Duplex Stainless Steel

Abstract

Duplex stainless steel which has: a chemical composition comprising, by mass%, 0.10% or less of C, 3.0% or less of Si, 8.0% or less of Mn, 0.040% or less of P, 0.020% or less of S, 20.0 to 38.0% of Cr, 3.00 to 12.00% of Ni, 1.0 to 6.5% of Mo, 3.0% or less of Cu, 0.200 to 0.700% of N, 0 to 1.0% of Al, 0 to 1.0% of Sn, 0 to 6.0% of W, 0 to 3.0% of Co, 0 to 0.50% of Nb, 0 to 1.5% of Ti, 0 to 1.0% of V, 0 to 0.50% of Zr, 0 to 0.100% of Ta, 0 to 0.100% of B, 0 to 0.50% of Ca, 0 to 0.50% of Mg, 0 to 0.10% of REM, and the balance Fe and impurities; a PRE of 41.0 or higher; a Creq / Nieq of 2.360 to 2.530; an average Md value of 0.9140 or less; and an σ-phase area ratio of 2.0% or less.

Description

[Technical Field]

[0001] The present invention relates to duplex stainless steel. [Background technology]

[0002] Duplex stainless steel is a stainless steel that has both austenite and ferrite phases in its structure. Duplex stainless steel has excellent corrosion resistance and high strength, and its high corrosion resistance is being utilized in a variety of fields, including petrochemical equipment materials, pump materials, and chemical tank materials.

[0003] For example, Patent Document 1 discloses an Sn-containing duplex stainless steel, a duplex stainless steel slab, and a duplex stainless steel material that are inexpensive and have good hot producibility.

[0004] PRE (Pitting Resistance Equivalent: Cr + 3.3Mo + 16N) is known as a parameter that indicates the corrosion resistance of duplex stainless steel, particularly its pitting corrosion resistance, and generally, the composition is designed by adjusting the contents of Cr, Mo, and N to increase the PRE value. In recent years, there has been a demand for steel materials with a PRE of 40 or more in order to improve corrosion resistance.

[0005] On the other hand, duplex stainless steels with increased Cr and Mo contents have the problem of proneness to the precipitation of intermetallic compounds such as the σ phase, which reduces mechanical properties and corrosion resistance. The precipitation of these σ phases significantly hardens the material, making it prone to cracking and significantly reducing hot workability. In addition, the toughness around the intermetallic compounds deteriorates even in the final product, making it difficult to ensure the desired performance.

[0006] Patent Document 2 discloses a method for continuously casting highly corrosion-resistant duplex stainless steel that has better embrittlement resistance, castability, and hot workability while maintaining high corrosion resistance by suppressing the precipitation of intermetallic compounds such as embrittling phases, such as the σ phase and X phase, during the production of the highly corrosion-resistant duplex stainless steel. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-119627 [Patent Document 2] Japanese Patent Application Publication No. 2017-80765 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in Patent Document 2, the content of Ni, which contributes to stabilizing the austenite phase, improving toughness, and suppressing nitride precipitation, is 7.0% or less, so there is a risk that these effects may not be fully achieved, and there is still room for improvement.

[0009] However, if the Ni content is high, Cr and Mo are concentrated in the ferrite phase, promoting the precipitation of the σ phase. Cast slabs made of steel containing a large amount of σ phase are very prone to cracking, which makes subsequent hot working difficult.

[0010] An object of the present invention is to provide a duplex stainless steel that can suppress cracking due to a decrease in toughness even when the PRE value is high and the Ni content is high. [Means for solving the problem]

[0011] As a result of extensive research by the present inventors to solve the above problems, the present inventors have come to the following findings.

[0012] (a) To prevent cracking of duplex stainless steel slabs, improvement of toughness is required.

[0013] (b) By properly managing the ratio of Cr equivalent (Creq) to Ni equivalent (Nieq) and controlling the solidification morphology, it is possible to suppress the decrease in toughness.

[0014] (c) By controlling the Md value, which is an index of the ease of forming intermetallic compounds, to a predetermined value or less and controlling the cooling conditions after casting, it is possible to suppress the precipitation of the σ phase.

[0015] (d) By satisfying these conditions, even if the PRE value is high and the Ni content is high, the resulting duplex stainless steel has good toughness and can suppress cracking.

[0016] The present invention has been made based on the above findings, and the gist of the present invention is the following duplex stainless steel.

[0017] (1) Chemical composition, in mass%, C: 0.10% or less, Si: 3.0% or less, Mn: 8.0% or less, P: 0.040% or less, S: 0.020% or less, Cr: 20.0~38.0%, Ni: 3.00~12.00%, Mo: 1.0-6.5% Cu:3.0% or less, N: 0.200~0.700%, Al: 0-1.0% Sn: 0 to 1.0% W: 0-6.0%, Co: 0-3.0% Nb: 0 to 0.50% Ti: 0 to 1.5% V: 0 to 1.0%, Zr: 0 to 0.50% Ta: 0 to 0.100%, B: 0~0.100%, Ca: 0-0.50% Mg: 0-0.50% REM: 0~0.10%, The balance is Fe and impurities. The value of PRE defined by the following formula (i) is 41.0 or more, The ratio Creq / Nieq of Creq defined by the following formula (ii) to Nieq defined by the following formula (iii) is 2.360 to 2.530, The average Md value defined by the following formula (iv) is 0.9140 or less, The area ratio of the σ phase contained in the metal structure is 2.0% or less. Duplex stainless steel. PRE = Cr + 3.3Mo + 16N (i) Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti...(ii) Nieq=Ni+0.31Mn+22C+14.2N+Cu (iii) Average Md value = ΣX i (Md) i (iv) In the above formulas (i) to (iii), the element symbols represent the content (mass %) of each element, and the symbols in the above formula (iv) have the following meanings: X i : atomic fraction of alloy component i (Md) i : Md value of alloy component i (eV)

[0018] (2) A continuous casting process for continuously casting molten steel having the chemical composition described in (1) above, In the continuous casting process, the slab is primarily cooled to a temperature range of 950 to 1050°C, then reheated until the maximum temperature reaches 1050°C or higher, and then cooled under conditions such that the residence time in the temperature range of 900 to 1000°C is 400 seconds or less. Manufacturing method of duplex stainless steel.

[0019] (3) The method further includes a hot rolling step of hot rolling the slab, In the hot rolling step, the slab is heated in a temperature range of 1150 to 1300°C for 1.5 hours or more, and then hot rolled under conditions where the finish rolling temperature is 900 to 1110°C, and then cooled to a temperature range of 500°C or less under conditions where the average cooling rate within a temperature range of 800 to 500°C is 0.1 to 1.0°C / s. The method for producing the duplex stainless steel according to (2) above. [Effects of the Invention]

[0020] According to the present invention, it is possible to obtain a duplex stainless steel that can suppress cracking due to a decrease in toughness even when the PRE value is high and the Ni content is high. DETAILED DESCRIPTION OF THE INVENTION

[0021] Each of the requirements of the present invention will be described in detail below.

[0022] 1.Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0023] C: 0.10% or less C is an element that dissolves in the austenite phase to increase strength. However, if it is contained in large amounts, carbide precipitation occurs, resulting in a decrease in corrosion resistance. Therefore, the C content is set to 0.10% or less, preferably 0.050% or less. In consideration of aging corrosion resistance, the C content is more preferably 0.030% or less. There is no need to set a lower limit for the C content, but if the above-mentioned effects are desired, it is preferably 0.010% or more, more preferably 0.015% or more.

[0024] Si:3.0% or less Si is used as a deoxidizing element and is sometimes added to improve oxidation resistance. However, a large amount of Si hardens the steel and deteriorates its workability. Therefore, the Si content is set to 3.0% or less, preferably 2.0% or less or 1.0% or less. There is no need to set a lower limit for the Si content, but if the above-mentioned effects are desired, it is preferably 0.10% or more, and more preferably 0.20% or more.

[0025] Mn: 8.0% or less Mn has the effect of increasing the austenite phase, increasing the solid solubility of nitrogen, and suppressing pore defects during manufacturing. However, a large amount of Mn reduces corrosion resistance. Therefore, the Mn content is set to 8.0% or less, preferably 3.0% or less or 1.0% or less. There is no need to set a lower limit for the Mn content, but if the above effects are desired, it is preferably 0.20% or more, and more preferably 0.40% or more.

[0026] P:0.040% or less P is an element that inevitably gets mixed into steel and is also contained in raw materials such as Cr, so it is difficult to reduce its content. However, a large amount of P reduces formability. The lower the P content, the better, and it should be 0.040% or less. The P content is preferably 0.030% or less.

[0027] S: 0.020% or less S is an element that inevitably gets mixed into steel, and it can combine with Mn to form inclusions that can become the starting point for rust. Therefore, the S content is set to 0.020% or less. The lower the S content, the better the corrosion resistance, so it is preferably 0.010% or less, and more preferably 0.0050% or less.

[0028] Cr: 20.0 to 38.0% Cr is an element necessary for ensuring corrosion resistance. In addition, Cr is a ferrite stabilizing element, and in order to obtain a two-phase structure of austenite and ferrite, a Cr content of 20.0% or more is required, taking into account the phase ratio. However, a large Cr content can actually result in a decrease in corrosion resistance. Therefore, the Cr content is set to 38.0% or less. The Cr content is preferably 22.0% or more or 24.0% or more, and preferably 33.0% or less, 28.0% or less, or 27.0% or less.

[0029] Ni: 3.00 to 12.00% Ni is an austenite stabilizing element. Ni also has the effect of improving corrosion resistance. Therefore, the Ni content is set to 3.00% or more. However, a large amount of Ni content increases raw material costs and may cause problems such as stress corrosion cracking. Therefore, the Ni content is set to 12.00% or less. The Ni content is preferably 5.00% or more, more preferably 7.50% or more, and preferably 10.00% or less.

[0030] Mo: 1.0 to 6.5% Mo is an element that improves corrosion resistance. Therefore, the Mo content is set to 1.0% or more. However, a large amount of Mo not only increases the raw material cost but also leads to a decrease in corrosion resistance. Therefore, the Mo content is set to 6.5% or less. The Mo content is preferably 2.0% or more, more preferably 3.0% or more, and is preferably 5.5% or less, and more preferably 4.4% or less.

[0031] Cu:3.0% or less Cu is an element that is very effective in improving sulfuric acid resistance. However, a large amount of Cu actually leads to a decrease in corrosion resistance. Therefore, the Cu content is set to 3.0% or less. The Cu content is preferably 2.0% or less, and more preferably 0.90% or less. There is no need to set a lower limit for the Cu content, but if the above effect is desired, the Cu content is preferably 0.10% or more, and more preferably 0.20% or more.

[0032] N: 0.200~0.700% Nitrogen (N) is an element that dissolves in the austenite phase to increase strength and corrosion resistance, contributing to alloy saving. Therefore, the N content is set to 0.200% or more. However, if the N content is too high, defects such as blowholes will occur, degrading the corrosion resistance of the steel. Therefore, the N content is set to 0.700% or less. The N content is preferably 0.240% or more, and 0.450% or less.

[0033] Al: 0 to 1.0% Al is an optional element and does not necessarily need to be included. When included, Al exhibits desulfurization and deoxidation effects. However, if a large amount of Al is included, it can precipitate hard spinel inclusions (MgO·Al2O3) that cause nozzle clogging, as well as increase manufacturing defects and raw material costs. Therefore, the Al content should be 1.0% or less. The Al content should preferably be 0.50% or less or 0.10% or less. To ensure the above effects, the Al content should preferably be 0.010% or more.

[0034] Sn: 0 to 1.0% Sn is an optional element and does not necessarily need to be contained. When contained, Sn increases the corrosion resistance of steel. However, Sn is an element that inhibits the workability of steel. Therefore, the Sn content is set to 1.0% or less. The Sn content is preferably set to 0.50% or less or 0.10% or less. To ensure the above effects, the Sn content is preferably set to 0.002% or more.

[0035] W: 0-6.0% W is an optional element and does not necessarily need to be included. When included, W improves the SCC resistance and pitting corrosion resistance of the steel. Furthermore, W is less likely to form σ phase than Mo. Therefore, W may be included in place of part of Mo. The above effects can be achieved to some extent even if even a small amount of W is included. However, if the W content is too high, the manufacturing cost increases. Therefore, the W content is set to 6.0% or less. The W content is preferably 3.0% or less, and more preferably 1.0% or less. To ensure the above effects, the W content is preferably 0.01% or more, and more preferably 0.10% or more.

[0036] Co: 0-3.0% Co is an optional element and does not necessarily need to be included. When included, Co increases the strength of the steel. Co also stabilizes austenite. Even if even a small amount of Co is included, the above effects can be obtained to a certain extent. However, if the Co content is too high, the corrosion resistance of the steel decreases and the manufacturing cost increases. Therefore, the Co content is set to 3.0% or less. The Co content is preferably 2.0% or less or 1.0% or less. To ensure the above effects, the Co content is preferably 0.01% or more, and more preferably 0.05% or more.

[0037] Nb: 0 to 0.50% Nb is an optional element and does not necessarily need to be contained. When contained, Nb increases the strength of steel. Even if even a small amount of Nb is contained, the above effects can be obtained to a certain extent. However, if the Nb content is too high, the corrosion resistance of the steel decreases. Therefore, the Nb content is set to 0.50% or less. The Nb content is preferably 0.30% or less, 0.10% or less, or 0.050% or less. To ensure the above effects, the Nb content is preferably 0.005% or more.

[0038] Ti: 0 to 1.5% Ti is an optional element and does not necessarily need to be contained. When contained, Ti increases the strength of the steel. Even if even a small amount of Ti is contained, the above effects can be obtained to a certain extent. However, if the Ti content is too high, the corrosion resistance of the steel decreases. Therefore, the Ti content is set to 1.5% or less. The Ti content is preferably 0.50% or less, 0.10% or less, or 0.050% or less. To ensure the above effects, the Ti content is preferably 0.005% or more.

[0039] V: 0 to 1.0% V is an optional element and does not necessarily need to be contained. When contained, V increases the strength of the steel. Even if even a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content is too high, the corrosion resistance of the steel decreases. Therefore, the V content is set to 1.0% or less. The V content is preferably 0.80% or less, 0.50% or less, or 0.30% or less. To ensure the above effect, the V content is preferably 0.01% or more, or 0.05% or more.

[0040] Zr: 0 to 0.50% Zr is an optional element and does not necessarily need to be contained. When contained, Zr contributes to improving corrosion resistance. Even if even a small amount of Zr is contained, the above effect can be obtained to a certain extent. However, if the Zr content is too high, the effect saturates. Therefore, the Zr content is set to 0.50% or less. The Zr content is preferably 0.40% or less or 0.30% or less. To ensure the above effect, the Zr content is preferably 0.005% or more.

[0041] Ta: 0 to 0.100% Ta is an optional element and does not necessarily need to be contained. When contained, Ta improves corrosion resistance by modifying inclusions. However, if the Ta content is too high, it will result in a decrease in ductility at room temperature. Therefore, the Ta content should be 0.100% or less. The Ta content is preferably 0.050% or less. To ensure the above effects, the Ta content is preferably 0.005% or more.

[0042] B: 0 to 0.100% B is an optional element and does not necessarily need to be contained. When contained, B improves hot workability. Even if even a small amount of B is contained, the above effect can be obtained to a certain extent. However, if the B content is too high, the above effect saturates. Therefore, the B content is set to 0.100% or less. The B content is preferably 0.0100% or less, and more preferably 0.0050% or less. To ensure the above effect, the B content is preferably 0.0001% or more, and more preferably 0.0003% or more.

[0043] Ca: 0 to 0.50% Ca is an optional element and does not necessarily need to be contained. When contained, Ca has the effects of desulfurization, deoxidation, and preventing the formation of spinel-based inclusions. However, a large amount of Ca content reduces corrosion resistance and increases the amount of spatter generated during welding. Therefore, the Ca content is set to 0.50% or less. The Ca content is preferably 0.050% or less, more preferably 0.010% or less, and even more preferably 0.0040% or less. To ensure the above effects, the Ca content is preferably 0.0010% or more, and more preferably 0.0015% or more.

[0044] Mg: 0 to 0.50% Mg is an optional element and does not necessarily need to be included. If included, Mg forms sulfides with S in the steel, reducing the segregation of S to grain boundaries. As a result, the corrosion resistance of the steel increases and contributes to improved hot workability. Even if even a small amount of Mg is included, the above effects can be achieved to some extent. However, if the Mg content is too high, coarse oxides or sulfides form, which become the starting point for pitting corrosion. As a result, the corrosion resistance of the steel decreases. Therefore, the Mg content is set to 0.50% or less. The Mg content is preferably 0.050% or less, more preferably 0.010% or less, and even more preferably 0.0040% or less. To ensure the above effects, the Mg content is preferably 0.0005% or more.

[0045] REM: 0 to 0.10% REM is an optional element and does not necessarily need to be contained. When contained, REM improves the hot workability of steel. Therefore, it is desirable to contain a small amount of REM. However, excessive REM content reduces the corrosion resistance of steel, so the REM content should be 0.10% or less. The REM content is preferably 0.050% or less, and more preferably 0.010% or less. To ensure the above effects, the REM content is preferably 0.0005% or more, or 0.005% or more.

[0046] In the present invention, REM refers to a total of 17 elements, including Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements. Note that lanthanoids are industrially added in the form of misch metals.

[0047] In the chemical composition of the duplex stainless steel of the present invention, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to raw materials such as ore and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0048] The chemical composition of the duplex stainless steel of the present invention must have the content of each element within the ranges described above, and in addition, the PRE value and Creq / Nieq value calculated by the formula shown below must each be within a specified range.

[0049] PRE: 41.0 or higher PRE is a general index of the corrosion resistance of stainless steel and is calculated from the chemical composition of the steel using the following formula (i). By designing an alloy so that the PRE value is 41.0 or higher, it is possible to ensure excellent corrosion resistance. While there is no need to set an upper limit for the PRE value, an excessively high value can lead to problems such as increased alloy costs. Therefore, it is preferable for the PRE value to be 60.0 or less. PRE = Cr + 3.3Mo + 16N (i) However, the element symbols in the above formula represent the content (mass %) of each element contained in the steel.

[0050] Creq / Nieq: 2.360~2.530 Creq and Nieq are defined by the following equations (ii) and (iii), respectively. By controlling the Creq / Nieq value to 2.360 or more, F-mode solidification can be achieved, making it possible to ensure toughness. The Creq / Nieq value is preferably 2.400 or more. On the other hand, if the Creq / Nieq is too high, a ferrite single-phase structure will result, and the properties of dual-phase steel will not be obtained. Therefore, the Creq / Nieq value should be 2.530 or less. Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti...(ii) Nieq=Ni+0.31Mn+22C+14.2N+Cu (iii) However, the element symbols in the above formula represent the content (mass %) of each element contained in the steel.

[0051] When the Creq / Nieq value is low and FA mode solidification occurs, the metal structure is mainly composed of vermicular ferrite, in which austenite crystallizes during solidification. In a metal structure mainly composed of vermicular ferrite, the interfacial coherence between the ferrite and austenite phases is low, making it easier for cracks to propagate along the phase boundary, resulting in a decrease in toughness.

[0052] On the other hand, when the Creq / Nieq value is equal to or greater than a predetermined value, the material undergoes F-mode solidification, which means that the material solidifies as a single ferrite phase. In the F-mode solidification, the material completely solidifies as ferrite, and then austenite precipitates through a solid-state transformation, resulting in a metal structure mainly composed of acicular ferrite. When the metal structure is mainly composed of acicular ferrite, the interfacial coherence between ferrite and austenite is high, and a decrease in toughness can be suppressed.

[0053] 2. Md value The Md value is an index of phase stability in a multi-component system and represents the electron orbital energy in the d orbital of each component of the alloy. The higher the Md value, the more unstable the phase becomes, making it easier for intermetallic compounds such as the σ phase to form. In the present invention, the average Md value defined by the following formula (iv) is set to 0.9140 or less in order to suppress the precipitation of intermetallic compounds. To further suppress the precipitation of intermetallic compounds, a value of 0.9090 or less is desirable. Average Md value = ΣX i (Md) i (iv) However, the meanings of the symbols in the above formula (iv) are as follows: X i : atomic fraction of alloy component i (Md) i : Md value of alloy component i (eV)

[0054] In order to suppress the precipitation of intermetallic compounds, the lower the average Md value, the better, so there is no need to set a lower limit. However, in the composition system specified in the present invention, it is difficult to make the average Md value less than 0.8800. Therefore, the average Md value may be 0.8800 or more.

[0055] The Md value of alloy component i can be calculated by cluster calculation (a molecular orbital calculation method using a cluster model consisting of several to several tens of atoms) (M. Morinaga et al., J. Phys. Soc. Jpn., 53 (1984), p. 653). The average Md value of an alloy can be calculated by converting the composition of the grain boundary and final solidification part, which is determined from the initial composition and segregation ratio, into atomic fractions, and then calculating X. i By calculating the above, it is possible to organize the precipitation of intermetallic compounds.

[0056] 3.Metal structure In the duplex stainless steel according to the present invention, the area fraction of the σ phase contained in the metal structure is 2.0% or less. As described above, in addition to the PRE value, when the Ni content is high, precipitation of the σ phase is promoted. In particular, when the area fraction of the σ phase exceeds 2.0%, the deterioration of toughness becomes significant. Therefore, the area fraction of the σ phase is set to 2.0% or less. The area fraction of the σ phase is preferably 1.0% or less, more preferably 0.10% or less, and even more preferably 0.05% or less. The lower the area fraction of the σ phase, the better, so there is no need to set a lower limit.

[0057] There are no particular restrictions on other metal structures. However, by adjusting the Creq / Nieq value within the above-mentioned range, a two-phase structure of ferrite and austenite is obtained, and F-mode solidification occurs. In this case, it is preferable that the metal structure contains 50% or less of acicular ferrite in terms of area fraction, with the remainder being austenite and unavoidable products. In the above-mentioned metal structure, the area fraction of austenite is relatively high, making it possible to improve toughness.

[0058] In addition to the σ phase mentioned above, unavoidable products may include Cr2N, etc., which are permissible if their total content is 2.0% or less.

[0059] In the present invention, the area ratios of ferrite and austenite are measured using a ferrite scope in accordance with JIS Z 3119: 2017. Whether the ferrite is mainly composed of vermicular ferrite or acicular ferrite can be determined by observing the structure using an optical microscope at a magnification of 50 times.

[0060] Furthermore, a sample for microscopic observation was cut out so that the observation surface was at a depth of 5 mm from the surface of the slab, and the σ phase was revealed by KOH electrolytic etching. Microstructure images were then taken of 60 fields of view at 400x magnification using an optical microscope. The obtained images were then binarized to measure the σ phase area ratio. Because the σ phase is distributed unevenly throughout the structure, samples were taken from five or more locations on the slab, and the average of the measured values ​​for each sample was used as the σ phase area ratio.

[0061] 4. Manufacturing method The duplex stainless steel according to the present invention can be produced, for example, by continuously casting molten steel having the above-described chemical composition. That is, the duplex stainless steel according to the present invention may be a cast piece. In this case, it is important to control the casting conditions. The present inventors first conducted the following investigation into the casting conditions for suppressing the precipitation of the σ phase.

[0062] In the continuous casting process, the slab is cooled mainly through two processes: primary cooling using a water-cooled copper mold, and secondary cooling in which cooling spray is sprayed from the slab surface. The temperature history at a depth of 5 mm from the slab surface was investigated using heat transfer analysis when the amount of water used in secondary cooling was varied. The casting speed was 1.1 m / min.

[0063] The precipitation nose of the σ phase is thought to occur at around 900 to 1000°C. Therefore, shortening the residence time within the temperature range of 900 to 1000°C during the cooling process after casting is effective in suppressing the precipitation of the σ phase.

[0064] As a result of the investigation, it was found that by setting the water flow rate during secondary cooling, which has traditionally been around 80 L / min, to 0 L / min and allowing the cast piece to cool naturally, the time that the surface layer (5 mm deep) of the cast piece remains in the temperature range of 900 to 1000°C, which is near the σ phase precipitation nose, can be minimized.

[0065] Specifically, after casting, the surface layer of the slab is primarily cooled to a temperature range of 950 to 1050°C, and then the temperature is raised to a maximum of 1050°C or higher by recuperating heat from the center of the slab, and then the slab is allowed to cool naturally.This makes it possible to reduce the residence time within the temperature range of 900 to 1000°C to 400 seconds or less and to reduce the area ratio of the σ phase to 2.0% or less.

[0066] In order to reduce the area ratio of the σ phase, the shorter the residence time, the better, and it is preferably 300 seconds or less.

[0067] The duplex stainless steel of the present invention may also be a hot-rolled material in the form of a plate or a rod. In this case, the method for producing a duplex stainless steel of the present invention further comprises a hot-rolling step of hot-rolling the slab. The conditions for the hot-rolling step are not particularly limited, but it is preferable to perform the hot-rolling step under the following conditions, for example.

[0068] Before hot rolling, the slab is preferably heated at a temperature in the range of 1150 to 1300°C for 1.5 hours or more. This allows the σ phase precipitated in the slab to be redissolved. Here, the heating temperature and heating time refer to the average temperature in the furnace and the time spent in the furnace, respectively.

[0069] The heated slab is then subjected to rough rolling and finish rolling. In this case, the finish rolling temperature is preferably 900 to 1110°C. After the finish rolling is completed, it is preferably cooled to a temperature range of 500°C or less under conditions where the average cooling rate within the temperature range of 800 to 500°C is 0.1 to 1.0°C / s. It is more preferable that the average cooling rate is 0.7°C / s or less. There are no particular limitations on the cooling method, and air cooling may be used, for example.

[0070] There are no particular restrictions on the cooling rate during cooling from 500°C or below to room temperature, and cooling can be done by air cooling, mist water cooling, water cooling, or the like. Here, the finish rolling temperature refers to the surface temperature of the hot-rolled material at the exit of the final stand of a rolling mill equipped with multiple stands. Furthermore, the cooling rate after finish rolling refers to the cooling rate at the surface of the hot-rolled material.

[0071] Furthermore, the duplex stainless steel of the present invention may be a cold-rolled material obtained by cold-rolling the above-mentioned hot-rolled material. Cold rolling may be performed using a conventional method. The above-mentioned hot-rolled material or cold-rolled material may be annealed to form an annealed hot-rolled material or an annealed cold-rolled material. In this case, from the viewpoint of suppressing precipitation of the σ phase, the annealing temperature is preferably, for example, 550 to 900°C.

[0072] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0073] Cylindrical cast slabs with a diameter of 180 mm and having the chemical compositions shown in Table 1 were produced under various production conditions. The continuous casting conditions for each cast slab are shown in Table 2.

[0074] [Table 1]

[0075] [Table 2]

[0076] Using the obtained cast slab, the metal structure was measured specifically according to the following procedure. First, the area ratios of ferrite and austenite were measured using a ferrite scope in accordance with JIS Z 3119:2017. Whether the ferrite was mainly composed of vermicular ferrite or acicular ferrite was determined by observing the structure using an optical microscope at 50x magnification.

[0077] Furthermore, specimens for microscopic observation were cut out from five locations so that the observation surface was a depth of 5 mm from the surface of the slab, and the σ phase was revealed by KOH electrolytic etching. Microstructure images were then taken of 60 fields of view at 400x magnification using an optical microscope, and the obtained images were binarized to measure the σ phase area ratio. The average value of the measurements for the five specimens was taken as the σ phase area ratio.

[0078] Furthermore, the toughness of each slab was evaluated. V-notch test pieces were prepared from the surface of each slab at a position 5 mm below the surface. The test pieces measured 10 mm x 10 mm x 55 mm and were subjected to a Charpy impact test in accordance with JIS Z 2242:2005. The impact properties were evaluated as follows: an impact value of 30.0 J / cm at 100°C. 2 Above 30.0J / cm is considered good. 2 If it was less than this, it was considered to be poor.

[0079] The results are also shown in Table 2. As can be seen from Table 2, the impact values ​​were poor in Test Nos. 1, 3, 5, 7, 9, and 11 to 13, where the area ratio of the σ phase exceeded 2.0% or the metal structure was mainly composed of vermicular ferrite. In these cases, cracks had occurred during the slab production stage.

[0080] On the other hand, in Test Nos. 2, 4, 6, 8, 10 and 14 to 16, which satisfied the requirements of the present invention, no cracks were generated in the resulting slabs and the impact values ​​were also good. These slabs were further evaluated for hot workability.

[0081] Test pieces with a diameter of 8 mm and a length of 110 mm were cut from the surface layer of each cast slab. The temperature was then raised from room temperature to 1250°C in 30 seconds and held at that temperature for 30 seconds. The pieces were then cooled to 1000°C at a cooling rate of 20°C / s and held at that temperature for 30 seconds. Tensile tests were then conducted to measure the tensile strength and reduction of area.

[0082] Furthermore, the slabs of Test Nos. 2, 4, 6, 8, 10, and 14 to 16, which satisfied the requirements of the present invention, were hot-rolled to obtain hot-rolled materials (wire rods) with a diameter of 5.5 mm. Specifically, the slabs were heated at 1200°C for 2 hours, and then hot-rolled under conditions where the finish rolling temperature was 1100°C. Subsequently, they were air-cooled to 400°C under conditions where the average cooling rate was 0.5°C / s within the temperature range of 800 to 500°C, and then water-cooled to room temperature.

[0083] The hot-rolled material was then cut into five specimens for microscopic observation, with the cross sections perpendicular to the longitudinal and radial directions serving as the observation surfaces. The σ-phase was then revealed by KOH electrolytic etching. Microstructure images were then taken of 60 fields of view at 400x magnification using an optical microscope, and the resulting images were binarized to measure the σ-phase area ratio. The average of the measurements for the five specimens was taken as the σ-phase area ratio.

[0084] As shown in Table 2, the examples of the present invention had good hot workability with a reduction of area of ​​60.0% or more at 1000°C, and furthermore, it was possible to suppress the area ratio of the σ phase in the hot-rolled material to 2.0% or less. [Industrial Applicability]

[0085] According to the present invention, it is possible to obtain a duplex stainless steel that can suppress cracking due to a decrease in toughness even when the PRE value is high and the Ni content is high.

Claims

1. The chemical composition, in mass%, is C: 0.10% or less, Si: 0.45% or less, Mn: 0.88% or less, P: 0.040% or less, S: 0.020% or less, Cr: 20.0-25.9%, Ni: 7.50-9.29%, Mo: 1.0 to 4.0%, Cu: 0.59% or less, N: 0.200-0.700%, Al: 0-1.0%, Sn: 0 to 1.0%, W: 0 to 6.0%, Co: 0-3.0%, Nb: 0 to 0.50%, Ti: 0 to 1.5%, V: 0 to 1.0%, Zr: 0 to 0.50%, Ta: 0-0.100%, B: 0-0.100%, Ca: 0-0.50%, Mg: 0 to 0.50%, REM: 0-0.10%, The balance is Fe and impurities. The value of PRE defined by the following formula (i) is 41.0 or more, a ratio Cr / Ni of Cr defined by the following formula (ii) to Ni defined by the following formula (iii) is 2.360 to 2.530, The average Md value defined by the following formula (iv) is 0.9140 or less, The area ratio of the σ phase contained in the metal structure is 2.0% or less. Duplex stainless steel. PRE=Cr+3.3Mo+16N...(i) Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti...(ii) Nieq=Ni+0.31Mn+22C+14.2N+Cu...(iii) Average Md value = ΣX ｉ (Md) ｉ ...(iv) In the above formulas (i) to (iii), the element symbols represent the content (mass%) of each element, and the symbols in the above formula (iv) have the following meanings: X ｉ : atomic fraction of alloy component i (Md) ｉ : Md value of alloy component i (eV)

2. A method for producing the duplex stainless steel of claim 1, comprising: a continuous casting process for continuously casting molten steel having the chemical composition according to claim 1, wherein the PRE value is 41.0 or more, the Cr / Ni value is 2.360 to 2.530, and the average Md value is 0.9140 or less, In the continuous casting process, the slab is primarily cooled to a temperature range of 950 to 1050 ° C, then reheated until the maximum temperature reaches 1050 ° C or higher, and then cooled under conditions such that the residence time in the temperature range of 900 to 1000 ° C is 400 seconds or less. Manufacturing method of duplex stainless steel.

3. The method further includes a hot rolling step of hot rolling the slab, In the hot rolling process, the cast piece is heated in a temperature range of 1150 to 1300 ° C. for 1.5 hours or more, and then hot rolled under conditions where the finish rolling temperature is 900 to 1110 ° C., and then cooled to a temperature range of 500 ° C. or less under conditions where the average cooling rate within a temperature range of 800 to 500 ° C. is 0.1 to 1.0 ° C. / s. The method for producing the duplex stainless steel according to claim 2.

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