JP2017160492A - Stainless steel for metal gasket - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel for metal gasket capable of being effectively manufactured and excellent in strength and leak resistance.SOLUTION: A stainless steel for metal gasket contains 0.05 mass% to 0.30 mass% of C, 1.50 mass% or less of Si, 2.5 mass% to 7.0 mass% of Mn, 0.06 mass% or less of P, 0.005 mass% or less of S, 1.0 mass% or more and less than 5.0 mass% of Ni, 15.0 mass% to 19.0 mass% of Cr, 2.0 mass% or less of Mo, 1.0 mass% to 3.5 mass% of Cu and 0.05 mass% to 0.30 mass% of N with C+N≥0.20 mass% and the balance Fe with inevitable impurities. It has a value of Mdof 10 to 30, a value of SFE of 15 or more and less than 35, a value of δcal of 2.0 or less and a dual-phase structure with an austenite phase and a processing induction martensite phase, and a surface hardness of the steel of 450 HV or more.SELECTED DRAWING: None

Description

  The present invention relates to stainless steel for metal gaskets.

  As shown in FIG. 1, a metal gasket 1 such as a cylinder head gasket or an exhaust manifold gasket used in a vehicle engine needs to have various characteristics necessary for maintaining the airtightness of the joint surface. It is necessary to have the ability to withstand the vibration stress specific to the engine repeatedly applied in the combustion gas atmosphere.

  Further, as shown in FIG. 2, the metal gasket 1 is formed with a bead (projection) 3 having a certain height around the opening 2 so as to maintain sufficient airtightness. High strength and good workability are required. The metal gasket 1 is used in a configuration in which beads 3 are formed in at least one layer and several sheets are laminated.

  In the case of an engine cylinder head gasket, a metal gasket 1 formed with beads 3 along the periphery of the combustion chamber and the periphery of the water holes and oil holes is formed and the bead 3 is tightened. It is common to seal gas, water and oil by surface pressure.

  As a material for this type of metal gasket, work hardening type metastable austenitic stainless steel represented by SUS301, which can obtain high strength by cold working, is frequently used.

  In addition, as a material for the metal gasket, for example, a metastable austenitic stainless steel described in Patent Documents 1 to 3, a work hardening type austenitic stainless steel in which work-induced martensite is generated, and a patent Stainless steel having a multiphase structure of ferrite and martensite as described in Document 4, and stainless steel having austenite structure stabilized by containing 10% by mass or more of Mn as described in Patent Document 5 Steel is known.

Japanese Patent No. 3347582 Japanese Patent No. 4019630 Japanese Patent No. 4785302 JP 2000-256802 A Japanese Patent No. 4116134

  For example, metal gaskets used for cylinder heads require high surface pressure (contact pressure) with a contact partner in order to ensure gas sealability because high pressure is applied during compression. For this reason, in the metal gasket, it is necessary to increase the bead molding height and improve the material strength.

  And in SUS301 system, in order to obtain high strength, it is necessary to perform high cold working, and therefore, rough skin and micro cracks are generated in the bead shoulder R portion at the time of bead generation processing due to reduction in ductility and workability, There is a problem that the leak resistance is lowered.

  The stainless steel of Patent Document 1 improves the high-temperature strength and the high-temperature oxidizability by increasing the N content effective for high-temperature characteristics and defining the Al content that is a harmful effect of N addition. However, in this patent document 1, since 7 to 15% by mass of Ni is added, there is a problem that the raw material cost is increased and it cannot be efficiently manufactured.

  The stainless steel of Patent Document 2 relates to an engine gasket, and the fatigue strength and sag resistance are improved by defining the metal structure of the temper rolled material. However, since Ni is added in an amount of 6 to 8% by mass, there is a problem that the raw material cost increases and the production cannot be efficiently performed.

  The stainless steel of Patent Document 3 relates to a metal gasket for high temperature having excellent sag resistance, but because Ni is added in an amount of 7 to 15% by mass, the raw material cost increases and it cannot be efficiently manufactured. There's a problem.

  Patent Document 4 has a problem that leakage resistance is insufficient from the viewpoint of strength because it has a multiphase structure of ferrite and martensite by a multiphase heat treatment, and therefore has a hardness as low as 400 HV or less.

  The stainless steel of Patent Document 5 contains 10 to 25% by mass of Mn in order to stabilize the austenite structure. Such high Mn steel produces fine Mn oxide particles that are harmful during steelmaking and refining. In addition, due to the high Mn content in the steel, the surface quality of the steel sheet is likely to deteriorate, and the productivity is reduced in the manufacturing process such as annealing pickling and bright annealing, so that it cannot be efficiently manufactured. There is.

  Therefore, as stainless steel for metal gaskets, there has been a demand for a material that can be efficiently manufactured from the viewpoint of raw material costs and from the viewpoint of productivity, and that is excellent in strength and leak resistance.

  This invention is made | formed in view of such a point, and it aims at providing the stainless steel for metal gaskets which can be manufactured efficiently and was excellent in intensity | strength and leak resistance.

The stainless steel for a metal gasket according to claim 1 is C: 0.05% by mass or more and 0.30% by mass or less, Si: 1.50% by mass or less, Mn: 2.5% by mass or more and 7.0% by mass. %: P: 0.06 mass% or less, S: 0.005 mass% or less, Ni: 1.0 mass% or more and less than 5.0 mass%, Cr: 15.0 mass% or more and 19.0 mass% or less Mo: 2.0% by mass or less, Cu: 1.0% by mass to 3.5% by mass and N: 0.05% by mass to 0.30% by mass, and C + N is 0.20% by mass or more, the balance being Fe and unavoidable _impurities, with Md 30 = 551-462 (C + N ) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo austenite stability index represented by the value of a certain _{Md 30} more than 10 30 or less, S The value of SFE, which is a stacking fault energy generation index indicated by E = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32, is 15 or more and less than 35, and δcal = -15-44.91C-0.88Mn-2.31Ni + 2 The value of δcal, which is a δ ferrite formation index after heating at 1230 ° C. for 2 hours, as shown by .20Cr-1.08Cu-28.8N, is 2.0 or less, and a multiphase structure of an austenite phase and a work-induced martensite phase. And having a surface hardness of 450 HV or more.

  The stainless steel for a metal gasket according to claim 2 is the stainless steel for a metal gasket according to claim 1, wherein V: 0.02% by mass to 0.20% by mass and Co: 0.01% by mass to 0%. It contains at least one of 20% by mass or less.

According to the present invention, in the alloy composition defined in the predetermined range, the value of Md ₃₀ is 10 or more and 30 or less, the value of SFE is 15 or more and less than 35, and the value of δcal is 2.0 or less. Since the components are adjusted, it can be produced efficiently and the strength and leak resistance can be improved.

It is a top view which shows typically the structure of a cylinder head gasket. It is explanatory drawing which shows typically the cross-section of the bead part of a metal gasket. (A) is a top view which shows the surface shape of a sag resistance evaluation test piece, (b) is sectional drawing which shows the cross-sectional shape of a sag resistance evaluation test piece. (A) It is explanatory drawing which shows roughly the structure of the bead metal mold | die which forms the bead of a fatigue characteristic test piece, (b) is a top view which shows the surface shape of a fatigue characteristic test piece, (c) is fatigue. It is sectional drawing which shows the cross-sectional shape before compression of a characteristic test piece, (d) is sectional drawing which shows the cross-sectional shape after compression of a fatigue characteristic test piece.

  Hereinafter, the configuration of an embodiment of the present invention will be described in detail.

  Stainless steel for metal gaskets is premised on having excellent sag resistance in order to exhibit excellent durability as a metal gasket. In order to improve sag resistance, it is effective to increase the strength of stainless steel.

  In general, effective means to obtain high strength based on austenitic stainless steel is to provide work such as cold rolling to work harden the austenite phase and harden part of the austenite phase. The so-called work-induced transformation plasticity (TRIP) phenomenon that transforms into a new work-induced martensite phase.

  Moreover, also in the molding process to the metal gasket, the portion to which processing strain is applied is cured by the TRIP phenomenon. The likelihood of such a TRIP phenomenon being affected by the austenite stability.

  However, such curing due to the TRIP phenomenon is effective in improving the sag resistance by increasing the strength, but causes a decrease in workability (for example, bendability) as a metal gasket.

  Therefore, in order to obtain good sag resistance while maintaining excellent workability as a metal gasket material in stainless steel for metal gasket, it is important to adjust the austenite stability and the amount of work-induced martensite. is there.

  Further, bendability, which is one of the criteria for workability, has a certain degree of correlation with the elongation evaluated by a tensile test or the like. This elongation is closely related to C and N, which are solid solution strengthening elements that affect the strength of the TRIP phenomenon and the processing-induced martensite phase. That is, in order to improve the bendability in order to ensure the workability, it is necessary to adjust the austenite stability, the C amount, and the N amount within appropriate ranges.

  And when the value of SFE, which is a generation index of stacking fault energy, is large, work hardening of the austenite phase is difficult to occur, so the hardness difference between the work-induced martensite phase and the austenite phase is large, and the value of SFE is small. Since work hardening of the austenite phase tends to occur, the ductility of the austenite phase decreases.

  Both the increase in the hardness difference between the work-induced martensite phase and the austenite phase and the decrease in the ductility of the austenite phase are factors that reduce the bendability. In order to ensure, it is important to adjust the value of SFE.

  The stainless steel for metal gasket according to one embodiment of the present invention is 0.05 mass% or more and 0.30 mass% or less of C (carbon), 1.50 mass% or less of Si (silicon), 2.5 mass. % Mn (manganese), 0.06 mass% or less P (phosphorus), 0.005 mass% or less S (sulfur), 1.0 mass% or more and less than 5.0 mass% Ni (nickel), 15.0 mass% to 19.0 mass% Cr (chromium), 2.0 mass% or less Mo (molybdenum), 1.0 mass% to 3.5 mass% Cu (Copper) and 0.05 mass% or more and 0.30 mass% or less of N (nitrogen), C + N ≧ 0.20 mass%, and the balance is composed of Fe (iron) and inevitable impurities The

  Moreover, 0.02 mass% or more and 0.20 mass% or less V (vanadium) and 0.01 mass% or more and 0.20 mass% or less Co (cobalt) are contained as needed.

Further, in the range of the content of each element, austenite represented by the formula (1) of Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo The components are adjusted so that the value of Md ₃₀ as a stability index is 10 or more and 30 or less.

  Moreover, in the range of the content of each element described above, the value of SFE, which is a stacking fault energy generation index represented by the formula (2) of SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32, is 15 or more and less than 35. The ingredients are adjusted so that

  Furthermore, within the range of the content of each element described above, δcal = -15-44.91C-0.88Mn-2.31Ni + 2.20Cr-1.08Cu-28.8N represented by the formula (3) at 1230 ° C. for 2 hours The components are adjusted so that the value of δcal, which is a δ ferrite formation index after heating, is 2.0 or less.

  And the stainless steel for metal gaskets which concerns on one Embodiment has a double phase structure of an austenite phase and a process induction martensite phase, and the surface hardness of steel is 450 HV or more. If the steel surface hardness is less than 450 HV, the sag resistance decreases, so when used as a metal gasket for a vehicle engine, the required contact pressure between the cylinder head and the cylinder block due to engine vibrations, etc. (Sealability) may not be obtained.

  C and N are austenite-generating elements. If the content of these elements is too small, the amount of δ ferrite phase increases and hot workability decreases. C and N are useful elements for strengthening the solution-induced martensite phase by solid solution strengthening. And, it is important for both the C content and the N content to be 0.05% by mass or more in order to stably obtain a remarkable ductility improving effect. On the other hand, when C and N are contained excessively exceeding 0.30 mass%, the steel becomes excessively hard and may be a factor that hinders workability.

  Moreover, in order to express sufficient ductility by TRIP at the time of production | generation of a process induction martensite phase, it is necessary to make C + N (total content of C and N) 0.20 mass% or more.

  Accordingly, the C content and the N content are both 0.05% by mass and 0.30% by mass, and C + N is 0.20% by mass or more.

  In addition, when C + N exceeds 0.40 mass%, there exists a possibility of inhibiting the workability by hardening. Therefore, C + N is preferably 0.40% by mass or less, and the C content and N content are both 0.05% by mass and 0.15% by mass, and C + N is 0.20% by mass or more. More preferably, it is 0.30 mass% or less.

  Si is an element useful for deoxidation in steelmaking and an element that contributes to solid solution strengthening, but if it exceeds 1.50% by mass, the steel becomes hard and the workability is impaired. It becomes. Further, since Si is a ferrite-forming element, excessive addition causes a large amount of δ-ferrite phase to be generated at a high temperature range, thereby impairing hot workability. Therefore, the Si content is set to 1.50 mass% or less.

  Mn is a useful austenite-forming element that is cheaper than Ni and can substitute for the action of Ni. In order to utilize this action, it is necessary to contain 2.5% by mass or more. On the other hand, when Mn is contained excessively in excess of 7.0 mass%, it becomes easy to cause environmental conservation problems in the steelmaking process, and causes a decrease in productivity due to surface properties, and MnS and the like. This is a factor that causes deterioration of workability and corrosion resistance due to the formation of inclusions. Therefore, the Mn content is set to 2.5% by mass or more and 7.0% by mass or less.

  P and S are mixed as inevitable impurities, but the lower the content, the better. In consideration of adverse effects on workability and other material characteristics and manufacturability, the P content is set to 0.06% by mass or less, and the S content is set to 0.005% by mass or less.

  Ni is an essential element for austenitic stainless steel. In order to obtain good hot workability in the above range of C, N, and Mn, for example, it is necessary to contain Ni so as to be a γ single phase at a heating temperature of 1200 ° C., and the lower limit is 1.0 mass%. It is. In addition, from the viewpoint of reducing raw material costs, component design is performed so as to keep the Ni content as low as possible. Therefore, the Ni content is 1.0% by mass or more and less than 5.0% by mass, preferably 4.0% by mass or less, and more preferably 3.0% by mass or less.

  Cr is an element essential for the formation of a passive film that ensures the corrosion resistance of stainless steel. By containing 15.0% by mass or more, sufficient corrosion resistance can be secured. On the other hand, since Cr is a ferrite-forming element, if it is contained excessively, the heating temperature before hot rolling becomes a (γ + δ) two-phase region, which causes a large amount of δ ferrite after heating, which is a factor that impairs hot workability. Become. In this embodiment, the content of the austenite-generating element can be adjusted to 19.0% by mass. Therefore, the Cr content is 15.0 mass% or more and 19.0 mass% or less.

  Mo is an element that is useful for improving corrosion resistance and contributes to solid solution strengthening. However, if it is excessively contained in an amount exceeding 2.0% by mass, hot workability is impaired. Therefore, the Mo content is set to 2.0 mass% or less.

  Since Cu is an austenite-generating element, the degree of freedom in setting the Ni content increases with an increase in the Cu content, and component design that suppresses Ni becomes easy. Moreover, it is an element effective in raising the value of SFE while suppressing the work hardening resulting from the production | generation of a work induction martensite phase. In order to obtain these effects effectively, it is necessary to contain Cu by 1.0 mass% or more. On the other hand, when Cu is contained in a large amount exceeding 3.5% by mass, hot workability may be hindered. Therefore, the Cu content is set to 1.0% by mass or more and 3.5% by mass or less.

  V has the effect | action which improves work hardening ability and has the effect of improving sag-proof property. In order to acquire this effect, it is necessary to contain V 0.02 mass% or more. On the other hand, if V is contained in an amount exceeding 0.20% by mass, hot workability may be reduced. Therefore, when V is contained, the content of V is set to 0.02% by mass or more and 0.20% by mass or less.

  Co has an effect of improving corrosion resistance and sag resistance. In order to obtain this effect, it is necessary to contain 0.01% by mass or more. On the other hand, when Co is contained in an amount exceeding 0.20% by mass, the sag resistance may decrease due to the decrease in strength. Therefore, when Co is contained, the Co content is 0.01% by mass or more and 0.20% by mass or less, and more preferably 0.01% by mass or more and 0.10% by mass or less.

Here, the larger the value of Md ₃₀ that is the austenite stability index represented by the formula (1), the easier the transformation from the austenite phase to the work-induced martensite phase occurs. High strength can be obtained and excellent ductility can be secured. Also in the molding process, a portion to which processing strain is applied, such as a bent portion, is further increased in strength by the TRIP phenomenon, and easily exhibits excellent sag resistance. Such an effect appears remarkably when the value of Md ₃₀ is 10 or more. On the other hand, if the value of Md ₃₀ exceeds 30, the amount of work-induced martensite generation in the part subjected to the bending process becomes too large, so that cracking may be induced and the bendability may deteriorate. Therefore, in order to stably secure a good ductility with a surface strength of 450 HV or more, the value of Md ₃₀ as an austenite stability index is set to 10 or more and 30 or less.

  Further, when the value of SFE, which is the stacking fault energy index represented by the formula (2), is large, specifically, when the value of SFE is 35 or more, the work hardening of the austenite phase becomes small, and Cracks are likely to occur due to the difference in hardness between the work-induced martensite phase and the austenite phase that sometimes occurs. On the other hand, when the value of SFE is less than 15, work hardening of the austenite phase becomes excessive, and ductility may be reduced. Therefore, the value of SFE, which is a stacking fault energy index, is set to 15 or more and less than 35 in order to prevent the occurrence of cracks due to the hardness difference and the decrease in ductility.

  Further, δcal represented by the expression (3) indicates the amount of δ ferrite in the slab (particularly the center portion of the slab) after being subjected to heat treatment at 1230 ° C. for 2 hours after continuous casting. When both the Mn content and the Cu content are increased, the Cu—Mn phase may be precipitated in the heating before hot rolling, and cracking may occur in the subsequent hot rolling. The Cu—Mn phase has a correlation with the δ ferrite phase, and when the value of δcal exceeds 2.0, a two-piece crack is likely to occur during hot rolling due to the precipitation of a large amount of the Cu—Mn phase. The δ ferrite phase also affects the deterioration of mechanical properties and fatigue properties. Therefore, in order to ensure good hot workability, mechanical properties and fatigue properties as a material for a metal gasket, the value of δcal which is a δ ferrite formation index after heating at 1230 ° C. for 2 hours is 2.0 or less. And

  The stainless steel for metal gasket according to the above embodiment can be manufactured by a general austenitic stainless steel sheet manufacturing process.

  Specifically, steel whose components are adjusted as described above is melted by a steel making facility to form a cast slab, and then hot rolled to obtain a hot rolled steel sheet. In addition, the slab heating temperature before hot rolling should just be the range of 1100 degreeC or more and 1350 degrees C or less.

  The hot-rolled steel sheet is subjected to intermediate annealing, and then subjected to finish annealing by reducing the sheet thickness by cold rolling. In addition, pickling is performed after intermediate annealing and finish annealing. Moreover, you may give an intermediate annealing in the middle of cold rolling as needed.

  The intermediate annealing and the finish annealing after the hot rolling are preferably performed in the range of 900 ° C. or higher and 1100 ° C. or lower.

  In addition, after finish annealing, temper rolling according to the target hardness is performed, and for example, a temper rolled steel sheet having a thickness of 0.1 mm to 3.0 mm can be obtained. Furthermore, after temper rolling, shape correction is performed as necessary.

According to the above-described embodiment, by adjusting the components so that the value of Md ₃₀ is 10 or more and 30 or less, the TRIP phenomenon can easily occur from the austenite phase to the work-induced martensite phase. In addition, it is possible to prevent a decrease in workability by suppressing a decrease in bendability.

  In addition, by adjusting the components so that the SFE value is 15 or more and less than 35, it is possible to suppress a decrease in ductility due to excessive work hardening of the austenite phase, and cracks due to a hardness difference between the austenite phase and the work-induced martensite phase. Generation can be suppressed and deterioration of workability can be prevented.

  By adjusting the components so that the value of δcal is 2.0 or less, it is possible to suppress the precipitation of the Cu—Mn layer and suppress the generation of cracks during hot rolling, thereby preventing the deterioration of hot workability. At the same time, it is possible to prevent the deterioration of the mechanical characteristics and fatigue characteristics due to the δ ferrite phase.

  By setting C + N to 0.20% by mass or more, ductility can be expressed by TRIP, the bendability can be improved, and the workability can be improved.

Therefore, within the above alloy composition range, the value of Md ₃₀ is 10 or more and 30 or less, the value of SFE is 15 or more and less than 35, the value of δcal is 2.0 or less, and C + N is 0.20% by mass or more. By adjusting the components as described above, the Ni content can be reduced to less than 5.0% by mass, and the increase in raw material costs can be suppressed, and the decrease in the surface quality due to the addition of a large amount of Mn is prevented. In addition to being able to be manufactured efficiently, the strength can be improved so that good durability and sag resistance can be secured as a metal gasket material, and workability can be improved to improve leakage resistance.

  And the stainless steel for metal gaskets according to the present invention has a spring property, durability and corrosion resistance equal to or higher than 300 series stainless steels such as SUS301 and SUS304, and has excellent sag resistance as a metal gasket material, Has bendability, fatigue properties and corrosion resistance.

  Hereinafter, this example and a comparative example will be described.

First, stainless steel having the composition shown in Table 1 was melted. In Table 1, a1 to a8 are subject steels having the chemical components defined in the present invention (this example), b1 to b5 are comparative steels (comparative examples), and c1 and c2 are conventional steels (comparative examples), respectively. SUS301 and SUS304. B1 is outside the range defined by the present invention in terms of Md ₃₀ and SFE, b2 is outside the range defined by the present invention in SFE, and b3 and b4 are defined by Md ₃₀ in the present invention. B5 is outside the range defined by the present invention in terms of C + N and δcal.

  Each steel obtained a steel ingot of 100 kg, and then hot rolled at an extraction temperature of 1230 ° C. to produce a hot rolled steel strip having a thickness of 3 mm.

  This hot-rolled steel strip was annealed at 1080 ° C. for 1 minute soaking, and then repeated cold rolling and annealing, so that the hardness was 450 ± 3HV5 and the thickness was 0.50 ± 0.003 mm. A rolled steel strip was obtained.

  In addition, the temper rolling rate at which the hardness after temper rolling becomes 450 HV5 is examined in advance for each steel, and the plate thickness at the time of finish annealing is set based on the temper rolling rate, and the plate thickness After performing cold rolling to 1, the finish annealing was carried out at 1080 ° C. for 1 minute.

  Furthermore, temper rolling was performed to a sheet thickness of 0.2 mm after finish annealing. This temper rolling was performed for 7 to 10 passes after heating the steel sheet to 70 ° C.

  Using the temper rolled material having a thickness of 0.2 mm manufactured as described above, the sag resistance and fatigue characteristics were investigated.

  The test for evaluating the sag resistance was performed using a sag resistance test piece 11 simulating the gasket shown in FIGS. 3 (a) and 3 (b). 3A shows the surface shape of the sag resistance test piece 11, and FIG. 3B shows the cross-sectional shape parallel to the plate thickness direction of the sag resistance evaluation test only on one side.

  The sag resistance test piece 11 is formed by punching a circular opening 12 of φ32 mm in the center of a circular test piece of φ50 mm, and then forming a bead 13 around the opening 12 along the opening 12 by press molding. did. The bead 13 was formed by press molding a bead having a width of 3 mm and a height of 0.5 mm.

The sag resistance is evaluated by using a test specimen 11 for sag resistance, compressing at 25 kN, and restoring and restoring up to 10 μm for up to 10 ⁵ cycles. did.

  It should be noted that according to such a test method, it is possible to simulate the sag resistance when molded into an actual spring shape and actually used through a process called setting. The higher the bead height, the better the sag resistance.

  In the test for evaluating the fatigue characteristics, a fatigue characteristic test piece 15 shown in FIG. 4B was used. This fatigue characteristic test piece 15 was formed by taking a strip-shaped sample having a longitudinal direction L direction and press-molding an initial bead 17 using a bead die 14 shown in FIG.

  In addition, as shown in FIG.4 (c), the fatigue characteristic test piece 15 formed the initial bead 17 whose width is 3 mm and whose height is 0.4 mm. The initial bead 17 was subjected to compression equivalent to the initial tightening of the metal gasket to obtain a bead 18 having a residual bead height of 0.1 mm as shown in FIG.

Such fatigue characteristics test piece 15 simulated bead portion using subjected to fatigue testing of imparting Reversed stresses (bead 18), the fatigue limit stress (fatigue limit: N / mm ²⁾ in the repeating number 10 ⁶ cycles determined It was.

And what has a fatigue limit stress of 300 N / mm ² or more can be evaluated as having excellent fatigue characteristics in a metal gasket having a bead press-formed part.

  Table 2 shows the results of the mechanical characteristics, the sag resistance evaluation test, and the fatigue characteristics evaluation test related to this example and the comparative example.

  In Table 2, H indicating the temper rolling ratio of Comparative Examples c1 and c2 is a tempering symbol defined by JIS G 4313 “Stainless Steel Band for Spring”.

  This example was confirmed to have sag resistance and fatigue characteristics equivalent to or better than those of conventional steels SUS301 and SUS304 (comparative examples c1 and c2).

Further, in all of the examples, the residual bead height is 0.111 mm or more, no crack is confirmed, the fatigue limit stress is 500 N / mm ² or more, and excellent sag resistance and fatigue characteristics are obtained. It was.

On the other hand, each comparative steel (comparative examples b1 to b5) has a residual bead height of 0.081 to 0.093 mm, which is lower than that of the present example, and the occurrence rate of cracks is higher than that of the present example. The stress was 460 N / mm ² or less, which was lower than that of this example. That is, all of the comparative examples were inferior in sag resistance and fatigue characteristics as compared with the present example.

  Therefore, it was confirmed that, by adjusting to the composition defined in the present invention, both sag resistance and fatigue characteristics excellent as a metal gasket material can be achieved in steel strengthened by temper rolling.

  The present invention relates to stainless steel for metal gaskets.

  As shown in FIG. 1, a metal gasket 1 such as a cylinder head gasket or an exhaust manifold gasket used in a vehicle engine needs to have various characteristics necessary for maintaining the airtightness of the joint surface. It is necessary to have the ability to withstand the vibration stress specific to the engine repeatedly applied in the combustion gas atmosphere.

  Further, as shown in FIG. 2, the metal gasket 1 is formed with a bead (projection) 3 having a certain height around the opening 2 so as to maintain sufficient airtightness. High strength and good workability are required. The metal gasket 1 is used in a configuration in which beads 3 are formed in at least one layer and several sheets are laminated.

  In the case of an engine cylinder head gasket, a metal gasket 1 formed with beads 3 along the periphery of the combustion chamber and the periphery of the water holes and oil holes is formed and the bead 3 is tightened. It is common to seal gas, water and oil by surface pressure.

  As a material for this type of metal gasket, work hardening type metastable austenitic stainless steel represented by SUS301, which can obtain high strength by cold working, is frequently used.

  In addition, as a material for the metal gasket, for example, a metastable austenitic stainless steel described in Patent Documents 1 to 3, a work hardening type austenitic stainless steel in which work-induced martensite is generated, and a patent Stainless steel having a multiphase structure of ferrite and martensite as described in Document 4, and stainless steel having austenite structure stabilized by containing 10% by mass or more of Mn as described in Patent Document 5 Steel is known.

Japanese Patent No. 3347582 Japanese Patent No. 4019630 Japanese Patent No. 4785302 JP 2000-256802 A Japanese Patent No. 4116134

  For example, metal gaskets used for cylinder heads require high surface pressure (contact pressure) with a contact partner in order to ensure gas sealability because high pressure is applied during compression. For this reason, in the metal gasket, it is necessary to increase the bead molding height and improve the material strength.

  And in SUS301 system, in order to obtain high strength, it is necessary to perform high cold working, and therefore, rough skin and micro cracks are generated in the bead shoulder R portion at the time of bead generation processing due to reduction in ductility and workability, There is a problem that the leak resistance is lowered.

  The stainless steel of Patent Document 1 improves the high-temperature strength and the high-temperature oxidizability by increasing the N content effective for high-temperature characteristics and defining the Al content that is a harmful effect of N addition. However, in this patent document 1, since 7 to 15% by mass of Ni is added, there is a problem that the raw material cost is increased and it cannot be efficiently manufactured.

  The stainless steel of Patent Document 2 relates to an engine gasket, and the fatigue strength and sag resistance are improved by defining the metal structure of the temper rolled material. However, since Ni is added in an amount of 6 to 8% by mass, there is a problem that the raw material cost increases and the production cannot be efficiently performed.

  The stainless steel of Patent Document 3 relates to a metal gasket for high temperature having excellent sag resistance, but because Ni is added in an amount of 7 to 15% by mass, the raw material cost increases and it cannot be efficiently manufactured. There's a problem.

  Patent Document 4 has a problem that leakage resistance is insufficient from the viewpoint of strength because it has a multiphase structure of ferrite and martensite by a multiphase heat treatment, and therefore has a hardness as low as 400 HV or less.

  The stainless steel of Patent Document 5 contains 10 to 25% by mass of Mn in order to stabilize the austenite structure. Such high Mn steel produces fine Mn oxide particles that are harmful during steelmaking and refining. In addition, due to the high Mn content in the steel, the surface quality of the steel sheet is likely to deteriorate, and the productivity is reduced in the manufacturing process such as annealing pickling and bright annealing, so that it cannot be efficiently manufactured. There is.

  Therefore, as stainless steel for metal gaskets, there has been a demand for a material that can be efficiently manufactured from the viewpoint of raw material costs and from the viewpoint of productivity, and that is excellent in strength and leak resistance.

  This invention is made | formed in view of such a point, and it aims at providing the stainless steel for metal gaskets which can be manufactured efficiently and was excellent in intensity | strength and leak resistance.

The stainless steel for a metal gasket according to claim 1 is C: 0.05% by mass or more and 0.30% by mass or less, Si: 1.50% by mass or less, Mn: 2.5% by mass or more and 7.0% by mass. %: P: 0.06 mass% or less, S: 0.005 mass% or less, Ni: 1.0 mass% or more and less than 5.0 mass%, Cr: 15.0 mass% or more and 19.0 mass% or less Mo: 2.0 mass% or less, Cu: 1.0 mass% or more and 3.5 mass% or less , N: 0.05 mass% or more and 0.30 mass% or less , V: 0.02 mass% or more, and 0.0. 20% by mass or less and Co: 0.01% by mass or more and 0.20% by mass or less , C + N is 0.20% by mass or more, the balance is made of Fe and inevitable impurities, and Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7C The value of _{Md 30} is austenite stability index indicated by -18.5Mo is 10 or more and 30 or less, the value of SFE = 2.2Ni + 6Cu-1.1Cr- 13Si-1.2Mn + 32 is a stacking fault energy generation index indicated by SFE Δcal is an index of δ ferrite formation after heating at 1230 ° C. for 2 hours, expressed as δcal = −15−44.91C−0.88Mn−2.31Ni + 2.20Cr−1.08Cu−28.8N. value at 2.0 or less, having a duplex structure of austenite phase and work-induced martensite phase, Ru der those surface hardness is above 450 HV.

According to the present invention, in the alloy composition defined in the predetermined range, the value of Md ₃₀ is 10 or more and 30 or less, the value of SFE is 15 or more and less than 35, and the value of δcal is 2.0 or less. Since the components are adjusted, it can be produced efficiently and the strength and leak resistance can be improved.

It is a top view which shows typically the structure of a cylinder head gasket. It is explanatory drawing which shows typically the cross-section of the bead part of a metal gasket. (A) is a top view which shows the surface shape of a sag resistance evaluation test piece, (b) is sectional drawing which shows the cross-sectional shape of a sag resistance evaluation test piece. (A) It is explanatory drawing which shows roughly the structure of the bead metal mold | die which forms the bead of a fatigue characteristic test piece, (b) is a top view which shows the surface shape of a fatigue characteristic test piece, (c) is fatigue. It is sectional drawing which shows the cross-sectional shape before compression of a characteristic test piece, (d) is sectional drawing which shows the cross-sectional shape after compression of a fatigue characteristic test piece.

  Hereinafter, the configuration of an embodiment of the present invention will be described in detail.

  Stainless steel for metal gaskets is premised on having excellent sag resistance in order to exhibit excellent durability as a metal gasket. In order to improve sag resistance, it is effective to increase the strength of stainless steel.

  In general, effective means to obtain high strength based on austenitic stainless steel is to provide work such as cold rolling to work harden the austenite phase and harden part of the austenite phase. The so-called work-induced transformation plasticity (TRIP) phenomenon that transforms into a new work-induced martensite phase.

  Moreover, also in the molding process to the metal gasket, the portion to which processing strain is applied is cured by the TRIP phenomenon. The likelihood of such a TRIP phenomenon being affected by the austenite stability.

  However, such curing due to the TRIP phenomenon is effective in improving the sag resistance by increasing the strength, but causes a decrease in workability (for example, bendability) as a metal gasket.

  Therefore, in order to obtain good sag resistance while maintaining excellent workability as a metal gasket material in stainless steel for metal gasket, it is important to adjust the austenite stability and the amount of work-induced martensite. is there.

  Further, bendability, which is one of the criteria for workability, has a certain degree of correlation with the elongation evaluated by a tensile test or the like. This elongation is closely related to C and N, which are solid solution strengthening elements that affect the strength of the TRIP phenomenon and the processing-induced martensite phase. That is, in order to improve the bendability in order to ensure the workability, it is necessary to adjust the austenite stability, the C amount, and the N amount within appropriate ranges.

  And when the value of SFE, which is a generation index of stacking fault energy, is large, work hardening of the austenite phase is difficult to occur, so the hardness difference between the work-induced martensite phase and the austenite phase is large, and the value of SFE is small. Since work hardening of the austenite phase tends to occur, the ductility of the austenite phase decreases.

  Both the increase in the hardness difference between the work-induced martensite phase and the austenite phase and the decrease in the ductility of the austenite phase are factors that reduce the bendability. In order to ensure, it is important to adjust the value of SFE.

The stainless steel for metal gasket according to one embodiment of the present invention is 0.05 mass% or more and 0.30 mass% or less of C (carbon), 1.50 mass% or less of Si (silicon), 2.5 mass. % Mn (manganese), 0.06 mass% or less P (phosphorus), 0.005 mass% or less S (sulfur), 1.0 mass% or more and less than 5.0 mass% Ni (nickel), 15.0 mass% to 19.0 mass% Cr (chromium), 2.0 mass% or less Mo (molybdenum), 1.0 mass% to 3.5 mass% Cu (Copper) , 0 . 05 mass% or more and 0.30 mass% or less N (nitrogen), 0.02 mass% or more and 0.20 mass% or less V (vanadium), and 0.01 mass% or more and 0.20 mass% or less Co containing (cobalt), a C + N ≧ 0.20 wt%, the balance Ru consists of Fe (iron) and unavoidable impurities.

Further, in the range of the content of each element, austenite represented by the formula (1) of Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo The components are adjusted so that the value of Md ₃₀ as a stability index is 10 or more and 30 or less.

  Moreover, in the range of the content of each element described above, the value of SFE, which is a stacking fault energy generation index represented by the formula (2) of SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32, is 15 or more and less than 35. The ingredients are adjusted so that

  Furthermore, within the range of the content of each element described above, δcal = -15-44.91C-0.88Mn-2.31Ni + 2.20Cr-1.08Cu-28.8N represented by the formula (3) at 1230 ° C. for 2 hours The components are adjusted so that the value of δcal, which is a δ ferrite formation index after heating, is 2.0 or less.

  And the stainless steel for metal gaskets which concerns on one Embodiment has a double phase structure of an austenite phase and a process induction martensite phase, and the surface hardness of steel is 450 HV or more. If the steel surface hardness is less than 450 HV, the sag resistance decreases, so when used as a metal gasket for a vehicle engine, the required contact pressure between the cylinder head and the cylinder block due to engine vibrations, etc. (Sealability) may not be obtained.

  C and N are austenite-generating elements. If the content of these elements is too small, the amount of δ ferrite phase increases and hot workability decreases. C and N are useful elements for strengthening the solution-induced martensite phase by solid solution strengthening. And, it is important for both the C content and the N content to be 0.05% by mass or more in order to stably obtain a remarkable ductility improving effect. On the other hand, when C and N are contained excessively exceeding 0.30 mass%, the steel becomes excessively hard and may be a factor that hinders workability.

  Moreover, in order to express sufficient ductility by TRIP at the time of production | generation of a process induction martensite phase, it is necessary to make C + N (total content of C and N) 0.20 mass% or more.

  Accordingly, the C content and the N content are both 0.05% by mass and 0.30% by mass, and C + N is 0.20% by mass or more.

  In addition, when C + N exceeds 0.40 mass%, there exists a possibility of inhibiting the workability by hardening. Therefore, C + N is preferably 0.40% by mass or less, and the C content and N content are both 0.05% by mass and 0.15% by mass, and C + N is 0.20% by mass or more. More preferably, it is 0.30 mass% or less.

  Si is an element useful for deoxidation in steelmaking and an element that contributes to solid solution strengthening, but if it exceeds 1.50% by mass, the steel becomes hard and the workability is impaired. It becomes. Further, since Si is a ferrite-forming element, excessive addition causes a large amount of δ-ferrite phase to be generated at a high temperature range, thereby impairing hot workability. Therefore, the Si content is set to 1.50 mass% or less.

  Mn is a useful austenite-forming element that is cheaper than Ni and can substitute for the action of Ni. In order to utilize this action, it is necessary to contain 2.5% by mass or more. On the other hand, when Mn is contained excessively in excess of 7.0 mass%, it becomes easy to cause environmental conservation problems in the steelmaking process, and causes a decrease in productivity due to surface properties, and MnS and the like. This is a factor that causes deterioration of workability and corrosion resistance due to the formation of inclusions. Therefore, the Mn content is set to 2.5% by mass or more and 7.0% by mass or less.

  P and S are mixed as inevitable impurities, but the lower the content, the better. In consideration of adverse effects on workability and other material characteristics and manufacturability, the P content is set to 0.06% by mass or less, and the S content is set to 0.005% by mass or less.

  Ni is an essential element for austenitic stainless steel. In order to obtain good hot workability in the above range of C, N, and Mn, for example, it is necessary to contain Ni so as to be a γ single phase at a heating temperature of 1200 ° C., and the lower limit is 1.0 mass%. It is. In addition, from the viewpoint of reducing raw material costs, component design is performed so as to keep the Ni content as low as possible. Therefore, the Ni content is 1.0% by mass or more and less than 5.0% by mass, preferably 4.0% by mass or less, and more preferably 3.0% by mass or less.

  Cr is an element essential for the formation of a passive film that ensures the corrosion resistance of stainless steel. By containing 15.0% by mass or more, sufficient corrosion resistance can be secured. On the other hand, since Cr is a ferrite-forming element, if it is contained excessively, the heating temperature before hot rolling becomes a (γ + δ) two-phase region, which causes a large amount of δ ferrite after heating, which is a factor that impairs hot workability. Become. In this embodiment, the content of the austenite-generating element can be adjusted to 19.0% by mass. Therefore, the Cr content is 15.0 mass% or more and 19.0 mass% or less.

  Mo is an element that is useful for improving corrosion resistance and contributes to solid solution strengthening. However, if it is excessively contained in an amount exceeding 2.0% by mass, hot workability is impaired. Therefore, the Mo content is set to 2.0 mass% or less.

  Since Cu is an austenite-generating element, the degree of freedom in setting the Ni content increases with an increase in the Cu content, and component design that suppresses Ni becomes easy. Moreover, it is an element effective in raising the value of SFE while suppressing the work hardening resulting from the production | generation of a work induction martensite phase. In order to obtain these effects effectively, it is necessary to contain Cu by 1.0 mass% or more. On the other hand, when Cu is contained in a large amount exceeding 3.5% by mass, hot workability may be hindered. Therefore, the Cu content is set to 1.0% by mass or more and 3.5% by mass or less.

V has the effect | action which improves work hardening ability and has the effect of improving sag-proof property. In order to acquire this effect, it is necessary to contain V 0.02 mass% or more. On the other hand, if V is contained in an amount exceeding 0.20% by mass, hot workability may be reduced. Therefore , the content of V is set to 0.02% by mass or more and 0.20% by mass or less.

Co has an effect of improving corrosion resistance and sag resistance. In order to obtain this effect, it is necessary to contain 0.01% by mass or more. On the other hand, when Co is contained in an amount exceeding 0.20% by mass, the sag resistance may decrease due to the decrease in strength. Therefore , the content of Co is 0.01% by mass or more and 0.20% by mass or less, and more preferably 0.01% by mass or more and 0.10% by mass or less.

Here, the larger the value of Md ₃₀ that is the austenite stability index represented by the formula (1), the easier the transformation from the austenite phase to the work-induced martensite phase occurs. High strength can be obtained and excellent ductility can be secured. Also in the molding process, a portion to which processing strain is applied, such as a bent portion, is further increased in strength by the TRIP phenomenon, and easily exhibits excellent sag resistance. Such an effect appears remarkably when the value of Md ₃₀ is 10 or more. On the other hand, if the value of Md ₃₀ exceeds 30, the amount of work-induced martensite generation in the part subjected to the bending process becomes too large, so that cracking may be induced and the bendability may deteriorate. Therefore, in order to stably secure a good ductility with a surface strength of 450 HV or more, the value of Md ₃₀ as an austenite stability index is set to 10 or more and 30 or less.

  Further, when the value of SFE, which is the stacking fault energy index represented by the formula (2), is large, specifically, when the value of SFE is 35 or more, the work hardening of the austenite phase becomes small, and Cracks are likely to occur due to the difference in hardness between the work-induced martensite phase and the austenite phase that sometimes occurs. On the other hand, when the value of SFE is less than 15, work hardening of the austenite phase becomes excessive, and ductility may be reduced. Therefore, the value of SFE, which is a stacking fault energy index, is set to 15 or more and less than 35 in order to prevent the occurrence of cracks due to the hardness difference and the decrease in ductility.

  Further, δcal represented by the expression (3) indicates the amount of δ ferrite in the slab (particularly the center portion of the slab) after being subjected to heat treatment at 1230 ° C. for 2 hours after continuous casting. When both the Mn content and the Cu content are increased, the Cu—Mn phase may be precipitated in the heating before hot rolling, and cracking may occur in the subsequent hot rolling. The Cu—Mn phase has a correlation with the δ ferrite phase, and when the value of δcal exceeds 2.0, a two-piece crack is likely to occur during hot rolling due to the precipitation of a large amount of the Cu—Mn phase. The δ ferrite phase also affects the deterioration of mechanical properties and fatigue properties. Therefore, in order to ensure good hot workability, mechanical properties and fatigue properties as a material for a metal gasket, the value of δcal which is a δ ferrite formation index after heating at 1230 ° C. for 2 hours is 2.0 or less. And

  The stainless steel for metal gasket according to the above embodiment can be manufactured by a general austenitic stainless steel sheet manufacturing process.

  Specifically, steel whose components are adjusted as described above is melted by a steel making facility to form a cast slab, and then hot rolled to obtain a hot rolled steel sheet. In addition, the slab heating temperature before hot rolling should just be the range of 1100 degreeC or more and 1350 degrees C or less.

  The hot-rolled steel sheet is subjected to intermediate annealing, and then subjected to finish annealing by reducing the sheet thickness by cold rolling. In addition, pickling is performed after intermediate annealing and finish annealing. Moreover, you may give an intermediate annealing in the middle of cold rolling as needed.

  The intermediate annealing and the finish annealing after the hot rolling are preferably performed in the range of 900 ° C. or higher and 1100 ° C. or lower.

  In addition, after finish annealing, temper rolling according to the target hardness is performed, and for example, a temper rolled steel sheet having a thickness of 0.1 mm to 3.0 mm can be obtained. Furthermore, after temper rolling, shape correction is performed as necessary.

According to the above-described embodiment, by adjusting the components so that the value of Md ₃₀ is 10 or more and 30 or less, the TRIP phenomenon can easily occur from the austenite phase to the work-induced martensite phase. In addition, it is possible to prevent a decrease in workability by suppressing a decrease in bendability.

  In addition, by adjusting the components so that the SFE value is 15 or more and less than 35, it is possible to suppress a decrease in ductility due to excessive work hardening of the austenite phase, and cracks due to a hardness difference between the austenite phase and the work-induced martensite phase. Generation can be suppressed and deterioration of workability can be prevented.

  By adjusting the components so that the value of δcal is 2.0 or less, it is possible to suppress the precipitation of the Cu—Mn layer and suppress the generation of cracks during hot rolling, thereby preventing the deterioration of hot workability. At the same time, it is possible to prevent the deterioration of the mechanical characteristics and fatigue characteristics due to the δ ferrite phase.

  By setting C + N to 0.20% by mass or more, ductility can be expressed by TRIP, the bendability can be improved, and the workability can be improved.

Therefore, within the above alloy composition range, the value of Md ₃₀ is 10 or more and 30 or less, the value of SFE is 15 or more and less than 35, the value of δcal is 2.0 or less, and C + N is 0.20% by mass or more. By adjusting the components as described above, the Ni content can be reduced to less than 5.0% by mass, and the increase in raw material costs can be suppressed, and the decrease in the surface quality due to the addition of a large amount of Mn is prevented. In addition to being able to be manufactured efficiently, the strength can be improved so that good durability and sag resistance can be secured as a metal gasket material, and workability can be improved to improve leakage resistance.

  And the stainless steel for metal gaskets according to the present invention has a spring property, durability and corrosion resistance equal to or higher than 300 series stainless steels such as SUS301 and SUS304, and has excellent sag resistance as a metal gasket material, Has bendability, fatigue properties and corrosion resistance.

  Hereinafter, this example and a comparative example will be described.

First, stainless steel having the composition shown in Table 1 was melted. In Table 1, a1 to a8 are subject steels having the chemical components defined in the present invention (this example), b1 to b5 are comparative steels (comparative examples), and c1 and c2 are conventional steels (comparative examples), respectively. SUS301 and SUS304. B1 is outside the range defined by the present invention in terms of Md ₃₀ and SFE, b2 is outside the range defined by the present invention in SFE, and b3 and b4 are defined by Md ₃₀ in the present invention. B5 is outside the range defined by the present invention in terms of C + N and δcal.

  Each steel obtained a steel ingot of 100 kg, and then hot rolled at an extraction temperature of 1230 ° C. to produce a hot rolled steel strip having a thickness of 3 mm.

  This hot-rolled steel strip was annealed at 1080 ° C. for 1 minute soaking, and then repeated cold rolling and annealing, so that the hardness was 450 ± 3HV5 and the thickness was 0.50 ± 0.003 mm. A rolled steel strip was obtained.

  In addition, the temper rolling rate at which the hardness after temper rolling becomes 450 HV5 is examined in advance for each steel, and the plate thickness at the time of finish annealing is set based on the temper rolling rate, and the plate thickness After performing cold rolling to 1, the finish annealing was carried out at 1080 ° C. for 1 minute.

  Furthermore, temper rolling was performed to a sheet thickness of 0.2 mm after finish annealing. This temper rolling was performed for 7 to 10 passes after heating the steel sheet to 70 ° C.

  Using the temper rolled material having a thickness of 0.2 mm manufactured as described above, the sag resistance and fatigue characteristics were investigated.

  The test for evaluating the sag resistance was performed using a sag resistance test piece 11 simulating the gasket shown in FIGS. 3 (a) and 3 (b). 3A shows the surface shape of the sag resistance test piece 11, and FIG. 3B shows the cross-sectional shape parallel to the plate thickness direction of the sag resistance evaluation test only on one side.

  The sag resistance test piece 11 is formed by punching a circular opening 12 of φ32 mm in the center of a circular test piece of φ50 mm, and then forming a bead 13 around the opening 12 along the opening 12 by press molding. did. The bead 13 was formed by press molding a bead having a width of 3 mm and a height of 0.5 mm.

The sag resistance is evaluated by using a test specimen 11 for sag resistance, compressing at 25 kN, and restoring and restoring up to 10 μm for up to 10 ⁵ cycles. did.

  It should be noted that according to such a test method, it is possible to simulate the sag resistance when molded into an actual spring shape and actually used through a process called setting. The higher the bead height, the better the sag resistance.

  In the test for evaluating the fatigue characteristics, a fatigue characteristic test piece 15 shown in FIG. 4B was used. This fatigue characteristic test piece 15 was formed by taking a strip-shaped sample having a longitudinal direction L direction and press-molding an initial bead 17 using a bead die 14 shown in FIG.

  In addition, as shown in FIG.4 (c), the fatigue characteristic test piece 15 formed the initial bead 17 whose width is 3 mm and whose height is 0.4 mm. The initial bead 17 was subjected to compression equivalent to the initial tightening of the metal gasket to obtain a bead 18 having a residual bead height of 0.1 mm as shown in FIG.

Such fatigue characteristics test piece 15 simulated bead portion using subjected to fatigue testing of imparting Reversed stresses (bead 18), the fatigue limit stress (fatigue limit: N / mm ²⁾ in the repeating number 10 ⁶ cycles determined It was.

And what has a fatigue limit stress of 300 N / mm ² or more can be evaluated as having excellent fatigue characteristics in a metal gasket having a bead press-formed part.

  Table 2 shows the results of the mechanical characteristics, the sag resistance evaluation test, and the fatigue characteristics evaluation test related to this example and the comparative example.

  In Table 2, H indicating the temper rolling ratio of Comparative Examples c1 and c2 is a tempering symbol defined by JIS G 4313 “Stainless Steel Band for Spring”.

  This example was confirmed to have sag resistance and fatigue characteristics equivalent to or better than those of conventional steels SUS301 and SUS304 (comparative examples c1 and c2).

Further, in all of the examples, the residual bead height is 0.111 mm or more, no crack is confirmed, the fatigue limit stress is 500 N / mm ² or more, and excellent sag resistance and fatigue characteristics are obtained. It was.

On the other hand, each comparative steel (comparative examples b1 to b5) has a residual bead height of 0.081 to 0.093 mm, which is lower than that of the present example, and the occurrence rate of cracks is higher than that of the present example. The stress was 460 N / mm ² or less, which was lower than that of this example. That is, all of the comparative examples were inferior in sag resistance and fatigue characteristics as compared with the present example.

  Therefore, it was confirmed that, by adjusting to the composition defined in the present invention, both sag resistance and fatigue characteristics excellent as a metal gasket material can be achieved in steel strengthened by temper rolling.

Claims (2)

C: 0.05 mass% or more and 0.30 mass% or less, Si: 1.50 mass% or less, Mn: 2.5 mass% or more and 7.0 mass% or less, P: 0.06 mass% or less, S: 0.005 mass% or less, Ni: 1.0 mass% or more and less than 5.0 mass%, Cr: 15.0 mass% or more and 19.0 mass% or less, Mo: 2.0 mass% or less, Cu: 1. 0 mass% or more and 3.5 mass% or less and N: 0.05 mass% or more and 0.30 mass% or less, C + N is 0.20 mass% or more, and the balance consists of Fe and inevitable impurities,
Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo is an austenite stability index represented by Md ₃₀ of 10 or more and 30 or less.
SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32 is a stacking fault energy generation index SFE value of 15 or more and less than 35,
δcal = -15-44.91C-0.88Mn-2.31Ni + 2.20Cr-1.08Cu-28.8N The value of δcal which is a δ ferrite formation index after heating at 1230 ° C. for 2 hours is 2.0. Below,
It has a multiphase structure of austenite phase and work-induced martensite phase,
Stainless steel for metal gaskets, characterized in that the surface hardness is 450 HV or higher. The stainless steel for metal gasket according to claim 1, comprising at least one of V: 0.02 mass% to 0.20 mass% and Co: 0.01 mass% to 0.20 mass%. steel.

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