Shape memory steel sheet and method for manufacturing same

Abstract

Provided are a shape memory steel sheet having a sufficient shape recovery strain amount, and a method for manufacturing the same. This shape memory steel sheet contains, in terms of mass%, 0.30-1.50% of C and 15.0-35.0% of Ni, and is such that the compositions of Si, Mn, P, S, Al, N, O, and the like are adjusted, wherein: the metal structure before a sub-zero treatment has an austenite content of 80% or greater and a graphite content of 15% or less in area ratio at a position of 1 / 4 of the sheet thickness; a part of the austenite is transformed into martensite by the sub-zero treatment; the martensite contains {112} twin crystals therein; the {112} twin crystals have a width of 50 nm or less; and the martensite is such that the value obtained by dividing the c-axis length of the crystal lattice (bct: body-centered square lattice) by the a-axis length thereof is greater than 1.00, the sub-zero treatment being a treatment in which the steel sheet is held in liquid nitrogen for 2 hours.

Description

Shape memory steel plate and its manufacturing method

[0001] The present invention relates to a shape memory steel sheet having excellent shape memory properties suitable as a material for use in industrial fields such as industry, energy, medicine, and electricity, and a method for producing the same.

[0002] Shape memory alloys are metallic materials that are expected to be applied to various fields such as industry, energy, and medicine by utilizing their unique functions.

[0003] Shape memory alloys in practical use include Ni-Ti-based alloys, Cu-Al-Ni-based alloys, and Fe-Mn-Si-based alloys. Among these, Ni-Ti-based alloys are the most mass-produced, boasting excellent shape memory properties, mechanical strength, and other characteristics. However, Ni-Ti-based alloys have drawbacks, such as poor cold workability and high material costs. Cu-Al-Ni-based alloys also have the drawback of high processing costs. Compared to these non-ferrous shape memory alloys, iron-based shape memory alloys have low material costs and excellent workability, making them promising for a variety of applications. However, iron-based shape memory alloys developed to date are significantly inferior to non-ferrous shape memory alloys in their as-manufactured form, making them unsuitable for practical applications.

[0004] The reason why conventional iron-based alloys, as produced, do not have a good shape memory effect is thought to be because permanent strain such as dislocations is introduced by deformation and because stress-induced transformation of irreversible lath or lenticular martensite occurs, which does not exhibit shape memory. To solve these problems, it is thought that it would be effective to introduce martensite with large tetragonal crystallinity and reversible fine twinning in iron-based shape memory alloys by adjusting the steel composition.

[0005] For example, Patent Document 1 discloses an Fe—Mn—Si-based alloy that exhibits a shape memory effect by utilizing the fcc-hcp transformation.

[0006] Japanese Patent Application Publication No. 09-176729

[0007] However, the conventional techniques have the following problems: The technique described in Patent Document 1 has a problem in that the amount of shape recovery strain is small in the as-manufactured state, and therefore processing and heat treatment must be repeated.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a shape memory steel sheet having a sufficient amount of shape recovery strain and a method for manufacturing the same.

[0009] In order to solve the above-mentioned problems, the inventors have conducted extensive research from the viewpoint of the component composition of the steel sheet and the manufacturing method in order to manufacture a shape memory steel sheet having an excellent shape memory effect, and have discovered the following.

[0010] Specifically, the steel sheet contains 0.30% by mass or more and 1.50% by mass or less of C and 15.0% by mass or more and 35.0% by mass or less of Ni, and the composition of other alloying elements such as Si and Mn is appropriately adjusted. The cold-rolled steel sheet is then hot-rolled and cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is then held at a holding temperature of 900°C or higher for 20 seconds to 3600 seconds, and then cooled to room temperature. As a result, a steel structure is obtained in which, by area ratio, austenite is 80% or more and graphite is 15% or less. During deformation, stress-induced transformation of martensite containing reversible twin crystals within the martensite and migration (growth) of the transformation interface occur at a stress lower than the plastic deformation stress of the steel sheet. Then, after heating, reverse transformation to austenite occurs, resulting in a shape memory effect in which macroscopic deformation is recovered.

[0011] From the above, it was found that in order for the shape memory effect to be exhibited, it is necessary that {112} twins are contained within the martensite structure that transforms upon cooling, and that the width of the {112} twins is 50 nm or less. Furthermore, it was found that when the value obtained by dividing the c-axis length by the a-axis length of the bct crystal lattice of martensite is greater than 1.00, it is possible to manufacture a shape memory steel sheet with excellent shape memory properties.

[0012] The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows: [1] A steel sheet having a chemical composition containing, by mass%, C: 0.30% to 1.50%, Si: 0.01% to 2.50%, Mn: 0.01% to 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, Ni: 15.0% to 35.0%, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities, and the metal structure before subzero treatment has an austenite content of 0.01% or less in area at a position of 1 / 4 of the sheet thickness. a shape memory steel sheet, the austenite being 80% or more and graphite being 15% or less; a portion of the austenite being transformed into martensite by sub-zero treatment; the martensite containing {112} twins and having a width of 50 nm or less; and the martensite having a bct (body-centered tetragonal lattice) crystal lattice in which the c-axis length divided by the a-axis length exceeds 1.00; the sub-zero treatment here refers to a treatment in which the steel sheet is held in liquid nitrogen for two hours. [2] The shape memory steel plate according to the above [1], wherein the chemical composition further contains, in mass%, at least one element selected from the group consisting of Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 10.00% or less, Mo: 1.00% or less, Co: 1.000% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less. [3] The shape memory steel plate according to the above [1] or [2], wherein the martensite has a transformation start temperature of -196°C or higher.[4] A method for producing a shape memory steel plate, comprising the steps of: a hot rolling step in which, after heating a steel material having the chemical composition described in [1] or [2] above, or directly after producing the steel material, the steel material is hot-rolled to produce a hot-rolled steel plate, the steel plate temperature on the finish rolling outlet side is set to a range of 750°C to 1000°C, a cold rolling step in which the hot-rolled steel plate is cold-rolled to produce a cold-rolled steel plate, and a heat treatment step in which the cold-rolled steel plate is then held at a holding temperature in a temperature range of 900°C or higher for a holding time in a range of 20 seconds to 3600 seconds, and then cooled from the held temperature to room temperature. [5] A method for producing a shape memory steel plate in the above [4], wherein, in the heat treatment step, when cooling from the holding temperature, the average cooling rate in the temperature range of 900°C to 600°C is set to 50°C / s or more.

[0013] According to the present invention, a shape memory steel sheet having an excellent shape memory effect, in which strain exceeding the elastic limit can be recovered by heating alone, can be obtained. The shape memory steel sheet obtained by the manufacturing method of the present invention has excellent cold workability and can be applied in industrial fields such as manufacturing, energy, medicine, and electricity, and has extremely great industrial utility value.

[0014] Hereinafter, embodiments of the present invention will be described in detail. The following embodiments are merely examples of steel structures and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0015] <Shape Memory Steel Plate> The shape memory steel plate according to this embodiment will be specifically described below. Note that "%" representing the content of component elements means "mass %" unless otherwise specified.

[0016] [Component Composition] The reasons for limiting the component composition of the steel sheet according to this embodiment will be explained.

[0017] C: 0.30% or more and 1.50% or less C is one of the important basic components of steel and is an important element that affects the c-axis length of martensite having a bct crystal lattice. If the C content is less than 0.30%, the c-axis length of martensite decreases, and the value obtained by dividing the c-axis length of martensite by the a-axis length becomes 1.00, i.e., the martensite becomes a body-centered cubic lattice (bcc crystal lattice), making it difficult to exhibit the shape memory effect. On the other hand, if the C content exceeds 1.50%, not only does the graphite area ratio increase, but the martensitic transformation start temperature also significantly decreases, making it difficult to exhibit the shape memory effect. Therefore, the C content is set to a range of 0.30% or more and 1.50% or less. Preferably, it is set to 0.32% or more. Preferably, it is set to 1.45% or less. More preferably, it is set to 0.35% or more. More preferably, it is set to 1.40% or less.

[0018] Si: 0.01% or more and 2.50% or less. Si is one of the basic components of steel. In this embodiment, Si suppresses graphite formation during cooling and promotes austenite formation, thereby affecting the austenite fraction. If the Si content is less than 0.01%, graphite is formed during cooling after annealing, reducing the austenite fraction and making it difficult to achieve the desired shape memory effect. On the other hand, if the Si content exceeds 2.50%, the stacking fault energy in austenite decreases and the width of {112} twins increases. As a result, the martensitic transformation-induced stress increases, and plastic deformation occurs prior to stress-induced martensitic transformation, preventing good shape memory properties. Therefore, the Si content is set to a range of 0.01% or more and 2.50% or less. Preferably, it is set to 0.05% or more. Preferably, it is set to 2.00% or less. More preferably, it is set to 0.10% or more. More preferably, it is set to 1.80% or less.

[0019] Mn: 0.01% or more and 5.00% or less Mn is one of the basic components of steel and, in this embodiment, is an important element that affects the austenite fraction. Mn is an element that stabilizes austenite, and this effect is observed when the Mn content is 0.01% or more. On the other hand, if the Mn content exceeds 5.00%, the austenite becomes excessively stable, the martensitic transformation start temperature falls below -196°C, and the desired shape memory effect cannot be obtained. Therefore, the Mn content is set to a range of 0.01% or more and 5.00% or less. Preferably, it is set to 0.10% or more. Preferably, it is set to 4.50% or less. More preferably, it is set to 0.50% or more. More preferably, it is set to 4.20% or less.

[0020] P: 0.100% or less P segregates at austenite grain boundaries and embrittles the grain boundaries, inhibiting martensitic transformation of the steel sheet and reducing the shape memory properties. Therefore, the P content must be 0.100% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to set it to 0.001% or more. Therefore, the P content is set to 0.100% or less. Preferably, it is set to 0.001% or more. Preferably, it is set to 0.070% or less.

[0021] S: 0.0200% or less S exists in steel as sulfides, inhibits the martensitic transformation of the steel sheet, and reduces the shape memory properties. Therefore, the S content must be 0.0200% or less. Although there is no particular lower limit for the S content, excessive reduction of S increases the refining load, so due to constraints on production technology, it is preferable to set it to 0.0001% or more. Therefore, the S content is set to 0.0200% or less. Preferably, it is set to 0.0001% or more. Preferably, it is set to 0.0050% or less.

[0022] N: 0.0100% or less N reduces the stacking fault energy of austenite and therefore increases the width of {112} twins. As a result, martensitic transformation-induced stress increases, and plastic deformation occurs prior to stress-induced martensitic transformation, preventing the desired shape memory properties from being obtained. Therefore, the N content must be 0.0100% or less. Although there is no particular lower limit for the N content, excessive reduction of N increases the refining load, so due to production technology constraints, the N content is preferably 0.0001% or more. Therefore, the N content is 0.0100% or less, preferably 0.0001% or more, and preferably 0.0050% or less.

[0023] Ni: 15.0% or more and 35.0% or less Ni is an important element that transitions the defect structure inside martensite crystals from dislocations to twins. If the Ni content is less than 15.0%, the defect structure inside martensite crystals is dominated by dislocations, so reversible macro-deformation does not occur after martensitic transformation, and good shape memory properties cannot be obtained. In order to introduce twins into the defect structure inside martensite crystals, the Ni content must be 15.0% or more. On the other hand, if the Ni content exceeds 35.0%, the austenite becomes too stable, and martensitic transformation does not occur upon cooling or processing below room temperature, making it difficult to exhibit the shape memory effect. Preferably, the Ni content is 16.5% or more and preferably 33.0% or less. More preferably, it is 18.0% or more and more preferably 31.0% or less.

[0024] Al: 0.100% or less Excessive content of Al ３ This increases the transformation point and causes a large amount of ferrite (bcc) to be included in the microstructure. As a result, it becomes difficult to achieve the desired shape memory effect. Therefore, the Al content must be 0.100% or less. Although there is no particular lower limit for the Al content, it is preferable that the Al content be 0.001% or more because it suppresses graphite formation during annealing and promotes austenite formation. Therefore, the Al content is 0.100% or less, preferably 0.001% or more, and preferably 0.050% or less.

[0025] O: 0.0100% or less O exists as an oxide and inhibits the martensitic transformation of the steel sheet. Therefore, if present in excess, it becomes difficult to exhibit the shape memory effect. Therefore, the O content must be 0.0100% or less. Although there is no particular lower limit for the O content, excessive reduction of O increases the refining load, so due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is 0.0100% or less. Preferably, it is 0.0001% or more. Preferably, it is 0.0050% or less.

[0026] The shape memory steel sheet according to this embodiment has a composition containing the above-mentioned basic components, with the balance including Fe (iron) and unavoidable impurities. Here, examples of the unavoidable impurities include Zn, Pb, and As. A total content of 0.100% or less of these unavoidable impurities is permitted. Here, it is preferable that the shape memory steel sheet according to one embodiment of the present invention contains only the above-mentioned basic components and the balance, with the balance being Fe (iron) and unavoidable impurities.

[0027] In addition to the above basic components, the shape memory steel sheet according to this embodiment may further contain, in mass%, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 10.00% or less, Mo: 1.00% or less, Co: 1.000% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less, either alone or in combination.

[0028] Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less. When Ti, Nb, and V are each 0.200% or less, carbides are not formed in large amounts, the value obtained by dividing the c-axis of martensite by the a-axis exceeds 1.00, and shape memory properties are not deteriorated. Therefore, the contents of Ti, Nb, and V are preferably each 0.200% or less. Although there are no particular lower limits for the contents of Ti, Nb, and V, the Ti, Nb, and V contents are more preferably 0.001% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, when Ti, Nb, and V are contained, their contents are each 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0029] Ta: 0.10% or less, W: 0.10% or less If Ta and W are each 0.10% or less, large amounts of coarse precipitates or inclusions are not formed, and the martensitic transformation of the steel sheet is not inhibited, so the shape memory properties are not reduced. Therefore, the Ta and W contents are preferably 0.10% or less. Although there are no particular lower limits for the Ta and W contents, the Ta and W contents are more preferably 0.01% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, when Ta and W are contained, their contents should each be 0.10% or less, more preferably 0.01% or more, and even more preferably 0.08% or less.

[0030] B: 0.0100% or less If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the martensitic transformation of the steel sheet will not be inhibited, so the shape memory properties will not be reduced. Therefore, the B content is preferably 0.0100% or less. Although there is no particular lower limit for the B content, since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is more preferably 0.0003% or more. Therefore, when B is contained, its content is 0.0100% or less, more preferably 0.0003% or more, and even more preferably 0.0080% or less.

[0031] Cr: 10.00% or less If Cr is 10.00% or less, the stacking fault energy of austenite does not decrease and twins become finer, so shape memory properties do not decrease. Therefore, the Cr content is preferably 10.00% or less. Although there is no particular lower limit for the Cr content, since Cr is an element that improves hardenability, it is more preferable that the Cr content be 0.01% or more. Therefore, when Cr is contained, its content is 10.00% or less. More preferably, it is 0.01% or more. Even more preferably, it is 8.00% or less.

[0032] Mo: 1.00% or less If Mo is 1.00% or less, a large amount of carbides are not formed, the value obtained by dividing the c-axis of martensite by the a-axis exceeds 1.00, and the shape memory properties are not deteriorated. Therefore, the Mo content is preferably 1.00% or less. Although there is no particular lower limit for the Mo content, since Mo is an element that improves hardenability, it is more preferable that the Mo content be 0.01% or more. Therefore, when Mo is contained, its content is 1.00% or less, more preferably 0.01% or more, and even more preferably 0.80% or less.

[0033] Co: 1.000% or less If Co is 1.000% or less, ferrite does not form in the microstructure, and therefore the shape memory properties do not deteriorate. Therefore, the Co content is preferably 1.000% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, its content is 1.000% or less, more preferably 0.002% or more, and even more preferably 0.80% or less.

[0034] Cu: 1.00% or less If Cu is 1.00% or less, coarse precipitates and inclusions do not increase and the martensitic transformation of the steel sheet is not inhibited, so the shape memory properties do not deteriorate. Therefore, the Cu content is preferably 1.00% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is more preferably 0.01% or more. Therefore, when Cu is contained, its content is 1.00% or less. More preferably, it is 0.01% or more. Even more preferably, it is 0.80% or less.

[0035] Sn: 0.200% or less If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the martensitic transformation of the steel sheet will not be inhibited, so the shape memory properties will not be reduced. Therefore, the Sn content is preferably 0.200% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability, the Sn content is more preferably 0.001% or more. Therefore, when Sn is contained, its content is 0.200% or less. More preferably, it is 0.001% or more. Even more preferably, it is 0.100% or less.

[0036] Sb: 0.200% or less If Sb is 0.200% or less, coarse precipitates and inclusions do not increase, and Sb does not inhibit the martensitic transformation of the steel sheet, so shape memory properties do not deteriorate. Therefore, the Sb content is preferably 0.200% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is more preferably 0.001% or more. Therefore, when Sb is contained, its content is 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0037] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. When Ca, Mg, and REM are each 0.0100% or less, coarse precipitates and inclusions do not increase, and martensitic transformation of the steel sheet is not inhibited, resulting in a deterioration in shape memory properties. Therefore, the contents of Ca, Mg, and REM are preferably each 0.0100% or less. Although there are no particular lower limits for the contents of Ca, Mg, and REM, since these elements spheroidize the shape of nitrides and sulfides and improve the strength of the steel sheet, it is more preferable that the contents of Ca, Mg, and REM are each 0.0005% or more. Therefore, when Ca, Mg, and REM are contained, their contents should each be 0.0100% or less, more preferably 0.0005% or more, and even more preferably 0.0050% or less.

[0038] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Co, Cu, Sn, Sb, Ca, Mg and REM is less than the preferable lower limit value, the effect of the present invention is not impaired, and therefore, these elements are included as unavoidable impurities.

[0039] [Metal Structure] Next, the metal structure of the shape memory steel sheet according to this embodiment will be described. First, the metal structure before the subzero treatment is set to the range specified below. Austenite area ratio: 80% or more In order to exhibit the shape memory effect, stress-induced martensite transformation from austenite is required. In order to obtain good shape memory properties, the austenite area ratio needs to be 80% or more. Preferably, it is 85% or more.

[0040] Graphite area ratio: 15% or less Graphite inhibits stress-induced martensitic transformation and the interfacial migration (growth) of martensitic transformation. Therefore, in order to obtain good shape memory properties, the graphite area ratio needs to be 15% or less. It is preferably 10% or less, and more preferably 8% or less. It may even be 0%.

[0041] The area ratios of austenite and graphite were determined by polishing a thickness cross section (L cross section) parallel to the rolling direction of the steel plate, followed by finish polishing with colloidal silica. Then, backscattered electron images of 10 fields of view were obtained at a magnification of 2000x using a scanning electron microscope (SEM) at a 1 / 4 position in the thickness direction (a position corresponding to 1 / 4 of the thickness in the depth direction from the steel plate surface). Next, the area ratios of each structure (austenite, graphite) for the 10 fields of view were calculated for the obtained structural images using Image-Pro from Media Cybernetics. These values ​​were then averaged to determine the area ratio of each structure. In the structural images, graphite exhibits a black structure, while austenite exhibits a structure with a white to dark gray color depending on the crystal orientation.

[0042] Martensitic transformation occurs during sub-zero treatment The shape memory effect is manifested due to martensitic transformation. In steel sheets in which martensitic transformation does not occur during sub-zero treatment, the austenite phase is excessively stable, and therefore stress-induced martensitic transformation does not occur, and the shape memory effect does not occur. Furthermore, since good shape memory properties can be obtained by causing martensitic transformation in an area fraction of 5% or more of the total austenite during sub-zero treatment, it is preferable that martensitic transformation occur in 5% or more of the total austenite during sub-zero treatment. More preferably, it is 10% or more. Note that sub-zero treatment here refers to a process in which the steel sheet is held in liquid nitrogen for two hours.

[0043] The area ratio of martensite is determined by polishing a thickness cross section (L cross section) parallel to the rolling direction of the steel sheet, finish-polishing it with colloidal silica, and then calculating 10 fields of view at a 1 / 4 position in the thickness direction (a position corresponding to 1 / 4 of the thickness in the depth direction from the steel sheet surface) using an EBSD (Electron Backscattered Diffraction) phase map, and averaging these values.

[0044] Inclusion of {112} twins within martensite Conventional martensite crystals contain dislocations, which are irreversible plastic deformation, but in this embodiment, it is necessary for the martensite crystals to contain reversible, coherent twins, and for these to be {112} twins, in order to exhibit the shape memory effect.

[0045] To confirm the presence of twins within martensite crystals, test specimens are prepared as follows. First, a 3 mm diameter disk is cut from a steel sheet by electrical discharge machining. Then, the disk is mechanically polished parallel to the sheet surface, including the 1 / 4 position of the sheet thickness, so that the thickness of the disk is 50 μm to 100 μm. Further, electrolytic polishing is performed to thin the specimen, and the specimen is prepared. Then, a selected area diffraction pattern within the martensite crystal is obtained using a TEM (transmission electron microscope) on the test specimen, confirming the presence of twins and determining the twin boundary orientation.

[0046] Width of {112} twin: 50 nm or less In order to exhibit the shape memory effect, it is important that the martensitic transformation-induced stress is equal to or less than the plastic flow stress of the parent phase. Furthermore, the martensitic transformation-induced stress is proportional to the width of the {112} twin. If the twin width is 50 nm or more, the martensitic transformation-induced stress exceeds the plastic flow stress, and the shape memory effect is not exhibited. Therefore, the width of the {112} twin must be 50 nm or less.

[0047] The width of the {112} twin can be determined using the TEM described above. First, a dark-field image is taken using diffraction from one variant of the twin from an orientation in which the twin plane is edge-on relative to the observation direction. Then, a line is drawn perpendicular to the trace of the twin plane inside the martensite, and the widths of 30 to 60 twins are measured. These values ​​are then averaged to determine the width of the {112} twin.

[0048] Value of c-axis length of martensite divided by a-axis length: greater than 1.00 In this embodiment, it is an extremely important constituent requirement that the value of the c-axis length of the bct (body-centered tetragonal lattice) crystal lattice of martensite divided by the a-axis length is greater than 1.00. By making the value of the c-axis length of the bct (body-centered tetragonal lattice) crystal lattice of martensite greater than 1.00, it is possible to reduce the stress that induces the transformation of martensite containing twins internally from a crystallographic perspective, and is effective in obtaining good shape memory properties. On the other hand, although there is no particular upper limit, if it exceeds 1.20, deformation twins in austenite are induced, so it is preferable to set it to 1.20 or less. It is more preferably 1.02 or more, and more preferably 1.18 or less.

[0049] The c-axis length and a-axis length, which are the lattice constants of martensite, are measured as follows. First, a plate material is subjected to subzero treatment with liquid nitrogen. Then, a diffraction profile is obtained for the plate material using a Cu Kα ray in an X-ray diffractometer using a general θ-2θ method in the range of 35°≦2θ≦155°. The lattice constant of martensite at room temperature is then calculated using the Pawly method. In this case, the crystal structure of martensite is treated as a bct (body-centered tetragonal lattice) structure, and the a-axis length and c-axis length, which are the lattice constants, are defined as a ≠ c and c > a. In other words, in this embodiment, the case where the martensite is a bcc (body-centered cubic lattice) crystal lattice is excluded, that is, the case where the c-axis length and the a-axis length are equal is excluded.

[0050] Transformation start temperature of martensite caused by cooling: -196°C or higher. If the transformation start temperature of martensite caused by cooling is below -196°C, the martensite transformation-induced stress will be higher than the plastic deformation stress during processing in the temperature range from room temperature to -196°C. Therefore, during processing, irreversible plastic deformation occurs prior to stress-induced martensite transformation, which may prevent the shape memory effect from occurring. Therefore, the transformation start temperature of martensite caused by cooling is preferably -196°C or higher. On the other hand, although there is no particular upper limit, if the transformation start temperature of martensite exceeds -20°C, dislocations rather than twins are introduced inside the martensite, which may prevent good shape memory properties from being obtained. Therefore, the transformation start temperature of martensite is preferably -20°C. More preferably, the transformation start temperature of martensite is -40°C or lower, more preferably -175°C or higher.

[0051] The martensite transformation start temperature can be measured using a differential scanning calorimetry (DSC). For example, a DSC curve is obtained in an Ar atmosphere at a cooling rate of 10°C / min in the measurement range of 25°C to -196°C, and the martensite transformation start temperature is calculated using the tangent intersection method for the obtained DSC curve.

[0052] In the metal structure before the subzero treatment according to this embodiment, in addition to austenite and graphite, the following metal structures may be contained in a total area ratio of 10% or less, and this does not impair the effects of the present invention. The metal structures that may be contained are unrecrystallized ferrite, polygonal ferrite, bainite, pearlite, and carbides such as cementite.

[0053] Reverse transformation temperature from martensite to austenite caused by heating In this embodiment, the reverse transformation temperature from martensite to austenite caused by heating is not limited. If this temperature is below room temperature, superelasticity, in which the shape is restored only by unloading stress rather than by heating, may be exhibited, and the shape memory effect may not be obtained. Therefore, the reverse transformation temperature from martensite to austenite caused by heating is preferably above room temperature. More preferably, it is 100°C or higher, and even more preferably, it is 500°C or higher.

[0054] <Method of Manufacturing Shape Memory Steel Plate> A method of manufacturing a shape memory steel plate according to this embodiment will be described. Steel Material When manufacturing a steel material, molten steel produced by a conventional melting method can be used. For example, the blast furnace-converter method or the electric furnace method may be used. Reduced iron obtained by direct reduction may also be melted. The molten steel may be subjected to secondary refining. The steel material, for example, a steel slab, is preferably manufactured by a continuous casting method. In addition, to prevent macrosegregation, it is preferable to employ segregation prevention measures such as a soft reduction method. However, manufacturing by an ingot casting method or a thin slab casting method is also possible. In addition to the conventional method in which the steel material is cooled to room temperature after production and then reheated, energy-saving processes such as direct rolling, in which the hot slab is charged into a heating furnace without being cooled to room temperature, or in which the hot slab is immediately rolled after a short heat retention, can also be applied without any problems. The steel material is also rough-rolled under normal conditions to form a sheet bar. When the heating temperature of the steel material is set to a low level, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[0055] Heating Temperature of Steel Material Although not particularly limited, the heating temperature of the steel material is preferably in the range of 1100°C or higher and 1300°C or lower. Precipitates present during the heating stage of the steel material exist as coarse precipitates in the final steel sheet and affect martensitic transformation. Therefore, it is preferable to redissolve the Ti and Nb-based precipitates precipitated during casting. Therefore, the heating temperature of the steel slab is preferably 1100°C or higher. In addition, from the viewpoint of scaling off (removing as oxides) defects such as bubbles and segregations on the slab surface, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, the heating temperature of the steel material is preferably 1100°C or higher. On the other hand, if the heating temperature of the steel material exceeds 1300°C, the amount of oxidation increases, resulting in increased scale loss, reduced yield, and reduced surface quality of the steel sheet. Therefore, the heating temperature of the steel material is preferably 1300°C or lower. More preferably, the heating temperature of the steel material is 1150°C or higher, and more preferably 1250°C or lower.

[0056] Steel sheet temperature at the finish rolling exit of hot rolling: 750°C or higher and 1000°C or lower. The heated steel material is hot-rolled by rough rolling and finish rolling to produce a hot-rolled steel sheet. If the steel sheet temperature at the finish rolling exit exceeds 1000°C, the amount of oxide (scale) generation increases rapidly, the interface between the base steel and the oxide becomes rough, and the surface quality after pickling and cold rolling tends to deteriorate. Furthermore, the presence of residual hot-rolled scale after pickling has a negative effect on the shape memory property. Furthermore, the grain size becomes excessively coarse, which reduces the plastic flow stress and may prevent the shape memory property from being exhibited. On the other hand, if the steel sheet temperature at the finish rolling exit is lower than 750°C, the rolling load increases, resulting in a large rolling load. In addition, the reduction ratio increases when austenite is not recrystallized, leading to the development of an abnormal texture and significant in-plane anisotropy in the final product. This not only impairs the uniformity (material stability) of the material, but also reduces the shape memory property itself. Therefore, the temperature of the steel sheet at the finish rolling exit side of hot rolling needs to be in the range of 750° C. or more and 1000° C. or less, preferably 800° C. or more and preferably 950° C. or less. The temperature of the steel sheet at the finish rolling exit side here is the measured surface temperature of the steel sheet.

[0057] Pickling of Hot-Rolled Steel Sheet The hot-rolled steel sheet produced in this way is subjected to pickling as needed. Pickling can remove oxides from the steel sheet surface, so it is preferable to perform this in order to ensure good chemical conversion treatability and plating quality of the final shape-memory steel sheet. When pickling is performed, pickling may be performed once or in multiple steps.

[0058] Cold Rolling After coiling, the hot-rolled steel sheet is pickled as necessary and then cold-rolled to obtain a cold-rolled steel sheet. There are no particular restrictions on the cold-rolling reduction, but a reduction ratio of 5% to 80% is preferred.

[0059] The cold-rolled steel sheet is then heat-treated to produce a shape-memory steel sheet. When the cold-rolled steel sheet is heat-treated at a temperature below 900°C, a large amount of graphite is generated, and the desired austenite content is not obtained. As a result, good shape-memory properties are not obtained. On the other hand, when the cold-rolled steel sheet is heat-treated at a temperature above 900°C for less than 20 seconds, sufficient recrystallization is not achieved, and the desired austenite content is not obtained. This results in poor shape-memory properties. Furthermore, when the cold-rolled steel sheet is heat-treated at a temperature above 900°C for more than 3600 seconds, a large amount of graphite is generated, and good shape-memory properties may not be obtained. Therefore, the cold-rolled steel sheet must be heat-treated at a temperature above 900°C for a time of 20 seconds to 3600 seconds. While the upper limit of the holding temperature is not limited, it is preferably 1250°C or less from the viewpoint of energy conservation.

[0060] The heat treatment method may be either continuous annealing or batch annealing. After the heat treatment, the steel sheet is cooled to room temperature. The cooling method is not particularly limited, and may be furnace cooling or air cooling in batch annealing, or gas jet cooling, mist cooling, or water cooling in continuous annealing. If pickling is performed, a conventional method may be used.

[0061] Cooling rate in the temperature range of 900°C to 600°C: 50°C / s or more If the cooling rate in the temperature range of 900°C to 600°C is 50°C / s or more, the formation of graphite during cooling can be suppressed. This promotes stress-induced martensitic transformation and provides good shape memory properties. There is no particular upper limit for the cooling rate in the temperature range of 900°C to 600°C, but due to production technology constraints, it is preferably 1000°C / s or less. It is more preferably 800°C / s or less.

[0062] The "shape memory steel sheet" of this embodiment can be subjected to skin pass rolling for the purpose of correcting the shape, adjusting the surface roughness, etc. The reduction ratio of the skin pass rolling is preferably in the range of 0.1% to 2.0%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit of the preferable range. If it exceeds 2.0%, productivity drops significantly, so this is the upper limit of the preferable range. The skin pass rolling may be performed online or offline. Furthermore, the skin pass may be performed at the desired reduction ratio in one go, or may be performed in several steps. Various painting treatments such as resin or oil coating may also be performed.

[0063] Steel having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities, was melted in a converter and continuously cast into slabs. The resulting slabs were reheated to 1,250°C, and steel sheets having thicknesses of 1.0 mm to 1.8 mm were obtained under the conditions shown in Table 2. The cross-sectional microstructure of the resulting steel sheets was observed using the method described above, and the shape memory properties were evaluated by a stress loading / unloading cycle tensile test, with the results shown in Table 3. In Table 2, "Finishing Rolling Outlet Temperature" refers to the steel sheet surface temperature at the outlet of finishing rolling in hot rolling. Furthermore, "Cooling Rate" in the cold-rolled sheet annealing treatment column in Table 2 refers to the cooling rate in the temperature range of 900°C to 600°C during cooling from the holding temperature to room temperature.

[0064]

[0065]

[0066]

[0067] For the stress loading / unloading cycle tensile test, a JIS No. 5 test piece was used, which was prepared so that the tensile direction was perpendicular to the rolling direction of the steel sheet. The test piece was stretched to a load strain of 5% at a temperature of -50°C, and after unloading, the gauge length was measured. The test piece was then heated to 800°C and cooled to room temperature, and the gauge length was measured again. Using the measurement results, the shape recovery strain due to heating was calculated, and the shape memory properties were evaluated. Furthermore, the shape memory properties were considered good when the shape recovery strain was 0.95% or more.

[0068] Under the test conditions indicated as examples in the remarks column, shape memory steel sheets with excellent shape memory properties were obtained. On the other hand, under the test conditions indicated as comparative examples in the remarks column, the shape memory properties were poor.

[0069] According to the present invention, a shape memory steel plate having an excellent shape memory effect, in which strain exceeding the elastic limit can be recovered by heating alone, can be obtained. The shape memory steel plate obtained by the manufacturing method of the present invention can be applied to industrial fields such as manufacturing, energy, medicine, and electricity, and has extremely high industrial utility value.

Claims

1. A steel sheet having a composition containing, by mass%, C: 0.30% or more and 1.50% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.01% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, Ni: 15.0% or more and 35.0% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; the metal structure before the sub-zero treatment is, in terms of area ratio at a position of 1 / 4 of the plate thickness, 80% or more austenite and 15% or less graphite; the sub-zero treatment causes a part of the austenite to be transformed into martensite; the martensite contains {112} twins inside and the width of the {112} twins is 50 nm or less; The martensite has a value obtained by dividing the c-axis length of a bct (body-centered tetragonal lattice) crystal lattice by the a-axis length of more than 1.

00. Here, the sub-zero treatment refers to a treatment in which the steel sheet is held in liquid nitrogen for two hours. Shape memory steel sheet.

2. The shape memory steel plate as described in claim 1, wherein the chemical composition further contains, by mass%, at least one element selected from the following: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 10.00% or less, Mo: 1.00% or less, Co: 1.000% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less.

3. A shape memory steel plate according to claim 1 or 2, wherein the martensite has a transformation start temperature of -196°C or higher.

4. A method for producing a shape memory steel plate, comprising: a hot rolling process in which, after heating a steel material having the chemical composition according to claim 1 or 2, or directly after producing the steel material, the steel material is hot rolled to produce a hot rolled steel plate, in which the steel plate temperature on the finish rolling exit side is set to a range of 750°C to 1000°C; a cold rolling process in which the hot rolled steel plate is cold rolled to produce a cold rolled steel plate; and a heat treatment process in which the cold rolled steel plate is then held at a holding temperature in a temperature range of 900°C or higher for a holding time in a range of 20 s to 3600 s, and then cooled from the holding temperature to room temperature.

5. A method for manufacturing a shape memory steel plate as described in claim 4, wherein in the heat treatment process, when cooling from the holding temperature, the average cooling rate in the temperature range of 900°C or less and 600°C or more is 50°C / s or more.

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