Shape memory steel plate and method for manufacturing the same

Abstract

Provided are a shape memory steel plate having a sufficient amount of shape recovery strain and a method for manufacturing the same. The steel plate contains, in mass%, C: 0.30% or more and 1.50% or less, and Ni: 15.0% or more and 35.0% or less, and has its composition such as Si, Mn, P, S, Al, N, and O adjusted. The metal structure before sub-zero treatment has an area ratio at a position of 1 / 4 of the plate thickness, where the austenite is 80% or more and the graphite is 15% or less. By sub-zero treatment, a part of the austenite is 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 the crystal lattice (bct: body-centered tetragonal lattice) by the a-axis length that exceeds 1.00. Here, the sub-zero treatment refers to a treatment of holding the steel plate in liquid nitrogen for 2 hours. It is a shape memory steel plate.

Description

Technical Field

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

Background Art

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

[0003] As shape memory alloys, Ni-Ti-based alloys, Cu-Al-Ni-based alloys, Fe-Mn-Si-based alloys, etc. have been put into practical use. Among them, the most mass-produced is the Ni-Ti-based alloy, which is excellent in characteristics such as shape memory and mechanical strength. However, the Ni-Ti-based alloy has drawbacks such as poor cold workability and high material cost. The Cu-Al-Ni-based alloy also has the drawback of high processing cost. For these non-ferrous shape memory alloys, ferrous shape memory alloys are expected to be used in various applications because of their low material cost and good workability. However, the ferrous shape memory alloys developed so far are significantly inferior to non-ferrous shape memory alloys as manufactured and are not suitable for applications.

[0004] It is considered that the reason why conventional ferrous alloys do not have a good shape memory effect as manufactured is that permanent strain such as dislocations is introduced by deformation, and stress-induced transformation of irreversible lath-like or lens-like martensite that does not exhibit shape memory occurs. To solve these problems, it is considered effective to introduce martensite by reversible fine twins with a large tetragonality of the ferrous shape memory alloy by devising the steel components.

[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.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 09-176729 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] However, the prior art has the following problems. In the technique described in Patent Document 1, there is a problem that since the amount of shape recovery strain is small as it is manufactured, it is necessary to repeat processing and heat treatment.

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

[0009] In order to solve the above-described problems, the inventors conducted intensive studies from the viewpoints of the component composition and manufacturing method of the steel sheet in order to manufacture a shape memory steel sheet having an excellent shape memory effect, and found the following.

[0010] That is, it contains C: 0.30% by mass or more and 1.50% by mass or less and Ni: 15.0% by mass or more and 35.0% by mass or less, and the component compositions of other alloy elements such as Si and Mn are appropriately adjusted. After hot rolling, cold rolling is performed to obtain a cold-rolled steel sheet. Then, the cold-rolled steel sheet is held at a holding temperature in a temperature range of 900°C or higher for 20 s or more and 3600 s or less, and then cooled to room temperature. As a result, a steel structure having an austenite area ratio of 80% or more and a graphite area ratio of 15% or less is obtained. Then, at a stress lower than the plastic deformation stress of the steel sheet during deformation, stress-induced transformation of martensite containing reversible twins and movement (growth) of the transformation interface occur inside. And after heating, by reverse transformation to austenite, a shape memory effect in which macro deformation is recovered is exhibited.

[0011] From the above, it was found that in order for the shape memory effect to occur, the martensite structure that transforms by cooling must contain {112} twins, and the width of the {112} twins must be 50 nm or less. Furthermore, it was found that by having a value obtained by dividing the c-axis length of the bct crystal lattice of martensite by the a-axis length exceeding 1.00, it becomes possible to manufacture a shape memory steel sheet having excellent shape memory characteristics.

[0012] The present invention was completed through further study based on the above findings. That is, the gist configuration of the present invention is as follows. [1] 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, and the balance being Fe and inevitable impurities, having a component composition, and the metal structure before sub-zero treatment is such that, at the position of 1 / 4 of the plate thickness, the area ratio of austenite is 80% or more and graphite is 15% or less. By sub-zero treatment, a part of the austenite is 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 the bct (body-centered tetragonal lattice) crystal lattice by the a-axis length exceeding 1.00. Here, sub-zero treatment means a treatment of holding the steel sheet in liquid nitrogen for 2 hours. It is a shape memory steel sheet. [2] In the above [1], the component composition further contains, by 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. It is a shape memory steel sheet. [3] In [1] or [2] above, the martensite is a shape memory steel plate with a transformation start temperature of -196°C or higher. [4] When hot rolling a steel material having the component composition described in [1] or [2] above, or directly after manufacturing the steel material, to obtain a hot-rolled steel plate, a hot rolling process in which the temperature of the steel plate on the finish rolling exit side is in the range of 750°C or higher and 1000°C or lower, a cold rolling process in which the hot-rolled steel plate is cold rolled to obtain a cold-rolled steel plate, and then, a heat treatment process in which the cold-rolled steel plate is held at a holding temperature in a temperature range of 900°C or higher for a holding time in the range of 20 s or longer and 3600 s or shorter, and then cooled from the holding temperature to room temperature, which is a method for manufacturing a shape memory steel plate. [5] In [4] above, in the heat treatment process, when cooling from the holding temperature, a method for manufacturing a shape memory steel plate in which the average cooling rate in a temperature range of 900°C or lower and 600°C or higher is 50°C / s or higher.

Advantages of the Invention

[0013] 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 only by heating is obtained. The shape memory steel plate obtained by the manufacturing method of the present invention has excellent cold workability and can be applied to industrial fields such as industry, energy, medical, and electricity, and the industrial utility value is extremely large.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be specifically described. Further, the following embodiments illustrate steel structures and methods for embodying the technical idea of the present invention, and do not specify the configuration to the following. That is, various changes can be made to the technical idea of the present invention within the technical scope described in the claims.

[0015] <Shape Memory Steel Plate> Hereinafter, the shape memory steel plate according to the present embodiment will be specifically described. Note that “%” representing the content of component elements means “mass %” unless otherwise specified.

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

[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. When 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, that is, a body-centered cubic lattice (bcc crystal lattice), making it difficult to exhibit the shape memory effect. On the other hand, when the C content exceeds 1.50%, in addition to the increase in the graphite area ratio, the martensite transformation start temperature significantly decreases, making it difficult to exhibit the shape memory effect. Therefore, the C content is in the range of 0.30% or more and 1.50% or less. Preferably, it is 0.32% or more. Preferably, it is 1.45% or less. More preferably, it is 0.35% or more. More preferably, it is 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, since it suppresses graphite generation during cooling and promotes austenite generation, it is an element that affects the austenite fraction. When the Si content is less than 0.01%, graphite is generated during cooling after annealing, and the austenite fraction decreases, making it difficult to exhibit the desired shape memory effect. On the other hand, when the Si content exceeds 2.50%, the stacking fault energy in austenite decreases, and the width of the {112} twin increases. As a result, the martensite transformation induced stress increases, and plastic deformation occurs prior to the stress-induced martensite transformation, so good shape memory characteristics cannot be obtained. Therefore, the Si content is in the range of 0.01% or more and 2.50% or less. Preferably, it is 0.05% or more. Preferably, it is 2.00% or less. More preferably, it is 0.10% or more. More preferably, it is 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 is an important element that affects the fraction of austenite in this embodiment. Mn is an element that stabilizes austenite, and such an effect is recognized when the Mn content is 0.01% or more. On the other hand, when the Mn content exceeds 5.00%, the stability of austenite becomes excessive, and the martensite transformation start temperature drops below -196°C, and the desired shape memory effect cannot be obtained. Therefore, the Mn content should be in the range of 0.01% or more and 5.00% or less. Preferably, it is 0.10% or more. Preferably, it is 4.50% or less. More preferably, it is 0.50% or more. More preferably, it is 4.20% or less.

[0020] P: 0.100% or less P segregates at the austenite grain boundaries and embrittles the grain boundaries, thus inhibiting the martensite transformation of the steel sheet and reducing the shape memory characteristics. Therefore, the P content needs to be 0.100% or less. Although the lower limit of the P content is not particularly specified, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more. Therefore, the P content is 0.100% or less. Preferably, it is 0.001% or more. Preferably, it is 0.070% or less.

[0021] S: 0.0200% or less S exists as sulfide in the steel, inhibits the martensite transformation of the steel sheet, and reduces the shape memory characteristics. Therefore, the S content needs to be 0.0200% or less. Although the lower limit of the S content is not particularly specified, since excessive reduction of S increases the refining load, it is preferably 0.0001% or more from the production technology constraints. Therefore, the S content is 0.0200% or less. Preferably, it is 0.0001% or more. Preferably, it is 0.0050% or less.

[0022] N: 0.0100% or less N increases the width of the {112} twin to reduce the stacking fault energy of austenite. As a result, the martensite transformation-induced stress increases, and plastic deformation occurs prior to the stress-induced martensite transformation, so that the desired shape memory characteristics cannot be obtained. Therefore, the N content needs to be 0.0100% or less. Although the lower limit of the N content is not particularly defined, since excessive reduction of N increases the refining load, from the production technology constraints, the N content is preferably 0.0001% or more. Therefore, the N content is 0.0100% or less. Preferably it is 0.0001% or more. Preferably it is 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 the martensite crystal from dislocations to twins. When the Ni content is less than 15.0%, the defect structure inside the martensite crystal is dominated by dislocations, so that reversible macro deformation does not occur after the martensite transformation, and good shape memory characteristics cannot be obtained. In order to introduce twins into the defect structure inside the martensite crystal, the Ni content needs to be 15.0% or more. On the other hand, when the Ni content exceeds 35.0%, the stability of austenite becomes excessive, and martensite transformation does not occur during cooling or processing below room temperature, making it difficult to exhibit the shape memory effect. Preferably, it 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 inclusion of Al raises the A3 transformation point and causes the microstructure to contain a large amount of ferrite (bcc). As a result, it becomes difficult to exhibit the desired shape memory effect. Therefore, the Al content needs to be 0.100% or less. Although the lower limit of the Al content is not particularly defined, since it suppresses graphite formation during annealing and promotes the formation of austenite, the Al content is preferably 0.001% or more. Therefore, the Al content is 0.100% or less. Preferably it is 0.001% or more. Preferably it is 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 it exists in excess, it becomes difficult to exhibit the shape memory effect. For this reason, the O content needs to be 0.0100% or less. Although the lower limit of the O content is not particularly defined, excessive reduction of O increases the refining load. Therefore, from the production technology constraints, the O content is preferably 0.0001% or more. Accordingly, 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 contains the above basic components and has a component composition in which the balance is Fe (iron) and unavoidable impurities. Here, examples of the unavoidable impurities include Zn, Pb, and As. The total content of these unavoidable impurities is allowed to be 0.100% or less. Here, the shape memory steel sheet according to one embodiment of the present invention preferably contains only the above basic components and the balance, and the balance is 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 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, by mass%.

[0028] Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less If Ti, Nb, and V are each 0.200% or less, a large amount of carbides will not be generated, the value obtained by dividing the c-axis of martensite by the a-axis will exceed 1.00, and the shape memory characteristics will not deteriorate. Therefore, it is preferable that the contents of Ti, Nb, and V are each 0.200% or less. Although the lower limit of the contents of Ti, Nb, and V is not particularly defined, since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, it is more preferable that the contents of Ti, Nb, and V are each 0.001% or more. Therefore, when containing Ti, Nb, and V, their contents are each 0.200% or less. More preferably, they are 0.001% or more. Even more preferably, they are 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, a large amount of coarse precipitates and inclusions will not be generated, and the martensite transformation of the steel sheet will not be inhibited, so the shape memory characteristics will not deteriorate. Therefore, it is preferable that the contents of Ta and W are each 0.10% or less. Although the lower limit of the contents of Ta and W is not particularly defined, since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing, it is more preferable that the contents of Ta and W are each 0.01% or more. Therefore, when containing Ta and W, their contents are each 0.10% or less. More preferably, they are 0.01% or more. Even more preferably, they are 0.08% or less.

[0030] B: 0.0100% or less If B is 0.0100% or less, cracks will not be generated inside the steel plate during casting or hot rolling, and since it does not inhibit the martensitic transformation of the steel plate, the shape memory characteristics will not deteriorate. Therefore, the B content is preferably 0.0100% or less. Although the lower limit of the B content is not particularly specified, since it is an element that segregates at the austenite grain boundary during annealing and improves hardenability, the B content is more preferably 0.0003% or more. Therefore, when containing B, the content is 0.0100% or less. More preferably, it is 0.0003% or more. Even more preferably, it is 0.0080% or less.

[0031] Cr: 10.00% or less If Cr is 10.00% or less, the stacking fault energy of austenite will not decrease, and twins will become fine, so the shape memory characteristics will not deteriorate. Therefore, the Cr content is preferably 10.00% or less. Although the lower limit of the Cr content is not particularly specified, since it is an element that improves hardenability, the Cr content is more preferably 0.01% or more respectively. Therefore, when containing Cr, the content is 10.00% or less respectively. 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 will not be generated, the value of the c-axis of martensite divided by the a-axis will exceed 1.00, and the shape memory characteristics will not deteriorate. Therefore, the Mo content is preferably 1.00% or less. Although the lower limit of the Mo content is not particularly specified, since it is an element that improves hardenability, the Mo content is more preferably 0.01% or more respectively. Therefore, when containing Mo, the content is 1.00% or less respectively. More preferably, it is 0.01% or more. Even more preferably, it is 0.80% or less.

[0033] Co: 1.000% or less If Co is 1.000% or less, ferrite is not generated in the microstructure, so the shape memory characteristics do not deteriorate. Therefore, it is preferable that the Co content is 1.000% or less. Although the lower limit of the Co content is not particularly defined, since it is an element that improves hardenability, it is more preferable that the Co content is 0.001% or more. Therefore, when Co is contained, its content is 1.000% or less. More preferably, it is 0.002% or more. Even more preferably, it is 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 do not inhibit the martensite transformation of the steel sheet, so the shape memory characteristics do not deteriorate. Therefore, it is preferable that the Cu content is 1.00% or less. Although the lower limit of the Cu content is not particularly defined, since it is an element that improves hardenability, it is more preferable that the Cu content is 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 Sn is 0.200% or less, cracks are not generated inside the steel sheet during casting or hot rolling, and it does not inhibit the martensite transformation of the steel sheet, so the shape memory characteristics do not deteriorate. Therefore, it is preferable that the Sn content is 0.200% or less. Although the lower limit of the Sn content is not particularly defined, since Sn is an element that improves hardenability, it is more preferable that the Sn content is 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, no coarse precipitates or inclusions will increase, and it will not inhibit the martensitic transformation of the steel sheet, so the shape memory characteristics will not deteriorate. Therefore, the Sb content is preferably 0.200% or less. Although the lower limit of the Sb content is not particularly defined, since it is an element that controls the surface softening thickness and enables strength adjustment, the Sb content is more preferably 0.001% or more. Therefore, when containing Sb, the content is 0.200% or less. More preferably, it is 0.001% or more. Even more preferably, it is 0.100% or less.

[0037] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less If Ca, Mg, and REM are each 0.0100% or less, no coarse precipitates or inclusions will increase, and it will not inhibit the martensitic transformation of the steel sheet, so the shape memory characteristics will not deteriorate. Therefore, the contents of Ca, Mg, and REM are preferably each 0.0100% or less. Although the lower limits of the contents of Ca, Mg, and REM are not particularly defined, since they are elements that spheroidize the shapes of nitrides and sulfides and improve the strength of the steel sheet, the contents of Ca, Mg, and REM are more preferably each 0.0005% or more. Therefore, when containing Ca, Mg, and REM, the contents are each 0.0100% or less. More preferably, they are 0.0005% or more. Even more preferably, they are 0.0050% or less.

[0038] Regarding Ti, Nb, V, Ta, W, B, Cr, Mo, Co, Cu, Sn, Sb, Ca, Mg, and REM described above, when each content is less than the preferable lower limit value, since it will not impair the effects of the present invention, they are included as inevitable impurities.

[0039] [Metallographic structure] Next, the metallographic structure of the shape memory steel sheet according to the present embodiment will be described. First, the metallographic structure before sub-zero treatment is within the range defined below. Area ratio of austenite: 80% or more In order to exhibit the shape memory effect, it is necessary to transform from austenite to stress-induced martensite transformation. In order to obtain good shape memory characteristics, the area ratio of austenite needs to be 80% or more. Preferably, it is 85% or more.

[0040] Area ratio of graphite: 15% or less Graphite inhibits stress-induced martensite transformation and the interface movement (growth) of martensite transformation. Therefore, in order to obtain good shape memory characteristics, it is necessary to make the area ratio of graphite 15% or less. Preferably, it is 10% or less, more preferably 8% or less. It may be 0.

[0041] Note that the area ratios of austenite and graphite are obtained by polishing the plate thickness cross-section (L cross-section) parallel to the rolling direction of the steel plate and then finish-polishing with colloidal silica. Then, for the position at 1 / 4 of the plate thickness (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface), 10 field-of-view backscattered electron images are acquired at a magnification of 2000 times using an SEM (scanning electron microscope). Next, for the obtained tissue images, the area ratios of each tissue (austenite, graphite) are calculated for 10 fields of view using Image-Pro of Media Cybernetics. Then, the area ratios of each tissue are obtained by averaging those values. Also, in the above tissue images, graphite exhibits a black tissue, and austenite exhibits a tissue having white to dark gray according to the crystal orientation.

[0042] Occurrence of martensite transformation by sub-zero treatment The shape memory effect is manifested due to martensitic transformation. In steel plates where martensitic transformation does not occur during sub-zero treatment, the stability of the austenite phase is excessive. Therefore, stress-induced martensitic transformation does not occur either, and the shape memory effect is not manifested. Also, since good shape memory characteristics can be obtained by causing martensitic transformation in an area ratio of 5% or more of all austenite by sub-zero treatment, it is preferable that martensitic transformation of 5% or more occurs by sub-zero treatment. More preferably, it is 10% or more. Here, the sub-zero treatment means a treatment of holding the steel plate in liquid nitrogen for 2 hours.

[0043] Note that for the area ratio of martensite, after polishing the plate thickness cross-section (L cross-section) parallel to the rolling direction of the steel plate, it is finish-polished with colloidal silica. Then, for the position at 1 / 4 of the plate thickness (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface), 10 fields are calculated by the Phase Map of EBSD (Electron Backscattered Diffraction), and their values are averaged.

[0044] Including {112} twins inside the martensite Conventional martensite crystals contain dislocations, which are irreversible plastic deformations. On the other hand, in this embodiment, it is necessary for the manifestation of the shape memory effect that the martensite crystals contain reversible and coherent twins, and that these are {112} twins.

[0045] Note that for the confirmation of twins inside the martensite crystals, a test piece is prepared as follows. First, a disk with a diameter of 3 mm is cut out from the steel plate by electrical discharge machining. Then, it is mechanically polished parallel to the plate surface so that the thickness of the disk becomes 50 μm to 100 μm and includes the position at 1 / 4 of the plate thickness. Further, electrolytic polishing is performed for thinning to obtain a test piece. After that, by using TEM (transmission electron microscope) for the test piece, it is possible to confirm that it is a twin by obtaining a restricted field diffraction pattern inside the martensite crystals, and it is also possible to determine the twin interface orientation.

[0046] Width of {112} twin: 50 nm or less For the manifestation of the shape memory effect, it is important that the martensite transformation-induced stress is below the plastic deformation stress of the parent phase. Further, the martensite transformation-induced stress is proportional to the width of the {112} twin. When the twin width becomes 50 nm or more, the martensite transformation-induced stress exceeds the plastic deformation stress and the shape memory effect is not manifested. Therefore, the width of the {112} twin needs to be 50 nm or less.

[0047] Note that the width of the {112} twin can be determined using the above-mentioned TEM. First, a dark-field image is taken using diffraction from one variant of the twin from an orientation where the twin plane is edge-on with respect to the observation direction. Then, a line is drawn perpendicular to the trace of the twin plane inside the obtained martensite, the widths of 30 to 60 twins are measured, and their values are averaged to obtain the width of the {112} twin.

[0048] Value obtained by dividing the c-axis length of martensite by the a-axis length: greater than 1.00 In this embodiment, it is an extremely important constituent requirement that the value obtained by dividing the c-axis length of the bct (body-centered tetragonal lattice) crystal lattice of martensite by the a-axis length is greater than 1.00. By setting the value obtained by dividing the c-axis length of the bct (body-centered tetragonal lattice) crystal lattice of martensite by the a-axis length to be greater than 1.00, it is effective in reducing the stress that induces the transformation of martensite containing twins inside from a crystallographic point of view and obtaining good shape memory characteristics. On the other hand, although the upper limit is not particularly defined, when it exceeds 1.20, deformation twins in austenite are induced, so it is preferably 1.20 or less. More preferably, it is 1.02 or more, and more preferably, it is 1.18 or less.

[0049] Incidentally, the c-axis length and the a-axis length, which are the lattice constants of martensite, are measured as follows. First, the plate material is sub-zero treated with liquid nitrogen. Then, for the plate material, using the Kα line of Cu with an X-ray diffractometer, a diffraction profile in the range of 35° ≤ 2θ ≤ 155° is obtained by the general θ-2θ method. And the lattice constants of martensite at room temperature are calculated by the Pawly method. At this time, the crystal structure of martensite is treated as a bct (body-centered tetragonal lattice) structure, and the a-axis length and the c-axis length, which are the lattice constants, are defined as a ≠ c and c > a. That is, in the present embodiment, except for the case where martensite is a bcc (body-centered cubic lattice) crystal lattice, that is, except for the case where the c-axis length and the a-axis length are equal.

[0050] Martensite transformation start temperature caused by cooling: -196°C or higher When the martensite transformation start temperature caused by cooling is lower than -196°C, in processing in the temperature range from room temperature to -196°C, the martensite transformation induced stress becomes higher than the plastic deformation stress. Therefore, during processing, irreversible plastic deformation occurs prior to stress-induced martensite transformation, so there is a possibility that the shape memory effect will not be exhibited. Therefore, it is preferable that the martensite transformation start temperature caused by cooling is -196°C or higher. On the other hand, although the upper limit is not particularly defined, when the martensite transformation start temperature exceeds -20°C, dislocations rather than twins are introduced inside the martensite, so there is a possibility that good shape memory characteristics cannot be obtained. Therefore, it is preferable that the martensite transformation start temperature is -20°C. More preferably, the martensite transformation start temperature is -40°C or lower, and more preferably -175°C or higher.

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

[0052] In the metal structure before sub-zero treatment according to this embodiment, in addition to austenite and graphite, even if the following metal structures are included in a total area ratio of 10% or less, the effects of the present invention will not be impaired and thus are allowed. The metal structures that may be included are unrecrystallized ferrite, polygonal ferrite, bainite, pearlite, and carbides such as cementite.

[0053] The 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 recovers only by stress relief rather than heating may occur, and there is a possibility that the shape memory effect cannot 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] <Manufacturing method of shape memory steel sheet> The manufacturing method of the shape memory steel sheet according to this embodiment will be described. Steel material In the production of steel materials, molten steel produced by common melting methods can be used. For example, it may be the blast furnace - converter method or the electric furnace method. It may also be molten reduced iron obtained by direct reduction. Secondary refining may be performed on the molten steel. Steel materials, such as steel slabs, are preferably produced by the continuous casting method. In addition, in order to prevent macrosegregation, it is preferable to take segregation prevention measures such as the soft reduction method. However, it is also possible to produce by the ingot method, the thin slab casting method, etc. Also, in addition to the conventional method of cooling the steel material to room temperature once after production and then reheating it, energy - saving processes such as direct - delivery rolling, where the steel material is charged into the heating furnace while still warm without cooling to room temperature or is rolled immediately after slight heat retention, can be applied without problems. Also, the steel material is usually made into a sheet bar by rough rolling under normal conditions. When the heating temperature of the steel material is set low, from the viewpoint of preventing troubles during hot rolling, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling.

[0055] Heating temperature of the steel material Although not particularly limited, the heating temperature of the steel material is preferably in the range of 1100°C or more and 1300°C or less. The precipitates present in the heating stage of the steel material exist as coarse precipitates in the finally obtained steel plate and affect the martensitic transformation. Therefore, it is preferable to redissolve the Ti and Nb - based precipitates that precipitated during casting. Thus, the heating temperature of the steel slab is preferably 1100°C or more. Also, from the viewpoint of scaling off (removing as oxides) defects such as bubbles and segregation on the slab surface layer, reducing cracks and unevenness on the steel plate surface, and achieving a smooth steel plate surface, the heating temperature of the steel material is preferably 1100°C or more. On the other hand, when the heating temperature of the steel material exceeds 1300°C, the scale loss increases with the increase in the oxidation amount, the yield decreases, and the surface quality of the steel plate deteriorates. Therefore, the heating temperature of the steel material is preferably 1300°C or less. More preferably, the heating temperature of the steel material is 1150°C or more, and more preferably 1250°C or less.

[0056] Steel plate temperature on the finish - rolling exit side of hot rolling: 750°C or more and 1000°C or less The steel material after heating is hot-rolled by rough rolling and finish rolling to become a hot-rolled steel sheet. At this time, if the temperature of the steel sheet on the exit side of the finish rolling exceeds 1000°C, the amount of oxide (scale) generated increases rapidly, the interface between the base metal and the oxide becomes rough, and the surface quality after pickling and cold rolling tends to deteriorate. In addition, if there is some remaining scale after pickling, it will have an adverse effect on the shape memory characteristics. Furthermore, the crystal grain size becomes excessively large, the plastic deformation stress decreases, and the shape memory characteristics may not be exhibited. On the other hand, when the temperature of the steel sheet on the exit side of the finish rolling is less than 750°C, the rolling load increases and the rolling load becomes large. In addition, the reduction ratio in the unrecrystallized state of austenite becomes high, abnormal textures develop, the in-plane anisotropy in the final product becomes prominent, not only the material uniformity (material stability) is impaired, but also the shape memory characteristics themselves deteriorate. Therefore, it is necessary to make the temperature of the steel sheet on the exit side of the finish rolling in the hot rolling range from 750°C to 1000°C. Preferably, it is 800°C or higher, and preferably 950°C or lower. The temperature of the steel sheet on the exit side of the finish rolling mentioned here is the measured surface temperature of the steel sheet.

[0057] Pickling of the hot-rolled steel sheet The hot-rolled steel sheet manufactured in this way is pickled as necessary. Since pickling can remove oxides on the steel sheet surface, it is preferably performed to ensure good chemical conversion treatability and plating quality of the final product shape memory steel sheet. When pickling is performed, pickling can be carried out once or divided into multiple times.

[0058] Cold rolling After the hot-rolled steel sheet is coiled and pickled as necessary, cold rolling is performed to obtain a cold-rolled steel sheet. The cold rolling reduction ratio is not particularly limited, but 5% - 80% is preferred.

[0059] Holding at a holding temperature in the temperature range of 900°C or higher for a holding time in the range of 20 s to 3600 s Next, the cold-rolled steel sheet is heat-treated to obtain a shape memory steel sheet. In the heat treatment of the cold-rolled steel sheet, if it is held in a temperature range below 900°C, a large amount of graphite is generated, and furthermore, a predetermined amount of austenite cannot be obtained. As a result, good shape memory characteristics cannot be obtained. On the other hand, if the holding time at a holding temperature in a temperature range of 900°C or higher is less than 20 s, sufficient recrystallization does not occur, and a predetermined amount of austenite cannot be obtained. Therefore, the shape memory characteristics deteriorate. Further, if the holding time at a holding temperature in a temperature range of 900°C or higher exceeds 3600 s, a large amount of graphite is generated, and good shape memory characteristics may not be obtained. Therefore, in the heat treatment of the cold-rolled steel sheet, it is necessary to hold it at a holding temperature in a temperature range of 900°C or higher for a holding time of 20 s or more and 3600 s or less. Although the upper limit of the holding temperature is not limited, from the viewpoint of energy saving, it is preferably 1250°C or lower.

[0060] Note that the heat treatment method may be either a continuous annealing method or a batch annealing method. Further, after the above heat treatment, it is cooled to room temperature, but the cooling method is not particularly specified, and it may be any cooling method such as furnace cooling in batch annealing, air cooling, gas jet cooling, mist cooling, or water cooling in continuous annealing. Also, when pickling treatment is performed, a conventional method may be used.

[0061] Cooling rate in the temperature range of 600°C or higher and 900°C or lower: 50°C / s or higher If the cooling rate in the temperature range of 600°C or higher and 900°C or lower is 50°C / s or higher, graphite generated during cooling can be suppressed. Therefore, stress-induced martensite transformation is promoted, and good shape memory characteristics can be obtained. Also, although the upper limit of the cooling rate in the temperature range of 600°C or higher and 900°C or lower is not particularly specified, from the constraints of production technology, it is preferably 1000°C / s or lower. More preferably, it is 800°C / s or lower.

[0062] In addition, skin pass rolling can be performed on the "shape memory steel plate" of the present embodiment for the purpose of shape correction, adjustment of surface roughness, etc. The reduction ratio of skin pass rolling is preferably in the range of 0.1% or more and 2.0% or less. If it is less than 0.1%, the effect is small and control is difficult, so this becomes the lower limit of the good range. On the other hand, if it exceeds 2.0%, the productivity will be significantly reduced, so this is set as the upper limit of the good range. Note that skin pass rolling may be performed online or offline. Also, the skin pass with the target reduction ratio may be performed at once, or it may be performed in several steps. In addition, various coating treatments such as resin and oil-grease coating can also be applied.

Example

[0063] Steel having the component composition shown in Table 1, with the balance being Fe and inevitable impurities, was melted in a converter and made into a slab by the continuous casting method. After reheating the obtained slab to 1250 °C, a steel plate with a thickness of 1.0 mm or more and 1.8 mm or less was obtained under the conditions shown in Table 2. The steel structure of the cross-section of the obtained steel plate was observed by the above method, and the shape memory characteristics were evaluated by a stress loading-unloading cycle tensile test, and the results are shown in Table 3. In Table 2, "finish rolling exit side temperature" represents the surface temperature of the steel plate on the exit side of the finish rolling in hot rolling. Also, in the cold rolled sheet annealing treatment column in Table 2, "cooling rate" represents the cooling rate in the temperature range of 600 °C or more and 900 °C or less during cooling from the holding temperature to room temperature.

[0064]

Table 1

[0065]

Table 2

[0066]

Table 3

[0067] In the stress loading-unloading cycle tensile test, a JIS No. 5 test piece was sampled so that the tensile direction was orthogonal to the rolling direction of the steel plate. Tension was applied up to a load strain of 5% at a temperature of -50°C, and after unloading, the distance between the gauge points was measured. Then, the test piece was heated up to 800°C, cooled to room temperature, and the distance between the gauge points was measured again. Using the measurement results, the shape recovery strain due to heating was calculated, and the shape memory characteristics were evaluated. Also, the shape memory characteristics were considered good when the shape recovery strain was 0.95% or more.

[0068] Under the test conditions shown as invention examples in the remarks column, shape memory steel plates with excellent shape memory characteristics were obtained in all cases. On the other hand, under the test conditions shown as comparative examples in the remarks column, the shape memory characteristics were inferior.

Industrial Applicability

[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 only by heating 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 industry, energy, medical, and electricity, and the industrial utility value is extremely large.

Claims

1. In mass percent, 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 The balance is Fe and unavoidable impurities, The metal structure before the sub-zero treatment has an area ratio at a position of 1 / 4 of the plate thickness of 80% or more of austenite, 15% or less of graphite, and a total of carbides including unrecrystallized ferrite, polygonal ferrite, bainite, pearlite, and cementite, which is 10% or less in total, and is 100% in total; The sub-zero treatment transforms a portion of the austenite into martensite, The martensite contains {112} twins therein, 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 kept in liquid nitrogen for two hours. Shape memory steel plate.

2. The composition further includes, in mass%, 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 The shape memory steel sheet according to claim 1, containing at least one element selected from the following:

3. The 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 hot rolling process in which, after heating the steel material having the composition according to claim 1 or 2, or directly after producing the steel material, the steel material is hot rolled to obtain a hot rolled steel sheet, the steel sheet temperature on the finish rolling delivery side is set to a range of 750°C to 1000°C. A cold rolling process of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; Then, the cold-rolled steel sheet is 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. The method for producing a shape memory steel plate, wherein the obtained shape memory steel plate has the metal structure according to claim 1.

5. The method for producing a shape memory steel plate according to claim 4, wherein in the heat treatment step, when cooling from the holding temperature, an average cooling rate in a temperature range of 900°C or less and 600°C or more is 50°C / s or more.

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