JPH07278758A - Stainless steel for engine gasket and its production - Google Patents

Abstract

PURPOSE:To produce a high strength stainless steel most suitable for engine gasket for automobile, etc., and excellent in spring characteristic, fatigue characteristic, and stress corrosion cracking resistance. CONSTITUTION:This steel is a stainless steel for engine gasket, which contains, by weight, 0.1-0.5% C, <=2% Si, <=5% Mn, 11-18% Cr, <=0.01% S, <=0.01% O, 0.01-0.2% N, and <=0.0005% H and in which the metallic structure of <=0.01% space factor of nonmetallic inclusions is composed of tempered martensite and Vickers hardness is regulated to 400-550. Further, if necessary, Al, Mg, Ca, Ni, Cu, Mo, Ti, and Nb are incorporated. Moreover, in order to attain this hardness, hardening heat treatment at 900-1050 deg.C and tempering heat treatment at 150-500 deg.C are carried out after cold rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel which is most suitable for engine gaskets for automobiles and the like, and has excellent spring characteristics, fatigue characteristics, stress corrosion cracking resistance, and manufacturability, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, SUS is used for engine gaskets.
Steels such as 301L (17% Cr-7% Ni-N added) that have been cold rolled of austenitic stainless steel to increase their strength have been used. However, in order to obtain sufficient strength, a considerable amount of cold rolling is required, and the strength after rolling is greatly affected by the reduction amount and rolling temperature, and there is a large variation in quality. Further, high temperature cooling water or exhaust gas condensed water in the engine may cause stress corrosion cracking.

[0003]

SUMMARY OF THE INVENTION The present invention is not an austenitic stainless steel which has been strengthened by cold rolling as in the prior art, but is inherently excellent in stress corrosion resistance and has a high strength and spring by an optimum heat treatment. It is an object of the present invention to provide a high-strength martensitic stainless steel having properties, which is excellent in spring properties, fatigue properties, stress corrosion cracking resistance, and manufacturability for engine gaskets of automobiles and the like.

[0004]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a steel material composition for satisfying the spring properties, fatigue properties and stress corrosion cracking resistance properties required for engine gaskets in martensitic stainless steel. , Inclusion area ratio,
The hardness is found, and its range and manufacturing conditions are specified. That is, the main component range of C, N, Cr, etc. and the optimum hardness are specified in order to satisfy the spring characteristics required for the gasket, and within this range, fatigue characteristics, elements harmful to stress corrosion cracking, and non-metallic elements are specified. Inclusions are restricted.

That is, the present invention has the following structures. (1) In% by weight, C: 0.1% to 0.5%, Si:
2% or less, Mn: 5% or less, Cr: 11% to 18%, S
: 0.01% or less, O: 0.01% or less, N:
0.01% to 0.2%, H: 0.0005% or less, the balance consisting of Fe and unavoidable impurity elements, the area occupancy of non-metallic inclusions is 0.01% or less, and the metal structure is A stainless steel for engine gasket, which has a Vickers hardness of 400 or more and 550 or less in tempered martensite. (2) In% by weight, C: 0.1% to 0.5%, Si:
2% or less, Mn: 5% or less, Cr: 11% to 18%, S
: 0.01% or less, O: 0.01% or less, N:
0.01% to 0.2%, H: 0.0005% or less is contained, and Al: 0.005% to 0.05% and Mg:
0.002% to 0.02%, Ca: 0.0005% to
It contains 0.005% of one or more kinds, and the balance is F
e and unavoidable impurity elements, the area occupancy of non-metallic inclusions is 0.01% or less, and the metal structure thereof has a Vickers hardness of 400 or more and 550 or less in tempered martensite. Stainless steel for engine gaskets. (3) In the steel according to the above (2) or (3), further, in weight%, Ni: 0.1% to 0.5%, Cu: 0.2%
~ 2%, Mo: 0.1% ~ 3%, Nb: 0.05% ~
0.5%, Ti: 0.05% to 0.5%, one or two
Stainless steel for engine gaskets, characterized by containing at least one kind. (4) After cold rolling a steel containing the components described in each of the above (1), (2) or (3) to a predetermined thickness, quenching heat treatment at 900 ° C to 1050 ° C is performed to form a gasket. Before or after processing 150 ℃ ~
A method for producing a stainless steel for an engine gasket, which comprises performing tempering heat treatment at 500 ° C.

[0006]

The present invention will be described in more detail. First, the reasons for limiting the components will be described. C: It is an important element for hardening the martensite phase and ensuring the strength required for the gasket. However, if excessively added, coarse carbides are formed and the fatigue characteristics and delayed fracture characteristics are impaired, so the content is 0.1% or more, and the content is 0.1.
It was set to 5% or less.

Si: An unavoidable contained element that hardens the martensite phase, but if contained in excess, the ductility required during gasket processing decreases, so its content was made 2% or less. Mn: an unavoidable contained element, but if contained excessively, ductility, corrosion resistance, and stress corrosion cracking resistance deteriorate, so its content was made 5% or less.

Cr: A basic element of stainless steel, at least 11 to obtain the corrosion resistance required for gaskets.
% Or more is required. However, if added excessively, the strength of the ferrite phase in the martensite phase will decrease, so the content was made 11% or more and 18% or less. S: Inevitably contained as an impurity element, but if the content is large, it remains in the steel as inclusions such as MnS and the fatigue properties and corrosion resistance are reduced, so its content is 0.01
% Or less.

O: Inevitably contained as an impurity element, but if the content is large, it will remain in the steel as oxide inclusions and the fatigue properties will be significantly reduced.
It was set to 01% or less. N: N hardens the martensite phase like C and increases the strength, finely disperses carbides, and ductility,
It is an important element that improves fatigue properties. In order to exert the effect, it is necessary to add 0.01% or more, but if added over 0.2%, blowholes are formed, so the content is 0.01% or more, 0 20% or less.

H: H is also an unavoidable impurity element,
Since the delayed fracture property of martensitic stainless steel is induced and cracking occurs when a gasket is used, its content is set to 0.0005% or less.

In addition to the above essential elements, it is effective to add the following elements as selective elements. Al, Mg, Ca: These elements have the effect of reducing and fixing oxygen in the steel, but excessive addition causes a large amount of oxide-based inclusions and reduces fatigue properties, so the content of each is 0.005% -0.05%,
0.002% -0.02%, 0.0005% -0.00
It was set to 5%. Ni, Cu, Mo, Nb, Ti: Ni is ductility, Cu is ductility and strength increase after tempering, Mo is corrosion resistance and ductility after tempering, Nb and Ti are effective for improving corrosion resistance, but if added excessively, Since the ductility and the manufacturability are impaired, the addition amount of each is Ni: 0.1% to 0.5.
%, Cu: 0.2% to 2%, Mo: 0.1% to 3%, N
b: 0.05% to 0.5%, Ti: 0.05% to 0.5
%.

Next, since the non-metallic inclusions remaining in the steel markedly deteriorate the fatigue property of the gasket, it is necessary to strictly limit the amount thereof. In order to obtain sufficient fatigue properties as a gasket material, the area content of non-metallic inclusions must be 0.01% or less.

Further, the metal structure must be subjected to quenching and tempering heat treatment to have a Vickers hardness of 400 or more and 550 or less. A steel plate having a bead shape as shown in Fig. 3 (the unit of the numbers in the figure indicates mm) was subjected to a compression fatigue test, and the result of examining the relationship between the number of times until the bead portion cracked and the material hardness was shown. As shown in FIG. In the compression fatigue test, the test piece is sandwiched between compression jigs, a compressive load is applied until the bead portion is completely crushed, and 10 μm from the fully bent state.
The process of releasing and compressing to full flexion was repeated. If the area ratio of non-metallic inclusions is 0.01% or less and the Vickers hardness is 550 or less, no cracks are found in the bead portion even after 10 ⁷ tests, but if the hardness exceeds 550, fatigue will increase rapidly. Service life is reduced. FIG. 2 shows the relationship between the load, that is, the restoring force of the bead and the material hardness when 10 μm is released from the fully bent state in the compression fatigue test. In order to maintain a good sealing property as a gasket, it is necessary to have excellent spring characteristics of the bead portion, that is, high restoring force, and 400 Vickers hardness or more is required to exert sufficient restoring force. Recognize.

From these results, the material hardness is 400 or more,
It was set to 550 or less. If the hardness exceeds 550, delayed fracture and stress corrosion cracking are likely to occur. In order to achieve this hardness, quenching heat treatment at 900 ° C to 1050 ° C and tempering heat treatment at 150 ° C to 500 ° C are effective after cold rolling. The tempering heat treatment may be performed either before or after forming. However, although it is possible to adjust the hardness within the above range only by quenching heat treatment, tempering heat treatment is essential for this application due to delayed fracture resistance.

[0015]

[Examples] Table 1 shows the chemical composition of the test steel. All the steels were melted, hot-rolled and cold-rolled, and the martensitic stainless steels of steel type symbols A to N were cold-rolled to 0.3 mm and then subjected to quenching heat treatment. Conventional steel O (SUS30
For 1 L), a solution heat-treated steel sheet having a thickness of 0.6 mm was temper-rolled to a thickness of 0.3 mm with a reduction rate of 50%. Test pieces having the bead shape shown in FIG. 3 were produced from these steel sheets and were subjected to tempering heat treatment or aging heat treatment.

After measuring the Vickers hardness and the area ratio of non-metallic inclusions, a compression fatigue test and a stress corrosion cracking test under the above conditions were carried out. In the stress corrosion cracking test, the bead portion was compressed until it was fully bent and immersed in 200 ° C. high-temperature water containing 1000 ppm of chlorine ions for 500 hours to examine the occurrence of cracking.

The heat treatment conditions and the above test results are shown in Tables 2 and 3. As is clear from Table 2, the steel whose chemical composition, hardness and area ratio of non-metallic inclusions are adjusted within the scope of the claims of the present invention has a bead restoring force required as a gasket,
No cracking due to fatigue or stress corrosion occurred. On the other hand, a comparative steel having a hardness of less than 400 does not have sufficient bead restoring force, and a steel having an area ratio of nonmetallic inclusions of more than 0.01% or a hardness of more than 550 is sufficient. Does not exhibit fatigue crack resistance or stress corrosion crack resistance.

Further, from Table 3, in order to achieve the hardness within the claimed range within the above-mentioned inventive components, 900 ° C. or higher, 10
It can be seen that quenching heat treatment at 50 ° C. or less and tempering heat treatment at 150 ° C. or more and 500 ° C. or less are effective, and thereby satisfy the bead restoring force, fatigue crack resistance, and stress corrosion cracking resistance required as a gasket. It can be seen from Table 3 that the temper-rolled material of SUS301L (steel type symbol O in the table) that has been conventionally used has insufficient stress corrosion cracking resistance.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

The high-strength stainless steel sheet of the present invention has excellent spring properties, fatigue properties, and stress corrosion cracking resistance required for gaskets, and further has a lower manufacturing cost than conventional materials.
It is most suitable as a gasket for engines of automobiles, etc., and its industrial contribution is extremely large.

[Brief description of drawings]

FIG. 1 shows the relationship between the material hardness (Hv) and the number of occurrences of bead cracking for each area ratio of non-metallic inclusions.

FIG. 2 is a diagram showing a relationship between material hardness (Hv) and bead restoring force.

FIG. 3 shows a bead shape of a test material, (a) is a plane,
(B) is a situation of the CC ′ cross section.

Claims (4)

[Claims] 1. By weight%, C: 0.1% to 0.5%, Si: 2% or less, Mn: 5% or less, Cr: 11% to 18%, S: 0.01% or less, O : 0.01% or less, N: 0.01% to 0.2%, H: 0.0005% or less, the balance consisting of Fe and unavoidable impurity elements, and the area occupancy of nonmetallic inclusions is 0.01. %, And the metallographic structure thereof has a Vickers hardness of 400 or more and 550 or less in tempered martensite. Stainless steel for engine gaskets. 2. By weight%, C: 0.1% to 0.5%, Si: 2% or less, Mn: 5% or less, Cr: 11% to 18%, S: 0.01% or less, O : 0.01% or less, N: 0.01% to 0.2%, H: 0.0005% or less, and further Al: 0.005% to 0.05%, Mg: 0.002% .About.0.02%, Ca: 0.0005% to 0.005%, containing 1 or 2 or more kinds, and the balance consisting of Fe and unavoidable impurity elements. A stainless steel for an engine gasket, which has a Vickers hardness of not less than 01% and a tempered martensite of 400 or more and 550 or less. 3. The steel according to claim 2 or 3, further comprising, by weight%, Ni: 0.1% to 0.5%, Cu: 0.2% to 2%, Mo: 0.1% to 3 %, Nb: 0.05% to 0.5%, Ti: 0.05% to 0.5%, and one or more kinds of stainless steel for engine gaskets. 4. A steel containing the components described in claim 1, 2 or 3 is cold-rolled to a predetermined thickness and then subjected to quenching heat treatment at 900 ° C. to 1050 ° C. to form a gasket. Before or after processing 150 ℃ ~ 5
A method for producing a stainless steel for engine gasket, which comprises performing tempering heat treatment at 00 ° C.