JP6621254B2 - Austenitic stainless steel sheet for exhaust parts with excellent heat resistance and surface smoothness and method for producing the same - Google Patents

Description

  The present invention relates to an austenitic stainless steel sheet as a material for a heat-resistant component that requires surface smoothness in addition to heat resistance, and a method for producing the same.

  Environment-friendly parts for automobile exhaust manifold, front pipe, center pipe, muffler and exhaust gas purification have high heat resistance such as oxidation resistance, high temperature strength, thermal fatigue characteristics, etc. in order to allow high temperature exhaust gas to flow stably. Excellent material is used. Moreover, since it is also a condensed water corrosive environment, it is also required to have excellent corrosion resistance. Stainless steel is often used for these parts from the viewpoints of stricter exhaust gas regulations, improved engine performance, and lighter body weight. In recent years, exhaust gas regulations have been further strengthened, and the exhaust gas temperature flowing through the exhaust manifold directly below the engine has been on the rise due to improvements in fuel efficiency and downsizing. In addition, turbochargers such as turbochargers are often installed, and stainless steel used for exhaust manifolds and turbochargers is required to have further improved heat resistance. Regarding the rise in the exhaust gas temperature, it is expected that the exhaust gas temperature, which was conventionally about 900 ° C., will rise to about 1000 ° C. On the other hand, the internal structure of the turbocharger is complicated, and it is important to increase the supercharging efficiency and to ensure the heat-resistant reliability. The use of heat-resistant austenitic stainless steel is mainly disclosed. Patent Documents 1 and 2 disclose the use of typical heat-resistant austenitic stainless steels such as SUS310S (25% Cr-20% Ni), SUSXM15J1 (19% Cr-13% Ni-3% Si), and Ni-based alloys. Has been. Patent Document 3 discloses steel used for exhaust parts of a nozzle vane turbocharger in a turbocharger. Here, the steel components are adjusted for the purpose of having good oxidation resistance and high-temperature strength, but these alone may not be sufficient as characteristics required for many parts constituting the turbocharger.

  Specifically, surface smoothness is important in addition to oxidation resistance and high temperature strength for parts called back plates and oil deflectors in turbochargers. The turbocharger is divided into three parts: a turbine part that collects exhaust gas and rotates the turbine, a compressor part that collects air and pushes it into the cylinder of the engine, and a center core that holds the rotating shaft. Parts such as a back plate and an oil deflector are located between the turbine part and the compressor part and the center core, and stably rotate the turbine and the compressor wheel while maintaining the sealing performance of each part. When the surface smoothness of these turbo parts is poor, the airtightness is deteriorated, leading to deterioration of supercharging efficiency. In addition, when the high temperature strength is insufficient, excessive thermal deformation (creep) occurs, which hinders the rotation of each wheel. Furthermore, if oxidation resistance is insufficient and scale peeling occurs, each wheel is damaged. Conventionally, these parts are manufactured using cast steel as a raw material, and are used after ensuring smoothness of the surface and thickness accuracy by cutting and grinding. However, there is a drawback that the cost of processing the raw material and parts becomes high. . The same applies to components that variably introduce and discharge exhaust gas called nozzle mounts and nozzle plates.

International Publication No. 2014-24905 JP 2002-332862 A Japanese Patent No. 4937277

  An object of the present invention is to solve the problems of the known technology, and in particular, to provide an austenitic stainless steel sheet that is a material for heat resistant parts that require surface smoothness in addition to heat resistance suitable for turbo parts in automobile exhaust parts. It is to provide.

  In order to solve the above-mentioned problems, the present inventors have conducted detailed studies on the austenitic stainless steel sheet and the manufacturing method thereof from the viewpoint of steel components, surface properties, metal structure, and high temperature characteristics. As a result, in addition to steel components, inclusion distribution, crystal grain size, etc., for materials that require heat resistance and surface smoothness in parts exposed to extremely harsh thermal environments such as turbochargers. It has been found that by controlling the surface roughness, heat and shape reliability can be ensured in parts made of stainless steel plate with excellent heat resistance and surface smoothness. Patent Documents 1 to 3 show that polishing treatment is performed after cutting or grinding of a cast product, or cold rolling annealing, but in the present invention, surface smoothness is ensured by pickling treatment after cold rolling annealing. At the same time, it contributes to omission of cutting and grinding processes or reduction of load.

The gist of the present invention for solving the above problems is as follows.
(1) By mass%, C: 0.005 to 0.20%, Si: 0.65 to 4.0%, Mn: 0.1 to 4.0%, P: 0.01 to 0.05% , S: 0.0001 to 0.010%, Ni: 5 to 25%, Cr: 15 to 30%, N: 0.01 to 0.30%, Al: 0.005 to 1.0%, Cu: Containing 0.1 to 3.0%, the balance is Fe and inevitable impurities, the inclusion cleanliness is 0.025% or less, the crystal grain size number is 6 or more, the surface roughness is centerline average roughness ( An austenitic stainless steel plate for exhaust parts having excellent heat resistance and surface smoothness, characterized by Ra) of 0.06 μm or less.
(2) The said steel plate is further mass%, Mo: 0.01-3.0%, V: 0.05-0.5%, Ti: 0.005-0.3%, Nb: 0. The heat resistance according to claim 1, comprising one or more of 005 to 0.3%, B: 0.0002 to 0.0050%, Ca: 0.0005 to 0.010%. Austenitic stainless steel plate for exhaust parts with excellent properties and surface smoothness.
(3) The steel sheet is further mass%, W: 0.10 to 3.00%, Zr: 0.05 to 0.30%, Sn: 0.01 to 0.50%, Co: 0.00. 03-0.30%, Mg: 0.0002-0.010%, Sb: 0.005-0.50%, REM: 0.001-0.2%, Ga: 0.0002-0.3% The austenitic stainless steel sheet for exhaust parts excellent in heat resistance and surface smoothness according to claim 1 or 2, characterized by containing at least one of the following. (4) When manufacturing the stainless steel sheet according to any one of claims 1 to 3, in the annealing step after cold rolling, after heating to 1000 to 1200 ° C, the cooling rate to 500 ° C is 10 ° C / Heat resistance and surface smoothness characterized by being cooled at a cooling rate of 10 ° C./sec or more to 100 ° C. or less after being immersed in a molten alkali salt held at 400 to 500 ° C. for 5 to 20 sec. A method for producing an austenitic stainless steel sheet for exhaust parts with excellent properties.

  According to the present invention, surface smoothness can be ensured by pickling treatment after cold rolling annealing without subjecting the austenitic stainless steel plate, which is the material of the heat resistant component, to grinding or polishing treatment. Austenitic stainless steel can be provided for automobile exhaust parts (particularly for turbo parts) that require heat resistance and surface smoothness. In addition, it contributes to omission of cutting and grinding processes or reduction of load.

  The reason for limitation of the present invention will be described below. An important property of the austenitic stainless steel sheet used for heat-resistant applications is high-temperature strength, and the present invention aims to have a high-temperature strength with a 0.2% proof stress at 1000 ° C. of 20 MPa or more. On the other hand, especially in the case of turbo parts as described above, the surface texture is also extremely important. As described above, if the surface smoothness of the product is inferior, the airtightness is deteriorated, and the wear characteristics with other parts are also inferior, leading to a decrease in reliability of the component performance at a high temperature. Here, the surface smoothness refers to the material surface and the surface properties after parts processing. As a result of detailed examination in consideration of the high temperature use environment, the material inclusion cleanliness, crystal grain size, surface roughness It was found that the degree was strongly involved.

  First, the inclusion cleanliness will be described. Main inclusions in the austenitic stainless steel sheet include Si-based or Al-based oxides, MnS, and the like. In the present invention, inclusions include carbon nitrides such as Cr carbonitride and σ phase, and intermetallic compounds resulting from the cooling rate in the annealing / pickling process after cold rolling. It has been found that, regardless of the inclusion composition, when the inclusion cleanliness deteriorates, that is, the inclusion ratio increases, creep voids are generated during high-temperature use and thermal deformation is likely to occur. When the thermal deformation increases, the dimensional accuracy of the turbo parts deteriorates and may be damaged by contact with other parts. In addition, it became clear that the cleanliness of inclusions affects not only high-temperature deformation but also surface smoothness, and when the inclusions increase, surface fine irregularities after parts processing tend to occur. The inclusion cleanliness is measured according to JIS G 0555 using, for example, a sample obtained by polishing a cross section of a steel plate (cross section perpendicular to the rolling direction). The cleanliness for maintaining sufficient high temperature deformation and surface smoothness is 0.025% or less. Furthermore, considering oxidation resistance, 0.020% or less is desirable. If the high temperature strength and the inclusion cleanliness are within the range of the present invention, the problem of creep thermal deformation does not occur.

  Next, the crystal grain size will be described. Various heat-resistant parts are used by processing a raw material. However, when rough skin (orange peel) generated after processing occurs remarkably, the sealing surface is uneven, and thus the airtightness is lowered. In the case of using conventional cast steel as a material, the surface is ground, so there was no such problem, but in the case of various press processing using a steel plate as a material, it is considered that minute unevenness becomes a problem. I found out in the process. As for the crystal grain size, the grain size number is measured according to JIS G 0551 using, for example, a sample obtained by polishing a cross section of a steel plate (cross section parallel to the rolling direction). As a result of investigating the correlation between the crystal grain size and the unevenness height after processing, it is possible to obtain an unevenness height after processing that satisfies the accuracy of turbo parts such as a back plate by making the grain size number 6.0 or more. Therefore, the grain size number is set to 6.0 or more. In addition, since the precision punching property deteriorates when the particle size is excessively reduced, the upper limit is desirably 10.5. Furthermore, considering the strength, ductility and manufacturability, 6.5 to 10.0 is desirable.

  In addition to the above, the surface roughness of the material is also important for the surface smoothness. If the surface roughness of the material is rough, the sealing performance between the parts becomes poor, leading to exhaust gas leakage and the like. This is also not a problem because conventional cast steel is processed by cutting parts. However, when steel plate is used as a raw material, it has been found that a minute surface roughness of the raw material is extremely important for ensuring reliability. Regarding the surface roughness measurement, the surface roughness is measured in parallel or perpendicular to the rolling direction with respect to the steel sheet surface in accordance with JIS B 0601. As a result of various investigations on the surface roughness of the material and the contact with other parts, if the center average roughness (Ra) in the direction perpendicular to the rolling direction is 0.06 μm or less, the turbine and the compressor wheel while maintaining the sealability of each part It became clear that the reliability for rotating the motor stably was satisfied. Furthermore, if the roughness which increases by productivity and a process is considered, 0.005-0.05 micrometer is desirable.

Next, the component range of steel will be described.
The following numerical values of content are all numerical values in mass%.
C has a lower limit of 0.005% in order to form an austenite structure and ensure high temperature strength. On the other hand, since the corrosion resistance, particularly the intergranular corrosion resistance of the welded portion is significantly deteriorated by the formation of Cr carbide, the upper limit is made 0.20%. Furthermore, if considering the manufacturing cost and scale peelability, 0.01 to 0.15% is desirable.

  In addition to being added as a deoxidizing element, Si is added in an amount of 0.1% or more in order to improve oxidation resistance and high-temperature strength. On the other hand, when the addition exceeds 4.0%, coarse Si-based oxides are generated, and the cleanliness of the inclusions is remarkably lowered, so the upper limit is made 4.0%. Furthermore, if considering the manufacturing cost, pickling property at the time of manufacturing the steel sheet, and solidification cracking property at the time of welding, 0.4 to 3.3% is desirable.

  In addition to being used as a deoxidizing element, Mn is added in an amount of 0.1% or more to ensure austenite structure formation and scale adhesion. On the other hand, addition of more than 4.0% significantly deteriorates the cleanliness of inclusions and increases the amount of oxidation significantly and lowers the abnormal oxidation limit temperature. Therefore, the upper limit is made 4.0%. Furthermore, if considering the manufacturing cost and pickling property at the time of manufacturing the steel sheet, 0.2 to 2.0% is desirable.

  P is an element that promotes hot workability and solidification cracking at the time of manufacture, and the inclusion cleanliness and oxidation resistance at 1000 ° C deteriorate, so the lower the content, the better. The upper limit is 0.05% and the lower limit is 0.01%. Furthermore, considering the manufacturing cost, 0.02 to 0.04% is desirable.

  S is an element that degrades hot workability during production and deteriorates corrosion resistance. Further, if coarse sulfides are formed, the cleanliness is remarkably deteriorated, so the upper limit is made 0.010%. On the other hand, excessive reduction leads to an increase in refining costs, so the lower limit is made 0.0001%. Furthermore, if considering the manufacturing cost and oxidation resistance, 0.0005 to 0.0050% is desirable.

  Ni is an element forming an austenite structure and an element that ensures corrosion resistance and oxidation resistance. Further, if it is less than 5%, the crystal grains are remarkably coarsened and the high-temperature strength is lowered, so 5% or more is added. On the other hand, excessive addition causes an increase in cost, so the upper limit is made 25%. Furthermore, if considering the manufacturability, high temperature strength and corrosion resistance, 10 to 20% is desirable.

  Cr is an element that improves corrosion resistance, oxidation resistance, and high-temperature strength. When considering the exhaust part environment, 15% or more is necessary from the viewpoint of suppressing abnormal oxidation. Further, if it is less than 15%, not a Cr-rich scale with high protection during annealing, but a Fe-rich scale is generated, and Fe is preferentially dissolved in subsequent descaling, so that the surface roughness of the matrix increases, Airtightness as a turbo component is deteriorated. On the other hand, excessive addition becomes hard and deteriorates moldability, and also leads to an increase in cost, so the upper limit was made 30%. Furthermore, if considering the manufacturing cost, steel plate manufacturability and workability, 17 to 25.5% is desirable.

  N, like C, is an effective element for forming an austenite structure and ensuring high temperature strength. The high temperature strength is known as a solid solution strengthening element, but in the present invention, in addition to the effect of N alone, the high temperature strength due to cluster formation with Cr is also taken into consideration, and 0.01% or more is added. On the other hand, the addition of more than 0.30% remarkably hardens the normal temperature material and deteriorates the cold workability in the steel plate manufacturing stage and deteriorates the formability during product processing, so the upper limit is made 0.30%. Furthermore, 0.02 to 0.25 is desirable from the viewpoint of suppressing pinholes during welding and suppressing intergranular corrosion of welds.

  Al is added as a deoxidizing element to improve the cleanliness of inclusions, and also has an effect of suppressing exfoliation of oxide scale. Since its action is manifested from 0.005%, the lower limit is set to 0.005%. . In addition, since it is a ferrite-forming element, addition of 1.0% or more lowers the stability of the austenite structure and lowers cleanliness and manufacturability, so the upper limit is made 1.0%. Furthermore, if considering refining costs and surface defects, 0.015 to 0.5% is desirable.

  Cu is an element effective for stabilizing the austenite phase, and is added in an amount of 0.1% or more. On the other hand, addition exceeding 3.0% causes abnormal oxidation in an oxidation test at 1000 ° C. and leads to a decrease in cleanliness and deterioration in manufacturability, so the upper limit is made 3.0%. Furthermore, if considering corrosion resistance and manufacturability, 0.3 to 1.0% is desirable.

Next, the selective additive element will be described.
Mo is an element that improves the corrosion resistance and contributes to the improvement of the high-temperature strength. In the present invention, in order to utilize precipitation strengthening by Mo carbide in addition to solid solution strengthening, the lower limit is made 0.01% and the upper limit is made 3.0%. Furthermore, considering that Mo is an expensive element, strengthening stability due to the precipitates, and inclusion cleanliness, 0.4 to 1.6% is desirable.

  V is an element that improves corrosion resistance, and is added in an amount of 0.05% or more in order to form V carbide and improve high temperature strength. On the other hand, excessive addition causes an increase in alloy cost and a decrease in abnormal oxidation limit temperature, so the upper limit is made 0.5%. Furthermore, if considering manufacturability and inclusion cleanliness, 0.1 to 0.3% is desirable.

  Ti is an element added to combine with C and N to improve corrosion resistance and intergranular corrosion resistance. Since the C and N fixing action starts from 0.005%, the lower limit was made 0.005%. Further, addition of more than 0.3% tends to cause nozzle clogging at the casting stage, which significantly deteriorates manufacturability and lowers cleanliness due to coarse Ti carbonitride, so the upper limit is 0.3. %. Furthermore, if considering the high temperature strength, the intergranular corrosion property of the weld and the alloy cost, 0.01 to 0.20% is desirable.

  Nb, like Ti, is an element that combines with C and N to improve corrosion resistance and intergranular corrosion resistance, as well as improve high-temperature strength. Since the C and N fixing action starts from 0.005%, the lower limit was made 0.005%. Further, addition of more than 0.3% significantly deteriorates the hot workability in the steel plate manufacturing stage and causes a decrease in cleanliness due to coarse Nb carbonitride, so the upper limit is made 0.3%. Furthermore, if considering the high temperature strength, the intergranular corrosion property of the weld and the alloy cost, 0.01 to 0.20% is desirable.

  B is an element that improves the hot workability in the steel plate manufacturing stage, and is set to 0.0002% or more. However, excessive addition causes a decrease in cleanliness and degradation of intergranular corrosion due to the formation of borocarbides, so the upper limit was made 0.0050%. Furthermore, if considering the refining cost and ductility reduction, 0.0003 to 0.0020% is desirable.

  Ca is added as necessary for desulfurization. Since this effect does not appear at less than 0.0005%, the lower limit is made 0.0005%. Further, if added over 0.010%, a water-soluble inclusion CaS is generated, leading to a decrease in cleanliness and a significant decrease in corrosion resistance, so the upper limit is made 0.010%. Furthermore, 0.0010 to 0.0030% is desirable from the viewpoint of manufacturability and surface quality.

  W contributes to improvement of corrosion resistance and high temperature strength, so 0.10% or more is added as necessary. Addition of more than 3.0% causes toughness deterioration during manufacturing of steel sheets, cost increase, and pickling performance to remarkably deteriorate and the surface roughness of the product increases remarkably, so the upper limit is made 3.0%. Furthermore, if considering refining costs and manufacturability, 0.1 to 2.0% is desirable.

  Zr is combined with C and N to improve the intergranular corrosion resistance and oxidation resistance of the welded portion, so 0.05% or more is added as necessary. However, the upper limit is 0.3%. Furthermore, considering refining costs and manufacturability, 0.05 to 0.1% is desirable.

  Sn contributes to the improvement of corrosion resistance and high-temperature strength, so 0.01% or more is added as necessary. The effect becomes remarkable at 0.03% or more, and becomes more remarkable at 0.05% or more. Since addition of more than 0.50% may cause slab cracking during steel sheet production, the upper limit is made 0.50%. Furthermore, considering refining costs and manufacturability, 0.05 to 0.30% is desirable.

  Co contributes to improving the high-temperature strength, so 0.03% or more is added as necessary. Addition of more than 0.3% leads to toughness deterioration at the time of steel plate production, reduction in cleanliness, and cost increase, so the upper limit is made 0.3%. Furthermore, considering refining costs and manufacturability, 0.03 to 0.1% is desirable.

  Mg may be added as a deoxidizing element, and is an element that contributes to improvement in the cleanliness of inclusions and refinement of the structure by refining and dispersing the oxide of the slab. Since this is expressed from 0.0002% or more, the lower limit was made 0.0002%. However, excessive addition leads to a decrease in cleanliness due to excessive formation of Mg—Al oxide and deterioration of weldability and corrosion resistance, so the upper limit was made 0.010%. Considering the refining cost, 0.0003 to 0.0050% is desirable.

  Sb is an element that segregates at the grain boundary to increase the high temperature strength. In order to obtain the addition effect, the content is made 0.005% or more. However, if it exceeds 0.50%, abnormal oxidation is caused in the oxidation test at 1000 ° C. and Sn segregation occurs and cracks occur during welding, so the upper limit is made 0.50%. Considering the high temperature characteristics, production cost and toughness, 0.03 to 0.30% is desirable. More desirably, it is 0.05 to 0.20%.

REM (rare earth element) is effective in improving the oxidation resistance, and is added at 0.001% or more as necessary. Moreover, since the effect will be saturated even if it adds exceeding 0.2% and the corrosion resistance fall by the granulated material of REM arises, it adds at 0.001-0.2%. Considering the workability and manufacturing cost of the product, it is desirable that the lower limit is 0.002% and the upper limit is 0.10%.
REM (rare earth element) follows the general definition. It is a generic term for two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu). It may be added alone or as a mixture.

  Ga may be added in an amount of 0.3% or less for improving corrosion resistance and suppressing hydrogen embrittlement. However, addition of more than 0.3% produces coarse sulfides and deteriorates the r value. The lower limit is made 0.0002% from the viewpoint of sulfide and hydride formation. Furthermore, 0.0020% or more is more preferable from the viewpoint of manufacturability and cost.

  The other components are not particularly defined in the present invention, but Ta and Hf may be added in an amount of 0.001 to 1.0% for improving the high temperature strength. Moreover, Bi may be contained in an amount of 0.001 to 0.02% as necessary. Note that it is desirable to reduce general harmful elements and impurity elements such as As and Pb as much as possible.

  Next, a manufacturing method will be described. The manufacturing method of the steel plate of this invention consists of each process of steelmaking-hot rolling-annealing and pickling-cold rolling-annealing and pickling. In steelmaking, a method in which steel containing the essential components and components added as necessary is subjected to electric furnace melting or converter melting, followed by secondary refining is preferable. The molten steel is made into a slab according to a known casting method (continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness by continuous rolling. As described above, since surface smoothness is required for the parts to which the present invention is applied, manufacturing conditions that ensure a predetermined surface roughness are set in the cold rolling process. Usually, steel sheets that require surface smoothness are heat-treated in a low-oxygen atmosphere by bright annealing after cold rolling, and are subjected to heat treatment that does not produce oxide scale. Generally, this is a surface finish called a BA product, but the BA product leads to an increase in cost. The present invention has found a technique for ensuring surface smoothness by utilizing a normal continuous annealing / pickling process, not bright annealing. Specifically, the cooling rate after heat treatment is adjusted to suppress minute irregularities on the surface of the base material accompanying the growth of the oxide scale on the surface, the temperature and time are adjusted in the subsequent immersion of molten alkali salt, and the cooling rate after molten alkali salt treatment Reduction of matrix erosion by adjustment is extremely effective in ensuring surface smoothness.

  In the present invention, the grain size number is adjusted in the annealing / pickling process after cold rolling, but when heated to over 1200 ° C., the coarsening of the crystal grains is remarkably promoted, and the grain size number becomes less than 6.0. Become. In addition, since the oxide scale is thickened and the internal oxide layer is developed by heating above 1200 ° C., the subsequent pickling property is inferior, and a lot of minute irregularities are generated on the surface after pickling, and the surface roughness is reduced. You will not meet the regulations. Therefore, the upper limit is set to 1200 ° C. On the other hand, if the heating is less than 1000 ° C., the recrystallized structure cannot be obtained and the ductility becomes poor, so the lower limit is set to 1000 ° C. Furthermore, when pickling property and manufacturing cost are considered, 1050-1150 degreeC is desirable.

  The steel strip heated to the above temperature is cooled and descaled by pickling treatment. In the present invention, the cooling rate up to 500 ° C. is set to 10 ° C./sec or more. When the cooling rate is less than 10 ° C./sec, carbonitride such as Cr carbonitride and σ phase, and intermetallic compounds are precipitated, which leads to deterioration of steel inclusion cleanliness, and deteriorates high temperature characteristics and surface quality. . Furthermore, in view of productivity and pickling properties, 15 to 50 ° C./sec is desirable. Cool to 500 ° C. Here, in the pickling pretreatment, scale modification is performed with a molten alkali salt mainly composed of sodium nitrate. In this invention, the temperature of molten alkali salt shall be 400-500 degreeC, and immersion time is prescribed | regulated to 5-20 sec. When the temperature is lower than 400 ° C., the scale modification is insufficient and the oxide scale remains after pickling. On the other hand, when the temperature exceeds 500 ° C., erosion of the matrix is large due to the molten salt, and it becomes impossible to make the surface roughness Ra after pickling 0.06 μm or less. Therefore, although the temperature of molten alkali salt shall be 400-500 degreeC, when productivity is considered, 400-440 degreeC is desirable. As for the immersion time, if it is less than 5 seconds, the reaction with the oxide scale is insufficient and the dissolution of the Cr oxide becomes poor, the scale residue is likely to occur, and it is necessary to strengthen the descale during the subsequent pickling, The surface roughness becomes rough because of the dissolution of the matrix. On the other hand, if it exceeds 20 seconds, the erosion of the base is large, and it becomes impossible to reduce the surface roughness Ra after pickling to 0.06 μm or less. Therefore, the immersion time in the molten alkali salt is 5 to 20 seconds, but 5 to 15 seconds is desirable in consideration of productivity and sheet passing property.

  After the molten alkali salt treatment, which is the above pickling pretreatment, cooling is performed until the nitric hydrofluoric acid immersion, nitric acid immersion or nitric acid electrolysis treatment for removing the oxide scale on the surface. The cooling rate until it becomes 10 degrees C / sec or more. When the cooling rate is less than 10 ° C./sec, erosion of the base is locally caused by the salt solution remaining on the steel sheet, and surface roughness may increase or surface unevenness may occur. Therefore, the lower limit is set to 10 ° C./sec or more. However, if the speed is excessively high, a shape defect is caused. Therefore, the upper limit is preferably set to 100 ° C./sec.

  As described above, the austenitic stainless steel plate for exhaust parts of the present invention is not brightly annealed and does not grind or cut the surface of the steel plate. Moreover, the steel plate of this invention is obtained as a steel strip after annealing pickling.

  Steels having the composition shown in Tables 1 and 2 are melted and cast into slabs, and after hot rolling and hot-rolled sheet annealing / pickling, cold rolling and final annealing / pickling are performed to obtain a 1.2 mmt product plate. It was. Some steels were subjected to intermediate annealing and pickling after hot-rolled sheet annealing and then cold-rolled twice. Each steel was measured for inclusion cleanliness, crystal grain size, and surface roughness by the methods described above, and a continuous oxidation test at 1000 ° C. in accordance with JISG0567 and a high-temperature tensile test in accordance with JISZ2281 were performed. In the continuous oxidation test, a continuous test was conducted for 200 hours in the atmosphere, and a pass / fail judgment was made based on whether or not abnormal oxidation occurred. In the high-temperature tensile test, 0.2% yield strength at 1000 ° C. was measured, and steel having the yield strength of 20 MPa or more was accepted. In addition, the specimen was processed into a partition part between a turbine and a center core called a back plate, mounted on a turbo, and when high temperature (1000 ° C.) exhaust gas was flowed, the airtightness was examined. At this time, the case where no exhaust gas leakage occurred was regarded as acceptable. As a result of manufacturing under the manufacturing conditions shown in Tables 3 to 5, it is confirmed that the steel of the present invention example is excellent in surface smoothness in addition to oxidation resistance and high temperature strength, and excellent in airtightness as a turbo component. In a comparative example in which one or more of inclusion cleanliness, crystal grain size, and surface roughness are out of the scope of the present invention, the surface smoothness of the back plate becomes poor, causing a problem that high-temperature exhaust gas leaks outside the turbo. Further, even when the high temperature strength and high temperature oxidation characteristics are poor, gas leakage occurs due to creep deformation or thinning due to oxidation.

  Note that other conditions in the manufacturing process may be appropriately selected. For example, what is necessary is just to design slab thickness, hot rolling board thickness, etc. suitably. In cold rolling, roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected. Intermediate annealing may be put in the middle of cold rolling, and batch annealing or continuous annealing may be used. Moreover, neutral salt electrolysis may be used in combination during pickling, and in addition to nitric acid and nitric acid electrolytic pickling, a pickling treatment using sulfuric acid or hydrochloric acid may be performed. Furthermore, the product plate may be lubricated to further improve press molding, and the type of lubricating film may be appropriately selected.

  According to the present invention, it is possible to provide an austenitic stainless steel sheet for an exhaust part that requires surface smoothness in addition to heat resistance, which has been a cast steel part, and a material to which the present invention is applied. In particular, by using it as an exhaust part for automobiles and motorcycle parts, it becomes possible to lead to exhaust gas regulation, weight reduction, and fuel efficiency improvement. Further, it is possible to omit the surface processing required for cast steel, which greatly contributes to cost reduction. Furthermore, the present invention can be applied not only to automobiles and motorcycles but also to parts used in high-temperature environments such as various boilers and fuel cell systems, and the present invention is extremely useful industrially.

Claims (4)

In mass%, C: 0.005 to 0.20%, Si: 0.65 to 4.0%, Mn: 0.1 to 4.0%, P: 0.01 to 0.05%, S: 0.0001-0.010%, Ni: 5-25%, Cr: 15-30%, N: 0.01-0.30%, Al: 0.005-1.0%, Cu: 0.1 Contain ~ 3.0%, the balance is Fe and inevitable impurities, inclusion cleanliness is 0.025% or less, grain size number is 6 or more, surface roughness is centerline average roughness (Ra) An austenitic stainless steel plate for exhaust parts having excellent heat resistance and surface smoothness, characterized by being 0.06 μm or less.   The steel sheet is further in mass%, Mo: 0.01-3.0%, V: 0.05-0.5%, Ti: 0.005-0.3%, Nb: 0.005-0. The heat resistance and surface smoothness according to claim 1, characterized by containing one or more of .3%, B: 0.0002 to 0.0050%, and Ca: 0.0005 to 0.010%. Excellent austenitic stainless steel sheet for exhaust parts.   The steel sheet is further in mass%, W: 0.10 to 3.00%, Zr: 0.05 to 0.30%, Sn: 0.01 to 0.50%, Co: 0.03 to 0 .30%, Mg: 0.0002 to 0.010%, Sb: 0.005 to 0.50%, REM: 0.001 to 0.2%, Ga: 0.0002 to 0.3% Or the austenitic stainless steel plate for exhaust parts excellent in heat resistance and surface smoothness of Claim 1 or 2 characterized by containing 2 or more types.   When manufacturing the stainless steel sheet according to any one of claims 1 to 3, in the annealing step after cold rolling, after heating to 1000 to 1200 ° C, the cooling rate to 500 ° C is 10 ° C / sec or more, Excellent heat resistance and surface smoothness characterized by being cooled at a cooling rate of 10 ° C./sec or more to 100 ° C. or less after being immersed in a molten alkali salt maintained at 400 to 500 ° C. for 5 to 20 sec. Manufacturing method of austenitic stainless steel sheet for exhaust parts.