JP4468137B2 - Ferritic stainless steel material and automotive exhaust gas path member with excellent thermal fatigue characteristics - Google Patents

Description

  The present invention relates to a ferritic stainless steel material excellent in thermal fatigue characteristics, and an automobile exhaust gas path member using the same. In particular, a member that constitutes the upstream portion of the exhaust gas path and is heated to 700 ° C. or higher when the automobile is used is a suitable target.

  Of the exhaust gas path members of automobiles, it is needless to say that heat resistance in a high temperature range exceeding 700 ° C. is required for an upstream side member that is heated to 700 ° C. or more during use. Therefore, in general, materials have been developed mainly for increasing the high-temperature strength in a high-temperature region such as 700 to 1000 ° C. In addition, the automobile exhaust gas path member repeatedly undergoes a heat cycle of temperature rise to a high temperature range and temperature drop to room temperature as the engine is started and stopped. For this reason, the strength in the middle temperature range of 500 to 700 ° C. that passes in the temperature raising process and the cooling process is also important, and component design that improves the high temperature strength in a wide temperature range of 500 to 1000 ° C. has been studied.

  On the other hand, from the standpoint that it is also important for automobile exhaust gas path members to be excellent in thermal fatigue characteristics as resistance to repeated temperature rise and fall, in Patent Document 1, in a ferritic stainless steel, a medium temperature range of 600 to 750 ° C. As an improvement in strength leads to an improvement in thermal fatigue properties, the strength is prevented from decreasing in the middle temperature range. The main means is to contain 1 to 3% of Cu in the steel and generate Cu fine precipitates in the middle temperature range.

  Also in Patent Document 2, 1.0 to 1.7% of Cu is contained in ferritic stainless steel, and the high temperature strength at 700 ° C. or 800 ° C. is improved by precipitating this during heating.

Japanese Patent No. 3468156 International Publication No. 03/004714 Pamphlet

  In the idea of increasing the high temperature strength in a wide range of 500 to 1000 ° C. as described above, it is necessary to rely on the component design in which a large amount of special elements are added, and the material cost increases. In addition, the moldability is disadvantageous.

In the technique of adding Cu in Patent Documents 1 and 2, the strength of the steel material is improved in a temperature range of 600 ° C. or 700 to 800 ° C. by using precipitation hardening by a Cu precipitation phase.
However, as a result of detailed investigations by the inventors, even when an annealed material of ferritic stainless steel simply added with an appropriate amount of Cu is repeatedly produced, the temperature rise to over 700 ° C. and the temperature drop to room temperature are repeated. In an automobile exhaust gas path upstream member, satisfactory thermal fatigue characteristics have not always been realized with good reproducibility.

  The present invention relates to a ferritic stainless steel material that has a basic high temperature strength level and molding processability when assuming an automobile exhaust gas path upstream member, and further relates to the durability of the member while having a low temperature toughness. The aim is to develop a technology that stably improves the “characteristic” by a relatively inexpensive component design, and to provide a steel material and an automobile exhaust gas member to which the technology is applied.

  As a result of various studies, the inventors have found that, in order to improve the durability and reliability of the upstream member of the automobile exhaust gas path, it is extremely advantageous to improve the thermal fatigue characteristics stably in a comprehensive manner. . The basic heat resistance (high temperature strength and oxidation resistance) in the high temperature range exceeding 700 ° C. is considerably high at present due to the accumulation of conventional technologies. In order to further improve this, a great increase in costs such as addition of special elements is required. On the other hand, there is still much room for improvement in durability (thermal fatigue characteristics) that can withstand the heat cycle of temperature rise / fall when using an automobile. It is considered to be extremely effective for improving reliability.

  As a result of detailed studies by the inventors, in a ferritic stainless steel material to which Cu has been added, a Cu precipitate (herein referred to as “ε-Cu phase”) at a stage prior to actual use as an exhaust gas member. It was found that the thermal fatigue characteristics were stably improved in the subsequent repeated cycle of temperature increase and decrease by keeping the existence form of in a certain state. The present invention has been completed based on such findings.

That is, the ferritic stainless steel material (for example, steel plate) excellent in thermal fatigue characteristics provided by the present invention is in mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 1.5% or less. Ni: 0.6% or less, Cr: 10 to 20%, Ti: 0.05 to 0.30%, Nb: 0.10 to 0.60%, Mo: 0 to less than 0.10%, Cu: 0.8 to 2.0%, Al: 0 to 0.10%, B: 0.0005 to 0.02%, V: 0.03 to 0.20%, N: 0.03% or less, balance Fe In addition, it has a composition consisting of inevitable impurities, satisfying the following formulas (1) and (2), and having a structure in which ε-Cu phases having a major axis of 0.5 μm or more are adjusted to 10/25 μm ² or less. is there.
Nb-8 (C + N) ≧ 0 (1)
10Si + 20Mo + 30Cu + 20 (Ti + V) + 160Nb- (Mn + Ni) ≥100 (2)

  Here, the lower limit of 0% for Mo, Al, and V is the case where the content of these elements is below the measurement limit in a normal analysis method performed in the steelmaking process. The presence of the ε-Cu phase can be confirmed by observation with a transmission electron microscope on a sample having a cross section (C-cross section) perpendicular to the rolling direction of the steel material. The content of the element expressed in mass% is substituted for the element symbol in the formulas (1) and (2).

  Moreover, in this invention, the motor vehicle exhaust gas path member comprised with the said steel material is provided. The member is manufactured, for example, by subjecting a steel plate, which is the steel material, to a process such as bending or welding pipe forming. Examples of the automobile exhaust gas path member include those constituting an exhaust manifold, a catalytic converter, a front pipe or a center pipe, and in particular, the temperature is raised to a temperature of 700 ° C. or higher during engine operation, and from the temperature rise temperature after the engine is stopped. A member cooled to an average cooling rate of 0.1 to 30 ° C./second up to 400 ° C. is a preferable application target.

  According to the present invention, the thermal fatigue characteristics of the ferritic stainless steel material could be remarkably improved without resorting to expensive elements such as Mo. This steel material adopts a rational component design that avoids increasing the high temperature strength more than necessary in a high temperature range exceeding 700 ° C., and repeatedly raises the temperature to a high temperature range exceeding 700 ° C. and lowers the temperature to room temperature. It exhibits excellent durability. Moreover, what is provided as a steel plate can be bent and welded, and has low temperature toughness. For this reason, this ferritic stainless steel material is suitable for an upstream member of an automobile exhaust gas path member that is heated to a temperature exceeding 700 ° C. in particular. And the automobile exhaust gas path member using this steel plate realizes simultaneously reduction of material cost and improvement of durability and reliability.

  For example, in the case of a ferritic stainless steel material used for a member that is used by being exposed to a high temperature exceeding 700 ° C. or 800 ° C., such as an upstream member of an automobile exhaust gas route, conventionally, Component design has been made with emphasis on improving high temperature strength and oxidation resistance. For this purpose, the addition of expensive elements is indispensable, and an increase in material cost is inevitable.

  However, according to the detailed examination by the inventors, in the application where the temperature rise to the high temperature region and the temperature fall to the room temperature are frequently repeated like the automobile exhaust gas path member, considering the durable life of the member comprehensively, Rather than improving the high-temperature strength in a wide range from 500 ° C to 900 ° C, for example, in an all-round manner, it is better to improve the thermal fatigue characteristics in the middle temperature range of 500-700 ° C. It has been found to be advantageous for improving the performance and reducing the cost.

  In the present invention, in order to improve the thermal fatigue characteristics, precipitation of ε-Cu phase is used in ferritic steel added with Cu. In ferritic stainless steel to which, for example, about 1 to 2% by mass of Cu is added, precipitation of ε-Cu phase occurs in the middle temperature range around 600 ° C., and precipitation strengthening occurs when this is finely dispersed in the matrix. . When the temperature exceeds about 900 ° C., solidification of the ε-Cu phase into the matrix proceeds. And it precipitates again in the subsequent temperature-fall process.

  Conventionally, there are examples in which the intermediate temperature region is strengthened by utilizing precipitation of ε-Cu phase (Patent Documents 1 and 2). However, when these are used for the upstream member of the automobile exhaust gas path, the thermal fatigue characteristics are not always stably improved, and the reliability cannot be satisfied. As a result of various studies conducted by the inventors to solve this problem, the metallographic structure of the steel material in the stage (hereinafter sometimes referred to as the “initial stage”) before being mounted on the automobile exhaust gas path and receiving the first temperature rise / fall history. The condition was found to have a significant impact on thermal fatigue properties in subsequent use.

That is, if the steel plate material is processed into an automobile exhaust gas path upstream member, at the steel plate stage before forming into the member, the number of ε-Cu phases having a major axis of 0.5 μm or more is 10/25 μm ² or less (ie, The tissue state adjusted to 10 or less per 25 μm ² may be realized. More preferably, the number is 5/25 μm ² or less.

If there are more than 10/25 μm ² ε-Cu phases with a major axis of 0.5 μm or more, the first temperature rise in actual use after processing the member, for example, when the engine is running at 800 ° C. or more When the temperature is reached, there will be cases where the ε-Cu phase does not sufficiently dissolve. In this case, when the engine is stopped, the temperature lowering is started in a state where a considerable ε-Cu phase already exists. Then, since a new ε-Cu phase precipitates with the surface of the existing ε-Cu phase as the main precipitation site in the temperature lowering process, sufficient precipitation strengthening due to fine dispersion does not occur. That is, the strength in the temperature lowering process that affects the thermal fatigue characteristics is not sufficiently ensured. Then, at the next temperature rise, the temperature rise starts from the textured state in which the coarsened ε-Cu phase exists, and then the structure in which the ε-Cu phase is finely dispersed in the process of repeating the heat cycle of temperature rise / fall. The state will not be realized indefinitely. As a result, no improvement in thermal fatigue properties is achieved.

On the other hand, it is desirable that the number of ε-Cu phases having a major axis of 0.5 μm or more in the initial stage is 0/25 μm ² (substantially not observed), but smaller ε-Cu phases are dispersed in the matrix. It does not matter if it exists. Even if a fine ε-Cu phase is present, it re-dissolves in the first heating process as an exhaust gas path member, and the ε-Cu phase precipitates finely in the matrix in the subsequent cooling process, resulting in a precipitation strengthening phenomenon. To do.

The structure state in which the ε-Cu phase having a major axis of 0.5 μm or more is adjusted to 10/25 μm ² or less can be realized by increasing the cooling rate of the finish annealing performed in the steel material manufacturing stage. However, if the cooling rate is too high, Cu remains in a solid solution, and the fine ε-Cu phase is not dispersed and is not preferable. In the experiment of the inventors, in an example of performing a finish annealing of 950 to 1050 ° C. × soaking 0 seconds to soaking 60 seconds in a continuous line in a manufacturing process of a ferritic stainless steel plate having a component composition described later, By controlling the average cooling rate from 900 ° C. to 400 ° C. to 10 to 30 ° C./second, a desirable tissue state was obtained.

Hereinafter, the component composition will be described.
C and N are generally effective elements for improving high-temperature strength such as creep strength, but if contained excessively, oxidation characteristics, workability, low-temperature toughness, and weldability deteriorate. In the present invention, both C and N are limited to 0.03 mass% or less.

  Si is effective in improving high-temperature oxidation characteristics, but if added excessively, hardness increases, and workability and low-temperature toughness decrease. In the present invention, the Si content is limited to 1.0 mass% or less.

  Mn improves the high temperature oxidation properties, particularly the scale peel resistance. However, excessive addition hinders workability and weldability. Further, since Mn is an austenite stabilizing element, if added in a large amount, a martensite phase is likely to be generated, which causes a decrease in thermal fatigue characteristics and workability. For this reason, Mn content shall be 1.5 mass% or less.

  Cr stabilizes the ferrite phase and contributes to the improvement of oxidation resistance, which is important for high temperature materials. However, excessive Cr content causes embrittlement and workability deterioration of the steel material. For this reason, Cr content shall be 10-20 mass%. The Cr content is preferably adjusted according to the use temperature of the material. For example, when high temperature oxidation resistance up to 950 ° C. is required, Cr content of 16% by mass or more is desired, and up to 900 ° C. may be in the range of 12-16% by mass.

Ti is effective in improving moldability. The mechanism is not necessarily clear, but when Nb-Ti-based precipitates are formed when heat-treating hot-rolled sheets, the formability of cold-rolled annealed plates is remarkably improved. It is inferred that this contributes to the formation of an effective texture, that is, a texture in which (111) or (211) faces are accumulated parallel to the rolling surface. It is conceivable that the precipitate itself acts directly, or a decrease in solid solution C accompanying the formation of the precipitate acts.
However, excessive addition of Ti causes deterioration of the surface properties due to the formation of TiN, and adversely affects weldability and low temperature toughness. The Ti content is specified to be 0.05 to 0.30 mass%.

  Nb is a very effective element for ensuring high temperature strength in a high temperature range exceeding 700 ° C. In this component system, the contribution by solid solution strengthening is considered to be large. However, excessive Nb addition is not preferable because it causes deterioration of workability, low-temperature toughness, and increased weld hot cracking sensitivity. The Nb content is defined as 0.10 to 0.60% by mass.

  Mo is effective for improving the high-temperature strength, but expensive Mo is not particularly required in the present invention. When a large amount of Mo is contained, workability, low temperature toughness and weldability deteriorate. The Mo content is limited to less than 0.10% by mass.

  Cu is an important element in the present invention. That is, in the present invention, as described above, the strength in the middle temperature range of 500 to 700 ° C. is increased by utilizing the fine dispersion precipitation phenomenon of the ε-Cu phase, and the thermal fatigue characteristics are improved. For this purpose, at least 0.8% by mass of Cu is required. However, since excessive Cu content reduces workability, low temperature toughness, and weldability, the upper limit of Cu content is limited to 2.0% by mass.

  Al is a deoxidizer and improves high-temperature oxidation resistance. However, if Al is contained in a large amount, the surface properties, workability, weldability, and low temperature toughness are adversely affected. For this reason, when adding Al, it carries out in the content range of 0.10 mass% or less.

  B is effective for improving secondary work brittleness. The mechanism is presumed to be due to the decrease in grain boundary solid solution C and the strengthening of grain boundaries. However, excessive addition of B deteriorates manufacturability and weldability. In the present invention, B is contained in the range of 0.0005 to 0.02 mass%.

V contributes to the improvement of the high-temperature strength by the combined addition with Nb and Cu. In addition, coexistence with Nb improves workability, low temperature toughness, intergranular corrosion susceptibility, and toughness of weld heat affected zone. However, excessive addition causes workability and low temperature toughness. Therefore , the V content is in the range of 0.03 to 0.20% by mass . And more preferably be 0.04 to 0.15 mass%.

It is necessary to adjust the composition so that the content of each element is in the above range and the following formulas (1) and (2) are satisfied.
Nb-8 (C + N) ≧ 0 (1)
10Si + 20Mo + 30Cu + 20 (Ti + V) + 160Nb- (Mn + Ni) ≥100 (2)
Here, equation (1) is a rule for securing solid solution Nb, and equation (2) is a rule for securing basic high-temperature strength.

  The ferritic stainless steel material of the present invention, for example, melts the steel whose composition has been adjusted as described above, and performs a hot rolling → annealing → pickling process, or further, “cold rolling → annealing → pickling” 1 It can be manufactured by a process performed once or multiple times. However, in order to obtain the precipitation form of the ε-Cu phase as described above, it is desirable to control the average cooling rate from 900 ° C. to 400 ° C. in the range of 10 to 30 ° C./second in the finish annealing. Here, "finish annealing" is the last annealing performed in the manufacture stage of steel materials, for example, the heat processing hold | maintained at 950-1050 degreeC for 0-3 minutes soaking.

  The steel material thus obtained is subjected to forming and welding to produce an automobile exhaust gas path member. For example, in the case of an exhaust manifold or a front pipe, a steel plate having a desired thickness can be formed by welding, and bending can be performed as necessary to form a member.

  Ferritic stainless steel shown in Table 1 was melted, and an annealed steel sheet having a thickness of 2 mm was obtained in the steps of hot rolling → hot rolled sheet annealing → cold rolling → finish annealing → pickling. Further, a round bar having a diameter of about 25 mm was made by hot forging using a part of the cast slab, and this was subjected to finish annealing. Finish annealing after cold rolling and finishing annealing after hot forging, except for steel No. 10, after holding at 1000 ° C. for 1 minute, the average cooling rate from 900 ° C. to 400 ° C. is 10 to 30 ° C. The cooling rate was controlled to be 1 / second. In Steel No. 10, after holding at 1000 ° C. for 1 minute, the average cooling rate from 900 ° C. to 400 ° C. was slowed to about 50 ° C./second (common conditions for both rolled material and bar material).

With respect to the plate material and the bar material after finish annealing, the metal structures were observed in cross sections perpendicular to the rolling direction and the longitudinal direction, respectively. The size of the ε-Cu phase was examined using a transmission electron microscope, and the number of ε-Cu phases having a major axis of 0.5 μm or more observed per 25 μm ² was determined. At least 10 visual fields were observed per sample, and the average was taken. The results are shown in Table 2, where ε-Cu phases having a major axis of 0.5 μm or more are 10/25 μm ² or less, and ◯ is the others. For each steel, there was no difference in the results between the plate and the bar, so the evaluation of the amount of ε-Cu shown in Table 2 applies to both the plate and the bar.

Using a plate material, a tensile test at normal temperature was performed to evaluate workability. The tensile direction was three types of 0 ° (parallel), 45 °, and 90 ° with respect to the rolling direction, and the test piece was a JIS 13B test piece. The tensile test of JIS Z2241 was conducted until breakage, and the test pieces after breakage were butted together to measure the elongation at breakage. The average elongation value EL _A was determined by the following formula.
EL _A = (EL _L + 2EL _D + EL _T )
However, EL _L is the elongation (%) in the 0 ° direction relative to the rolling direction, EL _D is the elongation (%) in the 45 ° direction relative to the rolling direction, and EL _T is the elongation (%) in the 90 ° direction relative to the rolling direction. is there.
What EL _A is more than 30% ○, it was evaluated as × those less than 30%.

  Using the plate material, an impact test was conducted to evaluate low temperature toughness. V-notch impact test specimens were collected so that the direction in which the impact was applied was the rolling direction of the plate, and the impact test of JIS Z2242 was performed at a pitch of 25 ° C. in the range of −75 to 25 ° C. to determine the ductile brittle transition temperature. . A transition temperature lower than −50 ° C. (those exhibiting a ductile fracture surface even at −50 ° C.) was evaluated as “◯”, and a transition temperature of −50 ° C. or higher was evaluated as “x”.

  Using the plate material, a high-temperature continuous oxidation test was conducted to evaluate high-temperature oxidation resistance. Using a 25 x 35 mm test piece with a surface and end surface of # 400 wet polished, a high-temperature continuous oxidation test according to JIS Z2281 is performed in the atmosphere at 900 ° C for 200 hours, and the test piece after the test is visually observed. Then, the formation of a knot-like thick oxide scale was defined as abnormal oxidation, and the presence or absence of abnormal oxidation was determined. The case where abnormal oxidation was not recognized was evaluated as ◯, and the case where abnormal oxidation was observed was evaluated as ×.

  Using the plate material, a high temperature tensile test was performed to evaluate the high temperature strength. A high temperature tensile test in accordance with JIS G0567 was performed at 600 ° C., and a 0.2% proof stress was obtained. The value was evaluated as ◯ when the value was 180 MPa or more, and x when the value was less than 180 MPa.

A thermal fatigue test was conducted using the bar material, and the thermal fatigue characteristics were evaluated. A notched round bar test piece having a diameter of 10 mm and a diameter of 7 mm in the center between the gauge points was prepared from the bar, and was held at 200 ° C. for 0.5 minutes in the atmosphere with a binding force of 20%. The heat cycle was repeated with the temperature rising to 900 ° C. at 3 ° C./second→holding at 900 ° C. × 0.5 min → cooling to 200 ° C. at a cooling rate of about 3 ° C./second ” The number of repetitions when the stress decreased to 75% of the initial stress was defined as the thermal fatigue life, and the thermal fatigue life of 900 cycles or more was evaluated as ◯, and the cycle of less than 900 cycles was evaluated as x.
These results are shown in Table 2.

  As can be seen from Table 2, the examples of the present invention satisfying the chemical composition defined in the present invention and the precipitation form of the ε-Cu phase exhibit excellent thermal fatigue properties, and have workability, low temperature toughness, and high temperature oxidation resistance. Both high-temperature strength and the characteristics suitable for the upstream member of the automobile exhaust gas path were provided. Although not shown in the table, each of these exhibited a structure in which fine ε-Cu phases having a major axis of less than 0.5 μm were dispersed in the matrix.

On the other hand, steel No. 10 of the comparative example has the chemical composition specified in the present invention, but the cooling rate after finish annealing was slower than 10 ° C./second, so that the ε-Cu phase having a major axis of 0.5 μm or more was present. The number exceeded 10/25 μm ² , and the high temperature strength and thermal fatigue properties at 600 ° C. were inferior. Steel Nos. 11 and 18 were low in Cu content, so that the precipitation of ε-Cu phase did not occur sufficiently, and the value of formula (2) was small, so that the high temperature strength and thermal fatigue characteristics were inferior. Steel No. 12 had an excessively high Cu content, so the number of ε-Cu phases having a major axis of 0.5 μm or more exceeded 10/25 μm ² and was inferior in high-temperature strength and thermal fatigue characteristics. Moreover, the low temperature toughness was insufficient due to the excessive addition of Cu. Steel No. 13 has a low Nb content and does not satisfy the formula (1), Steel No. 14 has a high C content, Steel No. 15 has a high Si content, and Steel No. 16 contains a large amount of Mo. As a result, the chairs were inferior in workability and low temperature toughness. In Steel No. 16, since the Cu content is further low, the thermal fatigue characteristics cannot be improved. Steel No. 17 was inferior in workability because the Mn content was too high. Steel No. 19 was inferior in low temperature toughness due to its high Al content. Steel No. 20 was inferior in workability because the B content was too high.

Claims (5)

% By mass
C: 0.03% or less,
Si: 1.0% or less,
Mn: 1.5% or less,
Ni: 0.6% or less,
Cr: 10 to 20%,
Ti: 0.05 to 0.30%,
Nb: 0.10 to 0.60%,
Mo: 0 to less than 0.10%,
Cu: 0.8-2.0%,
Al: 0 to 0.10%,
B: 0.0005 to 0.02%,
V: 0.03 to 0.20%,
N: 0.03% or less,
It has a composition composed of the balance Fe and inevitable impurities, satisfying the following formulas (1) and (2), and having a structure in which the ε-Cu phases having a major axis of 0.5 μm or more are adjusted to 10/25 μm ² or less. Ferritic stainless steel with excellent thermal fatigue properties.
Nb-8 (C + N) ≧ 0 (1)
10Si + 20Mo + 30Cu + 20 (Ti + V) + 160Nb- (Mn + Ni) ≥100 (2)   The ferritic stainless steel material having excellent thermal fatigue characteristics according to claim 1, wherein the steel material is a steel plate.   An automobile exhaust gas path member using the steel material according to claim 1.   The automobile exhaust gas path member according to claim 3, wherein the member constitutes an exhaust manifold, a catalytic converter, a front pipe or a center pipe.   The member is heated to a temperature of 700 ° C or higher during engine operation, and is cooled from the temperature rising temperature to 400 ° C at an average cooling rate of 0.1 to 30 ° C / second after the engine is stopped. 4. The automobile exhaust gas path member according to 4.