JP2011236465A - Austenitic stainless steel, stainless steel product and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide austenitic stainless steel which has oxidation resistance property and high-temperature strength, facilitates manufacturing of raw materials and processing to products by forming and welding, can be used for a large range of fields and exhibits excellent economy.SOLUTION: The austenite stainless steel has a chemical composition including: 0.05-0.20% ofC, less than 1.0% of Si, 2.0% or less of Mn, 0.04% or less of P, 0.01% or less of S, 20.0-30.0% of Cr, 10.0-15.0% of Ni, 0.10-0.42% of N, 0.0010-0.01% of B, 0.01-0.10% of La+Ce, 0.01-0.20% of sol.Al, Fe for balance and impurities. The C concentration at a surface layer in the thickness direction is higher than that at the center in the thickness direction by 0.03% or more, and/or the N concentration at the surface layer in the thickness direction is higher than that at the center in the thickness direction by 0.03% or more.

Description

  The present invention relates to austenitic stainless steel, stainless steel products, and methods for producing them, and more specifically, has excellent heat resistance when used in a high temperature environment, specifically oxidation resistance and high temperature strength, and is easy to use. The present invention relates to an austenitic stainless steel, a stainless steel product, and a manufacturing method thereof.

In recent years, it has been strongly desired to reduce the concentration of harmful gases such as NOx, SOx, and CO _{2 in} various exhaust gases due to global environmental problems. Efficient use of energy is emphasized, and improvements in fuel efficiency are being promoted in automobiles.

  These environmental issues are similarly addressed in many industrial fields including power generation, chemistry, steel, automobiles and home appliances. For this reason, operations at higher temperatures are directed in the fields of power generation, steel, and chemistry, and combustion at higher temperatures and pressures are directed at automobile engines.

  Conventionally, austenitic stainless steel has been frequently used for high temperature applications. For example, 18Cr-8Ni series stainless steel represented by SUS304, 25Cr-20Ni series stainless steel represented by SUS310S, and 20Cr-32N series high Cr-high Ni steel known as Alloy 800 are widely known.

  Further, AISI302B, JISXM15J1, AISI314 steel, and the like are also known as stainless steels that have improved oxidation resistance at high temperatures by increasing the Si content. In addition to these, Patent Documents 1 to 19 disclose materials for high-temperature applications.

  However, although 18Cr-8Ni series stainless steel is a material excellent in workability and economy of raw materials and products, both high-temperature strength and oxidation resistance are insufficient. Moreover, although high Cr-high Ni steel and high Si steel are excellent in oxidation resistance, the high temperature strength is insufficient. Further, the high Ni steel contains a large amount of Ni that is classified as a rare metal, so that the cost increases.

  For this reason, many automobiles have been studied to apply inexpensive ferritic stainless steel to heat-resistant applications because they contain almost no expensive Ni. Since the ferritic stainless steel has a smaller thermal expansion coefficient than the austenitic stainless steel, the exfoliation of the oxide hardly occurs due to repeated heating and cooling, and the oxidation of the material is delayed, so that the oxidation resistance is excellent. Such ferritic stainless steel is disclosed in Patent Documents 20 to 27, for example.

However, since ferritic stainless steel generally has a softening temperature lower than that of austenitic stainless steel, the high temperature strength is inferior.
Therefore, Patent Documents 28 and 29 and Patent Documents 30 to 32 disclose materials having excellent high-temperature strength and oxidation resistance by increasing the contents of C and N, which are effective strengthening elements.

  In particular, in Patent Document 28, C: 0.05 to 0.15% (in this specification, “%” relating to chemical composition means “mass%” unless otherwise specified), Si: 1.0 %: Mn: 2.0% or less, P: 0.04% or less, S: 0.01% or less, Cr: 20-30%, Ni: 10-15%, N: 0.10-0.30 %, B: 0.0010 to 0.01%, La + Ce: 0.01 to 0.10%, Al: 0.01 to 0.20%, the balance substantially consisting of Fe and inevitable impurities, Ni balance value Steel with {=% Ni + 0.5 ×% Mn + 30 × (% C +% N) −1.1 × (% Cr + 1.5 ×% Si) +8.2} in the range of −1.0% to + 3.0% An austenitic stainless steel for high temperature having a composition and excellent weldability is disclosed.

In Patent Document 29, C: 0.100% or less, Si: 1.50 to 4.00%, Mn: 2.00% or less, Cu: 0.05 to 2.00%, P: 0.00. 0040% or less, S: 0.0100% or less, Cr: 15.0 to 30.0%, Ni: 8.0 to 15.0%, N: 0.15 to 0.30%, B: 0.001 ~ 0.010%, total of one or more rare earth elements such as Ca and Y, La, Ce, etc., 0.01-0.10%, Al: 0.01-0.10%, the balance being substantially And Ni balance value {=% Ni + 0.5 × (% Mn +% Cu) + 30 × (% Cr +% N) −1.1 × (% Cr + 1.5 ×% Si) +8.2} is −1 An invention relating to high temperature austenitic stainless steel with excellent weldability in the range of 0.000% to + 2.00% is disclosed Patent The austenitic stainless steel disclosed in pp. 28 and 29 suppresses the growth and exfoliation of oxides containing Cr by adding REM (Rare Earth Metal), specifically, La and Ce, thereby improving the oxidation resistance. Is greatly improved.

  However, when a large amount of C and N is contained, a large amount and a coarse compound are precipitated, and the hot workability is remarkably deteriorated. When C and N are contained excessively, it is difficult to produce steel plates, steel pipes, steel bars and the like. In addition, REM-added steel has an active REM added to improve oxidation resistance, reacts with a large amount of C and N, and forms coarse compounds during melting, improving oxidation resistance. In addition to exhibiting a sufficient effect, the moldability also deteriorates.

  On the other hand, Patent Documents 33 to 38 disclose that the absorption of C and N in heat treatment is utilized instead of adjusting the components during melting. These all make use of N absorption. Patent Documents 34 to 36 disclose that corrosion resistance, time cracking, and workability are improved by solid solution of N, and Patent Document 37 improves the relationship between strength and ductility by utilizing solid solution and precipitation of N. Furthermore, Patent Document 38 discloses that the electrical resistance at the surface is improved by utilizing precipitates. However, Patent Documents 33 to 38 disclose nothing about the relationship between the absorption of C and N in the heat treatment and the high temperature characteristics.

  Patent Document 39 discloses that ferritic stainless steel improves high-temperature strength to the same level as conventional austenitic stainless steel by dispersing nitrogen compounds. However, since ferritic stainless steel has a small amount of nitrogen solid solution, it does not effectively utilize the solid solution strengthening of nitrogen, and cannot respond to combustion at higher temperatures and pressures required in the future.

Japanese Patent Publication No. 56-17424 Japanese Patent Publication No.57-54543 Japanese Patent Publication No.58-2268 Japanese Examined Patent Publication No. 7-65146 Japanese Patent No. 964343 Japanese Patent No. 997044 Japanese Patent No. 1015087 Japanese Patent No. 1112852 Japanese Patent No. 1163812 Japanese Patent No. 1167015 Japanese Patent No. 1173831 Japanese Patent No. 1209802 Japanese Patent No. 1534248 Japanese Patent No. 1612110 Japanese Patent Laid-Open No. 52-4418 JP-A-60-92454 JP 63-69950 A JP-A-63-69951 JP-A 63-157840 JP-A-6-088168 JP-A-7-011394 JP-A-8-260110 JP-A-11-256287 JP 2004-218013 A JP 2006-037176 A JP 2006-117985 A JP 2008-144199 A Japanese Patent No. 2970432 Japanese Patent No. 3381457 JP-A-8-319541 JP 2004-250783 A JP 2009-084606 A Japanese Patent Publication No.58-54186 Japanese Patent No. 4239718 Japanese Patent No. 4360136 Japanese Patent No. 4378773 Specification JP 2007-70696 A JP 2010-49980 A Japanese Patent Publication No.58-54186

  The object of the present invention was to solve the above-mentioned problems of the prior art and to have excellent heat resistance, specifically oxidation resistance and high temperature strength capable of withstanding the severe use environment at high temperature. Above, austenitic stainless steel that is easy to manufacture and process, and has excellent economy that can be used in a wide range of industrial fields, and steel products made of this austenitic stainless steel, and industrially stable these It is providing the manufacturing method to manufacture.

  In the present invention, C: 0.05 to 0.20%, Si: less than 1.0%, Mn: 2.0% or less, P: 0.04% or less, S: 0.01% or less, Cr: 20 0.0 to 30.0%, Ni: 10.0 to 15.0%, N: 0.10 to 0.42%, B: 0.0010 to 0.01%, La + Ce: 0.01 to 0.10 %, Sol. Al: 0.01 to 0.20%, having a chemical composition consisting of the balance Fe and impurities, the C concentration in the surface layer in the thickness direction being 0.03% or more higher than the C concentration in the center in the thickness direction, And / or an austenitic stainless steel characterized in that the N concentration in the surface layer in the thickness direction is 0.03% or more higher than the N concentration in the center in the thickness direction.

  In the austenitic stainless steel according to the present invention, it is preferable that the amount of compounds having a maximum diameter of 20 μm or more among compounds containing La or Ce is 30 or less per 5 g of mass.

  From another point of view, the present invention is a stainless steel product comprising the austenitic stainless steel according to the present invention described above. These stainless steel products include boiler heater tubes, reheater tubes, reaction furnace tubes for the chemical industry, automobile exhaust manifolds, front center pipes, flexible tubes, gaskets, heaters for cooking equipment, and exchangers. Illustrated.

  From another point of view, the present invention is a method for producing austenitic stainless steel by repeatedly performing a heat treatment on a material having the above-described chemical composition after performing hot working and cold working one or more times. The austenitic stainless steel manufacturing method is characterized in that at least the final heat treatment is performed at over 900 ° C. in an atmosphere containing C and / or N and having a dew point of less than −30 ° C.

In the manufacturing method according to the present invention, it is preferable to perform the surface oxide film removal treatment before the heat treatment in the final step.
From still another aspect, the present invention is a method for producing a stainless steel product, characterized in that processing and / or heat treatment is performed on the austenitic stainless steel produced by the production method according to the present invention described above.

  According to the present invention, it has excellent heat resistance that can withstand the use environment at severe temperatures, specifically oxidation resistance and high temperature strength, and can also be used for material production, product shaping, and processing by welding. Austenitic stainless steel that is easy and has excellent economy that can be used in a wide range of fields can be supplied industrially and stably.

Hereinafter, modes for carrying out the present invention will be described.
First, the reasons for limiting the chemical composition of the austenitic stainless steel according to the present invention will be described. In addition, the composition of the austenitic stainless steel according to the present invention basically conforms to the chemical composition of the high temperature austenitic stainless steel disclosed in Patent Document 28 described above. Is a rise.

[C: 0.05-0.20%]
C stabilizes the austenite structure and is an effective strengthening element. If the C content is less than 0.05%, this effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.20%, coarse massive carbides precipitate at the grain boundaries on the surface of the material during the heat treatment in the final step described later and cooling after heating to the environment of use, and heat resistance and processing In this case, the deterioration of corrosion resistance, which is the greatest feature of stainless steel, is unavoidable. For this reason, C content shall be 0.05% or more and 0.20% or less. The C content is preferably 0.18% or less. In the present invention, the C content is adjusted not only at the time of dissolution but also by the heat treatment in the final step, but the C content before the heat treatment in the final step is as described above. In view of the hot workability of the material, it is preferable to adjust to 0.15% or less during melting.

[Si: less than 1.0%]
Although Si is a deoxidizing element at the time of dissolution, it is considered that the absorption of C and N in the heat treatment in the final process described later is suppressed by the increase of the Si content, and the stability of the austenite structure is lowered. Therefore, the Si content is less than 1.0%. Preferably, it is less than 0.4%.

[Mn: 2.0% or less]
Mn is a deoxidizing element like Si, but stabilizes the austenite structure. However, when it contains excessively, oxidation resistance will be degraded. For this reason, Mn content shall be 2.0% or less. The Mn content is preferably 1.0% or less, and more preferably 0.5% or less.

[P: 0.04% or less]
The P content is preferably as small as possible to ensure high temperature strength, and is set to 0.04% or less in consideration of manufacturability and economy.

[S: 0.01% or less]
The S content is preferably as small as possible to ensure high temperature strength, and is 0.01% or less in consideration of manufacturability and economy.

[Cr: 20.0 to 30.0%]
Cr is an effective element that improves oxidation resistance and high-temperature strength. If the Cr content is less than 20.0%, this effect cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 30.0%, the austenite structure becomes unstable, and both heat resistance and workability deteriorate due to precipitation of coarse carbonitride. Therefore, the Cr content is 20.0% or more and 30.0% or less. The Cr content is preferably 21% or more and 25% or less.

[Ni: 10.0 to 15.0%]
Ni is an important element for stabilizing the austenite structure and improving heat resistance. This effect is small when the Ni content is less than 10.0%. On the other hand, Ni is an expensive element, and if the content exceeds 15%, weldability is significantly impaired. Therefore, the Ni content is set to 10.0% or more and 15.0% or less. The Ni content is preferably 10% or more and 13% or less.

[N: 0.10 to 0.42%]
N is an effective strengthening element while stabilizing the austenite structure. If the N content is less than 0.10%, this effect cannot be sufficiently obtained. However, if the N content exceeds 0.42%, a coarse compound may be deposited on the material during heat treatment in the final step described later and cooling after heating to the use environment, and both heat resistance and workability may deteriorate. Increases nature. For this reason, N content shall be 0.10% or more and 0.42% or less. The N content is preferably 0.38% or less. The N content is not only adjusted at the time of dissolution, but is also assumed to be added by the heat treatment in the final step, but the N content before the heat treatment in the final step is lower as described above. In view of the hot workability of the material, it is preferable to adjust to 0.30% or less during melting.

[B: 0.0010 to 0.01%]
B is an element that improves hot workability and is effective for high-temperature strength. In order to obtain this effect, the B content is set to 0.0010% or more. However, when the B content exceeds 0.01%, the hot workability is deteriorated. Therefore, the B content is set to 0.0010% or more and 0.01% or less.

[La + Ce: 0.01 to 0.10%]
La and Ce are both elements that are extremely effective for improving the oxidation resistance. If the total content of La and Ce is less than 0.01%, this effect cannot be sufficiently obtained. However, when the total content of La and Ce exceeds 0.10%, the compound of La and Ce increases, and hot workability and weldability deteriorate. Therefore, the total content of La and Ce is set to 0.01% or more and 0.10% or less. Note that La + Ce is added as a misch metal, and the ratio of both is not particularly limited.

[Sol. Al: 0.01-0.20%]
Since Al is a deoxidizing component for exerting the additive effect of La and Ce, sol. Al content shall be 0.01% or more. However, sol. If the Al content exceeds 0.20%, hot workability deteriorates. Therefore, sol. The Al content is 0.01% or more and 0.20% or less. sol. The Al content is preferably 0.10% or less.

  In addition to the above components, Cu or Mo inevitably contained when scrap is used as a raw material may be contained in an amount of 0.4% or less. In the present invention, Cu and Mo also act as elements for adjusting the austenite structure.

The balance other than the above is Fe and impurities.
Features other than the chemical composition of the austenitic stainless steel according to the present invention will be described.
[The C concentration in the surface layer in the thickness direction is 0.03% or more higher than the C concentration in the center in the thickness direction, and / or the N concentration in the surface layer in the thickness direction is higher than the N concentration in the center in the thickness direction. Is also 0.03% or higher]
In order to further improve the performance of heat-resistant austenitic stainless steel added with REM disclosed in Patent Documents 28 and 29, specifically, La and Ce, disclosed above, As a method for adding C and N, attention was focused on absorption by heat treatment to austenitic stainless steel, not conventional addition during melting.

As described above, heat-resistant austenitic stainless steel absorbs C and N, so that the strength at room temperature is improved, and the corresponding strength at high temperature is also improved.
On top of that, when C and N are absorbed in heat-resistant austenitic stainless steel to which La and Ce are added, the effects listed below greatly exceed expectations, specifically,
(A) workability is improved with high temperature strength, and (b) excellent oxidation resistance is obtained at the same time, and this excellent oxidation resistance is maintained even when heating and cooling are repeated. .

This unexpected effect is
(I) Since the C content and N content during hot working, which are most significantly affected, can be kept low, the final C content and N content are determined in the final step described below rather than the addition during dissolution. Affected by absorption due to heat treatment, increasing high-temperature strength,
(Ii) The formation of a coarse compound by REM and C, N formed during melting is suppressed, and further dispersion and improvement of workability are achieved by fine dispersion, and (iii) open air The amount of C and N dissolved in the direct contact surface and the vicinity thereof increases, and the yield of REM after melting is improved, so that it dissolves in the material, so that it exhibits better oxidation resistance. It is thought.

  That is, solid solution strengthening by C and N and precipitation strengthening by a compound of REM can be effectively utilized, thereby achieving both high temperature strength and workability and oxidation resistance by REM solid solution. Improvement is considered to be achieved most effectively. Therefore, there is a possibility that both excellent surface workability and oxidation resistance can be achieved by increasing the amount of dissolved C and N only on the surface that is in direct contact with the outside air, but in this sense, absorption by heat treatment is a coarse compound. It is considered that the addition at the time of dissolution, which forms a solution, is completely homogenized by a sufficient solution treatment and then strengthened by precipitation, and finally the same.

And the desired effect exceeding the above expectation is
Limiting the difference in the amount of C and N between the surface of the material and the inner center, and the size and number density of the compound containing REM, which are characteristics when C and N are absorbed by the heat treatment in the final process described later By doing so, it is thought that it is expressed stably.

  Thus, in the present invention, “the C concentration in the surface layer in the thickness direction is 0.03% or more higher than the C concentration in the center in the thickness direction, and / or the N concentration in the surface layer in the thickness direction is the thickness. The reason for limiting it to be 0.03% or more higher than the N concentration at the center in the direction is that the material is strengthened by absorption from the surface, and the coarseness at the surface that exerts particularly bad influence among the compounds containing La and Ce. It is because the production | generation of a compound can be suppressed. Preferably, the difference between the surface and the inner center is 0.05% or more.

  Here, “the C concentration in the surface layer in the thickness direction” and “the N concentration in the surface layer in the thickness direction” are measured values of the C and N concentrations by a glow discharge emission spectrometer (GDS) in the surface layer in the thickness direction. Specifically, it is obtained by measurement on the plate surface.

  The “C concentration at the center in the thickness direction” and the “N concentration at the center in the thickness direction” are measured values of C and N concentrations by a glow discharge emission spectrometer (GDS) at the center in the thickness direction. Specifically, it is obtained by measurement after removing the material to the center of the plate thickness by sputtering. The measurement is carried out twice each in the range of several mm in diameter, and the average value is calculated. The amount of reduction in thickness due to sputtering is based on the results of the previous test.

[Inherent amount of compound having maximum diameter of 20 μm or more among compounds containing La or Ce: 30 or less per 5 g of mass]
The reason why the intrinsic amount of a compound having a maximum diameter of 20 μm or more among the compounds containing La or Ce is limited to 30 or less per 5 g of mass is that the absorption at the heat treatment of C and N is relatively low. Melting is possible, and it becomes possible to suppress the formation of coarse La and Ce compounds at the time of melting, and as a result, the distribution of coarse compounds on the surface having particularly bad influences is suppressed, and workability is reduced. This is because the oxidation resistance is remarkably improved. The number is preferably 10 or less, and more preferably 6 or less.

  Since the austenitic stainless steel according to the present invention is constructed as described above, it can withstand the use environment at higher temperatures that are more severe, and can be easily manufactured and processed, and can be used in a wide range of industrial fields. It also has sex.

  By performing appropriate processing and / or heat treatment on the austenitic stainless steel according to the present invention, for example, a boiler heater tube, a reheater tube, a chemical reactor reactor tube, an automobile exhaust manifold, a front center pipe Stainless steel products such as flexible tubes, gaskets, heaters for cooking equipment, and exchangers can be obtained.

Next, the manufacturing method of the austenitic stainless steel which concerns on this invention is demonstrated.
According to the present invention, an austenitic stainless steel is produced by subjecting a material having the above-described chemical composition to appropriate hot working and cold working and then performing heat treatment one or more times.

  At this time, at least the final heat treatment is performed at a temperature exceeding 900 ° C. in an atmosphere containing C and / or N and having a dew point of less than −30 ° C. In addition, if it is after hot processing, you may make it implement heat processing of such a content not only once but multiple times.

  The reason why the atmosphere of the heat treatment in the final step includes C and N is that both elements are effective strengthening elements and elements that stabilize the austenite structure. Therefore, for example, methane or ethane can be used as a supply source of C, and ammonia or the like can be used as a supply source of N as well as nitrogen gas itself.

As described above, since the oxide film remarkably suppresses the absorption of both the C and N elements, it is preferable that the heat treatment in the final process is a reducing non-oxidizing atmosphere.
The reason why the dew point of the atmosphere of the heat treatment in the final step is less than −30 ° C. is to prevent the formation of the oxide film and further promote the reduction of the oxide film. The temperature is preferably lower, but it is preferably set to −40 ° C. or lower in consideration of industrial and economical aspects.

  Furthermore, the reason why the heat treatment temperature exceeds 900 ° C. is that absorption of C and N hardly occurs at temperatures below 900 ° C. The upper limit of the heat treatment temperature is not particularly limited, but is preferably 1200 ° C. or less, and more preferably 1100 ° C. or less.

Furthermore, according to the present invention, it is preferable to perform the surface oxide film removal treatment before the heat treatment in the final step, if necessary.
That is, it is thought that absorption of C and N in the heat treatment in the final process is accelerated and promoted by performing a well-known and commonly used surface oxide film removal treatment such as pickling or shot brass before the heat treatment in the final process. Because it is. Therefore, it is preferable to perform the surface oxide film removal treatment at least before the final heat treatment and, if necessary, before all the heat treatment.

  Table 1 shows the components of the material of the present invention (Material Nos. 1 to 6) and the comparative material (Material Nos. 7 to 16).

  Material No. Nos. 1 to 16 were manufactured by a general process using small ingots with components adjusted, and only the heat treatment conditions for the purpose of absorbing carbon and nitrogen in the final process were adjusted. The specimens were collected from each process and the characteristics were investigated.

Specifically, first, a small ingot is cut to a thickness of 40 mm, hot-rolled to a thickness of about 4 mm at 1200 ° C., and both width ends of the hot-rolled plate are visually observed, The presence or absence of an ear crack was confirmed.
Next, after holding at 1150 ° C. × 15 minutes for atmospheric annealing, descaling by cutting for cold rolling in the next step, and after adjusting the dimensions, cold rolling, atmospheric annealing at 1150 ° C., descaling by pickling The process was repeated.

  After that, it is cold-rolled to a thickness of 0.6 mm as the final process, heat-treated for the purpose of absorbing C and N, and investigated for C and N content distribution, maximum compound diameter, oxidation resistance, high-temperature strength, and formability. did. Some materials were pickled and shot blasted for the purpose of removing the oxide film immediately before the absorption heat treatment in the final step. The investigation method of various characteristics is as follows.

[Hot workability]
Both width end portions of the hot-rolled sheet were visually observed and judged by the presence or absence of ear cracks.
[C, N distribution]
A general chemical analysis was performed on the thin plate after the final heat treatment, and the nitrogen amount and carbon amount after absorption were measured by the average value of the entire plate thickness. Further, the distribution in the plate thickness direction was measured by a glow discharge optical emission spectrometer (GDS). Specifically, the measurement was performed twice after sputtering to the plate surface and the plate thickness center, and the difference in average value between the surface and the material center was calculated.

[Number of compounds]
A 5 g sample was taken from the thin plate after the final heat treatment, and the base metal part was removed by corrosion with a 10% bromine methanol solution. Thereafter, only the residue is extracted through a filter having a predetermined size of pore, the residue is observed using a scanning electron microscope (SEM), and analyzed by an energy dispersive X-ray fluorescence spectrometer (EDX). Among the compounds containing La and Ce, the total number having a maximum diameter of 20 μm or more was measured.

[Oxidation resistance]
The thin plate after the final heat treatment was subjected to 400 cycles (200 hours) of (a) cooling in air after continuous heating at 1000 ° C. for 200 hours, and (b) cooling for 5 minutes after heating at 1000 ° C. for 25 minutes. The change in weight was measured.

[High temperature strength]
A tensile test piece of JIS-B13 was collected from the thin plate after the final heat treatment by cutting, and the tensile strength at 1000 ° C. was measured.

[Formability]
About the thin plate before the final heat treatment, the portion located in the die hole having a diameter of 50 mm was stretched by hydraulic pressure, the hydraulic pressure was set to 0 immediately after the occurrence of cracking, and the depth at that stage was measured. The case where the same depth was 22 mm or more was rated as ◯, and the case where the depth was less than 22 mm was rated as x.

[Comprehensive evaluation]
(A) Hot rolling: No cracking, (b) Weight change due to oxidation resistance: −50 to 50 g, (c) High temperature strength: 60 MPa or more, (d) Formability: OK (◯) A comprehensive evaluation was made with ◎ when all were achieved, ◯ when one was not achieved, △ when two were not achieved, and × when the other was not achieved.

  The test conditions and test results are summarized in Table 2.

As shown in Table 2, the material of the present invention achieves C and N absorption by heat treatment, and the value on at least one surface thereof is higher by 0.03% or more than the inner center. In addition, the number of compounds exceeding 20 μm is small. From these results, the material of the present invention maintains excellent hot workability and has a continuous and repeated oxidation resistance at 1000 ° C. of 50 g / m ² or less (−50 to 50 g / m ² ), and it has high temperature strength exceeding 60 MPa and excellent formability at room temperature.

  Further, the sample No. 1 was pickled and shot blasted immediately before the heat treatment. As for 1D and 1E, the amount of absorption on the surface is clearly increased and both the oxidation resistance and the high temperature strength are superior to those of the sample 1C which is not implemented.

  On the other hand, when the same invention material is used as the comparative material (sample Nos. 4C to 6C, 6D to 6G, 7A, and 7B), in 4C and 5C in which C and N are excessively absorbed, a large number of coarse materials are used. The compound was identified. As a result, the characteristics were extremely inferior.

  Conversely, in sample 6C, C and N are not contained in the atmosphere, samples 6D and 6E have an atmospheric dew point of −30 ° C. or lower, and samples 6F and 6G have a heating temperature of 900 ° C. or lower, so C and N absorption occurs. In addition, the values of C and N on the surface were both almost the same as the inner center (less than 0.03%), inferior in oxidation resistance, and the high-temperature strength remained as low as around 40 MPa.

  Sample 7A does not absorb C and N because it does not contain C and N in the atmosphere, and does not show a difference between at least one of its surfaces and the inner center, and there are many coarse compounds produced during dissolution, Both formability is poor.

  From these results, it is confirmed that C and N show higher performance by increasing the amount of solid solution due to absorption in heat treatment compared to addition by dissolution (invention disclosed by Patent Documents 28 and 29). . In Sample 7B, N exceeds the upper limit of the present invention, and the characteristics are inferior.

  Samples 7A, 7B, 8A, 8B, 9A, and 9B, in which one or both of C and N exceed the upper limit of the present invention by addition during melting, a large number of coarse compounds are confirmed regardless of the presence or absence of absorption, Both heat resistance and workability were low. Furthermore, it can be seen from the results of Samples 11A to 16A that other characteristics cannot be obtained when the range of the present invention is exceeded.

  Furthermore, as shown in the column of comprehensive evaluation in Table 2, all of the materials of the present invention are evaluated as ◎ in the comprehensive evaluation, and can withstand the use environment at high temperatures that are more severe, and it is easy to manufacture and process the material. . On the other hand, the overall evaluation of the comparative materials is that the samples 6C to G in which C or N is not absorbed or insufficient in the heat treatment corresponding to the invention disclosed in Patent Documents 28 and 29 are Δ, Other than that became X.

Claims (6)

  In mass%, C: 0.05 to 0.20%, Si: less than 1.0%, Mn: 2.0% or less, P: 0.04% or less, S: 0.01% or less, Cr: 20 0.0 to 30.0%, Ni: 10.0 to 15.0%, N: 0.10 to 0.42%, B: 0.0010 to 0.01%, La + Ce: 0.01 to 0.10 %, Sol. Al: 0.01 to 0.20%, having a chemical composition consisting of the balance Fe and impurities, the C concentration in the surface layer in the thickness direction being 0.03% or more higher than the C concentration in the center in the thickness direction, And / or an austenitic stainless steel in which the N concentration in the surface layer in the thickness direction is 0.03% or more higher than the N concentration in the center in the thickness direction.   The austenitic stainless steel according to claim 1, wherein the compound having a maximum diameter of 20 μm or more among compounds containing La or Ce has 30 or less per 5 g of mass.   A stainless steel product comprising the austenitic stainless steel according to claim 1 or 2.   A method for producing austenitic stainless steel by repeatedly performing heat treatment one or more times after performing hot working and cold working on a material having the chemical composition according to claim 1, wherein at least the final step A method for producing an austenitic stainless steel, characterized in that the heat treatment is performed at 900 ° C. or higher in an atmosphere containing C and / or N and having a dew point of less than −30 ° C.   The method for producing an austenitic stainless steel according to claim 4, wherein a surface oxide film removal treatment is performed before the heat treatment in the final step.   A method for producing a stainless steel product, comprising processing and / or heat-treating the austenitic stainless steel produced by the production method according to claim 4 or 5.