then pickled. Then, cold rolling with a rolling reduction of 50%, annealing at 1150 ° C. for 60 seconds, pickling, and then temper rolling with a strain amount of 1% were performed to obtain a 5.0 mm thick steel sheet.

In the example of the present invention, the surface roughness of the roll used for the final pass during cold rolling was set to # 60. On the other hand, the test No. In 35 and 36, rolls having a surface roughness of # 40 were used in order to investigate the influence of the roll surface roughness described above. In addition, the test No. In 35 to 37, in order to investigate the influence of the surface roughness Rz due to the action of the scale self-repairing function by temper rolling, steel sheets not subjected to temper rolling were also manufactured.

Each steel was subjected to a roughness test of steel plate and scale, a surface hardness test before and after 850 ° C. aging, and a pin-on-disk method high temperature friction wear test.

The roughness test of the steel sheet was carried out in accordance with JIS B 0601 (2013) at a length of 3.0 mm and a speed of 0.3 mm / s. In the scale roughness test, the roughness of the scale formed on the surface of the air-cooled sample after holding at 850 ° C. for 60 minutes in the air was measured by the same method as described above.

When the Rz of the steel sheet was 0.1 μm or less, it was determined that the steel sheet roughness had sufficient surface texture to form a scale having good adhesion. The surface roughness of the scale is indicated by ⊚ when the surface roughness Rz is 0.6 μm or more, ○ when the surface roughness Rz is 0.5 μm or more, and less than 0.5 μm. Notated as x. For the measurement, Surfcom 1400 manufactured by Tokyo Seimitsu Co., Ltd. was used.

The surface hardness test before and after 850 ° C. aging was performed with a test force of 9.807 N in accordance with JIS Z 2244 (2009). The hardness test after aging at 850 ° C. was carried out on the surface of the air-cooled sample after holding at 850 ° C. for 60 minutes in an air atmosphere.

The sample for hardness measurement after aging at 850 ° C. was wet-polished in order to remove the scale formed on the surface by holding at high temperature, and was finished with # 600. The hardness after aging at 850 ° C. was determined to be good high temperature hardness if the HV hardness was 160 or more. Further, the case where the lvalue of the equation (i) is 1.1 or more is described as ⊚, the case of 1.01 or more is described as ◯, and the case of less than 1.01 is determined as ×. For the above hardness test, a micro Vickers hardness (micro Vickers hardness) tester manufactured by Shimadzu Corporation was used.

Pin-on-disc method The high-temperature friction and wear test is performed by processing a bullet-shaped pin: root diameter φ6 mm, tip diameter φ4 mm, total length 10 mm, disk-shaped disc: φ29 mm, height 5 mm into a test piece, and the tip of the pin with respect to the disc. We installed them so that they were in vertical contact with each other so that the pins would repeatedly wear by rubbing around the disc in a furnace at 850 ° C. in the atmosphere.

Initially, the temperature was raised in a state where the pin and the disc were not in contact with each other, and the temperature was maintained at 850 ° C. for 60 minutes, and then the pin and the disc were brought into contact with each other to start the test. The load was 0.5N and the test distance was 20m. When the wear mark depth of the disc after the test is 3.0 μm or less, it is indicated by ⊚, when it is 7.0 μm or less, it is indicated by ◯, and when it exceeds 7.0 μm, it is indicated by ×. A High-Temperature Tribometer manufactured by CSM Instruments was used for the above test.

In addition, the test material was processed into parts called nozzle mounts and nozzle plates, and mounted on a nozzle vane type turbocharger. Then, a high-temperature (900 ° C.) exhaust gas was flowed while repeatedly opening and closing the nozzle, and the high-temperature slidability in a state closer to the actual usage environment was investigated. Steels with a gas flow rate or flow rate change rate (before and after passing through the nozzle) of 95% or more during the test are marked with ◎, steels with 90% or more and less than 95% are marked with ○, and steels with less than 90% are marked with ○. Notated as x.

As a result of manufacturing under the manufacturing conditions shown in Tables 4 and 5, the steel sheets according to the examples of the present invention (steel sheets of Examples 1 to 23 of the present invention) were excellent in heat resistance and surface properties, and satisfied the performance as turbocharger parts. For those that do not satisfy even one of these characteristics, the performance of the exhaust parts is marked as x, if all are ○ or ◎, it is marked as ○, and if all are ◎, it is marked as ◎. In addition, the test No. in which one or more of the chemical composition or the surface roughness Rz of the steel sheet deviates from the provisions of the present invention. In the case of 24 to 37, the performance of the exhaust parts such as surface smoothness and high temperature slidability became poor and a problem occurred.

The results are summarized in Tables 2 to 5 below.

According to the present invention, it is possible to provide an austenitic stainless steel sheet having excellent characteristics for exhaust parts that require high temperature slidability in addition to heat resistance. In particular, by using it as a turbocharger part among exhaust parts, it is possible to provide parts that meet automobile exhaust gas regulations, weight reduction, and fuel efficiency improvement. Furthermore, it 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.

Claims (7)

The chemical composition is by mass%,
C: 0.005 to 0.3%,
Si: 0.1-5.0%,
Mn: 0.1 to 12.0%,
P: 0.01-0.05%,
S: 0.0001 to 0.01%,
Ni: 5.0-15.0%,
Cr: 15.0 to 25.0%,
N: 0.005 to 0.4%,
Al: 0.001-0.5%,
Cu: 0.05-4.0%,
Mo: 0.01-2.0%,
V: 0.01-1.0%,
Ti: 0-0.3%,
Nb: 0 to 0.3%,
B: 0 to 0.0050%,
Ca: 0-0.01%,
W: 0-3.0%,
Zr: 0-0.3%,
Sn: 0 to 0.5%,
Co: 0-0.3%,
Mg: 0-0.01%,
Sb: 0 to 0.5%,
REM: 0 to 0.20%,
Ga: 0-0.3%,
Ta: 0-1.0%,
Remaining: Fe and steel sheet, which is an unavoidable impurity,
The surface roughness Rz, which is the maximum height roughness measured in accordance with JIS B 0601 (2013) of the steel sheet, is 0.1 μm or less.
The surface roughness Rz of the scale formed on the surface of the steel sheet after being held at a temperature of 850 ° C. for 60 minutes in an air atmosphere and air-cooled is 0.5 μm or more.
The surface hardness of the steel sheet after being held at a temperature of 850 ° C. for 60 minutes in an air atmosphere and air-cooled is 200 or more in Vickers hardness.
Austenitic stainless steel plate for exhaust parts. The chemical composition is by mass%.
Ti: 0.005 to 0.3%,
Nb: 0.005 to 0.3%,
B: 0.0002 to 0.0050%, and Ca: 0.0005 to 0.01%,
Contains one or more selected from
The austenitic stainless steel sheet for exhaust parts according to claim 1. The chemical composition is by mass%.
W: 0.1-3.0%,
Zr: 0.05-0.3%,
Sn: 0.01-0.5%,
Co: 0.03 to 0.3%,
Mg: 0.0002-0.01%,
Sb: 0.005 to 0.5%,
REM: 0.001 to 0.20%,
Ga: 0.0002 to 0.3%, and Ta: 0.001 to 1.0%,
Contains one or more selected from
The austenitic stainless steel sheet for exhaust parts according to claim 1 or 2. The austenitic stainless steel sheet for exhaust parts according to any one of claims 1 to 3, which satisfies the following formula (i).
HV _aa / HV _ba ≧ 1.01 ... (i)
However, each symbol in the above equation (i) means the following.
HV _aa : Vickers hardness of the surface after aging at 850 ° C for 60 minutes in the atmosphere and air cooling HV _ba : Vickers hardness of the surface before aging The austenitic stainless steel sheet for exhaust parts according to any one of claims 1 to 4, which is used for a turbocharger. An exhaust component using the stainless steel plate according to any one of claims 1 to 5. The method for manufacturing an austenitic stainless steel sheet for exhaust parts according to any one of claims 1 to 5.
(A) A step of melting a steel having the chemical composition according to any one of claims 1 to 3, then producing a slab by casting, and hot rolling the produced slab to obtain a hot-rolled plate. ,
(B) The process of cold-rolling the hot-rolled plate to obtain a cold-rolled plate, and
(C) The step of annealing and pickling the cold-rolled plate of (b) above, and
(D) After the step of (c), there is a step of performing a temper rolling step or a step of performing a polishing step at # 60 or higher.
A method for manufacturing an austenitic stainless steel sheet for exhaust parts, in which a rolling roll ground by # 60 or higher is used for the final pass of cold rolling in the step (b).