Patent Application
20080279712
1. Field of the Invention
The present invention relates to a stainless steel sheet with excellent thermal fatigue properties, and to an automotive exhaust-gas path member using the same, and in particular, to a member that constitutes the upstream part of the exhaust-gas path and is heated to temperatures of 700° C. or higher upon the use of automobiles.
2. Description of the Related Art
Among automotive exhaust-gas path members, an upstream member, which is heated to 700° C. or higher during use thereof, is required to have, first of all, heat resistance in a range of high temperatures exceeding 700° C. In this regard, with the main goal of increasing high-temperature strength in the high-temperature range of 700˜1000° C., characteristic materials therefor have been developed. Further, the automotive exhaust-gas path member undergoes repeated heat cycles, including heating to the high-temperature range and cooling to room temperature, in response to the starting and stopping of an engine. Accordingly, strength in an intermediate temperature range of 500˜700° C. between the heating process and the cooling process is also regarded as important, and thus, component designs for increasing high-temperature strength in a wide temperature range from 500° C. to 1000° C. are under study.
Because it is important that the automotive exhaust-gas path member be imparted with excellent thermal fatigue properties for resistance to repeated heating and cooling, Japanese Patent No. 3468156 discloses ferritic stainless steel, which is intended to prevent the strength thereof from decreasing in an intermediate temperature range of 600˜750° C. because the thermal fatigue properties may be increased depending on an increase in the strength in the intermediate temperature range. To this end, 1˜3% of Cu is contained in steel, thus producing a fine precipitate of Cu in the intermediate temperature range.
In addition, WO 03/004714 discloses ferritic stainless steel containing 1.0˜1.7% Cu to precipitate Cu during heating so as to increase high-temperature strength at 700° C. or 800° C.
(Problems to be Solved by the Invetion)
As mentioned above, however, to increase the high-temperature strength in the wide temperature range from 500° C. to 1000° C. merely depends on component designs in which specific elements are added in large amounts, undesirably resulting in increased material costs. Moreover, forming workability is poor.
In Japanese Patent No. 3468156 and WO 03/004714, the strength of the steel sheet at 600° C. or in the temperature range of 700˜800° C. is increased using precipitation strengthening by the Cu precipitate through the addition of Cu.
However, the present inventors have conducted intensive investigation and thus discovered that, even if ferritic stainless steel simply containing a predetermined amount of Cu is annealed, thermal fatigue properties necessary for the upstream member of the automotive exhaust-gas path, which undergoes repeated heating to temperatures exceeding 700° C. and cooling to room temperature, may not always be repeatedly exhibited.
The present invention aims to provide a ferritic stainless steel sheet for use in the upstream member of an automotive exhaust-gas path, through the development of a technique for stably improving thermal fatigue properties, having a strong correlation with the durability of the member, using relatively inexpensive component designs while fundamentally imparting high-temperature strength, forming workability, and low-temperature toughness.
(Means for Solving the Problems)
As a result of further investigation by the present inventors, to increase the durability and reliability of the upstream member of the automotive exhaust-gas path, a stable improvement in thermal fatigue properties has been found to be very favorable, after taking everything into consideration. Fundamental durability (high-temperature strength and oxidation resistance) of the high-temperature range exceeding 700° C. reaches a considerably high level at present, due to technical advancement of the prior art. More enhancements in such properties need the addition of specific elements and so on, leading to high costs. Further, durability (thermal fatigue properties), which allows the automobiles to endure heat cycles of heating and cooling upon the use thereof, should be greatly improved. In the current situation, it is considered that the improvement of this durability is very effective in increasing the durability and reliability of the upstream member of the automotive exhaust-gas path.
Leading to the present invention, intensive and thorough research into a ferritic stainless steel sheet containing Cu, carried out by the present inventors, resulted in the finding that the form of the Cu precipitate (here, referred to as “ε-Cu phase”) is controlled to exist in any predetermined state in the step before the steel sheet is applied to actual use as the exhaust-gas member, and thereby thermal fatigue properties may be stably improved under repeated cycles of subsequent heating and cooling.
According to the present invention, the ferritic stainless steel sheet (e.g., steel plate) with excellent thermal fatigue properties includes, by mass %, 0.03% or less of C, 1.0% or less of Si, 1.5% or less of Mn, 0.6% or less of Ni, 10˜20% of Cr, 0.05˜0.30% of Ti, 0.51˜0.65% of Nb, 0˜less than 0.10% of Mo, 0.8˜2.0% of Cu, 0˜0.10% of Al, 0.0005˜0.02% of B, 0˜0.20% of V, and 0.03% or less of N, with the balance being Fe and inevitable impurities, has a chemical composition satisfying the following equations (1) and (2), and has a structure in which ε-Cu phase grains, each having a long diameter of 0.5 μm or more, are present in a density of 10 or less per 25 μm2.
Nb−8(C+N)≧0 (1)
10 Si+20 Mo+30 Cu+20(Ti+V)+160 Nb−(Mn+Ni)≧100 (2)
The lower limit of 0% of Mo, Al and V indicates the case in which the contents of these elements are below the measurable limit in a typical analysis method in a steel-making process. The presence of an ε-Cu phase may be confirmed through the observation of the cross-section (C-cross-section) of a specimen perpendicular to the rolling direction of a steel sheet using a transmission electron microscope. Into the elements of the equations (1) and (2), the contents of corresponding elements, represented by mass %, are substituted.
In addition, the present invention provides an automotive exhaust-gas path member formed using the above steel sheet. The path member is manufactured by subjecting the steel sheet, for example, the steel plate, to a process of forming a pipe, such as bending or welding. The automotive exhaust-gas path member is, for example, an exhaust manifold, a catalyst converter, a front pipe, or a center pipe. Particularly useful is a member that is heated to temperatures of 700° C. or higher while the engine is running and is then cooled to 400° C. from the increased temperature at an average cooling rate of 0.1˜30° C./sec after the engine is stopped.
According to the present invention, the thermal fatigue properties of the ferritic stainless steel sheet may be considerably improved without the use of expensive elements, including Mo. This steel sheet has a reasonable component design, which obviates the need for an excessive increase in high-temperature strength in the high-temperature range exceeding 700° C., and exhibits excellent durability in end uses, in which heating to the high-temperature range exceeding 700° C. and cooling to room temperature are repeated. The steel sheet, which is provided in the form of a steel plate, may be subjected to bending or welding to form a pipe, and has good low-temperature toughness. Thus, the ferritic stainless steel sheet is suitable for use in the automotive exhaust-gas path member, in particular, in the upstream member, which is heated to temperatures exceeding 700° C. The automotive exhaust-gas path member using such a steel plate may realize both low material costs and increased durability and reliability.
Typically, a ferritic stainless steel sheet for use in an upstream member of an automotive exhaust-gas path, for example, a member exposed to high temperatures exceeding 700° C. or 800° C., has been designed to have components in particular consideration of increasing high-temperature strength and oxidation resistance in such a high-temperature range. For this, the addition of expensive elements is inevitable and material costs are unavoidably increased.
However, according to the detailed investigation by the present inventors, in end uses in which heating to the high-temperature range and cooling to room temperature are frequently repeated, as in the automotive exhaust-gas path member, in the case where the durable lifetime of the member is considered, the improvement of thermal fatigue properties in the intermediate temperature range of 500˜700° C. has been found to be more important than the increase in high-temperature strength in all temperature ranges from 500° C. to, for example, 900° C., in order to increase durability and reliability and to decrease costs.
In the present invention, with the aim of improving the thermal fatigue properties of the steel sheet, in ferritic steel containing Cu, the precipitation of the ε-Cu phase is used. For instance, in ferritic stainless steel containing about 1˜2 mass % of Cu, because the ε-Cu phase is precipitated in the intermediate temperature range around 600° C., it is possible to manifest a precipitation strengthening phenomenon upon fine dispersion of the precipitate in a matrix. When the temperature exceeds about 900° C., the solid solution of the ε-Cu phase in the matrix is formed. Furthermore, in the subsequent cooling process, the above phase is precipitated again.
There have been conventional examples for strengthening the intermediate temperature range using the precipitation of the ε-Cu phase (Japanese Patent No. 3468156 and WO 03/004714). In these cases, however, thermal fatigue properties are not always improved stably upon the application of such a steel sheet to the upstream member of the automotive exhaust-gas path, resulting in unsatisfactory reliability. In order to solve these problems, the present inventors have conducted extensive investigation, resulting in the finding that the metal structure state of a steel sheet, in the step (hereinafter, referred to as “initial step”) before the steel sheet is mounted to the automotive exhaust-gas path and subjected to first heating and cooling hysteresis, has a great influence on the thermal fatigue properties in subsequent uses thereof.
That is, in the case where the material for a steel plate is formed into the upstream member of the automotive exhaust-gas path, the steel plate, before it is formed into the member, should preferably have a structure state in which ε-Cu phase grains, each having a long diameter of 0.5 μm or more, are present in a density of 10 or less per 25 μm2, and more preferably, 5 or less per 25 μm2.
In the case where the density of ε-Cu phase grains, each having a long diameter of 0.5 μm or more, exceeds 10 per 25 μm2, when the formed member is first heated for actual use, for example, when it reaches a temperature for engine starting, which is not lower than 800° C., the solid solution of the ε-Cu phase in the matrix may not be sufficiently taken place. In this case, when the engine is stopped, cooling is initiated in the state in which the ε-Cu phase exists in a considerable amount. Thereby, in the cooling process, because new ε-Cu phase grains are precipitated using the surface of the already-existing ε-Cu phase as a main precipitation site, precipitation strengthening through fine dispersion is not sufficiently exhibited. That is, the strength in the cooling process, which affects the thermal fatigue properties, is not sufficiently assured. Further, in the subsequent heating process, heating is initiated in a structure state in which the coarse ε-Cu phase is present. Consequently, in the subsequent course of repeated heat cycles of heating and cooling, the structure state in which the ε-Cu phase is finely dispersed is seldom realized even after a considerably long period of time, making it impossible to achieve an improvement in thermal fatigue properties.
In the initial step, the ε-Cu phase grains, each having a long diameter of 0.5 μm or more, are preferably present in a density of 0 per 25 μm2 (i.e. not actually observed). However, it doesn't matter if ε-Cu phase grains, each having a smaller size, are present in a state of being dispersed in the matrix. Even when such a fine ε-Cu phase is present, it is subjected to solid solution again in the first heating process of the exhaust-gas path member, and the ε-Cu phase is finely precipitated in the matrix in the cooling process subsequent thereto, thus exhibiting precipitation strengthening.
The structure state in which the density of the ε-Cu phase grains each having a long diameter of 0.5 μm or more is adjusted to be 10 or less per 25 μm2 is obtained by increasing the cooling rate of final annealing, which is conducted in the steel-making process. However, when the cooling rate is too high, Cu undesirably enters a complete solid solution state, instead of a structure state in which fine ε-Cu phase is dispersed. According to experiments by the present inventors, in the continuous line for the process of manufacturing the ferritic stainless steel plate having the composition mentioned below, when final annealing is conducted through soaking at 950˜1050° C. for 0˜60 sec, the average cooling rate to 400° C. from 900° C. is controlled to 10˜30° C./sec, thereby affording a preferable structure state.
Below, the components for steel are described.
C and N are elements that are effective in increasing high-temperature strength, including creep strength. However, when these elements are present in excessive amounts, oxidation properties, workability, low-temperature toughness, and weldability are decreased. In the present invention, the contents of both C and N are limited to 0.03 mass % or less.
Si is effective in improving high-temperature oxidation properties. However, when this element is used excessively, hardness is increased and workability and low-temperature toughness are decreased. In the present invention, the content of Si is limited to 1.0 mass % or less.
Mn functions to improve high-temperature oxidation resistance, in particular, scaling resistance. However, excessive addition thereof deteriorates workability and weldability. Further, because Mn is an element capable of stabilizing austenite, a large addition thereof enables the easy formation of martensite, undesirably decreasing thermal fatigue properties and workability. Thus, the content of Mn is set to be 1.5 mass % or less.
Cr functions to stabilize a ferritic phase and to improve oxidation resistance, which is important for high-temperature materials. However, when excessive Cr is added, the steel sheet becomes brittle or has deteriorated workability. Thus, the content of Cr is set to be 10˜20 mass %. The content of Cr is preferably adjusted to be suitable for the usage temperature of the material. For example, in the case in which outstanding high-temperature oxidation resistance to 950° C. is required, the content of Cr is preferably set to be 16 mass % or higher. Further, in the case in which resistance to 900° C. is required, Cr is preferably used in the range of 12˜16 mass %.
Ti is effective in improving formability. Although the mechanism thereof has not definitely been established, it is assumed that, base on the viewpoint in which the formability of a cold-rolled annealed plate is remarkably increased when an Nb—Ti-based precipitate is produced upon the thermal treatment of a hot-rolled plate, this precipitation phenomenon contributes to the formation of a texture structure in which the plane (111) or (211) is integrated parallel to the rolled surface, as the texture structure efficient for improving the formability. Although it is not definite that the precipitate itself functions directly, it is believed that the decrease in solid solution C due to the formation of the precipitate is connected therewith.
However, the excessive addition of Ti results in the deterioration of surface properties, attributable to the formation of TiN, and negatively influences weldability and low-temperature toughness. The content of Ti is set to be 0.05˜0.30 mass %.
Nb is an element that is very effective in assuring high-temperature strength in the high-temperature range exceeding 700° C. This element is considered to greatly contribute to the present component system through solid solution strengthening. However, when the content of Nb is less than 0.51 mass %, thermal fatigue strength is not satisfied. On the other hand, the excessive addition of Nb undesirably results in decreased workability and low-temperature toughness and increased hot cracking sensitivity of welds. Therefore, the upper limit of the content of Nb is set to be 0.65 mass %.
Mo is effective in enhancing high-temperature strength, but the present invention eliminates the need for expensive Mo. The addition of large amounts of Mo deteriorates workability, low-temperature toughness, and weldability. The content of Mo is limited to less than 0.10 mass %.
Cu is an important element in the present invention. As mentioned above, in the present invention, fine dispersion and precipitation of the ε-Cu phase facilitate the increase of the strength in the intermediate temperature range of 500˜700° C. and of the thermal fatigue properties. Thus, at least 0.8 mass % of Cu should be contained. However, the addition of excessive Cu results in decreased workability, low-temperature toughness, and weldability. The upper limit of the content of Cu is set to be 2.0 mass %.
Al functions as a deoxidant and acts to improve high-temperature oxidation resistance. However, when Al is contained in a large amount, it negatively affects surface properties, workability, weldability, and low-temperature toughness. Thus, in the case where Al is added, the content thereof is limited to 0.10 mass % or less.
B is effective in improving resistance to secondary working brittleness. The mechanism thereof is assumed to be due to the decrease in solid solution C at grain boundaries or the strengthening of the grain boundaries. However, the addition of excessive B worsens manufacturing ability or weldability. In the present invention, the content of B is set in the range of 0.0005˜0.02 mass %.
V contributes to increasing high-temperature strength along with the addition of Nb and Cu. Due to the co-existence with Nb, workability, low-temperature toughness, grain-boundary corrosion sensitization resistance, and toughness of weld heat affected zones are improved. However, the excessive addition thereof worsens workability and low-temperature toughness. Thus, in the case where V is added, the content thereof is set in the range of 0.20 mass % or less. The content of V is preferably set to be 0.03˜0.20 mass %, and more preferably 0.04˜0.15 mass %.
While individual elements are added in the ranges mentioned above, their contents are adjusted to satisfy the following equations (1) and (2).
Nb−8(C+N)≧0 (1)
10 Si+20 Mo+30 Cu+20(Ti+V)+160 Nb−(Mn+Ni)≧100 (2)
Here, the equation (1) is prescribed to assure Nb in solid solution, and the equation (2) is prescribed to assure fundamental high-temperature strength.
The ferritic stainless steel sheet of the present invention may be manufactured by preparing steel having the above composition using melting and then subjecting the prepared steel to a series of processes of hot rolling, annealing, and acid pickling, or an additional series of processes of cold rolling, annealing, and acid pickling, once or several times. In order to obtain the precipitated form of the ε-Cu phase, in final annealing, the average cooling rate to 400° C. from 900° C. is preferably set within the range from 10˜30° C./sec. Here, the final annealing is the last annealing in the steel-making process, and, for example, may be conducted through soaking at 950˜1050° C. for 0˜3 min.
The steel sheet thus obtained is subjected to forming or welding, thus producing an automotive exhaust-gas path member. For example, in the case of the exhaust manifold or front pipe, a steel plate having a desired thickness is welded, and may be subjected to bending depending on the need, thereby obtaining a desired member.
According to the present invention, the thermal fatigue properties of the ferritic stainless steel sheet may be drastically improved, without the use of the expensive element, such as Mo. This steel sheet has a reasonable component design to avoid the excessive increase in the high-temperature strength in the high-temperature range exceeding 700° C., and may exhibit excellent durability in end uses in which heating to the high-temperature range exceeding 700° C. and cooling to room temperature are repeated. Further, the steel sheet, which is provided in the form of a steel plate, may be subjected to bending or welding, resulting in good low-temperature toughness. Therefore, the ferritic stainless steel sheet is preferably used for the automotive exhaust-gas path member, in particular, the upstream member, which is heated to temperatures exceeding 700° C. The automotive exhaust-gas path member using the steel plate may realize both low material costs and high durability and reliability.
Ferritic stainless steel, shown in Table 1, below, was prepared using melting, and was then subjected to hot rolling, hot-rolled plate annealing, cold rolling, final annealing, and then acid pickling, thus obtaining an annealed steel plate having a thickness of 2 mm. In addition, part of a cast slab was subjected to hot forging, thus producing a round bar having a diameter of about 25 mm, which was then subjected to final annealing. Final annealing after the cold rolling and final annealing after the hot forging were conducted for all of the specimens, other than the steel No. 10, by maintaining the temperature of 1000° C. for 1 min and then adjusting the average cooling rate to 400° C. from 900° C. to 10˜30° C./sec. For the steel No. 10, maintaining the temperature of 1000° C. for 1 min and then adjusting the average cooling rate to 400° C. from 900° C. to about 50° C./sec were conducted (common conditions for rolled plate and bar).
For the plate and bar after the final annealing, the metal structure on the cross-sections perpendicular to the rolling direction and the longitudinal direction thereof was observed. Using a transmission electron microscope, the size of the ε-Cu phase grains was measured, and the density of ε-Cu phase grains each having a long diameter of 0.5 μm or more was determined per 25 μm2. One specimen was observed at least ten times, and then an average value was determined. The case in which the density of ε-Cu phase grains each having a long diameter of 0.5 μm or more was 10 or less per 25 μm2 was judged as “◯”, and the other case was judged as “X”. The results are shown in Table 2 below. Because there was no difference in the results of all steel for plates and bars, the ε-Cu content shown in Table 2 was evaluated to be suitable for both plates and bars.
A tensile test was conducted at room temperature using the plate, in order to evaluate workability. The tensile direction was divided into three types, that is, 0° (parallel), 45°, and 90° to the rolling direction. Using a JIS 13B specimen, a tensile test according to JIS Z2241 was conducted until breaking occurred, and after the breaking, two test pieces were butted to each other to thus measure elongation at the time of breaking. The average elongation ELA was determined from the following equation:
EL
A=(ELL+2ELD+ELT)
wherein ELL is the elongation (%) at 0° to the rolling direction, ELD is the elongation (%) at 45° to the rolling direction, and ELT is the elongation (%) at 90° to the rolling direction.
The case in which ELA was 30% or more was evaluated as “◯”, and the case in which ELA was less than 30% was evaluated as “X”.
An impact test was conducted using the plate, in order to evaluate low-temperature toughness. A V-notched impact specimen was prepared in a manner such that the impact direction coincided with the rolling direction of the plate, and the impact test according to JIS Z2242 was conducted at a pitch of 25° C. in the range of −75˜25° C., and a ductile-brittle transition temperature was determined. The case where the transition temperature was lower than −50° C. (ductile fracture surface was observed even at −50° C.) was evaluated as “◯”, and the case where the transition temperature was higher than −50° C. was evaluated as “X”.
A high-temperature continuous oxidation test was conducted using the plate, in order to evaluate high-temperature oxidation resistance. A 25 mm×35 mm specimen, the surface and cross-section of which were subjected to finishing through #400 wet polishing, was used. The high-temperature continuous oxidation test according to JIS Z2281 was conducted at 900° C. for 200 hours in an ambient atmosphere. After the test, the specimen was observed with the naked eye. The formation of a lump-shaped thick oxide scale was defined as abnormal oxidation, and the presence or absence of abnormal oxidation was observed. The case where abnormal oxidation was not observed was evaluated as “◯”, and the case where abnormal oxidation was observed was evaluated as “X”.
A high-temperature tensile test was conducted using the plate, in order to evaluate high-temperature strength. The high-temperature tensile test according to JIS G0567 was conducted at 600° C., and 0.2% proof stress was determined. The case not less than 180 MPa was evaluated as “◯”, and the case less than 180 MPa was evaluated as “X”.
A thermal fatigue test was conducted using the bar, in order to evaluate the thermal fatigue properties. A notched round bar specimen having a diameter of 10 mm in an un-notched cross-section and a diameter of 7 mm in a notched cross-section was manufactured. Under a constraint force of 20%, in an ambient atmosphere, a heat cycle, including 200° C.×0.5 min maintaining, heating to 900° C. at a heating rate of about 3° C./sec, 900° C.×0.5 min maintaining, and cooling to 200° C. at a cooling rate of about 3° C./sec, as one cycle, was repeated. The number of repeated cycles when the stress was decreased to 75% of initial stress was defined as thermal fatigue lifetime. The case where the thermal fatigue lifetime was 900 cycles or more was evaluated as “◯”, and the case where the thermal fatigue lifetime was less than 900 cycles was evaluated as “X”.
The results are shown in Table 2 below.
As is apparent from Table 2, the inventive examples satisfying the chemical composition and the precipitated form of the ε-Cu phase prescribed in the present invention had excellent thermal fatigue properties, and also had workability, low-temperature toughness, high-temperature oxidation resistance, and high-temperature strength suitable for use as the upstream member of the automotive exhaust-gas path. Although not shown in the above table, the inventive examples had a structure in which a fine ε-Cu phase having a long diameter less than 0.5 μm was dispersed in the matrix.
Although the steel No. 10 of the comparative examples had the chemical composition prescribed in the present invention, it had a cooling rate after the final annealing slower than 10° C./sec. Hence, the density of ε-Cu phase grains, each having a long diameter of 0.5 μm or more, exceeded 10 per 25 μm2, and the high-temperature strength at 600° C. and thermal fatigue properties were poor. The steel Nos. 11 and 18 had a low Cu content, and thus the ε-Cu phase was not sufficiently precipitated. Further, because the value of the equation (2) was small, high-temperature strength and thermal fatigue properties were poor. Because the steel No. 12 had too high Cu, the density of ε-Cu phase grains, each having a long diameter of 0.5 μm or more, exceeded 10 per 25 μm2, and high-temperature strength and thermal fatigue properties were poor. Moreover, the excessive addition of Cu resulted in insufficient low-temperature toughness. The steel No. 13 had a low Nb content, and undesirably unsatisfied the equation (1). The steel No. 14 had a high C content, the steel No. 15 had a high Si content, and the steel No. 16 had a high Mo content, resulting in poor workability and low-temperature toughness. In the steel No. 16, the content of Cu was much less, undesirably making it impossible to improve the thermal fatigue properties. The steel No. 17 had excessively high Mn, leading to poor workability. The steel No. 19 had poor low-temperature toughness due to its high Al content, and the steel No. 20 had poor workability due to excessively high B.
While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the spirit and scope of the invention as defined in the following claims.
