JP2017122244A - Metastable austenitic stainless steel and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metastable austenitic stainless steel for gasket excellent in balance of strength and elongation, as well as fatigue property.SOLUTION: There is provided a partially recrystallization structure containing, by mass%, C:0.01% or more and 0.1% or less, Si:2.0% or less, Mn:3.0% or less, Cr:10.0% or more and 20.0% or less, Ni:5.0% or more and 10.0% or less, N:0.01% or more and 0.2% or less and the balance Fe with inevitable impurities and having average recrystallization particle diameter of 5 μm or less and percentage of non-recrystallization part remaining effects of processing before a heat treatment of 20% or less.SELECTED DRAWING: None

Description

  The present invention relates to a metastable austenitic stainless steel for cylinder head gaskets (hereinafter simply referred to as “gaskets”) used in engines such as automobiles and motorcycles.

  Gaskets for automobiles and motorcycle engines are important sealing parts that are inserted between a cylinder head and a cylinder block and prevent leakage of combustion gas, engine coolant, and oil from the space (gap). Most of the gaskets used today have a basic structure in which a plurality of thin stainless steel plates are stacked, and convex parts called beads are formed around a bore (hole) corresponding to the combustion chamber of the engine. . Then, due to the adhesion (repulsion) force of the bead, high-pressure combustion gas and the like are sealed against the increase and decrease of the gap repeated by combustion. The cylinder head and the cylinder block are fixed with bolts.

  Conventionally, materials such as SUS301, 304, and 301L belonging to metastable austenitic stainless steel shown in JIS-G4305 have been widely used for gaskets. These materials are generally used after cold rolling (temper rolling) for the purpose of adjusting the strength, and a high strength can be obtained relatively easily by a large curing accompanied with a work-induced martensitic transformation. In addition, since the deformation at the undeformed portion is promoted by such a large hardening, the so-called TRIP effect that the local deformation of the material is suppressed and the whole is deformed is excellent in the workability of stainless steel. Furthermore, it exhibits the necessary corrosion resistance upon contact with cooling water.

  By the way, recent engines are required to satisfy both environmental problems and to achieve both high compression ratio and light weight of the fuel gas mixture effective for improving fuel efficiency. In order to realize these, the gasket material is required to have higher strength and excellent workability into a complicated shape at the same time. However, in the metastable austenitic stainless steel as described above, as with other metal materials, deterioration of workability due to high strength is inevitable, and both high strength and workability are sufficiently satisfied. There is no current situation.

  Furthermore, there has been a problem that during the gasket processing, defects such as wrinkles and minute cracks on the surface of the plate are generated in bead molding, and the fatigue characteristics are greatly reduced. This is because, when viewed from the engine, when the gap between the cylinder head and the cylinder block repeated due to combustion is increased or decreased, the gasket is subjected to fatigue failure earlier. The sealing performance will be insufficient, fuel consumption and output will be reduced, and air pollution will be caused by leakage of combustion gas. If it gets worse, it may cause engine failure.

In order to solve the above-mentioned problems, the crystal grain size of the gasket material is made finer, while maintaining the same high strength as before, while suppressing the occurrence of defects considered to occur mainly at the grain boundaries during bead molding. A material with improved fatigue characteristics and a method for producing the same have been proposed (see, for example, Patent Documents 1, 2, 3, and 4).
In addition, a material and a method for manufacturing the same characterized by a partially recrystallized structure have been proposed for electronic device spring parts and the like (see, for example, Patent Documents 5 and 6).

  These conventional techniques make the crystal grain size as fine as possible and utilize the excellent characteristics. In addition, the crystal grains are refined and thereby enhanced, and the required strength is realized by a synergistic effect with work hardening in the subsequent temper rolling.

JP-A-4-214484 JP-A-5-279802 Japanese Patent Laid-Open No. 5-117813 International Publication No. 2001/004292 JP 2001-247938 A International Publication No. 2008/013305

However, in the prior art described above, no study has been made regarding the nature of the grain boundaries.
Moreover, although it is assumed that the refinement of crystal grains uniformly improves the properties such as the balance between strength and elongation, fatigue strength, etc., it does not always work effectively.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide industrially stable metastable austenitic stainless steel for gaskets having excellent balance between strength and elongation and fatigue characteristics.

  In order to solve the above-mentioned problems, the present inventors have detailed the balance between the strength and elongation of metastable austenitic stainless steel, the microstructure on fatigue properties, specifically the influence of crystal grain size and grain boundary properties. Thus, the present invention has been achieved.

  That is, the metastable austenitic stainless steel of the present invention is, in mass%, C: 0.01% or more, 0.1% or less, Si: 2.0% or less, Mn: 3.0% or less, Cr: 10.0% or more, 20.0% or less, Ni: 5.0% or more, 10.0% or less, N: 0.01% or more, 0.2% or less, the balance being Fe and inevitable It is characterized by a partially recrystallized structure consisting of impurities, having an average recrystallized grain size of 5 μm or less, and a ratio of unrecrystallized parts leaving the influence of pre-processing of 20% or less.

  In the present invention, since the chemical composition is as described above, the average recrystallized grain size is 5 μm or less, and the proportion of unrecrystallized parts that leave the influence of processing before heat treatment is a partially recrystallized structure of 20% or less. Further, it is possible to provide a metastable austenitic stainless steel for gaskets having excellent balance of elongation and fatigue properties.

  The said structure of this invention WHEREIN: Furthermore, it is preferable to contain 0.5% or less of at least 1 sort (s) of Nb, Ti, and V.

  In addition, the metastable austenitic stainless steel of the present invention is rapidly rolled at an average rate of 5 ° C./s or higher after cold rolling at a processing rate of 50% or more, and undergoes reverse transformation from the work-induced martensite phase to the austenite matrix phase. By performing heat treatment at a completion temperature of 900 ° C. or less for 1 second or more and 10 minutes or less, it can be supplied industrially stably.

  According to the present invention, by utilizing the grain boundaries that are effective in grain refinement and effective action, it is possible to supply industrially stable metastable austenitic stainless steel for gaskets with excellent balance between strength and elongation and excellent fatigue characteristics. can do.

It is a figure (substitute photograph) which shows an example of the TEM structure | tissue of Example 13 of this invention. It is a figure (substitute photograph) which shows an example of the measuring method of the non-recrystallized part in FIG.

Embodiments of the present invention will be described below.
First, the composition of the material will be described.
[C: 0.01% or more, 0.1% or less]
C, together with N (described later), is the most powerful austenite stabilizing element and solid solution strengthening element. In order to obtain the effect of addition, 0.01% or more is added. Preferably it is 0.02% or more. However, excessive addition causes precipitation of a large amount and coarse carbides in a relatively low temperature heat treatment for the purpose of crystal grain refinement. As a result, the necessary and stable austenite stability cannot be obtained, the structure becomes non-uniform, it is difficult to obtain the target strength, and the fluctuation becomes large. For this reason, the upper limit was made 0.1% or less. Preferably, it is 0.08% or less.

[Si: 2.0% or less]
Si is an element that functions as a deoxidizer during melting and is a ferrite stabilizing element. However, if added excessively, coarse inclusions are generated, workability deteriorates, and the austenite phase becomes unstable, so the upper limit is made 2.0%. Preferably, it is 1.5% or less. The lower limit is not particularly defined, but 0.05% or more is preferable in order to reliably obtain the deoxidation effect.

[Mn: 3.0% or less]
Mn is an austenite stabilizing alloy element that is relatively inexpensive and effective. However, if added excessively, coarse inclusions are generated and workability deteriorates, so the upper limit is made 3.0%. Preferably it is 2.6% or less. The lower limit is not particularly defined, but is preferably 0.1% or more from the viewpoint of reliable stabilization of the austenite phase.

[Cr: 10.0% or more, 20.0% or less]
Cr is a basic element of stainless steel, and is an element for obtaining effective corrosion resistance. To obtain the effect of addition, 10.0% or more is added. Preferably it is 10.5% or more. However, Cr is a ferrite stabilizing element, and if added excessively, the austenite phase becomes unstable and the possibility of forming a compound with C and N increases. Therefore, the upper limit is made 20.0%. Preferably it is 19.4% or less.

[Ni: 5.0% or more and 10.0% or less]
Ni is the most powerful austenite stabilizing alloy element. In order to stabilize the austenite phase up to room temperature including addition of C and N, 5.0% or more is added. Preferably it is 5.4% or more. However, as described above, since it is an expensive and rare alloy element and it is desirable to reduce it as much as possible, the upper limit is made 10.0%. Preferably, it is 9.0% or less.

[N: 0.01% or more, 0.2% or less]
N, together with the aforementioned C, is the most powerful austenite stabilizing element and solid solution strengthening element. In order to obtain the effect of addition, 0.01% or more is added. Preferably it is 0.02% or more. However, excessive addition causes precipitation of a large amount and coarse nitride in a relatively low temperature heat treatment for the purpose of crystal grain refinement. As a result, the necessary and stable austenite stability cannot be obtained, the structure becomes non-uniform, it is difficult to obtain the target strength, and the fluctuation becomes large. For this reason, the upper limit was made 0.2% or less. Preferably, it is 0.15% or less.

[Nb, Ti, V: 0.5% or less of at least one kind]
Nb, Ti, and V are elements that combine with C and N to form a compound that suppresses crystal grain growth by a pinning effect. However, if any element exceeds 0.5%, a coarse compound is generated and the austenite phase formation is likely to be unstable, and the workability is deteriorated, and the coarse compound is a starting point of destruction. Therefore, the upper limit of each is 0.5%. Preferably, both are 0.4% or less. Although a minimum is not specifically limited, 0.01% or more of Nb, Ti, and V is preferable at the point which ensures an addition effect reliably.

Next, the reason for limiting the organization will be described.
The structure of the metastable austenitic stainless steel of the present invention has an average recrystallized grain size of 5 μm or less. This is because, basically, the refinement of recrystallized grains is an effective strengthening method with little deterioration in workability, and is considered to be effective in improving the balance between strength and elongation and fatigue characteristics. Preferably, it is 4 μm or less, more preferably 3 μm or less. The recrystallized grains referred to herein include crystal grains having a remarkably low dislocation density, which are lattice defects newly formed by heat treatment, including cases involving transformation (structural change), and the grains have grown. Say things. This recrystallized grain boundary effectively acts to improve the balance between strength and elongation and fatigue characteristics, for example, due to the reason that it is a large-angle grain boundary with a large crystal orientation difference.

  Further, the structure of the metastable austenitic stainless steel of the present invention is a partially recrystallized structure in which the proportion of the portion that remains affected by the processing before the heat treatment is 20% or less. The portion that remains affected by the processing before the heat treatment is an unrecrystallized portion in which dislocations introduced by rolling such as an austenite phase and a work-induced martensite phase remaining from the processing stage before the heat treatment remain. The reason why these ratios are set to 20% or less is that those portions, old grain boundaries, or subgrain boundaries formed in the inside thereof do not effectively act to improve the characteristics. That is, the recrystallized grain boundary acts effectively, it is desirable to make it finer, and it is desirable that there are few unrecrystallized parts. Further, coarse crystal grains from the pre-processing stage remain, and the effect of refining crystal grains cannot be obtained due to the origin of fracture, and the characteristics are not improved. For this reason, those coarse grains are divided and refined by new recrystallized grains and grain growth. Preferably, the proportion of the portion that remains affected by the pre-processing is 17% or less, more preferably 15% or less. Of course, the lower limit is over 0%.

Next, the manufacturing method of the metastable austenitic stainless steel of this invention is demonstrated.
First, cold rolling is performed at a processing rate of 50% or more. This is to saturate the processing-induced transformation and obtain a sufficient amount of martensite transformation. In general, the crystal grain refinement of metastable austenitic stainless steel is performed by reverse transformation to austenite matrix in the subsequent heat treatment after the processing-induced martensitic transformation. From this point, it is desirable that the processing-induced transformation is sufficiently performed to saturate the martensite amount. A sufficient amount is at least 50% or more. Preferably, it is 70% or more, more preferably 80% or more. In general steel materials, refinement of crystal grains is promoted by increasing the processing rate. From these, rolling is performed at a processing rate of 50% or more. Preferably, it is 60% or more, more preferably 70% or more.

  Next, the heat treatment to be performed is performed at an average temperature of 5 ° C./s or higher from room temperature to the heating temperature. This is unavoidable in order to obtain excellent target characteristics by making effective use of pre-processing and refinement of recrystallized grains that act effectively. In other words, heating is performed at high speed as much as possible, so that the reduction of recrystallization driving force, so-called recovery, is suppressed, and the recrystallization nuclei before grain growth are utilized as much as possible to refine the crystal grains and effectively operate recrystallization. It increases the density of grain boundaries and decreases the proportion of unrecrystallized parts. At least in the component system of the present invention, the effect of grain growth is large when the structure is only recrystallized grains. Therefore, when the density of effective recrystallized grain boundaries is compared, partial recrystallization limited in the present invention The decline is inevitable compared to the organization. Therefore, the heating rate is inevitably limited to obtain the structure defined in the present invention, preferably 7 ° C./s or more, more preferably 10 ° C./s or more. On the other hand, when the heating rate is slow, the recovery proceeds even in the non-recrystallized part, and a coarse grain shape consisting of a stretched grain shape elongated in the rolling direction or a part of those parts divided by the recrystallized grain. The possibility that the recrystallized portion remains is increased. This coarse non-recrystallized part becomes a factor which deteriorates the characteristic of a material.

  The heat treatment is performed at a temperature not lower than the completion temperature of reverse transformation of the work-induced martensite phase to the austenite matrix and not higher than 900 ° C. This is because, as described above, the recrystallized grains and the crystal grain boundaries in which they grow have effectively acted to improve the characteristics, and their proportion is increased, and the remaining unrecrystallized parts are divided. It is also for miniaturization. The term “reverse transformation completion” as used herein refers to a temperature at which changes such as a decrease in the ratio of the processing-induced martensite phase and an increase in the ratio of the austenite matrix show a saturation tendency. The remaining part of the non-recrystallized part is also divided by the grain growth. As a result, the density of recrystallized grain boundaries is increased, and the remaining non-recrystallized portion is divided and refined. The completion temperature of the reverse transformation varies depending on the component, but is generally 700 ° C. in this component system. The upper limit is 900 ° C. because it is desirable to suppress unnecessary crystal grain growth (coarse), but is preferably 890 ° C. or less, more preferably 880 ° C. or less. The lower limit is preferably 720 ° C., more preferably 730 ° C. or higher.

The heat treatment time is 1 second or more and 10 minutes or less. It is desirable that the time is basically short in order to avoid unnecessary growth of recrystallized grains. However, in order to obtain stable characteristics, holding is necessary, and the time is 1 second or longer. The upper limit was set to 10 minutes or less assuming continuous processing with a coil or the like. Desirably, it is 3 minutes or less, more preferably 1 minute or less.
During the heat treatment, tension may be applied for the purpose of adjusting the reverse transformation of the work-induced martensite phase to the austenite matrix with volume change. As described in Patent Document 6, the applied tension is 0.2% proof stress or less at the heating temperature so that the material does not break. More preferably, it is 40% or less of 0.2% proof stress.

  Furthermore, the same material is confirmed to have excellent characteristics even in a specific processed structure as described above. Therefore, as described in the background art, temper rolling can be subsequently carried out for the purpose of high strengthening, and the strength can be adjusted to the required strength by synergistic action with work hardening. In this case, the structure is a processed structure in which the average crystal grain size is 5 μm or less and the ratio of the portion where the dislocation density is twice or more higher than others is 20% or less. The dislocation density is twice or more because it is a lower limit that can be detected by measurement and can provide an effect. Preferably, it is 5 times or more, more preferably 10 times or more. The dislocation density can be measured by observation with an electron microscope. Moreover, it is considered that the same effect can be obtained when the difference in dislocation density expected to be equivalent to those is expected from the measurement results of EBSD due to recent advances in analytical technology.

Table 1 shows the composition of the test steel. The test steel is a laboratory-sized small ingot with adjusted components. Using laboratory-level equipment, hot rolling at 1100 ° C to a plate thickness of 4 mm, annealing at 1100 ° C x 12 minutes, and then a predetermined plate thickness Cut into pieces. Next, a predetermined heat treatment was performed after cold rolling to a plate thickness of 1 mm. The heat treatment was performed at a predetermined rate of temperature increase, and was carried out in a non-oxidizing atmosphere at each heating temperature for a holding time of 3 minutes. Then, test pieces were collected from them and various characteristics were investigated. The microstructure is embedded, polished and corroded with a predetermined aqueous acid mixture solution after embedding in the rolling direction, and after making an optical microscope, SEM, or thin film, a transmission electron microscope (TEM) is used. And observed. Then, a photograph of an average structure was taken, and the crystal grain size was measured from the photograph. Moreover, the ratio of the part (non-recrystallized part) in which the influence of pre-processing remains was measured from the TEM photograph. As an example, an example of the TEM structure of Example 13 of the present invention is shown in FIG. The lower part of the center of the photograph where lattice defects remain inside is an unrecrystallized portion that remains affected by pre-processing. In addition, the measuring method of the ratio of the non-recrystallized part in the same photograph is shown in FIG. The measurement was calculated based on the ratio of the total number of intersection points on the non-recrystallized portion where lines were drawn in a lattice pattern at regular intervals on the same photograph and ◯ was written with respect to the total number of intersection points. For tensile properties, specimens were taken in parallel with the rolling direction, and tensile strength and elongation were measured at room temperature using an Instron type tensile tester. For the fatigue characteristics, the fatigue limit (upper limit value of the stress that can withstand 10 ⁷ times repeated bending) was clarified using a double swing type plane bending tester for the test piece cut into a strip shape. Next, the strip-shaped test piece was bent at a right angle with a bending radius of 1 mm, repeatedly bent at a stress of 80% of the fatigue limit before bending, and examined for cracks after 10 ⁷ times repeated bending. The case where it broke was evaluated as x, and the case where it did not crack was evaluated as o.

  Table 2 shows the investigation results of various characteristics of the inventive examples and the comparative examples. The examples of the present invention form a partially recrystallized structure limited by the present invention, and exhibit an excellent balance exceeding 36000 in terms of the product of tensile strength and elongation, and excellent fatigue properties. These are achieved by processing a material having a composition limited in the present invention under predetermined conditions. In particular, Invention Example No. In 3-5 and 15-17, it is confirmed that the recrystallized grains become finer due to the increase in the heating rate, the proportion of unrecrystallized parts decreases, and the density of effective recrystallized grain boundaries increases. . On the other hand, the comparative example has a low balance between strength and elongation and inferior fatigue characteristics. Specifically, as in Comparative Examples Nos. 20 to 25, even in the raw materials having components that match the present invention, the target performance is not achieved if a predetermined structure is not achieved. This is because the manufacturing conditions are inappropriate. In particular, Comparative Example No. In Nos. 22 and 23, the temperature increase rate of the heat treatment did not reach 5 ° C./s, and it was confirmed that an unrecrystallized portion having a coarse expanded grain shape remained. Also, in the case of a material that deviates from a component that matches the present invention as in Comparative Examples Nos. 26 to 31, the target performance is not achieved in the same manner. In Comparative Example No. 26 using steel g, the austenite phase is not confirmed, and the main component is considered to be the martensite phase. In Comparative Examples Nos. 27 to 31 using other steels h to l, coarse and relatively large dispersions of compounds were confirmed, and these are considered to be a cause of performance deterioration.

  As described above, according to the present invention, by making use of crystal grain refinement and effective grain boundaries, it is possible to industrially stabilize metastable austenitic stainless steel for gaskets with excellent balance of strength and elongation, and excellent fatigue properties. Can be supplied.

Claims (3)

In mass%
C: 0.01% or more, 0.1% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
Cr: 10.0% or more, 20.0% or less,
Ni: 5.0% or more and 10.0% or less,
N: 0.01% or more, 0.2% or less,
Is a partially recrystallized structure in which the balance is made of Fe and inevitable impurities, the average recrystallized grain size is 5 μm or less, and the ratio of unrecrystallized parts leaving the influence of processing before heat treatment is 20% or less A metastable austenitic stainless steel characterized by that.   Furthermore, 0.5% or less of at least 1 sort (s) of Nb, Ti, and V is contained, The metastable austenitic stainless steel of Claim 1 characterized by the above-mentioned. A method for producing the metastable austenitic stainless steel according to claim 1 or 2,
After cold rolling at a processing rate of 50% or more, rapid heating is performed at an average of 5 ° C./s or more, and the reverse transformation from the work-induced martensite phase to the austenite matrix phase is performed at a temperature of 900 ° C. or less for 1 second or more. A method for producing a metastable austenitic stainless steel, characterized by performing a heat treatment for 10 minutes or less.

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