JP6369284B2 - Duplex stainless steel and method for producing the same - Google Patents

Description

  The present invention relates to an inexpensive Cr-based double phase having high strength and excellent workability, excellent wear resistance, and fatigue properties, and a method for producing the same. It is ideal as a material for household electrical appliances used after processing into a predetermined shape, and it transmits a scraping blade (doctor blade) and driving force to remove excess ink, powder, and chemicals used in printers. It is expected to be applied to gears, steel belts, etc., and can also be used for automobiles, various industrial equipments, etc., and is also widely expected to be applied to applications other than those described above.

  In recent years, home appliances and automobile parts have been promoted to be smaller and lighter from the viewpoints of environmental and energy problems and cost, and are used after being processed into a predetermined shape with strict accuracy. It has increased. Many of these parts are required to have high strength due to a decrease in rigidity and the like accompanying reduction in size and weight, and it is more difficult to achieve both workability and the like. Furthermore, durability may be required due to the problem of product reliability.

  Conventionally, tempered rolled material of metastable austenitic (A) stainless steel mainly made of SUS301, SUS304 for blades, gears, steel belts, etc. used for home appliances, specifically printers, etc. Has been used. Metastable austenitic stainless steels can have both high strength and excellent ductility due to the processing-induced transformation into a hard martensite (M) phase.

  Patent Document 1 discloses an improved metastable austenitic stainless steel. However, the large work hardening exhibited by metastable austenitic stainless steel is often a factor of variation, and there is a problem that the load during rolling increases. On the other hand, the austenite stabilizing element may be classified as a rare metal, and it is necessary to contain a large amount of expensive Ni.

  For this reason, martensitic stainless steels such as SUS403, SUS410, and SUS420 that can obtain high strength by transformation into a hard martensite phase as an intermediate phase by heat treatment (quenching) are also used. In many cases, martensitic stainless steel is used as a raw material to adjust the performance by utilizing a multiphase structure with a ferrite (F) phase. Martensitic stainless steel utilizing a multiphase structure with a ferrite (F) phase contains almost no Ni and is therefore less expensive than the metastable austenitic stainless steel described above.

  As such martensitic stainless steel, for example, Patent Document 2 discloses a high-strength duplex stainless steel, Patent Document 3 includes a high-strength duplex stainless steel strip or steel plate, and Patent Document 4 includes Ni. A double-phase stainless steel strip for steel belt containing 0.5 to 4% (in this specification, “%” means “mass%” unless otherwise specified) is disclosed in Patent Document 5 as a gasket. Double-phase stainless steel for use, Patent Document 6 for spring dual-phase stainless steel, Patent Document 7 for high-strength dual-phase stainless steel plate with high elasticity, and Patent Document 8 for high-strength with excellent ductility Stainless steel sheets are each disclosed. Further, Patent Document 9 discloses a further multi-phase structure stainless steel in which martensitic stainless steel is subjected to nitrogen absorption treatment to improve the austenite stability of the surface layer portion.

  However, these duplex martensitic stainless steels are also required to have higher strength and excellent workability due to recent downsizing and weight reduction of parts, and performance adjustment is difficult. Furthermore, the above-described applications are required to have excellent wear resistance and fatigue characteristics.

  On the other hand, Patent Documents 10 and 11 mainly focus on austenitic stainless steel and heat it at a relatively low temperature range of 400 to 600 ° C. where no material recovery or recrystallization occurs. Nitrogen treatment, carbonization treatment or carburization treatment or carbonitriding treatment (hereinafter referred to as “surface hardening” in this specification), which has improved wear resistance by diffusing the elements nitrogen and carbon or both. Processing "). The reason why the surface hardening treatment is mainly for austenitic stainless steel is that the solid solubility limit of carbon and nitrogen in the austenite phase is large, whereas the solid solubility limit of the martensite phase and ferrite phase is small. Yes, it is considered difficult to apply to martensitic stainless steel and ferritic stainless steel.

JP 2000-239800 A Japanese Patent No. 3363590 Japanese Patent No. 36002201 Japanese Patent No. 4252893 Japanese Patent No. 4353060 Japanese Patent No. 5257560 JP 2003-89851 A JP 2004-323960 A Japanese Patent No. 3521852 Japanese Patent Application Laid-Open No. 2004-124196 Japanese Patent No. 36002201

  The present invention has progressed in miniaturization and weight reduction, has high strength and excellent workability considered to be optimal for products and parts used after processing into a predetermined shape with strict accuracy, wear resistance, An object is to stably supply an inexpensive Cr-based duplex stainless steel having excellent fatigue characteristics industrially.

As a result of intensive studies to solve the above problems, the present inventor,
Using duplex stainless steel mainly composed of Cr having low cost, high strength and excellent ductility, it is processed into a predetermined shape (in the steel belt manufacturing process of the present invention shown in FIG. 1 described later, a steel plate as a material is cut, Ring welding, annealing for the purpose of softening including homogenization of the welded part, and then reducing to a predetermined plate thickness)
(I) The austenite (A) is heated to the highest temperature not lower than the Ac ₃ transformation point to complete the transformation, and the austenite phase is converted to the austenite phase by absorption of nitrogen, carbon, or both (N and / or C) as austenite stable elements. Then, then
(Ii) Heating from the Ac ₁ transformation point to the Ac ₃ transformation point at which the austenite transformation is initiated to the Ac ₃ transformation point and quenching from the two-phase region of the austenite (A) phase and the ferrite (F) phase, Perform performance adjustment with fine precipitation, and if necessary,
(Iii) It was conceived that the target excellent performance can be obtained by performing a surface hardening treatment at a low temperature below the Ac ₁ transformation point. Hereinafter, in the present specification, these treatments (i) to (iii) are referred to as absorption treatment, two-phase region quenching, and surface hardening treatment, respectively.

  In addition, at the same time as the above idea, the present inventor has conceived that in the two-phase region quenching whose main purpose is performance adjustment, adjustment can be facilitated by expanding the temperature range to a low temperature range.

  Based on these ideas, the present inventor continued earnest research and development, and after going through trial tests and actual machine prototype tests with small ingots with components adjusted, can obtain the main findings A to E listed below, Furthermore, the present invention was completed by obtaining the prospect of practical use with a duplex stainless steel mainly composed of Cr—Mn—C and N.

  (A) For refinement of crystal grains by two-phase quenching, performance adjustment with fine precipitation of compounds, and expansion of the temperature range to low temperature, the material is Cr-Mn-C, N steel with high austenite stability. To do. Thereby, it becomes easier to achieve the austenite stabilization of the surface layer portion by absorption of N and / or C which is also the most powerful austenite stabilizing element, which is performed prior to the two-phase region quenching. From these, Cr-Mn-C, N steel has a dual-phase structure in which the austenite phase may remain up to 5% by volume after quenching in the two-phase region, and the balance is mainly a hard martensite phase and a ferrite phase is mixed. . This multiphase structure corresponds to the vicinity of the center of the plate thickness.

(B) The steel is used as a starting material, and after forming into a predetermined shape by a general process of repeating hot working and cold working and annealing at a temperature below the Ac ₁ transformation point, the absorption treatment is performed in the vicinity of the plate surface. The proportion of the A phase is increased to 50% or more.

  (C) Next, the predetermined performance is adjusted by quenching from the two-phase region. At the same time, by utilizing the temperature difference from the absorption treatment at that time, a large number of fine compounds are precipitated in the austenite phase near the plate surface. By these, the target excellent performance can be obtained.

  (D) Furthermore, as a result of these, grain growth is suppressed by heating to the two-phase region, followed by separation at an angular difference of 5 ° or more that acts in the same way as the grain boundary due to martensitic transformation during cooling. The area to be formed can be finely divided to an average particle diameter equivalent to a circle of 5 μm or less, and higher performance can be achieved. This is considered to correspond to further lightening and downsizing in the future.

  (E) As described above, the two-phase quenching temperature range can be expanded to a low temperature by a general process using general-purpose Cr—Mn—C and N steel.

The present invention is listed below.
(1) The average chemical composition in the entire plate thickness direction is C + N: 0.12-0.6%, Si: 0.1-2.0%, Mn: 1.0-6.0%, Cr: It is composed of 10.0 to 20.0%, the balance Fe and impurities, the proportion of the austenite phase on the plate surface is 50% by volume or more, and a fine region containing C and N in a depth of up to 10 μm from the plate surface The compound is dispersed, the compound having a maximum equivalent particle diameter of 5 μm or more is 30 or less per 5 g of mass, the austenite phase ratio is within 5% by volume at the center of the plate thickness, and the balance is martensite. A duplex stainless steel characterized by comprising a complex phase structure of ferrite.

  (2) The chemical composition contains one or two selected from the group consisting of Ni: 2.0% or less and Cu: 2.0% or less. Duplex stainless steel.

  (3) The chemical composition contains one or more selected from the group consisting of Nb: 0.5% or less, V: 0.5% or less, and Ti: 0.5% or less. (1) The duplex stainless steel described in the item 1 or (2).

  (4) The circle-equivalent average grain size of the region separated at an angle difference of 5 ° or more of crystal orientation is 5 μm or less, which is described in any one of items (1) to (3) Duplex stainless steel.

(5) It has the average chemical composition described in any one of items (1) to (3), the austenite phase ratio is within 5% by volume, and the balance is a combination of martensite and ferrite. After cold working stainless steel mainly composed of phase structure to reduce the thickness to a predetermined thickness, it is heated to the Ac ₃ transformation point or higher to absorb nitrogen or carbon or nitrogen and carbon from the surface, and only the surface layer part Is made into an austenite stabilized state, cooled to near room temperature, then heated to the Ac ₃ transformation point or less and the Ac ₁ transformation point or more, and quenched.

  (6) After the quenching, the composite is further heated and subjected to carbonization or nitriding or carbonizing and nitriding surface hardening treatment with holding at 600 ° C. or lower. A method for producing phase stainless steel.

  With the present invention, miniaturization and weight reduction have progressed, and it has high strength and excellent workability that is considered to be optimal for products and parts used after processing into a predetermined shape with strict accuracy, wear resistance, fatigue It is possible to supply industrially stable inexpensive high-performance Cr-based duplex stainless steel with excellent characteristics.

FIG. 1 is an explanatory view showing an excerpt of a steel belt manufacturing process according to the present invention.

The present invention will be specifically described with reference to the accompanying drawings.
1. Chemical composition [C + N: 0.12-0.6%]
C and N are both strong austenite stabilizing elements and interstitial solid solution strengthening elements that harden the martensite phase. When the total content (C + N) is less than 0.12%, it is difficult to achieve the hardness (400 HV or higher) considered necessary for the above-mentioned application. On the other hand, if the total content (C + N) exceeds 0.6%, the material becomes too hard, a coarse compound is formed, and it is difficult to manufacture a thin plate and ring rolling, and it is difficult to adjust the necessary structure and properties. It becomes. For this reason, total content (C + N) shall be 0.12% or more and 0.6% or less. The lower limit of the total content (C + N) is preferably 0.13%, and the upper limit of the total content (C + N) is preferably 0.56%.

  These are average values over the entire plate thickness. In the present invention, N and / or C is absorbed and diffused from the plate surface in the final step. That is, the total content (C + N) of the starting material before that is lower than this, and is preferably 0.5% or less. This value also corresponds to the value at the center of the plate. That is, the total content (C + N) at the plate surface is a value higher by 0.1% or more than the total content (C + N) at the center of the plate. This is formed by an absorption process, and becomes a larger value in the subsequent surface hardening process.

[Si: 0.1 to 2.0%]
Si is used as a deoxidizer during melting. For this reason, the lower limit of the Si content is set to 0.1%. However, if the Si content is excessive, coarse inclusions are formed, and various properties deteriorate. On the other hand, Si is an effective solid solution strengthening element next to the interstitial solid solution strengthening elements such as C and N, but is also a ferrite stabilizing element and is contained in consideration of a balance with the austenite stabilizing element. For this reason, Si content shall be 0.1% or more and 2.0% or less. The upper limit of Si content is preferably 1.8%.

[Mn: 1.0 to 6.0%]
The Mn content is 1.0% or more and 6.0% or less. Mn is an effective austenite stabilizing alloy element and expands the two-phase region composed of austenite and ferrite at a high temperature to a lower temperature. Thereby, two-phase region quenching at a lower temperature and performance adjustment thereby can be achieved, and a fine grain structure can be obtained, and a plate surface in which the austenite phase is stabilized by absorption of N and / or C can be obtained. Since Mn is an alloy element responsible for adjusting the austenite stability of the starting material and plays an important role, the lower limit of the Mn content is 1.0%, preferably 1.2%. However, when Mn is added excessively, coarse inclusions are formed, making it difficult to manufacture a thin plate or process a product, and deteriorate various properties. For this reason, the upper limit of the Mn content is 6.0%, preferably 5.6%.

[Cr: 10.0-20.0%]
Cr is made 10.0% or more and 20.0% or less. Cr is a basic element of stainless steel, and in order to obtain effective corrosion resistance, the lower limit of the Cr content is 10.0%. Cr is considered to be an element effective for absorption of N and / or C. However, Cr is a ferrite stabilizing element, and excessive addition inhibits austenite stabilization on the plate surface and promotes formation of coarse carbonitride. For this reason, the upper limit of Cr content is set to 20.0.

  The duplex stainless steel according to the present invention may contain the following elements as optional additional elements as required.

[One or two selected from the group consisting of Ni: 2.0% or less and Cu: 2.0% or less]
The contents of Ni and Cu are each preferably 2.0% or less. Ni and Cu are both strong austenite stabilizing alloy elements, which lower the two-phase region composed of austenite and ferrite at high temperatures and enable quenching from lower temperatures. In order to supplement the effect of Mn, the upper limit of the content of each of Ni and Cu is 2.0%, more preferably 1.8%. Moreover, in order to acquire the said effect reliably, it is preferable that Ni and Cu content are each 0.1% or more.

[One or more selected from the group consisting of Nb: 0.5% or less, V: 0.5% or less, and Ti: 0.5% or less]
The contents of Nb, Ti, and V are each preferably 0.5% or less. Any of Nb, V, and Ti may be contained in order to form a compound with C and N and suppress crystal grain growth by their pinning effect. Therefore, the contents of Nb, V, and Ti are each 0.5% or less, preferably 0.4% or less. Moreover, in order to acquire the said effect reliably, it is preferable that content of Nb, V, and Ti is 0.01% or more, respectively.
The balance other than the above is Fe and inevitable impurities.

2. Metal structure [Ratio of austenite phase on the plate surface: 50% by volume or more]
The ratio of the austenite phase on the surface of the plate is 50% by volume or more because the austenite phase formed by solid solution of N and / or C in the absorption treatment with high-temperature heating maintains high strength and workability. This is because the wear resistance and fatigue characteristics are improved due to this. That is, it is a structure mainly composed of an austenite phase. Furthermore, precipitation of a coarse compound is suppressed in the surface hardening process performed as needed. Preferably, the proportion of the austenite phase is 60% by volume or more.

  In these cases, it is considered that at least 0.5% or more of the (C + N) amount is dissolved in the austenite phase on the plate surface.

[Fine compounds containing C and N are dispersed in a region with a depth of 10 μm from the surface of the plate, and 30 or fewer compounds per 5 g of mass with a maximum particle size equivalent to a circle of 5 μm or more]
When the fine compound containing C and N is dispersed in a region having a depth of 10 μm from the surface of the plate and the maximum particle size equivalent to a circle is 30 μm or less per 5 g of mass, the strength is high. In addition, while maintaining excellent workability, the wear resistance and fatigue characteristics are improved together with the above-described effect of solid solution of C and N.

  It is desirable that the compound is fine and numerous, and based on results, the number of equivalent-equivalent particle sizes of 5 μm or more is 30 or less per 5 g of mass. Preferably, it is up to 20 μm from the plate surface, the maximum particle size corresponding to a circle is 4 μm or less, and the number is 20 or less.

[Proportion of austenite phase at the center of the plate thickness: within 5% by volume, and the remainder: multi-phase structure of martensite and ferrite]
The reason why the metal structure of the material is a multiphase structure of a martensite phase and a ferrite phase is that the hard martensite phase shares strength and the soft ferrite phase shares elongation, and these adjust the performance, This is to achieve both high strength and excellent elongation. However, the austenite phase remaining in part is also a phase excellent in workability, and may remain at 5% by volume or less, preferably 4% or less.

[Equivalent circle average grain size of regions separated by an angle difference of 5 ° or more of crystal orientation: 5 μm or less]
It is preferable that an average grain size corresponding to a circle in a region separated at an angle difference of 5 ° or more in crystal orientation is 5 μm or less. When the angle difference is 5 ° or more, the boundary acts like a grain boundary, and it is thought that the effect of grain refinement can be obtained. The grain refinement achieves both high strength and elongation, wear resistance and fatigue strength. This is to improve both. This will correspond to future miniaturization and weight reduction. Preferably, the average particle size is 4 μm or less.

3. Manufacturing Method FIG. 1 is an explanatory view showing an excerpt of a steel belt manufacturing process according to the present invention.

The reason for limitation in each step of the production method according to the present invention will be described.
_First , Cr-based stainless steel having an average chemical composition as described above is used as a starting material, and the thickness is reduced to a predetermined thickness by a general process in which hot working and cold working and annealing at a temperature below the Ac ₁ transformation point are repeated. Thicken.

  These achieve excellent characteristics by adjusting the structure in the production process of the present invention, and consider economic and productivity.

Next, the stainless steel is cold worked to reduce the thickness to a predetermined thickness.
After reducing the thickness to a predetermined plate thickness, heating is performed to the Ac ₃ transformation point or higher, and nitrogen or carbon or nitrogen and carbon are absorbed from the surface to bring only the surface layer portion into an austenite stabilized state and cool to near room temperature.

Heating above the Ac ₃ transformation point to complete the austenite transformation to absorb N and / or C from the surface completes the transformation to the austenite phase with a large solid solubility limit, and allows efficient absorption at high temperatures ( This is because the surface of the plate is stabilized by a diffusion).

  It is desirable that the cooling rate after the austenite stabilized state is high. In addition, it is desirable that the unabsorbed interior is transformed into a martensite and then made into a finer structure by the subsequent two-phase quenching.

Although it depends on the chemical composition of the starting material, from the experimental results, heating and holding are carried out for 30 minutes or less after heating to 950 ° C. or higher, which is considered as the Ac ₃ transformation point. Furthermore, it is preferably maintained for 5 minutes or more and 20 minutes or less after heating to 1000 ° C. or more and 1200 ° C. or less which is considered as the upper limit of an industrial continuous heat treatment furnace.

  The heating atmosphere should just be what N and / or C can absorb. An atmosphere using a gas such as ammonia or methane as well as a mixed gas of N gas alone, commonly used N and reducing H is exemplified.

  However, absorption is suppressed by the formation of the oxide film. For this reason, it is desirable for the dew point of atmospheric gas to be low, and it shall be -40 degrees C or less. More preferably, it is -45 degrees C or less. Cooling is performed at 3 ° C./second or higher up to 200 ° C. when the martensitic transformation is completed. More preferably, the cooling rate is 5 ° C./second or more.

Next, it is quenched by heating to the Ac ₃ transformation point or less and the Ac ₁ transformation point or more. The reason for heating to the Ac ₃ transformation point or less to the Ac ₁ transformation point or more is to adjust the predetermined performance by two-phase region quenching to achieve both high strength and excellent elongation, and to suppress grain growth during holding. Furthermore, it is for making it a fine grain structure by martensitic transformation.

  Although this treatment itself is not affected by the atmosphere, it affects the subsequent surface hardening treatment, so that the atmosphere is preferably the same as described above. The cooling rate is desirably high in order to suppress the precipitation of the coarse compound and complete the martensitic transformation. Specifically, depending on the composition of the starting material, from the results of the experiment, after heating to 950 ° C. or lower and 750 ° C. or higher, the temperature up to 200 ° C. where the martensitic transformation is completed is set to 3 ° C./second or more, preferably 5 ° C / second or more.

  Furthermore, if necessary, after quenching, it is further heated to perform carbonization or nitriding or carbonizing and nitriding surface hardening treatment with holding at 600 ° C. or lower. The reason for this surface hardening treatment is to promote solid solution of carbon and nitrogen and fine precipitation of compounds, and to further improve the wear resistance and fatigue characteristics required for, for example, doctor blades, gears and steel belts. is there. The heating temperature is set to 600 ° C. or less so as not to change the structure adjusted by the previous two-phase quenching.

  In addition, even when nitrogen and carbon for absorption treatment and nitridation and carbonization for surface hardening treatment were replaced, it was confirmed that austenite stabilization and high performance on the plate surface could be achieved in the same way.

  As described above, the present invention maintains a multiphase state in which the central portion occupying most of the thickness direction is excellent in the balance between strength and ductility, and the solid solution and compound of N and / or C in the vicinity of the surface Thus, it is possible to stably supply a material having excellent wear resistance and fatigue characteristics on an industrial scale.

The present invention will be described with reference to examples.
The composition of the test material is shown in Table 1, and an excerpt of a steel belt manufacturing process using a SUS thin plate manufactured by a general process is shown in FIG.

  The sample was made to have a plate thickness of 4.0 mm by hot rolling and annealing from a real machine melted material and a small ingot at the laboratory level. Thereafter, cold rolling and annealing were repeated using laboratory-level equipment to obtain a plate thickness of 0.4 mm. Subsequently, after cold-rolling to plate thickness 0.2mm, it cut | disconnected in strip shape and used for the test.

  Table 2 shows various characteristics from the absorption treatment to after the two-phase region quenching. In addition, with respect to the heat treatment conditions other than those shown in Table 2, the absorption treatment was performed such that the dew point of the atmosphere was −40 ° C. and the holding time was 300 seconds. Quenching was performed in a similar mixed gas atmosphere with 75% by volume of hydrogen (hereinafter referred to as “%”) + 25% of nitrogen and a dew point of −40 ° C., and the holding time was 10 seconds. The cooling rate was 200 ° C for both treatments at 5 ° C / second.

  About the obtained sample, the metal structure, hardness, bendability, wear resistance, and fatigue characteristics were investigated by the methods listed below.

[Metal structure]
As for the ratio of each phase, the austenite phase (ferrite and martensite) phase and other diffraction peaks were measured using an X-ray diffractometer on the surface of the test piece after removing the plate surface and plate thickness by half. The ratio of the austenite phase was calculated from the peak integrated intensity ratio.

  The compound distribution was determined by corroding 5 g or more of the region up to 10 μm from both surfaces of the test piece with 10% by mass bromine-methanol solution, and then extracting the residue through a filter with a hole of a predetermined size. Observation was performed using (SEM: Scanning Electron Microscope), and the total number of compounds having a maximum diameter of 5 μm or more was obtained.

  The average grain size corresponding to a circle in a region separated by an angle difference of 5 ° or more of crystal orientation was measured by using EBSD (Electron BackScatter Diffraction) for some of the vertical cross sections in the rolling direction.

[Hardness]
For the plate surface and the plate thickness center portion in the vertical cross section in the rolling direction after embedding and polishing, using a micro Vickers hardness tester, the number of measurements (n) = 5 was carried out at 0.49 N, and the maximum and minimum The average value was calculated at the center 3 points excluding.

[Bendability]
About the strip-shaped test piece of the predetermined dimension extract | collected in parallel with the rolling direction, it implemented using the right angle bending metal mold | die with a bending radius of 4 mm, and a hydraulic press machine. Subsequently, using an optical microscope or SEM, the presence or absence of a crack on the outer peripheral surface of the bending was confirmed, and the case where there was no crack was evaluated as ◯, and the case where it was present was evaluated as x. Further, a part was investigated with a bending radius of 3 mm and evaluated in the same manner.

[Abrasion resistance]
About the disk-shaped test piece processed into the predetermined dimension, the test piece was rotated in the state which pressed the hard steel ball | bowl for bearing on fixed conditions using the pin-on-disk type testing machine. Then, after cutting the test piece, embedding and polishing the cross section, the reduction in plate thickness at the pressing (wear mark) portion was measured at n = 4, and the average value was calculated. In the evaluation, when the average value thereof was 8 μm or less, the evaluation was ○, and when it exceeded, the evaluation was ×.

[Fatigue test]
For a strip-shaped test piece of a predetermined size taken in parallel with the rolling direction, using a double swing plane bending fatigue tester, the bending axis is perpendicular to the rolling direction and the bending stress on the plate surface is 500 N / mm ² . Were repeatedly bent and examined for the presence or absence of fracture after 10 ⁷ repetitions. The investigation was carried out at n = 16, and all that did not break were marked with ◯, and one that broke even was marked with x. Further, a part was investigated at a bending stress of 600 N / mm ² on the plate surface and evaluated in the same manner. The bending stress on the surface of the plate was measured using a strain gauge at the start of measurement.

  Invention Example No. In Aa to Ah, the amount of austenite phase on the plate surface increases due to the absorption of N and / or C under predetermined conditions, and the metal structure defined in claim 1 is achieved. Thereby, the surface of a board hardens | cures with respect to a center, and the outstanding bendability, abrasion resistance, and a fatigue characteristic are shown. In addition, Invention Example No. This is also confirmed in the case of stainless steel having predetermined components shown in Ba to Ia.

  In contrast, Comparative Example No. Since Ai does not absorb N and / or C, it does not achieve a predetermined austenite phase amount on the plate surface, and no difference from the center portion of the plate thickness is recognized.

  Comparative Example No. Aj is considered to have a heating temperature as low as 900 ° C. with respect to the conditions of the absorption treatment, and sufficient N and / or C absorption is not achieved, and the increase in the amount of austenite phase on the plate surface is small and does not reach 50% by volume. .

  Furthermore, when the influence of the composition of the starting material is seen, Comparative Example No. Since Ja is low in both the (C + N) amount and the Mn amount in the starting material and has low austenite stability, the amount of austenite phase on the plate surface is low even under the processing conditions of the present invention.

  Comparative Example No. La is considered that a strong oxide film was formed by containing a large amount of Si and sufficient absorption was not achieved, and the amount of austenite phase on the plate surface was not achieved.

  Comparative Example No. Ma is a stable austenitic steel containing a large amount of Mn and Ni and does not show an increase in the austenite phase.

As a result, Comparative Examples Ai to Oa did not reach the target performance.
Next, a part of the material shown in Table 2 was subjected to surface hardening treatment. Table 3 shows various characteristics after the surface hardening treatment.

  Invention Example No. Ac1 to Ad3 have a hard plate surface while maintaining a structure almost the same as that after the previous treatment. No coarse compounds are found, and the required bendability, wear resistance, and fatigue properties are satisfied.

  On the other hand, Comparative Example No. Ad4 had a high temperature for surface hardening treatment, could not maintain the structure after the two-phase quenching, the material was softened, coarse compounds were precipitated, and the characteristics were unsatisfactory.

  Comparative Example No. Ai1 did not satisfy the predetermined structure after the two-phase quenching after the absorption treatment, and did not satisfy the required composition of the other starting materials, and the properties after the surface hardening treatment were poor.

  Further, in Table 2, with respect to some invention examples showing relatively close structure and hardness after quenching in the two-phase region after the absorption treatment, the equivalent of a circle separated by an EBSD with a crystal orientation angle difference of 5 ° or more The average particle size of was measured. In addition, the bendability and fatigue characteristics were investigated under more severe conditions. The survey results are shown in Table 4.

  Invention Example No. having an average particle size of 5 μm or less. Ac, Ad, and Bc satisfy bendability and fatigue strength even under more severe conditions, and have superior characteristics as compared to Ae and Bd having an average particle size of more than 5 μm. These have the potential to cope with further miniaturization, weight reduction and higher strength of future products and parts.

  In this way, high performance was confirmed by applying surface hardening treatment to inexpensive Cr-based duplex stainless steel with austenite stabilized in the vicinity of the plate surface by two-phase quenching after absorption treatment of N and / or C did it.

Claims (6)

  The average chemical composition in the entire plate thickness direction is mass%, C + N: 0.12-0.6%, Si: 0.1-2.0%, Mn: 1.0-6.0%, Cr : 10.0-20.0%, balance Fe and impurities, the proportion of austenite phase on the plate surface is 50% by volume or more, and a fine region containing C and N in a depth of 10 μm from the plate surface The compound having a maximum particle diameter equivalent to a circle of 5 μm or more is 30 or less per 5 g of mass, the austenite phase ratio is within 5% by volume at the center of the plate thickness, and the balance is martensite A duplex stainless steel characterized by having a metallic structure composed of a multilayered structure of ferrite and ferrite.   2. The chemical composition according to claim 1, wherein the chemical composition contains one or two selected from the group consisting of Ni: 2.0% or less and Cu: 2.0% or less in mass%. Duplex stainless steel.   The chemical composition contains, by mass%, one or more selected from the group consisting of Nb: 0.5% or less, V: 0.5% or less, and Ti: 0.5% or less. The duplex stainless steel according to claim 1 or 2, characterized by the above.   The circle-equivalent mean grain size of a region separated at an angle difference of 5 ° or more of the crystal orientation in the metal structure is 5 µm or less. Duplex stainless steel. A method for producing the duplex stainless steel according to any one of claims 1 to 4, comprising:
It has the average chemical composition described in any one of claims 1 to 3, the austenite phase ratio is within 5% by volume, and the balance is mainly composed of a multiphase structure of martensite and ferrite. After cold working stainless steel to reduce the thickness to a predetermined plate thickness, heat it to the Ac ₃ transformation point or higher and absorb nitrogen or carbon or nitrogen and carbon from the surface to stabilize only the surface layer austenite And cooling to near room temperature, followed by heating to the Ac ₃ transformation point or less and the Ac ₁ transformation point or more to quench, thereby producing a duplex stainless steel.   6. The duplex stainless steel according to claim 5, wherein after the quenching, the steel is further heated and subjected to carbonization or nitriding or carbonizing and nitriding surface hardening treatment with holding at 600 ° C. or lower. Production method.

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