JP7618443B2 - Ferrite-martensite duplex stainless steel with excellent bendability and method for producing same - Google Patents

Description

The present invention relates to a ferritic-martensite duplex stainless steel with excellent bendability and a method for manufacturing the same.

Martensitic stainless steel, which has been hardened through quenching and tempering, is known as a type of stainless steel with improved strength. However, this martensitic stainless steel has the problem of low ductility.

To address this issue, for example, multi-phase heat treatment technology is known to ensure both ductility and strength. For example, Patent Document 1 describes a method for manufacturing a highly ductile, high-strength multi-phase chromium stainless steel sheet by performing multi-phase heat treatment on a cold-rolled stainless steel sheet.

Patent Document 2 describes a method for producing a high-strength, multi-phase stainless steel strip with excellent spring properties, in which a cold-rolled stainless steel strip is subjected to a multi-phase heat treatment to produce a steel strip with a mixed structure of ferrite and martensite, and the steel strip is then subjected to a continuous aging treatment for 10 minutes or less.

Patent Document 3 describes a high-strength stainless steel sheet with improved bendability, which is obtained by decarburizing the surface layer of a two-phase structure of ferrite + martensite. Specifically, the high-strength stainless steel sheet has an area ratio of ferrite phase of 48% or more in the thickness cross section within a range of 25 μm from the steel sheet surface in the thickness direction, and an area ratio of ferrite phase of 48% or less in the other range.

Furthermore, Patent Document 4 describes a method for producing a high-strength stainless steel sheet having excellent ductility, in which the stainless steel sheet is subjected to a first heat treatment step as a multi-phase heat treatment, a cold rolling step, and a second heat treatment step of heating at a temperature of 400° C. or higher and lower than the _AC1 transformation point.

Patent Document 5 also describes a high-strength stainless steel material that has improved bendability by performing aging treatment after multi-phase heat treatment to make the difference in hardness between the ferrite phase and the martensite phase 300 HV.

Japanese Unexamined Patent Publication No. 63-7338 Japanese Patent Application Publication No. 3-56621 JP 2001-2349290 A JP 2004-323960 A International Publication No. WO2009/099035

However, in stainless steel that has been subjected to multi-phase heat treatment under the conditions described in Patent Document 1, cracks may occur when the stainless steel is bent. In addition, while the stainless steel obtained by the manufacturing methods described in Patent Documents 2 to 5 can improve bendability, it is necessary to carry out an additional heat treatment step, such as aging treatment, after the multi-phase heat treatment in the manufacturing process. This results in high manufacturing costs.

One aspect of the present invention aims to improve the bendability of duplex stainless steel and reduce manufacturing costs.

In order to solve the above problems, the stainless steel according to one embodiment of the present invention contains, by mass%, 0.01-0.2% C, 0.01-2.0% Si, 0.1-4.0% Mn, 0.05% or less P, 0.03% or less S, 10-20% Cr, 0.01-4.0% Ni, 0.12% or less N, 0.01% or less O, with the balance being Fe and unavoidable impurities. The stainless steel contains a ferrite phase and a martensite phase, has a hardness of 200-340 HV, and in any cross section of the stainless steel, the area ratio of the martensite phase is 60-75%, the area ratio of the carbides is 0.5-1.5%, and the major axis of each of the carbides is 1 μm or less.

In addition, a method for producing stainless steel according to one embodiment of the present invention includes a final annealing step in which stainless steel containing, by mass%, 0.01-0.2% C, 0.01-2.0% Si, 0.1-4.0% Mn, 0.05% or less P, 0.03% or less S, 10-20% Cr, 0.01-4.0% Ni, 0.12% or less N, 0.01% or less O, with the balance being Fe and unavoidable impurities, is heated to a temperature range of 800-1000°C after a cold rolling step, soaked in said temperature range for less than 1 minute, and then cooled at a cooling rate of 1°C/s or more.

According to one aspect of the present invention, it is possible to improve the bendability of duplex stainless steel and reduce manufacturing costs.

1 is a SEM photograph of an arbitrary cross section of a duplex stainless steel according to an embodiment of the present invention. 1 is a graph showing the effect of the holding time at the duplexing temperature and the duplexing temperature on the hardness of duplex stainless steel. 1 is a graph showing the influence of the holding time at the duplexing temperature and the duplexing temperature on the martensite area ratio.

One embodiment of the present invention will be described in detail below. Note that the following description is provided to facilitate a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. In this specification, "A-B" indicates A or more and B or less. In this specification, "%" in relation to chemical composition means "mass %" unless otherwise specified.

<Definition of terms>
The term "stainless steel" refers to a stainless steel material not limited to a specific shape, and examples of the stainless steel material include steel plates, steel pipes, and bar bars.

In this specification, unless otherwise specified, "duplex stainless steel" means stainless steel containing ferrite and martensite phases.

<General manufacturing method>
First, an example of a typical manufacturing process for stainless steel strip will be described in outline. In one example, the typical manufacturing process for stainless steel strip includes a steelmaking process, a hot rolling process, an annealing process, a pickling process, a cold rolling process, an annealing and pickling process, and a finish rolling process, in this order. Each of these steps in the conventional manufacturing process is publicly known, and therefore will not be described in detail except as described below.

<Summary of the findings of the invention>
Duplex stainless steel has a duplex metal structure consisting of a relatively soft and ductile ferrite phase and a strong martensite phase. Therefore, duplex stainless steel is known as a stainless steel that has both strength and ductility. However, due to the large difference in hardness between the ferrite phase and the martensite phase, duplex stainless steel may concentrate deformation in the soft ferrite phase during bending, causing cracks to occur starting from the ferrite phase.

In order to improve bendability, it is conceivable to soften the duplex stainless steel by lowering the phase ratio of the martensite phase in the duplex stainless steel. However, lowering the phase ratio of the martensite phase reduces the phase ratio of the martensite phase that ensures strength, and this reduces the strength of the stainless steel itself. In addition, other methods proposed to improve the bendability of duplex stainless steel require additional steps, such as further heat treatment, in the manufacturing process, which increases manufacturing costs.

After extensive research, the inventors have found that a high-strength duplex stainless steel with excellent bendability can be obtained by shortening the holding time in the duplex temperature range during the duplex heat treatment (see the Examples below). The duplex stainless steel in this embodiment maintains a high ratio of martensite phase, and does not require a heat treatment step to improve bendability to be added to the manufacturing process. According to one aspect of the present invention, it is possible to provide a duplex stainless steel with high strength and excellent bendability that can be manufactured with reduced possibility of increased manufacturing costs.

<Duplex stainless steel of the present invention>
A duplex stainless steel (stainless steel) according to one embodiment of the present invention includes a ferrite phase and a martensite phase, and has a hardness (Vickers hardness) of 200 to 340 HV, an area ratio of the martensite phase in a cross section of the duplex stainless steel is 60 to 75%, an area ratio of carbides is 0.5 to 1.5%, and the major axis of the carbides is 1 μm or less.

FIG. 1 is an SEM photograph of an arbitrary cross section of a duplex stainless steel according to an embodiment of the present invention. As shown in FIG. 1, in an arbitrary cross section of a duplex stainless steel according to an embodiment of the present invention, carbides can be observed as granular matter dispersed in the material. Examples of carbides present in the duplex stainless steel include (Fe, Cr) ₂₃ C _6. The area ratio of carbides is the ratio of the area where carbides are present (the sum of the areas of carbide particles) in a predetermined area of the cross section of the duplex stainless steel. The major axis of carbides means the diameter with the longest length among the diameters of particulate carbides. In the duplex stainless steel of the present invention, the major axis of each carbide confirmed in the cross section of the duplex stainless steel is 1 μm or less. The direction of the cross section of the duplex stainless steel when measuring the area ratio of martensite phase and carbides, and the major axis of carbides is not particularly limited. For example, the cross section may be parallel to the rolling direction and plate thickness direction of the duplex stainless steel.

The duplex stainless steel according to this embodiment has both ductility and strength by containing a ferrite phase having ductility and a martensite phase having strength. The higher the ratio of the hard martensite phase, the higher the strength of the duplex stainless steel itself. However, if the ratio of the martensite phase is excessively high, the ductility of the duplex stainless steel decreases, making processing difficult. Therefore, the area ratio of the martensite phase in the cross section of the duplex stainless steel according to this embodiment is 60 to 75%. This ensures the strength of the duplex stainless steel itself.

When bending duplex stainless steel, if the duplex stainless steel has a high hardness, its workability decreases. Also, if the hardness of the duplex stainless steel is too low, it is prone to deformation when made into a product. For this reason, the Vickers hardness of the duplex stainless steel in this embodiment is 200 to 340 HV.

In duplex stainless steels having the same composition, the difference in the area ratio of carbides in the cross section is believed to be due to the difference in the amount of C dissolved in the martensite phase. In other words, in duplex stainless steels having the same composition, the lower the area ratio of carbides, the greater the amount of C dissolved in the martensite phase. If the amount of C dissolved in the martensite phase is large, the hardness of the martensite phase increases, which causes the hardness of the duplex stainless steel itself to increase, adversely affecting bendability. Therefore, in duplex stainless steels having the composition of the present invention, the preferred area ratio of carbides in the cross section of the duplex stainless steel is 0.5 to 1.5%.

Furthermore, in duplex stainless steel, if the carbides are coarse, they are likely to become the starting point of cracks during bending. Therefore, in the duplex stainless steel according to this embodiment, the major axis of each carbide in the cross section is 1 μm or less, and more preferably 0.75 μm or less. This reduces the possibility of cracks occurring during bending of the duplex stainless steel.

From the above, it is possible to realize a duplex stainless steel with excellent bendability.

(Component Composition)
The duplex stainless steel according to this embodiment contains, as essential components, in mass%, 0.01 to 0.2% C, 0.01 to 2.0% Si, 0.1 to 4.0% Mn, 0.05% or less P, 0.03% or less S, 10 to 20% Cr, 0.01 to 4.0% Ni, 0.12% or less N, and 0.01% or less O, with the balance being Fe and unavoidable impurities.

The duplex stainless steel according to this embodiment may further contain at least one of the following optional components: 4.0% or less Cu, 1.0% or less Mo, 1.0% or less W, 0.5% or less Co, 0.2% or less Al, 1.0% or less V, 1.0% or less Nb, 1.0% or less Ti, 0.005% or less B, 0.005% or less Ca, 0.005% or less Mg, 0.5% or less Sn, 0.5% or less Sb, 0.01% or less Ga, 0.01% or less Ta, 0.5% or less Zr, 0.1% or less Y, 0.01% or less Hf, and 0.1% or less REM (rare earth elements). The significance of the content of each element contained in the duplex stainless steel according to this embodiment will be explained below.

C is an austenite-forming element that makes it easier to form the austenite phase. C stabilizes the austenite structure and improves the strength of the martensite formed during the annealing and/or cooling process. As the C content increases, C increases the volume fraction of the martensite phase and dissolves in the martensite, improving the strength of the stainless steel. Therefore, C is an important element in ensuring the strength of stainless steel. However, if the C content of stainless steel is too high, the volume fraction of the martensite phase becomes too high, reducing workability. In addition, if the C content of stainless steel is too high, it reduces toughness and corrosion resistance. Therefore, the duplex stainless steel according to this embodiment contains 0.01 to 0.2% C.

Si is an element that has a deoxidizing effect on stainless steel, but since it is a ferrite phase forming element that makes it easier to form the ferrite phase, if the Si content is high, a sufficient amount of martensite cannot be obtained. On the other hand, excessively low Si content in stainless steel leads to increased refining costs. Therefore, the duplex stainless steel according to this embodiment contains 0.01 to 2.0% Si.

Mn is an austenite-forming element and is effective for obtaining the martensite phase. However, the inclusion of a large amount of Mn leads to a decrease in workability because the volume fraction of the martensite phase becomes too high. Therefore, the duplex stainless steel according to this embodiment contains 0.1 to 4.0% Mn.

P and S are unavoidable impurities. Since P and S are elements that reduce toughness, in the duplex stainless steel according to this embodiment, the P content is 0.05% or less, and the S content is 0.03% or less.

Cr is an effective component for increasing the corrosion resistance of stainless steel. However, since Cr is a ferrite-forming element that makes it easier to form the ferrite phase, if the Cr content of stainless steel becomes too high, it will reduce the volume fraction of the martensite phase. Therefore, the duplex stainless steel according to this embodiment contains 10.0 to 20.0% Cr.

Ni is an austenite-forming element and is effective in forming the martensite phase. It is also effective in improving the toughness and corrosion resistance of stainless steel. However, if the Ni content is too high, the stainless steel will consist only of the martensite phase, and a multi-phase structure will not be obtained. Therefore, the multi-phase stainless steel according to this embodiment has a Ni content of 0.01 to 4.0%.

N is an element that increases the volume fraction of the martensite phase and contributes to improving the strength of stainless steel. N also improves the strength of stainless steel by dissolving in martensite. However, as the N content increases, the effects of phase ratio control and solid solution strengthening become saturated due to the solubility of N in austenite. Therefore, the duplex stainless steel according to this embodiment has an N content of 0.12% or less. More preferably, it has an N content of 0.050% or less.

O is an unavoidable impurity. O forms oxide-based inclusions and is a factor in reducing bendability, so the O content in the duplex stainless steel according to this embodiment is 0.01% or less.

(Other ingredients)
The duplex stainless steel according to this embodiment may selectively contain one or more elements selected from the following group of elements in addition to the above essential components.

Cu is an austenite-forming element and is also effective in maintaining the austenite phase. If excessive Cu is added, the volume fraction of the martensite phase becomes too high, resulting in reduced workability. Therefore, in the duplex stainless steel according to this embodiment, up to 4.0% Cu may be added as necessary.

Mo, W, and Co are elements that improve the corrosion resistance of stainless steel. However, if stainless steel contains excessive amounts of these elements, it becomes hard, its toughness decreases, and the material cost increases. Therefore, in the duplex stainless steel according to this embodiment, one or more of 1.0% or less Mo, 1.0% or less W, and 0.5% or less Co may be added as necessary.

Al is an effective element as a deoxidizer. However, since Al is a ferrite-forming element, adding too much Al requires increasing the amount of austenite-forming elements added. Furthermore, the effect of adding Al as a deoxidizer reaches saturation at a certain amount, and adding too much does not improve it. Therefore, the duplex stainless steel according to this embodiment may contain up to 0.2% Al.

Nb, V, and Ti are elements that have a high affinity with C and N, and are precipitated as carbides or nitrides during hot rolling, which has the effect of improving high-temperature strength. On the other hand, excessive addition of these elements hardens the steel and has a negative effect on bendability. Therefore, in the duplex stainless steel according to this embodiment, one or more of 1.0% or less Nb, 1.0% or less V, and 1.0% or less Ti may be added as necessary.

B, Ca, and Mg are elements that improve hot workability and secondary workability. On the other hand, excessive addition of these elements may reduce the manufacturability of stainless steel. Therefore, in the duplex stainless steel according to this embodiment, one or more of 0.005% or less B, 0.005% or less Ca, and 0.005% or less Mg may be added as necessary.

Sn and Sb are elements that improve corrosion resistance. However, excessive addition of these elements may reduce the manufacturability of stainless steel. Therefore, in the duplex stainless steel according to this embodiment, one or more of 0.5% or less Sn and 0.5% or less Sb may be added as necessary.

Ga and Ta are elements that improve corrosion resistance. However, excessive addition of these elements may result in increased alloy costs. Therefore, in the duplex stainless steel according to this embodiment, one or more of 0.01% or less Ga and 0.01% or less Ta may be added as necessary.

Zr, Y, Hf, and REM are elements that improve hot workability and cleanliness of steel. They are also effective as elements for improving oxidation resistance. However, excessive addition of these elements may result in increased alloy costs. Therefore, in the duplex stainless steel according to this embodiment, one or more of 0.5% or less Zr, 0.1% or less Y, 0.01% or less Hf, and 0.1% or less REM may be added as necessary.

The remainder of the duplex stainless steel according to this embodiment is Fe and unavoidable impurities.

<Method of manufacturing duplex stainless steel>
An example of a method for producing a duplex stainless steel according to an embodiment of the present invention will be described below. The method for producing a duplex stainless steel according to an embodiment of the present invention is characterized in that a duplex heat treatment is performed in the final annealing step in a general method for producing stainless steel.

(Pretreatment process)
In the pretreatment step, first, a vacuum melting furnace is used to produce steel having a composition adjusted to fall within the range of the present invention. This steel is then cast to produce a steel ingot.

(Hot rolling process)
In the hot rolling process, the steel ingot after the pretreatment process is hot rolled to produce a hot rolled steel strip. The temperature in the hot rolling process may be within a general range, for example, about 800 to 1250°C.

(First annealing step)
In the first annealing step, the hot rolled steel strip is annealed (batch annealing) using, for example, a batch annealing furnace (bell annealing furnace). This annealing step is referred to as the first annealing step.

(Pickling process)
In the pickling process, the annealed steel strip obtained in the first annealing process is subjected to a pickling treatment in the first pickling process. In this pickling process, a descaling treatment of the annealed steel strip is performed.

(Cold rolling process)
In the cold rolling step, the annealed steel strip descaled in the pickling step is cold rolled at a rolling reduction of, for example, 50 to 90% to produce a cold rolled steel strip.

(Final annealing process)
In the method for producing a duplex stainless steel according to the present embodiment, the cold-rolled steel strip cold-rolled by the cold rolling step is subjected to a duplex heat treatment as a final annealing step. Specifically, the cold-rolled steel strip is heated to a duplex temperature range of 800 to 1000°C, preferably 900 to 1000°C, and after soaking in the duplex temperature range for less than 1 minute, preferably 40 seconds or less, it is cooled at a cooling rate of 1°C/s or more, preferably 3°C/s. In the final annealing step, the cold-rolled steel strip is heated to a duplex temperature range of 800 to 1000°C to generate a two-phase metal structure of a ferrite phase and an austenite phase that is transformed into a martensite phase by subsequent cooling. Then, the heated cold-rolled steel strip is cooled at a cooling rate of 1°C/s or more to transform the austenite phase into a martensite phase.

In this way, in the manufacturing method of the duplex stainless steel according to this embodiment, a two-phase metal structure of ferrite and austenite is produced by a short period (less than 1 minute) of soaking at a temperature of 800 to 1000°C. The cooling rate from the duplex temperature range may be any rate that can transform the austenite phase into the martensite phase.

In the manufacturing method of the duplex stainless steel according to this embodiment, the characteristic (short) duplex heat treatment allows the production of duplex stainless steel with the desired bendability without the need for any additional heat treatment steps. Therefore, the duplex heat treatment can be used as the final annealing step.

(Pickling process, finish rolling process)
If necessary, the steel strip after the final annealing step is subjected to a final pickling treatment in a pickling step and a finish rolling step.

As described above, in the manufacturing method of the duplex stainless steel according to one embodiment of the present invention, the cold-rolled steel sheet is heated to a temperature range (duplex temperature range) where the steel sheet is in a two-phase region of ferrite and austenite, and then cooled as a final annealing step, and a duplex heat treatment is performed. During the duplex heat treatment, at least a part of the carbides in the cold-rolled steel strip is incorporated into the austenite phase, and is cooled to be in a state of solid solution in the martensite phase. That is, the duplex heat treatment increases the amount of solute C in the martensite phase of the duplex stainless steel, and the martensite phase becomes hard. From this, it is considered that the amount of C dissolved in the martensite phase of the duplex stainless steel is reduced by shortening the holding time in the duplex temperature range. As a result, the hardness of the martensite phase is reduced, reducing the difference in hardness with the ferrite phase, and the hardness of the duplex stainless steel itself is reduced, which is considered to have improved the bendability of the duplex stainless steel.

(Advantageous Effects)
The duplex stainless steel obtained by the above-mentioned exemplary method for producing a duplex stainless steel has excellent bendability.

In addition, the method for producing duplex stainless steel according to one embodiment of the present invention can produce duplex stainless steel with excellent bendability without performing additional heat treatment after the duplex heat treatment, which reduces production costs.

<Example>
The results of evaluation of stainless steel sheets according to examples of the present invention (invention examples) and comparative examples will be described below.

次に、当該鋼種Ａ～Ｕに対して、下記表２に示す条件で最終焼鈍工程（複相化熱処理）を実施した。表２における焼鈍時間とは、複相化温度での均熱保持時間を意味する。表２において、Ｎｏ．１～１３は、本発明の範囲内の条件により最終焼鈍工程を施した、本発明例の複相ステンレス鋼である。Ｎｏ．１４～２５は、本発明の範囲外の条件により最終焼鈍工程を施した、比較例としてのステンレス鋼である。また、表２には、本発明例および比較例についての、マルテンサイト面積率、炭化物面積率、炭化物径（炭化物の長径）、ビッカース硬さおよび曲げ性の評価の結果を示している。 Cold-rolled sheets having a thickness of 2.6 mm were prepared for stainless steels of steel types A to U having the chemical compositions shown in Table 1 below. Steel types A to M are steel types having compositions within the range of the present invention. Steel types N to T are steel types having compositions outside the range of the present invention. Note that the underlined items in Table 1 are items outside the range of the chemical composition of the duplex stainless steel according to one embodiment of the present invention. The same is true for Table 2 below.

Next, the steel types A to U were subjected to a final annealing process (multi-phase heat treatment) under the conditions shown in Table 2 below. The annealing time in Table 2 means the soaking holding time at the multi-phase temperature. In Table 2, Nos. 1 to 13 are multi-phase stainless steels of the present invention that were subjected to a final annealing process under conditions within the range of the present invention. Nos. 14 to 25 are stainless steels as comparative examples that were subjected to a final annealing process under conditions outside the range of the present invention. Table 2 also shows the results of evaluation of the martensite area ratio, carbide area ratio, carbide diameter (long diameter of carbide), Vickers hardness, and bendability for the present invention and comparative examples.

(Volume fraction of martensite phase)
The area ratio of the martensite phase in the cross section of each of the multi-phase stainless steel sheets obtained by the final annealing process under each condition was measured. For each stainless steel sheet, the center of the thickness of the cross section parallel to the rolling direction and the thickness direction was photographed at 1000 times using an optical microscope. The volume ratio of the martensite phase was calculated based on the photographed structure photographs by the point counting method (JIS G0555). The results are shown in Table 2 under "Martensite area ratio (%)".

(Carbide area ratio)
The area ratio of carbides in the cross section of each duplex stainless steel plate obtained by performing the final annealing process under each condition was measured. The center of the plate thickness in the cross section parallel to the rolling direction and plate thickness direction of each stainless steel plate was photographed at 2000 times using a SEM (scanning electron microscope). Based on the reflected electron images, the area ratio of carbides was calculated by the point counting method (JIS G0555). The results are shown in Table 2 under "Carbide area ratio (%)".

(Major axis of carbide)
The major axis of the carbides present in each duplex stainless steel sheet obtained by performing the final annealing process under each condition was measured. The center of the thickness of the cross section parallel to the rolling direction of each stainless steel sheet was photographed at 2000 times using an SEM. The major axis of the largest carbide in the photographed backscattered electron image was measured, and the results are shown in Table 2 under "Carbide diameter (μm)".

(Vickers hardness)
The Vickers hardness of each of the duplex stainless steel sheets obtained by the final annealing process under each condition was measured using a Vickers hardness tester with a test load of 5 kg in accordance with JIS Z2244. The evaluation results are shown in Table 2 under "Vickers hardness (HV)".

表２に示すように、本発明の範囲内条件での最終焼鈍工程を施すことによって得られた本発明例Ｎｏ．１～１３は、マルテンサイト面積率、炭化物径、およびビッカース硬さが本発明に規定する範囲内であり、良好な曲げ性を有していた。 (Bendability)
A bending test was carried out to evaluate the bendability of each duplex stainless steel sheet obtained by performing the final annealing process under each condition. A sample piece of 40 mm (rolling direction) x 150 mm (sheet width direction) was taken from the duplex stainless steel sheet obtained by performing the final annealing process under each condition. The sample piece was pressed against a V-block type jig with a tip of 2.5R and a tip angle of 90° so that the bending ridge was parallel to the rolling direction, and a 90° bending process was performed. The bending ridge was observed at a magnification of 50 times using a microscope to confirm the presence or absence of cracks. "◯" indicates that no cracks were generated, and "×" indicates that cracks were generated.

As shown in Table 2, the invention examples Nos. 1 to 13 obtained by performing the final annealing process under the conditions within the ranges of the invention had martensite area ratios, carbide diameters, and Vickers hardnesses within the ranges specified in the invention, and had good bendability.

On the other hand, in Comparative Examples 14 to 25, in which the final annealing process was performed under conditions outside the scope of the present invention, the martensite area ratio, carbide diameter, and Vickers hardness were all outside the ranges specified in the present invention, and the bendability was evaluated as poor. In Comparative Examples 14 and 17 to 20, the bendability was evaluated as good, but the martensite area ratio was outside the ranges specified in the present invention. In other words, it is believed that the strength of the duplex stainless steel was not ensured.

As an example showing the effect of different annealing times, Example No. 9 of the present invention and Comparative Example No. 16 are compared. In Example No. 9 of the present invention and Comparative Example No. 16, the same steel type I was subjected to the same multi-phase heat treatment at the same multi-phase temperature, but the annealing time was different. Since the multi-phase temperature was the same, the martensite area ratio of the two examples was the same, but Comparative Example No. 16, which had an annealing time of more than 1 minute, had increased hardness and poor bendability. This is thought to be because the carbide area ratio of No. 9 was 0.52%, while that of No. 16 was 0.23%, and therefore more carbides were dissolved in the martensite phase in No. 16, increasing the hardness of the martensite phase.

(Effect of holding time at duplex temperature and duplex temperature on hardness of duplex stainless steel)
2 is a graph showing the effect of the holding time at the duplexing temperature and the duplexing temperature on the hardness of duplex stainless steel. For a steel plate having the composition of steel type I, the Vickers hardness of the duplex stainless steel was measured when the holding time was 40 seconds and 90 seconds and the duplexing temperature was changed from 900 to 1100°C. The Vickers hardness was measured according to the method described above. Regarding the holding time, 40 seconds is within the range of the present invention, and 90 seconds is outside the range of the present invention.

From Figure 2, it can be seen that when stainless steels having the same composition are subjected to the same duplexing temperature within the duplexing temperature range of the present invention (900-1000°C), the Vickers hardness is lower when the holding time at the duplexing temperature is 40 seconds. It can also be seen that when the holding time at the duplexing temperature is 90 seconds, the hardness already exceeds 340 HV at a duplexing temperature of 1000°C.

This experiment demonstrated that by keeping the holding time at the duplex temperature within the duplex temperature range of the present invention at less than one minute, a duplex stainless steel with excellent bendability and a hardness of 200 to 340 HV can be obtained.

(Effect of holding time at duplex temperature and duplex temperature on martensite area ratio)
3 is a graph showing the effect of the holding time at the duplexing temperature and the duplexing temperature on the martensite area ratio of duplex stainless steel. For a steel plate having the composition of steel type I, the holding time was 40 seconds and 90 seconds, and the duplexing temperature was changed from 900 to 1100° C., and the martensite area ratio was measured. The measurement of the martensite area ratio was carried out according to the method described above.

Figure 3 shows that the change in martensite area ratio due to the change in the duplexing temperature is similar when the holding time at the duplexing temperature is 40 seconds and when it is 90 seconds. In other words, it has become clear that the martensite area ratio of duplex stainless steel is determined by the duplexing temperature, and does not depend on the holding time at the duplexing temperature.

[Additional Notes]
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

Claims (6)

A stainless steel containing, by mass%, 0.01 to 0.122% C, 0.01 to 2.0% Si, 0.1 to 4.0% Mn, 0.05% or less P, 0.03% or less S, 10 to 20% Cr, 0.01 to 4.0% Ni, 0.12% or less N, 0.01% or less O, with the balance being Fe and unavoidable impurities;
Contains ferrite and martensite phases,
The hardness is 200 to 340 HV.
In any cross section of the stainless steel, an area ratio of the martensite phase is 60 to 75%, an area ratio of carbides is 0.5 to 1.5%, and the major axis of each of the carbides is 1 μm or less. A stainless steel containing, by mass%, 0.01 to 0.122% C, 0.01 to 2.0% Si, 0.1 to 4.0% Mn, 0.05% or less P, 0.03% or less S, 10 to 20% Cr, 0.01 to 4.0% Ni, 0.12% or less N, 0.01% or less O, 4.0% or less Cu, with the balance being Fe and unavoidable impurities,
Contains ferrite and martensite phases,
The hardness is 200 to 335 HV.
In any cross section of the stainless steel, an area ratio of the martensite phase is 60 to 75%, an area ratio of carbides is 0.5 to 1.5%, and the major axis of each of the carbides is 1 μm or less . The stainless steel according to claim 1 or 2, further containing, by mass%, at least one of the following: 1.0% or less Mo, 1.0% or less W, 0.5% or less Co, 0.2% or less Al, 1.0% or less V, 1.0% or less Nb, 1.0% or less Ti, 0.005% or less B, 0.005% or less Ca, 0.005% or less Mg, 0.5% or less Sn, 0.5% or less Sb, 0.01% or less Ga, 0.01% or less Ta, 0.5% or less Zr, 0.1% or less Y, 0.01% or less Hf, and 0.1% or less REM. The stainless steel contains, by mass%, 0.01 to 0.122% C, 0.01 to 2.0% Si, 0.1 to 4.0% Mn, 0.05% or less P, 0.03% or less S, 10 to 20% Cr, 0.01 to 4.0% Ni, 0.12% or less N, 0.01% or less O, with the balance being Fe and unavoidable impurities, and includes a final annealing step in which the stainless steel is heated to a temperature range of 800 to 1000°C after a cold rolling step, soaked in the temperature range for less than 1 minute, and then cooled at a cooling rate of 1°C/s or more;
The stainless steel after the final annealing step includes a ferrite phase and a martensite phase,
The hardness is 200 to 340 HV.
In any cross section of the stainless steel after the final annealing step, the area ratio of the martensite phase is 60 to 75%, the area ratio of carbides is 0.5 to 1.5%, and the major axis of the carbides is 1 μm or less. The stainless steel contains, by mass%, 0.01 to 0.122% C, 0.01 to 2.0% Si, 0.1 to 4.0% Mn, 0.05% or less P, 0.03% or less S, 10 to 20% Cr, 0.01 to 4.0% Ni, 0.12% or less N, 0.01% or less O, 4.0% or less Cu , with the balance being Fe and unavoidable impurities, and includes a final annealing step in which the stainless steel is heated to a temperature range of 800 to 1000°C after a cold rolling step, soaked in the temperature range for less than 1 minute, and then cooled at a cooling rate of 1°C/s or more;
The stainless steel after the final annealing step includes a ferrite phase and a martensite phase,
The hardness is 200 to 335 HV.
In any cross section of the stainless steel after the final annealing step, the area ratio of the martensite phase is 60 to 75%, the area ratio of carbides is 0.5 to 1.5%, and the major axis of the carbides is 1 μm or less . The method for producing stainless steel according to claim 4 or 5, further comprising, by mass%, at least one of the following: 1.0% or less Mo, 1.0% or less W, 0.5% or less Co, 0.2% or less Al, 1.0% or less V, 1.0% or less Nb, 1.0% or less Ti, 0.005% or less B, 0.005% or less Ca, 0.005% or less Mg, 0.5% or less Sn, 0.5% or less Sb, 0.01% or less Ga, 0.01% or less Ta, 0.5% or less Zr, 0.1% or less Y, 0.01% or less Hf, and 0.1% or less REM.

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