JP4034129B2 - High-strength, high-thermal-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an austenitic stainless steel material excellent in high temperature sag resistance and corrosion resistance, and having high strength and high thermal expansion characteristics, and a method for producing the same.
[0002]
[Prior art]
Stainless steel spring materials are hard rolled by SUS301, SUS304, SUS631, etc., austenitic spring materials that have been strengthened by a combination of hard rolling and aging treatment, and strength is increased by quenching SUS420, SUS403, etc. There is an enhanced martensite spring material.
[0003]
Generally, an austenitic spring material is more excellent in high temperature sag resistance (high temperature sag resistance will be described later) than a martensitic spring material. Therefore, when high temperature sag resistance becomes an issue, a spring material using an austenitic steel material is used. However, even in austenitic stainless steels such as SUS301, the high temperature sag characteristics may be insufficient. Therefore, for example, Japanese Patent Application Laid-Open Nos. 10-330889 and 11-241145 disclose techniques for positively adding Mo, W, and the like to improve high temperature resistance.
[0004]
[Problems to be solved by the invention]
In the prior art, since Mo and W are both expensive elements, an alloy to which these are added has a high manufacturing cost. Therefore, there has been a demand for austenitic stainless steel that is inexpensive and excellent in high temperature resistance. On the other hand, the corrosion resistance has a problem that rust is generated depending on the use environment, and there is a demand for austenitic stainless steel having higher corrosion resistance.
[0005]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, a first aspect of the present invention is a high-strength, high-thermal-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance, characterized by comprising the following component composition.
(A) C: 0.17 to 0.30 mass%, Si: 3.0 mass% or less, Mn: 3.0 mass% or less, Ni: 5.0 to 12.0 mass%, Cr: 18.0 to 30.0 mass %, N: more than 0.10 to 0.20 mass%, and the balance is a stainless steel material made of Fe and inevitable impurities,
(B) the area ratio occupied by carbides and carbonitrides precipitated in the metal structure of the stainless steel material is less than 3%;
(C) The Ni equivalent represented by the following formula (1) is 24.0 to 30.0 mass%.
Ni equivalent = 24.2C + 13.7N + 0.35Si + 0.5Mn + Ni + 0.65Cr --- (1)
[0006]
A second aspect of the present invention, the stainless steel, as a component composition, further 4.0 mass% or less in the case of Mo, a range of 3.0 mass% or less in the case of Cu, any one The Ni equivalent represented by the following formula is 24.0 to 30.0 mass %, and has high strength and high heat resistance with excellent high temperature sag resistance and corrosion resistance. It is an expanded austenitic stainless steel material.
Ni equivalent = 24.2C + 13.7N + 0.35Si + 0.5Mn + Ni + 0.65Cr + 0.98Mo + 0.6Cu --- (2)
[0007]
A third aspect of the present invention is a high-strength, high-thermal-expansion austenitic stainless steel material excellent in high temperature sag resistance and corrosion resistance, characterized in that the martensite content of the metal structure of the stainless steel material is 10% or less. is there.
[0008]
A fourth aspect of the present invention is a method for producing a high-strength, high-thermal-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance, characterized by comprising the following steps.
(A) C: 0.10 to 0.30 mass %, Si: 3.0 mass % or less, Mn: 3.0 mass % or less, Ni: 5.0 to 12.0 mass %, Cr: 18.0 ~30.0 mass%, N: 0.05~0.30 containing mass%, and a balance of Fe and unavoidable impurities, Ni eq represented by the following formula from 24.0 to 30.0 mass% Prepare a slab of stainless steel material, and perform plate processing and heat treatment to obtain a plate material,
(B) The plate material is annealed at a temperature range of 1050 to 1200 ° C., and then cold-rolled at a reduction rate of 20 to 85%.
[0009]
A fifth aspect of the present invention is a high-strength, high-thermal-expansion austenitic stainless steel excellent in high-temperature sag resistance and corrosion resistance, characterized by further performing heat treatment in the temperature range of 200 to 700 ° C. after the cold rolling. It is a manufacturing method of steel materials.
[0010]
In a sixth aspect of the present invention, the stainless steel material further includes, as a component composition, Mo in a range of 4.0 mass % or less, or Cu in a range of 3.0 mass % or less. The Ni equivalent represented by the formula is 24.0 to 30.0 mass %, and is a method for producing a high-strength, high-thermal-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below. The inventors first examined the relationship between the alloy composition and strength of austenitic stainless steel, strength, high temperature erosion, corrosion resistance, and the like. Regarding strength and high temperature settling, it was decided not to use expensive components such as Mo and W, and to add component elements in a balanced manner using Ni equivalents. Further, investigation was made by adding solid solution strengthening elements such as C and N components.
[0012]
When it is desired to increase the strength of the austenitic stainless steel material, generally, the Ni equivalent is lowered, and a large amount of martensite (work-induced martensite) is generated by cold rolling. Actually, SUS301, SUS304, SUS631, etc. are called metastable austenite, and a large amount of martensite is generated by cold rolling to ensure strength. However, when the amount of martensite produced increases, the resistance to high temperature resistance deteriorates, so that it cannot be used for severe applications.
[0013]
When the Ni equivalent is increased, the amount of martensite produced is reduced or no longer produced. However, since high strength was not obtained, it was not used as a high-strength spring material. Therefore, in order to ensure high-temperature sag resistance, the Ni equivalent is set to a high value, and the amount of martensite produced is limited to an appropriate amount. The decrease in strength was compensated by increasing the contents of C and N, which are solid solution strengthening elements.
[0014]
For example, Japanese Patent Laid-Open No. 63-1000015 proposes an austenitic stainless steel having high strength and high fatigue strength with a C content of 0.10 to 0.25 mass %. However, since the Ni equivalent is as low as 22.05 to 23.28 (calculated from the examples), it is difficult to use in fields where high temperature sag resistance is required.
[0015]
Regarding corrosion resistance, it is known that addition of Cr, Mo, N is effective. In the present invention, expensive Mo is avoided and Cr is added in addition to N in order to increase the strength. Addition of Cr not only increases corrosion resistance, but also increases manufacturability.
[0016]
In other words, C and N added to increase the strength are strong austenite forming elements, and thus γ solidifies easily during solidification. When γ-solidifies, it causes solidification cracking and causes cracking in the subsequent hot rolling process, resulting in a decrease in yield and an increase in costs, such as requiring an extra process.
[0017]
In austenitic stainless steel, it is effective to form about 5% δ ferrite in order to prevent cracking during solidification and to prevent ear cracking during hot rolling. For this purpose, it is necessary to add a ferrite forming element that promotes the formation of δ ferrite. For example, since Cr is a strong ferrite forming element, it promotes the formation of δ ferrite and effectively acts to prevent cracking.
[0018]
In the present invention, high strength and high thermal expansion austenitic stainless steel has a component composition as described below. The reason for defining the component composition of the present invention will be described below.
[0019]
C is a strong solid solution strengthening element, and it is more preferable for increasing the strength, and it is necessary to contain 0.10% by mass or more. However, if the content exceeds 0.30 mass %, coarse undissolved carbides are generated to deteriorate the corrosion resistance, and the risk of breaking during cold rolling increases. Therefore, it is set as the range of 0.10-0.30 mass %. Preferably it is set as the range of 0.15-0.25 mass %, More preferably, it is set as the range of 0.17-0.22 mass %.
[0020]
Si is an element necessary for deoxidation and an element effective for increasing the strength. However, if the content exceeds 3.0 mass %, the material becomes extremely brittle and the risk of cold rolling breakage is increased. Therefore, the content is preferably 3.0 mass % or less.
[0021]
Mn is an element necessary for deoxidation like Si. However, if it contains more than 3.0 mass %, it causes abnormal oxidation during annealing pickling, and deteriorates the surface quality and lowers the yield, so it is desirable to make it 3.0 mass % or less.
[0022]
Ni is effective in preventing time cracking, and it is necessary to contain 5.0 mass % or more from the viewpoint of manufacturability. However, since an expensive element, the upper limit was made 12.0 mass%, is preferably in the range of 5.0 to 12.0 mass%. Preferably, it is the range of 5.0-10.0 mass %, More preferably, it is set as the range of 5.0-8.0 mass %.
[0023]
Since Cr is effective in improving corrosion resistance and preventing cracks in solidification cracking and hot rolling, it is necessary to contain 18.0 mass % or more. However, if the content exceeds 30.0 mass %, a σ phase is precipitated, which adversely affects corrosion resistance and deteriorates manufacturability. For example, it breaks during cold rolling, so the upper limit is 30.0 mass %. It is desirable to be in the range of 0.0 to 30.0 mass %. Preferably, it is in the range of more than 20.0 to 25.0 mass %.
[0024]
Mo is an element effective for improving corrosion resistance and high temperature resistance, but is an extremely expensive element, so it is desirable to set it in a range of 4.0 mass % or less.
[0025]
Cu is an element contained in scrap and is an element that is difficult to remove by refining. 3.0 Beyond mass% hot workability is degraded when contained in the hot rolling reduces the yield causing edge cracks, because it causes an increase in manufacturing cost, 3.0 mass% or less of A range is desirable.
[0026]
N is a solid solution strengthening element like C and is not only indispensable for increasing the strength, but also effective for improving high temperature sag resistance and corrosion resistance. 0.05 mass % or more is necessary, but if it exceeds 0.30 mass %, insoluble nitride is formed, and not only the corrosion resistance is deteriorated, but also there is a risk of fracture during cold rolling. Rise. For this reason, the upper limit is set to 0.30 mass%, is preferably in the range of 0.05 to 0.30 mass%. Preferably, it is in the range of more than 0.10 to 0.20 mass %, more preferably in the range of more than 0.10 to 0.15 mass %.
[0027]
Ni equivalent = 24.2C + 13.7N + 0.35Si + 0.5Mn + Ni + 0.65Cr --- (1)
Ni equivalent = 24.2C + 13.7N + 0.35Si + 0.5Mn + Ni + 0.65Cr + 0.98Mo + 0.6Cu --- (2)
Ni equivalent is an index calculated by the formula shown in the above (1) or (2), and is an index indicating the stability of austenite with respect to cold working.
[0028]
In the austenitic stainless steel represented by SUS301, when processing such as cold rolling is performed, processing-induced martensite is generated, but the generated amount decreases as the Ni equivalent increases. If the formation of this martensite can be suppressed, the high temperature resistance is improved. For that purpose, Ni equivalent needs to be 24.0 mass % or more.
[0029]
However, when the Ni equivalent is 30.0 mass % or more, martensite is not generated at all, and it becomes difficult to increase the strength, and the effect on high temperature resistance is saturated. Therefore, the upper limit was made 30.0 mass%, is preferably in the range of 24.0 to 30.0 mass%. Preferably, it is set as the range of 26.0-28.0 mass %.
[0030]
In the present embodiment, an alloy having the chemical component and Ni equivalent is melted and cast. Next, the cast slab is hot-rolled, annealed, and then cold-rolled. After cold rolling, annealing is performed using a continuous annealing furnace for a short time, for example, so that the temperature of the plate material is 1050 to 1200 ° C. Subsequently, the rolling reduction is set to 20 to 85%, and further cold rolling is performed to obtain an austenitic stainless steel material.
[0031]
The structure of the stainless steel material obtained by the above method is mainly composed of austenite, and is composed of work-induced martensite formed by cold rolling, and carbide and carbonitride. In the present invention, the area ratio occupied by the carbide and carbonitride precipitated in the metal structure of the stainless steel material is less than 3%. In the present invention, the amount of martensite in the metal structure of the stainless steel material is 10% or less.
[0032]
The reason why the area ratio occupied by carbides and carbonitrides in the present invention is limited to less than 3% is to stabilize the austenite structure by almost completely dissolving the carbides. When 3% or more of carbides and carbonitrides are contained, the composition of the austenite structure changes, and a large amount of martensite is formed when cold-rolled, resulting in deterioration of high temperature sag resistance. Therefore, it is desirable to make it less than 3%. Preferably it is less than 2%, more preferably 1% or less.
[0033]
Therefore, in the present invention, the manufacturing conditions are defined as follows. In the present invention, a large amount of C and N is contained. Therefore, in order to dissolve carbides and carbonitrides, it is necessary to anneal at a high temperature of 1050 ° C. or higher. However, a temperature range of 1050 to 1200 ° C. is desirable because annealing is performed at 1200 ° C. or higher to abnormally oxidize and deteriorate the surface quality. Preferably, it is set as the temperature range of 1100-1150 degreeC.
[0034]
Moreover, in this invention, in order to ensure intensity | strength by final cold rolling, 20% or more of rolling reduction is required. However, when the rolling reduction exceeds 85%, a large amount of martensite is generated to deteriorate the resistance to high temperature resistance, and the risk of rolling breakage is increased by increasing the sensitivity to time cracking. Therefore, the upper limit is set to 85%, preferably 40 to 70%. After cold rolling, an aging heat treatment such as tension annealing or stress release may be performed in the range of 200 to 700 ° C. in order to improve the spring property and correct the shape.
[0035]
【Example】
Embodiments of the present invention will be described with reference to Table 1 as FIG. 1 and Table 2 as FIG. First, 10 kg of steel ingot having the composition shown in Table 1 was melted. No. in Table 1 1-9 are examples of the present invention. No. 10 to 14 are comparative examples, and any one of the components, Ni equivalent, or carbide area ratio, or some items are out of the range defined in the present invention. No. Nos. 15 and 16 are conventional steels. 15 is a SUS304 equivalent product. 16 is a SUS301 equivalent product.
[0036]
Next, the steel ingot after ingot forming was heated at 1250 ° C. for 30 minutes, followed by hot forging to produce a cross-sectional plate material having a thickness of 10 mm and a width of 60 mm. Next, it was kept in air at 1120 ° C. for 30 minutes and then cooled with water.
[0037]
Next, the plate material was subjected to surface grinding to obtain a plate material having a thickness of 8 mm. Next, cold rolling was performed to make the plate thickness 4 mm. Next, heat treatment was performed by holding in the atmosphere at 1120 ° C. for 1 minute, followed by forced air cooling. Next, cold rolling was performed to make the plate thickness 2 mm. Next, after heat-treating at 1120 ° C. in the atmosphere for 30 seconds, forced air cooling was performed.
[0038]
Next, cold rolling was performed to obtain a plate thickness of 0.8 mm. The rolling reduction during the cold rolling is 60%. The cold rolled material with a plate thickness of 0.8 mm was subjected to various tests. The test confirmed hardness, martensite amount, high temperature sag amount, corrosion resistance, thermal expansion measurement, and the presence or absence of precipitation of carbides and carbonitrides. The results are shown in Table 2 as FIG.
[0039]
About the area ratio of the carbide | carbonized_material and the carbonitride, it calculated | required using the following method. The plate material cold-rolled to a thickness of 0.8 mm was cut at a right angle to the rolling direction, surface-polished, and then etched using a mixture of hydrochloric acid and picric acid alcohol. The obtained tissue was photographed with an electron microscope at a magnification of 3000 times. Use this photo to find the point.
[0040]
Specifically, a grid having vertical and horizontal intervals of 5 mm is provided on the photographic field, and the number of carbides that touch or are on the grid points of a total of 100 grids, for example, 10 vertically. This operation is repeated 10 times, and the average value is defined as the area ratio.
[0041]
In other words, the area ratio is given by the following equation.
n ÷ (p × f) × 100 = Carbide area ratio (%). n, p, and f are as follows.
p: total number of lattice points in the field of view f: number of fields of view n: number of lattice point centers occupied by carbides in f fields of view
The hardness was measured by the Vickers hardness test method defined in JISZ2244. The measurement position was the plate surface, and the measurement load was 10 kg.
[0043]
The amount of martensite was determined using a ferrite scope manufactured by Fischer. When the plate thickness is 2 mm or less, the measured value depends on the plate thickness. Therefore, the measurement was performed so that the influence of the plate thickness was negligible by stacking three plates to a thickness of 2.4 mm.
[0044]
The measurement method for high temperature settling is as follows. A jig 1 made of SUS310S as shown in FIG. 3 (a) is prepared, and a bolt 5 is passed through a hole provided in a flat plate test piece 3 having a thickness of 0.8 mm, a width of 20 mm, and a length of 100 mm. Set. A block 7 having a height of 10 mm is provided at a position 80 mm away from the fixed end of the test piece. By fixing the test piece 3 with the bolts 5, a bending stress is forcibly applied. Next, with this state maintained, heat treatment was performed by holding at 500 ° C. for 15 minutes and then air cooling.
[0045]
As shown in FIG. 3 (b), the amount of sag was determined as 9 from the height at a position 80 mm away from the fixed end after removing the block. In addition, even when the heat treatment was not performed, the test piece was attached to a jig, removed after holding for 15 minutes, and the amount of sag was measured, and it was confirmed that no large sag occurred.
[0046]
The corrosion resistance was determined by a pitting potential measurement method defined in JISG0577. The larger the measured value, the better the corrosion resistance.
[0047]
The thermal expansion coefficient was determined using a test piece cut out to a thickness of 0.8 mm, a width of 5 mm, and a length of 20 mm. At this time, the test piece is cut out so that the direction perpendicular to the rolling direction is 5 mm and the rolling direction is 20 mm, and is maintained at 550 ° C. for 1 minute to stabilize the measurement, and then air-cooled to room temperature. An aging treatment for cooling was performed. The measuring device is MTC1000S manufactured by Mac Science. The test direction was the rolling parallel direction, the reference temperature was 30 ° C., the rate of temperature increase was 5 ° C./min, and the temperature was raised to 200 ° C., and the average thermal expansion coefficient of 30 to 100 ° C. was taken as the measured value.
[0048]
The test results are shown in Table 2 as FIG. The comparative example No. No. 14 could not be adjusted to the target thickness (0.8 mm) because it broke during cold rolling.
[0049]
The relationship between the amount of martensite and Ni equivalent is shown in FIG. The amount of martensite decreases sharply until the Ni equivalent is around 24% (about 10% in terms of martensite amount), then decreases gradually, and no martensite is formed around 30%.
[0050]
The relationship between the amount of sagging and the Ni equivalent is shown in FIG. The amount of sag decreased rapidly until the Ni equivalent reached around 24%, and then decreased gradually. That is, the amount of sag can be suppressed by setting the Ni equivalent to 24% or more.
[0051]
The hardness is affected by the Ni equivalent and the amounts of C and N. FIG. 6 shows the relationship between hardness and Ni equivalent. Hardness decreases as the Ni equivalent increases. Further, the reason why each data in FIG. 6 varies is that the C and N contents for each data are different. For example, No. 1 of the present invention. 2 and Comparative Example No. When comparing 13, the amount of N and the amount of Ni are almost the same. However, the hardness is 446 (No. 2) and 417 (No. 13) corresponding to the C amount being different from 0.18% (No. 2) and 0.06% (No. 12). No. containing a large amount of C 2 has a higher hardness value.
[0052]
In addition, Invention Example No. 2 and Comparative Example No. Compared to 12, the C amount and Ni equivalent are almost the same, but the N amount is different from 0.12% (No. 2) and 0.04% (No. 12), and the hardness is 446 (No. 2). ) 422 (No. 12) and No. containing a large amount of N. 2 has a higher hardness value. That is, in order to obtain high strength, it is necessary to lower the Ni equivalent and to increase the C and N contents.
[0053]
Regarding corrosion resistance, Cr was positively added, and all of them exhibited excellent characteristics compared to conventional steel. The product of the present invention exhibited higher thermal expansion characteristics than SUS301 and SUS304, which are conventional steels. That is, the invention product has high strength, high corrosion resistance, and high thermal expansion, and also has excellent high temperature sag resistance. Therefore, the product of the present invention has applications such as metal gaskets, steel belts, various leaf springs, and press plates.
[0054]
【The invention's effect】
As is apparent from the above, according to the present invention, it is possible to provide an austenitic stainless steel having excellent high-temperature sag resistance and high strength and high corrosion resistance. Therefore, the present invention can be applied to applications requiring these characteristics, for example, metal gaskets. Moreover, since it also has a high thermal expansion characteristic, it can be applied to applications requiring high thermal expansion such as bimetal.
[Brief description of the drawings]
FIG. 1 is Table 1 shown as FIG. 1, which is a list showing the composition of components of the present invention products and comparative products.
FIG. 2 is a table 2 shown as FIG. 2, and is a list showing test results of the present invention products and comparative products.
FIG. 3 is a diagram showing a setting test method.
FIG. 4 is a graph showing the relationship between the amount of martensite and Ni equivalent.
FIG. 5 is a diagram showing the relationship between the amount of sag and the Ni equivalent.
FIG. 6 is a diagram showing the relationship between hardness and Ni equivalent.
1 Stool measuring jig 3 Test piece 5 Bolt 7 Block 9 Sag amount (mm)

Claims (6)

A high-strength, high-thermal-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance, characterized by comprising the following component composition.
(A) C: 0.17 to 0.30 mass%, Si: 3.0 mass% or less, Mn: 3.0 mass% or less, Ni: 5.0 to 12.0 mass%, Cr: 18.0 to 30.0 mass %, N: more than 0.10 to 0.20 mass%, and the balance is a stainless steel material made of Fe and inevitable impurities,
(B) the area ratio occupied by carbides and carbonitrides precipitated in the metal structure of the stainless steel material is less than 3%;
(C) The Ni equivalent represented by the following formula (1) is 24.0 to 30.0 mass%.
Ni equivalent = 24.2C + 13.7N + 0.35Si + 0.5Mn + Ni + 0.65Cr --- (1) The stainless steel, as a component composition, further, 4.0 mass% or less in the case of Mo, a range of 3.0 mass% or less in the case of Cu, contain one or more one, and the following The high strength and high thermal expansion austenitic stainless steel material excellent in high temperature sag resistance and corrosion resistance according to claim 1, wherein the Ni equivalent represented by the formula is 24.0 to 30.0 mass %.
Ni equivalent = 24.2C + 13.7N + 0.35Si + 0.5Mn + Ni + 0.65Cr + 0.98Mo + 0.6Cu --- (2) 3. The high-strength, high-thermal-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance according to claim 1, wherein the martensite content of the metal structure of the stainless steel material is 10% or less. A method for producing a high-strength, high-thermal-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance, characterized by comprising the following steps.
(A) C: 0.10 to 0.30 mass %, Si: 3.0 mass % or less, Mn: 3.0 mass % or less, Ni: 5.0 to 12.0 mass %, Cr: 18.0 ~30.0 mass%, N: 0.05~0.30 containing mass%, and a balance of Fe and unavoidable impurities, Ni eq represented by the following formula from 24.0 to 30.0 mass% Prepare a slab of stainless steel material, and perform plate processing and heat treatment to obtain a plate material,
(B) The plate material is annealed at a temperature range of 1050 to 1200 ° C., and then cold-rolled at a reduction rate of 20 to 85%. The high-temperature high-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance according to claim 4, wherein heat treatment is further performed in the temperature range of 200 to 700 ° C after the cold rolling. Method. The stainless steel, as a component composition, further below 4.0 mass% in the case of Mo, a range of 3.0 mass% or less in the case of Cu, contain one or more one, and the following formula The Ni equivalent represented by the formula is 24.0 to 30.0 mass %, and the production of a high-strength, high-thermal-expansion austenitic stainless steel material excellent in high-temperature sag resistance and corrosion resistance according to claim 4 or 5 Method.

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