JP6223351B2 - Ferritic stainless steel, exhaust system member using the same, and method for producing ferritic stainless steel - Google Patents

Description

  The present invention relates to a material for a thin plate structure used at a high temperature, and particularly to a ferritic stainless steel that is not easily embrittled by being used at a high temperature as well as corrosion resistance at a normal temperature, such as an automobile exhaust system material. is there.

  Ferritic stainless steel is inferior in workability, toughness and high-temperature strength compared to austenitic stainless steel, but it is inexpensive because it does not contain a large amount of Ni, and its thermal expansion is small. It is used for applications where thermal distortion becomes a problem, such as materials and automotive exhaust system parts that become hot. Particularly when used as an exhaust system part material for automobiles, high temperature strength, corrosion resistance at normal temperature, and high toughness associated with high temperature use are important. In general, steel types such as SUH409L, SUS429, SUS430LX, SUS436J1L, SUS432, and SUS444 are used as ferritic stainless steel suitable for these applications.

  Among these materials, Patent Document 1 discloses a material in which high temperature strength is increased by using 0.05 to 2% of Sn. Patent Document 2 discloses a technique for improving the surface quality of a stainless steel plate by adding 0.005 to 0.10% Sn. In recent years, a large amount of Sn exceeding 0.05% has been contained in stainless steel as an unavoidable impurity by using scrap iron including a surface-treated steel sheet as a raw material.

JP 2000-169943 A Japanese Patent Laid-Open No. 11-92872

  It has been found that when stainless steel containing Sn described in the background art is used at a high temperature, a grain boundary embrittlement phenomenon, which has not been known in the past, occurs and the strength of the component is impaired. An object of the present invention is to provide a ferritic stainless steel that does not deteriorate toughness at room temperature even when exposed to a high temperature for a long time, such as an automobile exhaust system material.

  The inventors of the present invention have made various studies on the reduction in toughness at normal temperature after high-temperature and long-term aging of ferritic stainless steel containing Sn. First, when SUS430LX contained 0.3% of Sn, it was found that the temperature range was 500 to 800 ° C. when it was examined in which temperature range the toughness was lowered. In addition, it was found that the temperature at which the toughness drop occurs particularly in a short time is 700 ° C., and the toughness drop significantly occurs in only 1 hour. As shown in FIG. 1, the fracture surface form in which brittle fracture occurred was different from a general cleavage fracture surface and had a characteristic of showing a grain boundary fracture surface. When the sample was destroyed after cooling to a low temperature in an AES (Auger Electron Spectroscopy) apparatus and the grain boundary fracture surface was analyzed, noticeable Sn segregation was observed at a thickness of about 1 nm. That is, it was considered that the decrease in toughness due to long-term use at high temperature was caused by segregation of Sn grain boundaries.

  In order to prevent such grain boundary embrittlement, it is most effective to reduce the Sn content. However, since the recycling of the surface-treated steel sheet is unavoidable for environmental protection, the actual situation is that scrap containing Sn must be used. Further, it is difficult to remove Sn by refining with the existing technology, and a material that does not easily cause grain boundary embrittlement even if Sn is eagerly desired.

  Therefore, in order to prevent embrittlement due to grain boundary segregation of Sn, the effect of various alloy elements is investigated in detail, and a stabilizing element added to fix C and N in stainless steel to ensure corrosion resistance. It was found that the influence of Ti and Nb was great. That is, as shown in FIGS. 1 and 2, when steel stabilized with Ti contains Sn, grain boundary embrittlement due to high temperature use is remarkable, and steel stabilized with Nb is brittle even if it contains Sn. I found that it is hard to happen.

  Based on this knowledge, it is possible to investigate the effect on the toughness when the stabilizing elements Ti and Nb are added alone or in combination, and to develop a steel that does not easily deteriorate toughness due to high temperature use. It was.

The present invention has been made based on these findings, and means for solving the problems of the present invention, that is, the ferritic stainless steel sheet of the present invention is as follows.
(1) By mass%, Cr: 13.0 to 21.0%, Sn: 0.01 to 0.50%, Nb: 0.05 to 0.60%, C: 0.015% or less , Si: 1.5% or less, Mn: 1.5% or less, N: 0.020% or less, P: 0.035% or less, and S: 0.015% or less, Formula 1 and Formula 2 Grain boundary Sn concentration when the balance is Fe and inevitable impurities, and heat treatment is performed at a temperature of 600 to 750 ° C. and the L value represented by Formula 3 is 1.91 × 10 ⁴ or more Is a ferritic stainless steel characterized by having a grain size number of 5.0 or more and 9.0 or less after cold-rolled sheet annealing.
8 ≦ CI = 0.52 Nb / (C + N) ≦ 26 (Formula 1)
GBSV = Sn−2Nb−0.2 ≦ 0 (Expression 2)
L = (273 + T) (log (t) +20) (Equation 3)
Where T: temperature (° C.), t: time (h)

(2) By mass%, Cr: 13.0 to 21.0%, Sn: 0.01 to 0.30%, Nb: 0.20 to 0.60%, Ti: more than 0.05, 0.32% In addition, in addition, by mass%, Ni: 0.5% or less, Cu: 1.5% or less, Mo: 2.0% or less, V: 0.3% or less, Al: 0.3% Hereinafter, B: contains one or more of 0.0020% or less, C: 0.015% or less, Si: 1.5% or less, Mn: 1.5% or less, N: 0.020% Hereinafter, it is limited to P: 0.035% or less and S: 0.015% or less, the balance is Fe and inevitable impurities, satisfies the formulas 1 ′ and 2 ′, and 600 to 750 ° C. at temperatures, grain boundaries Sn concentration when subjected to a heat treatment L value shown in equation 3 is 1.91 × 10 ⁴ or more and 2 atomic% or less, the grain size number after cold-rolled sheet annealing 5.0 or more, ferrites stainless steel, wherein to Rukoto and 9.0 or less.
8 ≦ CI = (Ti + 0.52Nb) / (C + N) ≦ 26 Formula 1 ′
GBSV = Sn + Ti-2Nb-0.3Mo-0.2≤0 Formula 2 '
L = (273 + T) (log (t) +20) (Equation 3)
Where T: temperature (° C.), t: time (h)

(3) the heat treatment is characterized by 1 hour der Rukoto at 700 ° C. (1) or ferritic stainless steel according to (2).

(4) Further, in mass%, W: 0.20% or less, Zr: 0.20% or less, Sb: 0.5% or less, Co: 0.5% or less, Ca: 0.01% or less, Mg The ferritic stainless steel according to any one of (1) to (3), characterized by containing one or more of: 0.01% or less and REM: 0.1% or less.

(5) The ferritic stainless steel according to any one of (1) to (4), wherein the grain size number after cold-rolled sheet annealing is 6.0 or more and 8.5 or less.

(6) When the stainless steel having the composition described in (1), ( 2 ) or (4) is set to a cold-rolled sheet annealing temperature of 850 ° C to 1100 ° C and then cooled from the cold-rolled plate annealing temperature, 800 to 500 The method for producing a ferritic stainless steel according to any one of (1) to (5), wherein the cooling rate is 5 ° C./s or more in a temperature range of ° C.
(7) An exhaust system member using the ferritic stainless steel of any one of (1) to (5).
(8) By mass%, Cr: 13.0 to 21.0%, Sn: 0.01 to 0.50%, Nb: 0.05 to 0.60%, W: 0.01% -0.20% and Sb: 0.001% -0.5% at least one type, C: 0.015% or less, Si: 1.5% or less, Mn: 1.5% or less, N : 0.020% or less, P: 0.035% or less, and S: 0.015% or less, with the balance being Fe and inevitable impurities, satisfying Formula 1 and Formula 2, and 600 A ferritic stainless steel having a grain boundary Sn concentration of 2 atomic% or less when a heat treatment is performed at a temperature of ˜750 ° C. and an L value represented by Formula 3 is 1.91 × 10 ⁴ or more.
8 ≦ CI = 0.52 Nb / (C + N) ≦ 26 (Formula 1)
GBSV = Sn−2Nb−0.2 ≦ 0 (Expression 2)
L = (273 + T) (log (t) +20) (Equation 3)
Where T: temperature (° C.), t: time (h)
(9) By mass%, Cr: 13.0 to 21.0%, Sn: 0.01 to 0.30%, Nb: 0.20 to 0.60%, Ti: more than 0.05, 0.32% Or less, containing at least one of W: 0.01% to 0.20% and Sb: 0.001% to 0.5%, and further, by mass%, Ni: 0.5 % or less , Cu: 1.5% or less, Mo: 2.0% or less, V: 0.3% or less, Al: 0.3% or less, B: 0.0020% or less of one or comprise two or more C: 0.015% or less, Si: 1.5% or less, Mn: 1.5% or less, N: 0.020% or less, P: 0.035% or less, and S: 0.015% or less, is limited to, balance being Fe and unavoidable impurities, and satisfying the formula 1 ') and (2', and, at a temperature of 600 to 750 ° C., L value shown in equation 3 is 1.91 × 10 ⁴ or more Ferrites stainless steel to intergranular Sn concentration characterized der Rukoto 2 atomic% or less when subjected to become heat treatment.
8 ≦ CI = (Ti + 0.52Nb) / (C + N) ≦ 26 (Formula 1 ′)
GBSV = Sn + Ti-2Nb-0.3Mo-0.2≤0 (Formula 2 ')
L = (273 + T) (log (t) +20) (Equation 3)
Where T: temperature (° C.), t: time (h)
(10) The ferritic stainless steel according to (8) or (9), wherein the heat treatment is performed at 700 ° C. for 1 hour.
(11) Further, by mass%, Zr: 0.20% or less, Co: 0.5% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less 1 type or 2 types or more are contained, Ferritic stainless steel in any one of (8)-(10) characterized by the above-mentioned.
(12) The ferritic stainless steel according to any one of (8) to (11), wherein the grain size number after cold-rolled sheet annealing is 5.0 or more and 9.0 or less.
(13) When the stainless steel having the composition described in (8), ( 9) or (11) is set to a cold-rolled sheet annealing temperature of 850 ° C to 1100 ° C and then cooled from the cold-rolled plate annealing temperature, 800 to 500 The method for producing a ferritic stainless steel according to any one of (8) to (12), wherein the cooling rate is 5 ° C./s or more in a temperature range of ° C.
(14) An exhaust system member using the ferritic stainless steel according to any one of (8) to (12).

  According to the ferritic stainless steel containing Sn of the present invention, since the stabilizing elements Nb and Ti are optimized, the stainless steel is excellent in corrosion resistance with little deterioration in toughness even when used at high temperatures. A steel plate can be obtained.

The ferritic stainless steel and comparative steel in this embodiment were subjected to a Charpy impact test after being heat-treated at 700 ° C. for 1 hour with a hot rolled annealed sheet having a thickness of 4.0 mm. It is a fracture surface photograph of a test piece. After subjecting the ferritic stainless steel and the comparative steel in this embodiment to a hot-rolled annealed sheet having a thickness of 4.0 mm and a hot-rolled annealed sheet held at 700 ° C. for 1 hour, a V-notch Charpy impact test is performed. It is the graph which showed the ductile-brittle transition temperature measured with the subsize test piece of plate | board thickness 4.0mm, and was measured. When the ferritic stainless steel and the comparative steel in this embodiment are hot-rolled annealed plates having a thickness of 4 mm, and further heat-treated at 700 ° C. for 1 hour, the V-notch Charpy impact test piece is a sub-size test piece having a thickness of 4.0 mm. It is a graph which shows the relationship between the ductility-brittle transition temperature (DBTT) and the parameter | index (GBSV) showing the grain-boundary segregation tendency of Sn measured by doing. When the ferritic stainless steel and comparative steel in this embodiment are hot-rolled annealed plates with a thickness of 4 mm and further heat-treated at 700 ° C. for 1 hour, the Sn concentration at the grain boundary fracture surface is measured by AES, and the Charpy impact test is performed. It is a graph which measures the ductility-brittle transition temperature (DBTT), and shows the relationship between Sn density | concentration of a grain boundary, and DBTT.

  Embodiments of the present invention will be described below. First, the reason which limited the steel composition of the stainless steel plate of this embodiment is demonstrated. In addition, the description of% about a composition means the mass% unless there is particular notice.

C: 0.015% or less Since C deteriorates formability, corrosion resistance, and hot-rolled sheet toughness, the lower the content thereof, the better. Therefore, the upper limit is made 0.015%. However, excessive reduction leads to an increase in refining costs, so the lower limit may be 0.001%. From the viewpoint of corrosion resistance, it is desirable that the lower limit is 0.002% and the upper limit is 0.009%.

N: 0.020% or less N, like C, deteriorates formability, corrosion resistance, and hot-rolled sheet toughness, so the smaller the content, the better. However, excessive reduction leads to an increase in refining costs, so the lower limit should be 0.001%. In order to more surely avoid the deterioration of corrosion resistance and the deterioration of toughness, the upper limit is preferably set to 0.018%, and more preferably set to 0.015%.

Si: 1.5% or less Excessive addition of Si lowers the room temperature ductility, so the upper limit is made 1.5%. However, Si is an element that is also useful as a deoxidizer and is an element that improves high-temperature strength and oxidation resistance. The deoxidation effect is improved with an increase in the amount of Si. The effect is manifested at 0.01% or more, and is stable at 0.05% or more. Therefore, the lower limit may be 0.01%. In addition, when adding Si in consideration of oxidation resistance, it is more desirable to set the lower limit to 0.1% and the upper limit to 0.7%.

Mn: 1.5% or less Excessive addition of Mn causes a decrease in hot-rolled sheet toughness due to precipitation of the γ phase (austenite phase), and also forms MnS to lower the corrosion resistance. And On the other hand, Mn is an element added as a deoxidizer and an element contributing to an increase in high-temperature strength in the middle temperature range. In addition, Mn-based oxides form on the surface layer during long-time use, and are also elements that contribute to the adhesion of scale (oxide) and the effect of suppressing abnormal oxidation. In order to exhibit such an effect, Mn may be added so that the Mn content of the stainless steel of the present invention is 0.01% or more. In consideration of high temperature ductility, scale adhesion, and suppression of abnormal oxidation, it is more desirable to set the lower limit to 0.1 and the upper limit to 1.0%.

P: 0.035% or less P is an element having a large solid solution strengthening ability, but it is a ferrite stabilizing element and is also an element harmful to corrosion resistance and toughness.
P is contained as an impurity in ferrochrome which is a raw material of stainless steel. Since it is very difficult to remove P from the molten steel, it may be 0.010% or more. The P content is almost determined by the purity and amount of the ferrochrome raw material to be used. The content of P in the ferrochrome raw material is preferably low. However, since low P ferrochrome is expensive, it is set to 0.035% or less, which is a range in which the material and corrosion resistance are not greatly deteriorated. In addition, Preferably it is 0.030% or less.

S: 0.015% or less S forms sulfide inclusions and degrades the general corrosion resistance (entire corrosion and pitting corrosion) of steel materials. Therefore, the S content is preferably as small as possible, and the upper limit is made 0.015% in consideration of the range that does not affect the corrosion resistance. Further, the smaller the S content, the better the corrosion resistance. However, the lowering of the S content increases the desulfurization load and the manufacturing cost, so the lower limit may be 0.001%. Preferably, the lower limit is 0.001% and the upper limit is 0.008%.

Cr: 13.0-21.0%
In the present invention, Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If it is less than 13.0%, these effects do not appear. On the other hand, if it exceeds 21.0%, workability and toughness are deteriorated, so the lower limit is 13.0 and the upper limit is 21.0%. To do. Furthermore, when manufacturability and high temperature ductility are taken into consideration, the upper limit is desirably set to 18.0%.

Sn: 0.01 to 0.50%
Sn is an element effective for improving corrosion resistance and high temperature strength. In addition, there is an effect that the mechanical properties at room temperature are not greatly deteriorated. Since the effect on corrosion resistance is manifested at 0.01% or more, the lower limit is made 0.01%. The contribution to the high-temperature strength is stably expressed by addition of 0.05% or more, so the preferable lower limit is made 0.05%. On the other hand, if added excessively, manufacturability and weldability deteriorate significantly, so the upper limit is made 0.50%. In view of oxidation resistance and the like, the lower limit is preferably 0.1%. In consideration of weldability and the like, the upper limit is preferably set to 0.3%. The development of the embrittlement phenomenon at high temperature use becomes remarkable when 0.05% or more of Sn is contained, but the embrittlement phenomenon caused by the inclusion of Sn can be suppressed by adding Nb described below in combination. . Moreover, in order to make DBTT (ductility-brittle transition temperature) less than 50 degreeC, it is still more preferable to make the upper limit of Sn content 0.21%.

Nb: 0.05-0.60%
Nb is an element having an effect of suppressing sensitization due to precipitation of chromium carbonitride in stainless steel and deterioration of corrosion resistance by forming carbonitride. This effect is manifested at 0.05% or more. Furthermore, the present inventors have found that it has an effect of suppressing grain boundary embrittlement in Sn-containing steel. Since both effects of improving corrosion resistance and suppressing grain boundary embrittlement are manifested at 0.05% or more, the lower limit is made 0.05%. In order to obtain the effect more reliably, it is preferably set to 0.09% or more, and if it is 0.2% or more, the effect can be obtained almost certainly. On the other hand, excessive addition causes a problem of decrease in manufacturability due to the generation of the Laves phase. Considering these, the upper limit of Nb is set to 0.60%. Furthermore, from the viewpoint of weldability and workability with a thin plate, the lower limit may be 0.3% and the upper limit may be 0.5%. Moreover, the grain boundary embrittlement suppressing effect in the Sn-containing steel can be obtained even when Ti and Nb are added in combination. Also in this case, the effect is obtained when the amount of Nb added is 0.05% or more. However, it is necessary to adjust the CI value, which will be described later, to be within a predetermined range both when adding Nb alone and when adding Ti and Nb together.

  CI = (Ti + 0.52Nb) / (C + N) is 8 or more and 26 or less. When Ti is not contained, CI = 0.52Nb / (C + N) is 8 or more and 26 or less. Ti and Nb form carbonitrides and suppress deterioration of corrosion resistance due to formation and sensitization of chromium carbonitrides. That is, the addition amount corresponding to the amount of C and N in steel is necessary. The CI value is an index for precipitating C and N in steel as Ti and Nb carbonitrides to suppress sensitization, and as the CI value increases, sensitization is suppressed. In order to stably suppress precipitation of chromium carbonitride even in a welding heat cycle, CI is required to be 8 or more. However, if Ti and Nb are added excessively, large inclusions are formed and the workability is lowered, so the CI is made 26 or less. In order to ensure stable corrosion resistance and workability, the CI is preferably 10 or more and 20 or less.

  Furthermore, in the present invention, GBSV = Sn + Ti-2Nb-0.3Mo-0.2 is set to 0 or less. When Ti and Mo are not contained, GBSV = Sn-2Nb-0.2 is set to 0 or less. GBSV is an index indicating the grain boundary segregation tendency of Sn, and the grain boundary segregation becomes more prominent as the value is larger. The coefficient of the elements constituting GBSV is an evaluation of the effect on grain boundary segregation. Sn is an element effective for high-temperature strength and corrosion resistance, but lowers the toughness of the material at 400 ° C. or lower due to grain boundary segregation. On the other hand, Nb and Mo have the effect of increasing the grain boundary strength in addition to the action of suppressing the grain boundary segregation of Sn, and the action of suppressing embrittlement due to the grain boundary segregation of Sn. As shown in FIG. 3, the ductile-brittle transition temperature decreases as GBSV decreases. If GBSV becomes 0 or less, the ductile-brittle transition temperature is 150 in a hot-rolled annealed sheet having a thickness of 4.0 mm. It can be seen that the toughness is greatly improved at a temperature below ℃. For this reason, GBSV was made 0 or less.

  Next, the Sn concentration (atomic%) at the grain boundary fracture surface was used as an index of grain boundary segregation of Sn, and the relationship with the ductile brittle transition temperature was examined. As shown in FIG. 4, it was found that when the Sn concentration at the grain boundary exceeds 2.0 atomic%, the ductile-brittle transition temperature increases rapidly, and grain boundary embrittlement is likely to occur. Even in a high temperature use environment, it is important for the Sn concentration at the grain boundary to be 2.0 atomic% or less in order to suppress grain boundary embrittlement due to Sn.

Here, as an index for uniformly treating the temperature and time in the case of high temperature and long time use, an L value represented by Formula 3 that is usually used as an evaluation index for heat treatment was introduced. When heat treatment is performed at a temperature of 600 to 750 ° C. so that the L value shown in Formula 3 is 1.91 × 10 ⁴ or more, Sn segregation to the grain boundary is noticeably observed when Ti is added, and the grain boundary is reached. The present inventors have found that the Sn segregation of N will adversely affect the characteristics (transition temperature). In addition, the present inventors also confirmed that the grain boundary Sn concentration is 2 atomic% or less when heat treatment is performed with an L value of 1.91 × 10 ⁴ or more in the case of the component composition in the present invention. did.
As a condition that further simplifies the definition of the heat treatment condition by the L value, the grain boundary Sn concentration after heat treatment at 700 ° C. for 1 hour is preferably 2.0 atomic% or less.

The Sn concentration of the grain boundary is measured by breaking in an AES apparatus under ultra high vacuum. Since Auger electrons are emitted not only from the surface but also from atoms within several nm from the surface, this value does not represent only the Sn concentration at the grain boundary. Moreover, the analysis accuracy differs for each apparatus. However, in principle, the Sn concentration at the cleavage fracture surface is the same as the average Sn concentration of the base material. Therefore, the Sn concentration at the grain boundary was determined by calibrating the measured value of the Sn concentration on the cleavage fracture surface so that the Sn concentration measured on the cleavage fracture surface becomes the average Sn concentration of the base material.
In order to stably reduce grain boundary embrittlement, it is preferable to set the Sn concentration at the grain boundary to 1.7 atomic% or less. Moreover, since it is difficult to make it below the Sn concentration of the base material, it is preferable to set 0.02 atomic% as the lower limit.

  In the present invention, in addition to the above elements, Ti: 0.32% or less, Ni: 1.5% or less, Cu: 1.5% or less, Mo: 2.0% or less, V: 0.3% Hereinafter, it is preferable to add one or more of Al: 0.3% or less and B: 0.0020% or less.

Ti: 0.32% or less Ti is an element that suppresses deterioration of sensitization and corrosion resistance due to precipitation of chromium carbonitride in stainless steel by forming carbonitride similarly to Nb. However, since it has a greater effect of promoting grain boundary embrittlement in Sn-containing steel than Nb, it is an element to be reduced in Sn-containing steel. The influence of Sn on grain boundary segregation appears when the Ti content exceeds 0.05%. However, when Nb is contained, adverse effects due to Ti can be reduced. In the case of adding Nb in combination, it was confirmed that the Sn grain boundary concentration was 2.0 atomic% or less in the heat treatment if the upper limit was 0.32%. A preferable upper limit in the case of containing Nb is 0.15%. In addition, since it is difficult to reduce excessively since it mixes as an inevitable impurity from a raw material, it is preferable to make content of Ti 0.001% or more. From the viewpoint of improving workability by reducing inclusions, it is more preferable to set the lower limit to 0.001 and the upper limit to 0.03%.

Ni: 1.5% or less Ni is mixed as an inevitable impurity in the ferritic stainless steel alloy raw material and is generally contained in the range of 0.03 to 0.10%. Moreover, it is an element effective in suppressing the progress of pitting corrosion, and the effect is stably exhibited by addition of 0.05% or more. Therefore, the lower limit is preferably 0.05%. More preferably, the lower limit is 0.1%.
On the other hand, since a large amount of addition may cause material hardening due to solid solution strengthening, the upper limit is made 1.5%. In consideration of the alloy cost, the upper limit is preferably 1.0%. More preferably, the upper limit is 0.5%. From the above, Ni is preferably 0.1 to 0.5%.
In the present invention, Ni is an element that improves the corrosion resistance by a synergistic effect with Sn. It is useful to add together with Sn. Further, Ni also has an effect of improving the decrease in workability (elongation, r value) accompanying the addition of Sn. When combined with Sn, it is preferable that the lower limit of Ni is 0.2 and the upper limit is 0.4%.

Cu: 1.5% or less Cu is effective in improving the corrosion resistance. This is particularly effective in reducing the rate of progress after crevice corrosion. It is desirable to contain 0.1% or more in order to improve corrosion resistance. However, excessive addition deteriorates workability. Therefore, it is desirable to contain Cu with a lower limit of 0.1 and an upper limit of 1.5%.
Cu is an element that improves the corrosion resistance by a synergistic effect with Sn. It is useful to add together with Sn. Further, Cu also has an effect of improving a decrease in workability (elongation, r value) accompanying the addition of Sn. When combined with Sn, it is preferable to contain Cu with a lower limit of 0.1 and an upper limit of 0.5%.
From the above, in the present invention, it is useful to add Sn and Ni and / or Cu in combination to improve the corrosion resistance.
Cu is also an element necessary for increasing the high-temperature strength required for use as a high-temperature environment member typified by a high-temperature exhaust system of an automobile. Cu mainly exhibits precipitation strengthening ability at 500 to 750 ° C., and at higher temperatures, suppresses plastic deformation of the material by solid solution strengthening and exhibits a function of improving thermal fatigue characteristics. Such precipitation hardening effect and solid solution strengthening of Cu are manifested by addition of 0.2% or more. On the other hand, excessive addition causes abnormal oxidation during hot rolling and causes surface defects, so the upper limit is made 1.5%. In order to utilize the high temperature strengthening ability of Cu and stably suppress surface defects, it is desirable that the lower limit is 0.5 and the upper limit is 1.0%.

Mo: 2.0% or less Mo may be added as necessary in order to improve the high temperature strength and thermal fatigue characteristics. In order to exert these effects, the lower limit is preferably made 0.01%.
On the other hand, excessive addition may cause the generation of a Laves phase, which may cause a reduction in hot-rolled sheet toughness. Considering these, the upper limit of Mo is set to 2.0%. Further, from the viewpoint of productivity and manufacturability, it is desirable that the lower limit is 0.05% and the upper limit is 1.5%.

V: 0.3% or less V is mixed as an inevitable impurity in ferritic stainless steel alloy raw materials, and is difficult to remove in the refining process, so generally contained in the range of 0.01 to 0.1% Is done. Moreover, since it has the effect which forms a fine carbonitride and a precipitation strengthening effect | action produces and contributes to a high temperature strength improvement, it is an element also added intentionally as needed. Since the effect is stably manifested by addition of 0.03% or more, the lower limit is preferably 0.03%.
On the other hand, if added excessively, the precipitates may be coarsened. As a result, the high-temperature strength decreases and the thermal fatigue life decreases, so the upper limit is made 0.3%. In view of manufacturing cost and manufacturability, it is desirable that the lower limit is 0.03% and the upper limit is 0.1%.

Al: 0.3% or less In addition to being added as a deoxidizing element, Al is an element that improves oxidation resistance. Moreover, it is useful for the strength improvement in 600-700 degreeC as a solid solution strengthening element. Since the action is stably expressed from 0.01%, the lower limit is preferably set to 0.01%.
On the other hand, excessive addition hardens and significantly reduces the uniform elongation, and also significantly reduces the toughness, so the upper limit is made 0.3%. Furthermore, considering the occurrence of surface flaws, weldability and manufacturability, the lower limit is preferably 0.01% and the upper limit is preferably 0.07%.

B: 0.0020% or less B is effective for fixing N which is harmful to workability and improving secondary workability, and is added at 0.0003% or more as necessary. Moreover, since the effect will be saturated even if it adds exceeding 0.0020% and the workability deterioration by B and corrosion resistance fall, it adds at 0.0003-0.002%. In consideration of workability and manufacturing cost, it is desirable that the lower limit is 0.0005% and the upper limit is 0.0015%.

W: 0.20% or less W is effective in improving the high-temperature strength, and is added at 0.01% or more as necessary. Further, if added over 0.20%, the solid solution strengthening is too large and the mechanical properties are lowered, so 0.01 to 0.20% is added. Considering the manufacturing cost and hot-rolled sheet toughness, it is desirable that the lower limit is 0.02% and the upper limit is 0.15%.

Zr: 0.20% or less Zr is added in an amount of 0.01% or more as necessary in order to form a carbonitride and suppress the formation of Cr carbonitride and improve the corrosion resistance like Nb and Ti. To do. Moreover, even if added over 0.20%, the effect is saturated, and it causes surface flaws due to the formation of large oxides, so it is added at 0.01 to 0.20%. Since it is an expensive element compared with Ti and Nb, considering the manufacturing cost, it is desirable that the lower limit is 0.02% and the upper limit is 0.05%.

Sb: 0.5% or less Sb is effective in improving sulfuric acid resistance, and is added at 0.001% or more as necessary. Further, even if added over 0.5%, the effect is saturated and embrittlement due to segregation of Sb grain boundaries occurs, so 0.001 to 0.20% is added. In consideration of workability and manufacturing cost, it is desirable to set the lower limit to 0.002% and the upper limit to 0.05%.

Co: 0.5% or less Co is effective for improving wear resistance and high-temperature strength, and is added at 0.01% or more as necessary. Moreover, even if added over 0.5%, the effect is saturated, and mechanical properties are deteriorated due to solid solution strengthening, so 0.01 to 0.5% is added. From the viewpoint of production cost and high temperature strength stability, it is desirable that the lower limit is 0.05% and the upper limit is 0.20%.

Ca: 0.01% or less Ca is an important desulfurization element in the steelmaking process, and also has a deoxygenating effect, so is added at 0.0003% or more as necessary. Moreover, even if added over 0.01%, the effect is saturated, resulting in a decrease in corrosion resistance due to Ca granulated materials and a deterioration in workability due to oxides. Add in%. Considering manufacturability such as slag treatment, it is desirable that the lower limit is 0.0005% and the upper limit is 0.0015%.

Mg: 0.01% or less Mg is an element effective for refining the solidified structure in the steel making process, and is added in an amount of 0.0003% or more as necessary. Moreover, even if added over 0.01%, the effect is saturated, and the corrosion resistance due to Mg sulfide or oxide is liable to be lowered, so 0.0003 to 0.01% is added. In consideration of the fact that the Mg addition in the steelmaking process causes oxidative combustion of Mg and the yield decreases and the cost increases greatly, the lower limit is preferably 0.0005% and the upper limit is preferably 0.0015%.

REM: 0.1% or less REM is effective in improving oxidation resistance, and is added at 0.001% or more as necessary. Moreover, since the effect will be saturated even if it adds exceeding 0.1% and the corrosion resistance fall by the granulated material of REM arises, it adds at 0.001-0.1%. Considering the workability and manufacturing cost of the product, it is desirable that the lower limit is 0.002% and the upper limit is 0.05%.

The grain size number after cold rolling annealing is set to 5.0 or more and 9.0 or less.
When the Sn-added steel is exposed to a high temperature environment, it is considered that even if the component control is performed by the GBSV value, the toughness is not completely reduced. In that case, grain boundary embrittlement can be alleviated by increasing the area of grain boundaries where Se segregates. For this purpose, the crystal grain size number needs to be 5 or more. However, if the crystal grain size number is too large, the mechanical properties become low ductility and high strength due to fine graining, so 5.0 or more and 9.0 or less. Considering the optimization of the Rankford value that governs the improvement of deep drawability and the reduction of rough skin during processing, it is desirable that the value be 6.0 or more and 8.5 or less.

  Even if Sn-added steel is not used in a high-temperature environment, if Sn segregates at the grain boundaries in the manufacturing process, it causes a reduction in the toughness of the thin plate product. It is necessary to do. The cold-rolled sheet annealing temperature is set to 850 ° C. or more at which Sn grain boundary segregation hardly occurs, and is set to 1100 ° C. or less at which crystal grain size coarsening hardly occurs. It is desirable that the cooling rate is 5 ° C./s or higher in the temperature range.

Example 1
Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

  In this example, first, steels having the component compositions shown in Table 1-1 and Table 1-2 were melted and cast into slabs. After heating this slab to 1190 degreeC, finishing temperature was made into the range of 800-950 degreeC, and it hot-rolled to plate | board thickness 4mm, and was set as the hot-rolled steel plate. In Table 1-1 and Table 1-2, numerical values that are outside the scope of the present invention are underlined. The hot-rolled steel sheet was cooled to 500 ° C. by air-water cooling and then wound into a coil.

  In Table 1-1 and Table 1-2, examples of the present invention and comparative examples that do not contain Ti and Mo have Ti and Mo contents indicated by “−”. Moreover, in Table 1-1 and Table 1-2, each value of CI and GBSV of the present invention example and the comparative example which do not contain Ti and Mo was calculated based on the above-described formula 1 and formula 2, respectively. Further, the CI and GBSV values of the inventive examples and comparative examples containing Ti and Mo were calculated based on the above-described formulas 1 'and 2', respectively.

Subsequently, the hot rolled coil was annealed at 900 to 1100 ° C. and cooled to room temperature. At this time, the average cooling rate in the range of 800 to 550 ° C. was set to 20 ° C./s or more. Subsequently, the hot-rolled annealed plate was pickled and cold-rolled to obtain a thin plate having a thickness of 1.5 mm, and then the cold-rolled plate was annealed and pickled to obtain a thin plate product. No. in Table 1-1. 1 to 5, 7, 9 to 14, 16, 18 to 21, 28 to 31, and 34 are examples of the present invention, No. 1 in Table 1-1. 2-2, 6, 8, 15, 17, 22, 27, and 32 to 33-2 are reference examples, No. 1 in Table 1-2. 35 to 56 are comparative examples.

The hot-rolled annealed sheet thus obtained was subjected to a heat treatment (L value: 19460) at 700 ° C. for 1 hour, and then a Charpy impact test was performed according to JIS Z 2242 to obtain a ductile-brittle transition. The temperature (DBTT) was measured. The measurement results are shown in Table 2-1 and Table 2-2. In addition, since the test piece in a present Example is a subsize test piece with the plate | board thickness of a hot-rolled annealing board, the hot-rolled annealing board in each Example is obtained by dividing absorbed energy by a cross-sectional area (unit cm ² ). The toughness was compared and evaluated. In addition, the evaluation criteria of toughness made ductility-brittle transition temperature (DBTT) 150 degrees C or less favorable.

  Moreover, the test piece of 14x4x4mm was created for the Auger electron spectroscopy analysis (AES) from the hot-rolled annealing board. A notch having a depth of 1 mm and a width of 0.2 mm was placed in the center of the test piece in the longitudinal direction. In ultra-high vacuum in the AES apparatus, it was cooled with liquid nitrogen, ruptured by impact, and the Sn concentration at the grain boundary fracture surface was measured. The measurement results are shown in Tables 2-1 and 2-2 as “grain boundary Sn concentration (at%)”. As the AES apparatus, SAM-670 (PHI, FE type) was used. The beam size was 0.05 μm. The concentration was calibrated so that the analytical value on the cleavage plane was the same as the concentration of the base material. Auger electrons are emitted not only from the outermost surface of the grain boundary fracture surface, but also from a depth of several nanometers. In this method, the Sn concentration of the grain boundary is not accurate, but this method is used as a general measurement value. 2 atomic% (at%) or less was considered good.

  Further, the hot-rolled annealed sheet is cold-rolled to 1.5 mm, pickled after annealing at 840 to 980 ° C. for 100 seconds, Mig bead-on-plate welding is performed on the cold-rolled annealed sheet, and is specified in JIS G 0575. The stainless steel was subjected to a sulfuric acid / copper sulfate corrosion test to investigate the presence or absence of sensitization in the welded HAZ part. However, the sulfuric acid concentration was 0.5%, and the test time was 24 hours. Those in which intergranular corrosion was observed were considered to have failed corrosion resistance. The evaluation results are shown in Tables 2-1 and 2-2 as “improved Strauss test”.

  In addition, after the surface of the cold-rolled annealed pickled plate is polished to # 600, the salt spray test method specified in JIS Z 2371 is performed for 24 hours, and the presence or absence of rust is confirmed. It was rejected. The evaluation results are shown in Table 2-1 and Table 2-2 as “salt spray test”.

Table 3 shows the results of tests similar to the items described in Table 2-1 and Table 2-2, with different heat treatment conditions for the hot-rolled annealed plate. Some steels shown in Table 3 were evaluated by a wet and dry repeated test. The test solution was nitrate ion NO ₃ ⁻ : 100 ppm, sulfate ion SO ₄ ²⁻ : 10 ppm, chloride ion Cl ⁻ : 10 ppm, pH = 2.5. Filled with 10 ml of the test solution in a test tube with an outer diameter of 15 mm, height of 100 mm, and thickness of 0.8 mm, various stainless steels cut into 1t x 15 x 100 mm and wet-polished with # 600 emery paper on the entire surface. Semi-immersed. The test tube was put in a warm bath at 80 ° C., and a sample completely dried after 24 hours was lightly washed with distilled water. Then, the test solution was again filled in the newly washed test tube, and the sample was half-immersed again. And holding for 24 hours was performed 14 cycles.

  Table 4 shows the results of a V-notch Charpy impact test with the plate thickness changed after changing the annealing conditions of the cold-rolled annealed plate to a 1.5 mm thin plate product and aging treatment at 600 ° C. for 1 week. Indicated. At this time, the condition that the ductile-brittle transition temperature was −20 ° C. or lower was regarded as acceptable.

  As is clear from Table 1-1, Table 1-2, Table 2-1, Table 2-2, and Table 3, the steel of the component composition and grain boundary Sn concentration to which the present invention is applied is evaluated by a hot-rolled annealing plate. The ductility-brittle transition temperature (DBTT) was low, the corrosion resistance evaluated with the cold-rolled annealed sheet was good, and the total elongation evaluated with a tensile test was also good at 30% or more. Moreover, no surface flaws were observed. On the other hand, in the comparative example which deviates from the present invention, one or more of Charpy impact values (absorbed energy), corrosion resistance, material, and surface flaws were rejected. Thereby, it turns out that the heat resistance of the ferritic stainless steel in a comparative example and corrosion resistance are inferior.

  Specifically, no. 35, 39 to 41, 43, 44, 46, 49, 50 have GBSV larger than 0, and the grain boundary Sn segregation amount after heat treatment at 700 ° C. for 1 hour is larger than 2 at% by AES measurement. -It was low toughness so that the brittle transition temperature was over 150 ° C. No. Since CI values of 43 to 45 and 47 to 49 were less than 8, the intergranular corrosion resistance evaluated by the improved Strauss test and the weather resistance evaluated by the salt spray test were poor. No. 36, 37, 38, 52, 53, and 51 had high Si, Mn, P, Ni, Cu, and Mo, respectively, and their mechanical properties were poor because elongation decreased due to solid solution strengthening. No. No. 39 has a high S. No. 40 has low Cr. No. 42 has a low Sn. Since 55 had a high B, the corrosion resistance evaluated in the salt spray test was poor. No. Since No. 42 has low Sn, toughness was good even when GBSV was larger than 0. No. No. 45 has a high Nb. 47, 45, 50 are Ti, No. No. 54 had a high V, so wrinkles due to large inclusions occurred, and it was judged that the quality was poor. No. 41 is Cr, No. No. 56 was judged to be of poor quality because Al was high and hot rolling occurred.

Symbols a1 to a3 in Table 3 indicate that the DBTT exceeds 150 ° C. because the grain boundary Sn concentration after the heat treatment with an L value of 1.91 × 10 ⁴ or more is 2 atomic% or more. And toughness was poor. Further, as in a4, when the L value is less than 1.91 × 10 ⁴ , Sn does not segregate at the grain boundary, so DBTT is as low as 80 ° C. However, when the L value increases, Sn segregates at the grain boundary. From the fact that DBTT becomes high, it was confirmed that Sn segregation at the grain boundary must be evaluated at an L value of 1.91 × 10 ⁴ or more.
Moreover, the maximum corrosion depth of all the steels in the range of the present invention was 50 μm or less. In the case of steel containing Ni or Cu in the range of the present invention, the maximum corrosion depth was 20 μm or less, indicating a result of extremely excellent corrosion resistance.

  Further, as apparent from Table 4, the thin plate to which the component composition to which the present invention is applied, the crystal grain size number after cold rolling annealing, the cold rolling plate annealing temperature, and the cooling rate are applied has a low ductile-brittle transition temperature and is good. Showed toughness.

  On the other hand, the symbol b1 has a cold-rolled sheet annealing temperature of 1100 ° C. or higher, and the crystal grain size number defined by the steel-crystal grain size microscopic test method defined in JIS G0551 is less than 5.0. The cooling rate at 20 ° C. was 20 ° C./s, but the ductile-brittle transition temperature was high. The symbol b2 had a cold rolled sheet annealing temperature of less than 850 ° C. and a crystal grain size number exceeding 9.0, so the mechanical properties were poor. Moreover, since b3 and b6 had a cooling rate of less than 5 ° C./s at 800 to 500, the annealing temperature was appropriate and the grain size number was 8.0, but the ductile-brittle transition temperature was high. Furthermore, since b4 and b5 were comparative example components, the cold-rolled sheet annealing temperature, cooling rate, and crystal grain size number were within the proper ranges, but the ductile-brittle transition temperature was high.

  From these results, the above-mentioned findings could be confirmed, and the grounds for limiting the above-described steel compositions and calibration could be supported.

  As is clear from the above description, according to the ferritic stainless steel containing Sn of the present invention, since the stabilizing elements Nb and Ti are optimized, the toughness is deteriorated even when used at a high temperature. It is possible to manufacture a stainless steel plate that is small and excellent in corrosion resistance of a thin plate. Further, by applying the material to which the present invention is applied to exhaust system members of automobiles and two-wheeled vehicles in particular, it becomes possible to extend the life of parts and increase the social contribution. In other words, the present invention has sufficient industrial applicability.

Claims (14)

% By mass
Cr: 13.0-21.0%,
Sn: 0.01 to 0.50%,
Nb: 0.05 to 0.60%
Containing
C: 0.015% or less,
Si: 1.5% or less,
Mn: 1.5% or less,
N: 0.020% or less,
P: 0.035% or less, and S: 0.015% or less,
Limited to
The balance is Fe and inevitable impurities,
Satisfying Equation 1 and Equation 2,
And the grain boundary Sn density | concentration is 2 atomic% or less when the heat processing which the L value shown by Formula 3 becomes 1.91 * 10 < ⁴ > or more is performed at the temperature of 600-750 degreeC, and after cold-rolled sheet annealing A ferritic stainless steel having a grain size number of 5.0 or more and 9.0 or less.
8 ≦ CI = 0.52 Nb / (C + N) ≦ 26 (Formula 1)
GBSV = Sn−2Nb−0.2 ≦ 0 (Expression 2)
L = (273 + T) (log (t) +20) (Equation 3)
Where T: temperature (° C.), t: time (h) % By mass
Cr: 13.0-21.0%,
Sn: 0.01-0.30%,
Nb: 0.20 to 0.60%,
Ti: more than 0.05 and 0.32% or less,
Containing
Furthermore, in mass%,
Ni: 0.5 % or less,
Cu: 1.5% or less,
Mo: 2.0% or less,
V: 0.3% or less,
Al: 0.3% or less,
B: 0.0020% or less of 1 type or 2 types or more ,
C: 0.015% or less,
Si: 1.5% or less,
Mn: 1.5% or less,
N: 0.020% or less,
P: 0.035% or less, and
S: 0.015% or less,
Limited to
The balance is Fe and inevitable impurities,
Satisfying Equation 1 ′ and Equation 2 ′,
And the grain boundary Sn density | concentration is 2 atomic% or less when the heat processing which the L value shown by Formula 3 becomes 1.91 * 10 < ⁴ > or more is performed at the temperature of 600-750 degreeC , and after cold-rolled sheet annealing grain size number of 5.0 or more, ferrites stainless steel, characterized by 9.0 or less.
8 ≦ CI = (Ti + 0.52Nb) / (C + N) ≦ 26 Formula 1 ′
GBSV = Sn + Ti-2Nb-0.3Mo-0.2≤0 Formula 2 '
L = (273 + T) (log (t) +20) (Equation 3)
Where T: temperature (° C.), t: time (h) The ferritic stainless steel according to claim 1 or 2, wherein the heat treatment is performed at 700 ° C for 1 hour. Furthermore, in mass%,
W: 0.20% or less,
Zr: 0.20% or less,
Sb: 0.5% or less,
Co: 0.5% or less,
Ca: 0.01% or less,
Mg: 0.01% or less,
REM: 0.1% or less,
The ferritic stainless steel according to any one of claims 1 to 3, wherein the ferritic stainless steel contains one or more of the following.   The ferritic stainless steel according to any one of claims 1 to 4, wherein the grain size number after cold-rolled sheet annealing is 6.0 or more and 8.5 or less. The stainless steel having the composition according to claim 1, 2 or 4 has a cold-rolled sheet annealing temperature of 850 ° C to 1100 ° C, and then cooled from the cold-rolled plate annealing temperature in a temperature range of 800 to 500 ° C. The manufacturing method of the ferritic stainless steel as described in any one of Claims 1-5 characterized by setting it as 5 degrees C / s or more.   An exhaust system member using the ferritic stainless steel according to any one of claims 1 to 5. % By mass
Cr: 13.0-21.0%,
Sn: 0.01 to 0.50%,
Nb: 0.05 to 0.60%
Containing
Containing at least one of W: 0.01% to 0.20% and Sb: 0.001% to 0.5%,
C: 0.015% or less,
Si: 1.5% or less,
Mn: 1.5% or less,
N: 0.020% or less,
P: 0.035% or less, and S: 0.015% or less,
Limited to
The balance is Fe and inevitable impurities,
Satisfying Equation 1 and Equation 2,
And the ferrite type | system | group characterized by the grain boundary Sn density | concentration being 2 atomic% or less when performing the heat processing which L value shown by Formula 3 becomes 1.91 * 10 < ⁴ > or more at the temperature of 600-750 degreeC. Stainless steel.
8 ≦ CI = 0.52 Nb / (C + N) ≦ 26 (Formula 1)
GBSV = Sn−2Nb−0.2 ≦ 0 (Expression 2)
L = (273 + T) (log (t) +20) (Equation 3)
Where T: temperature (° C.), t: time (h) % By mass
Cr: 13.0-21.0%,
Sn: 0.01-0.30%,
Nb: 0.20 to 0.60%,
Ti: more than 0.05 and 0.32% or less,
Containing
Containing at least one of W: 0.01% to 0.20% and Sb: 0.001% to 0.5%,
Furthermore, in mass%,
Ni: 0.5 % or less,
Cu: 1.5% or less,
Mo: 2.0% or less,
V: 0.3% or less,
Al: 0.3% or less,
B: 0.0020% or less of 1 type or 2 types or more ,
C: 0.015% or less,
Si: 1.5% or less,
Mn: 1.5% or less,
N: 0.020% or less,
P: 0.035% or less, and
S: 0.015% or less,
Limited to
The balance is Fe and inevitable impurities,
Satisfying Equation 1 ′ and Equation 2 ′,
And, at a temperature of 600 to 750 ° C., off to intergranular Sn concentration characterized der Rukoto 2 atomic% or less when subjected to a heat treatment L value shown in Equation 3 is 1.91 × 10 ⁴ or more Ellite stainless steel.
8 ≦ CI = (Ti + 0.52Nb) / (C + N) ≦ 26 (Formula 1 ′)
GBSV = Sn + Ti-2Nb-0.3Mo-0.2≤0 (Formula 2 ')
L = (273 + T) (log (t) +20) (Equation 3)
Where T: temperature (° C.), t: time (h) The ferritic stainless steel according to claim 8 or 9, wherein the heat treatment is performed at 700 ° C for 1 hour. Furthermore, in mass%,
Zr: 0.20% or less,
Co: 0.5% or less,
Ca: 0.01% or less,
Mg: 0.01% or less,
REM: 0.1% or less,
The ferritic stainless steel according to any one of claims 8 to 10, comprising one or more of the following.   The ferritic stainless steel according to any one of claims 8 to 11, wherein the grain size number after cold-rolled sheet annealing is 5.0 or more and 9.0 or less. The stainless steel having the composition according to claim 8, 9, or 11 is set to a cold-rolled sheet annealing temperature of 850 ° C. to 1100 ° C., and then cooled from the cold-rolled plate annealing temperature in a temperature range of 800 to 500 ° C. The method for producing a ferritic stainless steel according to any one of claims 8 to 12, wherein the temperature is 5 ° C / s or more.   An exhaust system member using the ferritic stainless steel according to any one of claims 8 to 12.