JP6093210B2 - Heat-resistant ferritic stainless steel sheet with excellent low-temperature toughness and method for producing the same - Google Patents

Description

  The present invention relates to a heat-resistant ferritic stainless steel sheet excellent in low-temperature toughness after processing, which is optimal for use in automobile exhaust system members that particularly require high-temperature strength and oxidation resistance, and a method for producing the same.

  High-temperature strength and oxidation resistance are required for exhaust system members such as automobile exhaust manifolds, converters, front pipes, and mufflers, and heat-resistant steel containing Cr is used. When these members are manufactured from a steel plate by press working, since the steel plate is formed into a predetermined shape after the steel plate is formed, workability of the raw steel plate is required. On the other hand, the use environment temperature has been increasing year by year, and it has become necessary to increase the addition amount of alloys such as Cr, Mo and Nb to increase the high temperature strength and thermal fatigue characteristics. When the additive element increases, there is a problem that the press formability of the raw steel plate and the workability after pipe forming are lowered, and the low temperature toughness after processing is deteriorated. Deterioration of low-temperature toughness after processing causes brittle cracks during parts transportation, assembly, and use, especially in cold regions, which is a major problem. Therefore, a heat-resistant ferritic stainless steel sheet with excellent low-temperature toughness is desired. Yes.

  In order to solve the problems related to the low temperature toughness of the heat-resistant ferritic stainless steel sheet, contrivances with steel components have been made.

  Patent Document 1 discloses a ferritic stainless steel containing 1.0 to 2.5% Cu and 0.2 to 1.2% Al and excellent in heat resistance and toughness. Patent Document 2 discloses a heat-resistant and oxidation-resistant ferritic stainless steel excellent in workability and toughness containing Al of 0.5 to 3.5%. Patent Document 3 and Patent Document 4 disclose ferritic stainless steel for exhaust gas passage members having excellent low-temperature toughness containing 0.8 to 2.0% and more than 1 to 2% of Cu, respectively. Patent Document 5 and Patent Document 6 disclose steel components in which Cu is 0.1% or more and Mn / S is defined. All of these evaluate the toughness of the product plate (cold-rolled annealed plate) by Charpy impact test, but the low temperature toughness becomes a problem in actual parts. It is a brittle crack that occurs after molding, and conventional knowledge alone is insufficient.

  Moreover, about the improvement of the low temperature toughness by a steel plate manufacturing method, it is cooled at 30 degrees C / sec or more by making the temperature range of 850-700 degreeC into an average cooling rate after hot rolling in patent document 7, and after 850 after annealing a cold-rolled sheet. A manufacturing technique of a ferritic stainless steel sheet excellent in secondary work brittleness resistance that is cooled at 30 ° C./sec or more with a cooling rate between 600 ° C. is disclosed. In addition, Patent Document 8 discloses that the temperature range of 820 ° C. to 500 ° C. in hot-rolled sheet annealing is 15 ° C./sec or more, and the average cooling rate to 500 ° C. in cold-rolled sheet annealing is 15 ° C./sec or more. A technique relating to a method for producing a ferritic stainless steel sheet excellent in secondary workability is disclosed. In addition, although not related to low temperature brittleness, Patent Document 9 discloses a cooling rate during hot-rolled sheet annealing of 5 ° C./sec or less, and Patent Document 10 discloses a cooling rate during hot-rolled sheet annealing of 50 ° C./sec or less. A technique relating to the manufacturing method is disclosed.

JP 2009-235573 A Japanese Patent Publication No.57-4699 JP 2008-297631 A JP 2009-120893 A Japanese Patent No. 3219099 Japanese Patent No. 2696584 Japanese Patent No. 3142427 (Japanese Patent Laid-Open No. 7-126812) Japanese Patent No. 4265751 (Japanese Patent Laid-Open No. 2004-323957) JP 2011-149101 A JP-A-4-224634

  An object of the present invention is to solve the problems of known techniques and to provide a heat-resistant ferritic stainless steel sheet having excellent low-temperature toughness after forming.

  Conventional knowledge on steel sheet manufacturing may specify the cooling rate of the hot-rolled sheet annealing and cold-rolled sheet annealing cooling process, but controls the growth of precipitates during hot-rolled sheet annealing to improve low-temperature toughness. For this reason, there is no example in which the influence of the heating rate is examined.

  In order to solve the above-mentioned problems, the present inventors conducted detailed studies on the steel composition, the structure in the manufacturing process, and the crystal orientation formation regarding the workability of the heat-resistant ferritic stainless steel sheet and the low-temperature toughness after processing.

The gist of the present invention for solving the above problems is as follows.
(1) In mass%, C: 0.001 to 0.02%, Si: 0.1 to 0.5%, Mn: more than 0.1% to less than 1.5%, P: 0.01 to 0.04%, S: 0.0001-0.01%, Cr: 13-20%, N: 0.001-0.03%, Nb: 0.1-0.6%, Mo: 0.2 -3%, Ti: 0.001-0.3%, B: 0.0002-0.005%, Al: 0.003-0.5% or less, with the balance consisting of Fe and inevitable impurities , {111} <112> azimuth strength and {111} <011> azimuth strength is 4 or more, ratio of 0.2% proof stress (YP) and average r value (rm) at normal temperature (YP / rm) ) Is 350 or less, a heat resistant ferritic stainless steel sheet excellent in low temperature toughness.
(2) V: 0.05 to 1.0%, Cu: 0.01 to 2.0%, Ni: 0.1 to 2.0%, W: 0.1 to 3.0%, Zr: 0 0.05 to 0.30%, Sn: 0.05 to 0.50%, Co: 0.05 to 0.50%, Mg: 0.0002 to 0.0100% (1) A heat-resistant ferritic stainless steel sheet having excellent low-temperature toughness.
(3) After hot-rolling the slab having the component composition described in (1) or (2), the temperature is raised from 700 to 900 ° C. at 5 ° C./sec or more during heating, and then kept in the temperature range of 900 to 980 ° C. for 30 sec or more. (1) or (2), characterized in that after hot-rolled sheet annealing is performed to 400 ° C. at a rate of 10 ° C./sec or more, the sheet is cold-rolled to a predetermined thickness and then heated to 1000 to 1100 ° C. The manufacturing method of the heat-resistant ferritic stainless steel plate excellent in low-temperature toughness of description .

  According to the present invention, a heat-resistant ferritic stainless steel sheet excellent in low-temperature toughness of a processed product can be efficiently provided without requiring special new equipment.

It is a figure which shows the influence which the ratio of {111} <011> azimuth | direction intensity | strength and {111} <112> azimuth | direction intensity | strength and the ratio (YP / rm) of YP and rm in normal temperature exerts on the low temperature toughness of a workpiece.

  The reason for limitation of the present invention will be described below. Unless otherwise specified,% means mass%.

  Since C deteriorates toughness, workability, corrosion resistance and oxidation resistance, the lower the content, the better. Therefore, the upper limit was made 0.02%. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.001%. Furthermore, if considering the manufacturing cost and corrosion resistance, 0.002 to 0.01% is desirable.

  In addition to being added as a deoxidizing element, Si is an element that improves oxidation resistance and high-temperature strength. Further, since it is an element that promotes precipitation of the Laves phase, a coarse Laves phase is precipitated during hot-rolled sheet annealing by addition of 0.1% or more, and the development of {111} -oriented grains during cold-rolled sheet annealing, r Contributes to improved value. On the other hand, excessive addition reduces normal temperature ductility and low temperature toughness, so the upper limit was made 0.5%. Furthermore, if considering the material and oxidation characteristics, 0.15 to 0.4% is desirable.

Mn forms MnCr ₂ O ₄ and MnO at a high temperature and improves scale adhesion. Since this effect appears at more than 0.1%, the lower limit was made more than 0.1%. On the other hand, since the increase in oxidation is increased, abnormal oxidation is likely to occur when 1.5% or more is added. In the exhaust gas component, when scale peeling or abnormal oxidation occurs, a failure occurs in a subsequent component, or the reliability as a structure decreases due to a reduction in the plate thickness. Furthermore, if considering workability and manufacturability, 0.6 to 1.1% is desirable.

  Since P is a solid solution strengthening element like Si, its content is preferably as small as possible from the viewpoint of material and toughness, and the upper limit is made 0.04%. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.01%. Furthermore, if considering the manufacturing cost and oxidation resistance, 0.015 to 0.025% is desirable.

  The lower the S content, the better from the viewpoint of material, corrosion resistance and oxidation resistance, so the upper limit was made 0.01%. In particular, excessive addition generates Ti and a compound, and recrystallization and grain growth of the hot-rolled annealed plate are promoted too much to degrade the r value, and sulfide becomes a fracture starting point and lowers toughness. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.0001%. Furthermore, if considering the manufacturing cost and corrosion resistance, 0.003 to 0.0050% is desirable.

  Cr needs to be added in an amount of 13% or more in order to improve the high-temperature strength and the oxidation resistance. However, the addition of 20% or more deteriorates the manufacturability due to the deterioration of toughness and also deteriorates the material. Therefore, the Cr range is set to 13 to 20%. Furthermore, 15 to 18% is desirable from the viewpoint of cost and corrosion resistance.

  N, like C, deteriorates low-temperature toughness, workability, and oxidation resistance. The lower the content, the better. Therefore, the upper limit was made 0.03%. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.001%. Furthermore, if considering cost and toughness, 0.005 to 0.02% is desirable.

  Nb is an essential element in order to improve high temperature strength and high temperature fatigue characteristics by solid solution strengthening and precipitation strengthening. In addition, C and N are fixed as carbonitrides, and the recrystallized texture of the product plate is developed, and an intermetallic compound of Fe and Nb called a Laves phase is formed, and the recrystallized texture is determined depending on the volume ratio and size. It affects the formation and contributes to the improvement of r value and toughness. Since these effects are manifested at 0.1% or more, the lower limit was made 0.1%. On the other hand, excessive addition leads to hardening, leading to a decrease in room temperature ductility and a decrease in toughness due to the generation of a large amount of precipitates, so the upper limit was made 0.6%. Furthermore, if considering cost and manufacturability, 0.3 to 0.55% is desirable.

  Mo improves the corrosion resistance and improves the high-temperature strength and thermal fatigue properties due to the solid solution Mo. Since this effect appears at 0.2% or more, the lower limit was set to 0.2%. However, excessive addition causes toughness deterioration and elongation reduction. In addition, the Laves phase is formed too much, and {011} oriented grains are easily formed, resulting in a decrease in the r value. In addition, the oxidation resistance deteriorates when added over 3%, so the upper limit was made 3%. Furthermore, considering the manufacturing cost and manufacturability, the high temperature characteristics after being exposed to a high temperature for a long time, particularly high temperature strength and high temperature high cycle fatigue characteristics, is preferably 1.5 to 1.9%.

  Ti is an element added to combine with C, N, and S to further improve the corrosion resistance, intergranular corrosion resistance, and deep drawability. In particular, the development of {111} crystal orientation that improves the r value is manifested by addition of 0.001% or more, so the lower limit was made 0.001%. However, excessive addition causes coarse TiN to precipitate and lowers the low temperature toughness, so the upper limit was made 0.3%. Furthermore, if considering the production cost, low temperature toughness, surface flaws and scale peelability, 0.05 to 0.15% is desirable.

B is an element that improves the grain boundary strength by segregating at the grain boundaries and improves the secondary workability and the low temperature toughness, and improves the high temperature strength in the intermediate temperature range. Since these effects are manifested at 0.0002% or more, the lower limit was made 0.0002%. Addition of more than 0.005% produces B compounds such as Cr ₂ B, which degrades intergranular corrosion and fatigue characteristics, and also causes an increase in {011} oriented grains and lowers the r value, so the upper limit is set. 0.005%. Furthermore, considering weldability and manufacturability, 0.0003 to 0.001% is desirable.

  Al may be added as a deoxidizing element and improves high-temperature strength and oxidation resistance. Moreover, it becomes a precipitation site of TiN or a Laves phase, contributes to fine precipitation, and contributes to improvement of low temperature toughness. Since the effect is manifested from 0.003%, the lower limit was made 0.003%. Further, addition of 0.50% or more brings about a decrease in elongation, a deterioration in weldability and surface quality, and a decrease in low temperature toughness due to the formation of coarse Al oxide, and the generation of {011} oriented grains is promoted. In order to bring about a decrease in value, the upper limit was made 0.5%. Furthermore, 0.01 to 0.1% considering the refining cost is desirable.

  The present invention may further contain the following elements as necessary.

  V may be added in an amount of 0.05% or more as necessary from the viewpoint of corrosion resistance and heat resistance by combining with C and N. However, the addition of 1.0% or more reduces the toughness by forming coarse carbonitrides and leads to an increase in cost, so the upper limit was made 1.0%. Furthermore, if manufacturability is considered, 0.06 to 0.2% is desirable.

  Cu is an element that improves the corrosion resistance and raises the high-temperature strength particularly in the middle temperature region by ε-Cu precipitation, and is added as necessary. This effect is manifested by the addition of 0.01% or more, so the lower limit was made 0.01%. On the other hand, excessive addition causes toughness reduction and ductility reduction by hardening, so the upper limit was made 2.0%. Furthermore, if considering oxidation resistance and manufacturability, 0.01 to less than 0.1% is desirable.

  Ni is an element that improves toughness and corrosion resistance, so it is added as necessary. Since the contribution to toughness is manifested at 0.1% or more, the lower limit was made 0.1%. On the other hand, an austenite phase is produced by addition of more than 2.0%, and the upper limit is made 2.0% in order to lower the r value. Furthermore, if considering the cost, 0.1 to 0.5% is desirable.

  W is an element that is added as necessary to increase the high-temperature strength. Since its action is manifested from 0.1%, the lower limit was made 0.1%. However, excessive addition causes toughness deterioration and elongation reduction. In addition, the upper limit is set to 3.0% in order that the Laves phase is generated too much and {011} oriented grains are easily generated and the r value is lowered. Furthermore, if considering the manufacturing cost and manufacturability, 0.1 to 2.0% is desirable.

  Zr is an element that improves oxidation resistance and is added as necessary. The effect is manifested at 0.05% or more, so the lower limit was made 0.05%. However, addition of 0.30% or more significantly deteriorates the manufacturability such as toughness and pickling properties, and the compound of Zr, carbon, and nitrogen is coarsened to coarsen the hot-rolled annealed plate structure to reduce the low r Therefore, the upper limit was made 0.30%. Furthermore, if considering the manufacturing cost, 0.05 to 0.20% is desirable.

  Sn is an element that is added as necessary in order to segregate at the grain boundaries and increase the high-temperature strength. Since its action is manifested from 0.05%, the lower limit was made 0.05%. However, Sn segregation occurs due to the addition of more than 0.5%, and {011} -oriented grains are generated in the segregation part to lower the r value and lower the low temperature toughness. Therefore, the upper limit was made 0.5%. Furthermore, if considering the high temperature characteristics, production cost and toughness, 0.10 to 0.30% is desirable.

  Co is an element that improves the high-temperature strength, and is added in an amount of 0.05% or more as necessary. However, excessive addition deteriorates toughness and workability, so the upper limit was made 0.50%. Furthermore, if considering the manufacturing cost, 0.05 to 0.30% is desirable.

  Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizer, and finely crystallized Mg oxide serves as a nucleus, and Nb and Ti-based precipitates are finely precipitated. When these are finely precipitated in the hot rolling process, in the hot rolling process and hot-rolled sheet annealing process, the fine precipitates become recrystallization nuclei and a very fine recrystallized structure is obtained, contributing to the development of the texture and toughness. Contributes to improvement. Since this effect appears from 0.0002%, the lower limit was made 0.0002%. However, excessive addition causes deterioration of oxidation resistance, decrease in weldability, etc., so the upper limit was made 0.0100%. Furthermore, considering refining costs, 0.0003 to 0.0020% is desirable.

  It is well known that ductile fracture and brittle fracture occur due to the temperature dependence of material strength (0.2% yield strength) and brittle fracture strength, and that low temperature toughness is improved by lowering the material strength (low yield strength). It is a fact. This concept is applied to the low temperature toughness of product plates and steel pipes, and is effective when it is necessary to improve the Charpy impact value of the product plate as in the above-mentioned conventional knowledge. However, the purpose of the present invention is to improve low-temperature toughness after forming a product plate or steel pipe, and the toughness after forming is not sufficient only by adjusting the strength (proof strength) of the material. Therefore, as a result of detailed research on the material of the steel plate, the crystal orientation distribution, and the low temperature toughness of the molded product used as the material for the exhaust parts, the following was found. Since the steel sheet is polycrystalline, each crystal grain has various crystal orientations. In the case of a ferritic stainless steel plate that is a body-centered cubic metal, the crystal orientation after cold rolling annealing is mainly {111} crystal grains. Of these, the {111} <011> orientation and the {111} <112> orientation are mainly used. However, in the case of heat-resistant steel, the amount of Nb or the like is large, and the manufacturing process is caused by the thick plate thickness. Since the cold rolling reduction rate cannot be ensured in these materials, the development of these crystal orientations is unlikely to occur in thick materials having a thickness of 1.2 mm or more. However, in the present invention, it has been found that the sum of both affects the low temperature toughness after processing. In addition, the 0.2% proof stress (hereinafter referred to as YP, unit: MPa) at room temperature in the raw material stage before processing also affects the low temperature toughness after processing, and the higher the YP, the smaller the deformability after processing, and the remaining of the processed product. Since the stress increases, the low temperature toughness deteriorates. Furthermore, the deformability after processing is not determined only by YP, and the average r value (hereinafter referred to as rm) of the material, which is an index of the deep drawability of the material, also affects the material. The deformability of the product increases. That is, low YP and high rm as the material material have an advantageous effect on the low temperature toughness after processing, and the ratio of these (the ratio of the YP measurement value to the rm measurement value is defined as YP / rm) is lower. It was found that it is effective in reducing residual stress after processing and ensuring impact deformability.

FIG. 1 shows the results of examining the influence of the sum of {111} <011> orientation strength and {111} <112> orientation strength, and the ratio of YP and rm at normal temperature (YP / rm) on the low temperature toughness of the processed product. is there. Here, various steel components were melted in a vacuum and subjected to hot rolling, cold rolling, and annealing to obtain a 2.5 mm thick cold rolled annealing plate. The {100} <012> azimuth strength is obtained by using an X-ray diffractometer (manufactured by Rigaku Denki Kogyo Co., Ltd.) and using Mo-Kα rays, in the vicinity of the plate thickness center layer (combination of mechanical polishing and electropolishing) (200), (310), and (211) positive pole figure of (200), (311), and (211) are obtained, and a three-dimensional crystal orientation density function is obtained from these using the spherical harmonic function method. (Intensity ratio with the random sample) was obtained, and the sum of {111} <011> azimuth intensity and {111} <112> azimuth intensity was obtained. Moreover, a JIS No. 13 B tensile test piece was collected so that the rolling direction and the tensile direction were parallel, and 0.2% yield strength was obtained in accordance with JIS Z2241. Further, the r value is obtained by collecting JIS No. 13 B tensile test pieces from cold-rolled annealed plates and applying a 14.4% strain in the rolling direction, the rolling direction and 45 ° direction, and the rolling direction and 90 ° direction (1). The average r value was calculated using the equation and the equation (2).
r = ln (W ₀ / W) / ln (t ₀ / t) (1)
Here, W ₀ is the plate width before tension, W is the plate width after tension, t ₀ is the plate thickness before tension, and t is the plate thickness after tension.
rm = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 (2)
Here, r ₀ is the r value in the rolling direction, r ₄₅ is the r value in the rolling direction and the 45 ° direction, and r ₉₀ is the r value in the direction perpendicular to the rolling direction. About low temperature toughness, the low temperature drop test was done with respect to the cylindrical drawing goods, and it evaluated by the presence or absence of a crack. A disc of 100 mmφ was collected from the product plate, and a cup molded product was obtained by cylindrical drawing (wrinkle restraining force: 10 ton, punch: 50 mmφ × 8 mmr, die: 55.7 mmφ × 18 mmr). After holding this molded product at various temperatures below room temperature, a weight (12 kf) was dropped from the height of 1 m on the side surface, and the presence or absence of cracks in the molded product was visually observed. In FIG. 1, the case where an impact crack occurs at −40 ° C. is indicated by x, and the case where no crack occurs is indicated by ◯. Ordinary exhaust parts are sufficient if they do not crack under impact at -40 ° C. From this result, when the sum of {111} <112> orientation strength and {111} <011> orientation strength is 4 or more and YP / rm is 350 or less, impact cracking does not occur at −40 ° C., that is, cracking It was revealed that the generation temperature was less than −40 ° C.

  The sum of the above {111} <112> azimuth strength and {111} <011> azimuth strength is 4 or more, and the ratio between the 0.2% yield strength (YP) and the average r value (rm) at room temperature (YP / In order to obtain a stainless steel sheet having a rm) of 350 or less, in addition to the steel components of the present invention, it is important to control the structure by optimizing hot-rolled sheet annealing conditions and cold-rolled sheet annealing in steel sheet production.

  In the present invention, after hot-rolling a slab having the component composition defined in the present invention, after heating at 700 to 900 ° C. at 5 ° C./sec or more during heating, after maintaining in the temperature range of 900 to 980 ° C. for 30 sec or more , By performing hot-rolled sheet annealing that is cooled to 400 ° C. at 10 ° C./sec or more, and after cold rolling to a predetermined thickness, heating to 1000 to 1100 ° C., even if the thickness is 1.2 mm or more, The sum of {111} <112> azimuth strength and {111} <011> azimuth strength is 4 or more, and the ratio (YP / rm) of 0.2% proof stress (YP) and average r value (rm) at room temperature A stainless steel plate having a thickness of 350 or less can be obtained. This will be described in detail below.

  A hot-rolled sheet that has been hot-rolled to a predetermined thickness using a slab as a starting material is subjected to hot-rolled sheet annealing. In order to obtain the above crystal orientation after cold rolling annealing and to improve the toughness, hot-rolled sheet annealing conditions are important. However, in steels to which a large amount of alloy elements such as Nb and Mo are added, carbonitride In addition, an intermetallic compound mainly composed of Fe, Nb, and Mo called a Laves phase is generated in the heating stage. If a large amount of these precipitates during heating, the recrystallization is delayed and the development of the recrystallized texture is delayed, and the precipitate is coarsened, and the sum of crystal orientation strength and / or YP / rm is outside the scope of the present invention. Degradation of low temperature toughness. In order to prevent these, the heating rate at 700 to 900 ° C. is set to 5 ° C./sec or more during hot-rolled sheet annealing. Moreover, although the heat-rolled sheet annealing temperature of heat-resistant ferritic stainless steel is usually 1000 ° C. or higher, in the present invention, the time for holding at 900 to 980 ° C. is set to 30 seconds or longer to promote dissolution of precipitates. The hot rolled texture remains. When the holding time becomes long, productivity is lowered and coarsening of undissolved precipitates progresses. Therefore, the upper limit of the holding time is preferably 120 sec or less. When heated to over 980 ° C., particularly 1000 ° C. or higher, the hot-rolled texture is randomized by recrystallization, and the development of the {111} <112> orientation does not occur particularly in the cold-rolled annealed plate. In addition, since the recovery / recrystallization does not occur when the heating is less than 900 ° C., the {100} <011> orientation existing in the hot-rolled sheet remains after cold rolling annealing and lowers the r value. Set to 900 ° C.

  Furthermore, in consideration of productivity, plate shape, and toughness after hot-rolled sheet annealing, the heating rate at 700 to 900 ° C. is desirably 10 to 100 ° C./sec, and the time for maintaining at 900 to 980 ° C. is desirably 30 to 60 sec. .

  The plate heated to the above temperature in the hot-rolled sheet annealing step is cooled to room temperature, but in the present invention, it is cooled to 400 ° C at 10 ° C / sec or more. This is because, when cooled at less than 10 ° C./sec, the precipitates are reprecipitated and coarsened, the sum of crystal orientation strengths and / or YP / rm becomes outside the scope of the present invention, and the low temperature toughness deteriorates. Considering productivity, plate shape, and toughness after hot-rolled sheet annealing, it is desirable to be less than 20-30 ° C / sec.

  In the prior art, a technique for improving the characteristics by controlling the cooling rate of hot-rolled sheet annealing is disclosed, but in the present invention, in the hot-rolled sheet annealing process, the Laves phase precipitation is suppressed by controlling the heating rate, and the holding temperature is controlled. It has been found that excellent low-temperature toughness can be achieved after pressing a product by satisfying the control of texture by control, the promotion of dissolution and coarsening of precipitates by the control of holding time, and the suppression of precipitation by controlling the cooling rate. It was.

  After hot-rolled sheet annealing, after cold rolling to a predetermined thickness, a recrystallized structure is obtained by cold-rolled sheet annealing. At this time, when the heating temperature is less than 1000 ° C., an unrecrystallized structure is obtained, the yield strength is remarkably increased, Yp / rm exceeds 350, and {111} recrystallized texture does not develop, so the lower limit is set to 1000 ° C. . On the other hand, when heated to over 1100 ° C., the crystal grains become coarse, the sum of the crystal orientation strength and / or YP / rm is outside the present invention, and the surface becomes rough during processing, cracking, and the low temperature toughness is significantly reduced. The upper limit is 1100 ° C. Furthermore, considering the material and high temperature characteristics, 1010 to 1070 ° C. is desirable. The present invention is particularly effective for thick products in which texture development is unlikely to occur, and the product plate thickness is preferably 1.2 mm or more, more preferably 1.5 to 3.0 mm. . The cold rolling reduction ratio and hot rolled sheet thickness at this time may be selected according to the product sheet thickness.

  Steel having the composition shown in Table 1 was melted and cast into a slab, and the slab was hot-rolled to obtain a hot-rolled sheet having a thickness of 5.0 mm. Thereafter, the hot-rolled sheet was subjected to continuous annealing treatment, pickled, cold-rolled to a thickness of 2.5 mm, and subjected to continuous annealing-pickling to obtain a product plate. As for the hot-rolled sheet annealing conditions, the heating rate at 700 to 900 ° C. was 10 ° C./sec, and the time for staying 900 to 980 ° C. was 30 sec, and the cooling rate to 400 ° C. was 20 ° C./sec. The cold-rolled sheet annealing temperature was 1050 ° C.

  The cold-rolled annealed sheet thus obtained was subjected to the above-described tensile test, r value measurement, and crystal orientation strength measurement. Further, the low temperature toughness at −40 ° C. was evaluated for the cylindrical drawn product by the above-described method, and the presence or absence of cracks (circle without cracking, with cracking x) was visually observed. Moreover, in order to investigate the heat resistance in the heat-resistant component in which this material is mainly used, 0.2% proof stress at 900 ° C. was measured. At this time, a tensile test piece was taken from the raw steel plate so that the rolling direction was the tensile direction, and the heating rate up to 900 ° C. was 45 ° C./min, the holding time was 15 min, and the tensile rate was 0.3% / min. A test was conducted. If the proof stress at 900 ° C. is 20 MPa or more, it can be applied to main applications, and therefore 20 MPa or more was considered acceptable.

  As is clear from Table 1, the steel having the component composition defined in the present invention has a higher sum of {111} <011> azimuth strength and {111} <112> azimuth strength than the comparative example, and YP / rm. Is low and no cracking occurs even in the low temperature drop test, indicating that the low temperature toughness of the processed product is excellent. Moreover, also about high temperature proof stress, all this invention steel is satisfying 20 MPa or more.

  Table 2 shows the characteristics when the manufacturing conditions are variously changed. In the case of a comparative example that deviates from the manufacturing conditions specified in the present invention, the sum of crystal orientation strength and / or YP / rm is out of the present invention, low temperature toughness deteriorates, and cracking occurs when processed into a complex shaped part. End up. Moreover, although the high temperature proof stress of the steel manufactured by the method of the present invention satisfies 20 MPa or more, when the steel manufacturing method is outside the range of the present invention, coarse precipitates remain in the product or the crystal grains become coarse The pressure is less than 20 MPa.

  In addition, what is necessary is just to design slab thickness, hot-rolled sheet thickness, etc. suitably. In cold rolling, the rolling reduction, roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected. The annealing may be bright annealing, which is performed in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, if necessary, or in the air. Further, a tension leveler process for temper rolling and shape correction may be passed after annealing.

Claims (3)

  In mass%, C: 0.001 to 0.02%, Si: 0.1 to 0.5%, Mn: more than 0.1% to less than 1.5%, P: 0.01 to 0.04 %, S: 0.0001-0.01%, Cr: 13-20%, N: 0.001-0.03%, Nb: 0.1-0.6%, Mo: 0.2-3% Ti: 0.001 to 0.3%, B: 0.0002 to 0.005%, Al: 0.003 to 0.5% or less, with the balance being Fe and inevitable impurities, {111 } The sum of <112> orientation strength and {111} <011> orientation strength is 4 or more, and the ratio (YP / rm) of 0.2% proof stress (YP) to average r value (rm) at room temperature is 350. A heat-resistant ferritic stainless steel sheet excellent in low-temperature toughness characterized by the following:   V: 0.05-1.0%, Cu: 0.01-2.0%, Ni: 0.1-2.0%, W: 0.1-3.0%, Zr: 0.05- One or more of 0.30%, Sn: 0.05-0.50%, Co: 0.05-0.50%, Mg: 0.0002-0.0100% are contained. Item 2. A heat-resistant ferritic stainless steel sheet excellent in low-temperature toughness according to item 1. After hot-rolling the slab having the component composition according to claim 1 or 2 and heating at 700 to 900 ° C. at 5 ° C./sec or more at the time of heating, after maintaining at a temperature range of 900 to 980 ° C. for 30 sec or more The hot-rolled sheet annealing which cools to 400 degreeC at 10 degree-C / sec or more is given, It heats to 1000-1100 degreeC after cold-rolling to predetermined | prescribed board thickness, The Claim 1 or Claim 2 characterized by the above-mentioned. A method for producing heat-resistant ferritic stainless steel sheets with excellent low-temperature toughness.

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