JP2006274391A - Stainless steel for strain detection sensor substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a strain detection sensor in which the increase of strength is attained by a means which does not check its adhesion with a glass based material and toughness. <P>SOLUTION: When the stainless steel is subjected to heat treatment so as to be heated at 700 to 1,000°C and to be cooled to a room temperature, martensite transformation occurs in the range from 200°C to a room temperature; wherein its linear expansion strain in the range from 200°C to a room temperature reaches ≤0.5%, and, after the heat treatment, the stainless steel has the properties of exhibiting a ferrite+martensite two phase structure. As the preferable composition, the one comprising, by mass, ≤0.15% C, ≤2.0% Si, ≤3.0% Mn, ≤0.10% P, ≤0.03% S, >0.6 to 5.0% Ni, 14 to 20% Cr and ≤0.10% N, and in which G value defined by formula (1) is 60 to 90 can be cited; wherein the formula (1) is G=420C+470N+11.5(Cr+Si)+23Ni+7Mn+189. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to stainless steel having a high strength level for use in a strain detection sensor.

  As schematically illustrated in the cross-sectional structure in FIG. 1, the strain detection sensor has an electrode 3 and a resistor 4 buried between the electrodes 3 via an insulating layer 2 on the substrate 1, and further includes an electrode 3 and a resistor 4. It has a structure covered with an insulating layer 5 from above. When strain in the tensile direction or compression direction is applied to the substrate 1, the resistor 4 is also strained with the deformation of the substrate 1, and the electric resistance value changes. This change in electrical resistance is read by a circuit including the electrode 3, and the amount of strain is detected.

  Usually, the substrate 1 is made of a metal material, and the insulating layer 2 is made of a glass material. The board | substrate 1 and the insulating layer 2 are joined by baking at high temperature, such as 700-1000 degreeC. Therefore, when a metal material that causes a phase transformation at the firing temperature is used for the substrate 1, it is necessary to strictly manage the process so as not to cause a bonding failure with the insulating layer 2 due to a volume change accompanying the transformation. This is a heavy burden in the manufacturing process.

  Conventionally, in the case of a strain detection sensor of a type that is bonded by an adhesive without firing at a high temperature, special steel has been mainly used as a member corresponding to the substrate 1, but depending on the application, corrosion resistance may be insufficient. It was. On the other hand, examples of the metal material used for the substrate 1 of the type that is fired at a high temperature as described above include “stainless steel” that has good corrosion resistance and does not cause phase transformation at the firing temperature as described above. However, since austenitic stainless steel and ferrite (α) + austenite (γ) two-phase stainless steel have a large difference in thermal expansion coefficient from that of glass-based materials, poor bonding due to the difference tends to be a problem. On the other hand, ferritic stainless steel has a thermal expansion coefficient closer to that of glass-based materials and can prevent austenite from occurring even at high temperatures exceeding 800 ° C. by adjusting the composition. It can be said that this is an advantageous steel type.

JP 2000-180255 A JP 2003-4553 A JP 2003-114153 A JP 2004-83937 A

  The strain detection sensor has a structure in which the stress applied thereto is borne by the substrate 1. For this reason, in the use which requires high intensity | strength, it respond | corresponded by making the thickness of the board | substrate 1 thick. However, in various machine structures including automobiles, there is a strong demand for miniaturization and weight reduction of parts, and a method for improving the strength by increasing the thickness is not a good solution.

  As described above, ferritic stainless steel is advantageous in terms of bondability with glass-based materials, but among stainless steel, the strength level after heat treatment is low, and it is not suitable for applications that require high strength. You may have to rely on growth. Therefore, in order to improve the strength of the strain sensor, a method has been proposed in which ferrite + austenite duplex stainless steel in which a σ phase is generated is used as a substrate and reinforcement by the σ phase is used (Patent Documents 3 and 4). ). However, since the σ phase itself is a brittle phase, there is a lack of reliability in terms of securing the toughness of the substrate.

  In view of such a current situation, the present invention has a volume change (volume change caused by transformation and coefficient of thermal expansion) in a firing process when joining with a glass-based material to such an extent that a problem of poor bonding does not occur, that is, the function of the sensor. In order to provide high-strength stainless steel suitable for strain detection sensors that can achieve significant strength improvement over conventional ferritic stainless steel after the firing process without relying on fragile precipitated phases. It is.

  The above object includes Cr: 14 to 20% by mass, and when subjected to heat treatment that is heated to 700 to 1000 ° C. and then cooled to room temperature, a martensitic transformation occurs between 200 ° C. and room temperature, provided that 200 ° C. to room temperature. This is achieved by the stainless steel for the strain detection sensor substrate, which has a property of exhibiting a ferrite + martensite two-phase structure after the heat treatment, and the linear expansion strain during the above is 0.5% or less.

Here, “room temperature” is 20 ° C. ± 15 ° C. (JIS K0050). “Generate martensitic transformation between 200 ° C. and room temperature” means that the steel has an Ms point of 200 ° C. or lower and an Mf point of 35 ° C. or higher. “The linear expansion strain between 200 ° C. and room temperature is 0.5% or less” means that the volume of the material at 200 ° C. is V ₂₀₀ , and the volume at room temperature after undergoing martensitic transformation with volume expansion is V _{When RT} , in linear expansion measurement,
(V _RT −V ₂₀₀ ) / V ₂₀₀ × 100 ≦ 0.5
Means that V _RT may be employed a value in the can measure adopted at any temperature in the room, for example, 20 ° C.. “Cooling” in the heat treatment is not particularly required as long as it is a cooling rate of a level in a normal firing process performed at the time of manufacturing the strain detection sensor.

After the heat treatment, those having a martensite amount of 40% by volume or more, or those having an austenite phase of less than 1% by volume and a σ phase of less than 1% by volume are suitable targets. Moreover, what can obtain the high intensity | strength in which 0.2% yield strength will be 600 N / mm < ² > or more after this heat history becomes a suitable object.

  As the amount of martensite increases, the linear expansion strain resulting from the martensitic transformation increases, so the upper limit of the amount of martensite is limited so that the linear expansion strain is 0.5% or less. For this reason, although it is not necessary to prescribe | regulate especially the upper limit of the amount of martensite after the said heat history in this invention, what becomes a martensite amount of 90 volume% or less, for example is a suitable object.

The specific chemical composition of the steel having such properties is, by mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0 or less, P: 0.10% or less. S: 0.03% or less, Ni: more than 0.6 to 5.0%, Cr: 14 to 20%, N: 0.10% or less, Nb: 0 to 1.0%, Ti: 0 to 0 0.5%, Mo: 0 to 4.0%, Cu: 0 to 3.0%, Al: 0 to 6.0%, B: 0 to 0.01%, the balance Fe and inevitable impurities, and the following ( 1) The composition whose G value defined by a formula will be 60-90 is mentioned.
G = 420C + 470N + 11.5 (Cr + Si) + 23Ni + 7Mn + 1189 (1)

Here, Nb, Ti, Mo, Cu, Al, and B are arbitrary elements, and the lower limit of 0% means that the content of the element is below the detection limit in the analysis method in the normal steelmaking process. . As those containing these optional elements, Nb: 0.1-1.0%, Ti: 0.05-0.5%, Mo: 0.2-4.0%, Cu: 0.2-3. Those containing one or more of 0%, Al: 0.2-6.0%, B: 0.0002-0.01% are suitable targets.
The value of the content of the element expressed in mass% is substituted for the element symbol in the formula (1).

According to the present invention, when subjected to a firing process at the time of manufacturing a strain detection sensor, a strengthening action by martensite is obtained by exhibiting a ferrite + martensite two-phase structure after firing, and in conventional ferritic stainless steel types, A 0.2% proof stress of 600 N / mm ² or more, which was difficult, was realized. For this reason, when the high-strength steel of the present invention is used, the substrate of the strain detection sensor can be thinned, and in particular, a strain detection sensor for applications requiring high strength can be expected to be significantly smaller and lighter than before. . In addition, the elastic deformation range of the substrate can be widened, which is advantageous for improving the sensitivity of the sensor. In addition, the high strength steel of the present invention has a small volume change in the firing process due to the difference in thermal expansion coefficient from the glass-based material and transformation, and the reliability of bonding between the base material and the insulating layer Is also expensive. Therefore, the present invention contributes to the construction of a strain detection sensor that is excellent in terms of size, performance, and reliability.

  As a result of detailed studies, the inventors have reinforced martensite in a stainless steel in which a moderate amount of martensite is generated in the firing process of the strain detection sensor and exhibits a two-phase structure of ferrite + martensite. While realizing, it has been found that it is possible to prevent a volume change that causes poor bonding with the glass-based material during the firing process.

Hereinafter, “%” in element content means “% by mass” unless otherwise specified.
As a substrate material for the strain detection sensor, a Cr content of 14% or more is required from the viewpoint of corrosion resistance in consideration of application to various uses.

In addition, in order to sufficiently contribute to the reduction in size and weight of the strain detection sensor, for example, after holding a heat history such as holding at 700 to 1000 ° C. for 1 hour or more and then cooling, 0.2% proof stress of 600 N / mm ² or more. The property of exhibiting is desired. In order to obtain such a high strength, it is desirable that martensite of about 40% by volume or more is generated after the thermal history.

  However, the martensitic transformation is accompanied by volume expansion. If martensitic transformation occurs at a very high temperature, bonding defects are likely to occur between the glass-based material. Therefore, it is desirable that martensitic transformation occurs in the low temperature region of the cooling process as much as possible. Specifically, it was found that good results were obtained when the Ms point and Mf point were between 200 ° C. and room temperature.

  Further, it was found that the amount of martensite produced was allowed in a range where the linear expansion strain between 200 ° C. and room temperature was 0.5% or less. If the amount of martensite is larger than this, poor bonding due to volume expansion due to transformation becomes obvious.

Such an appropriate martensitic transformation can be realized in stainless steel controlled to a composition in which the G value defined by the following formula (1) is 60 to 90.
G = 420C + 470N + 11.5 (Cr + Si) + 23Ni + 7Mn + 1189 (1)
When the G value is less than 60, the amount of martensite produced is insufficient, and it becomes difficult to achieve a 0.2% yield strength of 600 N / mm ² or more after firing. On the other hand, when the G value exceeds 90, the volume change due to the martensitic transformation becomes large, resulting in poor bonding between the substrate and the insulating layer (glass-based material). The G value is more preferably 85 or less.

  If a large amount of austenite phase remains after firing, the strength improvement effect by martensite cannot be obtained sufficiently. Moreover, the thermal expansion difference with a glass-type material becomes large and may be unpreferable. As a result of various studies, it was found that the austenite content is desirably less than 1% by volume after providing a heat history of cooling from 700 to 1000 ° C. In addition, if a large amount of σ phase is generated, the toughness is impaired, which is not preferable. The σ phase is also preferably less than 1% by volume after the thermal history is applied. Such a desirable structure state can be realized in the appropriate range of the G value and the appropriate ranges of the following elements.

  The strength of the substrate after firing largely depends on the content of each component element constituting the stainless steel. Specifically, it is necessary to set Cr: 14% or more and Ni: more than 6%. In addition, it is desirable to ensure C: 0.005% or more and N: 0.005% or more.

  However, when the content of the ferrite-forming element is excessively large, toughness and workability deteriorate. Specifically, Si: 2.0% or less, Cr: 20% or less, Nb: 1.0% or less, Ti: 0.5% or less, Mo: 4.0% or less, Al: 6.0% or less Is desirable. In particular, care should be taken because an increase in Cr facilitates the formation of the σ phase.

  If the content of the austenite-forming element is excessively large, the bondability with the glass due to phase transformation is deteriorated. Specifically, it is desirable that C: 0.15% or less, Mn: 3.0 or less, Ni: 5.0% or less, N: 0.10% or less, and Cu: 3.0%.

  In addition to these, for example, Co: 3.0% or less, W: 3.0% or less, Zr: 0.5% or less, Ta: 1.0% or less are appropriately added as elements effective for improving the strength. be able to. Further, as elements effective for improving manufacturability (mainly hot workability), Y: 0.1% or less, REM (rare earth element): 0.1% or less, Mg: 0.01% or less, Ca: 0.01% or less can be appropriately added. Preferably, Y is added in the range of 0.005 to 0.1%, REM: 0.005 to 0.1%, Mg: 0.0002 to 0.01%, Ca: 0.0002 to 0.01% It is effective to do. These elements may be added singly or in combination of two or more, as long as the desired properties are not impaired.

  In order to manufacture a material for a strain detection sensor substrate using steel having such a composition, a normal stainless steel melting process and a stainless steel sheet manufacturing process can be employed. Finally, a hot-rolled steel sheet or a cold-rolled annealed steel sheet having a thickness of about 0.5 to 3.5 mm may be used. The surface finish may be pickled.

  Steel having the composition shown in Table 1 was melted, and a cold-rolled annealed steel sheet having a thickness of 2.0 mm was obtained through steps of hot rolling, cold rolling, and annealing. About these steel plates, as a heat treatment simulating a firing process when manufacturing a strain detection sensor, “temperature rising at 3 ° C./min→holding at 900 ° C. × 1 h → cooling to room temperature at 3 ° C./min cooling rate” The heat history of was given.

The following tests were performed on the samples after the heat treatment to evaluate each characteristic.
-Amount of martensite after heat treatment: The structure of the sample cross section was observed using an optical microscope, and the amount of martensite was determined by the area ratio method. A martensite amount of 40% by volume or more was marked with ◯, and a martensite amount of less than 40% by volume was marked with x.
-Austenite amount after heat treatment: The austenite amount was determined by the same method. The austenite amount is less than 1% by volume (including those that are not detected).
-Amount of σ phase after heat treatment: The amount of σ phase was determined in the same manner. The case where the amount of the σ phase is less than 1% by volume (including the case where it is not detected) is ○, and the case where the amount is 1% by volume or more is ×.
-0.2% proof stress at normal temperature: Using a JIS 13B test piece parallel to the rolling direction, a tensile test was performed according to JIS Z2241, and a 0.2% proof stress of 600 N / mm ² or more A value of less than 600 N / mm ^{2 was} evaluated as x.
- Toughness: perform V-notch Charpy impact test in accordance with JIS Z2242, the impact value at room temperature is 40 J / cm ² or more of the ○, and as × of less than 40 J / cm ^2.
・ Corrosion resistance: After 0.26 g / L CuCl ₂ in 5% NaCl and adjusted to pH 3.0 with acetic acid was sprayed at 50 ° C. for 100 h, the appearance of the sample surface was visually observed, and it was confirmed that it was rusting. Those that did not exist were marked with ◯, and those that were recognized were marked with ×.

  In addition, a test piece having a thickness of 2 × width 5 × length 50 (mm) was cut out from the cold-rolled annealed steel sheet, set in an apparatus, and given the same heat history as the heat treatment, from 200 ° C. to room temperature (here The linear expansion strain up to 30 ° C. was measured. The case where no linear expansion strain occurred was marked with ◎, the case where the value of linear expansion strain was 0.5% or less was marked with ○, and the case where the linear expansion strain exceeded 0.5% was marked with ×.

Further, a plate having a width of 25 × 100 (mm) was cut out from the cold-rolled annealed steel plate, and crystalline glass was applied almost uniformly to the plate, which was then placed in a furnace to give the same thermal history as the heat treatment. . About the sample cooled to room temperature, the peeling state of glass was investigated by visual observation, the thing in which peeling was not recognized was set to (circle), and the recognized thing was set to x.
These results are shown in Table 2. The numbers in Table 2 correspond to the numbers in Table 1.

As can be seen from Tables 1 and 2, the examples of the present invention exhibited a ferrite + martensite two-phase structure, and a high strength of 0.2% proof stress 600 N / mm ² or more was obtained. Despite the martensitic transformation, the volume change (linear expansion strain) resulting from the transformation was sufficiently small, and there was no problem with the adhesion to the glass-based material. Furthermore, toughness and corrosion resistance were also good. Therefore, these are suitable as substrate materials for strain detection sensors that require high strength.
In all of the inventive examples, martensitic transformation occurred between 200 ° C. and room temperature.

  In contrast, Comparative Example No. 14 had a low G value, so the amount of martensite produced was insufficient, and the yield strength was inferior by 0.2%. Since No. 15 had a high G value, a large amount of martensite was generated, resulting in a large linear expansion strain and poor adhesion to the glass-based material. Since No. 16 had a high Cr content, a σ phase was generated and the toughness was poor. In No. 17, since the Ni content was high, the martensite transformation did not occur sufficiently, the retained austenite increased, and the yield strength was inferior by 0.2%. No. 18 was inferior in toughness due to its high Si content. No. 19 was inferior in corrosion resistance because of its low Cr content.

Sectional drawing which represented the structure of the strain detection sensor typically.

Explanation of symbols

1 substrate 2 insulating layer 3 electrode 4 resistor 5 insulating layer

Claims (6)

  When Cr: 14 to 20% by mass and subjected to a heat treatment that is heated to 700 to 1000 ° C. and then cooled to room temperature, a martensitic transformation occurs between 200 ° C. and room temperature, but between 200 ° C. and room temperature. Stainless steel for strain detection sensor substrates, having a linear expansion strain of 0.5% or less and exhibiting a ferrite + martensite two-phase structure after the heat treatment.   The stainless steel for strain detection sensor substrates according to claim 1, wherein the martensite amount becomes 40 vol% or more after the heat treatment. The stainless steel for strain detection sensor substrate according to claim 1 or 2, wherein a 0.2% proof stress becomes 600 N / mm ² or more after the heat treatment.   The stainless steel for strain detection sensor substrates according to claim 1, wherein the austenite phase is less than 1% by volume and the σ phase is less than 1% by volume after the heat treatment. In mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0 or less, P: 0.10% or less, S: 0.03% or less, Ni: more than 0.6 5.0%, Cr: 14 to 20%, N: 0.10% or less, Nb: 0 to 1.0%, Ti: 0 to 0.5%, Mo: 0 to 4.0%, Cu: 0 -3.0%, Al: 0-6.0%, B: 0-0.01%, the balance Fe and inevitable impurities, and the G value defined by the following formula (1) is 60-90. The stainless steel for strain detection sensor substrates according to claim 1 having a composition.
G = 420C + 470N + 11.5 (Cr + Si) + 23Ni + 7Mn + 1189 (1)   Nb: 0.1-1.0%, Ti: 0.05-0.5%, Mo: 0.2-4.0%, Cu: 0.2-3.0%, Al: 0.2 The stainless steel for a strain detection sensor substrate according to claim 5, satisfying one or more of 6.0% and B: 0.0002 to 0.01%.

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