Steel strip and method for manufacturing same

Abstract

Provided are: a method for manufacturing a steel strip, wherein quenching is easily performed even in local quenching and warpage during local quenching in the manufacturing can be suppressed; and a steel strip.　A quench-hardened phase having a hardness of at least 600 HV is formed by radiating a laser from the side of a strip-shaped quenching material 10, which has a component composition containing, in terms of mass%, 0.50-0.85% of C, 0.1-0.95% of Si, 0.1-1.5% of Mn, 0.01-0.80% of Ni, 8.0-14.0% of Cr, at most 3.0% of Mo and W independently or Mo+0.5W in combination, and at most 0.05% (excluding 0%) of N, with the remainder consisting of Fe and impurities and has a hardness of at most 360 HV and a thickness of at most 1.0 mm, toward one end portion 11 of the quenching material in the width direction. Consequently, a steel strip having the hardened phase and a softened phase, which is provided at the other end portion 13 of the steel strip in the width direction while maintaining a hardness of at most 360 HV, is obtained.

Description

Steel strip and method for producing the same

[0001] The present invention relates to a steel strip and a method for producing the same.

[0002] Conventionally, as steel for blades such as cutters and razors, high-carbon steel equivalent to SK1 and martensitic stainless steel containing 12 to 13% of Cr have been used. The former can obtain high hardness through quenching and tempering heat treatment, but has poor corrosion resistance, so it is only used for light-duty applications. On the other hand, the latter martensitic stainless steel can not only obtain high hardness through quenching and tempering, but also has excellent corrosion resistance, so it is not easy to rust and is widely used in general.

[0003] Quenching of high-carbon steel and martensitic stainless steel is mostly performed by a heat treatment method in which the steel is inserted into a furnace heated to a constant temperature and then rapidly cooled, but there are also induction hardening, laser quenching and other methods that perform quenching by locally heating a part of a steel product. For example, in Patent Document 1, a passage for flowing liquid nitrogen is provided inside a cooling plate, the cooling plate is provided with a narrow passage through which most of each blade body of a razor blade in which individual blade bodies are continuous in a tape shape passes, and a quenching apparatus for a razor blade is proposed, which comprises a cooling plate having a portion exposing the blade edge of the razor blade passing therethrough, and a laser irradiation apparatus for heating the exposed blade edge.

[0004] Japanese Unexamined Patent Publication No. Sho 62-204788

[0005] The above-described local quenching performs quenching by heating only a part of the product instead of the entire furnace, so it has high manufacturing rationality. However, when applied to thin sheets with small thickness, self-cooling is difficult and quenching is not easily achieved, and warpage caused by shrinkage and expansion between the quenched portion and the base material is a problem. The quenching apparatus described in Patent Document 1 has a cooling device using liquid nitrogen as a cooling medium, so it is a useful invention that can solve the problem of quenching by self-cooling. However, since the laser is irradiated in the thickness direction of the razor blade, there is no description about the problem of warpage caused by shrinkage and expansion between the quenched portion and the base material, leaving room for further study.

[0006] Therefore, the object of the present invention is to provide a method for manufacturing steel strips and a steel strip that allows for easy hardening even during localized hardening and suppresses warping during localized hardening during manufacturing.

[0007] To solve the above problems, the inventors investigated a method to simultaneously address the self-cooling and warping of steel strips. As a result, they discovered that by precisely adjusting the content of C and N, as well as Cr and Mo, which are the main components of the steel strip, hardening can be easily achieved even with short-duration laser irradiation, and by directing the laser irradiation direction to the steel strip from one end in the width direction, the above two problems can be solved simultaneously, leading to the present invention.

[0008] That is, one aspect of the present invention is a method for manufacturing a steel strip, the method comprising the steps of: preparing a strip-shaped material for quenching with a thickness of 1.0 mm or less having a component composition of, in mass%, C: 0.50 to 0.85%, Si: 0.1 to 0.95%, Mn: 0.1 to 1.5%, Ni: 0.01 to 0.80%, Cr: 8.0 to 14.0%, Mo and W individually or in combination as Mo + 0.5W: 3.0% or less, N: 0.05% or less (excluding 0%), the remainder being Fe and impurities, and having a hardness of 360 HV or less; and a laser quenching step of irradiating the material for quenching from the side toward one end in the width direction of the material for quenching to form a quenched hardened phase with a hardness of 600 HV or more.

[0009] Another aspect of the present invention is a steel strip having a composition in mass%, C: 0.50 to 0.85%, Si: 0.1 to 0.95%, Mn: 0.1 to 1.5%, Ni: 0.01 to 0.80%, Cr: 8.0 to 14.0%, Mo and W individually or in combination as Mo + 0.5W: 3.0% or less, N: 0.05% or less (excluding 0%), with the remainder being Fe and impurities, comprising a hardened phase with a hardness of 600 HV or more and a softened phase with a hardness of 360 HV or less, wherein the hardened phase is provided at one end in the width direction of the steel strip and the softened phase is provided at the other end in the width direction of the steel strip, the widthwise length of the hardened phase is 1 to 10% of the width of the steel strip and the thickness is 1.0 mm or less.

[0010] According to the present invention, it is possible to obtain a steel strip that is easily hardened even in localized hardening and in which warping during localized hardening is suppressed.

[0011] This is a schematic diagram illustrating one embodiment of the steel strip manufacturing method according to the present invention. This is a schematic diagram illustrating the steel strip manufacturing method in a comparative example. This is a photograph of the appearance of the steel strip according to the present invention. This is a photograph of the appearance of the steel strip according to the comparative example.

[0012] Hereinafter, an embodiment of the steel strip and its manufacturing method according to the present invention will be described with reference to the attached drawings. Note that the drawings are not necessarily to scale, and some features may be exaggerated in order to clearly illustrate and explain the characteristics of the exemplary embodiment. Furthermore, the present invention is not limited to the embodiment described herein, and can be appropriately combined and improved without departing from the technical spirit of the invention.

[0013] First, the reason for limiting the component composition of the steel strip according to the present invention will be explained. The hardening material used in the manufacture of the steel strip has a similar component composition. The applications of the stainless steel according to the present invention are not particularly limited, but it is preferable to apply it to applications that require thin sheets and high hardness characteristics, for example, to cutlery applications.

[0014] C: 0.50–0.85% Carbon (C) is an important element that determines the hardness of martensite formed during quenching, as it dissolves from carbides into the matrix at the austenitization temperature during quenching. Here, carbon in steel is divided into carbon that dissolves in the matrix and carbon that precipitates as carbides, and the ratio of these is determined by the interaction with Cr, Mo, W, and N, so it is important that Cr, Mo, W, and N are also within the composition range described later. If the amount of carbon is low, the hardness of the quenched hardened phase described later will be low, so the lower limit of the amount of carbon is set at 0.50%. The preferred lower limit is 0.53%, and the more preferred lower limits are 0.55%, 0.57%, and 0.60%. On the other hand, if there is too much carbon, there will be an excessive amount of carbides, and the carbides will also tend to be coarse, which may cause cracks to form when processing into steel strip, leading to fracture failure. Therefore, the upper limit of the amount of carbon is set at 0.85%. The preferred upper limit is 0.83%, and the more preferred upper limit is 0.81%.

[0015] Si: 0.1–0.95% Si is used as a deoxidizing agent during the refining of steel strips and also dissolves in the steel, suppressing softening during low-temperature tempering. Therefore, the lower limit is set at 0.1%. On the other hand, excessive content significantly increases the hardness of the steel strip before quenching and tempering, so adding too much will increase the hardness of the softened phase described later. For this reason, the upper limit of Si is set at 0.95%. A preferred upper limit is 0.90%, a more preferred upper limit is 0.70%, and an even more preferred upper limit is 0.50%.

[0016] Mn: 0.1–1.5% Like Si, Mn acts as a deoxidizing agent during refining, dissolves in the matrix, and enhances hardenability. If the amount of Mn is too low, the hardenability of the steel decreases, and there is a possibility that the steel may not harden at all, especially in the center of the wall thickness, so the lower limit is set at 0.1%. On the other hand, excessive Mn content reduces hot workability, so the upper limit is set at 1.5%. A preferred upper limit is 1.2%, and a more preferred upper limit is 1.0%.

[0017] Ni: 0.01–0.80% Ni is an effective element for improving corrosion resistance to non-oxidizing acids such as sulfuric acid, and is also effective for increasing the toughness of the steel strip. At least 0.01% must be added to obtain this effect. On the other hand, Ni dissolves in the matrix and significantly increases the hardness of the steel strip before quenching and tempering, so adding too much will increase the hardness of the softened phase described later. For this reason, the upper limit of the Ni amount is set at 0.80%. The preferred upper limit of Ni is 0.50%, and the more preferred upper limit of Ni is 0.30%.

[0018] Cr: 8.0–14.0% Cr is an element necessary for forming a strong insulated film on steel and obtaining excellent corrosion resistance. To exhibit this corrosion resistance, it is necessary for the steel to contain at least 8.0% Cr. The preferred lower limit for Cr is 8.3%, the more preferred lower limit is 8.5%, and the even more preferred lower limit is 8.7%. On the other hand, an excessive amount of Cr leads to a decrease in the martensitic transformation initiation temperature (Ms point), and the increase in retained austenite reduces the hardness of the quenched hardened phase described later. For this reason, the upper limit for Cr is set at 14.0%. The preferred upper limit for Cr is 13.7%, and the more preferred upper limit for Cr is 13.5%.

[0019] Mo and W, either individually or in combination, should be present in an amount of Mo + 0.5W: 3.0% or less. Mo and W have similar effects and are defined by (Mo + W / 2) based on their atomic weight relationship. Mo and W can be present individually or in combination. Mo and W have a high effect in stabilizing passivation and are effective elements in improving corrosion resistance by increasing the pitting potential in chloride solutions. On the other hand, excessive addition of Mo and W drastically reduces workability during hot working and, for similar reasons as Cr, lowers the hardness of the quenched hardened phase described later. Therefore, the amount of Mo + 0.5W should be 3.0% or less. The upper limit for a preferred amount of Mo + 0.5W is 2.5%, the upper limit for a more preferred amount of Mo + 0.5W is 2.3%, and the upper limit for a particularly preferred amount of Mo + 0.5W is 2.1%. While it is acceptable to omit Mo and W (0%), it is preferable to include 0.01% or more if you wish to obtain the aforementioned corrosion resistance improvement effect. A more preferable lower limit is 0.5%.

[0020] N: 0.05% or less (excluding 0%). N is an essential element because it dissolves in the martensitic structure and improves the hardness and corrosion resistance of the quenched hardened phase. To reliably obtain the effect of N, it is preferable to include 0.01% or more. However, if there is too much N, for the same reasons as Cr, it will reduce the hardness of the quenched hardened phase described later. Therefore, the N content should be 0.05% or less. The preferred upper limit is 0.04% and 0.03%.

[0021] The steel strip according to the present invention may contain the following elements. For any of the contained elements, the lower limit for each element is zero percent (below the level of no additive).

[0022] Co: 0.5% or less. Co is an element that dissolves in martensite and increases the resistance to tempering and softening. On the other hand, for applications that may come into contact with the human body, such as razor blades, it may be included in the steel of this embodiment in a range of 0.5% or less, as it may cause metal allergies.

[0023] In this embodiment, components other than those mentioned above are Fe and impurities. Examples of impurity elements include unavoidable impurities such as P, S, Al, Ti, and O, but they may be included as long as they do not hinder the effects of the present invention, as shown below: P ≤ 0.04%, S ≤ 0.03%, Al ≤ 0.1%, Ti ≤ 0.1%, and O ≤ 0.05%.

[0024] In addition to the unavoidable impurities mentioned above, other elements may be included as impurities, as long as they do not hinder the effects of the present invention. The elements included as impurities vary greatly depending on the raw materials used in steel strip production, such as iron ore and iron scrap, so examples of elements are not given here. However, the total content of elements included as impurities is preferably 0.2% or less.

[0025] Next, an embodiment of the steel strip manufacturing method according to the present invention will be described. As shown in Figure 1, the manufacturing method of this embodiment includes a laser quenching step in which a laser is irradiated from the side of a strip-shaped quenching material 10 having the above-described component composition toward one end 11 in the width direction of the quenching material 10 to form a quenched hardened phase. Note that Figure 1 shows the quenching material 10 with its length extending from back to front. As shown in Figure 1, multiple quenching materials 10 may be bundled together in the thickness direction and fixed by jigs 20a and 20b for simultaneous processing, or a single quenching material 10 may be processed by sandwiching it between jigs 20a and 20b. A quenched hardened phase is formed on one end 11 in the width direction of the quenching material 10 that has been irradiated with the laser L.

[0026] The thickness of the hardening material 10 is 1.0 mm or less, preferably 0.5 mm or less, more preferably 0.2 mm or less, and even more preferably 0.1 mm or less. The lower limit of the thickness of the hardening material 10 is not particularly limited, but for example it can be 0.05 mm. The length of the hardening material 10 is not particularly limited, and it may be prepared in a coil shape wound around a core, the hardening material 10 may be flattened and irradiated with the laser L, and then wound around the core again to form a coil shape, or it may be cut to a predetermined length to form, for example, a thin plate shape. Alternatively, the hardening material 10 may be cut to a predetermined length and then irradiated with the laser L.

[0027] The laser L can be used to irradiate the hardening material 10 with the laser L by fixing the hardening material 10 and moving the laser L side, so that the laser L is irradiated along the length direction of the hardening material 10 to one end 11 in the width direction. Alternatively, the laser L side may be fixed and the hardening material 10 may be moved in the length direction.

[0028] In this way, by irradiating the hardened material 10 from the side to one end 11 in the width direction of the hardened material 10 to form a hardened phase, the warping of the resulting steel strip can be minimized. In contrast, as shown in Figure 2, when the hardened material 10 is clamped between jigs 20a and 20b so that the surface 12 at one end in the width direction of the hardened material 10 is exposed, and the laser L is irradiated from the thickness direction of the hardened material 10 to the surface 12 at one end in the width direction of the hardened material 10, a temperature difference occurs between the surface irradiated by the laser L and the opposite surface. The temperature of the laser-irradiated surface is high, and more carbides in the hardened material 10 dissolve, while the temperature of the opposite surface is low, and carbides do not dissolve easily. Therefore, not only is there a temperature difference between the laser-irradiated surface and the opposite surface, but the difference in carbide dissolution causes a difference in the amount of carbon dissolved in the matrix, which in turn causes a difference in the amount of expansion during martensitic transformation, resulting in warping of the steel strip. Therefore, in order to suppress the occurrence of a temperature difference between the front and back surfaces of the hardening material 10 due to the irradiation of the laser L, in this embodiment, as shown in Figure 1, the laser is irradiated from the side of the hardening material 10 to one end 11 in the width direction of the hardening material 10.

[0029] Here, "irradiating the hardening material with a laser from the side" is not limited to irradiating the hardening material 10 from the side in a direction parallel to the front or back surface of the hardening material 10. The laser irradiation angle may be changed as long as it does not cause warping due to the temperature difference between the front and back surfaces of the hardening material 10 as described above. For example, the laser irradiation angle can be within ±10° of the direction parallel to the front or back surface of the hardening material 10.

[0030] The matrix of the hardening material 10 having the above-described component composition is a steel mainly composed of the ferrite phase, or in other words, a steel having a ferrite structure. The hardness of the hardening material 10 is 360 HV or less. As described above, when the laser L is irradiated to one end 11 in the width direction of the hardening material 10, a hardened phase is formed, and this hardened phase has a martensite structure. Here, "mainly ferrite phase" means that when the microstructure of the steel is observed at room temperature, the proportion of the ferrite phase in the matrix excluding carbides is 90% or more in terms of area. Hereafter, the hardened phase will also be simply referred to as the "hardened phase," and the part of the matrix that is mainly composed of the ferrite phase and in which the hardened phase has not been formed (laser hardening has not been performed) will be referred to as the softened phase. Although a hardened phase with a hardness of 600 HV or more is formed by laser hardening, the hardness of the softened phase remains 360 HV or less.

[0031] Next, an embodiment of the steel strip according to the present invention will be described. The steel strip of this embodiment has the above-described component composition and comprises a hardened phase with a hardness of 600 HV or more at one end in the width direction of the steel strip and a softened phase with a hardness of 360 HV or less at the other end in the width direction of the steel strip. Such a steel strip can be obtained by the manufacturing method described above. With this configuration, the steel strip of the present invention is suitable for applications that require high hardness and wear resistance in certain areas, such as cutting tools.

[0032] If the entire surface of the steel strip is made of a hardening phase, the workability of the steel strip, such as winding and bending, deteriorates drastically, so it is important to have both a hardening phase and a softening phase. On the other hand, if the entire surface of the steel strip is made of a softening phase, the hardness and wear resistance of the steel strip will be low, and its durability will be extremely low, so it is also important to have both a hardening phase and a softening phase. In this invention, in order to balance the workability and durability of the steel strip as described above, the widthwise length of the hardening phase is set to be in the range of 1 to 10% of the steel strip width. The preferred upper limit is 7%, and more preferably 5%.

[0033] In this embodiment, the hardened phase of the steel strip has a hardness of 600 HV or higher, and the softened phase has a hardness of 360 HV or lower. This makes it possible to achieve both good workability and durability. The preferred lower limit of hardness for the hardened phase is 610 HV, a more preferred lower limit is 620 HV, and an even more preferred lower limit is 630 HV. There is no particular upper limit to the hardness of the hardened phase, but if the hardness is too high, the ductility of the steel strip will decrease, increasing the risk of breakage during winding or bending, so it may be set to, for example, 900 HV.

[0034] Furthermore, the preferred upper limits for the hardness of the softened phase are 350 HV, 340 HV, and 330 HV. The lower limit for the hardness of the softened phase is not particularly limited, but if the hardness is too low, the tensile strength will inevitably be low, raising concerns that the steel strip may break due to inability to withstand the tension during winding. Therefore, it may be set to, for example, 200 HV or higher.

[0035] In this embodiment, it is preferable to form a quenched hardened phase continuously for at least 30 mm in the length direction of the steel strip. Furthermore, the hardened phase may be omitted in parts that are not used as a product (for example, cut sections).

[0036] The thickness of the steel strip in this embodiment is 1.0 mm or less, similar to the specifications for the hardening material described above. If the thickness of the steel strip is too large, the strength of the steel strip will increase, requiring a very large force when winding it. A preferred thickness is 0.50 mm or less, a more preferred thickness is 0.20 mm or less, and an even more preferred thickness is 0.10 mm or less. There is no particular lower limit to the thickness of the steel strip, but for example, it may be 0.05 mm.

[0037] A 1.6 mm thick hot-rolled material having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was annealed in a batch-type annealing furnace, and then cold-rolled and annealed to obtain a cold-rolled material with a thickness of 0.07 mm. This was then cut into strips with a width of 6 to 10 mm and a length of 100 mm to prepare the material for quenching the steel strips of the present invention (alloys No. A to C). The chemical composition can be analyzed in accordance with the analytical methods specified in JIS and ASTM. C, S, N, and O can be analyzed by combustion-infrared absorption spectroscopy and inert gas fusion-infrared absorption spectroscopy in accordance with ASTM-E1019, and other elements can be analyzed by emission spectroscopy and X-ray fluorescence spectroscopy in accordance with JIS-G0320 or G0321.

[0038] Then, a laser was irradiated onto one end of the steel strip in the width direction to form a hardened phase, and steel strips of the present invention example (steel strip No. A1-A2, B1-B2, C1-C2) and comparative example (steel strip No. A3-A4, B3-B4, C3-C4) were obtained as shown in Table 2. As shown in Table 2, irradiation No. 1-2 is when the laser was irradiated from the side of the material to be hardened, and irradiation No. 3-4 is when the laser was irradiated from the thickness direction of the material to be hardened. In addition, the amount of heat input by the laser differs for irradiation No. 1-4. The amount of heat input for irradiation No. 1 is less than for irradiation No. 2, and the amount of heat input for irradiation No. 3 is less than for irradiation No. 4.

[0039]

[0040]

[0041] The hardness of the steel strip was measured by cutting a hardness test specimen along the width direction of the steel strip after laser hardening as described above, and measuring the surface hardness of the cross-sectional micro in the width direction on a mirror-polished surface. The hardness was measured according to JIS-Z2244-1:2024 "Vickers hardness test," measuring the surface hardness of the hardened and softened phases of the steel strip. The load was set to 50 gf, and the average value was calculated from three measured points. The evaluation results are shown in Table 3. Furthermore, the steel strip of the present invention example after laser hardening is shown in Figure 3, and the steel strip of the comparative example after laser hardening is shown in Figure 4.

[0042]

[0043] From the results in Table 3, in the examples of the present invention (Nos. A1 to A2, B1 to B2, C1 to C2), the hardness of the quenched hardened phase is 600 HV or more, and the hardness of the softened phase is 360 HV or less. Further, the length of the quenched hardened phase in the width direction is 1.7 to 6.1% relative to the width of the steel strip, and as shown in Figure 3, the steel strip had a good appearance without warpage. On the other hand, in the comparative examples (Nos. A3 to A4, B3 to B4, C3 to C4), although there are portions where the hardness of the quenched hardened phase is 600 HV or more, the comparative examples are unfavorable because there are scattered portions where no quenched hardened phase is formed due to humping (that is, softened phases with a hardness of 360 HV or less). Further, as shown in Figure 4, significant warpage occurred in the length direction. This confirms that compared with conventional examples, the examples of the present invention can provide a favorable steel strip without warpage while including a high-hardness quenched hardened phase and a low-hardness softened phase.

[0044] 10 Quenching raw material 11 One end portion in the width direction 12 Surface of quenching raw material 13 Other end portion in the width direction 21 Jig

Claims

1. A method for manufacturing a steel strip, comprising the steps of: preparing a strip-shaped material for quenching with a thickness of 1.0 mm or less having a composition in mass%, of C: 0.50 to 0.85%, Si: 0.1 to 0.95%, Mn: 0.1 to 1.5%, Ni: 0.01 to 0.80%, Cr: 8.0 to 14.0%, Mo and W individually or in combination as Mo+0.5W: 3.0% or less, N: 0.05% or less (excluding 0%), the remainder being Fe and impurities, and having a hardness of 360 HV or less; and laser quenching, wherein a laser is irradiated from the side of the material for quenching toward one end in the width direction of the material for quenching to form a quenched hardened phase with a hardness of 600 HV or more.

2. A steel strip having a composition in mass%, C: 0.50-0.85%, Si: 0.1-0.95%, Mn: 0.1-1.5%, Ni: 0.01-0.80%, Cr: 8.0-14.0%, Mo and W individually or in combination as Mo+0.5W: 3.0% or less, N: 0.05% or less (excluding 0%), with the remainder being Fe and impurities, comprising a hardened phase with a hardness of 600 HV or more and a softened phase with a hardness of 360 HV or less, wherein the hardened phase is provided at one end in the width direction of the steel strip and the softened phase is provided at the other end in the width direction of the steel strip, and the thickness of the steel strip is 1.0 mm or less, with the widthwise length of the hardened phase being 1-10% of the width of the steel strip.

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