JP6552385B2 - Austenitic stainless steel plate with excellent heat resistance and workability, its manufacturing method, and exhaust parts made of stainless steel - Google Patents

Description

  The present invention relates to an austenitic stainless steel plate as a heat-resistant component material that requires heat resistance and workability, and is particularly applicable to automobile exhaust hold, converter, and turbocharger parts. In particular, the present invention relates to materials suitable for internal precision parts and housings such as nozzle mounts, nozzle plates, vanes, and back plates of turbochargers mounted on gasoline vehicles and diesel vehicles.

  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. Good materials are 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 and other turbochargers are often installed, and stainless steel used in 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. In addition to SUS310S (25% Cr-20% Ni), which is a typical heat-resistant austenitic stainless steel, Ni-based alloy, and the like, Patent Document 1 discloses a high Cr, Mo-added steel. Further, Patent Document 2 discloses an exhaust guide part of a nozzle vane type turbocharger using austenitic stainless steel added with 2 to 4% of Si. In Patent Document 2, although steel components are defined in consideration of hot workability, it cannot be said that the high temperature characteristics required for the above parts are sufficiently satisfied. In addition, it is important to maintain the hole expansion workability of the punched holes, but sufficient hole expandability could not be obtained with the steel components specified from the hot workability.

  The main object parts of the present invention are the precision parts inside the nozzle vane type turbocharger in the turbocharger. The internal precision parts are parts called back plate and oil deflector, which are located between the turbine part and compressor part and the center core, and stably rotate the turbine and compressor wheel while maintaining the seal of each part. Therefore, surface smoothness is important in addition to oxidation resistance and high temperature strength. In addition, in order to adjust the flow rate and flow rate of exhaust gas, there is a nozzle component composed of precision components such as a nozzle mount, a nozzle plate, a nozzle vane, a drive ring, and a drive lever. Since these are in contact with high-temperature exhaust gas, high-temperature strength, creep characteristics, and oxidation resistance are important. In addition, since the exhaust gas flow rate and flow rate are adjusted by opening and closing the vanes, the friction and wear characteristics at high temperatures such as high temperature slidability are important.

  Furthermore, the nozzle parts are subjected to drilling and hole expansion, but since extremely high dimensional accuracy and plate thickness accuracy are required, workability is also important. Conventionally, materials with excellent heat resistance are hard, so castings are used, and in processing using steel sheets, after processing and cutting and grinding treatment from the viewpoint of cracks, cracks and processing accuracy during parts processing It was used. However, there is a drawback that the cost of processing the material and parts is high. In particular, with the high Si component as described in Patent Document 2, defects such as cracking are likely to occur during plate processing, and sheet metallizing of the part has been difficult.

JP, 2013-069220, A Patent No. 4937277 gazette

  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 required to have heat resistance and workability suitable for turbocharger parts in automobile exhaust parts.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, the present inventors conducted detailed research regarding the austenitic stainless steel plate and its manufacturing method from the viewpoint of steel composition, processing characteristics, surface characteristics, metal structure, high temperature characteristics. As a result, for components that are required to have heat resistance among parts exposed to extremely severe thermal environments such as turbochargers, for example, the heat resistance is ensured by the steel component, and the metal structure and hardness are controlled. It has been found that, by doing this, it is possible to obtain parts with excellent board thickness and dimensional accuracy.
From the viewpoint of formability, it is not satisfied only with the steel components as described in Patent Document 2, so that we have succeeded in significantly improving the precision formability by adjusting the grain size and the cross-sectional hardness. In addition, in order to improve exhaust efficiency and thermal efficiency, we have found a method of controlling the surface roughness, and have made it possible to provide an austenitic stainless steel sheet excellent in heat resistance and processability.
In order to obtain surface smoothness, it is conceivable to grind the casted article after cutting processing, grinding processing or cold rolling annealing, but in the present invention, surface smoothness can be ensured by pickling processing after cold rolling annealing. Therefore, it is possible to contribute to omission or load reduction of the cutting and grinding processes.

The gist of the present invention for solving the above problems is as follows.
(1)
C: 0.005 to 0.2%, Si: 0.1 to 4%, Mn: 0.1 to 5%, P: 0.01 to 0.05%, S: 0.0001 to C in mass% 0.01%, Ni: 5 to 20%, Cr: 15 to 30%, N: 0.01 to 0.4%, Al: 0.005 to 1%, Cu: 0.05 to 3%, Mo: 0.01 to 3%, V: 0.05 to 1%, the balance is Fe and inevitable impurities, the cross-sectional hardness Hv is 190 or less, the grain size number is 8.5 or less Austenitic stainless steel sheet with excellent heat resistance and workability.
Hereinafter, the element additionally contained is additionally contained in place of a part of Fe.
(2)
The austenitic stainless steel sheet having excellent heat resistance and workability as described in (1), wherein the stainless steel sheet has a surface roughness Rz of 30 μm or less.
(3)
The stainless steel sheet is further, in mass%, Ti: 0.005-0.3%, Nb: 0.005-0.3%, B: 0.0002-0.0050%, Ca: 0.0005-0. The austenitic stainless steel sheet excellent in heat resistance and workability according to (1) or (2), characterized by containing 0.01% or one or more.
(4)
The stainless steel sheet further contains, in mass%, W: 0.1 to 3%, Zr: 0.05 to 0.3%, Sn: 0.01 to 0.5%, Co: 0.03 to 0. 3%, Mg: 0.0002 to 0.01%, Sb: 0.005 to 0.5%, REM: 0.001 to 0.2%, Ga: 0.0002 to 0.3%, Ta: 0 An austenitic stainless steel sheet excellent in heat resistance and processability according to any one of (1) to (3) characterized by containing one or more of 0.001 to 1.0%.
(5)
It is the manufacturing method of the stainless steel plate of any one of Claims 1-4, Comprising: The steel which has the component of any one of Claims 1, 3, and 4 was hot-rolled, and was obtained. The heat-rolled steel plate is heated to 1100 ° C. to 1200 ° C., and then cooled to 400 ° C. at 20 ° C./sec or more, and then subjected to pickling treatment, which is an austenitic stainless steel plate excellent in heat resistance and workability. Production method.
(6)
Furthermore, after cold rolling the stainless steel plate at a rolling reduction of 50% or less and heating the obtained cold rolled steel plate to 1100 ° C. to 1200 ° C., it is cooled to 400 ° C. at 20 ° C./sec or more and then pickling treatment (5) The method for producing an austenitic stainless steel sheet excellent in heat resistance and processability according to (5).
(7)
An austenitic stainless steel exhaust part characterized in that the stainless steel plate according to any one of (1) to (4) is used for the exhaust part.

  According to the present invention, austenite for automobile exhaust parts (particularly for turbocharger parts) having excellent processing accuracy when processing parts, reducing grinding and polishing processes, and excellent high-temperature slidability, gas flow and thermal efficiency. We can provide stainless steel. At the same time, it contributes to the omission or reduction of load of the cutting and grinding processes.

  The reasons for limitation of the present invention will be described below. High temperature strength is important as a characteristic of an austenitic stainless steel sheet used for heat-resistant applications, but particularly in the case of turbocharger parts as described above, workability and surface properties are also extremely important. As mentioned above, if the processing accuracy when processing into the internal parts of the turbocharger is poor, the parts may be in contact with each other or burned in, gas tightness and poor gas flow, etc., leading to damage and reduced thermal efficiency. It leads to the deterioration of performance reliability.

[Section hardness Hv: 190 or less]
First, the cross-sectional hardness Hv will be described. Exhaust parts are often formed into parts by forming steel sheets, but precision parts such as turbochargers are required to have stricter processing accuracy. For example, in the case of the above-mentioned nozzle plate or part called a nozzle mount, since processing such as drilling and hole expansion is performed, hole dimensions at the time of drilling, hole inner surface condition, presence or absence of cracks, end cracks at hole expansion, rising An angle etc. correspond to this. Further, in the case of the housing, a springback etc. and a thinning rate are relevant. These are affected by the hardness and inclusions of the austenitic stainless steel plate that is the material. In the present invention, the cross-sectional hardness of the material is 190 or less in Vickers hardness (Hv), so that processing defects occur in hole expansion processing of the turbocharger. It was found that it did not occur. Specifically, when the nozzle plate or mount is expanded, the crack depth at the end is suppressed to 0.1 mm or less, and sufficient processing accuracy is obtained. As a result, it is not necessary to apply an excessive cutting process to the end after the hole expanding process, and the cost is also beneficial. In addition, with regard to the hole dimension accuracy at the time of drilling, by satisfying the above-mentioned hardness, excellent hole dimension accuracy can be obtained. Furthermore, in order to ensure processability, the upper limit of the cross-sectional hardness is preferably Hv160. Here, the cross-sectional hardness is measured according to JIS Z 2244 using, for example, a sample obtained by polishing a cross section of a steel plate (a cross section in a direction parallel to the rolling direction). Should be used. In addition, since excessive softening affects heat resistance, it is desirable to set the lower limit of cross-sectional hardness to Hv110.

[Grain size number: 8.5 or less]
As described above, in the present invention, a soft material is provided in consideration of the workability of various heat-resistant components, but in order to satisfy Hv190 or less, the crystal grain size number is set to 8.5 or less. However, since the surface roughening (orange peel) at the time of processing occurs notably by excessive coarse graining, No. 4 or more is desirable. As for the grain size number, for example, the grain size number is measured according to JIS G 0551 using a sample obtained by polishing a cross section of a steel plate (a cross section in the direction parallel to the rolling direction). Furthermore, in consideration of manufacturability and heat resistance, it is preferable to set the lower limit of the grain size to 4.5 and the upper limit to 7.5.

[Surface roughness Rz: 30 μm or less]
In addition to the above, it has been found in the present application that the surface roughness of the material is important particularly in the case of a turbocharger or an exhaust manifold. The surface roughness is preferably a smooth surface also from the point of load of the cutting process when cutting parts. In the case of precision parts, it is better to be smooth in terms of gas sealability. In the present application, in addition to these, they have found that they have the effect of improving the gas flow to increase the thermal efficiency and the efficiency of the turbocharger, and found that these effects can be obtained by setting the maximum roughness (Rz) to 30 μm. The In conventional cast steel parts, this effect can be obtained by polishing or cutting because the surface roughness is large. However, in the present application, by controlling the surface roughness of the material, the merit of omitting the polishing or cutting can be obtained. Regarding the surface roughness measurement, the surface roughness is measured in a direction parallel to and perpendicular to the rolling direction with respect to the steel sheet surface in accordance with JIS B 0601, and the average value of each Rz is obtained. As a result of various investigations of surface roughness of the material and contact with other parts, exhaust efficiency, and thermal efficiency, if the average value of the rolling direction and the roughness (Rz) in the direction perpendicular to the rolling direction is 30 μm or less, the sealability is maintained. It was found that the gas flow smoothly flows without being disturbed locally. The uniform gas flow leads to the improvement of the exhaust efficiency of the turbocharger and the improvement of the thermal efficiency of the whole exhaust pipe system, which directly leads to the improvement of the fuel efficiency of the car. Furthermore, in consideration of the productivity at the time of steel plate production and the increase in roughness due to pressing, Rz is desirably 5 μm or less.

[component]
Next, the component range of steel will be described. The% with respect to the component content indicates% by mass unless otherwise specified.

  C has a lower limit of 0.005% in order to form an austenite structure and ensure high temperature strength. On the other hand, excessive addition leads to hardening, corrosion resistance due to Cr carbide formation, especially degradation of intergranular corrosion of welds, degradation of high-temperature slidability due to carbides, grain boundaries during pickling of cold-rolled annealed plates The upper limit is made 0.2% because the surface roughness becomes rough due to the formation of the erosion groove. Furthermore, in consideration of manufacturing cost and hot workability, the lower limit of the C content is preferably 0.008% and the upper limit is 0.1%.

  In addition to being added as a deoxidizing element, Si is added in an amount of 0.1% or more because internal oxidation of Si brings about an improvement in oxidation resistance, high temperature slidability and high temperature strength. On the other hand, the addition of 4.0% or more makes the steel hard and coarse Si-based oxides are formed, and the processing accuracy of the parts is significantly reduced, so the upper limit is made 4%. In view of the manufacturing cost, the pickling property at the time of steel plate manufacture, and the solidification cracking property at the time of welding, it is desirable to set the lower limit of the Si content to 0.4% and the upper limit to 3.5%. Furthermore, considering the high temperature slidability, the lower limit of Si is desirably 2%.

  Mn is used as a deoxidizing element, and 0.1% or more is added to secure austenite structure formation and scale adhesion. On the other hand, addition of more than 5% significantly deteriorates the cleanliness of inclusions and lowers the hole expansibility, and the pickling property deteriorates significantly and the product surface becomes rough, so the upper limit is made 5%. Furthermore, considering the manufacturing cost and the pickling property when manufacturing the steel sheet, the lower limit of the Mn content is preferably 0.2% and the upper limit is preferably 3%.

  P is an element that promotes hot workability and solidification cracking at the time of manufacture, and its content is preferably as low as possible because it hardens. However, considering refining costs, the upper limit is 0.05% and the lower limit is 0. .01%. Furthermore, considering the manufacturing cost, it is desirable that the lower limit of the P content is 0.02% and the upper limit is 0.04%.

  S is an element that degrades hot workability during production and deteriorates corrosion resistance. In addition, when coarse sulfides (MnS) are formed, the degree of cleanliness significantly deteriorates and the hole expansibility is deteriorated, so the upper limit is made 0.01%. On the other hand, the excessive reduction leads to an increase in the refining cost, so the lower limit is made 0.0001%. Furthermore, considering the manufacturing cost and oxidation resistance, it is desirable that the lower limit of the S content is 0.0005% and the upper limit is 0.0050%.

  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, so 5% or more is added. On the other hand, excessive addition causes cost rise and hardening, so the upper limit is made 20%. Furthermore, in consideration of manufacturability, high temperature strength and corrosion resistance, the lower limit of the Ni content is desirably 10%.

  Cr is an element that improves the corrosion resistance, oxidation resistance and high temperature sliding property, and needs 15% or more from the viewpoint of abnormal oxidation suppression in consideration of the exhaust part environment. On the other hand, excessive addition makes it hard and degrades moldability, and also increases the cost, so the upper limit is made 30%. Furthermore, considering the manufacturing cost, steel plate manufacturability and workability, it is desirable that the lower limit of the Cr content is 17% and the upper limit is 25.5%.

  N, like C, is an effective element for austenite structure formation, high temperature strength, and high temperature slidability. The high temperature strength is known as a solid solution strengthening element, but in the present application, in addition to the effect of N alone, the high temperature strength due to the formation of clusters with Cr is also taken into consideration, and 0.01% or more is added. On the other hand, addition of more than 0.4% remarkably hardens the normal temperature material and deteriorates the cold workability at the steel plate manufacturing stage, and also deteriorates the formability and part accuracy during part processing. I assume. From the viewpoints of softening, suppression of pinholes during welding, and suppression of intergranular corrosion of welds, the lower limit of the N content is preferably 0.02% and the upper limit is preferably 0.3%. Furthermore, from the viewpoint of high temperature strength and slidability, the lower limit of the N content is preferably 0.04%.

  Al is added as a deoxidizing element, and improves hole expansibility by improving inclusion cleanliness. In addition, there is an effect that contributes to the improvement of high temperature slidability by suppressing peeling of oxide scale and a small amount of internal oxidation, and since its action is manifested from 0.005%, the lower limit was made 0.005%. Further, since it is a ferrite-forming element, addition of 1% or more lowers the stability of the austenite structure, and also causes an increase in surface roughness due to a decrease in pickling properties, so the upper limit is made 1%. Furthermore, in consideration of refining costs and surface defects, the lower limit of the Al content is preferably 0.007% and the upper limit is 0.5%.

  Cu is an element effective for stabilizing and softening the austenite phase, and is added at 0.05% or more. On the other hand, excessive addition leads to deterioration of oxidation resistance and manufacturability, so the upper limit is made 3%. Furthermore, considering corrosion resistance and manufacturability, it is desirable that the lower limit of the Cu content is 0.1% and the upper limit is 1%.

  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%. Furthermore, considering that Mo is an expensive element, strengthening stability due to the precipitates, and inclusion cleanliness, the lower limit of the Mo content is preferably 0.4% and the upper limit is preferably 1.6%.

V is an element that improves the corrosion resistance, and is added by 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 1%. Furthermore, in consideration of the manufacturability and the cleanliness of inclusions, the lower limit of the V content is preferably 0.1% and the upper limit is 0.5%.
Although the above is a main element, it may contain one or more of the following elements as a substitute for a part of the other Fe.

  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%. In addition, addition of more than 0.3% tends to cause nozzle clogging at the casting stage, which significantly deteriorates manufacturability and causes deterioration of hole expansion workability due to coarse Ti carbonitride, so the upper limit is 0. .3%. Furthermore, in view of high temperature strength, intergranular corrosion of welds and alloy cost, the lower limit of the Ti content is preferably 0.01% and the upper limit is 0.2%.

  Nb is an element that, like Ti, 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%. In addition, addition of more than 0.3% significantly deteriorates hot workability in the steel plate manufacturing stage, and also causes deterioration of hole expansibility due to coarse Nb carbonitride, so the upper limit is made 0.3%. . Furthermore, in view of high temperature strength, intergranular corrosion of welds and alloy cost, the lower limit of the Nb content is preferably 0.01% and the upper limit is 0.20%.

  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 hole expansibility and deterioration of intergranular corrosion due to the formation of borocarbides, so the upper limit was made 0.0050%. Furthermore, in consideration of the refining cost and the decrease in ductility, the lower limit of the B content is desirably 0.0003% and the upper limit is 0.002%.

  Ca is added as needed for desulfurization. Since this effect does not appear at less than 0.0005%, the lower limit is made 0.0005%. In addition, when it is added in excess of 0.01%, water-soluble inclusions CaS are formed to cause a decrease in cleanliness and a marked decrease in corrosion resistance, so the upper limit is made 0.01%. Furthermore, from the viewpoint of manufacturability and surface quality, it is desirable that the lower limit of the Ca content is 0.0010% and the upper limit is 0.0030%.

  W contributes to the improvement of corrosion resistance and high temperature strength, so 0.1% or more is added as necessary. The upper limit is made 3% because addition of more than 3% leads to hardening, toughness deterioration at the time of steel plate production and cost increase. Furthermore, considering refining costs and manufacturability, it is desirable that the lower limit of the W content is 0.1% and the upper limit is 2%.

  Zr is added in an amount of 0.05% or more as needed in order to combine with C and N to improve intergranular corrosion resistance and oxidation resistance of the weld. However, the addition of more than 0.3% increases the cost and remarkably deteriorates manufacturability and hole expandability, so the upper limit is made 0.3%. Furthermore, considering refining costs and manufacturability, it is desirable that the lower limit of the Zr content is 0.05% and the upper limit is 0.1%.

  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 0.5% may cause slab cracking during steel sheet production, the upper limit is made 0.5%. Furthermore, considering the refining cost and manufacturability, the upper limit is preferably 0.3%.

  Co contributes to improving the high-temperature strength, so 0.03% or more is added as necessary. The addition of more than 0.3% leads to hardening, deterioration of toughness during steel plate production and cost increase, so the upper limit is made 0.3%. Furthermore, considering refining costs and manufacturability, the lower limit of the Co content is preferably 0.03%, and the upper limit is preferably 0.1%.

  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 deterioration of weldability and corrosion resistance, and reduction of hole expansibility due to coarse inclusions, so the upper limit is made 0.01%. Considering the refining cost, it is desirable that the lower limit of the Mg content is 0.0003% and the upper limit is 0.005%.

  Sb is an element that segregates at the grain boundary to increase the high temperature strength. In order to obtain the effect of addition, the content is made 0.005% or more. However, if it exceeds 0.5%, Sb segregation occurs and cracks occur during welding, so the upper limit is made 0.5%. Considering the high temperature characteristics, production cost and toughness, the lower limit of the Sb content is preferably 0.03% and the upper limit is preferably 0.3%. More preferably, the lower limit of the Sb content is 0.05%, and the upper limit is 0.2%.

REM (rare earth element) is effective in improving the oxidation resistance and the high temperature sliding property, and is added by 0.001% or more as needed. Moreover, even if it is added in excess of 0.2%, the effect is saturated and corrosion resistance is reduced due to REM particulates, so 0.001 to 0.2% is added. In consideration of the processability and the manufacturing cost of the product, it is preferable to set the lower limit to 0.002% and the upper limit to 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 at 0.3% or less for the purpose of improving corrosion resistance and suppressing hydrogen embrittlement, but addition of more than 0.3% causes formation of coarse sulfides and deterioration of the r value. From the viewpoint of sulfide and hydride formation, the lower limit is made 0.0002%. Furthermore, from the viewpoint of manufacturability and cost, 0.002% or more is more preferable.

  Ta and Hf may be added in an amount of 0.001 to 1.0% for improving the high temperature strength. An effect is obtained at 0.001% or more, and even higher strength is obtained at 0.01% or more. 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.

[Production method]
Next, the manufacturing method will be described. The manufacturing method of the steel plate of the present invention comprises steelmaking-hot rolling-annealing / pickling or steelmaking-hot rolling-annealing / pickling-cold rolling-annealing / 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 etc.). The slab is heated to a predetermined temperature and hot rolled by continuous rolling to a predetermined thickness. As described above, manufacturing conditions in which predetermined grain size, cross-sectional hardness, and surface roughness are secured in the steps after hot rolling are set for parts to which the present invention is applied.
The hot rolled steel sheet is subjected to hot rolled sheet annealing and pickling treatment. Normally, the hot-rolled sheet annealing temperature is less than 1000-1100 ° C. to obtain a recrystallized structure, but in the present application, heat treatment is performed at a high temperature of 1100-1200 ° C., the cross-sectional hardness Hv is 190 or less, and the grain size number is 8. 5 or less to ensure soft, hole-piercing ability, hole-expanding ability and various processing accuracy.

  On the other hand, by performing high temperature annealing, an oxide scale is excessively generated, and subsequent pickling properties are inhibited. When pickling is hindered, mechanical descaling such as shot blasting or bender is strengthened, or chemical descaling such as sulfuric acid or nitric acid hydrofluoric acid is used to increase the concentration, increase the temperature, or decrease the plate speed. Processing is required. These not only reduce the production efficiency but also increase the roughness of the product surface and lead to the reduction of the efficiency of the exhaust parts. In the present application, it has been found that by increasing the cooling rate during heat treatment, the surface roughness can be ensured without deteriorating the pickling property. In order to make the surface roughness Rz be 30 μm or less, it was found that it is effective to set the cooling rate to 400 ° C. from the maximum temperature to 20 ° C./sec or more. The method for setting the cooling rate to 20 ° C./sec may be appropriately selected, such as water injection cooling or ventilation cooling, but water injection cooling is desirable as a method for promoting scale peeling in the cooling process, considering the pickling efficiency and the steel plate shape. The cooling rate is preferably 30 to 100 ° C./sec.

  In this application, a smooth surface is further obtained by performing cold rolling after hot-rolled sheet annealing and pickling, and performing cold-rolled sheet annealing and pickling after that. The cold rolling process may be performed by tandem rolling, Sendzimir rolling, cluster rolling, or the like. Usually, the rolling reduction of cold rolling is over 50%, but when rolling at a reduction of over 50%, subsequent annealing tends to form a fine grain structure, and a grain size number of 8.5 or less is obtained. Since there is no case, the rolling reduction is 50% or less. Furthermore, when considering the manufacturing cost and the steel plate shape, the rolling reduction is preferably 15 to 45% or less.

  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. In the present application, softening and surface smoothness are ensured by utilizing a normal continuous annealing / pickling process instead of bright annealing. This is the same idea as the hot-rolled sheet annealing / pickling step described above, and heat treatment is performed at a high temperature of 1100 to 1200 ° C., which is higher than the normal annealing temperature of 1000 to 1100 ° C., and the cross-sectional hardness Hv is 190. Hereinafter, the crystal grain size number is ensured to be 8.5 or less, and the hole is easy to drill, expandability and various processing accuracy are ensured.

  On the other hand, by performing high temperature annealing, an oxide scale is excessively generated, and subsequent pickling properties are inhibited. If pickling is hindered, it is necessary to strengthen pretreatment such as neutral salt electrolysis and molten alkali treatment or pickling treatment such as nitric hydrofluoric acid and nitric acid electrolysis, which leads to a reduction in sheet feeding speed and an increase in surface roughness. . In the present application, it has been found that by increasing the cooling rate during heat treatment, the surface roughness can be ensured without deteriorating the pickling property. In order to make the surface roughness Rz be 30 μm or less, it was found that it is effective to set the cooling rate to 400 ° C. from the maximum temperature to 20 ° C./sec or more. The method for setting the cooling rate to 20 ° C./sec may be appropriately selected, such as water injection cooling or ventilation cooling, but water injection cooling is desirable as a method for promoting scale peeling in the cooling process, considering the pickling efficiency and the steel plate shape. The cooling rate is preferably 30 to 100 ° C./sec.

Steels of the composition shown in Table 1 and Table 2 are melted and cast into slabs, and hot rolled, hot rolled sheet annealing and pickling, and then cold rolling and final annealing and pickling are applied to 4.3 to 7 A product plate of .3 mm thickness was obtained. For each steel, the cross-sectional hardness, grain size, and surface roughness were measured by the method described above, and a hole expansion test and a high temperature tensile test at 900 ° C. in accordance with JIS Z 2281 were performed.
In the hole expansion test, a punched hole with a diameter of 25 mm was expanded to an inner diameter of 32 mm, and then the cracking state at the end of the rising portion having a height of 6 mm was confirmed. If the crack depth is 0.4 mm or less, it does not affect the turbo parts performance, so the case of 0.4 mm or less is passed (○) and the case of cracks exceeding 0.4 mm is rejected (×) did. Regarding high temperature tension, a steel having a proof stress of 20 MPa or more at 900 ° C. was accepted (○), and a case of less than 20 MPa was rejected (×).

In addition, the sample material was processed into parts called nozzle mounts and nozzle plates, mounted on a nozzle vane type turbocharger, and exhaust gas of high temperature (900 ° C) was flowed while repeating opening and closing of the nozzles, and gas flowability was examined. . Under the present circumstances, the steel in which the problem in the gas flow did not occur was made into pass ((circle)), and the steel in which the defect in gas flow defect and nozzle opening and closing had a failure was made into rejection (x).
As a result of manufacturing under the manufacturing conditions shown in Tables 3 and 4, it is confirmed that the steel of the example of the present invention is excellent in workability, heat resistance and surface properties, and satisfies the performance as a turbocharger part. If one or more of the cross-sectional hardness, the grain size, and the surface roughness are out of the range of the present invention, the processing accuracy and the turbocharger performance become poor and a problem occurs. In addition, even when the high-temperature strength is poor, creep deformation causes a defect in turbocharger performance.

  The other conditions in the manufacturing process may be appropriately selected. For example, the slab thickness, the hot-rolled sheet thickness, etc. may be designed as appropriate. In cold rolling, the roll roughness, the roll diameter, the rolling oil, the number of rolling passes, the rolling speed, the rolling temperature and the like may be appropriately selected. Intermediate annealing may be put in the middle of cold rolling, and may be batch annealing or continuous annealing. In addition, any of the neutral salt electrolytic treatment and the salt bath immersion treatment may be omitted as a pretreatment at the time of pickling, and the pickling process may be omitted in addition to nitric acid, nitric acid electrolytic pickling, sulfuric acid and hydrochloric acid. You may process using. After annealing and pickling of the cold-rolled sheet, shape and material adjustment may be performed by temper rolling, a tension leveler or the like. Furthermore, the product plate may be lubricated to further improve press molding, and the type of lubricating film may be appropriately selected. In addition, heat resistance may be further improved by performing special surface treatment such as nitriding treatment or carburizing treatment after parts processing.

  According to the present invention, it is possible to provide an austenitic stainless steel sheet having excellent characteristics for exhaust parts that require workability in addition to heat resistance, and exhaust is excellent because of excellent surface smoothness. By improving the gas fluidity, the thermal efficiency and turbocharger efficiency are improved. By using the material to which the present invention is applied, particularly as an exhaust part of an automobile, it becomes possible to lead to exhaust gas regulation, weight reduction, and fuel efficiency improvement. Also, it is possible to omit parts cutting and grinding and surface processing, which greatly contributes to cost reduction. Furthermore, the present invention can be applied not only to automobile and two-wheeled exhaust parts, 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 (7)

  C: 0.005 to 0.2%, Si: 0.1 to 4%, Mn: 0.1 to 5%, P: 0.01 to 0.05%, S: 0.0001 to C in mass% 0.01%, Ni: 5 to 20%, Cr: 15 to 30%, N: 0.01 to 0.4%, Al: 0.005 to 1%, Cu: 0.05 to 3%, Mo: 0.01 to 3%, V: 0.05 to 1%, the balance is Fe and inevitable impurities, the cross-sectional hardness Hv is 190 or less, the grain size number is 8.5 or less Austenitic stainless steel sheet with excellent heat resistance and workability.   2. The austenitic stainless steel plate excellent in heat resistance and workability according to claim 1, wherein the stainless steel plate has a surface roughness Rz of 30 μm or less.   The stainless steel sheet is further, in mass%, Ti: 0.005-0.3%, Nb: 0.005-0.3%, B: 0.0002-0.0050%, Ca: 0.0005-0. The austenitic stainless steel sheet having excellent heat resistance and workability according to claim 1 or 2, characterized by containing 0.01% of one kind or two or more kinds.   The stainless steel sheet further contains, in mass%, W: 0.1 to 3%, Zr: 0.05 to 0.3%, Sn: 0.01 to 0.5%, Co: 0.03 to 0. 3%, Mg: 0.0002 to 0.01%, Sb: 0.005 to 0.5%, REM: 0.001 to 0.2%, Ga: 0.0002 to 0.3%, Ta: 0 The austenitic stainless steel sheet excellent in heat resistance and workability according to any one of claims 1 to 3, comprising 0.001 to 1.0% of one kind or two or more kinds.   It is the manufacturing method of the stainless steel plate of any one of Claims 1-4, Comprising: The steel which has the component of any one of Claims 1, 3, and 4 was hot-rolled, and was obtained. An austenitic stainless steel sheet excellent in heat resistance and workability characterized by heating a hot-rolled steel sheet to 1100 ° C. to 1200 ° C., cooling to 400 ° C. at 20 ° C./sec or more, and then performing pickling treatment. Production method.   Further, the stainless steel plate is cold-rolled at a reduction rate of 50% or less, and the obtained cold-rolled steel plate is heated to 1100 ° C. to 1200 ° C., then cooled to 400 ° C. at 20 ° C./sec or more, and then pickled. The method for producing an austenitic stainless steel sheet excellent in heat resistance and processability according to claim 5, characterized in that   An exhaust part made of austenitic stainless steel, wherein the stainless steel plate according to any one of claims 1 to 4 is used as an exhaust part.