Steel strip and method for manufacturing same

Abstract

Provided are a method for manufacturing a steel strip having excellent cold workability, and a steel strip that can contribute to improvement in handleability while having excellent carbide density.　Provided are a steel strip and a method for manufacturing a steel strip, the method comprising: a continuous annealing step for continuously annealing a material for cold rolling having a component composition containing, by mass%, 0.55 to 0.95% C, 0.1 to 0.95% Si, 0.1 to 1.5% Mn, 8.0 to 11.0% Cr, and Mo or W alone or in combination such that (Mo + W / 2) is 0.5 to 3.0%, with the remainder consisting of Fe and unavoidable impurities, at a temperature higher than or equal to an Ac1 transformation point to obtain a continuously annealed material; and a cold rolling step for subjecting the continuously annealed material to at least one time each of cold rolling and stress-relief annealing, wherein the continuously annealed material after the continuous annealing step has a hardness increase of 140 HV or less when cold-rolled at a rolling reduction of 50%.

Description

Steel strip and method for manufacturing the same

[0001] This invention relates to a steel strip and a method for manufacturing the same.

[0002] Currently, martensitic stainless steel containing 12.0% to 14.0% by mass of Cr, which is widely used in industrial parts and other applications, can achieve a high hardness exceeding 600 HV through quenching and tempering heat treatment, and is used, for example, as steel for cutlery. Furthermore, martensitic stainless steel is superior to high-carbon steel in terms of rust prevention and corrosion resistance.

[0003] The martensitic stainless steel described above is typically manufactured by a combination of hot rolling, cold rolling, and annealing, and is supplied to the next process as a strip of steel. In the next process, after cold working, it undergoes heat treatment such as quenching and tempering in a continuous furnace, as well as surface treatment, to become the final product.

[0004] The microstructure of the martensitic stainless steel described above after annealing is a state in which carbides are dispersed within a ferrite structure. The particle size and distribution of these carbides have a significant impact on the workability and the properties of the part after heat treatment.

[0005] Many proposals have been made for martensitic stainless steels as described above. For example, in Patent Document 1, the applicant of this application has proposed a stainless steel for razors in which the number of carbides has been increased and the hardenability has been dramatically improved. Patent Document 1 describes a material consisting of more than 0.55 mass% and less than or equal to 0.73 mass% of C, less than or equal to 1 mass% of Si, less than or equal to 1 mass% of Mn, 12 mass% to 14 mass% of Cr, with the remainder being Fe and impurities, and having a carbide number density of 140 to 600 particles / 100 μm in a continuous furnace annealing state. ２ A stainless steel for razors with excellent short-time hardenability is disclosed. The carbide number density shown in Patent Document 1 represents the carbide number density of steel that has been annealed by inserting a stainless steel strip for razors into a continuous furnace set to Ac1 or higher, which is the transformation temperature of the steel, prior to or during cold rolling.

[0006] Furthermore, Patent Document 2 describes a method for manufacturing steel for cutting tools having a metallic composition consisting of 0.55% to 0.80% by mass of C, 1.0% or less by mass of Si, 1.0% or less by mass of Mn, 12.0% to 14.0% by mass of Cr, 1.0% or less by mass of Mo, 1.0% or less by mass of Ni, with the remainder being Fe and unavoidable impurities, wherein a cold-rolling material having the metallic composition is subjected to batch annealing at a temperature range of 500°C to less than 720°C for 3 to 30 hours. The process includes a batch annealing step to obtain a batch annealed material, a continuous annealing step to obtain a continuously annealed material by continuously annealing the batch annealed material, which has been heated to above the Ac1 transformation point of the metal composition, for 5 to 30 minutes after the batch annealing step, and a cold rolling step to cold roll the continuously annealed material after the continuous annealing step, wherein the continuous annealing step and the cold rolling step are performed at least once, and the carbide content in the ferrite structure of the steel for cutting tools after the cold rolling step is 100 μm. ２ A method for manufacturing steel for blades has been proposed in which the number of particles in the region is greater than 200 but less than or equal to 1000.

[0007] Japanese Patent Publication No. 5-39547, International Publication No. 2014-162997

[0008] The stainless steel for razors disclosed in Patent Document 1, mentioned above, achieves excellent hardenability by dramatically increasing the carbide number density through the mandatory application of continuous annealing in a specific temperature range. Furthermore, Patent Document 2 further improves the carbide number density by repeatedly performing cold rolling and annealing above the Ac1 transformation point multiple times, following annealing above the Ac1 transformation point.

[0009] While Patent Documents 1 and 2, mentioned above, dramatically increase the carbide number density to obtain excellent hardenability, there are concerns that excessively high carbide numbers can lead to a decrease in cold-rollability, leaving room for improvement. Furthermore, as the thickness of the steel strip decreases (for example, to a thickness of 0.2 mm or less), handling during cold working, winding, and transportation becomes more difficult (handling performance decreases), so improvement in handling performance is also desired. In other words, to improve cold workability, for example, when using steel strip as a material to make a cutting tool, it is necessary to maintain the required carbide number density while improving workability during cold rolling. However, methods for achieving both of these are not disclosed, leaving room for further investigation.

[0010] The object of the present invention is to provide a method for manufacturing steel strips with good cold workability while adjusting to an appropriate carbide number density, and to provide steel strips that contribute to improved handling while adjusting to an appropriate carbide number density.

[0011] The inventors of the present invention discovered that by heat-treating a cold-rolling material of a predetermined metal composition in a batch annealing process and a continuous annealing process, and then performing cold rolling and stress-relieving annealing one or more times, the carbide number density is improved, resulting in excellent hardenability and excellent cold workability, and thus arrived at the present invention.

[0012] In other words, one aspect of the present invention is a method for manufacturing a steel strip, comprising: a preparation step of preparing a material for cold rolling having a component composition of mass%, C: 0.55 to 0.95%, Si: 0.1 to 0.95%, Mn: 0.1 to 1.5%, Cr: 8.0 to 11.0%, Mo and W alone or in combination (Mo + W / 2): 0.5 to 3.0%, with the remainder being Fe and unavoidable impurities; a continuous annealing step of performing continuous annealing on the material for cold rolling at a temperature above the Ac1 transformation point to obtain a continuously annealed material; and a cold rolling step of performing cold rolling and stress-relieving annealing on the continuously annealed material at least once each, wherein the continuously annealed material after the continuous annealing step has a hardness increase of 140 HV or less when cold-rolled at a reduction ratio of 50%.

[0013] Another aspect of the present invention is a steel strip having a composition by mass%, containing C: 0.55-0.95%, Si: 0.1-0.95%, Mn: 0.1-1.5%, Cr: 8.0-11.0%, Mo and W individually or in combination (Mo + W / 2): 0.5-3.0%, with the remainder being Fe and unavoidable impurities, and having an average equivalent circle diameter of carbides in the cross-sectional structure of 0.10-0.30 μm and a carbide number density of 280-530 particles / 100 μm ２ The hardness is 280-400 HV.

[0014] According to the present invention, it is possible to provide a method for manufacturing steel strips with excellent cold workability. Furthermore, it is possible to obtain steel strips that have excellent carbide density while also contributing to improved handling properties.

[0015] This is a scanning electron microscope image showing the cross-sectional structure of a steel strip according to an example of the present invention.

[0016] One embodiment of the present invention will be described below. However, 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. First, in the present invention, the alloy composition was considered in order to simultaneously achieve the above-mentioned "optimization of carbide number density," "good cold workability," and "improvement of handling properties." The reasons for limiting the component composition of the steel strip according to the present invention will be explained below. The steel strip of the present invention can be used, for example, for cutlery applications and automobile parts applications. Normally, when used for cutlery applications or automobile parts applications, a steel strip in which carbides are dispersed in a ferrite structure is used as the material, and it is quenched and tempered to adjust it to a martensite structure. C: 0.55 to 0.95% C is an important element that not only provides the appropriate carbide number density as defined in the present invention, but also solid dissolves from the carbides into the matrix at the austenitization temperature during quenching, and determines the hardness of the martensite produced by quenching. To obtain sufficient hardness for a steel strip, and to achieve a carbide density of 280 particles / μm in the ferrite structure, ２To increase the carbon content, a carbon content of 0.55% or more by mass is required. Furthermore, depending on the balance of carbon and chromium content, large eutectic carbides may crystallize during solidification. In steel strips, especially for blades such as kitchen knives and razor blades, which are about 0.1 mm thick and have sharp edges, the presence of such large carbides can cause chipping. For this reason, considering the balance with the chromium content, the upper limit of the carbon content is set at 0.95%. The preferred lower limit of the carbon content is 0.60%, more preferably 0.63%, and even more preferably 0.65%. The preferred upper limit of the carbon content is 0.90%, more preferably 0.85%, even more preferably 0.82%, and particularly preferably 0.75%.

[0017] Si: 0.10–0.95% 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 0.10%. On the other hand, Si increases the dislocation density by dissolving in the ferrite matrix of the steel strip, hindering dislocation movement. For this reason, excessive Si content reduces the cold workability during cold rolling. For this reason, the upper limit of Si content is set at 0.95%. A preferred upper limit is 0.90%, a more preferred upper limit is 0.85%, an even more preferred upper limit is 0.80%, and a particularly preferred upper limit is 0.70%.

[0018] Mn: 0.1–1.5% Like Si, Mn acts as a deoxidizing agent during refining, dissolving in the matrix and enhancing hardenability. If the amount of Mn is too low, the hardenability of the steel strip decreases, and there is a possibility that the steel may not harden at all, especially in the center of the thickness of the steel strip, 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%, a more preferred upper limit is 1.0%, and even more preferably 0.80%.

[0019] Cr: 8.0–11.0% Cr is an important element that not only forms a non-conductive film on steel, providing excellent corrosion resistance, but also increases hardness for superior handling. In other words, to obtain excellent handling, it tends to be effective to reduce the Cr content compared to general martensitic stainless steel. To exhibit this corrosion resistance, it is necessary for the steel to contain at least 8.0% Cr. The lower limit of preferred Cr is 8.3%, the lower limit of more preferred Cr is 8.5%, and the lower limit of even more preferred Cr is 8.8%. Furthermore, the Cr contained in the steel strip either dissolves in the ferrite matrix or precipitates as Cr carbides. If there is an excessive amount of Cr, it increases the dislocation density by increasing the solid solution in the ferrite matrix, hindering dislocation movement and thus reducing cold workability. For this reason, the upper limit of Cr should be 11.0% to exhibit excellent cold workability. The preferred upper limit for Cr is 10.5%, and the more preferred upper limit for Cr is 10.2%.

[0020] Mo and W, individually or in combination, (Mo + W / 2): 0.5 to 3.0% Mo and W have similar effects and are defined by (Mo + W / 2) based on their atomic weight relationship. Mo and W can be included individually or in combination. Mo and W are elements that have a high effect in stabilizing the passivation of Cr and are effective in improving corrosion resistance by making the pitting potential in chloride solutions noble, so it is necessary that they be included at least 0.5% in (Mo + W / 2) proportions. The preferred lower limit of (Mo + W / 2) is 0.7%, more preferably 0.8%, and even more preferably 0.9%. On the other hand, Mo and W, like Si, solid-solve in the ferrite matrix of the steel strip, increasing the dislocation density, hindering dislocation movement, and reducing cold workability, so the upper limit of (Mo + W / 2) is 3.0%. The preferred upper limit for (Mo + W / 2) is 2.8%, more preferably 2.5%, and even more preferably 2.3%.

[0021] The steel strip according to the present invention may contain the following elements. For any element to be included, the lower limit for each element is zero percent (below the level of no additive). 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 knife blades and razor blades, it may cause metal allergies, so it may be included in the steel strip of this embodiment in a range of 0.5% or less.

[0022] In this embodiment, components other than those mentioned above are Fe and unavoidable impurities. Examples of unavoidable impurity elements include P, S, Ni, Cu, V, Nb, Al, Ti, N, 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%, Ni ≤ 0.5%, Cu ≤ 0.5%, V ≤ 0.1%, Nb ≤ 0.1%, Al ≤ 0.1%, Ti ≤ 0.1%, N ≤ 0.05%, O ≤ 0.05%.

[0023] Next, the method for manufacturing steel strip according to the present invention will be described. The present invention allows for the production of continuously annealed material from a cold-rolling material having the above-described component composition using two methods.

[0024] In the manufacturing method of this embodiment, as described above, the cold-rolling material is continuously annealed at a temperature above the Ac1 transformation point to obtain a continuously annealed material (continuous annealing step). Then, the obtained continuously annealed material is subjected to cold rolling and strain-relieving annealing at least once each (cold rolling step). Here, the cold-rolling material is a hot-worked material (hot-forged material or hot-rolled material) having the alloy composition described above. Before proceeding to the continuous annealing step, the surface of the hot-rolled material may be polished to remove scale and rolling defects present on the surface of the material after hot rolling. According to the present invention, the continuous annealing step promotes the solid solution of carbides into the austenite matrix, temporarily eliminating fine carbides and promoting recrystallization, and further releasing strain, which makes it less likely for the hardness of the steel strip to increase during the cold working step. Furthermore, this effect of making it difficult for the hardness to increase allows the hardness increase when the continuously annealed material described later is cold-rolled at a reduction ratio of 50% to be kept below 140 HV. The annealing time in this continuous annealing process is preferably 5 to 30 minutes. The continuous annealing process can be performed at least once, and is preferably performed at least twice. In order to control the carbides to obtain good cold workability, the temperature should be above the Ac1 transformation point + 20°C (preferably above the Ac1 transformation point + 30°C). In addition, in order to further enhance the effect of continuous annealing, it is preferable to use a cold-rolling material with a thickness of 3 mm or less (preferably 2.5 mm or less).

[0025] Another manufacturing method of this embodiment preferably further includes a batch annealing step, in which the cold-rolling material described above is batch annealed at a temperature range of 500°C to less than 720°C for 3 to 30 hours to obtain batch-annealed material. In this case, the obtained batch-annealed material is continuously annealed at a temperature above the Ac1 transformation point. The reason for performing batch annealing on the cold-rolling material is that it is easy to adjust the heating and cooling rates, and the holding time at the desired temperature can be shortened or lengthened. By utilizing these characteristics of batch annealing, it is possible to easily adjust the carbide number density of the steel strip. Furthermore, by applying batch annealing, the number of long coils of cold-rolling material that can be processed at once can be increased, thereby increasing productivity. The number of coil-shaped cold-rolling material coils processed in a batch annealing operation at one time should preferably be 8 or more, depending on their length, as this is advantageous for increasing productivity. More preferably, it should be 10 or more coils. If carbides can be precipitated at the grain boundaries in the batch annealing process, and then sufficiently precipitated within the crystal grains in the continuous annealing process heated above the Ac1 transformation point, then it is possible to reduce the number of continuous annealing processes heated above the Ac1 transformation point compared to the case where only continuous annealing is performed without batch annealing. It is also possible to introduce a preliminary batch annealing process before the batch annealing process of this embodiment described above, to the extent that the effects of the present invention are not impaired. Furthermore, to control the carbides in order to obtain good cold workability, it is preferable to hold the material at a temperature in the range of 550 to 650°C for 5 hours or more.

[0026] In the manufacturing method of this embodiment, the obtained continuous annealed material is subjected to cold rolling and stress relief annealing at least once each (cold rolling step). A known rolling mill can be used for the cold rolling step. For example, cold rolling may be performed using a tandem or reverse type cold rolling mill. The material is adjusted to the desired plate thickness during cold rolling. If the hardness of the cold-rolled material becomes excessively high during cold rolling, the rolling rate will not increase even if the number of passes in the cold rolling step is increased. Therefore, the cold rolling rate is determined while considering the increase in the hardness of the cold-rolled material, and stress relief annealing at a temperature lower than the Ac1 transformation point is combined. The stress relief annealing described above has the effect of removing the strain caused by processing the material for cold rolling and softening the cold-rolled material that has become excessively work-hardened. This stress relief annealing step is preferable if a continuous annealing furnace is used, as it does not reduce productivity. It is preferable to perform the cold rolling step at least twice. In addition to the above, the cold rolling process may also include other processes, such as trimming, which involves cutting the edges of the cold-rolled material.

[0027] The continuously annealed material of the present invention, which has undergone the continuous annealing process described above (or a batch annealing process and a continuous annealing process), exhibits excellent cold workability because the increase in hardness is suppressed. Specifically, the increase in hardness when the continuously annealed material is rolled at a reduction ratio of 50% can be kept to 140 HV or less. A preferred increase in hardness is 120 HV or less, and a more preferred increase in hardness is 110 HV or less. The lower limit of the increase in hardness is not particularly limited and may be set to, for example, 50 HV or more.

[0028] Next, the steel strip of the present invention obtained by the manufacturing method of the present invention will be described. The steel strip of the present invention is a martensitic steel, but because it is in an annealed state before quenching, it exhibits a form in which carbides are dispersed in the ferrite structure. In other words, by performing quenching on the steel strip of the present invention, a steel strip having a martensitic structure can be obtained. Here, since a few percent of residual austenite may occasionally be found in the ferrite structure, steel strips in which austenite is found to be less than 3% are also included in the category of the steel strip of the present invention. The thickness of the steel strip of the present invention is not particularly limited, but for example, for cutlery applications, it is preferable to have a thickness of 0.5 mm or less, preferably 0.3 mm or less, and preferably 0.2 mm or less.

[0029] The steel strip of this embodiment has an average equivalent circle diameter of carbide in the cross-sectional structure (hereinafter, also simply referred to as "equivalent circle diameter") of 0.10 to 0.30 μm. When the average equivalent circle diameter of carbide is extremely small, the density of dislocations introduced during cold rolling increases, and the cold workability extremely decreases. Therefore, the lower limit of the average equivalent circle diameter of carbide is 0.10 μm. The lower limit of the average of the preferable equivalent circle diameter of carbide is 0.12 μm, more preferably 0.13 μm, and still more preferably 0.15 μm. On the other hand, when the average equivalent circle diameter of carbide is large, it is difficult for carbide to rapidly dissolve into the matrix during quenching, and sufficient hardness cannot be obtained for products (for example, cutting tools) using the steel strip of the present invention with a short-time quenching hold. Therefore, the upper limit of the average equivalent circle diameter of carbide is 0.30 μm. The average equivalent circle diameter of carbide in this embodiment is obtained by observing the carbide in the cross-sectional structure parallel to the rolling direction (drawing direction of rolling process) of the steel strip and performing image analysis on the field area of 100 μm ２ in the above-mentioned field of view. It can be calculated by observing the carbide in the field of view and performing image analysis on it. The carbide targeted for image analysis is limited to those with an equivalent circle diameter of 0.05 μm or more, and those less than that are not targeted. In addition, the identification of carbide can be confirmed by element mapping using EPMA (electron probe microanalyzer) attached to the scanning electron microscope. The above-mentioned equivalent circle diameter and number density of carbide are, for example, preferably obtained by observing at least 5 randomly selected fields of view and averaging the obtained average equivalent circle diameter, because the sparse and dense parts of carbide are averaged.

[0030] The steel strip of this embodiment has a carbide number density in the cross-sectional structure of 280 to 530 pieces / 100 μm ２ When the number density of carbide is small, it is difficult for carbide to rapidly dissolve into the matrix during quenching, and sufficient hardness cannot be obtained for the steel strip that has become martensite after a short-time quenching hold. Therefore, the lower limit of the number density of carbide is 280 pieces / 100 μm ２ is set. The lower limit of the preferable number density of carbide is 300 pieces / 100 μm ２ and more preferably 320 pieces / 100 μm ２More preferably, 350 particles / 100 μm ２ On the other hand, if the number density of carbides becomes too high, dislocations accumulate around the finely dispersed carbides during cold working, making work hardening easier and thus worsening cold workability. Therefore, the upper limit of the number density of carbides should be 530 particles / 100 μm². The preferred upper limit of the number density of carbides is 500 particles / 100 μm². ２ More preferably, 470 particles / 100 μm ２ More preferably 450 particles / 100 μm ２ In this embodiment, the carbide number density is determined under the same conditions as when observing the average equivalent diameter of the carbides described above. This can then be confirmed by elemental mapping using EPMA (electron beam microanalyzer). For example, it is preferable to observe at least five randomly selected fields of view and obtain the average number density obtained for each field of view.

[0031] The steel strip of this embodiment has a hardness of 280 to 400 HV. By keeping the hardness of the steel strip within this range, it is expected that the handling properties of the steel strip will be improved. If the hardness of the steel strip is greater than 400 HV, the strength will be too high, making it difficult to process in cold working processes, winding, and handling during transport, so the upper limit of the hardness of the steel strip is set to 400 HV. The preferred upper limit of hardness is 380 HV, and more preferably 370 HV. On the other hand, if the hardness of the steel strip is less than 280 HV, there is a concern that it may break when tension is applied during processing, and it will be more prone to plastic deformation when deformed, making it difficult to handle, so the lower limit of the hardness of the steel strip is set to 280 HV. The preferred lower limit of hardness is 300 HV, and more preferably 320 HV.

[0032] For a hot-rolled material with a thickness of 2.0 mm having the component composition shown in Table 1, continuous annealing (or batch annealing + continuous annealing) shown in Table 2 was carried out to obtain a continuously annealed material. The continuous annealing was carried out at least twice or more by holding at 850 °C (a temperature above the Ac1 point) for 10 minutes and cooling to room temperature. The number of continuous annealing times in Examples 3 to 5 and Comparative Examples 10 to 12 of the present invention is more than the number of continuous annealing times in Examples 1 and 2 of the present invention. The batch annealing was carried out under the condition of holding at 600 °C for 7 hours. Table 3 shows the hardness of the continuously annealed material after continuous annealing and the increase in the hardness of the steel strip when it was cold-rolled by 50%. Subsequently, cold rolling and stress relief annealing were each repeated twice on the obtained continuously annealed material to finish it to a thickness of about 0.1 mm, and the steel strips of Examples 1 to 5 and Comparative Examples 10 to 12 of the present invention shown in Table 4 were obtained.

[0033] The hardness of the continuously annealed material and the steel strip was measured with a Vickers hardness tester for the surface hardness of the samples of the present invention examples and comparative examples. The equivalent circle diameter and the carbide number density of the carbides in the cross-sectional structure of the strip steel were measured by collecting observation samples from the steel strips of Examples 1 to 5 and Comparative Examples 10 to 12 of the present invention prepared. The average of the equivalent circle diameter of the carbides and the carbide number density were measured in a cross-sectional structure parallel to the rolling direction of the steel strip by using an image analyzer for carbides with an equivalent circle diameter of 0.05 μm or more in a visual field area of 100 μm 2 The above. The scanning electron microscope of Example 1 of the present invention is shown in FIG. 1. The chemical composition can be analyzed in accordance with the analysis methods defined by JIS and ASTM. For C, S, N, and O, combustion-infrared absorption method and inert gas fusion-infrared absorption method in accordance with ASTM-E1019 can be used for analysis, and for other elements, component analysis can be carried out by emission analysis method and fluorescent X-ray analysis method in accordance with JIS-G0320 or G0321.

[0034]

[0035]

[0036]

[0037]

[0038] As shown in Tables 3 and 4, in Examples 1 to 5 of the present invention, the hardness increase when the continuously annealed material was reduced by 50% was 100 HV or less, indicating good cold workability. The hardness of the steel strip was 290 to 370 HV, the equivalent circle diameter of the carbides was 0.16 to 0.30 μm, and the carbide number density was 310 to 510 particles, suggesting that it can be expected to exhibit sufficient hardness for cutting tool applications after quenching. On the other hand, in Comparative Example 10, the hardness increase when the continuously annealed material was reduced by 50% was 147 HV, resulting in inferior cold workability compared to the examples of the present invention. This is because the carbide number density of the steel strip was 537 particles / 100 μm ２ This is considered to be one of the reasons why it is higher than in the present invention example. In addition, in Comparative Examples 11 and 12, although the hardness increase when the continuously annealed material is reduced by 50% is 80 HV or less, the carbide number density is 200 to 273 particles / 100 μm because the amount of C in the component composition is low and the amount of Cr is high. ２ In this case, the carbides were not finely distributed. Therefore, in Comparative Examples 11 and 12, even if quenching is performed, it may not be possible to obtain sufficient hardness for applications such as cutting tools. Based on the above, it is considered that, compared to the conventional example, the continuously annealed material with controlled carbide number density has excellent workability and good cold rolling properties, and the resulting steel strip can achieve both a fine carbide distribution and good handling properties due to its appropriate hardness.

Claims

1. A method for manufacturing a steel strip, comprising: a preparation step of preparing a material for cold rolling having a composition in mass%, of C: 0.55 to 0.95%, Si: 0.1 to 0.95%, Mn: 0.1 to 1.5%, Cr: 8.0 to 11.0%, Mo and W individually or in combination (Mo + W / 2): 0.5 to 3.0%, with the remainder being Fe and unavoidable impurities; a continuous annealing step of performing continuous annealing on the material for cold rolling at a temperature above the Ac1 transformation point to obtain a continuously annealed material; and a cold rolling step of performing cold rolling and stress-relieving annealing on the continuously annealed material at least once each, wherein the continuously annealed material after the continuous annealing step has a hardness increase of 140 HV or less when cold-rolled at a reduction ratio of 50%.

2. The method for manufacturing a steel strip according to claim 1, further comprising a batch annealing step of performing batch annealing on the cold-rolling material at a temperature range of 500°C to less than 720°C for 3 to 30 hours to obtain a batch-annealed material, and then performing the continuous annealing step on the batch-annealed material.

3. The composition, by mass%, consists of C: 0.55–0.95%, Si: 0.1–0.95%, Mn: 0.1–1.5%, Cr: 8.0–11.0%, Mo and W individually or in combination (Mo + W / 2): 0.5–3.0%, with the remainder being Fe and unavoidable impurities. The average equivalent circle diameter of carbides in the cross-sectional structure is 0.10–0.30 μm, and the carbide number density is 280–530 particles / 100 μm. ２ A steel strip with a hardness of 280 to 400 HV.

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