JP7166082B2 - Austenitic stainless steel sheet and manufacturing method thereof - Google Patents

Description

The present invention relates to an austenitic stainless steel sheet that is used as a material for heat-resistant parts that require heat resistance, and is particularly applicable to exhaust holds of automobiles and motorcycles, converters, turbocharger parts, plants, and the like.

Exhaust manifolds, front pipes, center pipes, mufflers, and environmentally friendly parts for purifying exhaust gas in automobiles are required to have heat resistance such as oxidation resistance, high-temperature strength, and thermal fatigue properties in order to stably ventilate high-temperature exhaust gas. Good materials are used. In addition, it is also required to be excellent in corrosion resistance because it is also a condensed water corrosive environment.

Stainless steel is often used for these parts from the standpoint of tightening exhaust gas regulations, improving engine performance, and reducing vehicle weight. In recent years, exhaust gas regulations have become even more stringent, fuel efficiency has improved, and automobiles have been downsized, so the temperature of the exhaust gas passing through the exhaust manifold directly below the engine is on the rise. In addition, the number of cases where a supercharger such as a turbocharger is installed is increasing, and the stainless steel used for exhaust manifolds and turbochargers is required to have further improved heat resistance. As for the rise in 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 a turbocharger is complicated, and it is important to improve supercharging efficiency and ensure heat resistance reliability. In addition to SUS310S (25% Cr-20% Ni), which is a typical heat-resistant austenitic stainless steel, Ni-based alloys, and the like, Patent Document 1 discloses steel with high Cr and Mo additions. Further, Patent Document 2 discloses an exhaust guide component for a nozzle vane type turbocharger using austenitic stainless steel to which Si is added by 2 to 4%.

In Patent Document 2, the steel composition is specified in consideration of the hot workability during steel production, but it cannot be said that the high-temperature properties required for the above parts are sufficiently satisfied. In addition, it is important to maintain the hole expandability of punched holes, but it was not possible to obtain sufficient hole expandability with the steel composition specified from the hot workability. Furthermore, the housing of a turbocharger is made of cast stainless steel, which has a large wall thickness.

In Patent Document 3, the optimum range of Nb, V, C, N, Al, and Ti contents is determined, and the manufacturing process is optimized to improve the high-temperature strength and creep properties of heat-resistant austenitic stainless steel sheets. is disclosed. However, the technical problem of the invention disclosed in Patent Document 3 is to improve high temperature strength and creep characteristics at 800 ° C. is insufficient.

Patent Document 4 discloses a heat-resistant austenitic stainless steel having a hardness of 40 HRC or more at room temperature after heat treatment at 700° C. for 400 hours by optimizing the material composition and treatment conditions. However, the problem of the invention disclosed in Patent Document 4 is to have high temperature strength that can withstand use environments of 550 ° C. or higher, and Patent Document 4 only shows high temperature strength at 700 ° C. , the heat-resistant austenitic stainless steel according to the invention disclosed in Patent Document 4 is insufficient for handling exhaust gas exceeding 900°C.

In addition, Patent Document 5 discloses that by controlling the low ΣCSL grain boundary frequency and the crystal average grain size, etc., it is possible to improve intergranular corrosion resistance and high temperature strength with a small grain size material. It is However, the "high-temperature strength" in Patent Document 5 means high-temperature strength in water, and no specific means for achieving strength against exhaust gas exceeding 900° C. is disclosed.

Further, the stainless steel for nuclear power use disclosed in Patent Document 6 is characterized by ensuring excellent intergranular corrosion resistance in high-temperature water by increasing the twin grain boundary ratio in the steel. However, Patent Document 6 does not disclose the high-temperature strength of the stainless steel for nuclear power use, and Patent Document 6 discloses a specific solution for achieving strength against exhaust gas exceeding 900 ° C. Not disclosed.

In addition, the corrosion-resistant austenitic alloy disclosed in Patent Document 7 is subjected to cold working and heat treatment of more than 30% to the austenitic alloy to form twin boundaries in the austenite grains, and the austenite grain boundaries and/or precipitates are dispersedly formed on twin boundaries. Due to the above characteristics, intergranular sliding is suppressed and intergranular strength is increased, so that the corrosion-resistant austenitic alloy has higher resistance to stress corrosion crack growth. However, the stress corrosion cracking resistance shown in Patent Document 7 is a characteristic in high-temperature water, and Patent Document 7 describes a specific solution for achieving strength against exhaust gas exceeding 900 ° C. , not disclosed.

The main purpose of improving properties by increasing the twin grain boundary ratio is to improve corrosion resistance. In addition, the steel composition and manufacturing conditions for that purpose are variously described in Patent Documents 8 to 14. As for the steel composition, SUS304 and SUS316 series are targeted, and there is no description of a material containing more than 1% of Si, which is a feature of the present application. As for the rolling conditions, a low rolling reduction is the basic, for example, 2 to 15% in Patent Document 9 and 2 to 5% in Patent Documents 12 and 13. In addition, the heat treatment conditions after that are relatively long heat treatments. 11 at 900-950° C. for 10-48 hours. On the other hand, in Patent Literature 12, it is within 2 minutes at 1052° C. or higher. If recrystallization is accelerated and a large number of large-angle grain boundaries are generated, the twin grain boundary frequency will decrease. It is considered that the heat treatment is performed at a relatively low temperature for a long time as described above in order to move the grain boundary.

Further, the austenitic stainless steel sheet disclosed in Patent Document 15 is a steel sheet invented based on the knowledge that the higher the frequency of annealing twins, the higher the high temperature strength at 900 ° C. It discloses that a high-strength material of 70 MPa or more can be obtained by increasing the frequency of annealing twins in an austenitic stainless steel sheet to 40% or more. However, the strength against exhaust gas exceeding 900° C. is insufficient.

WO2014/157655 Japanese Patent No. 4937277 JP 2013-209730 A JP 2005-281855 A JP 2011-168819 A JP-A-2005-15896 Japanese Patent Application Laid-Open No. 2008-63602 JP-A-11-80905 Japanese Patent Application Laid-Open No. 2003-253401 Japanese Patent Application Laid-Open No. 2009-161802 Japanese Patent Application Laid-Open No. 2009-191341 JP 2009-287104 A JP 2010-275569 A JP 2014-5509 A WO2017/164344

F.B. Pickering, in :G.D. Dunlup(Ed.), Proceedings of the Stainless steels 84, Chalmers University of Technology, Goteborg, September 3-4, (1984), The institute of Metals, London(1985)12.

When heat-resistant parts are exposed to high-temperature environments, excessive deformation or, in extreme cases, destruction occurs due to insufficient high-temperature strength and rigidity. In addition, high-cycle or low-cycle fatigue failure due to vibration is also an issue. In conventional austenitic stainless steel sheets, addition of alloying elements to increase high-temperature strength results in insufficient room-temperature ductility and leads to an increase in cost. SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and improve heat resistance by controlling the structure of an austenitic stainless steel sheet used for heat-resistant parts.

The parts to be solved by the present application are heat-resistant parts such as exhaust parts for transportation such as automobiles and motorcycles, and plant parts. Among them, it is especially the exhaust system parts of automobiles, such as exhaust manifolds and turbochargers. For turbochargers, the housing that makes up the outer frame, precision parts inside the nozzle vane type turbocharger (for example, back plate, oil deflector, compressor wheel, nozzle mount, nozzle plate, nozzle vane, drive ring, drive lever) be.

In austenitic stainless steel, grain boundaries are formed after cold rolling and annealing, resulting in a polycrystalline body. At the grain boundary, the atomic arrangement becomes regular or irregular. If the atomic arrangement at the crystal grain boundary is regular and there are few gaps, the structure will have a low energy, and the grain boundary deterioration phenomenon will not easily occur. Corresponding grain boundaries are cited as representatives of these special grain boundaries. On the other hand, when the atomic arrangement at the crystal grain boundary is not regular, it is called a random grain boundary and has a high energy structure. Corresponding orientation relationships appear geometrically in many combinations, and grain boundaries formed by corresponding orientation relationships of Σ3 to Σ29 are called corresponding grain boundaries. This Σ value is an odd number, and the smaller this value is, the higher the corresponding lattice point density is and the stronger the character as a low-energy grain boundary is.

Most of the above conventional knowledge improves corrosion resistance by increasing the corresponding grain boundary frequency. is 75% or more, intergranular corrosion is suppressed. On the other hand, Patent Document 9 requires heat treatment at 900 to 1000° C. for 5 hours or more to increase the frequency of corresponding grain boundaries, which is inefficient for industrial mass production. In addition, the invention disclosed in Patent Document 9 increases the frequency of corresponding grain boundaries to suppress precipitation at grain boundaries and improve corrosion resistance, but the effect on mechanical properties at high temperatures is unknown. there were.

In order to solve the above problems, the present inventors conducted detailed research on the relationship between the metallographic structure of an austenitic stainless steel sheet, high-temperature properties, and room-temperature workability. As a result, for materials that require heat resistance in parts that are exposed to extremely harsh thermal environments, such as turbochargers, the heat resistance is ensured by the steel composition, and the cold rolling process and subsequent annealing It has been found that by controlling the characteristics of the grain boundaries in the metallographic structure through the process, remarkably excellent high-temperature strength properties can be obtained. Also, in terms of workability, the steel composition described in Patent Document 2 alone is not satisfactory, and by controlling the characteristics of the grain boundaries described above through the cold rolling process and the subsequent annealing process, we have succeeded in achieving both high-temperature strength. .

Furthermore, the annealing time for increasing the corresponding grain boundary frequency by making the steel composition such that the stacking fault energy (stacking fault energy) value SFE obtained from the following formula described in Non-Patent Document 1 becomes small is shortened. I found that I can make it.
SFE (mJm ⁻² )=25.7+2×Ni+410×C−0.9×Cr−77×N−13×Si−1.2×Mn
However, the element symbol in the formula means the content (% by mass) of the element, and 0 is substituted when the element is not contained.

The gist of the present invention for solving the above problems is
(1) In mass%, C: 0.005 to 0.300%, Si: more than 1.00 to 4.00%, Mn: 0.10 to 10.00%, Ni: 2.00 to 25.00 %, Cr: 15.0 to 30.0%, N: 0.005 to less than 0.400%, Al: 0.001 to 1.000%, Cu: 0.05 to 4.00%, Mo: 0 .02 to 3.00%, V: 0.02 to 1.00%, P: 0.050% or less, S: 0.0100% or less, the balance being Fe and unavoidable impurities, the following ( 1) The corresponding grain boundary frequency of corresponding grain boundaries with a formula value of 50 or less and a Σ value of 3 to 29 is 80% or more in total, and the corresponding grain boundary frequency of corresponding grain boundaries with a Σ value of 5 to 29 is a total of 10% to 25%, an austenitic stainless steel sheet.
25.7+2×Ni+410×C−0.9×Cr−77×N−13×Si−1.2×Mn (1)
However, the element symbol in the formula means the content (% by mass) of the element, and 0 is substituted when the element is not contained.
(2) The steel sheet further contains, in mass%, Ti: 0.005 to 0.300%, Nb: 0.005 to 0.300%, B: 0.0002 to 0.0050%, Ca: 0.0005 ~0.0100%, W: 0.10-3.00%, Zr: 0.05-0.30%, Sn: 0.01-0.50%, Co: 0.03-0.30%, Mg: 0.0002-0.0100%, Sb: 0.005-0.300%, REM: 0.002-0.200%, Ga: 0.0002-0.3000%, Ta: 0.01- 1.00%, Hf: 0.01 to 1.00%, Bi: 0.001 to 0.020%, characterized by containing one or more of (1), the austenitic system according to (1) stainless steel plate.
(3) The austenitic stainless steel sheet according to (1) or (2), which is used for exhaust parts.
(4) In the method for producing a stainless steel sheet according to (1) or (2), the steel sheet is rolled at a rolling reduction of 80% or less in the cold rolling step, and the subsequent cold-rolled sheet annealing is performed at a heating rate of 10°C/sec. As described above, after annealing at a temperature of 1000 to 1200 ° C., cold rolling is performed at a reduction rate of 10% or less, and in the subsequent cold-rolled sheet annealing, the heating rate is 10 ° C./sec or more to less than 900 ° C., and the heating is 900 ° C. or more. A method for producing an austenitic stainless steel sheet, characterized by performing cold-rolled sheet annealing at a rate of 1° C./sec or more and less than 10° C./sec at 950 to 1150° C. for 20 minutes or less.

According to the present invention, it is possible to provide an austenitic stainless steel sheet with excellent room temperature formability and high temperature properties.

The reasons for limitation of the present invention will be described below. High-temperature strength and creep properties are important characteristics of austenitic stainless steel sheets used for heat-resistant applications. In particular, the previously mentioned turbocharger housing has a complex shape, and if it deforms excessively in a high-temperature environment, contact between parts and poor gas flow will occur, leading to damage and a decrease in thermal efficiency, which will lead to deterioration in part performance. lead to a decrease in the reliability of Therefore, in order to ensure the reliability of these, the following findings were obtained by diligently conducting microscopic research on the grain boundary structure of austenitic stainless steel.

First, the point that the corresponding grain boundary frequency is 80% or more will be described. In austenitic stainless steel, grain boundaries are formed after cold rolling and annealing, resulting in a polycrystalline body. As described above, when the grain boundary is a corresponding grain boundary, the grain boundary has a low energy structure with regular atomic arrangement and few gaps, so that the grain boundary deterioration phenomenon is less likely to occur. The Σ value of the corresponding lattice formed by two adjacent crystal grains is 3 to 29, and the corresponding grain boundary is formed in the corresponding lattice within this range. Also, the smaller the Σ value, the higher the corresponding lattice point density, and the stronger the character of the low-energy grain boundary.

The Σ3 grain boundary, which is the lowest Σ grain boundary among the corresponding grain boundaries, consists of annealing twins. Annealing twins are closely related to stacking fault energies that are generated during heat treatment and are caused by the steel composition. Therefore, from the viewpoint of preventing grain boundary deterioration, it is preferable that the ratio of corresponding grain boundaries, particularly the ratio of annealing twins, to the grain boundaries of the entire material is large.

The corresponding grain boundary frequency is determined by the ratio of the length of the corresponding grain boundary to the total length of the grain boundaries in the cross section of the material. Using EBSP (Electron Back-Scatering Diffraction pattern), crystal orientation analysis is performed on a region of about 300 μm thickness × about 100 μm width in the range of about 1/4 to 1/2 of the plate thickness from the center of the plate thickness of the material, and observed. The total length of grain boundaries and the length of corresponding grain boundaries are measured.

The higher the corresponding grain boundary frequency, the higher the proportion of grain boundaries with a low-energy structure with a regular atomic arrangement and few gaps. resulting in an improvement in The excellent heat resistance of the austenitic stainless steel sheet of the present invention is brought about by setting the corresponding grain boundary frequency to 80% or more.

Therefore, the corresponding grain boundary frequency of the austenitic stainless steel sheet manufactured by the manufacturing method specified in the present invention is 80% or more due to both the increase in the Σ3 grain boundary frequency and the increase in the corresponding grain boundary frequency other than the Σ3 grain boundary. ing. Therefore, the frequency of corresponding grain boundaries other than the Σ3 grain boundary in the austenitic stainless steel sheet of the present invention is preferably 10% to 25%, more preferably 12% to 20%.

In the present invention, the steel composition is specified in order to increase the corresponding grain boundary frequency. In the present application, by adjusting the composition so that the stacking fault energy (SFE) value obtained by the following formula (1) is 50 (mJm ⁻² ) or less, annealing twins are easily generated even in a short heat treatment. , it is possible to increase the corresponding grain boundary frequency. In order to lower the SFE value, it is effective to decrease Ni and C and increase Cr, N, Si and Mn. -50 or more. Further, in order to more effectively improve the creep property, which is the effect of the present invention, and in consideration of the oxidation property and the like, it is desirable to be 30 or less.
SFE (mJm ⁻² )=25.7+2×Ni+410×C−0.9×Cr−77×N−13×Si−1.2×Mn Formula (1)
However, the element symbol in the formula means the content (% by mass) of the element, and 0 is substituted when the element is not contained.

Next, the composition range of the austenitic stainless steel of the present invention will be explained.
The lower limit of C is set to 0.005% in order to form an austenite structure, ensure high-temperature strength and creep life. On the other hand, excessive addition causes hardening, deterioration of corrosion resistance, especially intergranular corrosion of welds due to the formation of Cr carbides, deterioration of high-temperature slidability due to carbides, and grain boundaries during pickling of cold-rolled annealed sheets. The formation of erosion grooves increases the surface roughness. Also, since C raises the stacking fault energy and lowers the frequency of annealing twins, the upper limit is made 0.300%. Furthermore, considering the manufacturing cost and hot workability, the C content is desirably 0.010% or more and 0.200% or less.

Si is sometimes added as a deoxidizing element, and internal oxidation of Si improves oxidation resistance and high-temperature slidability. % is added. On the other hand, addition of 4.00% or more results in hardening and formation of coarse Si-based oxides, which significantly lowers the processing accuracy of parts, so the upper limit is made 4.00%. Considering the production cost, hot workability during steel sheet production, pickling property, and solidification crack resistance during welding, the Si content is preferably more than 1.00% and 3.50% or less. From the standpoint of stacking fault energy, the lower limit is set to more than 1.50%, and considering high-temperature slidability and precipitation suppression, 2.00% or more and 3.00% or less is desirable.

Mn is used as a deoxidizing element, and is added in an amount of 0.10% or more in order to form an austenite structure and secure scale adhesion. On the other hand, if the addition exceeds 10.00%, the cleanness of inclusions is remarkably deteriorated, the hole expansibility is lowered, and the pickling property is remarkably deteriorated, resulting in a rough product surface. Moreover, in order to effectively lower the stacking fault energy, it is desirable to be 8.00% or less. Furthermore, considering the production cost, the pickling property during the steel plate production, and the oxidation property, the Mn content is preferably 0.80% or more and 5.00% or less.

Ni is an austenitic structure-forming element and an element that secures corrosion resistance and oxidation resistance. Also, if it is less than 2.00%, crystal grain coarsening occurs remarkably, so 2.00% or more is added. On the other hand, excessive addition causes an increase in cost and a decrease in the frequency of annealing twins, so the upper limit is made 25.00%. Also, in consideration of manufacturability, normal temperature ductility and corrosion resistance, it is desirable to be 5.00 to 20.00% or less in order to lower the stacking fault energy and effectively increase the corresponding grain boundary frequency. Furthermore, 13% or less is desirable from the aspect of cost.

Cr is an element that improves corrosion resistance, oxidation resistance, and high-temperature slidability, and is a necessary element from the viewpoint of suppressing abnormal oxidation in consideration of the environment of exhaust parts. In addition, 15.0% or more is required to sufficiently generate twin crystals. On the other hand, excessive addition causes hardness and deterioration of moldability, and also leads to an increase in cost, so the upper limit was made 30.0%. Furthermore, considering the manufacturing cost, steel sheet manufacturability and workability, the Cr content is desirably 17.0% or more and 25.5% or less.

N, like C, is an effective element for forming an austenitic structure and securing high-temperature strength, creep, and high-temperature slidability. N is known as a solid-solution strengthening element with respect to high-temperature strength, and is also effective for twinning. In the present application, considering the high-temperature strength due to cluster formation with Cr in addition to the effect of N alone, 0.005% or more is added. On the other hand, addition of 0.400% or more significantly hardens the room temperature material, degrades cold workability in the steel sheet manufacturing stage, and deteriorates formability and part accuracy during parts processing, so the upper limit is 0.400%. Less than From the viewpoint of softening, suppression of pinholes during welding, and suppression of intergranular corrosion of welded portions, the N content is preferably 0.020% or more and 0.350% or less. Furthermore, from the viewpoint of high-temperature strength, slidability, and room-temperature ductility, it is desirable that the content is more than 0.040% and less than 0.350%. Also, from the viewpoint of creep properties, it is desirable that the N content is more than 0.150% and less than 0.350%.

Al is added as a deoxidizing element to improve cleanliness of inclusions, thereby improving hole expansibility. In addition, it has the effect of suppressing the peeling of oxide scale and contributing to the improvement of high-temperature slidability due to a small amount of internal oxidation, and the action is manifested from 0.001%, so the lower limit is 0.001%. Further, since it is a ferrite-forming element, addition of 1.000% or more lowers the stability of the austenitic structure and causes an increase in surface roughness due to a decrease in pickling property, so the upper limit is 1%. Furthermore, considering refining costs and surface defects, the Al content is preferably 0.007% or more and 0.500% or less, and more preferably 0.010% or more and 0.100% or less from the viewpoint of weldability.

Cu is an element effective for stabilizing and softening the austenite phase, and is added in an amount of 0.05% or more. On the other hand, excessive addition leads to deterioration of oxidation resistance and productivity, so the upper limit is made 4.00%. In addition, in the steel of the present invention, if the content exceeds 4.00%, the frequency of annealing twins is lowered. Furthermore, considering corrosion resistance and manufacturability, the Cu content is desirably 0.30% or more and 1% or less.

Mo is an element that improves corrosion resistance and contributes to improvement of high-temperature strength. The improvement of high-temperature strength is mainly due to solid-solution strengthening, but since it is a precipitation-promoting element such as the σ phase, it also contributes to fine precipitation strengthening at the twin interface. In the present invention, the lower limit is set to 0.02% in order to utilize precipitation strengthening by Mo carbide in addition to solid solution strengthening. However, excessive addition lowers the frequency of annealing twins, so the upper limit is made 3.00%. Furthermore, considering that Mo is an expensive element, the strengthening stability due to the precipitates, and the cleanliness of inclusions, the Mo content is desirably 0.40% or more and 1.60% or less, and abnormal oxidation characteristics are improved. Taking this into account, it is more preferable to be 0.40% or more and 1.00% or less.

V is an element that improves corrosion resistance, promotes the formation of V carbide and σ phase, and is added in an amount of 0.02% or more to improve high-temperature strength. On the other hand, excessive addition causes an increase in alloy cost and a decrease in the abnormal oxidation limit temperature, so the upper limit is made 1.00%. Furthermore, considering manufacturability and cleanliness of inclusions, the V content is desirably 0.10% or more and 0.50% or less.

P is an impurity, and is an element that promotes hot workability and solidification cracking during production, and hardens to reduce ductility, so the lower the content, the better. 050%, lower limit of 0.010%. Furthermore, considering the manufacturing cost, the P content is desirably 0.020% or more and 0.040% or less.

S is an impurity, and is an element that lowers the hot workability during manufacturing and also deteriorates the corrosion resistance. In addition, when coarse sulfides (MnS) are formed, cleanliness is remarkably deteriorated and normal temperature ductility is deteriorated. On the other hand, excessive reduction leads to an increase in refining costs, so the lower limit may be 0.0001%. Furthermore, considering the manufacturing cost and oxidation resistance, the S content is desirably 0.0005% or more and 0.0050% or less.

The austenitic stainless steel sheet for exhaust parts of the invention may contain the following components in addition to the elements described above.

Ti is an element added to improve corrosion resistance and intergranular corrosion resistance by combining with C and N. Since the C and N fixing action is manifested from 0.005%, it may be added as necessary with the lower limit set to 0.005%. In addition, addition of more than 0.300% tends to cause nozzle clogging in the casting stage, which significantly deteriorates manufacturability and causes deterioration of ductility due to coarse Ti carbonitrides, so the upper limit is 0.3%. and Furthermore, considering the high-temperature strength, the intergranular corrosion resistance of the weld zone, and the alloy cost, the Ti content is desirably 0.010% or more and 0.200% or less. From the viewpoint of creep properties, the Ti content is preferably more than 0.030% and 0.300% or less.

Nb, like Ti, is an element that combines with C and N to improve corrosion resistance and intergranular corrosion resistance as well as high-temperature strength. In addition to the action of fixing C and N, high-temperature strength enhancement due to solute Nb and high-strength enhancement due to Laves phase twin crystal interface precipitation occur from 0.005%, so the lower limit is 0.005% and added as necessary You can Further, addition of more than 0.300% significantly deteriorates hot workability in the steel sheet manufacturing stage and causes deterioration of ductility due to coarse Nb carbonitrides, so the upper limit is made 0.300%. Furthermore, considering the high-temperature strength, the intergranular corrosion resistance of the weld zone, and the alloy cost, the Nb content is desirably 0.010% or more and 0.200% or less. From the viewpoint of creep properties, the Nb content is preferably more than 0.005% and 0.050% or less.

B is an element that improves hot workability in the steel sheet manufacturing stage, and may be added as necessary in an amount of 0.0002% or more. In addition, the segregation of B at the twin crystal interface also acts to increase the strength. However, excessive addition results in a decrease in cleanliness and ductility and intergranular corrosion due to the formation of borocarbides, so the upper limit was made 0.0050%. Furthermore, considering the refining cost and the reduction in ductility, the content of B is desirably 0.0003% or more and 0.0030% or less.

Ca is added as needed for desulfurization. Since this effect is not exhibited at less than 0.0005%, it may be added as necessary with the lower limit of 0.0005%. Further, if added in excess of 0.0100%, water-soluble inclusions CaS are formed, leading to a decrease in cleanliness and a significant decrease in corrosion resistance, so the upper limit is made 0.0100%. Furthermore, from the viewpoint of manufacturability and surface quality, the Ca content is preferably 0.0010% or more and 0.0030% or less.

W contributes to the improvement of corrosion resistance and high-temperature strength, so it may be added in an amount of 0.10% or more, if necessary. Addition of more than 3.00% leads to hardening, deterioration of toughness during steel sheet production, and an increase in cost, so the upper limit is made 3.00%. Furthermore, considering refining cost and manufacturability, the W content is preferably 0.10% to 2.00%, and more preferably 0.10% to 1.50% in consideration of abnormal oxidation characteristics.

Zr combines with C and N to improve the intergranular corrosion resistance and oxidation resistance of the weld zone, so 0.05% or more may be added as necessary. However, addition of more than 0.30% increases the cost and significantly deteriorates manufacturability and hole expandability, so the upper limit is made 0.30%. Furthermore, considering the refining cost and manufacturability, the Zr content is desirably 0.05% or more and 0.10% or less.

Sn contributes to the improvement of corrosion resistance and high-temperature strength, so it may be added in an amount of 0.01% or more, if necessary. The effect becomes remarkable at 0.03% or more, and becomes more remarkable at 0.05% or more. Addition of more than 0.50% may cause slab cracking during steel sheet production, so the upper limit is made 0.50%. Furthermore, considering the refining cost and manufacturability, the Sn content is desirably 0.05% or more and 0.30% or less.

Co contributes to the improvement of high-temperature strength, so 0.03% or more may be added as necessary. Addition of more than 0.30% leads to hardening, deterioration of toughness during steel sheet manufacturing, and an increase in cost, so the upper limit is made 0.30%. Furthermore, considering the refining cost and manufacturability, the Co content is desirably 0.03% or more and 0.10% or less.

Mg is sometimes added as a deoxidizing element, and is an element that contributes to improving the cleanliness of inclusions and refining the structure of the slab by refining and dispersing oxides. Since this is manifested from 0.0002% or more, it may be added as necessary with the lower limit of 0.0002%. However, excessive addition leads to deterioration of weldability and corrosion resistance, and deterioration of hole expansibility due to coarse inclusions, so the upper limit was made 0.0100%. Considering the refining cost, the Mg content is desirably 0.0003% or more and 0.0050% or less.

Sb is an element that segregates at grain boundaries to increase high-temperature strength. In order to obtain the effect of addition, it may be added in an amount of 0.005% or more, if necessary. However, if it exceeds 0.300%, Sb segregation occurs and cracks occur during welding, so the upper limit is made 0.300%. Considering high-temperature properties, manufacturing costs, and toughness, the Sb content is preferably 0.030% or more and 0.300% or less, more preferably 0.050% or more and 0.200% or less.

REM (rare earth element) is effective in improving oxidation resistance and high-temperature slidability, and may be added in an amount of 0.002% or more, if necessary. Further, even if it is added in excess of 0.200%, the effect is saturated, and the corrosion resistance is lowered due to REM granules. Considering the workability and manufacturing cost of the product, it is desirable to set the lower limit to 0.002% and the upper limit to 0.10%. In addition, REM (rare earth element) follows a general definition. A general term for two elements, scandium (Sc) and yttrium (Y), and fifteen elements (lanthanoids) from lanthanum (La) to lutetium (Lu). They may be added singly or as a mixture.

Ga may be added at 0.3000% or less as necessary in order to improve corrosion resistance and suppress hydrogen embrittlement. . From the viewpoint of formation of sulfides and hydrides, the lower limit is made 0.0002%. Furthermore, 0.0020% or more is more preferable from the viewpoint of manufacturability and cost.

Other components are not particularly specified in the present invention, but Ta and Hf may be added in an amount of 0.01% or more and 1.00% or less in order to improve high-temperature strength. Also, Bi may be contained in an amount of 0.001 to 0.020%, if necessary. In addition, it is desirable to reduce general harmful elements such as As and Pb and impurity elements as much as possible.

Next, a manufacturing method will be described. The method for producing the steel sheet of the present invention comprises steelmaking-hot rolling-annealing/pickling-cold rolling-annealing/pickling-cold rolling-annealing/pickling. Annealing may be omitted.

In steelmaking, a method of smelting steel containing the above essential components and optionally added components in an electric furnace or a converter, followed by secondary refining is preferable. The melted molten steel is made into a slab according to a known casting method (continuous casting), and the slab is heated to a predetermined temperature according to a known hot rolling method and continuously hot rolled to a predetermined thickness. be. As described above, manufacturing conditions for ensuring a predetermined crystal grain size, cross-sectional hardness, and surface roughness are set according to known methods in the processes after hot rolling for the parts to which the present invention is applied. In order to set the corresponding grain boundary frequency to 80%, the following manufacturing conditions are defined.

After hot rolling or after annealing and pickling after the hot rolling process, in the present application, two cold rolling and annealing and pickling processes are applied. First, in the first cold rolling step, the rolling reduction is set to 80% or less. This is because when the rolling reduction in this step exceeds 80%, grain growth is suppressed in subsequent recrystallization, and the corresponding grain boundary frequency of corresponding grain boundaries with a Σ value of 3 to 29 becomes difficult to increase. Further, if the rolling reduction is too low, recrystallization becomes difficult to occur, so the rolling reduction is preferably 3% or more. Furthermore, 70 to 80% is desirable considering the material and plate shape.

Next, in the first annealing/pickling process, the heating rate up to 900°C and above 900°C is increased in order to increase the corresponding grain boundary frequency after the second cold-rolling/annealing/pickling process, especially the annealing twins. The annealing temperature is set to 1000 to 1200°C at 10°C/sec or more. By setting the entire temperature range to a high heating rate, it is possible to suppress the formation of the σ phase and Cr carbonitrides that precipitate at low temperatures, and to suppress excessive grain growth at high temperatures. Precipitates formed at low temperatures particularly inhibit the formation of corresponding grain boundaries during the second cold rolling and annealing. In order to promote the formation of corresponding grain boundaries during the second annealing, it is desirable that the crystal grains after the first cold rolling, annealing and pickling are fine. From the above technical point of view, the rate is set to 10° C./sec or more. From the viewpoint of manufacturability, 100° C./sec or less is desirable.

This is because if the annealing temperature exceeds 1200° C., crystal grain growth occurs excessively, resulting in problems such as material deterioration and orange peel during processing. In the case of turbo parts, if orange peel occurs, high-temperature slidability and flow of exhaust gas deteriorate, so the upper limit is made up to 1200°C. On the other hand, if the annealing temperature is less than 1000°C, poor recrystallization occurs, so the lower limit temperature was set to 1000°C. Furthermore, considering the stability of the material, 1100 to 1180°C is desirable.

In the second cold rolling step, the cold rolling reduction is set to 10% or less. This is because when the rolling reduction exceeds 10%, recrystallization proceeds in the subsequent annealing step and new random grain boundaries are formed, thereby reducing the frequency of corresponding grain boundaries. Since an excessive reduction in the rolling reduction results in a defective steel sheet shape, the lower limit of the rolling reduction is preferably 1% or more. Further, 3 to 7% is desirable in consideration of manufacturability and formation of annealing twins.

In the second annealing and pickling process, in order to increase the corresponding grain boundary frequency, especially annealing twins, the heating rate to below 900 ° C. is 10 ° C./sec or more, the heating rate to 900 ° C. or more is 1 ° C./sec or more, Heat treatment is performed at a maximum temperature of 950 to 1150° C. for 20 minutes or less at a rate of less than 10° C./sec.

A high heating rate is used in the temperature range up to 900° C. to suppress the formation of the σ phase and Cr carbonitrides that precipitate at low temperatures. This is because these precipitates inhibit the formation of corresponding grain boundaries and become nuclei for newly generated random grain boundaries. From the above technical point of view, the rate is set to 10° C./sec or more. From the viewpoint of manufacturability, 100° C./sec or less is desirable.

On the other hand, the heating rate is set to be low at 900° C. or higher. This is for the purpose of softening in a temperature range where recrystallization is not promoted, promoting the movement of random grain boundaries, and sufficiently dissolving the precipitates. As a result, grain boundary migration can be promoted while suppressing nucleation of random grain boundaries, and the corresponding grain boundary frequency can be increased. From the above technical point of view, the rate should be 1° C./sec or more and less than 10° C./sec. 5° C./sec or less is desirable. The above range is appropriate also from the viewpoint of manufacturability.

If the maximum annealing temperature is excessively high, a large number of recrystallized grain boundaries having new random grain boundaries are formed, and the corresponding grain boundary frequency is reduced. From the viewpoint of formability of the material, the temperature is preferably 1030° C. or higher, and 1100° C. or lower is desirable for suppressing excessive grain growth. The holding time at the maximum temperature is set to 20 minutes or less in order to promote movement of random grain boundaries due to grain growth and increase the frequency of corresponding grain boundaries. This is because if the time exceeds 20 minutes, excessive grain growth occurs, which causes material deterioration and orange peel, and also accelerates the movement of corresponding grain boundaries. Moreover, from the viewpoint of productivity, 20 sec or more is desirable. Furthermore, it is desirable to set the time to 30 seconds or more in consideration of the improvement of the quality of the material due to the softening.

In the present application, the hot-rolled sheet is annealed and pickled, then cold-rolled, and then the cold-rolled sheet is annealed and pickled to obtain a smoother surface. The cold rolling process may be performed by tandem rolling, Sendzimir rolling, cluster rolling, or the like. 2B or 2D products are generally applied to functional applications such as automobile exhaust parts, but if high surface smoothness and gloss are required, bright annealing is applied after cold rolling to produce BA products. Also good. pickling treatment. Pretreatment such as neutral salt electrolysis or molten alkali treatment, or pickling treatment such as nitric hydrofluoric acid or nitric acid electrolysis may be appropriately selected.

Steels having the chemical compositions shown in Tables 1-1 and 1-2 are melted, cast into slabs, hot-rolled, hot-rolled sheet annealing and pickled, and then shown in Tables 2-1 to 2-3. Cold rolling and final annealing were performed under the conditions, and pickling was further performed to obtain a product sheet with a thickness of 2.0 mm. The values in the columns marked with a symbol "*" in Tables 1-1 and 1-2 indicate that the corresponding component does not satisfy the requirements of the present invention.

For each product sheet shown in Tables 2-1 to 2-3, the corresponding grain boundary frequency (%) was measured by the method described above, and a creep test was performed at 900°C and 950°C. Here, the creep test was carried out by cutting out a test piece so that a load acts in the rolling direction of the product sheet, and performing a constant load creep test at 900° C.-20 MPa and 950° C.-15 MPa with a creep tester equipped with a heating furnace. At this time, the no-load time was set to 1 hour at the test temperature, and the load was applied after 1 hour. Also, a test piece was cut out so that tension acts in the rolling direction of the product sheet (JIS No. 13 B test piece), and a tensile test was performed at room temperature to determine the elongation at break. In addition, the item "Σ3 to Σ29" in the table indicates the total (%) of corresponding grain boundary frequencies of corresponding grain boundaries with a Σ value of 3 to 29, and the item "Σ5 to Σ29" indicates that the Σ value is from 5 to The sum (%) of corresponding grain boundary frequencies of up to 29 corresponding grain boundaries is shown.

The values marked with a symbol “*” in the “annealing twinning frequency (%)” column in Tables 2-1 to 2-3 do not satisfy the requirements for the corresponding grain boundary frequency in the present invention. show. In addition, materials marked with a symbol “x” in the column “creep test” in Tables 2-1 to 2-3 indicate that the rupture life was less than 100 hours. Further, the materials marked with a symbol "x" in the column "elongation at break" in Tables 2-1 to 2-3 indicate that the elongation at break was less than 30%.

It is confirmed that the steel manufactured under the composition and manufacturing conditions specified in the present invention has a long creep rupture life and is extremely excellent in heat resistance. In addition, it has high elongation at break and excellent moldability. On the other hand, as shown in Tables 2-1 to 2-3, the comparative steels have a short creep rupture life. It can be seen that the comparative steel, which has a long creep rupture life, is also unsuitable for application as a heat-resistant part because its formability is extremely poor.

For example, the thickness of the slab, the thickness of the hot-rolled plate, etc. may be appropriately designed. In cold rolling, the roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected. Intermediate annealing may be performed during cold rolling, and batch annealing or continuous annealing may be used. In addition, as a pretreatment for pickling, either neutral salt electrolytic treatment or salt bath immersion treatment may be performed or omitted. may be used for processing. After the cold-rolled sheet is annealed and pickled, the shape and material may be adjusted by temper rolling, a tension leveler, or the like. Further, the product sheet may be coated with a lubricating coating to further improve press forming, and the type of lubricating film may be appropriately selected. In addition, special surface treatment such as nitriding treatment or carburizing treatment may be applied after the parts are processed to further improve the heat resistance.

According to the present invention, it is possible to provide an austenitic stainless steel sheet having excellent properties for exhaust parts requiring heat resistance. By using the material to which the present invention is applied, especially for automotive exhaust manifolds and turbochargers, it is possible to significantly reduce weight compared to conventional castings, leading to exhaust gas regulations, weight reduction, and fuel efficiency improvement. Become. In addition, it becomes possible to omit cutting and grinding of parts, and omit surface treatment, which greatly contributes to cost reduction. Note that the present invention can be applied to any of these parts when used for a turbocharger. Specifically, the housing that makes up the outer frame of the turbocharger, precision parts inside the nozzle vane type turbocharger (for example, back plate, oil deflector, compressor wheel, nozzle mount, nozzle plate, nozzle vane, drive ring, drive lever) etc.). Also, in the case of the exhaust manifold, any one of plate press parts, pipe parts, and double pipe parts may be used. Furthermore, the present invention is industrially very useful because it can be applied not only to automobiles and motorcycles, but also to various types of boilers, fuel cell systems, and exhaust parts used in high-temperature environments such as plants.

Claims (4)

% by mass, C: 0.005 to 0.300%, Si: more than 1.00 to 4.00%, Mn: 0.10 to 10.00%, Ni: 2.00 to 25.00%, Cr : 15.0-30.0%, N: 0.005-0.400%, Al: 0.001-1.000%, Cu: 0.05-4.00%, Mo: 0.02- 3.00%, V: 0.02 to 1.00%, P: 0.050% or less, S: 0.0100% or less, the balance being Fe and inevitable impurities,
The value of the following (1) formula is 50 or less,
The corresponding grain boundary frequency of corresponding grain boundaries with a Σ value of 3 to 29 is 80% or more in total, and the corresponding grain boundary frequency of corresponding grain boundaries with a Σ value of 5 to 29 is 10% to 25% in total. An austenitic stainless steel sheet, characterized by:
25.7+2×Ni+410×C−0.9×Cr−77×N−13×Si−1.2×Mn (1)
However, the element symbol in the formula means the content (% by mass) of the element, and 0 is substituted when the element is not contained. The steel sheet further contains Ti: 0.005-0.300%, Nb: 0.005-0.300%, B: 0.0002-0.0050%, Ca: 0.0005-0. 0100%, W: 0.10-3.00%, Zr: 0.05-0.30%, Sn: 0.01-0.50%, Co: 0.03-0.30%, Mg: 0 .0002-0.0100%, Sb: 0.005-0.300%, REM: 0.002-0.200%, Ga: 0.0002-0.3000%, Ta: 0.01-1.00 %, Hf: 0.01 to 1.00%, and Bi: 0.001 to 0.020%. 3. The austenitic stainless steel sheet according to claim 1, wherein the austenitic stainless steel sheet is used for exhaust parts. A method for manufacturing a stainless steel plate according to claim 1 or 2,
Rolled at a reduction rate of 80% or less in the cold rolling process, and then annealed at a heating rate of 10 ° C./sec or more and a temperature of 1000 to 1200 ° C. in the subsequent cold-rolled sheet annealing, and then cold rolled at a reduction rate of 10% or less. In the subsequent cold-rolled sheet annealing, the heating rate to less than 900°C is set to 10°C/sec or more, the heating rate to 900°C or more is set to 1°C/sec or more and less than 10°C/sec, and the temperature is 950 to 1150°C for 20 minutes or less. A method for producing an austenitic stainless steel sheet, characterized in that cold-rolled sheet annealing is performed.