JP2009052120A - Stainless steel member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive component while satisfying strength properties required in a high-temperature environment by the reduction of an Ni content. <P>SOLUTION: The austenitic stainless steel member comprises, by mass, 0.04 to 0.10% C, 0.8 to 2.0% Si, 1.0 to 2.0% Mn, 17 to 25% Cr, 5 to 10% Ni and 0.15 to 0.30% N, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a stainless steel wire used for automobile exhaust system parts and the like, and more particularly, to an austenitic stainless steel material excellent in high temperature oxidation resistance and high temperature strength characteristics.

Stainless steel materials with excellent oxidation characteristics and strength characteristics in high-temperature environments are used for exhaust system parts such as automobile exhaust manifolds. In recent years, high temperature combustion has progressed due to the need to reduce nitrogen oxides, and accordingly, parts exposed to exhaust gas are also required to have durability at high temperatures. For this reason, an austenitic stainless steel material that is superior in durability at a higher temperature than a ferrite type is employed (Patent Document 1).
JP 2000-291430 A

  As described above, the austenitic stainless steel material can be used to satisfy the durability in a high temperature environment. However, since Ni contained in the austenitic stainless steel material is expensive, an austenitic stainless steel material was used. There is a problem that the cost of parts increases. In order to reduce the cost of the stainless steel material, it is conceivable to suppress the Ni content. However, if the Ni content contributing to the formation of austenite is reduced, the phase stability is lowered. On the other hand, the Ni content affects the strength of the stainless steel material. For example, the strength of SUS316 having a higher Ni content than SUS304 is lower than SUS306 in a room temperature environment, but higher than SUS306 in a high temperature environment. Therefore, when it is assumed to be used in a high temperature environment of 800 ° C., 900 ° C., and 950 ° C. in which the heat resistant mesh is used, it is preferable that the Ni content is large.

  This invention is made | formed in view of the said situation, and it aims at enabling provision of a low-cost component, without impairing the phase stability when content of Ni is suppressed.

  The present inventors have obtained knowledge that phase stability when the Ni content is suppressed can be ensured by adding N, which is an austenite-forming element. Moreover, when N and Si accounted for a fixed ratio, the knowledge that oxidation-resistant performance could be improved was acquired.

  That is, the present invention, in mass%, C: 0.04-0.10%, Si: 0.8-2.0%, Mn: 1.0-2.0%, Cr: 17-25%, Ni : 5 to 10%, N: 0.15 to 0.30%, with the balance being Fe and inevitable impurities, austenitic stainless steel material, 0.125 × Si% ≦ N% ≦ 0. It contains 125 × Si% + 0.15, and further contains Mo: 0.4 to 3.0% and Co: 0.2 to 1.0% by mass. The stainless steel material of the present invention preferably has a tensile strength at room temperature of 1200 MPa or less.

  Hereinafter, the reason for specifying each element constituting the stainless steel material will be described.

Ni: 5 to 10% by mass
Normally, 10% or more is required to stabilize the austenite phase at a high temperature. However, since the material of the present invention contains 0.15 to 0.30% N, the amount of Ni is less than 10%. Stability can be ensured. However, 10% or less is preferable in order to provide a material cheaper than SUS316. Therefore, in order to obtain the necessary minimum phase stability, it is set to 5% or more, and in order to realize low cost, it is set to less than 10%.

N: 0.15-0.30 mass%
Since N is an austenite generating element, the austenite phase can be stabilized by adding N. Further, since N contributes to the strength characteristics, the addition of N can further improve the strength characteristics of the stainless steel material in combination with the improvement of the strength characteristics due to the decrease in the Ni content. Addition of 0.15% or more is preferable for stabilizing the phase and improving strength properties. In addition, since excessive addition causes blowhole generation at the time of casting and is economically disadvantageous, 0.30% or less is preferable.

Si: 0.8-2.0 mass%
Si is a deoxidizer and is effective in improving corrosion resistance. In order to compensate for the corrosion resistance, which is reduced by suppressing the Ni content, by increasing the Si content, it is preferably 0.8% or more. Since Si is a ferrite-forming element, it contributes to improving the mechanical properties by solid solution strengthening. However, excessive addition decreases the austenite phase stability and decreases the toughness, so 2.0% or less. preferable.

  In relation to N, the corrosion resistance can be improved in the range of 0.125 × Si (mass%) ≦ N (mass%) ≦ 0.125 × Si (mass%) + 0.15.

C: 0.04 to 0.10% by mass
C, as well as N, is an austenite-forming element and has an effect of improving phase stability. However, in an actual steel wire manufacturing process, carbide is likely to precipitate and does not contribute much to phase stability. Therefore, the lower limit for obtaining a certain degree of strength as a structural material is set to 0.04%, and the upper limit for losing toughness is set to 0.10%.

Mn: 1.0 to 2.0% by mass
Mn is an austenite generating element. Like Si, it is used as a deoxidizer during melting and refining. Therefore, the lower limit is set to 1.0% as an addition amount necessary for the deoxidizer. In addition, since excessive addition of Mn causes deterioration of oxidation resistance, the upper limit was made 2.0%.

Cr: 17 to 25% by mass
Cr is an important element contributing to the corrosion resistance of stainless steel. If the Cr concentration is high, the passive film becomes strong and difficult to corrode. Therefore, in order to obtain the effect, the lower limit was made 17%. However, excessive addition reduces the stability of the austenite phase and lowers the mechanical properties, so the upper limit was made 25%.

Mo: 0.4-3.0 mass%
Mo is an element that has a function of repairing the passive state film and improves the corrosion resistance. It is also an element that improves strength characteristics. Therefore, since it is possible to compensate for the decrease in corrosion resistance and the lack of strength due to the suppression of the Ni content, the lower limit is made 0.4% in order to obtain the effect. However, excessive addition causes a decrease in toughness, so the upper limit was made 3.0%.

Co: 0.2-1.0 mass%
Co is an element that improves strength characteristics. In order to obtain the effect, the upper limit was made 0.2%. However, excessive addition increases the cost, so the upper limit was made 1.0%.

Tensile strength of 1200 MPa or less Even if the steel wire is excellent in high temperature strength and high temperature oxidation resistance, if it cannot be knitted as a mesh, its high temperature characteristics cannot be utilized. Considering workability and yield as a heat-resistant mesh, 1200 MPa or less is desirable.

  According to the present invention, since the decrease in phase stability due to the suppression of the Ni content can be compensated by the addition of N, the temperature range where the heat resistant mesh is used while suppressing the Ni content is 800 ° C., 900 It is possible to provide low-cost parts while satisfying the oxidation resistance and strength characteristics required in a high temperature environment of 950 ° C. and 950 ° C.

(Example 1)
(High temperature oxidation test)
Stainless steel (invention material) having the components shown in Table 1 was processed into a soft wire having a wire diameter of 0.148 mm, and then a high temperature oxidation test was performed. The invented material is based on SUS316, which is an austenitic stainless steel, in which Ni is reduced, N is added, and the addition amounts of Si, Mo, Co and other elements are optimized. FIG. 1 is a graph showing the relationship between the amount of N and the amount of Si in stainless steel, and the ratio of N and Si in the inventive material is 0.125 × Si% ≦ N% ≦ 0.125 × Si% + 0.15. It is in the shaded area that satisfies

  The high temperature oxidation test was performed by putting about 30 g of a coiled sample wound tightly into a ceramic crucible. In three conditions of 800 ° C., 900 ° C., and 950 ° C. in which the heat-resistant mesh is used, after heating to the test temperature, a cycle of holding for 30 minutes and then allowing to cool for 20 minutes was repeated 100 times. . Since the sample took 20 minutes to reach 150 ° C. or less, the holding time at each test temperature took 30 minutes, and the temperature rise and fall took about 5 minutes, the test was conducted at about 55 minutes for each cycle. Although there is a difference in ascending / descending time at each test temperature, since the difference in ascending / descending time is within 5 minutes, there is no difference in oxidation characteristics due to this difference. Further, the excessive temperature rise when the temperature is raised to the test temperature is about ± 5 ° C., and therefore, the influence on the sample is small.

  2 to 4 are diagrams showing changes in oxidation increase, in which the vertical axis indicates the increase in oxidation and the horizontal axis indicates the number of cycles. The increase in oxidation of Inventive Material 1 indicated by ● is about ½ of Comparative Material 1 (SUS316) at 800 ° C, about 1/6 at 900 ° C, and about 1/8 at 950 ° C. It can be seen that the oxidation characteristics of Invention Material 1 are better than those of Material 1. Moreover, as a result of observing the cross sections of the inventive material 1 and the comparative material 1 (SUS316), the oxide scale increases as the test temperature increases. However, the comparative material 1 has a steel wire in addition to the oxidation of the steel wire surface. It was confirmed that the grain boundary was oxidized inside, the internal oxidation proceeded, and the surface was spalled up. This is considered to cause a significant increase in oxidation of the comparative material 1. Table 2 shows the increase in oxidation after 96 cycles in the high temperature oxidation test.

  Further, as a result of an EPMA (electron beam microanalysis) surface analysis in the vicinity (cross section) of the sample surface, it was confirmed that the inventive material 1 had a Cr oxide layer even at 950 ° C. Since this layer functions as a passive film, it is considered that explosive oxidation does not occur even at high temperatures. On the other hand, although the comparative material has confirmed the presence of Cr itself, it does not function as a passive film because it does not exist as a layer. Furthermore, since the comparative material was confirmed to have a Cr oxide having a low diffusion rate at a depth of about 30 μm from the sample surface, it was found that the oxide scale had a thickness of 30 μm or more. From this fact, it is considered that the comparative material promoted upward diffusion of Fe and Ni and downward diffusion of O, and explosive oxidation occurred.

  FIG. 5 shows the distribution of elements that are likely to affect the oxidation resistance of the elements in the vicinity of the sample surface. In the figure, symbol a represents Cr, symbol b represents Si, and symbol c represents Ni distribution. . The invention material confirmed the presence of a thin Si layer between the Cr existing portion and the Ni existing portion ((A) in the figure). Since a thin Si oxide layer exists on the lower side (metal side) of Cr, which is a passive film, the adhesion to the passive film is improved. As a result, the downward diffusion of O and the upward diffusion of Fe This is considered to suppress the oxidation resistance and improve the oxidation resistance. On the other hand, the layer structure like the invention material cannot be confirmed in the comparative material, and Cr and Si oxides exist under the Fe oxide, and the state where high-temperature oxidation has progressed can be confirmed ((B) in the figure). .

(High temperature strength test)
Tensile strength was measured at room temperature, 800 ° C., 900 ° C. and 950 ° C. using stainless steel (invention material) and SUS316 (comparative material) used in the high-temperature oxidation test. Table 3 shows the measurement results. It can be confirmed that the inventive material shows higher strength characteristics than the comparative material at any high temperature range of 800 ° C., 900 ° C., and 950 ° C.

(Example 2)
The high temperature oxidation test and the high temperature strength test were performed under the same conditions as in Example 1, using the stainless steel (Invention materials 4 to 10) shown in Table 4 in which the addition amounts of Mo and Co were increased or decreased based on the invention material 1. . In Table 4, invention materials 4 to 7 show stainless steel with variable Mo, and invention materials 8 to 10 show stainless steel with variable Co.

  Table 5 shows the increase in oxidation after 96 cycles in the high temperature oxidation test.

  When the Mo content is in the range of 0.4 to 3%, which is a specific matter of the present invention, the higher the Mo content, the smaller the oxidation increase. Thus, it was confirmed that high oxidation resistance was exhibited. A large correlation could not be confirmed between the Co content and the oxidation increase.

  It was confirmed that the higher the Mo and Co content, the higher the tensile strength. In addition, since the tensile strength of the inventive material 8 containing no Co is reduced, it can be confirmed that Mo is effective for improving the strength characteristics, and Co is an effective element for further improving the strength. It was.

The figure which shows the relationship between N quantity and Si quantity of stainless steel Diagram showing change in oxidation increase (800 ° C) Diagram showing change in oxidation increase (900 ° C) Diagram showing change in oxidation increase (950 ° C) Element distribution near the sample surface (Cr, Si, Ni)

Claims (5)

  C: 0.04 to 0.10% by mass%, Si: 0.8 to 2.0%, Mn: 1.0 to 2.0%, Cr: 17 to 25%, Ni: 5 to 10%, N: An austenitic stainless steel material containing 0.15 to 0.30%, the balance being Fe and inevitable impurities.   The stainless steel material according to claim 1, comprising 0.125 x Si% ≤ N% ≤ 0.125 x Si% + 0.15 by mass%.   Furthermore, the stainless steel material of Claim 1 or 2 containing Mo: 0.4-3.0% by mass%.   Furthermore, the stainless steel material as described in any one of Claim 1 to 3 which contains Co: 0.2-1.0% by mass%.   The stainless steel material according to any one of claims 1 to 4, wherein the tensile strength at room temperature is 1200 MPa or less.