JP5141296B2 - Ferritic stainless steel with excellent high temperature strength and toughness - Google Patents

Description

  The present invention relates to ferritic stainless steel, and more particularly, to exhaust gas path members (hereinafter referred to as “exhaust system”) used in high-temperature environments such as exhaust pipes and catalyst outer cylinders of automobiles and motorcycles and exhaust ducts of thermal power plants. It is related to ferritic stainless steel having both high-temperature strength and toughness, which is used for "member".

  Materials used for exhaust system members typified by automobile exhaust manifolds, exhaust pipes, converter cases, and mufflers are required to be excellent in moldability and heat resistance. Therefore, for such applications, type 429 (14Cr-0.9Si-0.4Nb) steel added with Nb and Si, which has excellent moldability at room temperature and relatively high proof stress at high temperature, has been conventionally used. Many Cr-containing steels are used. However, as the engine performance improves, the exhaust gas temperature tends to rise, and when the temperature rises to near 900 ° C., Type 429 steel has become insufficient in high-temperature proof stress.

  In order to solve this problem, Cr-containing steel in which Nb and Mo are added to improve high-temperature yield strength, SUS444 (19Cr-0.2Nb-1.8Mo) steel defined in JIS G4305, and the like have been developed. Yes. However, in response to improvements in automobile fuel efficiency and stricter exhaust gas regulations, the temperature of exhaust gas from engines tends to rise further, and materials used for automobile exhaust system members have better heat resistance. Is becoming required. In addition, increasing the high-temperature strength of the materials used for exhaust system members enables thinning of the members and contributes to the weight reduction of the automobile body, so that the demand for improvement of the high-temperature strength is increasing.

Under such circumstances, various materials for exhaust system members have been developed. For example, Patent Documents 1 to 5 disclose Cr-containing steel and ferritic stainless steel in which high temperature strength and oxidation resistance are improved by adding W in addition to addition of Nb and Mo. Further, Patent Document 6 discloses a steel having a low thermal expansion and improved thermal fatigue characteristics by adding Cu in addition to the addition of Nb, Mo and W. Patent Documents 7 and 8 disclose Cr-containing steel and ferritic stainless steel in which high temperature strength is improved by adding Cu in addition to addition of Nb, Mo and W.
Japanese Patent Laid-Open No. 2002-212585 JP 2003-213377 A JP 2004-018921 A JP 2004-018914 A JP 2004-076154 A JP 2005-206944 A JP 2001-303202 A JP 2002-004011 A

  However, the Cr-containing steel and ferritic stainless steel disclosed in Patent Documents 1 to 5 are insufficient in heat resistance for use in exhaust system members. In addition, Cr-containing steels and ferritic stainless steels disclosed in Patent Documents 6 to 8 need to add a large amount of alloy elements such as Nb, Mo, W, and Cu, so that the workability of the steel sheet is lowered. There is a problem that the processing of parts must be performed warmly. In particular, when Cu is added in excess of 1%, there is a problem that ε-Cu is precipitated at the time of final annealing cooling in the steel sheet manufacturing process, and the room temperature workability is lowered.

  Accordingly, an object of the present invention is to advantageously solve the above-mentioned problems of the prior art, and to provide a ferritic stainless steel having excellent high temperature strength and excellent toughness. Here, “excelling in high temperature strength” in the present invention means that the 0.2% yield strength at 900 ° C. is 27 MPa or more and the 0.2% yield strength at 650 ° C. is 280 MPa. Further, “excellent toughness” means that the brittle fracture surface ratio is 5% or less when a Charpy impact test is performed at 0 ° C.

  In order to solve the above-mentioned problems, the inventors paid attention to the component system possessed by the ferritic stainless steel and conducted extensive studies. As a result, it is possible to obtain high high-temperature strength in a wide temperature range by adding 3.0 to 5.8 mass% of (Mo + W) and 1.2 to 1.8 mass% of Cu to ferritic stainless steel. In addition, the present inventors have found that the reduction in toughness associated with the addition of Cu can be improved by reducing the Si content and adding an appropriate amount of Al.

That is, the present invention includes C: 0.015 mass% or less, Si: 0.10 mass% or less, Mn: 2.0 mass% or less, P: 0.040 mass% or less, S: 0.010 mass% or less, Al: 0.00. 01 to 0.10 mass%, N: 0.015 mass% or less, Cr: 12.0 to 20.0 mass%, Ni: 1.0 mass% or less, Cu: 1.2 to 1.8 mass%, Mo: 0.8 ~3.0mass%, W: 1.0~ 3.01 mass % and (Mo + W): containing meets 3.0~5.8mass%, further, Nb: not more than 0.3 mass% or more and the following formula;
0.25 ≦ Nb− (93/12) × C− (93/14) × N ≦ 0.50
However, Nb, C and N in the above formula are the mass of each element (mass%)
Is a ferritic stainless steel having a component composition consisting of Fe and inevitable impurities.

  In addition to the above component composition, the present invention further includes B: 0.003 mass% or less, Ti: 0.25 mass% or less, REM: 0.08 mass% or less, Zr: 0.5 mass% or less, V: 0.5 mass% 1 or 2 or more types chosen from below and Co: 0.5 mass% or less are characterized by the above-mentioned.

  According to the present invention, a ferritic stainless steel suitable for use in an automobile exhaust system member having high strength at a high temperature of 900 ° C. and excellent toughness at 0 ° C. can be provided. Therefore, the ferritic stainless steel of the present invention can be used as an exhaust path member of a thermal power generation system and a member for a solid oxide fuel cell that require similar characteristics. Furthermore, since the ferritic stainless steel of the present invention contains Mo and W which are effective for improving corrosion resistance, it can be suitably used for applications requiring corrosion resistance.

First, basic experiments that have triggered the development of the present invention will be described.
C: 0.006 mass%, N: 0.007 mass%, Mn: 0.4 mass%, Cr: 15 mass%, Nb: 0.45 mass%, Mo: 1.5 mass%, W: 2.7 mass%, Cu: 1 .51 mass% and Al: 0.041 mass% as the base composition, and variously changing the content of Si in the range of 0 to 0.4 mass%, melting, hot rolling, cold rolling, Finish annealed to produce a cold-rolled steel sheet with a thickness of 2 mm, take a sub-size Charpy impact test piece with a width of 2 mm from this cold-rolled steel sheet, conduct a Charpy impact test at a temperature of 0 ° C, and determine the brittle fracture surface ratio. Asked. In addition, the direction of the V notch of the test piece was 90 ° direction (TD direction) with respect to the rolling direction.

  FIG. 1 shows the results of the above test. As can be seen from FIG. 1, the toughness is remarkably improved by reducing the Si content, and when the Si content is 0.10 mass% or less, the brittle fracture surface ratio is 5% or less.

  Next, in the same manner as described above, C: 0.006 mass%, N: 0.007 mass%, Si: 0.06 mass%, Mn: 0.4 mass%, Nb: 0.49 mass%, Cr: 16 mass%, Mo Cold rolled steel sheet (sheet thickness: 1.7 mass%, W: 2.3 mass% and Cu: 1.39 mass%) and variously changing the Al content in the range of 0 to 0.2 mass% 2 mm), a Charpy impact test was performed, and the brittle fracture surface ratio at 0 ° C. was determined.

FIG. 2 shows the results of the above test, and it can be seen that by setting the Al content to 0.01 mass% or more, the toughness is improved and the brittle fracture surface ratio is 5% or less. From the above results, it was found that in order to ensure toughness, it was necessary to add 0.01 mass% or more of Al after Si was 0.1 mass% or less.
The present invention has been developed based on the above findings and further studies.

Next, the component composition that the ferritic stainless steel of the present invention should have will be described.
C: 0.015 mass% or less C is a component effective for increasing the strength of steel, and is preferably contained in an amount of 0.001 mass% or more in order to ensure a desired strength. However, if C is contained in excess of 0.015 mass%, the deterioration of toughness and formability becomes remarkable, so in the present invention, it is set to 0.015 mass% or less. In addition, from the viewpoint of ensuring moldability, the lower the C content, the better, and 0.008 mass% or less is preferable. More preferably, C is in the range of 0.002 to 0.008 mass%.

Si: 0.10 mass% or less Si is an important element that needs to be strictly limited in content in order to improve the toughness of the ferritic stainless steel of the present invention. As shown in FIG. 1, the effect of improving toughness by reducing Si can be obtained for the first time by limiting the Si content to 0.10 mass% or less after setting the Al content to 0.01 mass% or more. Therefore, the upper limit is 0.10 mass%. Preferably, Si: 0.08 mass% or less

Mn: 2.0 mass% or less Mn is added as a deoxidizer and as a component for increasing the strength of the steel sheet. However, excessive addition generates a γ phase at a high temperature to lower the heat resistance. Therefore, in the present invention, the Mn content is set to 2.0 mass% or less. Preferably it is 1.5 mass% or less.

P: 0.040 mass% or less P is an impurity that is inevitably mixed in steel, and is a harmful element that lowers toughness. Therefore, it is desirable to reduce it as much as possible. Therefore, in this invention, it is 0.040 mass% or less. Preferably it is 0.030 mass% or less.

S: 0.010 mass% or less S is an impurity inevitably mixed in the steel, and lowers the elongation and r-value of the steel sheet, promotes precipitation of the Laves phase, hardens the steel, and lowers formability. It is an element. Moreover, since it is also an element which reduces the corrosion resistance which is the basic characteristic of stainless steel, it is desirable to reduce it as much as possible. Therefore, in this invention, S is made into 0.010 mass% or less.

Al: 0.01-0.10 mass%
Al is an important element necessary for improving the toughness of the ferritic stainless steel of the present invention. As shown in FIG. 2, after limiting Si to 0.10 mass% or less, Al is 0.01 mass%. The effect is acquired for the first time by adding above. On the other hand, when it is added in excess of 0.10 mass%, the toughness improving effect is saturated, and the descalability after cold rolling annealing is lowered, resulting in poor productivity. Therefore, the Al content is 0.01 to 0.10 mass%. Preferably, it is the range of 0.015-0.08 mass%.

N: Less than 0.015 mass% N is an element that lowers the toughness and formability of steel, and when it exceeds 0.015 mass%, this effect becomes significant. For this reason, it is desirable to reduce N as much as possible, and it is 0.015 mass% or less. Preferably it is 0.010 mass% or less.

Cr: 12.0 to 20.0 mass%
Cr is a component necessary for ensuring the corrosion resistance and oxidation resistance of ferritic stainless steel. In order to acquire the said effect, addition of 12.0 mass% or more is required. On the other hand, Cr dissolves in the steel to increase the room temperature strength and harden, which causes a decrease in ductility. In particular, when the Cr content exceeds 20.0 mass%, this effect becomes significant, so the upper limit of Cr is 20.0 mass%. Therefore, in this invention, Cr is taken as the range of 12.0-20.0 mass%. Preferably it is the range of 14.0-19.0 mass%.

Ni: 1.0 mass% or less Ni is an element that improves the toughness of steel. However, Ni is not only expensive but also a strong γ-phase-forming element, promotes the formation of γ-phase at high temperatures, and reduces oxidation resistance. Therefore, Ni is set to 1.0 mass% or less. Preferably, it is the range of 0.05-0.6 mass%.

Cu: 1.2-1.8 mass%
Cu is an important component that contributes to improving the strength in a wide temperature range from room temperature to the precipitation temperature by precipitating as ε-Cu in the temperature range of 500 to 750 ° C. In particular, in order to achieve the target 0.2% proof stress at 650 ° C. of 280 MPa or more, which is the target of the present invention, it is necessary to add 1.2 mass% or more of Cu. On the other hand, if the Cu content exceeds 1.8 mass%, a large amount of ε-Cu precipitates during cooling in the final annealing in steel sheet production, and even if the Si and Al contents are optimized, the toughness of the steel sheet The decrease is remarkable. Therefore, in the present invention, the Cu content is set to a range of 1.2 to 1.8 mass% from the viewpoint of improving strength and securing toughness.

Mo: 0.8-3.0 mass%
Mo is an important component having the effect of increasing the high temperature proof stress and improving the corrosion resistance and oxidation resistance by being present in a solid solution state in the steel. Such an effect is recognized by addition of 0.8 mass% or more. On the other hand, when the content exceeds 3.0 mass%, precipitation of the Laves phase becomes prominent, and the amount of Mo existing in a solid solution state decreases, so that the contribution to improving high-temperature proof stress and corrosion resistance is reduced, and the strength at room temperature is increased. Processability is reduced. Therefore, Mo is set to a range of 0.8 to 3.0 mass%. Preferably, it is the range of 1.0-3.0 mass%.

W: 1.0-5.0 mass%
W, like Mo, is an important component in the present invention because it has the effect of increasing high-temperature proof stress and improving corrosion resistance and oxidation resistance by being present in a solid solution state in steel. Such an effect is recognized by containing 1.0 mass% or more. On the other hand, if it exceeds 5.0 mass%, the Laves phase precipitates significantly, the amount of W present in the solid solution state is saturated, and the toughness and workability deteriorate. Therefore, W is set to a range of 1.0 to 5.0 mass%. Preferably it is the range of 2.0-4.0 mass%.

(Mo + W): 3.0 to 5.8 mass%
Mo and W are components having the same effect as described above. However, in order to achieve the high temperature strength targeted by the present invention, that is, 0.2% proof stress at 900 ° C .: 27 MPa or more and 0.2% proof stress at 650 ° C .: 280 MPa or more, the total amount of Mo and W ( Mo + W) needs to be 3.0 mass% or more. On the other hand, when (Mo + W) exceeds 5.8 mass%, the above-described effects are saturated, and toughness and workability are lowered. Therefore, (Mo + W) is in the range of 3.0 to 5.8 mass%. Preferably it is the range of 3.5-5.0 mass%.

Nb: 0.3 mass% or more and
Nb- (93/12) × C- (93/14) × N): 0.25 to 0.50 mass%
Nb is an element that has the effect of fixing C and N and enhancing high-temperature strength, formability, corrosion resistance, and intergranular corrosion resistance of welds. Such an effect is Nb: 0.3 mass% or more and
Nb- (93/12) × C- (93/14) × N): recognized at 0.25 mass% or more. Here, the above formula means a so-called “effective Nb” obtained by subtracting the amount of Nb consumed for fixing C and N from the amount of added Nb. On the other hand, when the effective Nb content exceeds 0.50 mass%, a large amount of Laves phase is precipitated, the high temperature strength improving effect of Nb is saturated, and toughness and surface properties are deteriorated. For this reason, in this invention, Nb shall be 0.3 mass% or more and it is set as the range of effective Nb: 0.25-0.50 mass%. When particularly excellent high-temperature strength is required, the effective Nb is preferably 0.30 mass% or more.

In addition to the above components, the ferritic stainless steel of the present invention may further contain one or more selected from B, Ti, REM, Zr, V and Co in the following range.
B: 0.003 mass% or less B is an element effective for improving workability, particularly secondary workability. This effect is manifested at B: 0.0005 mass% or more. On the other hand, if the content exceeds 0.003 mass%, a large amount of BN is generated, resulting in a decrease in workability. Therefore, when adding B, it is preferable to set it as 0.003 mass% or less.

Ti: 0.25 mass% or less Ti is an effective component for improving elongation and r value. However, if added in excess of 0.25 mass%, the toughness is significantly reduced even in the component system of the present invention. Therefore, when adding Ti, the content is limited to 0.25 mass% or less.

REM: 0.08 mass% or less, Zr: 0.5 mass%
REM (rare earth element) and Zr are both elements that improve oxidation resistance, and can be contained as necessary. However, the content exceeding REM: 0.08 mass% embrittles the steel. Further, if the Zr content exceeds 0.5 mass%, a Zr intermetallic compound precipitates, and the steel is also embrittled. For this reason, when adding REM, it is preferable to set it as 0.08 mass% or less, and when adding Zr, it is preferable to set it as 0.5 mass% or less.

V: 0.5 mass% or less V is an element effective for improving moldability. However, excessive content exceeding 0.5 mass% causes coarse V (C, N) to precipitate and deteriorates the surface properties. For this reason, when adding V, it is preferable to set it as 0.5 mass% or less.

Co: 0.5 mass% or less Co is an element effective for improving toughness, but the effect is saturated even when added in excess of 0.5 mass%. Moreover, since Co is also an expensive component, when adding it, 0.5 mass% or less is preferable.
In the steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

  The manufacturing method of the ferritic stainless steel according to the present invention is not particularly limited, and a known method can be applied. For example, steel having a component composition suitable for the present invention is melted by a known method such as a converter or an electric furnace, and further subjected to secondary refining such as ladle refining or vacuum refining, and then continuous casting. Steel strip (slab) is formed by the method or ingot-bundling rolling method. Then, it is preferable to make it a cold-rolled annealing board through each process, such as hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, pickling. The cold rolling may be performed once or twice or more with intermediate annealing. Further, in addition to cold rolling, finish annealing and pickling steps may be repeated. Moreover, you may abbreviate | omit hot-rolled sheet annealing. Furthermore, when glossiness is required on the steel sheet surface, skin pass rolling may be performed.

  No. shown in Table 1. Steel having a component composition of 1 to 16 is melted in a vacuum melting furnace to form 50 kg steel ingots, and these steel ingots are heated to 1170 ° C. and hot-rolled to obtain a hot rolled sheet having a thickness of 5 mm. did. Subsequently, these hot-rolled sheets are subjected to hot-rolled sheet annealing (annealing temperature: 1040 ° C.), pickling, cold rolling (cold rolling reduction ratio: 60%), and finish annealing (annealing temperature: 1050 ° C., average). (Cooling rate: 20 ° C./sec) and pickled to obtain a cold-rolled annealed plate having a thickness of 2 mm. As reference examples, cold-rolled annealed plates were prepared in the same manner for the steel sheets described as Examples in Patent Documents 6 to 8, and No. 1 is shown in Table 1. Shown as 17-20.

The various cold-rolled annealed plates obtained as described above were subjected to the following evaluation tests.
(1) High-temperature strength Two tensile test pieces each having the rolling direction as the tensile direction were sampled from each cold-rolled annealed sheet, and strains were measured at 900 ° C. and 650 ° C. in accordance with JIS G0567. Speed: Perform high-temperature tensile test at 0.3% / min and measure 0.2% yield strength (σ _{0.2 at} 900 ° C.) at 900 ° C. and 0.2% yield strength (σ _{0.2 at} 650 ° C.) at 650 ° C. did. The evaluation of the high-temperature strength is as follows: σ _{0.2 at} 900 ° C. is good (◯) for 27 MPa or more, poor (×) if it is less than 27 MPa, and σ _{0.2 at} 650 ° C. is good (◯) for 280 MPa or more. Less than 280 MPa was determined to be defective (x).
(2) Toughness From each cold-rolled annealed plate, sub-size Charpy impact test pieces with a width of 2 mm and a notch direction of 90 ° in the rolling direction were collected, and five Charpy impact tests were conducted at 0 ° C. Then, the obtained fracture surface was observed to determine the average value of the brittle fracture surface ratio. In the evaluation of toughness, a case where the brittle fracture surface ratio was 5% or less was judged as good (◯), and a case where it exceeded 5% was judged as bad (×).

  The results of the above test are shown in Table 2. From Table 2, the steel sheets of the examples of the present invention all have excellent high-temperature strength such that the 0.2% proof stress at 900 ° C. is 27 MPa or more, the 0.2% proof stress at 650 ° C. is 280 MPa or more, and at 0 ° C. It can be seen that the brittle fracture surface ratio is 5% or less and excellent in toughness. On the other hand, it can be seen that none of the comparative examples or the prior art steel sheets that are outside the scope of the present invention satisfy one or more of both properties.

  The steel sheet of the present invention can be used as an exhaust path member of a thermal power generation system or a solid oxide fuel cell member in addition to an automobile exhaust system member. The steel plate of the present invention can also be used as a corrosion resistant steel plate.

It is a graph which shows the relationship between the brittle fracture surface rate in 0 degreeC, and Si content. It is a graph which shows the relationship between the brittle fracture surface rate in 0 degreeC, and Al content.

Claims (2)

C: 0.015 mass% or less, Si: 0.10 mass% or less, Mn: 2.0 mass% or less, P: 0.040 mass% or less, S: 0.010 mass% or less, Al: 0.01 to 0.10 mass% N: 0.015 mass% or less, Cr: 12.0 to 20.0 mass%, Ni: 1.0 mass% or less, Cu: 1.2 to 1.8 mass%, Mo: 0.8 to 3.0 mass%, W: 1.0 to 3.01 mass% and (Mo + W): 3.0 to 5.8 mass%, and Nb: 0.3 mass% or more and the following formula:
0.25 ≦ Nb− (93/12) × C− (93/14) × N ≦ 0.50
However, Nb, C and N in the above formula are the mass of each element (mass%)
And ferritic stainless steel having a component composition in which the balance is composed of Fe and inevitable impurities. In addition to the above component composition, B: 0.003 mass% or less, Ti: 0.25 mass% or less, REM: 0.08 mass% or less, Zr: 0.5 mass% or less, V: 0.5 mass% or less, and Co: The ferritic stainless steel according to claim 1, comprising one or more selected from 0.5 mass% or less.

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