JP7616857B2 - Austenitic stainless steel sheet and method for producing same - Google Patents

Description

The present invention relates to an austenitic stainless steel sheet and a manufacturing method thereof.

Exhaust parts, among other automotive parts, are required to stably ventilate high-temperature exhaust gases. In addition, exhaust parts are used in environments where condensation water produced when the exhaust gas cools causes severe corrosion. For this reason, exhaust parts are made of materials with excellent heat resistance, such as oxidation resistance, high-temperature strength, and thermal fatigue properties, as well as corrosion resistance.

One example of an exhaust part is a turbocharger part. In recent years, turbochargers are often installed in automobiles from the perspectives of improving fuel efficiency, reducing engine displacement (also known as "downsizing"), and achieving appropriate engine displacement (also known as "rightsizing").

For this reason, heat-resistant austenitic stainless steel, which has excellent heat resistance and corrosion resistance, is used as the material for turbocharger parts. For example, Patent Documents 1 to 4 disclose austenitic stainless steels intended for use in turbochargers.

International Publication No. 2014/157655 JP 2017-14538 A JP 2017-160493 A JP 2019-143186 A

In addition, some turbocharger parts are used in conditions where friction occurs repeatedly at high temperatures without lubrication. In such cases, parts that come into contact with each other may wear out and adhere to each other in the operating environment. As a result, for example, vanes and valves that regulate the flow rate of exhaust gas inside the turbocharger may malfunction when opening and closing, causing poor gas flow.

Therefore, turbocharger parts are also required to have high-temperature sliding properties, which are friction and wear resistance at high temperatures. In particular, from the perspective of improving efficiency and reducing environmental impact, in turbocharger conditions where the temperature of the exhaust gas passing through is becoming higher, friction and adhesion between parts are likely to occur.

However, the austenitic stainless steels disclosed in Patent Documents 1 to 3 have not been thoroughly studied for their high-temperature sliding properties. As a result, there is room for further improvement in terms of high-temperature sliding properties. Furthermore, the austenitic stainless steel disclosed in Patent Document 4 has a high Mn and Cu content, and has relatively good high-temperature sliding properties. On the other hand, if the usage environment becomes even hotter and thick scale is formed, the scale may be more likely to peel off. Furthermore, in a usage environment where heating and cooling are repeated, the repeated peeling and regeneration of the scale increases the surface irregularities. This makes it easier for parts to adhere to each other, and may result in a decrease in high-temperature sliding properties.

In light of the above, the present invention aims to solve the above problems and provide an austenitic stainless steel sheet with good high-temperature sliding properties and a manufacturing method thereof.

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

(1) Chemical composition, in mass%,
C: 0.005-0.30%,
Si: 1.00-4.00%,
Mn: 0.90-8.00%,
P: 0.01-0.05%,
S: 0.0001-0.01%,
Ni: 5.0 to 15.0%,
Cr: 19.50-26.00%,
N: 0.02-0.40%,
Al: 0.001-0.50%,
Cu: 0-3.0%,
REM: 0-0.100%,
Mo: 0-2.0%,
V: 0 to 1.0%,
Ti: 0 to 0.30%,
Nb: 0 to 0.30%,
B: 0 to 0.005%,
Ca: 0-0.010%,
W: 0 to 3.0%,
Zr: 0 to 0.30%,
Sn: 0 to 0.50%,
Co: 0 to 0.30%,
Mg: 0 to 0.010%,
Sb: 0 to 0.50%,
Ga: 0-0.30%,
Ta: 0-1.0%,
Hf: 0-1.0%,
Bi: 0-0.02%,
The balance is Fe and impurities.
An austenitic stainless steel sheet satisfying the following formulas (i) and (ii):
11.5≦(Cr+Mn)/(Si+Al)≦15.0...(i)
0.80≦X/Y≦1.00...(ii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, it is set to zero. Each symbol in the above formula is defined as follows.
X: Maximum Mn concentration (mass%) in the region from the surface to a depth of 1.0 μm in the sheet thickness direction
Y: Mn concentration (mass%) at a depth of 10.0 μm from the surface in the sheet thickness direction

(2) The chemical composition is, in mass%,
Cu: 0.01 to 3.0%,
REM: 0.0001-0.100%,
Mo: 0.1-2.0%,
V: 0.05-1.0%,
Ti: 0.005-0.30%,
Nb: 0.005-0.30%,
B: 0.0002 to 0.005%,
Ca: 0.0005-0.010%,
W: 0.1-3.0%,
Zr: 0.05-0.30%,
Sn: 0.01-0.50%,
Co: 0.03 to 0.30%,
Mg: 0.0002 to 0.010%,
Sb: 0.005 to 0.50%,
Ga: 0.0002-0.30%,
Ta: 0.001 to 1.0%,
Hf: 0.001 to 1.0%, and Bi: 0.001 to 0.02%,
The austenitic stainless steel sheet according to the above (1), comprising one or more selected from the following:

(3) An austenitic stainless steel sheet according to (1) or (2) above, which, after being held in air at 850°C for 1 hour, exhibits an average wear scar depth of 5.0 μm or less when subjected to a pin-on-disk friction and wear test under conditions of a load of 0.5 N and a test distance of 20 m.

(4) An austenitic stainless steel sheet according to any one of (1) to (3) above, used for turbocharger parts.

(5) An exhaust part using an austenitic stainless steel sheet according to any one of (1) to (4) above.

(6) A method for manufacturing an austenitic stainless steel sheet according to any one of (1) to (5) above, which involves annealing heat treatment in a temperature range of 1140 to 1250°C for 5 to 60 seconds.

According to the present invention, it is possible to obtain an austenitic stainless steel sheet having good high-temperature sliding properties.

FIG. 1 is a graph showing the relationship between the wear scar depth after a high-temperature sliding test and (Cr+Mn)/(Si+Al). FIG. 2 is a diagram showing a schematic diagram of the change in Mn concentration.

The inventors conducted detailed studies on the high-temperature sliding properties of austenitic stainless steel sheets and obtained the following findings (a) to (c).

(a) The temperature range in which turbochargers are used is high, so thick scale forms in the operating environment. The inventors therefore focused on this oxide scale in order to improve high-temperature sliding properties. Depending on the oxide scale that is formed, when parts rub against each other, it transfers to the sliding surfaces and reduces the coefficient of friction, resulting in a so-called self-lubricating effect. When this self-lubricating effect occurs, high-temperature sliding properties are improved. Therefore, it is desirable to form a scale that has a self-lubricating effect.

(b) For example, when scale containing oxides such as Si-based oxides and Mn-based oxides is formed, a self-lubricating effect occurs. On the other hand, the inventors have clarified that when scale containing a large amount of Mn-based oxides is formed, the formed scale becomes thick and may become more prone to peeling. And, although a self-lubricating effect occurs even in the area where the scale has peeled off, such an area has a large surface irregularity, and therefore the high-temperature sliding properties are easily deteriorated over time.

(c) Therefore, the inventors have clarified that it is effective to control the Mn concentration in the scale within an appropriate range, as well as to control the Cr concentration, which is effective in forming a scale with a self-lubricating effect, and the concentrations of Si and Al, which contribute to improving high-temperature sliding properties in the scale. In order to control the concentrations of these elements in the scale, it is preferable to set the contents of the above elements in the steel within an appropriate range and to perform annealing heat treatment after cold rolling at a relatively high temperature for a short period of time.

The present invention was made based on the above findings. Each aspect of the present invention will be explained in detail below.

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

C: 0.005-0.30%
C is an element that stabilizes the austenite structure and improves high-temperature strength and high-temperature sliding properties. It is also an element necessary for ensuring high-temperature hardness. For this reason, the C content is From the viewpoints of high-temperature sliding properties and manufacturing costs, the C content is preferably 0.030% or more, and more preferably 0.050% or more. When the steel contains an excessive amount of C, the steel is easily work-hardened. In addition, the excess C combines with Cr to form Cr carbides, which reduces the corrosion resistance and intergranular corrosion resistance. The C content is set to 0.30% or less. From the viewpoint of hot workability, the C content is preferably set to 0.20% or less, and more preferably set to 0.19% or less.

Si: 1.00-4.00%
Silicon has a deoxidizing effect. In addition, the internal oxidation of silicon makes it difficult for scale to peel off, and as a result, high-temperature sliding properties are improved. In other words, the internal oxidation of silicon improves high-temperature strength and high-temperature hardness. The improvement in the high temperature sliding property contributes to the improvement of the high temperature sliding property. Therefore, the Si content is set to 1.00% or more. When it is desired to further improve the high temperature sliding property, the Si content is set to It is preferably 1.20% or more, and more preferably 1.50% or more.

However, if too much Si is added, the steel plate becomes excessively hard, reducing part workability and manufacturability. For this reason, the Si content is set to 4.00% or less. From the standpoints of manufacturing costs, pickling properties, and solidification cracking during welding, the Si content is preferably set to 3.00% or less, and more preferably 2.80% or less.

Mn: 0.90-8.00%
Mn has a deoxidizing effect. In addition, Mn has the effect of stabilizing the austenite structure and improving scale adhesion. Furthermore, by including Mn, the scale has a self-lubricating effect and a self-repairing function. Therefore, the Mn content is set to 0.90% or more.

However, if the Mn content exceeds 8.00%, thick scale will form in the usage environment. This will cause the scale to peel off easily, resulting in increased surface unevenness and reduced high-temperature sliding properties. It is also believed that inclusion cleanliness and pickling properties will be significantly reduced, resulting in a rough product surface. For this reason, the Mn content is set to 8.00% or less. From the viewpoint of inclusion cleanliness and pickling properties, the Mn content is preferably set in the range of 0.90 to 1.50%. Furthermore, if it is desired to further improve high-temperature sliding properties, the Mn content is preferably set in the range of 1.85 to 8.00%.

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. The P content is preferably set to 0.04% or less. It is preferable that the P content is small, but if the P content is excessively reduced, the refining cost increases. Therefore, the P content is set to 0.01% or more. Furthermore, from the viewpoint of reducing the refining cost, The P content is preferably 0.02% or more.

S: 0.0001-0.01%
S is an element that reduces hot workability and corrosion resistance. Furthermore, when coarse sulfides such as MnS are formed, the inclusion cleanliness of the steel sheet is significantly reduced. For this reason, the S content is set to 0. From the viewpoint of oxidation resistance, the S content is more preferably 0.005% or less. However, if the S content is excessively reduced, the refining cost increases. For this reason, the S content is From the viewpoint of refining costs, the S content is preferably 0.0005% or more.

Ni: 5.0-15.0%
Ni is an element that stabilizes the austenite structure and improves corrosion resistance and oxidation resistance. For this reason, the Ni content is set to 5.0% or more. From the viewpoints of high-temperature strength, corrosion resistance, and manufacturability, The Ni content is preferably 9.0% or more. However, excessive Ni content increases the manufacturing cost and hardens the material. For this reason, the Ni content is set to 15.0% or less. The Ni content is preferably 14.0% or less.

Cr: 19.50-26.00%
Cr is an element that improves corrosion resistance, oxidation resistance, and high-temperature sliding properties. From the viewpoint of suppressing abnormal oxidation in an exhaust part environment, the Cr content is set to 19.50% or more. The amount of Cr is preferably 20.00% or more, and more preferably 20.50% or more. However, if the Cr content is excessive, the material becomes hard and the manufacturing cost increases. Therefore, the Cr content is set to 26.00% or less. Furthermore, from the viewpoints of workability, manufacturability, and manufacturing costs, the Cr content is preferably set to 25.30% or less, and more preferably set to 24.00% or less. More preferred.

N: 0.02-0.40%
Like C, N stabilizes the austenite structure. It is also an effective element for improving high-temperature strength, high-temperature hardness, and high-temperature sliding properties. In particular, it acts as a solid solution strengthening element. In order to improve high-temperature strength, the N content is set to 0.02% or more. In order to improve high-temperature sliding properties, the N content is preferably set to more than 0.05%, and more preferably 0.05% or less. It is more preferable that the content be 0.10% or more, and even more preferable that the content be 0.20% or more.

However, if the N content exceeds 0.40%, the room temperature material becomes significantly hard. As a result, the cold workability during steel plate manufacturing deteriorates, and part workability decreases. For this reason, the N content is set to 0.40% or less. From the viewpoint of suppressing pinholes and intergranular corrosion of welds during welding, it is preferable that the N content be 0.33% or less.

Al: 0.001~0.50%
Al acts as a deoxidizing element and improves inclusion cleanliness. In addition, Al improves scale adhesion, suppresses the spalling of oxide scale, and improves high-temperature sliding properties. The Al content is set to 0.001% or more, preferably 0.010% or more, and more preferably 0.040% or more.

However, Al is a ferrite-forming element, and if it is contained in an amount exceeding 0.50%, it reduces the stability of the austenite structure. Furthermore, not only does it reduce pickling properties, but it also increases the amount of inclusions, which reduces surface properties. For this reason, the Al content is set to 0.50% or less. From the viewpoint of surface properties, the Al content is preferably set to 0.30% or less. Also, from the viewpoint of hot workability, the Al content is preferably set to 0.10% or less.

Of the above elements, it is necessary to control the content of each of Cr, Mn, Si, and Al, which affect the scale formed in the usage environment. Of the above elements, Cr and Mn constitute the outside of the scale, i.e., the outer layer scale, in the usage environment. This outer layer scale tends to grow thick, but if it has an appropriate thickness, it improves high-temperature sliding properties due to its self-lubricating action. On the other hand, Si and Al constitute the base steel plate side of the scale, i.e., the inner layer scale. This inner layer scale is easy to form thin. And since it contains oxides of Si and Al and hardens the base steel plate, it not only improves high-temperature sliding properties, but also improves oxidation resistance.

Therefore, by controlling the relationship between the content of Cr and Mn, which thicken the scale, and the content of Si and Al, which thin the scale, within an appropriate range and controlling the thickness of the scale so that it does not become too thick, it is possible to maintain good high-temperature sliding properties. Figure 1 shows the relationship between the wear scar depth after high-temperature sliding tests and (Cr+Mn)/(Si+Al) for austenitic stainless steels containing 19-25% Cr, 9-20% Ni, 0.5-3.3% Si, 0.8-8.4% Mn, and 0.01-0.04% Al.

The numbers in parentheses next to each plot in Figure 1 indicate the thickness (μm) of the scale. The scale was defined as a dark gray layer in which 25% (mass%) or more of O was detected by SEM/EDS analysis of the sliding surface side. Figure 1 shows that when the wear scar depth after high-temperature sliding tests is 5 μm or less and high-temperature sliding properties are good, (Cr+Mn)/(Si+Al) is in the range of 11.5 to 15.0, and in this case, the thickness of the scale is also relatively thin.

Therefore, the respective contents of Cr, Mn, Si and Al need to satisfy the following formula (i).
11.5≦(Cr+Mn)/(Si+Al)≦15.0...(i)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero.

If the value in formula (i) is less than 11.5, the scale is thin, but the high-temperature sliding properties are likely to be poor. For this reason, the value in formula (i) is set to 11.5 or more. The value in formula (i) is preferably set to 11.8 or more. On the other hand, if the value in formula (i) exceeds 15.0, the scale becomes too thick and the scale is more likely to peel off. As a result, in a high-temperature, long-term usage environment, the high-temperature sliding properties are actually reduced. For this reason, the value in formula (i) is set to 15.0 or less. The value in formula (i) is preferably set to 14.0 or less.

In addition to the above elements, one or more selected from Cu, REM, Mo, V, Ti, Nb, B, Ca, W, Zr, Sn, Co, Mg, Sb, Ga, Ta, Hf, and Bi may be contained within the ranges shown below. The reasons for limiting each element are explained below.

Cu: 0 to 3.0%
Cu has the effect of stabilizing austenite and improving oxidation resistance. In addition, Cu also has the effect of imparting a self-lubricating action and a self-repairing function to the scale and improving high-temperature sliding properties. For this reason, it may be contained as necessary. However, if Cu is contained in excess, oxidation resistance and manufacturability will decrease. Therefore, the Cu content is set to 3.0% or less. From the viewpoint of manufacturability, the Cu content is preferably set to 2.8% or less, and more preferably set to 2.5% or less. On the other hand, in order to obtain the above effect, the Cu content is preferably set to 0.01% or more, and more preferably set to 0.10% or more. From the viewpoint of corrosion resistance, the Cu content is more preferably set to 0.20% or more.

REM: 0~0.100%
REM (rare earth elements) have the effect of improving oxidation resistance and thereby improving high-temperature sliding properties. Therefore, they may be contained as necessary. However, if REM is contained in excess, During casting, oxides containing REM are formed in the nozzle through which molten steel flows, causing nozzle blockage, and sulfides containing REM are formed, reducing corrosion resistance. From the viewpoint of production costs, the REM content is preferably 0.090% or less. On the other hand, in order to obtain the above effects, the REM content is 0.0001% or more. It is preferable that the content of Si is 0.0010% or more, and more preferable that the content of Si is 0.0010% or more.

REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

Mo: 0 to 2.0%
Mo is an element that improves corrosion resistance. In addition, Mo has the effect of improving high-temperature strength. When Mo is contained, in addition to solid solution strengthening, precipitation strengthening occurs by precipitating Mo carbides. In addition, the formation of precipitates can improve wear resistance, i.e., high-temperature sliding properties. For this reason, Mo may be contained as necessary. However, Mo is an expensive element, and if it is contained in excess, the manufacturing cost increases. Therefore, the Mo content is set to 2.0% or less. From the viewpoint of inclusion cleanliness, the Mo content is preferably set to 1.7% or less. On the other hand, in order to obtain the above effect, the Mo content is preferably set to 0.1% or more.

V: 0 to 1.0%
V has the effect of improving corrosion resistance. In addition, V forms carbides and has the effect of improving high-temperature strength and wear resistance, i.e., high-temperature sliding properties. Therefore, V may be contained as necessary. However, if V is contained in excess, not only will the manufacturing cost increase, but the abnormal oxidation limit temperature will decrease. For this reason, the V content is set to 1.0% or less. From the viewpoint of manufacturability and inclusion cleanliness, the V content is preferably set to 0.8% or less. On the other hand, in order to obtain the above effect, the V content is preferably set to 0.05% or more.

Ti: 0-0.30%
Ti combines with C and N to improve corrosion resistance and intergranular corrosion resistance, and therefore may be contained as necessary. However, Ti should not be contained in an amount exceeding 0.30%. If Ti is contained, nozzle clogging occurs easily during the casting stage, and manufacturability is significantly reduced. For this reason, the Ti content is set to 0.30% or less. From the viewpoint of manufacturing costs, the Ti content is set to 0. It is preferable that the Ti content is 20% or less. On the other hand, in order to obtain the above-mentioned effects, the Ti content is preferably 0.005% or more. From the viewpoints of high temperature strength and intergranular corrosion resistance of welded parts, More preferably, the Ti content is 0.010% or more.

Nb: 0-0.30%
Like Ti, Nb has the effect of improving corrosion resistance and intergranular corrosion resistance by bonding with C and N. In addition, Nb improves high-temperature strength, thereby improving high-temperature sliding properties. It also has the effect of improving the hardness of the steel. Therefore, Nb may be added as necessary. However, if the Nb content exceeds 0.30%, the hot workability is significantly reduced. Therefore, the Nb content is From the viewpoint of production costs, the Nb content is preferably less than 0.15%. On the other hand, in order to obtain the above effects, the Nb content is preferably 0.005% or less. From the viewpoints of high temperature strength and intergranular corrosion resistance of welds, the Nb content is preferably 0.010% or more.

B: 0-0.005%
B has the effect of improving hot workability. It also has the effect of improving oxidation resistance. Therefore, it may be contained as necessary. However, if B is contained in excess, boron may be easily formed. Carbide is formed. This reduces the inclusion cleanliness and intergranular corrosion resistance of the steel sheet. For this reason, the B content is set to 0.005% or less. From the viewpoint of ductility, the B content is It is preferable that the B content is 0.002% or less. On the other hand, in order to obtain the above-mentioned effects, the B content is preferably 0.0002% or more. From the viewpoint of refining costs, the B content is 0. It is more preferable that the content is 0.003% or more.

Ca: 0-0.010%
Ca has a desulfurizing effect and may be added as necessary. However, if the Ca content exceeds 0.010%, water-soluble inclusions called CaS are formed. As a result, the steel sheet The inclusion cleanliness and corrosion resistance are significantly reduced. For this reason, the Ca content is set to 0.010% or less. From the viewpoint of surface properties, the Ca content is preferably set to 0.003% or less. In order to obtain the above effects, the Ca content is preferably 0.0005% or more. Furthermore, from the viewpoint of manufacturability, the Ca content is more preferably 0.0010% or more.

W: 0 to 3.0%
W has the effect of improving high-temperature strength and corrosion resistance, so it may be contained as necessary. However, if W is contained in an amount exceeding 3.0%, it leads to hardening of the steel plate, deterioration of toughness during manufacturing, and increase in manufacturing costs. For this reason, the W content is set to 3.0% or less. Furthermore, from the viewpoint of manufacturability and refining costs, the W content is preferably set to 2.0% or less. On the other hand, in order to obtain the above effect, the W content is preferably set to 0.1% or more.

Zr: 0-0.30%
Zr has the effect of improving the intergranular corrosion resistance and oxidation resistance of the welded portion by combining with C or N. Therefore, Zr may be contained as necessary. However, Zr should not be more than 0.30%. If the Zr content exceeds 0.30%, it will significantly decrease manufacturability and increase manufacturing costs. Therefore, the Zr content is set to 0.30% or less. From the viewpoint of manufacturability and refining costs, the Zr content is On the other hand, in order to obtain the above-mentioned effects, the Zr content is preferably 0.05% or more.

Sn: 0-0.50%
Sn has the effect of improving corrosion resistance and high-temperature strength, and may be contained as necessary. However, if the Sn content exceeds 0.50%, slab cracks may occur during manufacturing. For this reason, the Sn content is set to 0.50% or less. Furthermore, from the viewpoint of manufacturability and refining costs, the Sn content is preferably set to 0.30% or less. To achieve this, the Sn content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

Co: 0-0.30%
Co has the effect of improving high-temperature strength and may be added as necessary. However, if the Co content exceeds 0.30%, the steel plate becomes hard and the toughness during manufacturing decreases. , and leads to an increase in production costs. For this reason, the Co content is set to 0.30% or less. In consideration of manufacturability and refining costs, the Co content is preferably set to 0.20% or less. On the other hand, in order to obtain the above effects, the Co content is preferably 0.03% or more.

Mg: 0-0.010%
Mg has a deoxidizing effect. In addition, Mg has the effect of refining or dispersing the slab structure as an oxide, thereby refining the steel sheet structure and improving the cleanliness of inclusions. However, excessive Mg content reduces weldability and corrosion resistance. In addition, coarse Mg inclusions are formed, which reduces part workability. In view of the refining cost, the Mg content is preferably 0.005% or less. On the other hand, in order to obtain the above-mentioned effects, the Mg content is preferably 0.010% or less. The Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0003% or more.

Sb: 0-0.50%
Sb has the effect of improving high-temperature strength by segregating at grain boundaries. Therefore, it may be contained as necessary. However, if the Sb content exceeds 0.50%, segregation occurs. If the Sb content is too high, cracks will occur during welding. Therefore, the Sb content is set to 0.50% or less. Taking into consideration high temperature properties and manufacturing costs, the Sb content is preferably set to 0.30% or less, and more preferably 0. It is more preferable that the Sb content is 20% or less. On the other hand, in order to obtain the above-mentioned effects, the Sb content is preferably 0.005% or more. In consideration of toughness, the Sb content is preferably 0.030% or less. It is more preferable that the content is 0.050% or more, and further more preferable that the content is 0.050% or more.

Ga: 0-0.30%
Ga may be added as necessary to improve corrosion resistance and suppress hydrogen embrittlement. However, if Ga is added in an amount exceeding 0.30%, coarse sulfides are generated, which reduces part workability. Therefore, the Ga content is set to 0.30% or less. On the other hand, in order to obtain the above effect, the Ga content is preferably set to 0.0002% or more. Furthermore, manufacturability is improved. From the viewpoint of production costs, the Ga content is more preferably 0.0020% or more.

Ta: 0 to 1.0%
Hf: 0 to 1.0%
Bi: 0~0.02%
Ta has the effect of improving high-temperature strength, so it may be contained as necessary. However, if Ta is contained in excess, the manufacturing cost increases, so 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 set to 0.001% or more. In order to further increase the strength, the Ta content is more preferably set to 0.010% or more. For the same reason, the Hf content is set to 1.0% or less. Also, for the same reason, the Hf content is preferably set to 0.001% or more, more preferably set to 0.010% or more.

Bi may also be included as necessary for the same reason. However, excessive Bi content increases manufacturing costs. Therefore, the Bi content is set to 0.02% or less. On the other hand, to obtain the same effect as above, it is preferable that the Bi content be 0.001% or more.

In the chemical composition of the present invention, the balance is Fe and impurities. Here, "impurities" refers to components that are mixed in due to various factors in the raw materials such as ores and scraps and the manufacturing process when industrially manufacturing austenitic stainless steel sheets, and are acceptable within the range that does not adversely affect the present invention. It is desirable to reduce general harmful elements such as As and Pb, as well as impurity elements, as much as possible.

2. Maximum Mn Concentration In the austenitic stainless steel sheet according to the present invention, it is desirable to form a scale containing Mn by oxidation in the usage environment. This is because this scale exerts a self-lubricating action and a self-repairing function in the usage environment, and improves high-temperature sliding properties. For this reason, it is necessary that a sufficient amount of Mn is present near the surface of the steel sheet before use.

Here, simply increasing the Mn content is insufficient from the viewpoint of improving high-temperature sliding properties in order to ensure a sufficient amount of Mn near the surface of the steel sheet before use. This is because the scale formed during use may become thicker and may not satisfy formula (i).

Normally, in austenitic stainless steel, before use, thick scales are not formed after pickling, and the Mn concentration near the surface is not constant due to the effects of diffusion, etc. Specifically, as shown in Figure 2, the Mn concentration increases as you move from the outermost surface in the thickness direction. Then, at a depth of about 10.0 μm in the thickness direction, the increase in Mn concentration slows down and becomes close to the Mn content of the steel, i.e., the average composition.

Therefore, the maximum Mn concentration in the region from the surface to a depth of 1.0 μm in the plate thickness direction, which is the Mn concentration near the surface, is specified as a requirement, compared to the Mn concentration at a depth of 10.0 μm from the surface in the plate thickness direction, where the increase in Mn concentration is gradual. In other words, the austenitic stainless steel sheet according to the present invention must satisfy the following formula (ii). The maximum Mn concentration is measured in the region from the surface to a depth of 1.0 μm in the plate thickness direction. The reason for this is that in the region from the surface to a depth of 1.0 μm in the plate thickness direction, the concentration does not increase at a constant rate but fluctuates, and if sufficient Mn concentration occurs up to 1.0 μm, high-temperature sliding properties can be improved.

0.80≦X/Y≦1.00...(ii)
In the above formula, each symbol is defined as follows.
X: Maximum Mn concentration (mass%) in the region from the surface to a depth of 1.0 μm in the sheet thickness direction
Y: Mn concentration (mass%) at a depth of 10.0 μm from the surface in the sheet thickness direction

(ii) If the value X/Y in the formula is less than 0.80, a good scale that exhibits self-lubricating and self-repairing properties is not formed. As a result, high-temperature sliding properties cannot be improved. For this reason, X/Y is set to 0.80 or more. X/Y is preferably set to 0.85 or more. On the other hand, if X/Y exceeds 1.00, Mn is excessively concentrated in the scale, and the scale becomes thick. As a result, high-temperature sliding properties are degraded. For this reason, X/Y is set to 1.00 or less. X/Y is preferably set to 0.95 or less.

The above-mentioned X and Y can be measured by the following procedure. Specifically, the steel sheet surface is degreased by pickling to remove the scale formed by annealing, and the Mn, Fe, Cr, Si, Ni, Al, N, C, and O concentrations are analyzed from the steel sheet surface in the sheet thickness depth direction using a glow discharge optical emission analyzer, and the maximum Mn concentration up to a depth position of 1.0 μm and the concentration at a depth position of 10.0 μm are calculated. Here, the Mn concentration is the analytical concentration of Mn when the sum of all the above analyzed elements is 100%.

3. Evaluation of high-temperature sliding properties In the austenitic stainless steel sheet according to the present invention, the high-temperature sliding properties are evaluated by a pin-on-disk friction and wear test at 850°C. In the pin-on-disk friction and wear test, the test material is held in a furnace at 850°C in air for 1 hour. Thereafter, the pin and the disk are brought into contact in a furnace at 850°C, and the test material is rotated around a circumference with a rotation diameter of 20 mm for a test distance of 20 m at a load of 0.5 N and a sliding speed of 3.3 mm/s.

After the test, the wear scar depth on the surface of the disk material is measured using a laser microscope. The wear scar depth is the difference between the non-contacting parts of the pin and disk and the contacting parts, and is taken to be the largest value. If the average value of the six measured wear scar depths (hereinafter simply referred to as the "average wear scar depth") is 5.0 μm or less, the high-temperature sliding properties are judged to be good, and if the average wear scar depth exceeds 5.0 μm, the high-temperature sliding properties are judged to be poor.

The austenitic stainless steel sheet according to the present invention is suitable for use in exhaust parts, particularly turbocharger parts, such as nozzle parts and valve parts made up of precision parts, such as nozzle mounts, nozzle plates, nozzle rings, nozzle vanes, drive rings, drive levers, lever plates, shrouds, and wastegate valves.

5. Manufacturing Method A preferred manufacturing method for the austenitic stainless steel sheet according to the present invention will now be described. It has been confirmed that the austenitic stainless steel sheet according to the present invention can be manufactured by any manufacturing method, for example, the following manufacturing method, regardless of the manufacturing method.

The austenitic stainless steel sheet according to the present invention is preferably manufactured in a process including steelmaking-hot rolling-annealing and pickling, or steelmaking-hot rolling-annealing and pickling-cold rolling-annealing and pickling, as follows:

5-1. Steelmaking process It is preferable to melt steel having the above-mentioned chemical composition in an electric furnace or converter, followed by secondary refining. The melted steel is made into slabs by a known casting method (continuous casting, etc.). The thickness of the slab to be produced may be appropriately set depending on the subsequent rolling conditions, etc.

5-2. Hot rolling process Next, the obtained slab is preferably heated at 1200 to 1300°C and hot rolled by continuous rolling until the plate thickness reaches a predetermined value. The slab is hot rolled to become a hot rolled plate. The reduction ratio during hot rolling may be appropriately selected.

After hot rolling, the hot-rolled sheet is generally subjected to hot-rolled sheet annealing and pickling treatment, but hot-rolled sheet annealing may be omitted. If hot-rolled sheet annealing is performed, the annealing temperature is preferably in the range of 1100 to 1200°C, and the annealing time is preferably in the range of 10 to 60 seconds.

5-3. Cold rolling process It is preferable to cold roll the sheet to a predetermined thickness to obtain a cold rolled sheet. The reduction ratio during hot rolling may be appropriately selected.

5-4. Cold-rolled sheet annealing process Next, it is preferable to perform a cold-rolled sheet annealing heat treatment (hereinafter simply referred to as "annealing"). Mn diffuses into the scale formed during annealing of the cold-rolled sheet, and the Mn concentration decreases at the interface between the scale formed during annealing and the base material. This area of reduced Mn concentration appears on the steel sheet surface by pickling after annealing. That is, the Mn concentration near the surface decreases, making it difficult to satisfy formula (ii). If the Mn concentration near the surface (also called the "surface layer") is reduced when used as a turbocharger part, the formation of a scale having self-lubricating and self-repairing functions may be inhibited, and high-temperature sliding properties may decrease.

Table 1 shows the Mn concentration when the annealing conditions are changed for an austenitic stainless steel containing 19% Cr, 13% Ni, and 3% Si. When the cold-rolled sheet annealing conditions are changed to high temperature and short time conditions, such as 1120°C for 60 seconds to 1180°C for 30 seconds, the maximum Mn concentration in the region from the surface to a depth of 1.0 μm in the sheet thickness direction increases from 0.63% to 0.88%, suppressing the decrease in Mn concentration in the surface layer. As a result, high-temperature sliding properties can be improved.

Therefore, as is clear from Table 1, it is preferable to anneal at a high temperature for a short time so as not to cause a decrease in the Mn concentration. This is because annealing at a high temperature for a short time suppresses the decrease in the Mn concentration in the surface layer and produces a steel sheet with a structure in which recrystallization is complete.

Specifically, the annealing temperature when annealing the cold-rolled sheet is preferably in the range of 1140 to 1250°C. If the annealing temperature is less than 1140°C, recrystallization does not occur sufficiently, and sufficient high-temperature strength cannot be obtained. In addition, high-temperature sliding properties also deteriorate. For this reason, the annealing temperature is preferably 1140°C or higher. On the other hand, if the annealing temperature exceeds 1250°C, the diffusion of Mn is excessively promoted, resulting in a decrease in the Mn concentration at the interface between the scale formed after annealing and the base material. As a result, formula (ii) cannot be satisfied, and high-temperature sliding properties deteriorate. For this reason, the annealing temperature is preferably 1250°C or lower, and more preferably 1190°C or lower.

The annealing time is preferably 5 to less than 60 seconds. If the annealing time is less than 5 seconds, recrystallization does not occur sufficiently, and sufficient high-temperature strength cannot be obtained. In addition, high-temperature sliding properties also deteriorate. For this reason, the annealing time is preferably 5 seconds or more, and more preferably 20 seconds or more. On the other hand, if the annealing time is 60 seconds or more, the diffusion of Mn is excessively promoted, and the Mn concentration decreases at the interface between the scale formed after annealing and the base material. As a result, formula (ii) cannot be satisfied, and high-temperature sliding properties deteriorate. For this reason, the annealing time is preferably less than 60 seconds, and more preferably 50 seconds or less. In other words, it is preferable to hold the cold-rolled sheet for the above-mentioned time in the above-mentioned temperature range.

The cold-rolled sheet may be annealed between passes of cold rolling, and may be batch-type or continuous-type annealing. After annealing, the sheet may be cooled at a rate that results in the structure of the steel sheet becoming that of austenitic stainless steel.

5-5. Pickling process After annealing the cold-rolled sheet, pickling is preferably performed to remove the scale formed by annealing. The pickling method may be any chemical descaling method such as sulfuric acid, nitric hydrofluoric acid, or nitric acid electrolysis, and immersion in a molten alkali salt may be performed as a pretreatment. The temperature and time conditions for immersion in a molten alkali salt can be appropriately changed depending on the desired properties of the steel sheet. After that, temper rolling and polishing may be performed as appropriate and necessary.

The steel plate obtained through the above steps can be processed into a desired part shape by precision machining such as fine blanking to manufacture turbocharger parts. After processing, the parts may be subjected to special surface treatment such as nitriding or carburizing to improve high-temperature sliding properties.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

After the steel having the chemical composition shown in Tables 2 and 3 was melted, the following process was carried out to obtain steel plate. Specifically, a block of 90 mm length x 150 mm width x 40 mm thickness was cut out from an ingot cast in a 13 kg flat mold. This was hot rolled to 10 mm at 1250°C, hot-rolled sheet annealed at 1150°C for 60 seconds, and then pickled. It was then cold-rolled and annealed under the conditions shown in Tables 4 and 5. It was then pickled to obtain a 5.0 mm thick austenitic stainless steel plate.

(Maximum Mn concentration)
The X and Y of the formula (ii) were measured for the obtained steel sheet. For the measurement, a sample having a size of 30 mm length x 30 mm width was cut out, and after degreasing with acetone, a depth analysis was performed from the steel sheet surface to a depth of 10.0 μm in the sheet thickness direction. For the measurement, a glow discharge optical emission analyzer manufactured by Rigaku was used, and the measured elements were Mn, Fe, Cr, Ni, Si, Al, C, N, and O. The maximum Mn concentration in the region from the steel sheet surface to a depth position of 1.0 μm in the sheet thickness direction was taken as X, and the Mn concentration at a depth position of 10.0 μm in the sheet thickness direction from the steel sheet surface was taken as Y. Then, the median value of the formula (ii) was 0.85 to 0.95, ◎, the value not included in ◎ and satisfying the formula (ii) was O, and the value not satisfying the formula (ii) was ×.

(Pin-on-disk method high temperature friction and wear test)
In the pin-on-disk high-temperature friction and wear test, a circular disk was taken from each of the obtained steel plates, and a pin was placed so that the tip of the pin was in vertical contact with the disk, and the pin was adjusted so that it would repeatedly undergo friction and wear by orbiting the disk in a furnace at 850° C. in air. The bullet-shaped pin had a base diameter of φ6 mm, a tip diameter of φ4 mm, and a total length of 10 mm, and the circular disk was machined to a shape of φ29 mm and a height of 5 mm.

In the above test, the pin and disk were initially heated without contacting each other, and after holding at 850°C for 1 hour, the pin and disk were brought into contact in the furnace to start the test. The load was 0.5 N, the test distance was 20 m, the rotation diameter was 20 mm, and the sliding speed was 3.3 mm/s. After the test, the wear scar depth of the disk was measured from the disk surface using a laser microscope, and the average value of six points was taken as the average wear scar depth. When the average wear scar depth was 5.0 μm or less, it was marked as ◯, and when it was over 5.0 μm, it was marked as ×. As mentioned above, the wear scar depth is the difference between the non-contact part of the pin and disk and the contact part, and is the largest value.

A High-Temperature Tribometer manufactured by CSM Instruments was used for the above tests, and a VF-8500 manufactured by Keyence was used to measure the wear scar depth.

The results are summarized in Tables 4 and 5 below. In Tables 4 and 5, those that do not satisfy the requirements of the present invention and do not satisfy the performance as turbocharger parts are given an overall rating of x. The results are summarized in Tables 4 and 5 below.

According to the present invention, it is possible to provide an austenitic stainless steel sheet with excellent properties for exhaust parts that require high-temperature sliding properties. In particular, by using it as a turbocharger part among exhaust parts, it is possible to provide parts that comply with automobile exhaust gas regulations, weight reduction, and improved fuel efficiency. Furthermore, it is not limited to exhaust parts for automobiles and motorcycles, but can also be applied to parts used in high-temperature environments, particularly high-temperature friction environments, such as various boilers and fuel cell systems.

Claims (6)

The chemical composition, in mass%, is
C: 0.005-0.30%,
Si: 1.00-4.00%,
Mn: 0.90-8.00%,
P: 0.01-0.05%,
S: 0.0001-0.01%,
Ni: 5.0 to 15.0%,
Cr: 19.50-26.00%,
N: 0.02-0.40%,
Al: 0.001-0.50%,
Cu: 0 to 3.0% (excluding 0.05% or more) ,
REM: 0-0.100%,
Mo: 0-2.0%,
V: 0 to 1.0%,
Ti: 0 to 0.30%,
Nb: 0 to 0.30%,
B: 0 to 0.005%,
Ca: 0-0.010%,
W: 0 to 3.0%,
Zr: 0 to 0.30%,
Sn: 0 to 0.50%,
Co: 0 to 0.30%,
Mg: 0 to 0.010%,
Sb: 0 to 0.50%,
Ga: 0-0.30%,
Ta: 0-1.0%,
Hf: 0-1.0%,
Bi: 0-0.02%,
The balance is Fe and impurities.
An austenitic stainless steel sheet satisfying the following formulas (i) and (ii):
11.5≦(Cr+Mn)/(Si+Al)≦15.0...(i)
0.80≦X/Y≦1.00...(ii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, it is set to zero. Each symbol in the above formula is defined as follows.
X: Maximum Mn concentration (mass%) in the region from the surface to a depth of 1.0 μm in the sheet thickness direction
Y: Mn concentration (mass%) at a depth of 10.0 μm from the surface in the sheet thickness direction The chemical composition, in mass%,
Cu: 0.01 to 3.0% (excluding 0.05% or more) ,
REM: 0.0001-0.100%,
Mo: 0.1-2.0%,
V: 0.05-1.0%,
Ti: 0.005-0.30%,
Nb: 0.005-0.30%,
B: 0.0002 to 0.005%,
Ca: 0.0005-0.010%,
W: 0.1-3.0%,
Zr: 0.05-0.30%,
Sn: 0.01-0.50%,
Co: 0.03 to 0.30%,
Mg: 0.0002 to 0.010%,
Sb: 0.005-0.50%,
Ga: 0.0002-0.30%,
Ta: 0.001-1.0%,
Hf: 0.001 to 1.0%, and Bi: 0.001 to 0.02%,
The austenitic stainless steel sheet according to claim 1, further comprising one or more selected from the group consisting of: 3. The austenitic stainless steel sheet according to claim 1, wherein when the sheet is held in air at 850°C for 1 hour and then subjected to a pin-on-disk friction and wear test in a furnace at 850°C under conditions of a load of 0.5 N and a test distance of 20 m, the average wear scar depth is 5.0 μm or less. The austenitic stainless steel sheet according to any one of claims 1 to 3, used for turbocharger parts. An exhaust part using the austenitic stainless steel sheet according to any one of claims 1 to 4. The method for producing an austenitic stainless steel sheet according to any one of claims 1 to 4 , wherein a cold-rolled sheet annealing heat treatment is performed by retaining the sheet in a temperature range of 1140 to 1250°C for 5 seconds or more and less than 60 seconds.

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