JPWO2017061487A1 - Austenitic stainless steel sheet and manufacturing method thereof - Google Patents

Abstract

In mass%, C: 0.03-0.15%, Si: 0.20-2.5%, Mn: 0.2-4.5%, P: 0.010-0.030%, S: 0.0001-0.0010%, Cr: 20.0-26.0%, Ni: 10.0-15.0%, Cu: 0.01-2.0%, Mo: 0.01-2. 0%, Co: 0.05-2.50%, Al: 0.01-0.20%, N: 0.1-0.6%, V: 0.02-0.15%, B: 0 .0002-0.0050%, Nb: 0-0.10%, Ti: 0-0.10%, Y: 0-0.10%, Ca: 0-0.010%, Mg: 0-0. 010%, REM: 0 to 0.10%, balance is Fe and impurities, Mn content [Mn] (mass%), S content [S] (mass%) is [Mn] × [ S] ≦ 0.0020, board Is 0.5 mm or less, and when the length of the major axis of the crystal grain is L1 and the length of the minor axis of the crystal grain is L2, the aspect ratio value L1 / L2 ≧ 1.5 is satisfied, and at 600 ° C. An austenitic stainless steel sheet having a surface hardness (HV) of 300 or more after holding for 400 hours. This austenitic stainless steel sheet is excellent in corrosion resistance and heat resistance.

Description

  The present invention relates to an austenitic stainless steel sheet.

  Stainless steel for springs such as SUS301 and SUS403 is used for a gasket (head gasket) between a cylinder head and an engine block of an automobile engine. SUS301 is partly transformed into work-induced martensite and strengthened by cold rolling. SUS403 is mostly strengthened into a martensite phase by quenching and tempering. Since the head gasket is used in an environment of 200 ° C. or lower, the martensite phase exists as a stable phase even in the environment of use, and exhibits a sealing property as a gasket.

  On the other hand, the gasket used between the flanges that connect the exhaust system parts of the automobile is heated to a maximum temperature of 500 to 700 ° C. in order to connect parts that are hotter than the engine block or cylinder head. Since the heat resistance temperature of SUS301 and SUS403 is about 350 ° C. and the strength cannot be maintained in such a high temperature environment, NCF625 specified in JIS G 4902 (corrosion-resistant heat-resistant superalloy plate) is used as a gasket for exhaust system parts. NCF718, SUH660, SUH310 etc. which are prescribed | regulated to JISG4312 (heat-resistant steel plate and steel strip) are used.

  NCF625 and NCF718 are precipitation strengthened alloys made of intermetallic compounds, and contain 50% or more of Ni. SUH660 is a precipitation strengthened steel containing 24 to 27% Ni, and SUH310 is an austenitic heat resistant steel containing 19 to 22% Ni. Both are expensive because they contain a large amount of Ni. Therefore, a steel type containing N, which is the same austenite stabilizing element, and having reduced Ni has been developed and used.

  However, in recent years, demands for lower fuel consumption of engines and exhaust gas regulations have increased, and exhaust system parts such as EGR, turbocharger, DPF, and exhaust heat recovery device have been installed even in small cars. The downsizing of the engine is progressing to improve the fuel efficiency, and the miniaturization and the high output are aimed at. Therefore, the exhaust gas temperature becomes high, and the residual stress more than conventional is applied to the gasket.

  In addition, because some exhaust system parts are miniaturized and a water cooling mechanism is added to increase cooling efficiency, recently developed Ni-saving heat-resistant materials can maintain the strength while being heated to high temperatures, but the engine There has been a case where stress corrosion cracking (hereinafter referred to as SCC) peculiar to austenitic stainless steel appears due to exhaust gas that has condensed after the stoppage.

  In Patent Document 1, the solid solubility limit of N is increased by increasing Mn to 1.0 to 10.0%, and the high temperature strength is improved by solid solution strengthening of N by increasing N to 0.35 to 0.8%. An enhanced steel sheet is disclosed.

  In Patent Document 2, it is used as a metal gasket that maintains excellent sag resistance even when exposed to a high temperature atmosphere by increasing C-Si-N and adjusting C + 2N + 0.12Si + 1.4Nb to 0.45% or more. An austenitic stainless steel is disclosed. C and N are elements for strengthening the solid solution of the matrix, and Si and Nb are elements that suppress the movement of dislocations in a high temperature atmosphere and increase the high temperature strength.

  In patent document 3, in order to ensure the shape flatness required for a gasket in a correction process, the usage-amount of N is reduced as a solid solution strengthening element, N is made into 0.05% or less, and Si is 2 instead. A heat-resistant austenitic stainless steel for metal gaskets of more than 0.0% and 5.0% or less is disclosed.

  In Patent Document 4, as a stainless steel having a stable austenite phase, suppressing recovery / recrystallization, and capable of causing age hardening, 7 to 25% Ni, 16 to 30% Cr, 0.1 to 0.4 Cold rolled austenitic stainless steel containing% N and having a spring limit value of 220 MPa or more is disclosed.

Japanese Patent Laid-Open No. 9-279315 JP 2003-82441 A JP 2011-252208 A JP 2012-2111348 A

However, the above prior art has the following problems.
The invention of Patent Document 1 is a material excellent in strength, high-temperature strength, sag resistance, and high-temperature oxidation property, but has not been examined for corrosion resistance and cannot provide sufficient characteristics for corrosion resistance.

  The steel type described in Patent Document 2 has not been studied except for sagability and hardness at high temperatures, and sufficient corrosion resistance cannot be obtained.

  Even in the invention of Patent Document 3, it seems that the workability and sag resistance at room temperature are improved, but the corrosion resistance has not been studied.

  Also in the invention of Patent Document 4, only the spring limit value and hardness at the time of high temperature use are studied, and the corrosion resistance is not sufficiently studied.

  Based on the above, the present invention has been well studied in terms of heat resistance, which is suitable as a material for metal gaskets used when connecting exhaust gas flow path components such as automobile engines, and conventional metal gasket materials. An object of the present invention is to supply a low-cost austenitic stainless steel sheet that is compatible with corrosion resistance that has not been achieved.

  The present inventors have intensively studied to solve the above problems. First, corrosion that occurs in heat-resistant gaskets is likely to occur in parts in which exhaust system parts are partially cooled by a cooling device, occurs when residual stress occurs due to thermal cycles due to heating and cooling, and corrosion. From the form, it was presumed to be mainly SCC.

  In order to reduce SCC sensitivity, it is effective to use ferritic stainless steel, but sufficient high-temperature strength has not been obtained. In order to improve SCC resistance in austenitic stainless steel, elements such as Si and Mo are effective, but excessive inclusion of Si and Mo may impair fatigue life due to the formation of sigma phase. Was considered inappropriate. In general, it is also effective to increase Cr or N as the corrosion resistance of stainless steel is expressed as Cr + 3Mo + 16N as the pitting corrosion index, but Cr has a problem due to the formation of sigma phase like Si and Mo. , N excessive content increases the strength at normal temperature and impairs manufacturability, and it has been found that there is a problem in shape freezing property in press molding into a gasket shape.

  Therefore, as a result of individually examining the influence of other elements in the SCC test simulating the actual environment, the following knowledge was obtained as a method for improving SCC resistance without impairing heat resistance, workability, and press formability. Obtained.

  1) In order to improve the SCC resistance, it is effective to reduce inclusions (MnS) that are the starting point of SCC generation. Therefore, it is conceivable to reduce Mn and S, but Mn is also an element that increases the solid solution amount of N. From the viewpoint of high-temperature strength, N is preferably contained in a certain amount within a range that does not cause problems of manufacturability such as bubble defects during solidification and ear cracks during rolling. Therefore, it is essential not to reduce Mn but to reduce S to 10 ppm or less (0.0010% or less).

  2) Inclusion of a small amount of Co is effective for corrosion resistance and high temperature strength without promoting precipitation of the sigma phase.

  The present invention has been completed based on such findings, and the gist thereof is as follows.

  (1) By mass%, C: 0.03 to 0.15%, Si: 0.20 to 2.5%, Mn: 0.2 to 4.5%, P: 0.010 to 0.030% , S: 0.0001 to 0.0010%, Cr: 20.0 to 26.0%, Ni: 10.0 to 15.0%, Cu: 0.01 to 2.0%, Mo: 0.01 -2.0%, Co: 0.05-2.50%, Al: 0.01-0.20%, N: 0.1-0.6%, V: 0.02-0.15%, B: 0.0002 to 0.0050%, Nb: 0 to 0.10%, Ti: 0 to 0.10%, Y: 0 to 0.10%, Ca: 0 to 0.010%, Mg: 0 -0.010%, REM: 0-0.10%, the balance is Fe and impurities, Mn content [Mn] (mass%), S content [S] (mass%) is [Mn ] × [S] ≦ 0.0020 When the plate thickness is 0.5 mm or less, the length of the major axis of the crystal grains is L1, and the length of the minor axis of the crystal grains is L2, the aspect ratio value L1 / L2 ≧ 1.5 is satisfied, An austenitic stainless steel sheet having a surface hardness (HV) of 300 or more after being held at 600 ° C. for 400 hours.

  (2) The austenitic stainless steel sheet according to (1), which contains Nb: 0.01 to 0.10% and / or Ti: 0.01 to 0.10% by mass.

  (3) By mass%, Y: 0.01-0.10%, Ca: 0.001-0.010%, Mg: 0.0002-0.010%, and REM: 0.01-0.10 The austenitic stainless steel sheet according to (1) or (2), which contains one or more selected from%.

  (4) The austenitic stainless steel sheet according to any one of (1) to (3), which has been subjected to temper rolling with a rolling reduction of 20% or more in cold rolling.

  According to the present invention, it is possible to provide an austenitic stainless steel sheet that achieves both high temperature strength and corrosion resistance with a small amount of Ni without using 20% or more of Ni, such as SUH310, SUH660, NCF625, and NCF718. Suitable for heat-resistant metal gasket for exhaust system.

FIG. 1 is a view showing a test piece used in Examples. (A) is a top view, (b) is the AA 'cross-section enlarged view in (a).

  The chemical composition of the stainless steel plate according to the present invention and a preferred method for producing the steel plate will be described. In the following description, “%” representing the content of each element means “mass%” unless otherwise specified.

<C: 0.03-0.15%>
C is effective for increasing the stability of the austenite structure and the high temperature strength. C forms carbides with Cr, suppresses the growth of austenite grains, moderately grows grain boundary oxidation, and improves the scale peeling resistance. In order to obtain this effect, the C content is 0.03% or more. Further, in order to stably suppress grain growth, the C content is preferably 0.10% or more. On the other hand, if C is contained in an amount exceeding 0.15%, the amount of Cr carbide increases and the chromium-deficient layer at the grain boundary increases, and even a high Cr austenitic stainless steel such as this steel is an automobile. Corrosion resistance required for exhaust manifold members, turbo parts, etc. cannot be maintained. Therefore, the C content is 0.15% or less. From the viewpoint of corrosion resistance, the C content is preferably 0.12% or less.

<Si: 0.20% to 2.5%>
Si is effective in oxidation resistance, and is particularly effective in preventing scale peeling during intermittent oxidation. In order to form grain boundary oxidation in an environment exceeding 1000 ° C. and suppress surface scale peeling, 0.20% or more of Si is required. Therefore, the Si content is 0.20% or more. In order to improve oxidation resistance, the Si content is preferably 0.50% or more. Further, Si is a ferrite stabilizing element, and increases the amount of δ ferrite in the solidified structure, which causes a problem of a decrease in hot workability in hot rolling. Therefore, the Si content is 2.5% or less. In addition, since Si promotes the formation of a sigma layer and there is a risk of embrittlement when used at a high temperature for a long time, the Si content is preferably 2.0% or less, more preferably 1.5% or less. preferable.

<Mn: 0.2 to 4.5%>
Mn is an element used as a deoxidizer and expands the austenite single phase region and contributes to the stabilization of the structure. Moreover, Mn contributes to securing high temperature strength by expanding the solid solubility limit of N. The effect appears clearly at 0.2% or more. Therefore, the Mn content is 0.2% or more. Moreover, since there exists an effect which improves hot workability by forming sulfide and reducing the amount of solid solution S in steel, it is preferable that Mn content is 0.5% or more. On the other hand, excessive content reduces corrosion resistance. Therefore, the Mn content is 4.5% or less. Further, from the viewpoint of oxidation resistance, an oxide mainly composed of Cr ₂ O ₃ is desirable, and Mn oxide is not preferable. Therefore, the Mn content is preferably 2.0% or less.

<P: 0.010 to 0.030%>
P is an element contained as an impurity in the main raw materials such as hot metal and ferrochrome as raw materials. It is an element harmful to hot workability. Therefore, the P content is 0.030% or less. An excessive reduction leads to an increase in cost, for example, making it necessary to use a high-purity raw material, so the P content is 0.010% or more. Economically, it is preferable to contain 0.020% or more.

<S: 0.0001 to 0.0010%>
S forms sulfide inclusions and degrades the general corrosion resistance (entire corrosion or pitting corrosion) of the steel material, so that the upper limit of its content is preferably small. Moreover, in order to reduce the inclusion (MnS) which becomes the starting point of SCC generation | occurrence | production, the content shall be 0.0010% or less. Further, since the corrosion resistance becomes better as the S content is smaller, 0.0008% or less is preferable. However, since the desulfurization load increases and the production cost increases for lowering the S content, the content is 0.0001%. The above is preferable. Generally, from the viewpoint of production cost, there is little adjustment of the S content in the range of 0.0001 to 0.0010%. However, in the present invention, in order to reduce inclusions (MnS), the above range is set, and it can be said that the S content is extremely low.

<Cr: 20.0 to 26.0%>
In the present invention, Cr is an essential element for securing oxidation resistance and corrosion resistance. If the Cr content is less than 20.0%, these effects are not exhibited. On the other hand, if it exceeds 26.0%, the austenite single phase region is reduced, and the hot workability during production is impaired. Therefore, the Cr content is 20.0 to 26.0%. In addition, from the viewpoint of oxidation resistance, the Cr content is preferably 24.0% or more. In addition, when the Cr content is increased, embrittlement occurs due to formation of a sigma phase. Therefore, the Cr content is preferably 25.0% or less.

<Ni: 10.0-15.0%>
Ni is an element that stabilizes the austenite phase, and unlike Mn, is an element that is effective in oxidation resistance. These effects are obtained at 10.0% or more. Therefore, the Ni content is 10.0% or more. The Ni content is preferably 11.0% or more because it also has an effect of suppressing the formation of the sigma phase. On the other hand, excessive inclusion increases susceptibility to solidification cracking and also decreases hot workability. Therefore, the Ni content is 15.0% or less. Furthermore, in order to suppress scale peeling in intermittent oxidation, the Ni content is preferably 14.0% or less.

<Cu: 0.01 to 2.0%>
Cu is a relatively inexpensive element that substitutes for Ni as an austenite stabilizing element. Furthermore, it is effective in suppressing the progress of crevice corrosion and pitting corrosion. For that purpose, it is preferable to contain 0.01% or more. However, in the production of austenitic stainless steel, Cu is often mixed from raw materials such as scrap, and is often included in an amount of about 0.2% as an inevitable impurity. However, if it exceeds 2.0%, the hot workability is lowered, so the Cu content is set to 2.0% or less.

<Mo: 0.01 to 2.0%>
Mo, together with Si or Cr, is effective for forming a protective scale on the surface, and the effect is obtained at 0.01%. Therefore, the Mo content is set to 0.01% or more. Moreover, since it is an element effective also in improving corrosion resistance, the Mo content is preferably 0.3% or more. On the other hand, it is also a ferrite stabilizing element, and when the Mo content increases, the Ni content also needs to be increased, so excessive content is not preferable. Moreover, the formation of a sigma phase may be promoted to cause embrittlement. Therefore, the Mo content is set to 2.0% or less. The effect of improving corrosion resistance and oxidation resistance is almost saturated when it exceeds 0.8%. Therefore, the Mo content is preferably 0.8% or less.

<Co: 0.05-2.50%>
Co is extremely effective in improving heat resistance even when contained in a trace amount. Therefore, the Co content is 0.05% or more. However, excessive content impairs hot workability, so the Co content is 2.50% or less. Since it is an element effective for corrosion resistance, the Co content is preferably 0.10% or more. In order to suppress the formation of the sigma phase, the Co content is preferably 2.0% or less.

<Al: 0.01-0.20%>
In addition to being used as a deoxidizing element, Al is an element that improves oxidation resistance. Therefore, the Al content is 0.01% or more. In order to increase the deoxidation efficiency, the Al content is preferably 0.05% or more. On the other hand, excessive inclusion forms nitrides, lowers the amount of solute N, and lowers the high temperature strength. Therefore, the Al content is 0.20% or less. In consideration of weldability, the Al content is preferably 0.15% or less.

<N: 0.1 to 0.6%>
N is one of the very important elements in the present invention. In addition to increasing the high temperature strength in the same manner as C, it is possible to reduce Ni by increasing the austenite stability. Further, since the corrosion resistance lowering effect due to sensitization is smaller than that of C, it can be contained in a larger amount than C. In order to obtain a high temperature strength that can withstand a high temperature environment, the N content is 0.10% or more. Considering the Ni reduction effect, the N content is preferably 0.25% or more. On the other hand, if it is contained in a large amount, a bubble defect occurs during solidification in the steel making process. In addition, the N content should be 0.4% or less because a pressure melting facility is required and the strength at normal temperature is too high, increasing the load during cold rolling and impairing productivity. Preferably, it is 0.3% or less.

<V: 0.02 to 0.15%>
V is generally contained in the range of 0.02 to 0.15% because it is mixed into the alloy raw material of stainless steel as an impurity and is difficult to remove in the refining process. Moreover, since fine carbonitride is formed and it has a grain growth inhibitory effect, it contains intentionally as needed. In order to stably exhibit the effect, the V content is set to 0.02% or more. In order to keep the crystal grain size within a certain range, the V content is preferably 0.08% or more. On the other hand, when it contains excessively, a precipitate may coarsen and, as a result, the toughness after hardening will fall. Therefore, the V content is 0.15% or less. In view of manufacturing cost and manufacturability, the V content is preferably 0.10% or less.

<B: 0.0002 to 0.0050%>
B is an element effective for improving hot workability, and its effect is manifested at 0.0002% or more. Therefore, the B content is set to 0.0002% or more. In order to improve the hot workability in a wider temperature range, the B content is preferably 0.0005% or more. On the other hand, excessive content causes surface flaws due to a decrease in hot workability, so the B content is 0.0050% or less. In consideration of corrosion resistance, the B content is preferably 0.0025% or less.

<Nb: 0 to 0.10%>
Nb is an element that suppresses deterioration of sensitization and corrosion resistance due to precipitation of chromium carbonitride in stainless steel by forming carbonitride, and thus may be contained. However, the formation of large steel-making inclusions can easily cause surface flaws and can also cause a reduction in the fatigue life of the gasket. Therefore, the Nb content is 0.10% or less. Considering the improvement in high temperature strength by securing the solid solution C and N content, the Nb content is preferably 0.05% or less. In order to acquire said effect, it is preferable to make it contain 0.01% or more.

<Ti: 0 to 0.10%>
Ti is an element that suppresses deterioration of sensitization and corrosion resistance due to precipitation of chromium carbonitride in stainless steel by forming carbonitride, and thus Ti may be contained. However, since the formation of large steel-making inclusions causes a decrease in the fatigue life of the gasket, the Ti content is 0.10% or less. Considering the improvement in high-temperature strength by securing the amount of dissolved C and N, the Ti content is preferably 0.05% or less. In order to acquire said effect, it is preferable to make it contain 0.01% or more.

<Y: 0 to 0.10%>
Y is an element for desulfurization as well as an effect of improving oxidation resistance, and therefore may be contained. However, excessive content causes the problem of nozzle clogging during continuous casting, and the fatigue life of the gasket is impaired due to the formation of large oxide inclusions, so the Y content is preferably 0.10% or less. In order to acquire the said effect, it is preferable that Y content is 0.01% or more.

<Ca: 0 to 0.010%>
Ca is used as a desulfurization element, and has the effect of reducing S in steel and improving hot workability. Therefore, Ca may be contained. Generally, it is added as CaO in the slag at the time of melting and refining, and a part of this is dissolved as Ca in the steel. _Moreover, also included in steel as a composite oxide such as _{CaO-SiO 2 -Al 2 O 3} -MgO. On the other hand, when it is contained in a large amount, a relatively coarse water-soluble inclusion CaS is precipitated, and the corrosion resistance is lowered. Therefore, the Ca content is preferably 0.010% or less. In order to obtain the effect of improving hot workability, the Ca content is preferably 0.001% or more.

<Mg: 0 to 0.010%>
Similar to Ca, Mg is contained as a desulfurization element. In general, the equilibrium amount is solid-solved from slag into molten steel, and may be contained as MgO in the composite oxide. In addition, MgO in the refractory may be dissolved into the molten steel. On the other hand, excessive inclusion causes coarse precipitation of the water-soluble inclusion MgS and lowers the corrosion resistance. Therefore, the Mg content is preferably 0.010% or less. The Mg content is preferably 0.0002% or more.

<REM: 0 to 0.10%>
Since REM is a desulfurization element as well as an effect of improving oxidation resistance, it may be contained. However, excessive inclusion causes a problem of nozzle clogging during continuous casting, and also deteriorates the fatigue life of the gasket due to the formation of large oxide inclusions. Therefore, the REM content is preferably 0.10% or less. In order to obtain the above effects, the REM content is preferably 0.01% or more.

  REM is a general term for Sc and lanthanoid, and the content of REM means the total amount of the above elements, and usually Y is also included in REM, but the description is divided in the present invention.

  In the steel sheet of the present invention, the balance is Fe and impurities. Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something.

<[Mn] × [S] ≦ 0.0020>
Mn and S form sulfide MnS, and lower the corrosion resistance in the environment where the heat resistant gasket is used. In particular, SCC is a problem, and in order to suppress the occurrence of cracking, the product of Mn content [Mn] (mass%), S content [S] (mass%), [Mn] × [S] It is necessary to satisfy ≦ 0.0020. By reducing MnS that is the starting point of corrosion, it becomes possible to significantly suppress the occurrence of SCC. Lowering the sulfur content increases the desulfurization load, and lowering the Mn content requires more Ni to stabilize the austenite and increases the manufacturing cost. Therefore, the lower limit is preferably set to 0.0001. Considering workability and the like, it is preferable to satisfy [Mn] × [S] ≦ 0.0015.

<Aspect ratio of crystal grains>
In the present invention, no heat treatment is performed after cold working. For this reason, the crystal grains at the end have a rolled structure. In the present invention, the length of the major axis of the crystal grain is L1, the length of the minor axis of the crystal grain is L2, and the aspect ratio L1 / L2 is defined, and the aspect ratio is obtained in order to obtain sufficient surface hardness. Ratio: L1 / L2 ≧ 1.5. When the aspect ratio is L1 / L2 <1.5, the surface hardness of the metal gasket is not sufficient, and more specifically, when exposed to a high temperature for a long time, specifically, at 600 ° C. for 400 hours. It is not satisfied that the surface hardness (HV) when exposed is 300 or more. Aspect ratio: L1 / L2 is preferably L1 / L2 ≦ 25, and preferably L1 / L2 ≧ 5. In order to measure the aspect ratio, the metal structure observed and recorded with an optical microscope was subjected to image analysis and measured.

<Surface hardness (HV) after holding at 600 ° C. for 400 hours>
The present invention is an austenitic stainless steel plate used for a metal gasket, and the metal gasket is usually different in its use part, but when used in the vicinity of exhaust system parts, it is exposed for a long time in a high temperature range of 500 to 700 ° C. Is done. For this reason, deformation occurs during use, and so-called “sagging” occurs in which the sealing performance and hardness are reduced. In order to suppress sag during use at 600 ° C., the surface hardness (HV) when exposed at 600 ° C. for 400 hours needs to be 300 or more.
For this reason, in the Example mentioned later, the said use environment is simulated and the surface hardness etc. after holding | maintaining 600 degreeC 400 hours are measured.

  In addition, after adding various heat histories, the present inventors further examined the hardness after being held at 600 ° C. for 400 hours, and it was confirmed that the surface hardness (HV) was 300 or more. . That is, even if a material that is actually used or a material with an unknown previous heat history is used, the surface hardness (HV) after exposure at 600 ° C. for 400 hours is 300 or more, assuming the actual usage environment. Thus, this requirement of the present invention will be satisfied. The surface hardness (Vickers hardness) is measured at a load of 4.903N (HV0.5) at 5 points or more by a method based on JIS Z 2244, and an average value is taken as a representative value.

  Moreover, since it is necessary to evaluate about a sagging also in the point of a deformation | transformation at high temperature, it evaluated by the variation | change_quantity of the bead height after hold | maintaining at 600 degreeC for 400 hours. The bead height is the height of the portion of the cross-sectional shape protruding in an arc shape, and the change in the height of the portion after holding at 600 ° C. for 400 hours was measured.

<SCC resistance>
As described above, the SCC resistance is improved by setting the quantity ratio of Mn and S to [Mn] × [S] ≦ 0.0020. SCC resistance is evaluated in an autoclave test at 150 ° C. for 40 hours in a 0.08% NaCl aqueous solution. The autoclave test is a test performed using a pressure-resistant container in order to obtain a hot aqueous solution corrosive environment. The SCC test method was carried out in accordance with JIS G0576, and the temperature and composition of the liquid were adjusted.

<Manufacturing process>
In the method for producing austenitic stainless steel according to the present invention, the step of producing a steel plate to be subjected to cold temper rolling is not particularly limited. A steel rolled by a known means (for example, an electric furnace) is cast into a slab having a thickness of 150 to 250 mm by a continuous casting machine, and in some cases, after grinding the surface, the steel is heated to 1200 ° C. or higher to be a hot rolling mill. Is hot rolled to obtain a hot rolled steel strip having a thickness of about 3 to 6 mm. The hot-rolled steel strip is annealed at a temperature of about 1100 ° C. and pickled. Subsequently, cold rolling and annealing are repeated to obtain a thin plate having a thickness of 0.5 mm or less. More preferably, the thickness is 0.3 mm or less. The finish annealing may be an annealed pickling finish (2B finish) or a BA finish annealed in a non-oxidizing atmosphere. In addition, finish annealing here means the annealing process before temper rolling.

  On the other hand, the cold temper rolling process after finish annealing is performed by changing the rolling reduction ratio according to the required strength (surface hardness) in order to obtain the strength (surface hardness) required for the spring material for gaskets. Is called. In order to obtain the strength required for the heat-resistant gasket, the temper rolling is desirably a rolling reduction of 20% or more. In order to satisfy the surface hardness (HV) of 300 or more after being held at 600 ° C. for 400 hours, it is preferable to use the component system of the present invention and the temper rolling reduction to be 20% or more. It is more preferable to set the rolling reduction to be at least%. The upper limit of the rolling reduction is preferably 80% or less and more preferably 60% or less in order to impair the productivity due to an increase in the number of temper rolling passes.

  In the present invention, the annealing process is not performed after temper rolling, but the process after temper rolling is not particularly limited as long as it is a process other than annealing. A shape forced or degreasing cleaning process may be applied.

  Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

  First, steel having the composition shown in Table 1 was melted and cast into a 200 mm thick slab. This slab is heated to 1250 ° C., then subjected to rough hot rolling and finish hot rolling to obtain a hot rolled steel plate having a thickness of 4 mm, which is inserted into a heat treatment at 800 ° C. in order to simulate winding in a temperature range of 800 ° C. After holding for an hour, it was air-cooled. Subsequently, hot-rolled sheet annealing was performed at 1100 ° C. for 20 seconds and then water-cooled. Then, it was shot blasted and pickled to remove scale. Cold rolling, annealing and pickling were repeated several times to obtain a cold rolled steel sheet having a thickness of 0.25 to 0.5 mm. The aspect ratio was measured using the method described above.

  As shown in FIG. 1, each stainless steel plate 1 has a circular opening 2 having an inner diameter of 80 mm, and a bead 3 having a width of 2.5 mm, a height of 0.25 mm, and a protrusion 2R is press-formed around the opening 2; A test piece 10 simulating a metal gasket was produced. After holding the test piece 10 at 600 ° C. for 400 hours, the surface hardness was measured. The surface hardness was measured at 5 points or more with a load of 4.903N (HV0.5) by a method based on JIS Z 2244, and an average value was taken as a representative value.

  Further, the change in bead height was measured and evaluated as a gasket flange. A bead height change of 30% or less was accepted. Furthermore, SCC was evaluated in an autoclave test at 150 ° C. for 40 hours in a 0.08% NaCl aqueous solution. As a comparative example, the same evaluation was performed on a sample having a composition outside the present invention. The SCC test method was carried out in accordance with JIS G0576, and the temperature and composition of the liquid were adjusted.

  The results are shown in Table 2. The rolling reduction of the last cold rolling (temper rolling) is shown as “temper rolling reduction”.

  Test No. which is an example of the present invention. 1 to 25 satisfy the definition of the composition of the present invention, and [Mn] × [S] ≦ 0.0020, aspect ratio L1 / L2 ≧ 1.5, 600 ° C., surface hardness (HV) after holding for 400 hours. It was 300 or more, the bead height change satisfied the target value (30% or less), and SCC and surface flaws did not occur.

  On the other hand, test No. which is a comparative example. Nos. 26 to 47 do not satisfy the definition of the composition of the present invention, the surface hardness after holding at 600 ° C. for 400 hours is less than 300, SCC generation, bead height change exceeds 30%, or surface flaws occur. did.

  Moreover, test No. which is a comparative example. No. 48 did not satisfy the definition of the composition of the present invention, had a low aspect ratio, and a low surface hardness (HV) after holding at 600 ° C. for 400 hours. In addition, the bead height change exceeded 30%.

  Test No. which is a comparative example. 49 is within the range defined by the present invention for each element, but does not satisfy [Mn] × [S] ≦ 0.0020, has a low aspect ratio, and has a surface hardness after holding at 600 ° C. for 400 hours ( HV) was a low value. In addition, SCC occurred and the bead height change exceeded 30%.

  Test No. which is a comparative example. 50 is within the range of the present invention in each element, but [Mn] × [S] ≦ 0.0020 was not satisfied and SCC was generated.

  Test No. which is a comparative example. No. 51 satisfies the definition of the composition of the present invention, but has a low aspect ratio, a low surface hardness (HV) after holding at 600 ° C. for 400 hours, and a bead height change of more than 30%. became.

1 Stainless steel plate 2 Opening 3 Bead 10 Test piece

The present invention relates to an austenitic stainless steel sheet and a method for producing the same .

(4) The method for producing an austenitic stainless steel sheet according to any one of (1) to (3), wherein temper rolling with a rolling reduction of 20% or more is performed in cold rolling.

Claims (4)

% By mass
C: 0.03-0.15%,
Si: 0.20 to 2.5%,
Mn: 0.2 to 4.5%
P: 0.010-0.030%,
S: 0.0001 to 0.0010%,
Cr: 20.0-26.0%,
Ni: 10.0-15.0%,
Cu: 0.01 to 2.0%,
Mo: 0.01 to 2.0%,
Co: 0.05-2.50%
Al: 0.01-0.20%,
N: 0.1-0.6%
V: 0.02-0.15%,
B: 0.0002 to 0.0050%,
Nb: 0 to 0.10%,
Ti: 0 to 0.10%,
Y: 0 to 0.10%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
REM: 0 to 0.10%,
The balance is Fe and impurities,
Mn content [Mn] (mass%), S content [S] (mass%)
[Mn] × [S] ≦ 0.0020 is satisfied,
The plate thickness is 0.5 mm or less,
When the length of the major axis of the crystal grain is L1, and the length of the minor axis of the crystal grain is L2,
Satisfies the aspect ratio value L1 / L2 ≧ 1.5,
An austenitic stainless steel sheet having a surface hardness (HV) of 300 or more after being held at 600 ° C. for 400 hours. % By mass
Nb: 0.01-0.10%, and / or
Ti: 0.01-0.10%
The austenitic stainless steel sheet according to claim 1, comprising: % By mass
Y: 0.01-0.10%,
Ca: 0.001 to 0.010%,
Mg: 0.0002 to 0.010%, and REM: 0.01 to 0.10%
The austenitic stainless steel sheet according to claim 1, comprising at least one selected from the group consisting of:   The austenitic stainless steel sheet according to any one of claims 1 to 3, which has been subjected to temper rolling with a rolling reduction of 20% or more in cold rolling.

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