Method for manufacturing martensitic stainless steel strip and martensitic stainless steel strip

Abstract

The present invention provides a method for manufacturing a martensitic stainless steel strip and a martensitic stainless steel strip. A method for manufacturing a martensitic stainless steel strip, comprising the following steps: a quenching step, wherein a steel strip containing C: 0.3% to 1.2% and Cr: 10.0% to 18.0% by mass and having a thickness of 1 mm or less is passed through a quenching furnace, heated to a quenching temperature, and then cooled to a temperature below the Ms point; a heat-retaining and conveying step, wherein the steel strip cooled to a temperature below the Ms point in the quenching step is conveyed to a tempering furnace while being heat-retained so as not to drop to a temperature below 80°C; and a tempering step, wherein the steel strip conveyed while being heat-retained so as not to drop to a temperature below 80°C in the heat-retaining and conveying step is passed through a tempering furnace in a non-oxidizing gas atmosphere, and heated to a tempering temperature. In addition, a martensitic stainless steel strip, wherein the amount of retained austenite is 10% to 25% by volume.

Description

Technical Field

[0001] The invention relates to a method for manufacturing a martensitic stainless steel strip and the martensitic stainless steel strip. Background Art

[0002] Martensitic stainless steel strips are excellent in corrosion resistance, hardness, and fatigue properties, and are used in a wide range of applications, such as knives, spring materials, valve materials, and cover materials that are subjected to repeated stress. In particular, in the application of spring materials and valve materials, martensitic stainless steel strips with sufficiently high fatigue strength are required to suppress fatigue failure caused by repeated stress.

[0003] In order to improve the fatigue strength of the above-mentioned martensitic stainless steel strip, various proposals have been made in the past. For example, in Reference 1, in order to obtain a spring steel strip having a higher fatigue limit than before, a spring steel strip having excellent durability is described, characterized in that, in terms of weight %, it substantially contains C: 0.35% to 0.45%, Si: 0.10% to 0.50%, Mn: 0.10% to 0.50%, Cr: 10% to 15%, Mo: 1.0% to 1.5%, P: 0.05% or less, S: 0.005% or less, O: 0.002% or less, N: 0.02% or less, Al: 0.005% or less, Ti: 0.01% or less, and the remainder is Fe.

[0004] In addition, Patent Document 2 describes a flapper valve body including martensitic stainless steel, which has compressive residual stress on the plate surface and a solid solution nitrogen-rich layer on the plate surface layer in order to improve the corrosion resistance and fatigue characteristics of the flapper valve body. Patent Document 2 also describes that if the residual stress on the surface of the martensitic stainless steel is adjusted to compressive stress by heating to a temperature above the temperature at which the stainless steel undergoes phase transformation into an austenite single phase and then rapidly cooling the temperature in an environment containing 20% ​​or more nitrogen and 10% or less (including 0%) oxygen (percentages are in volume %).

[0005] Furthermore, the applicant of the present application proposed in Patent Document 3 a method for manufacturing a martensitic stainless steel strip for the purpose of suppressing shape defects without reducing productivity, wherein the following steps are continuously performed: a drawing step of drawing out a martensitic stainless steel strip having a thickness of less than 1 mm; a quenching step of passing the steel strip through a quenching furnace in a non-oxidizing gas environment and heating it, and then cooling it; a tempering step of passing the quenched steel strip through a tempering furnace in a non-oxidizing gas environment and tempering it; and a winding step of winding the tempered steel strip, wherein the quenching furnace in the quenching step has at least a heating section and a holding section.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Laid-Open No. 4-48050

[0009] Patent Document 2: Japanese Patent Laid-Open No. 10-274161

[0010] Patent Document 3: Japanese Patent Application Publication No. 2018-111881 Summary of the invention

[0011] Problems to be solved by the invention

[0012] In recent years, compressors for air conditioners have been continuously advancing towards higher compression, and valves used in compressors are also required to improve fatigue properties and mechanical properties in order to cope with higher pressures. The invention of Patent Document 1 is an invention that can improve the fatigue limit of the steel strip, but there are cases where the fatigue limit is insufficient depending on the use environment, leaving room for further improvement. In addition, the invention described in Patent Document 2 is an invention that specifies the compressive residual stress caused by the nitrogen-rich layer formed on the surface of the plate, but the nitrogen-rich layer is difficult to form uniformly on the edge and outer periphery of the valve shape, and it may not be possible to obtain the desired residual stress. Furthermore, the formation range of the nitrogen-rich layer varies depending on the thickness of the material, so whenever the plate thickness changes, the gas amount or gas composition must be changed, and there is a concern that productivity will be reduced. In addition, Patent Document 3 is an excellent invention that can obtain a martensitic stainless steel strip with excellent flatness without reducing productivity, but there is no description of the improvement of fatigue properties and mechanical properties, leaving room for research. Therefore, an object of the present invention is to provide a martensitic stainless steel strip having superior fatigue properties and mechanical strength compared with conventional products, and a method for easily producing the martensitic stainless steel strip.

[0013] Technical means of solving problems

[0014] The present invention has been made in view of the above-mentioned problems.

[0015] That is, one embodiment of the present invention is a method for manufacturing a martensitic stainless steel strip, wherein the following steps are performed: a quenching step, wherein a steel strip containing, by mass%, C: 0.3% to 1.2% and Cr: 10.0% to 18.0% and having a thickness of 1 mm or less is passed through a quenching furnace in a non-oxidizing gas atmosphere, and is heated to a quenching temperature, and then cooled to a temperature below the Ms point; a heat-retaining and conveying step, wherein the steel strip, which has been cooled to a temperature below the Ms point in the quenching step, is conveyed to a tempering furnace while being heat-retained so as not to drop to less than 80° C.; and a tempering step, wherein the steel strip, which has been conveyed while being heat-retained so as not to drop to a temperature below 80° C. in the heat-retaining and conveying step, is passed through a tempering furnace in a non-oxidizing gas atmosphere, and is heated to a tempering temperature.

[0016] Another embodiment of the present invention is a martensitic stainless steel strip, which contains C: 0.3% to 1.2%, Cr: 10.0% to 18.0% in mass %, has a martensitic structure and a thickness of 1 mm or less, and the amount of retained austenite in the martensitic stainless steel strip is 10 volume % to 25 volume %, the tensile strength is 1600 MPa to 2300 MPa, and the ratio of 0.2% yield strength to tensile strength, i.e., yield strength ratio, is 75% or less.

[0017] It is preferred that the ratio of the compressive residual stress in the rolling direction of the martensitic stainless steel strip to the compressive residual stress in the width direction be 75% or more.

[0018] Effects of the Invention

[0019] According to the present invention, a martensitic stainless steel strip having excellent fatigue properties and mechanical properties compared with conventional products can be obtained. DETAILED DESCRIPTION

[0020] The present invention is described in detail below. However, the present invention is not limited to the embodiments listed here, and can be appropriately combined or improved within the scope of the technical concept of the present invention. The present invention can be applied to a martensitic stainless steel strip having a martensitic stainless steel composition. The composition range is not limited, for example, the composition of the steel strip of the present invention preferably includes C: 0.3% to 1.2%, Cr: 10.0% to 18.0% in mass%. Furthermore, the steel strip of the present invention is preferably a martensitic stainless steel having a component composition of C: 0.3% to 1.2% (more preferably 0.3% to 1.0%, and even more preferably 0.3% to 0.8%), Si: 1% or less, Mn: 2% or less, Mo: 3.0% or less (more preferably 2.5% or less, and even more preferably 2.0% or less), Ni: 1.0% or less (including 0%), Cr: 10.0% to 18.0% (more preferably 11.0% to 16.0%, and even more preferably 12.0% to 15.0%), and the remainder Fe and inevitable impurities.

[0021] First, the method for manufacturing a martensitic stainless steel strip of the present invention is described. The present invention is a method for manufacturing a martensitic stainless steel strip, wherein the following steps are performed: a quenching step, in which a steel strip having a thickness of 1 mm or less is passed through a quenching furnace in a non-oxidizing gas atmosphere, and is heated and then cooled; a heat-retaining and conveying step, in which the steel strip after the quenching step is conveyed to a tempering furnace while being kept warm so that the temperature does not drop below 80°C (in other words, while being kept in a temperature range of 80°C or above and below the Ms point or below the tempering temperature); and a tempering step, in which the steel strip conveyed in the heat-retaining and conveying step is passed through a tempering furnace in a non-oxidizing gas atmosphere and tempered. The quenching step, the heat-retaining and conveying step, and the tempering step can be performed continuously, and other steps such as a preheating step may be added as long as the effect of the present invention is not impaired. Below, the manufacturing method of an embodiment of the present invention is described.

[0022] (Quenching process)

[0023] In this embodiment, the following quenching process is performed: the prepared steel strip is passed through a quenching furnace in a non-oxidizing gas environment, and is heated, and then the steel strip is cooled. Before being transported to the quenching furnace, the following extraction process may also be performed: the rolled steel strip wound in a coil shape is assembled on an extractor, and the steel strip is transported to the quenching furnace. The set heating temperature in the quenching furnace is preferably 850°C to 1200°C. In the case of less than 850°C, there is a tendency for the solid solution of carbides to become insufficient. In contrast, in the case of exceeding 1200°C, there is a tendency for the solid solution amount of carbides to increase, and the hardness during tempering tends to decrease. In addition, the temperature of the quenching furnace can be set to a certain temperature from the entrance to the exit of the furnace, and at least one of a heating section or a cooling section can be set before and after the holding section for quenching at a certain temperature.

[0024] In order to further improve production efficiency, the present invention can also set a preheating process between the extraction process and the quenching process. In the preheating process, an existing heating device can be used, preferably an induction heating device that can rapidly heat the steel strip.

[0025] In order to make the preheating effective, the preheating temperature in the preheating step is preferably set to 600° C. or higher. On the other hand, in order to more reliably suppress deformation caused by a rapid temperature rise, it is preferably set to less than 800° C.

[0026] Then, the heated steel strip is quenched in a quenching furnace. As a method for quenching, a salt bath, molten metal, oil, water, polymer aqueous solution, saline solution, or gas can be used. Preferably, a spray cooling method using sprayed water or a gas cooling method using a non-oxidizing gas is used. When a gas cooling method is selected as the quenching method, the non-oxidizing gas is preferably a mixed gas of hydrogen, helium, nitrogen, argon, and hydrogen. The temperature of the steel strip is cooled to below the Ms point by the quenching process, but in order to obtain the effect brought about by the heat-insulating conveying process described later, the steel strip is cooled in such a manner that the temperature does not drop to less than 80°C. In addition, the rapid cooling process can also combine rapid cooling and slow cooling to implement two-stage quenching to avoid pearlite nose. For example, it is preferred to perform a secondary cooling process after the primary cooling process. The primary cooling process is to cool the steel strip to a temperature exceeding the Ms point and below 350°C by spray cooling, and the secondary cooling process is to use a water-cooled press plate to clamp the steel strip to constrain it, correct the shape while cooling it to below the Ms point.

[0027] (Insulation and transportation process)

[0028] Then, in the present embodiment, the following heat preservation and conveying process is implemented: the steel strip is conveyed to the tempering furnace while being kept warm in such a manner that the temperature of the steel strip after the non-quenching process drops to less than 80°C. By including the above process, the amount of retained austenite in the steel strip and the compressive residual stress on the surface of the steel strip can be increased, and the effect of improving fatigue strength can be obtained. When the temperature during the heat preservation and conveying process is less than 80°C, it is difficult to obtain the desired amount of retained austenite. In addition, although the steel strip in the heat preservation and conveying process is cooled to a temperature below the Ms point through the quenching process, it is also considered that the temperature is raised to a temperature exceeding the Ms point through subsequent reheating. However, a temperature exceeding the tempering temperature set in the next tempering process should be avoided. Moreover, when the temperature during the heat preservation becomes too high (for example, exceeding 300°C), including this case, there is a tendency for the quenching hardness to decrease. As a method for insulating the steel strip during the heat-insulating conveying process, for example, a metal cover or a tunnel furnace equipped with an insulating material is provided between the quenching furnace and the tempering furnace as an insulation device, and the steel strip is conveyed in the cover or the furnace. The insulation material can be an existing inorganic fiber-based or plastic-based insulation material. In addition, the tunnel furnace can also use an existing device, and in order to obtain a higher surface oxidation prevention effect and a more stable effect of the steel strip temperature during conveying, it is preferred to use a gas atmosphere furnace. Ideally, the insulation device is most preferably directly connected to the outlet of the quenching device after quenching and the entrance of the tempering furnace, but as long as the temperature of the steel strip is not less than 80°C before passing through the tempering furnace, a gap can be provided between the quenching device and the tempering furnace and the insulation device. In addition, in the present embodiment, a continuous furnace is used as the quenching furnace and the tempering furnace described later, but the present invention can also be implemented when the quenching furnace and the tempering furnace are batch furnaces.

[0029] (Tempering process)

[0030] In the present embodiment, there is a tempering process as follows: the steel strip after the heat-retaining and conveying process is tempered in a tempering furnace in a non-oxidizing gas environment, and the steel strip is adjusted to the desired hardness. The temperature of the tempering furnace can be set to a desired temperature according to the application. For example, in the case where a higher hardness property is required, it can be set to 200°C to 300°C. In addition, in order to make the shape processability such as pressing good, it can also be set to 300°C to 400°C. In addition, if the passing speed in the tempering process is too fast, there is a possibility that the temperature range is not reached. Therefore, when the time required for the steel strip to pass through the tempering furnace is set to M [min] and the plate thickness of the steel strip is set to t [mm], it is preferably set in a manner that M / t becomes 5 to 9. The quenching process, the heat-retaining and conveying process, and the tempering process are carried out continuously, so that a steel strip with excellent mechanical properties and fatigue strength can be obtained without reducing productivity, so it is preferred.

[0031] The steel strip after the tempering step may be subjected to a grinding step to remove the surface rust of the steel strip. As the grinding method, grinding by mechanical processing such as grinding with a grindstone, grinding with a belt, grinding with a brush, grinding with a buffing wheel, etc. may be selected. Among them, buffing with a buffing wheel is preferred because the surface rust can be removed without causing significant damage to the surface of the steel strip.

[0032] Next, the martensitic stainless steel strip of the present embodiment is described. One of the characteristics of the martensitic stainless steel strip of the present embodiment is that the amount of retained austenite is 10 volume % to 25 volume %. In order to improve the mechanical properties, the amount of retained austenite in the steel strip after quenching is usually reduced. In the present invention, by setting the amount of retained austenite in the steel strip after quenching and tempering to 10 volume % or more, the crack development of the steel strip can be suppressed without significantly reducing the mechanical properties of the steel strip, and the fatigue strength characteristics can be greatly increased. On the other hand, if the amount of retained austenite is too much, there is a tendency for the mechanical properties to be significantly reduced, so the upper limit of the amount of retained austenite is set to 25 volume %. The lower limit of the preferred amount of retained austenite is 12 volume %, and the upper limit of the preferred amount of retained austenite is 20 volume %. In addition, in the method for determining the amount of retained austenite in the present embodiment, an X-ray diffraction device is used to derive the amount of retained austenite (volume %) based on the obtained diffracted X-ray intensity distribution.

[0033] In order to further improve the durability of the product, the tensile strength of the steel strip of the present embodiment is greater than 1600MPa and less than 2300MPa. The more preferred lower limit of the tensile strength is 1700MPa, and the more preferred upper limit of the tensile strength is 2200MPa. The more preferred upper limit of the tensile strength is 2000MPa. In addition, the steel strip of the present embodiment satisfies the tensile strength range, and the ratio of the 0.2% yield strength to the tensile strength, i.e., the yield strength ratio, is less than 75%. By setting the yield strength ratio, the steel strip can be given appropriate toughness and the fatigue strength can be further improved. There is no particular limitation on the lower limit of the yield strength ratio, but an excessively low yield strength ratio tends to reduce mechanical properties such as hardness, so it can be set to 50% or more, for example.

[0034] The steel strip of the present embodiment is preferably a martensitic stainless steel strip in which the ratio of the compressive residual stress in the rolling direction to the compressive residual stress in the width direction is 75% or more. As a result, the anisotropy of the compressive residual stress of the steel strip of the present invention is small, so the deviation of the characteristics in the cutting direction can be suppressed. The effect is particularly effective for a baffle valve having a plurality of leads arranged radially and having a rotationally symmetrical shape. Preferably, the ratio of the compressive residual stress in the rolling direction to the compressive residual stress in the width direction is 77% or more. In addition, there is no particular limitation on the value of the compressive residual stress, but in order to more effectively obtain the effect of improving fatigue strength, it is preferred to set the lower limit of the compressive residual stress to 300MPa. The more preferred lower limit of the compressive residual stress is 330MPa, and the more preferred lower limit of the compressive residual stress is 360MPa. In addition, the residual stress on the surface of the steel strip in the present embodiment can be measured by an X-ray residual stress measuring device. In the present embodiment, 2θ-sin is used. 2 The residual stress is measured by the Ψ method. The rolling perpendicular direction is a direction perpendicular to the rolling direction, and in a long steel strip, it corresponds to the width direction when the longitudinal direction is the rolling direction.

[0035] The steel strip of this embodiment can be applied to a martensitic stainless steel strip with a thickness of 1 mm or less. Since the thinner the steel strip, the more likely it is to have a poor shape caused by heating during quenching, it is preferably applied to a martensitic stainless steel strip with a thickness of 0.5 mm or less. In addition, the lower limit of the plate thickness does not need to be specifically set. For example, as a steel plate manufactured by rolling, if the plate thickness is too thin, it is difficult to manufacture, so it can be set to about 0.01 mm. A more preferred lower limit of the plate thickness is 0.05 mm, and a further preferred lower limit of the plate thickness is 0.1 mm.

[0036] [Example]

[0037] First, prepare a martensitic stainless steel strip having a width of about 300 mm and a thickness of 0.15 mm. The composition is shown in Table 1. The prepared steel strip is wound in a coil shape and set in an extractor 1, the steel strip is extracted from the extractor, and the extracted steel strip is passed through a quenching furnace set in an argon atmosphere and the temperature is adjusted to 850°C to 1200°C. Then, the following quenching process is performed: after a primary cooling is performed by spraying pure water on the steel strip using a coolant spray device set on the outlet side of the quenching furnace for rapid cooling, the steel strip is cooled to 290°C to 350°C, and is cooled to below the Ms point (about 270°C) by secondary cooling by pressing using a water-cooled press plate. In addition, the steel strip after the quenching process is subjected to a heat preservation conveying process in which the steel strip passes through an asbestos cylinder as an example of the present invention, and the steel strip without the heat preservation conveying process is set as a comparative example. The temperatures of the steel strips of the present invention example and the comparative example immediately before being conveyed to the tempering furnace are shown in Table 2. The steel strip of the present invention example after the heat-retaining conveying step and the steel strip of the comparative example after the quenching step were passed through a tempering furnace set to an argon atmosphere, and the temperature was adjusted to about 350° C. for tempering. Finally, the tempered steel strip was mechanically polished by polishing with a buff, and the steel strip was wound with a coiler to produce the martensitic stainless steel strip of the present invention example.

[0038] [Table 1]

[0039] (mass%)

[0040] C Si Mn Cr Mo The remainder 0.39 0.30 0.31 13.37 1.23 Fe and inevitable impurities

[0041] Then, the amount of retained austenite, residual stress, tensile strength and 0.2% yield strength were measured based on the samples of the examples of the present invention and the comparative examples produced. The amount of retained austenite was measured using a rotating cathode automatic X-ray diffraction device. The residual stress was measured using the residual stress measuring device AUTOMATE-II manufactured by Rigaku. The tensile strength and 0.2% yield strength were measured according to the method specified in Japanese Industrial Standards (JIS)-Z2241, and the test piece used a JIS 13 No. B test piece. The results of the amount of retained austenite and tensile strength are shown in Table 2, and the results of the measurement of the compressive residual stress are shown in Table 3.

[0042] [Table 2]

[0043]

[0044] [Table 3]

[0045]

[0046] As shown in Table 2, it was confirmed that the retained austenite amount of the samples No. 1 and No. 2, which were subjected to the heat preservation and conveying process, increased compared with the sample No. 11 as the comparative example. In addition, it was also confirmed that the tensile strength was at the same level as the comparative example, and the yield strength was lower than that of the comparative example. From the above results, it can be seen that the examples of the present invention are conducive to improving fatigue resistance while maintaining the same mechanical strength as the conventional products.

[0047] The compressive residual stress also shows a larger value than that of the comparative example, and the compressive residual stress ratio is also increased, so the deviation of the compressive residual stress in the rolling direction and the width direction is small. For example, when applied to products such as baffle valve materials, improved productivity can be expected.

Claims

1. A method for manufacturing a martensitic stainless steel strip, comprising the steps of: performing the following steps to obtain a martensitic stainless steel strip: In a quenching step, a steel strip containing, by mass%, 0.3% to 1.2% C and 10.0% to 18.0% Cr and having a thickness of 1 mm or less is passed through a quenching furnace in a non-oxidizing gas atmosphere, heated to a quenching temperature, and then cooled to a temperature below the Ms point; a heat-insulating and conveying step, in which a heat-insulating device is provided between the quenching furnace and the tempering furnace, and the steel strip cooled to a temperature below the Ms point in the quenching step is conveyed to the tempering furnace while being kept at a temperature range of 80° C. or more and below the Ms point; and In the tempering step, the steel strip conveyed while being kept at a temperature range of 80° C. or higher and below the Ms point in the heat-retaining conveying step is passed through a tempering furnace in a non-oxidizing gas atmosphere and heated to a tempering temperature. The martensitic stainless steel strip: The thickness is less than 1 mm, the amount of retained austenite is 10 volume % to 25 volume %, The tensile strength is 1600MPa or more and 2300MPa or less. The yield strength ratio, which is the ratio of the 0.2% yield strength to the tensile strength, is 75% or less, and The ratio of the compressive residual stress in the rolling direction to the compressive residual stress in the width direction is 75% or more.

2. A martensitic stainless steel strip, comprising, by mass%, 0.3% to 1.2% C and 10.0% to 18.0% Cr, having a martensitic structure and a thickness of 1 mm or less, and The amount of retained austenite in the martensitic stainless steel strip is 10 volume % to 25 volume %, The tensile strength is 1600MPa or more and 2300MPa or less. The yield strength ratio, which is the ratio of the 0.2% yield strength to the tensile strength, is 75% or less. The ratio of the compressive residual stress in the rolling direction to the compressive residual stress in the width direction is 75% or more.

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