JP2022098633A - Austenitic stainless steel sheet, method for producing austenitic stainless steel sheet and automobile exhaust component - Google Patents

Abstract

To provide an austenitic stainless steel sheet having excellent heat resistance and corrosion resistance after long-aging in a σ phase precipitation temperature area.SOLUTION: An austenitic stainless steel sheet is adopted that has a predetermined chemical composition and satisfies formula (i): 5[Cr]+6[Si]+9[Mo]-2[Ni]-30[C]-70[N]-100[*Ni]/[Ni]≤0 and formula (ii): 0.80≤[*Ni]/[Ni]<1.00, where, [element symbol] in the formulae each denotes the content (mass%) of the element in steel and [*Ni] denotes an average content of Ni (mass%) in the Ni negative-segregated portion.SELECTED DRAWING: None

Description

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

Exhaust system members such as automobile exhaust manifolds, converters, front pipes and center pipes allow high-temperature exhaust gas discharged from the engine to pass through. Various characteristics such as are required. Further, since the exhaust system member is in a condensed water corrosive environment, it is also required to have excellent corrosion resistance. Furthermore, stainless steel is often used for these exhaust system members from the viewpoints of tightening exhaust gas regulations, improving engine performance, and reducing the weight of the vehicle body. Moreover, in recent years, the exhaust gas temperature has been on the rise due to the further tightening of exhaust gas regulations, improvement of fuel efficiency, downsizing and the like. In addition, there are many cases where a supercharger such as a turbocharger is installed, and stainless steel used for exhaust manifolds and turbochargers is required to have further improved heat resistance. The maximum temperature of exhaust gas has been about 900 ° C in the past, but it is expected to rise to about 1000 ° C in the future, and there is a possibility that it will rise to about 1050 ° C.

On the other hand, the internal structure of the turbocharger is complicated, and it is important to improve the feeding efficiency and ensure heat resistance, and Ni-based alloys and heat-resistant austenitic stainless steels are mainly used. A typical heat-resistant austenitic stainless steel is SUS310S (25% Cr-20% Ni). In addition, although casting is used for the housing of the turbocharger, there is a demand for thinning by sheet metal using heat-resistant austenitic stainless steel from the viewpoint of reducing the weight of the vehicle body.

From the viewpoint of such an increase in the exhaust gas temperature and the thinning of parts, it is necessary to further improve the heat resistance of the heat-resistant austenitic stainless steel. For example, when the maximum temperature of the exhaust gas rises to about 1050 ° C, the maximum temperature of the exhaust system members and the parts constituting the turbocharger becomes about 950 to 1000 ° C, and the heat resistance such as creep characteristics in that temperature range becomes high. You will need it. However, the component temperature does not always reach the maximum temperature, and it is often the case that the time when the temperature is about 900 ° C., which is lower than the maximum temperature, is longer. However, in austenitic stainless steel, the σ phase precipitates when aged at about 900 ° C. for a long time. When the σ phase is precipitated, there is a concern that the creep characteristics may be deteriorated. Further, since the σ phase is a phase having a high Cr concentration, there is a concern that the Cr concentration in the steel in the vicinity thereof is locally reduced and the corrosion resistance is lowered. That is, in the development of heat-resistant austenitic stainless steel, it is necessary to consider the creep characteristics and corrosion resistance at about 950 to 1000 ° C after aging at about 900 ° C for a long time, and the heat resistance and corrosion resistance of heat-resistant austenitic stainless steel are simplified. It is necessary to develop a technique for suppressing the precipitation of the σ phase in a long-term aging of about 900 ° C.

Patent Document 1 describes that the creep characteristics of heat-resistant austenitic stainless steel are improved by defining the optimum range of Nb, V, C, N, Al, and Ti contents and optimizing the manufacturing process. ing. However, the technique described in Patent Document 1 relates to improvement of creep characteristics at 800 ° C., and does not consider heat resistance in a range exceeding 900 ° C.

Patent Document 2 describes a technique for improving creep characteristics by controlling the characteristics of grain boundaries in the metal structure of heat-resistant austenitic stainless steel. However, the technique described in Patent Document 2 relates to creep characteristics at 900 ° C. and 950 ° C., and creep characteristics at 1000 ° C. have not been studied. In addition, no studies have been conducted on creep characteristics and corrosion resistance in consideration of σ precipitation due to long-term aging at about 900 ° C.

Patent Document 3 describes that by controlling the component balance of N and Nb, the high-temperature strength of heat-resistant austenitic stainless steel at 1000 ° C. after aging at 1000 ° C. is improved. However, since the σ phase is difficult to precipitate at 1000 ° C., the precipitation of the σ phase is not considered in Patent Document 3. Moreover, the corrosion resistance in consideration of the precipitation of the σ phase has not been investigated.

Japanese Unexamined Patent Publication No. 2013-209730 Japanese Unexamined Patent Publication No. 2019-218588 Japanese Patent No. 5605996

An object of the present invention is to provide an austenitic stainless steel sheet, a method for manufacturing an austenitic stainless steel sheet, and an automobile exhaust system component having excellent heat resistance and corrosion resistance after long-term aging in the σ-phase precipitation temperature range.

In order to solve the above problems, the present inventors have diligently investigated the effects of various components on the creep characteristics and corrosion resistance of austenitic stainless steel sheets after aging and the segregation of components inside the base metal. As a result, it was found that the σ-phase precipitation after aging deteriorates the creep characteristics and corrosion resistance. It was also found that the σ phase precipitation after aging is affected by the component balance of the steel, and that the σ phase precipitation increases when the Ni segregation is large. When the Ni segregation is large, especially in the Ni negative segregation part where the Ni concentration is lower than the Ni content in the steel in the cross section of the steel plate, σ precipitation occurs because the Ni concentration is lower than the Ni content in the steel. Furthermore, it is considered that other elements are further microsegregated in the Ni negative segregation part, causing microsegregation in which local σ precipitation is likely to occur, and σ precipitation is likely to occur. Based on these findings, we have invented an austenitic stainless steel sheet having excellent heat resistance and corrosion resistance after long-term aging in the σ-phase precipitation temperature range.

That is, the gist of the present invention for solving the above problems is as follows.
(1) By mass%,
C: 0.001% or more, 0.300% or less,
N: 0.001% or more, 0.45% or less,
Si: 0.10% or more, 5.00% or less,
Mn: 0.10% or more and 12.00% or less,
P: 0.060% or less,
S: 0.0200% or less,
Cr: 15.0% or more, 35.0% or less,
Ni: 2.0% or more, 35.0% or less,
Cu: 0.001% or more, 5.00% or less,
Mo: 0.001% or more, 5.00% or less,
Al: 0.001% or more, 1.000% or less,
V: 0.01% or more, 1.00% or less,
B: 0.0001% or more, 0.0100% or less,
Ca: 0.0001% or more, 0.0200% or less,
O: 0.0150% or less,
An austenitic stainless steel sheet containing Fe and impurities, and satisfying the following formulas (i) and (ii).
5 [Cr] + 6 [Si] + 9 [Mo] -2 [Ni] -30 [C] -70 [N] -100 [* Ni] / [Ni] ≤ 0 ... Equation (i)
0.80≤ [* Ni] / [Ni] <1.00 ... Equation (ii)
However, the [element symbol] in the formula (i) and the formula (ii) means the content (mass%) of the element in the steel, and [* Ni] is the average content of Ni in the Ni negative segregation part. It means (mass%).
(2) By mass%,
C: 0.001% or more, less than 0.080%,
N: Over 0.20%, 0.39% or less,
Si: Over 1.00%, 3.00% or less,
Mn: 0.80% or more, 3.00% or less,
P: 0.050% or less,
S: 0.0018% or less,
Cr: 20.0% or more, 35.0% or less,
Ni: Over 8.0%, 25.0% or less,
Cu: 0.001% or more, 3.40% or less,
Mo: 0.001% or more, less than 0.50%,
Al: Over 0.005%, 0.130% or less,
V: 0.01% or more, 1.00% or less,
B: 0.0001% or more, 0.0100% or less,
Ca: 0.0001% or more, 0.0200% or less,
O: 0.0150% or less,
An austenitic stainless steel sheet containing Fe and impurities, and satisfying the following formulas (iii) and (iv).
5 [Cr] + 6 [Si] + 9 [Mo] -2 [Ni] -30 [C] -70 [N] -100 [* Ni] / [Ni] ≤ 0 ... Equation (iii)
0.80≤ [* Ni] / [Ni] <1.00 ... Equation (iv)
However, the [element symbol] in the formula (iii) and the formula (iv) means the content (mass%) of the element in the steel, and [* Ni] is the average content of Ni in the Ni negative segregation part. It means (mass%).
(3) Instead of a part of Fe, in mass%, Ti: 0.001% or more, 1.00% or less,
Nb: 0.001% or more, 1.00% or less,
W: 0.01% or more, 3.00% or less,
Y: 0.001% or more, 0.50% or less,
REM: 0.001% or more, 0.50% or less,
Zr: 0.01% or more, 0.50% or less,
Hf: 0.001% or more, 1.0% or less,
Sn: 0.001% or more, 1.0% or less,
Mg: 0.0001% or more, 0.0100% or less,
Co: 0.01% or more, 1.00% or less,
Sb: 0.001% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0001% or more, 0.50% or less,
The austenitic stainless steel sheet according to (1) or (2), which contains one or more of the above.
(4) The austenitic stainless steel sheet according to any one of (1) to (3), which is used for an automobile exhaust system member.
(5) The method for manufacturing an austenite-based stainless steel sheet according to any one of (1) to (4), wherein the thickness A (mm) of the slab at the time of casting, the thickness B (mm) of the final product, and the like. A method for producing an austenite-based stainless steel sheet, wherein the final annealing temperature C (° C.) and the final annealing time D (sec) satisfy the following formula (v).
50 ≦ (C × D) / (A × B) ≦ 700 ・ ・ ・ Equation (v)
(6) An automobile exhaust system component including an automobile exhaust system member made of the austenitic stainless steel plate according to any one of (1) to (4).

According to the present invention, it is possible to provide an austenitic stainless steel sheet, a method for producing an austenitic stainless steel sheet, and an automobile exhaust system component, which are excellent in heat resistance and corrosion resistance after long-term aging in the σ-phase precipitation temperature range.

The austenitic stainless steel sheet according to the present invention can be applied to automobile exhaust system members such as turbocharger parts, exhaust manifolds, and converters of automobiles. In particular, it can be suitably applied to a turbocharger mounted on a gasoline vehicle or a diesel vehicle. The turbocharger can be applied to the housing that constitutes the exterior. Further, it can be suitably applied to a back plate, an oil deflector, a compressor wheel, a nozzle mount, a nozzle plate, a nozzle vane, a drive ring, a drive lever and the like, which are precision parts inside a nozzle vane type turbocharger. However, the applications of the austenitic stainless steel sheet of the present invention are not limited to these applications.

Hereinafter, the present invention will be described in detail.
In the following description, the “X negative segregation portion” means a region where the average content of the element X in the cross section of the steel sheet is less than the content of the element X in the steel. Further, the “X positive segregation portion” means a region in which the average content of the element X exceeds the content of X in the steel in the cross section of the steel sheet. For example, the Ni negative segregation portion refers to a region where the average Ni content in the steel sheet cross section is less than the Ni content in the steel. Further, the Cr positive segregation portion refers to a region where the Cr average content exceeds the Cr content in the steel in the cross section of the steel sheet.

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

(C: 0.001% or more, 0.300% or less)
Since C is an element that improves high temperature strength and creep characteristics, the C content is 0.001% or more. Further, considering the production cost, the C content is preferably 0.010% or more. The C content is more preferably 0.020% or more. However, since excessive C content lowers oxidation resistance and intergranular corrosion resistance, the C content is set to 0.300% or less. Further, in consideration of further improvement of corrosion resistance after aging in the σ precipitation temperature range, the C content is preferably less than 0.080%. More preferably, it is 0.075% or less.

(N: 0.001% or more, 0.45% or less)
Since N is an element that improves high-temperature strength and creep characteristics, the N content is 0.001% or more. Further, in consideration of the production cost, the N content is preferably 0.030% or more. Further, considering the further improvement of the creep characteristics after aging in the σ precipitation temperature range, the N content is more preferably more than 0.20%. However, excessive N content causes bubble defects, so the N content is set to 0.45% or less. Further, considering the high temperature fatigue characteristics, the N content is preferably 0.39% or less. Further, in consideration of processability, the N content is more preferably 0.32% or less. Even more preferably, it is 0.28% or less.

(Si: 0.10% or more, 5.00% or less)
Since Si is an element contained as a deoxidizing agent and also an element for improving oxidation resistance, the Si content is set to 0.10% or more. Further, since Si is an element that also improves workability, it is preferable that Si is 0.40% or more. Further, considering the scale peeling resistance, the Si content is more preferably more than 1.00%. Even more preferably, it is 1.50% or more. However, since an excessive Si content lowers hot workability and pickling property during production, the Si content is set to 5.00% or less. Further, Si is an element that promotes σ precipitation, and the Si content is preferably 3.00% or less in consideration of further improvement of creep characteristics after aging in the σ precipitation temperature range. Further, considering the solidification crackability at the time of welding, the Si content is more preferably 2.50% or less. Even more preferably, it is 2.20% or less.

(Mn: 0.10% or more and 12.00% or less)
Mn is an element contained as a deoxidizing agent, and the Mn content is 0.10% or more. Further, considering the refining cost, the Mn content is more preferably 0.40% or more. Further, Mn is an element that improves the solid solution limit of N, and considering that the N content is increased in order to further improve the creep characteristics after aging in the σ precipitation temperature range, the Mn content is 0. It is more preferably .80% or more. Even more preferably, it is 1.00% or more. However, since the excessive Mn content lowers the oxidation resistance, the Mn content is set to 12.00% or less. Further, in consideration of scale peeling resistance, the Mn content is preferably 3.00% or less. Further, considering the pickling property and processability at the time of production, the Mn content is more preferably 2.30% or less. Even more preferably, it is 1.80% or less.

(P: 0.060% or less)
Since P is an impurity mainly mixed from the raw material during steelmaking refining and is an element that deteriorates toughness and weldability, the P content is 0.060% or less. Further, in consideration of high temperature fatigue characteristics, the P content is preferably 0.050% or less. Further, in consideration of manufacturability, the P content is more preferably 0.040% or less. Even more preferably, it is 0.035% or less. However, since excessive reduction leads to an increase in refining cost, the P content may be 0.001% or more. Further, the P content may be 0.010% or more.

(S: 0.200% or less)
Since S is an impurity mainly mixed from the raw material during steelmaking refining and is an element that deteriorates hot workability and corrosion resistance during manufacturing, the S content is 0.200% or less. Further, in consideration of scale peeling resistance, the S content is preferably 0.0018% or less. More preferably, it is 0.0014% or less. Even more preferably, it is 0.0012% or less. However, since excessive reduction leads to an increase in refining cost, the S content may be 0.0001% or more. Further, the S content may be 0.0003% or more.

(Cr: 15.0% or more and 35.0% or less)
Since Cr is an element that improves corrosion resistance, the Cr content is 15.0% or more. Further, in consideration of oxidation resistance, the Cr content is preferably 18.0% or more. Further, Cr is an element that improves the solid solution limit of N, and considering that the N content is increased in order to further improve the creep characteristics after aging in the σ precipitation temperature range, the Cr content is 20. More preferably, it is 0.0% or more. However, since the excessive content of Cr reduces the hot workability and pickling property at the time of production, the Cr content is set to 35.0% or less. Further, in consideration of processability and raw material cost, the Cr content is preferably 30.0% or less. More preferably, it is 26.0% or less.

(Ni: 2.0% or more and 35.0% or less)
Since Ni is an element that improves corrosion resistance, the Ni content is set to 2.0% or more. Further, in consideration of further improvement of creep characteristics after aging in the σ precipitation temperature range, the Ni content is preferably more than 8.0%. More preferably, it is 10.0% or more. However, since the excessive Ni content lowers the moldability, the Ni content is set to 35.0% or less. Further, Ni is an element that easily segregates, and if the content thereof is large, the segregation of Ni expands, and σ precipitation is promoted in the Ni negative segregation portion where the Ni concentration is lower than the surroundings. Therefore, the Ni content is preferably 25.0% or less in consideration of further improvement of the creep characteristics after aging in the σ precipitation temperature range. Further, considering the raw material cost, the Ni content is more preferably 20.0% or less. Even more preferably, it is 15.0% or less.

(Cu: 0.001% or more, 5.00% or less)
Since Cu is an element that improves corrosion resistance, the Cu content is 0.001% or more. Further, in consideration of manufacturability, the Cu content is preferably 0.05% or more. However, the Cu content is set to 5.00% or less because excessive Cu content reduces hot workability during production. Further, considering the scale peeling resistance, the Cu content is preferably 3.40% or less. Further, considering the raw material cost, the Cu content is more preferably 2.00% or less. Further, considering the oxidation resistance, the Cu content is further preferably 1.00% or less. Even more preferably, it is 0.40% or less.

(Mo: 0.001% or more, 5.00% or less)
Since Mo is an element that improves corrosion resistance, the Mo content is 0.001% or more. Further, in consideration of manufacturability, the Mo content is preferably 0.01% or more. More preferably, it is 0.05% or more. However, since the excessive Mo content lowers the processability, the Mo content is set to 5.00% or less. Further, considering the raw material cost, the Mo content is preferably 1.50% or less. Further, Mo is an element that promotes σ precipitation, and the Mo content is more preferably less than 0.50% in consideration of further improvement of creep characteristics after aging in the σ precipitation temperature range.

(Al: 0.001% or more, 1.000% or less)
Since Al is an element contained as a deoxidizing element, the Al content is 0.001% or more. Further, in consideration of oxidation resistance and refining cost, the Al content is preferably more than 0.005%. More preferably, it is 0.010% or more. However, the excessive content of Al deteriorates the surface quality, so the content is set to 1.000% or less. Further, in consideration of high temperature fatigue characteristics, the Al content is preferably 0.130% or less. Further, considering the manufacturability, it is more preferably 0.080% or less. Even more preferably, it is 0.070% or less.

(V: 0.01% or more, 1.00% or less)
Since V is an element that improves corrosion resistance and high-temperature strength, the V content is 0.01% or more. Further, considering the refining cost, the V content is preferably 0.03% or more. More preferably, it is 0.05% or more. However, since the excessive content of V lowers the manufacturability, the V content is set to 1.00% or less. Further, considering the surface quality, the V content is preferably 0.20% or less. More preferably, it is less than 0.15%.

(B: 0.0001% or more, 0.0100% or less)
Since B is an element that improves corrosion resistance and high-temperature strength, the B content is 0.0001% or more. Further, considering the hot workability at the time of production, the B content is preferably 0.0002% or more. More preferably, it is 0.0003% or more. However, since the excessive content of B deteriorates the surface texture of the steel surface, the B content is set to 0.0100% or less. Further, in consideration of manufacturability, the B content is preferably 0.0050% or less. Further, in consideration of moldability, the B content is more preferably 0.0030% or less. Even more preferably, it is 0.0015% or less.

(Ca: 0.0001% or more, 0.0200% or less)
Since Ca is an element that promotes desulfurization, the Ca content is 0.0001% or more. Further, considering the scale peeling resistance, the Ca content is preferably 0.0002% or more. However, since excessive Ca content may lead to a decrease in corrosion resistance due to the formation of CaS, which is a water-soluble inclusion, the Ca content is set to 0.0200% or less. Further, in consideration of manufacturability, the Ca content is more preferably 0.0050% or less. Even more preferably, it is 0.0030% or less.

(O: 0.0150% or less)
O is an impurity inevitably contained, which causes surface defects due to air bubbles and inclusions, and also reduces corrosion resistance and oxidation resistance. Therefore, the O content is 0.0150% or less. Further, in consideration of manufacturability, the O content is preferably 0.0050% or less. More preferably, it is 0.0035% or less. However, since excessive reduction of O increases the refining cost, the O content may be 0.0001% or more. The O content may be 0.0003% or more. Here, the O content means the total content including oxygen dissolved in the steel and oxygen of the oxide intervening in the steel.

In the austenitic stainless steel sheet according to the present embodiment, the balance other than the above-mentioned elements is Fe and impurities. However, elements other than the above-mentioned elements can also be contained within a range that does not impair the effects of the present embodiment. The impurities referred to here are components mixed with raw materials such as ore and scrap and various factors in the manufacturing process when the austenitic stainless steel according to the present invention is industrially manufactured, and are included in the present invention. It means something that is acceptable as long as it does not have an adverse effect.

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

In investigating various effects on the creep characteristics and corrosion resistance of austenitic stainless steel sheets after aging, the present inventors have investigated the effects of Ni segregation of the base metal in addition to Cr, Si, Mo, Ni, C, and N. We have found that these balances are important. Therefore, in this embodiment, it is necessary to satisfy the formulas (i) to (iv). However, in the present invention, the formula (i) and the formula (iii) are the same formula, and the formula (ii) and the formula (iv) are the same formulas. By explaining ii), the equation (ii) and the equation (iv) will also be described.

5 [Cr] + 6 [Si] + 9 [Mo] -2 [Ni] -30 [C] -70 [N] -100 [* Ni] / [Ni] ≤ 0 ... Equation (i)

0.80≤ [* Ni] / [Ni] <1.00 ... Equation (ii)

However, the [element symbol] in the formulas (i) and (ii) means the content (mass%) of the element in the steel. Further, [* Ni] means the average content (mass%) of Ni in the Ni negative segregation portion.

When the value on the left side of the equation (i) is more than 0, the component balance of the steel is a balance in which σ precipitation is likely to occur, and the Ni segregation is large. When the Ni negative segregation is large, the Ni content is low in the Ni negative segregation part, so that σ precipitation is likely to occur, and other elements are further microsegregated in the Ni negative segregation part to cause local σ precipitation. Easy microsegregation occurs, and σ precipitation is likely to occur. Therefore, it is necessary to satisfy the equation (i). More preferably, the value on the left side of the formula (i) is −65 or more and -1 or less.

Further, when the value of the middle side of the equation (ii) is less than 0.80, the Ni negative segregation becomes large. Further, when the value of the middle side of the formula (ii) is 1.00 or more, it means that there is no segregation of Ni. However, eliminating Ni segregation requires a large amount of heat treatment cost and time, which greatly impairs economic efficiency. Therefore, it is necessary to satisfy the equation (ii). The value on the middle side of the formula (ii) is more preferably 0.85 or more and 0.98 or less.

Further, when the negative Ni segregation is large, the positive segregation of Cr, Mo, and Si is also large, and it is considered that σ is likely to be precipitated in the positive segregation portion of these elements. It is considered that the balance of Cr, Mo, and Si also has an effect, and it is preferable to satisfy the conditions 1 to 3 as preferable conditions. It should be noted that the condition 2 may satisfy either the formula (B) or the formula (C), and the condition 3 may satisfy either the formula (D) or the formula (E).

(Condition 1)
1.00 <[** Cr] / [Cr] ≤ 1.30 ... Equation (A)

(Condition 2)
[** Mo] ≤ 0.80 ... (B)
Or 1.00 <[** Mo] / [Mo] ≤ 1.30 ... Equation (C)

(Condition 3)
[** Si] ≤ 0.80 ・ ・ ・ Equation (D)
Or 1.00 <[** Si] / [Si] ≤ 1.30 ... Equation (E)

However, the [element symbol] in the formulas (A) to (E) means the content (% by mass) of the element in the steel, and the [** element symbol] is the element in the positive segregation portion of the element. Means the average content (% by mass) of.

The identification of the Ni negative segregation part and the average content of Ni in the Ni negative segregation part of the austenitic stainless steel sheet (hereinafter, may be referred to as a test material) according to the present embodiment are as follows. Calculated by analysis with an analyzer (EPMA). For EPMA, for example, JXA-8230 manufactured by JEOL Ltd. is used. LaB ₆ is used as the electron gun, the analysis conditions are set to an acceleration voltage of 15 kV, an irradiation current of 325 nA, and a beam diameter of 5 μm. As the spectroscope, for example, XM-86030 type manufactured by JEOL Ltd. is used. LiF is used for the spectroscopic crystal, the relative intensity of NiKα ray is obtained as the characteristic X-ray, and the concentration is converted using the calibration curve.

As a calibration curve sample for measuring the Ni concentration, an austenitic stainless steel sample in which the Ni content is changed to 8%, 12%, 20%, and 30% by mass% based on SUS304 is prepared. For the Ni content in the steel of the calibration curve sample and the test material, the value analyzed to the second decimal place by the ICP emission spectroscopic analysis method specified by JIS G 1258 is used as a positive value.

The position to be analyzed by EPMA is as follows. Calibration curve In the measurement area covering the entire thickness of 2 mm in the length direction of the cross section of the sample or test material, analysis points are set at intervals of 5 μm in a grid pattern, and point analysis is performed at each measurement point. This is done in 5 measurement areas. Then, the average value of the element content at each measurement point is taken as the measured value of each element in the steel. Calibration curve A basic calibration curve is created using the relative intensity of the content of each element (for example, Ni content) measured by ICP of the sample and the characteristic X-ray (Kα ray in the case of Ni) measured by EPMA. In addition, the content of each element in the steel measured by ICP (for example, Ni content) and the content of each element in the steel measured by EPMA (for example, Ni content) for each test material are calibrated so as to match. Correct the line.

The Ni negative segregation part is specified and the average content of Ni in the Ni negative segregation part is determined as follows. When the Ni content at each analysis point at 5 μm intervals measured by EPMA is less than the Ni content in the above steel, the analysis point is regarded as a Ni negative segregation part. The average value of the Ni content at each analysis point in the Ni negative segregation section is taken as the average Ni content in the Ni negative segregation section. The identification of the positive segregation part of Cr, Mo and Si and the average content are also obtained in the same manner as the method of specifying the Ni negative segregation part and analyzing and calculating the average content of Ni. As the characteristic X-rays, NiKα rays, CrKα rays, MoLα rays, and SiKα rays are used, respectively.

As a calibration curve sample, an austenitic stainless steel sample in which the content of each element is changed based on SUS304 is prepared for each element. The Cr content of the calibration curve sample for Cr concentration measurement is 10%, 18%, 26%, 35% by mass%, and the Mo content of the calibration curve sample for Mo concentration measurement is 0.5% by mass%, 1 %, 3%, 5%, and the Si content of the calibration curve sample for measuring the Si concentration is 0.5% by mass, 1%, 3%, and 5%.

The steel of the austenitic stainless steel sheet of the present embodiment includes the above-mentioned X negative segregation section and X positive segregation section. Specifically, at least a Ni negative segregation section may be included, and a Cr positive segregation section, a Mo positive segregation section, or a Si positive segregation section may be included.

Further, in the austenitic stainless steel sheet according to the present embodiment, one of Ti, Nb, W, Y, REM, Zr, Hf, Sn, Mg, Co, Sb, Bi, Ta, and Ga is selectively used. Alternatively, by containing two or more kinds, the characteristics can be further improved. Hereinafter, these elements will be described. Since these elements do not have to be contained, the lower limit of the content of each of these elements is 0%.

(Ti: 0.001% or more, 1.00% or less)
Ti is an element that binds to C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawing resistance, and is preferably contained in an amount of 0.001% or more, if necessary. It is more preferably 0.003% or more, and even more preferably 0.005% or more. However, the Ti content is preferably 1.00% or less because excessive Ti content may lead to deterioration of hole-expanding workability due to formation of coarse Ti-based precipitates. Further, in consideration of uniform elongation, the Ti content is more preferably 0.30% or less. Further, considering the occurrence of surface defects and toughness, the Ti content is even more preferably 0.25% or less.

(Nb: 0.001%, 1.00% or less)
Nb is an element that contributes to the improvement of high-temperature strength and the improvement of intergranular corrosion resistance, and is preferably contained in an amount of 0.001% or more, if necessary. More preferably, it is 0.005% or more, and even more preferably 0.01% or more. However, since excessive Nb content may lead to an increase in raw material cost and a decrease in manufacturability, the Nb content is preferably 1.00% or less. Further, the Nb content is more preferably 0.60% or less in consideration of uniform elongation, hole expandability, and manufacturability. Even more preferably, it is 0.50% or less.

(W: 0.01% or more, 3.00% or less)
W is an element that improves high-temperature strength, and is preferably contained in an amount of 0.01% or more, if necessary. Further, since W is also an element for improving corrosion resistance, the W content is more preferably 0.05% or more. However, since the excessive content of W may cause a decrease in toughness, the W content is preferably 3.00% or less. Further, considering the raw material cost, the W content is more preferably 0.50% or less. Further, in consideration of manufacturability, the W content is further preferably 0.40% or less.

(Y: 0.001% or more, 0.50% or less)
Y is an element that improves the cleanliness of steel, improves rust resistance and hot workability, and also improves oxidation resistance, and is preferably contained in an amount of 0.001% or more, if necessary. More preferably, it is 0.003% or more. However, since the excessive content of Y may lead to a decrease in manufacturability, the Y content is preferably 0.50% or less. Further, considering the raw material cost, the Y content is more preferably 0.20% or less. Even more preferably, it is 0.10% or less.

(REM: 0.001% or more, 0.50% or less)
REM (Rare earth element) is an element that improves the cleanliness of steel, improves rust resistance and hot workability, and also improves oxidation resistance, and is 0.001% as needed. It is preferable to contain the above. More preferably, it is 0.003% or more. However, since the excessive content of REM may lead to a decrease in manufacturability, the REM content is preferably 0.50% or less. Further, considering the raw material cost, the REM content is more preferably 0.20% or less. Even more preferably, it is 0.10% or less. REM is a general term for scandium (Sc) and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). As the REM, one of the above elements may be contained alone, or two or more of them may be contained. When two or more of the above elements are contained as REM, the REM content is the total content of those elements.

(Zr: 0.01% or more, 0.50% or less)
Zr is an element that improves corrosion resistance, intergranular corrosion resistance, high temperature strength, and oxidation resistance, and is preferably contained in an amount of 0.01% or more, if necessary. More preferably, it is 0.03% or more. However, since the excessive content of Zr may lead to a decrease in manufacturability, the Zr content is preferably 0.50% or less. Further, considering the raw material cost, the Zr content is more preferably 0.30% or less. Even more preferably, it is 0.20% or less.

(Hf: 0.001% or more, 1.0% or less)
Hf is an element that improves corrosion resistance, intergranular corrosion resistance, high temperature strength, and oxidation resistance, and is preferably contained in an amount of 0.001% or more, if necessary. More preferably, it is 0.003% or more. However, since the excessive content of Hf may lead to a decrease in manufacturability, the Hf content is preferably 1.0% or less. More preferably, it is 0.5% or less.

(Sn: 0.0001% or more, 1.0% or less)
Sn is an element that improves corrosion resistance and high-temperature strength, and is preferably contained in an amount of 0.0001% or more, if necessary. More preferably, it is 0.001% or more, or 0.003% or more. However, since the excessive Sn content may cause a decrease in toughness and manufacturability, the Sn content is preferably 1.0% or less. More preferably, it is 0.5% or less.

(Mg: 0.0001% or more, 0.0100% or less)
Mg may be contained as a deoxidizing element and is also an element for improving moldability, and is preferably contained in an amount of 0.0001% or more as needed. More preferably, it is 0.0003% or more. However, since the excessive content of Mg may cause deterioration of the surface quality, the Mg content is preferably 0.0100% or less. Further, in consideration of corrosion resistance and weldability, the Mg content is more preferably 0.0030% or less. Even more preferably, it is 0.0020% or less.

(Co: 0.01% or more, 1.00% or less)
Co is an element that improves high-temperature strength, and is preferably contained in an amount of 0.01% or more, if necessary. More preferably, it is 0.03% or more. However, since excessive Co content may cause a decrease in toughness, the Co content is preferably 1.00% or less. Further, in consideration of manufacturability, the Co content is more preferably 0.50% or less. Further, in consideration of processability, the Co content is even more preferably 0.30% or less.

(Sb: 0.001% or more, 0.50% or less)
Sb is an element that improves high-temperature strength, and is preferably contained in an amount of 0.001% or more, if necessary. More preferably, it is 0.005% or more. However, since excessive Sb content may lead to a decrease in toughness, the Sb content is more preferably 0.50% or less. Further, considering the weldability, the Sb content is further preferably 0.40% or less.

(Bi: 0.001% or more, 1.0% or less)
Bi is an element that suppresses roping that occurs during cold rolling and improves manufacturability, and is preferably contained in an amount of 0.001% or more, if necessary. More preferably, it is 0.003% or more. However, since the excessive content of Bi may cause a decrease in hot workability, the Bi content is preferably 1.0% or less. More preferably, it is 0.5% or less.

(Ta: 0.001% or more, 1.0% or less)
Ta is an element that improves high-temperature strength, and is preferably contained in an amount of 0.001% or more, if necessary. More preferably, it is 0.003% or more. However, since excessive Ta content may cause a decrease in toughness, the Ta content is preferably 1.0% or less. Further, in consideration of manufacturability, the Ta content is more preferably 0.5% or less.

(Ga: 0.0001% or more, 0.50% or less)
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance, and is preferably contained in an amount of 0.0001% or more, if necessary. More preferably, it is 0.0003% or more. However, since the excessive content of Ga may cause a decrease in manufacturability, the Ga content is preferably 0.50% or less. More preferably, it is 0.30% or less.

Next, a method for manufacturing an austenitic stainless steel sheet according to the present invention will be described. The austenitic stainless steel sheet according to the present embodiment may be manufactured by any method, and for example, it can be manufactured by the following manufacturing method.

As for the method for producing an austenitic stainless steel sheet of the present invention, a general process for producing an austenitic stainless steel can be adopted. Generally, molten steel is made in a converter or an electric furnace, refined in an AOD furnace, a VOD furnace, etc. to make steel pieces by a continuous casting method or an ingot method, and then hot-rolled-annealing of hot-rolled plates-pickling-cooling. Manufactured through the steps of inter-rolling-finish annealing-pickling. If necessary, annealing of the hot-rolled plate may be omitted, or cold rolling-finish annealing-pickling may be repeated.

The conditions of each of these steps may be general conditions, for example, a hot-rolled plate annealing temperature of 1000 to 1300 ° C., a hot-rolled plate annealing temperature of 900 to 1200 ° C., a cold-rolled plate annealing temperature of 800 to 1200 ° C., and the like. However, the present invention is not characterized by manufacturing conditions, and the manufacturing conditions are not limited. Therefore, the hot-rolled conditions, the hot-rolled plate thickness, the presence or absence of hot-rolled plate annealing, the cold-rolled conditions, the hot-rolled and cold-rolled plate annealing temperatures, the atmosphere, and the like can be appropriately selected.

In addition, the treatment before finishing pickling may be a general treatment, for example, a mechanical treatment such as shot blasting or a grinding brush, or a chemical treatment such as a melt salt treatment or a neutral salt electrolysis treatment. can. Further, temper rolling or tension leveler may be applied after cold rolling and annealing. Further, the product plate thickness may be selected according to the required member thickness. Further, this steel plate may be used as a welded pipe by a normal method for manufacturing a stainless steel pipe for an exhaust system member such as electric resistance welding, TIG welding, and laser welding.

However, in order to improve the heat resistance and corrosion resistance after long-term aging in the σ phase precipitation temperature range, it is necessary to further reduce Ni segregation, and for that purpose, the thickness A (mm) of the slab at the time of casting, the final Make sure that the product thickness B (mm), final annealing temperature C (° C.) and final annealing time D (seconds) satisfy the following formula (v).

50 ≦ (C × D) / (A × B) ≦ 700 ・ ・ ・ Equation (v)

For Ni segregation, the smaller the thickness A (mm) of the slab at the time of casting, the smaller the thickness B (mm) of the final product, and the higher the final annealing temperature C (° C.), the longer the final annealing time D (seconds). The longer it is, the more it decreases. When the value of the middle side of the formula (v) is less than 50, the Ni segregation may become large. Further, when the value of the middle side of the formula (v) exceeds 700, it becomes overannealed, and the corrosion resistance may decrease due to the consumption of Cr due to oxidation during annealing and the promotion of precipitation of carbonitride. Therefore, the equation (v) is satisfied. More preferably, the value on the middle side of the formula (v) is 60 or more and 600 or less.

According to the austenitic stainless steel sheet according to the present embodiment, it is possible to provide an austenitic stainless steel sheet having excellent heat resistance and corrosion resistance after long-term aging in the σ-phase precipitation temperature range. Further, according to the method for producing an austenitic stainless steel sheet according to the present embodiment, the Ni segregation can be further reduced, and the austenitic stainless steel sheet is excellent in heat resistance and corrosion resistance after long-term aging in the σ phase precipitation temperature range. Can be provided.

Further, the automobile exhaust system parts according to the present embodiment can be exemplified by automobile turbocharger parts, exhaust manifolds, and converters, and the austenitic stainless steel plate of the present embodiment can be applied to the materials of these parts. As turbocharger parts for automobiles, it can be applied to turbocharger parts mounted on gasoline-powered vehicles and diesel-powered vehicles.

As the turbocharger component, for example, the austenitic stainless steel plate of the present embodiment can be applied to the material of the housing constituting the exterior of the turbocharger.

In addition, the austenitic stainless steel plate of this embodiment is used as the material for the back plate, oil deflector, compressor wheel, nozzle mount, nozzle plate, nozzle vane, drive ring, drive lever, etc., which are precision parts inside the nozzle vane type turbocharger. Applicable. However, the applications of the austenitic stainless steel sheet of the present invention are not limited to these applications.

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

Test materials having the component compositions shown in Tables 1 and 2 (Examples A to S of the present invention, Comparative Examples a to q) were melted in a vacuum melting furnace and cast into a 150 kg ingot. This ingot was hot-rolled to obtain a 5.0 mm thick hot-rolled steel sheet. Then, the hot-rolled steel sheet was annealed, pickled, cold-rolled to a thickness of 2 mm, and finally annealed and pickled at 1000 to 1200 ° C., which became a recrystallized structure, and used as a product plate.

Various manufacturing conditions were carried out within the scope of the present invention. That is, the thickness A (mm) of the ingot, the thickness B (2 mm) of the final product, the final annealing temperature C (° C.), and the final annealing time D (seconds) satisfy the following formula (v). However, the steels l to q in Tables 2 and 4 were manufactured under conditions deviating from the following formula (v).

50 ≦ (C × D) / (A × B) ≦ 700… (v)

Further, the product plate was held at 900 ° C., which is the σ precipitation temperature range, for 400 hours, and this was used as an aging material.

For the obtained product plate, the average content of Ni in the Ni negative segregation part and the average content of each element in the positive segregation part of each element of Cr, Mo, Si are analyzed by EPMA on the plate cross section as described above. It was calculated by doing.

For the evaluation of creep characteristics, an aging material having a plate thickness of 2 mm was used, and tests were conducted under two conditions: a load stress of 20 MPa at 950 ° C and a load stress of 10 MPa at 1000 ° C. Those having a breaking time of 100 hours or more under the condition of 950 ° C. and a load stress of 20 MPa were evaluated as "0 (good)", and those having a breaking time of less than 100 hours were evaluated as "x (defective)". Further, those having a breaking time of 100 hours or more under the condition of 100 ° C. and a load stress of 10 MPa were designated as “⊚ (further better)”.

For the evaluation of corrosion resistance, the pitting potential of product plates of various thicknesses and aging materials was measured. The pitting potential is determined by measuring the pitting corrosion potential V'C100 in a 1 mol / L sodium chloride aqueous solution at 30 ° C. under Ar degassing according to the method for measuring the pitting potential of stainless steel specified by JIS G 0557. evaluated. As the surface treatment of the test material, polishing, passivation treatment, and last-minute polishing were all carried out. If the pitting potential of the aging material is 0.7 times or more the pitting potential of the product plate, it is called "○ (good)", and if it is less than 0.7 times, it is called "x (bad)". did. Further, the aging material having a pitting potential of 0.60 V or more was designated as “◎ (further better)”.

Tables 3 and 4 show [* Ni], [** Cr], [** Mo], [** Si] of Examples A to S of the present invention and Comparative Examples a to q, and the values on the left side of the formula (i). [* Ni] / [Ni], [** Cr] / [Cr], [** Mo] / [Mo], [** Si] / [Si], and the evaluation results of creep characteristics and corrosion resistance are shown. However, [element symbol] means the content (mass%) of the element in steel, and [* element symbol] means the average content (mass%) of the element in the negative segregation part of the element. , [** Element symbol] means the average content (mass%) of the element in the positive segregation part of the element.

As is clear from Tables 1 to 4, the examples of the present invention, which are the component compositions specified in the present invention and satisfy the formulas (i) and (ii), are excellent in creep characteristics and corrosion resistance as compared with the comparative examples. You can see that. The example of the present invention also satisfies the conditions 1 to 3.

Further, as described above, there are preferable or more preferable ranges of C, N, Si, Mn, Cr, Ni and Mo for further improving creep characteristics and corrosion resistance. It can be seen that the examples of the present invention within these ranges have further improved evaluation of creep characteristics and corrosion resistance.

In addition, steel sheets were produced under various production conditions for the material steels B, D, and E having a composition within the scope of the present invention. Table 5 shows the material steel, the thickness A (mm) of the slab, the thickness B (mm) of the product plate, the final annealing temperature C (° C), the final annealing time D (seconds), and the value on the middle side of the formula (v). The average content of Ni in the Ni negative segregation part, the value on the left side of the formula (i), the value on the middle side of the formula (ii), and the evaluation results of creep characteristics and corrosion resistance are shown. Regarding the evaluation of creep characteristics, only those having a thickness B (mm) of 2.0 mm of the product version were evaluated.

As is clear from Table 5, by satisfying the formula (v), the negative segregation of Ni is reduced, the formulas (i) and (ii) are satisfied, and the creep characteristics and the corrosion resistance are improved.

No. in Table 5 In No. 5, since the final annealing was over-annealing, the value of the formula (v) became more than 700, and the corrosion resistance was clearly lowered due to the consumption of Cr due to oxidation during annealing and the promotion of precipitation of carbonitride, and as a product. It wasn't available. Therefore, the average content of Ni in the Ni segregation section and the formulas (i) and (ii) were not measured or calculated.
No. In 2, 3, 6, 7, 9, and 10, the value of the formula (v) was less than 50, one or both of the formulas (i) and (ii) were not satisfied, and the corrosion resistance was lowered.

As is clear from these, it can be seen that the steel sheet having the individual component composition specified in the present invention and satisfying the formulas (i) to (ii) is excellent in creep characteristics and corrosion resistance. Further, it can be seen that steels whose individual composition satisfies various preferred or more preferred ranges further improve creep properties and corrosion resistance.

According to the present invention, it is possible to provide an austenitic stainless steel sheet having excellent creep characteristics and corrosion resistance for applications such as exhaust members for transportation such as automobiles and motorcycles, and heat-resistant parts such as plant members. Among them, specific examples of heat-resistant parts for automobiles include exhaust manifolds, converters, front pipes, flexible pipes, etc. as exhaust system parts, housings that make up the exterior as turbocharger parts, and precision parts inside nozzle vane type turbochargers. For example, there are a back plate, an oil deflector, a compressor wheel, a nozzle mount, a nozzle plate, a nozzle vane, a drive ring, a drive lever) and the like. By applying these, it is possible to contribute to the reduction of environmental load by purifying the exhaust gas of automobiles and improving fuel efficiency.

Claims (6)

By mass%,
C: 0.001% or more, 0.300% or less,
N: 0.001% or more, 0.45% or less,
Si: 0.10% or more, 5.00% or less,
Mn: 0.10% or more and 12.00% or less,
P: 0.060% or less,
S: 0.0200% or less,
Cr: 15.0% or more, 35.0% or less,
Ni: 2.0% or more, 35.0% or less,
Cu: 0.001% or more, 5.00% or less,
Mo: 0.001% or more, 5.00% or less,
Al: 0.001% or more, 1.000% or less,
V: 0.01% or more, 1.00% or less,
B: 0.0001% or more, 0.0100% or less,
Ca: 0.0001% or more, 0.0200% or less,
O: 0.0150% or less,
An austenitic stainless steel sheet containing Fe and impurities, and satisfying the following formulas (i) and (ii).
5 [Cr] + 6 [Si] + 9 [Mo] -2 [Ni] -30 [C] -70 [N] -100 [* Ni] / [Ni] ≤ 0 ... Equation (i)
0.80≤ [* Ni] / [Ni] <1.00 ... Equation (ii)
However, the [element symbol] in the formula (i) and the formula (ii) means the content (mass%) of the element in the steel, and [* Ni] is the average content of Ni in the Ni negative segregation part. It means (mass%). By mass%,
C: 0.001% or more, less than 0.080%,
N: Over 0.20%, 0.39% or less,
Si: Over 1.00%, 3.00% or less,
Mn: 0.80% or more, 3.00% or less,
P: 0.050% or less,
S: 0.0018% or less,
Cr: 20.0% or more, 35.0% or less,
Ni: Over 8.0%, 25.0% or less,
Cu: 0.001% or more, 3.40% or less,
Mo: 0.001% or more, less than 0.50%,
Al: Over 0.005%, 0.130% or less,
V: 0.01% or more, 1.00% or less,
B: 0.0001% or more, 0.0100% or less,
Ca: 0.0001% or more, 0.0200% or less,
O: 0.0150% or less,
An austenitic stainless steel sheet containing Fe and impurities, and satisfying the following formulas (iii) and (iv).
5 [Cr] + 6 [Si] + 9 [Mo] -2 [Ni] -30 [C] -70 [N] -100 [* Ni] / [Ni] ≤ 0 ... Equation (iii)
0.80≤ [* Ni] / [Ni] <1.00 ... Equation (iv)
However, the [element symbol] in the formula (iii) and the formula (iv) means the content (mass%) of the element in the steel, and [* Ni] is the average content of Ni in the Ni negative segregation part. It means (mass%). Instead of a part of Fe, in mass%, Ti: 0.001% or more, 1.00% or less,
Nb: 0.001% or more, 1.00% or less,
W: 0.01% or more, 3.00% or less,
Y: 0.001% or more, 0.50% or less,
REM: 0.001% or more, 0.50% or less,
Zr: 0.01% or more, 0.50% or less,
Hf: 0.001% or more, 1.0% or less,
Sn: 0.0001% or more, 1.0% or less,
Mg: 0.0001% or more, 0.0100% or less,
Co: 0.01% or more, 1.00% or less,
Sb: 0.001% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0001% or more, 0.50% or less,
The austenitic stainless steel sheet according to claim 1 or 2, wherein the austenitic stainless steel sheet contains one or more of the above. The austenitic stainless steel sheet according to any one of claims 1 to 3, which is used for an automobile exhaust system member. The method for manufacturing an austenite-based stainless steel sheet according to any one of claims 1 to 4, wherein the thickness A (mm) of the slab at the time of casting, the thickness B (mm) of the final product, and the final annealing temperature C ( A method for producing an austenite-based stainless steel sheet, wherein the temperature) and the final annealing time D (seconds) satisfy the following formula (v).
50 ≦ (C × D) / (A × B) ≦ 700 ・ ・ ・ Equation (v) An automobile exhaust system component including an automobile exhaust system member made of the austenitic stainless steel plate according to any one of claims 1 to 4.