JP2002038243A - Stainless steel having plural phase structure and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel for a spring of such a gasket for engine, low in cost, high in strength, excellent in workability, and high in fatigue property, and to provide a technique capable of stably supplying the same. SOLUTION: This stainless steel is composed of 0.06% or above and 0.50% or below C+N, 0.1% or above and 2.0% or below Si, 0.1% or above and 3.0% or below Mn, 10.0% or above and 17.0% or below Cr, below 1.0% Ni, below 2.0% Cu, and the balance Fe with unavoidable impure elements, wherein the outer layer part of material composed of single phase structure of martensite, or two phase structure of martensite and farrite, is controlled to be single phase of austenite or plural layer structure containing this phase. In production, the stainless steel composed of the single phase structure of martensite, or the two phase structure of martensite and ferrite, is diminished in sheet thickness to required thickness, is heated and kept to above Ac1 transformation point in atmosphere of nitrogen gas or mixed gas of nitrogen gas and reductive gas, and thereafter is hardened.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inexpensive dual phase stainless steel sheet having high strength, excellent workability and high fatigue properties, and a method for producing the same.

The stainless steel sheet according to the present invention is suitable for use in springs or spring parts used after processing into various shapes, electronic equipment requiring spring properties, and mechanical parts in general. More specifically, it is most suitable for gaskets for engines of automobiles and motorcycles.

[0003]

2. Description of the Related Art An engine gasket is a seal component inserted into a gap between a cylinder head and a block for the purpose of sealing off combustion gas, cooling water and lubricating oil. It has the appearance of multiple (1-3) stainless steel sheets with holes (pores) corresponding to (shape, number).

The basic structure is such that an annular convex portion (bead) is formed around the pore by press working or the like, and the above-mentioned gap generated due to combustion is sealed by the repulsive force. Conventionally, gasket materials have been required to have high strength but excellent workability.
A temper-rolled material of metastable austenitic stainless steel centering on S301, SUS304 and the like has been used.

[0005] This material has a high work hardening rate due to the formation of a martensite phase which is a high hardness intermediate phase due to work induced transformation, and high strength can be obtained relatively easily by rolling. In addition, necking (constriction) is suppressed due to the hardening of the deformed portion due to the transformation, showing high uniform elongation, and excellent workability.

[0006] The strength and workability of a material are generally contradictory characteristics, and it becomes difficult to maintain the required workability as the strength increases. These facts indicate that metastable austenitic steel is a very excellent material. However, the material was expensive because it contained a large amount of expensive Ni. Further, it is difficult to obtain a stable strength because the strength is strongly dependent on conditions such as a rolling ratio and a rolling temperature,
There was also a problem that the quality was large.

In view of these problems, application of a stainless steel quenching material containing almost no expensive Ni and containing about 10 to 20% of Cr as a main component has been studied in Japanese Patent Application Laid-Open No. 7-278758. I have. Although this stainless steel should originally have a ferrite phase structure at room temperature, it is heated to a temperature higher than the Ac ₁ transformation point and quenched (quenched) from the austenite phase region at a high temperature to form an interstitial solid solution element (C
, N) are transformed into a martensite phase, which is a high hardness intermediate phase in which the solid solution is supersaturated, and high hardness is obtained. The Ac ₁ transformation point is the temperature at which transformation to the austenite phase starts during heating.

However, in general, when the steel has a martensite single phase structure, the material after quenching becomes too hard, and it is often difficult to process the product into a subsequent product shape. For this reason, tempering (at around 500 ° C.) is required to improve workability.
However, this tempering causes embrittlement due to precipitation of carbides and the like and generation of a Cr-deficient phase, thereby impairing the corrosion resistance required for stainless steel, and increasing the production cost.

[0009] Under these circumstances, the present inventors have disclosed in
In -109957 Patent Publication (Japanese Patent Application No. Hei 10-282758), interstitial solid solution elements in an amount of a stainless steel containing 10-17% of Cr adjusted above Ac ₁ transformation point of the austenite and ferrite two-phase temperature Material for gasket that achieves both high hardness and excellent workability without tempering by quenching from the zone to form a two-phase structure of martensite and residual ferrite with hardness adjusted by optimizing the phase ratio Has been proposed.

[0010]

However, recent engines have tended to have a high output due to user needs and the like, and at the same time, have been reduced in weight by various methods in order to cope with the improvement of fuel efficiency (environmental problem) due to strict exhaust gas regulations. It is proceeding at the same time. Such weight reduction indicates that the engine rigidity is reduced and the gap generated between the cylinder head and the block tends to increase with combustion.

That is, the usage environment of the gasket is becoming severer. Along with this, the gasket material is also required to be inexpensive, have high strength and excellent workability as described above, and also have high fatigue properties that can withstand repeated stresses (increase / decrease in gaps).

An object of the present invention is to provide a stainless steel sheet which is inexpensive, has high strength and is excellent in workability and has high fatigue characteristics, and is useful for, for example, a spring, and a manufacturing method capable of stably supplying the stainless steel sheet.

[0013]

In the case where at least the above-described duplex stainless steel sheet containing Cr as a main component is subjected to severe processing such as further increasing a bead in response to an increased gap, a gasket is required. In addition to being capable of processing itself, a material that does not generate defects such as wrinkles or minute cracks in the surface layer of the bead where the most severe processing is performed is required. When a material having only such defects is subjected to a fatigue test, defects in the surface layer grow rapidly due to stress concentration, and a gasket may crack at an early stage through the plate.

It is considered that the surface layer defects mainly occur at the grain boundaries of the hard martensite phase and the soft ferrite phase. In other words, when processing beyond a certain limit is performed, it is considered that such a defect is unavoidably generated in the surface layer even if processing into a gasket is possible. However, this conversely involves processing, and gaskets in which fluctuating stress is repeatedly applied to the processed part, and furthermore, various types of springs or spring parts used in similar situations have a long service life. In other words, it is extremely effective to suppress the occurrence of defects that occur at the same time, in other words, the occurrence of fatigue fracture starting points.

[0015] That is, the present invention is _one of the most powerful austenite stabilizing elements for heating and holding a stainless steel containing Cr as a main component to a temperature higher than the Ac ₁ transformation point prior to quenching. By performing the process in a gas atmosphere containing nitrogen, the workability of the material is greatly improved by nitriding the surface layer to form an austenite phase, and the occurrence of defects in the processed surface layer is suppressed and the fatigue characteristics are reduced. It is based on the discovery of a dramatic improvement.

In this case, the (internal) structure after quenching can obtain sufficient performance as a single phase of martensite together with a conventional two phase of martensite and ferrite. N is
It is the most powerful austenite stabilizing element together with C. It is an experiment to convert a material to an austenite single phase with almost no expensive Ni by dissolving both elements or one of them in a large amount during melting. It is possible in a room.

However, it is difficult to dissolve such amounts of N and C in a normal industrial dissolution apparatus, and it is considered that an extremely expensive material is used when a special apparatus is used. Even if a solid solution is formed, the subsequent manufacture of the plate becomes extremely difficult.

The present invention provides a stainless steel containing Cr as a main component, which can be manufactured by a commonly used industrial apparatus and is inexpensive in the final quenching step without burdening other manufacturing steps. By austeniticizing only the surface layer of (a), (metastable) high performance equivalent to or higher than that of austenitic stainless steel is obtained.

Here, the present invention is as follows. (1) In mass% C + N: 0.06% or more, 0.50% or less Si: 0.1% or more, 2.0% or less Mn: 0.1% or more, 3.0% or less Cr: 10.0% or more, 17.0% or less Ni: 1.0% or less Cu : 2.0% or less The balance has a chemical composition consisting of Fe and impurity elements, and the surface layer of a material consisting of a single phase of martensite or a dual phase structure of martensite and ferrite has a single phase of austenite or a double phase structure containing the same phase. steel sheet.

(2) having the chemical composition described in (1) above,
After reducing the thickness of a stainless steel sheet composed of a single phase of martensite or a two-phase structure of martensite and ferrite to a predetermined thickness, Ac _{1 is obtained} in an atmosphere containing nitrogen gas or a mixture of nitrogen gas and a reducing gas (eg, hydrogen). A method for producing a stainless steel sheet having a dual phase structure, wherein the steel sheet is heated and maintained at a temperature equal to or higher than the transformation point and then quenched.

(3) An engine gasket using the duplex stainless steel according to (1).

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the reason for defining the chemical composition and the manufacturing process as described above will be described. In the present specification, “%” defining the chemical composition is:
Unless otherwise specified, it is “% by mass”.

C + N: 0.06% or more and 0.50% or less C and N are not only strong austenite stabilizing elements but also interstitial solid solution elements, and mainly harden the martensite phase. These effects are considered to be almost the same for both elements. If the total content of both elements is less than 0.06%, Hv after quenching
It is difficult to obtain the required hardness of 300 or more. Conversely, if it is contained excessively, it will be too hard, so subsequent board production,
Processing into products becomes difficult. In addition, an increase in the content exceeding 0.50% suppresses nitriding, and makes it difficult to austenitize the surface layer.

Therefore, the total content of both elements is set to 0.06% or more and 0.50% or less. More preferably, 0.10% or more,
0.40% or less. Note that the total content of both elements was the average value of the material after nitriding, "the surface layer of which nitrogen was concentrated".

Si: 0.1% or more and 2.0% or less Si is a solid solution strengthening element, and it is necessary to adjust the content according to the C + N content, but if the content is less than 0.1%, it is difficult to obtain a hardness of Hv300 or more after quenching. . However, excessive addition hardens the material too much and forms inclusions and the like, making it difficult to manufacture a plate and process it into a product. Further, Si is a ferrite stabilizing element, and is considered to suppress nitriding by decreasing the amount of solid solution of nitrogen, and inhibits austenitization of a surface layer portion.
Therefore, the content is set to 0.1% or more and 2.0% or less. More preferably, it is 0.15% or more and 1.8% or less.

Mn: 0.1% or more and 3.0% or less Mn is an austenite stabilizing element. Further, it is considered that nitriding is promoted by increasing the amount of solid solution of nitrogen, and is an element effective for austenitizing the surface layer portion, and the content is required to be 0.1% or more. However, if added excessively, it becomes difficult to form inclusions and the like and subsequently manufacture the plate and work into a product, and the material becomes austenite and conversely, it becomes difficult to obtain the required hardness after quenching. Therefore, the Mn content is set to 0.1% or more and 3.0% or less. More preferably, 0.
3% or more and 2.6% or less.

Cr: 10.0% or more, 17.0% or less Cr is a basic element of stainless steel, and is added at 10.0% or more to obtain effective corrosion resistance. Although it is thought to promote nitriding, it is a ferrite stabilizing element, and excessive addition impairs austenitization of the surface layer. Therefore, the upper limit was set to 17.0% or less. Preferably, 1
It is at most 6.0%, more preferably at most 15.6%.

Ni: 1.0% or less Ni is the most powerful austenite stabilizing element.
Adjustment is required depending on the Cu content. However, it is considered that it is an expensive element as described above and also suppresses nitriding. Therefore, its content is set to 1.0% or less. More preferably, it is 0.9% or less.

Cu: 2.0% or less Cu is an austenite stabilizing element and needs to be adjusted depending on the contents of Mn and Ni. The effect on nitriding is considered to be small. However, if it is added excessively, it will precipitate at the grain boundaries and the like, making it difficult to manufacture a plate and process it into a product. Therefore, the content is set to 2.0% or less. More preferably, it is at most 1.3%.

The balance consists of Fe and impurity elements. In addition, elements added from an industrial aspect other than these components, for example, Ca, Al, Ti used as a deoxidizing agent during melting.
Alternatively, a total of 1.0% or less of REM (rare earth metal), Mo, for which hot workability is expected to be improved, and Mo, which is expected to greatly improve corrosion resistance, may be included as necessary.

Next, the reasons for limiting the structure of the starting material will be described. The reason why the structure of the starting material is a martensite single phase or a two-phase structure of martensite and ferrite is to strengthen the material after quenching by a martensite phase having high hardness.

It is considered that the hardness of the material can maintain a sufficient bead height even after the gasket is completely compressed.
Hv300 or more is desirable. The phase ratio at which the material exhibits excellent workability while satisfying the same hardness is a martensite content of 40%.
Volume% or more and 90 volume% or less, the two-phase state of the remaining ferrite phase, but after nitriding the surface layer, the martensite content is 30 volume%.
As described above, the range was expanded to the range of the remaining ferrite phase.

The material having a surface layer having an austenite single phase or a multi-phase structure containing the same phase improves the workability and suppresses the occurrence of defects such as wrinkles and microcracks on the surface during the processing. This is because high fatigue characteristics can be obtained after processing.

The reason why the austenite phase is destabilized to have a dual phase structure containing the same phase is that the generation of defects in the surface layer is expected to be further suppressed by the hardening of the deformed portion due to the work-induced martensitic transformation. .

Here, the “surface layer” is a region where an austenite phase is formed by the nitriding treatment, and is usually 20 minutes from the surface layer.
μm, preferably a region having a depth of about 15 μm. From the experimental results, a sufficient effect was recognized when the austenite phase ratio in the surface layer was 10% by volume or more. The thickness is 3 μm
It is desirable to make the above.

Next, the reasons for limiting each step in the manufacturing method according to the present invention will be described. First, in the present invention, a stainless steel sheet composed of a single phase of martensite or a two-phase structure of martensite and ferrite is used as a starting material, and this is a result of securing predetermined strength and considering economic efficiency. It is.

The starting material thus prepared is reduced to a predetermined thickness by a suitable means such as hot rolling or cold rolling.
The reason for quenching after heating to a temperature range equal to or higher than the Ac ₁ transformation point is to obtain a martensite phase. From the experimental results, it is desirable that the heating temperature be 850 ° C or higher, and the cooling rate during quenching be 10 ° C / sec or higher up to about 200 ° C, which is considered to complete martensitic transformation. The atmosphere during heating and holding is set to nitrogen gas or a mixed state of nitrogen gas and reducing gas.
Nitrogen is one of the strongest austenite stabilizing elements, and the surface layer can have a single phase of austenite or a multiphase structure containing the same phase by nitriding. In the case of using nitrogen gas alone, a part thereof may be replaced with an inert gas such as Ar gas.

Although there are differences depending on the steel material composition, the state of the surface coating, and the like, nitriding proceeds as the heating temperature is higher and the holding time is longer. Therefore, the heating temperature is 900 ° C. or more in consideration of the above-mentioned Ac ₁ transformation temperature, and 1200 ° C. from the industrial aspect.
C. or lower, the holding time is preferably at least 10 seconds or more, and 300 seconds or less from an industrial viewpoint. The reason why the reducing gas such as hydrogen is mixed is to reduce an oxide film or the like that inhibits nitriding.

Therefore, immediately before or before the heat treatment, if necessary, the film may be removed by pickling and dried at a low temperature or pre-annealed in a reducing gas atmosphere. The thus obtained dual-phase stainless steel sheet according to the present invention is typically used as a gasket, but exhibits excellent properties when used in other horns (diaphragms) of automobiles and the like. Can be.

[0040]

EXAMPLE A stainless steel containing Cr as a main component having the seven chemical compositions shown in Table 1 was smelted in a 10 kg ingot and subjected to hot rolling, annealing, descaling, cold rolling and softening annealing. Was repeated to prepare a cold-rolled sheet having a thickness of 0.25 mm, and subjected to the following quenching heat treatment.

The heat treatment atmosphere is 50% by volume nitrogen and 50% by volume.
The test was also carried out mainly on a mixed gas of% hydrogen, and partially on 100% nitrogen and vacuum. The heating temperature is 900-1200 ℃,
The heating rate to each temperature was fixed at 20 ° C./sec. The holding time after the heating was mainly 30 seconds, and was partially 180 seconds. The cooling rate during the subsequent quenching was mainly 20 ° C./sec, and was partially performed at 10 ° C./sec.

In this embodiment, no special treatment for removing an oxide film or the like was performed before the quenching heat treatment. The nitrogen content, carbon content, phase ratio, hardness, bending workability, and fatigue strength of the steel sheet after the quenching heat treatment were investigated.

The nitrogen content was determined by chemical analysis of the average value of the entire material. The phase ratio was measured by X-ray diffraction and microstructure observation with an optical microscope for the plate surface as it is (surface layer portion) and the surface after removing the plate thickness to half by one side (inner layer portion).

The cross section of the test piece after corrosion was observed by SEM, and the thickness of the nitrided surface layer was also measured. The hardness was measured by measuring the Vickers hardness under a load of 9.8 N with respect to the surface of the plate at the surface layer portion and the inner layer portion.

The bending workability is the width taken parallel to the rolling direction.
(w) 20 mm × length (L) 60 mm using JIS-Z2
Perform the V-bending test specified in 248, and ratio of the thickness (t: 0.25mm) of the thin steel sheet to the minimum radius (R) that can be bent.
(R / t) was measured.

The fatigue strength was w10m taken in parallel with the rolling direction.
a strip specimen of m × L40mm, using a specimen subjected to beading shown denoted by the dimensions in Figure 1, the maximum does not lead to fracture after 10 ^six times with pulsating plane bending test bending stress Was measured.

Table 2 shows the results of an examination of each characteristic. As shown in Table 2, in Examples Nos. 1 to 6 and 8 to 13 satisfying the composition of the steel of the present invention, a portion composed of an austenite phase having a thickness of 5 μm or more or a multiphase structure containing the same phase was formed on the surface layer, Hv300
With excellent bending workability of a high hardness and from 1 to 1.4 are compatible exceeding displays high fatigue strength of 800 ~1000N / mm ^2. In particular, Example No. 12, in which the inner layer became a martensite single phase, also exhibited higher performance than Example No. 7, which corresponds to a conventional two-phase structure material that was not quenched in a nitrogen atmosphere.

It has been confirmed that the austenitization of the surface layer by nitriding proceeds with an increase in the heating temperature, an increase in the holding time, and a mixed state of nitrogen gas and hydrogen gas as compared with nitrogen gas alone. At the same time, the fatigue strength tended to increase.

On the other hand, in Example No. 7 in which quenching was not performed in a nitrogen atmosphere, a portion composed of a structure containing an austenite phase was not formed on the surface layer, and a high hardness exceeding Hv 300 and 1.6 Although it shows excellent bending workability,
Many microcracks occurred on the surface layer of the bead, and the fatigue characteristics were reduced by half.

Similarly, Example No. 14 was composed of a martensitic single-phase structure and exhibited high strength around Hv460, but the bending workability was extremely poor and the fatigue strength was further reduced.
Regarding materials deviating from the composition of the steel of the present invention, Examples Nos. 15 to 16
Has not reached hardness Hv300 and has fatigue strength of 300
It remained low value of ~400N / mm ^2. Example No.17 is Hv30
Although high strength exceeding 0 was exhibited, bending workability was poor and fatigue strength remained at the same value.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

According to the present invention, it is suitable for a spring or a spring part used after machining into various shapes, electronic equipment and mechanical parts in general requiring a spring property, and has a high strength which is most suitable for an engine gasket. An inexpensive duplex stainless steel sheet having excellent workability and high fatigue properties can be stably supplied in a general manufacturing process without causing an increase in manufacturing cost.

[Brief description of the drawings]

FIG. 1 is a schematic view showing the shape of a fatigue test piece after bead processing.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shinji Tsuge 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (72) Inventor Kenichi Goshokubo 2--12 Minatomachi, Joetsu-shi, Niigata 1 Sumitomo Metal Naoetsu, Ltd. (72) Inventor Masahiro Aoki 2-12-1, Minatomachi, Joetsu-shi, Niigata Prefecture Sumitomo Metal Naoetsu, Ltd.

Claims (3)

[Claims] [Claim 1] In mass%, C + N: 0.06% or more, 0.50% or less Si: 0.1% or more, 2.0% or less Mn: 0.1% or more, 3.0% or less Cr: 10.0% or more, 17.0% or less Ni: 1.0% Cu: 2.0% or less The balance has a chemical composition consisting of Fe and impurity elements, and the surface layer of a material consisting of a single phase of martensite or a two-phase structure of martensite and ferrite has a single-phase structure of austenite or a double-phase structure containing the same phase Stainless steel plate. 2. A stainless steel sheet having a chemical composition according to claim 1 and having a single phase of martensite or a dual phase structure of martensite and ferrite is reduced to a predetermined plate thickness, and then is subjected to nitrogen gas or nitrogen gas. A method for producing a stainless steel sheet having a dual-phase structure, comprising heating and holding at a temperature higher than the Ac ₁ transformation point in an atmosphere containing a reducing gas, followed by quenching. 3. A gasket for an engine using the stainless steel sheet having a dual phase structure according to claim 1.