JP4353060B2 - Stainless steel for gasket - Google Patents

Description

  The present invention relates to an inexpensive martensitic stainless steel for gaskets having high hardness and excellent workability and corrosion resistance, a method for producing the same, and further relates to an engine gasket for automobiles and the like.

  The engine gasket is an important part provided between the cylinder head and the block. As shown in FIGS. 1 (a) and 1 (b), the beaded projection basically closes the gap between the combustion gas, Seal parts that prevent leakage of cooling water and lubricating oil. FIG. 1 (b) is a partial perspective view of the engine gasket when the frame portion of FIG. 1 (a) is cut out.

Therefore, the gasket material is required to have high strength (high hardness), excellent workability and corrosion resistance, and the like.
Conventionally, metastable austenitic stainless steels mainly made of SUS301 stainless steel mainly composed of Cr and Ni have been used as gasket materials. This is because these cause transformation to a high-hardness martensite phase with deformation, and thus have a high work hardening rate and excellent workability.

  However, since the hardness after processing strongly depends on the processing rate and processing temperature, there is a problem that the variation in quality is large. There is also a problem that stress corrosion cracking is likely to occur. Furthermore, it contained a large amount of expensive Ni, and the product was also expensive.

  As a solution to such a problem, recently, Patent Document 1 proposes stainless steel having a tempered martensite structure mainly composed of Cr. Martensitic stainless steel generally has better stress corrosion cracking properties than the austenitic stainless steels mentioned above, and it is relatively easy to obtain high hardness by utilizing transformation to a hardened martensite phase by quenching heat treatment. It is done. In addition, it is inexpensive because it contains almost no expensive Ni.

However, tempering heat treatment is inevitable due to deterioration of workability due to elongation reduction, etc., which causes embrittlement due to carbide precipitation and corrosion resistance deterioration due to the generation of Cr-deficient phases, and also increases the manufacturing cost.
Japanese Unexamined Patent Publication No. 7-278758

  The present invention relates to the stable supply of high-performance and inexpensive stainless steel for engine gaskets. Unlike conventional austenitic stainless steel, the martensite system is improved in its as-quenched finish for high strength, workability, and corrosion resistance. The aim is to develop an excellent and inexpensive martensitic stainless steel.

  The present inventors performed quenching from the austenite + ferrite two-phase region instead of the austenite single-phase region at a high temperature, and transformed the austenite phase at that time into a high-hardness martensite phase. It has been found that the workability required for the gasket can be obtained while maintaining a high hardness from the relationship between the hardness and the amount of martensite without performing tempering treatment with the structure of martensite phase + ferrite phase. The present invention was completed by clarifying the steel composition range to be obtained and the heat treatment conditions for obtaining excellent corrosion resistance.

  Therefore, in the present invention, the hardness, the amount of martensite and the manufacturing method thereof are defined together with the steel material components. Further, according to the present invention, it is possible to provide a cheaper material by reducing the amount of expensive Ni or the like added as much as possible.

That is, the present invention is weight percent,
C + N: 0.1 to 0.3%, Si: 0.5% or less, Mn: 0.7% or less, Cr: 10 to 17%, Ni: 0 to 0.6% , and at least one selected from the group of Nb, V and Ti It has a steel composition consisting of 0-2.0%, the balance Fe and impurities , has a structure consisting of 40% to 80% martensite phase and the balance ferrite phase, and has a Vickers hardness of 300 to 500 Stainless steel for gaskets.

  From another aspect, the present invention is a method for producing stainless steel for gaskets, wherein the steel having the above steel composition is processed into a predetermined thickness and then finished by quenching heat treatment heated to 850 to 1000 ° C. .

From yet another aspect, the present invention provides, by weight, C + N: 0.1 to 0.3%, Si: 0.5% or less, Mn: 0.7% or less, Cr: 10 to 17%, Ni: 0 to 0.6%, and Nb And a steel composition composed of at least one selected from the group of V and Ti in a total of 0 to 2.0%, the balance Fe and impurities, and a structure composed of 40% to 80% martensite phase and the balance ferrite phase An engine gasket having a Vickers hardness of 300 to 500.

  Furthermore, from another aspect, the present invention shows a martensitic structure comprising a martensite phase of 40% or more and 80% or less and a remaining ferrite phase, and has an Vickers hardness of 300 or more and 500 or less. It is a gasket.

  According to the present invention, it is possible to stably supply a high-performance and inexpensive stainless steel plate as a material for engine gaskets used in automobiles and the like.

The embodiment of the present invention will be described in more detail.
First, the reason why the steel composition is limited as described above in the present invention will be described.
C + N: 0.1 to 0.3%
C and N are elements that harden the martensite phase in a small amount, and their effects are considered to be equivalent to C and N. If the total amount of C + N is less than 0.1%, the required hardness of Hv 300 or more cannot be obtained after quenching. Therefore, the total content of C + N is 0.1% or more and 0.3% or less.

Si: 0.5% or less
Si is an element that hardens the martensite phase, and if it exceeds 0.5%, the workability becomes insufficient. Therefore, the Si content is set to 0.5% or less. Preferably, it is 0.4% or less.

Mn: 0.7% or less
Mn is an element that expands the austenite phase region at high temperatures, and has the effect of stabilizing the same phase to lower temperatures and increasing the amount of martensite after quenching. When Mn exceeds 0.7%, it becomes a martensite single phase, resulting in poor workability. Furthermore, when it adds excessively, the appearance of a retained austenite phase will be caused and required hardness will not be obtained. Therefore, the Mn content is set to 0.7% or less. Preferably, it is 0.5% or less.

Cr: 10-17%
Cr is a basic additive element of stainless steel, and in order to obtain effective corrosion resistance, addition of at least 10% is necessary. Further, the presence of Cr is considered to delay the transformation of the austenite phase at a high temperature, and when it exceeds 17%, the appearance of the retained austenite phase is caused and the required hardness cannot be obtained. Therefore, the Cr content is set to 10% or more and 17% or less. Preferably, it is 12 to 15%.

Ni: 0 to 0.6%
Ni, like Mn, is an element that expands the austenite phase region at high temperatures, and if desired, 0.6% or less can be added to stabilize the same phase to lower temperatures and increase the amount of martensite after quenching. However, if it exceeds 0.6%, it becomes a martensite single phase and the workability becomes poor. Therefore, when Ni is added, the Ni content is set to 0.6% or less. Preferably, it is 0.5% or less.

  As described above, the steel according to the present invention adjusts the martensite phase hardness mainly by C, N, and Si, and the martensite amount by Cr, Mn, and Ni. Stipulated. In addition, if necessary for improving the strength, addition of at least one of a small amount of elements such as Nb, V, Ti and the like in a total amount of 0 to 2.0% is allowed, but the balance is usually made of Fe and inevitable impurity elements.

Next, the reasons for limiting the hardness of the material and the amount of martensite will be described using experimental results.
Table 1 shows the results of component analysis of the thin plate material made of Fe-13Cr steel used here. After melting steel with such a steel composition, hot rolling and cold rolling to a predetermined thickness (usually 0.3 to 0.1 mm, 0.2 mm in this example), and then changing the quenching temperature, the martensite amount and hardness The test piece was cut out from the obtained material.

FIG. 2 shows the shape of the bead portion in the strip-shaped test piece of the same thin plate material. The numbers in the figure indicate dimensions (mm).
FIG. 3 shows the influence of the hardness of the material on the bead height after releasing the beaded test piece from the fully bent state using a compression tester. The bead height at the start was 0.3 mm, but after the test it was 0.06 to 0.15 mm.

  The bead height increases especially at Hv300 and above and 0.10 mm and above. These indicate that effective sealability can be obtained at the same value or more. For this reason, the lower limit of the hardness of the material was set to Hv300. Moreover, when Hv500 was exceeded, the crack at the time of bead processing was confirmed in some specimens.

  FIG. 4 shows the influence of the amount of martensite and the hardness of the material on the presence or absence of cracking during bead processing using a die press. Although hardening with an increase in the amount of martensite is recognized, they have a wide range.When the amount of martensite exceeds 80%, the hardness of the material should be Hv500 even if the amount of martensite is low. When it exceeds, cracks occur frequently.

  From these facts, it is considered that the hardness and the amount of the martensite phase itself influence the workability and show that there is an appropriate relationship between them. On the other hand, if the amount of martensite is less than 40%, it is difficult to obtain the lower limit of the hardness of Hv300.

  From the above, in the present invention, the required hardness is defined as Hv300 or more and Hv500 or less, and the martensite amount is defined as 40% or more and 80% or less in order to clarify the range in which necessary workability can be obtained.

In the present specification, the structural ratio of the martensite phase and the ferrite phase is expressed by volume%.
Finally, the reason for limiting the manufacturing method will be described using experimental results.

  FIG. 5 shows the influence of the heat treatment temperature on the corrosion resistance of the thin plate specimen made of Fe-13Cr steel in the salt spray test (JIS-Z-2371). When the quenching temperature is 800 ° C or less, the rating No. (R.No.) is greatly reduced, and the corrosion resistance deteriorates rapidly.

It was also confirmed that the corrosion resistance deteriorated rapidly when tempering was performed after quenching at 1000 ° C. These are presumed to be caused by the occurrence of Cr-deficient phases due to carbide precipitation.
If the quenching temperature exceeds 1000 ° C, the microstructure at high temperature is considered to be an austenite single phase. The structure after quenching has a martensite content exceeding 80% (Table 2, No. B6), and the workability deteriorates. To do.

  From the above, the quenching temperature was set to 850 ° C or higher and 1000 ° C or lower. Although there are differences depending on the steel material composition, quenching temperature, etc., the holding time is preferably 10 seconds or more, and the cooling rate during quenching is preferably 10 ° C./second or more.

  Stainless steel with the composition shown in Table 2 is melted in a 10 kg vacuum melting furnace, hot rolled, annealed, descaled (pickled), and then cold rolled to a thickness of 0.2 mm to create a thin plate material Then, after holding for 10 seconds by heating at 750 to 1050 ° C., quenching was performed by air cooling.

  Then, using the obtained sheet material, the hardness, martensite amount, corrosion resistance in the salt spray test, the presence or absence of cracks after processing a bead similar to that in FIG. 2 and the corrosion resistance in the salt spray test were investigated. .

Table 2 shows the survey results.
The crack occurrence rate was the number of cracks in five test pieces, and the corrosion resistance was evaluated by R.No. according to JIS Z 2371.

  As is clear from the test results shown in the table, No. D4, F4, G4, and H4, where the martensite content exceeds 90% and the hardness exceeds Hv500, cracks frequently during bead processing, and only the hardness exceeds Hv500. No.E4 also cracks. The same applies to No. B6, whose heating temperature exceeds 1000 ° C. No. A1 and B1 below 850 ° C cannot obtain the required hardness of Hv300 or higher, and the corrosion resistance is greatly deteriorated.

  In contrast, Nos. A2 to No. 5 and B2 to 5, which are examples of the present invention, exhibit high hardness and high corrosion resistance, and the occurrence rate of cracks is extremely low.

1 (a) and 1 (b) are explanatory views showing the arrangement of the gasket and the basic structure, respectively. It is sectional drawing which shows the shape of the bead part in the strip-shaped thin-plate material test piece which consists of Fe-13Cr steel. It is a graph which shows the influence of the hardness of the material which acts on the bead height after releasing the test piece after the bead processing by a compression tester from a full bending state. It is a graph which shows the influence of the amount of martensite and the hardness of material which has on the presence or absence of the crack generation at the time of bead processing. It is a graph which shows the influence of heat processing temperature on the corrosion resistance of the Fe-13Cr thin plate material test piece in a salt spray test (JIS-Z-2371).

Claims (3)

% By weight, C + N: 0.1 to 0.3%, Si: 0.5% or less, Mn: 0.7% or less, Cr: 10 to 17%, Ni: 0 to 0.6% , and at least selected from the group of Nb, V and Ti It has a steel composition consisting of 0 to 2.0% balance Fe and impurities in total of 1 type, shows a structure consisting of 40% to 80% martensite phase and the balance ferrite phase, and has a Vickers hardness of 300 to 500 Stainless steel for gaskets. % By weight, C + N: 0.1 to 0.3%, Si: 0.5% or less, Mn: 0.7% or less, Cr: 10 to 17%, Ni: 0 to 0.6%, and at least selected from the group of Nb, V and Ti It has a steel composition consisting of 0 to 2.0% balance Fe and impurities in total of 1 type, shows a structure consisting of 40% to 80% martensite phase and the balance ferrite phase, and has a Vickers hardness of 300 to 500 An engine gasket characterized by that. The stainless steel for gaskets according to claim 1, wherein the steel having the steel composition according to claim 1 is processed to a predetermined thickness, and then finished by a heat treatment in which the steel is heated to 850 to 1000 ° C and quenched. Steel manufacturing method .

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