Ferritic-austenitic duplex stainless steel sheet and method for manufacturing the same

Abstract

To provide a ferrite and austenite two-phase stainless steel plate excellent in hardness and toughness and its manufacturing method.SOLUTION: Steel having a predetermined chemical composition is hot rolled under the conditions that the rolling reduction ratio of rolling at a temperature Tc (°C) or lower is 1.2 or more, is hot-rolled under the condition where the final rolling pass temperature is (Tc-100)°C or higher, by air cooling for 20 seconds or more after the hot rolling, and then cooling at a cooling rate of 1°C / s or more in the temperature range from 800°C to 600°C, a PREN_Mn value is 35.0 or less, and Charpy impact value vE-20°C at -20°C is 100 J / cm2 or more, and a ferrite-austenite two-phase stainless steel plate having a Brinell hardness HBW3000 of 230 or more can be obtained. PREN_Mn value=Cr+3.3(Mo+0.5W)+16N-Mn Tc(°C)=930+50Mo(°C)SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic-austenite duplex stainless steel sheet and a method for manufacturing the same. [Background technology]

[0002] Duplex stainless steel is a type of stainless steel that has both austenite and ferrite phases in its microstructure. Compared to austenitic stainless steel, which generally has comparable corrosion resistance, duplex stainless steel is attracting attention as a material that can achieve both strength and corrosion resistance at a low cost because it has a low nickel content, resulting in lower alloy costs and high strength.

[0003] In structures such as rivers and dam facilities, if ordinary steel is used in areas that come into contact with rainwater and river water, corrosion will occur in those areas. Therefore, painting or plating is applied, or stainless steel is used. In particular, in sliding parts included in gates, friction occurs due to the operation of equipment such as gate opening and closing, and in lining materials, contact with stones and gravel occurs, significantly accelerating the deterioration of the coating or plating. For this reason, stainless steel, which is less affected by friction, is widely used.

[0004] In sliding parts, SUS304N2, which contains nitrogen (N) to increase hardness, is widely used from the viewpoint of wear resistance. However, in river facilities with gates, such as sluice gates at river mouths, seawater mixes in, significantly increasing the chloride ion concentration of the river water and creating a harsh corrosive environment. In such places, SUS304N2 with a low chromium content may not be able to ensure the desired corrosion resistance. Therefore, there is a need for an alternative using duplex stainless steel that is both high-strength and highly corrosion-resistant.

[0005] The surface hardness of solution-treated SUS304N2 is approximately 200 HBW (Brinell hardness). If further improvement in wear resistance is required, the material is hot-rolled at a lower temperature than usual, and post-rolling annealing is omitted to retain processing strain and increase strength before application. Similar manufacturing methods may be necessary when using duplex stainless steel as a substitute, depending on the required wear resistance.

[0006] Patent documents 1 and 2 state that PRE W、Mn A method for producing duplex stainless steel with a surface hardness of HBW230 or higher and excellent wear resistance and weld corrosion resistance is described, by hot rolling with a finishing rolling temperature of 1000°C to 800°C and a cooling rate of 1°C / s or higher in the 800°C to 600°C section. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-100872 [Patent Document 2] Japanese Patent Publication No. 2021-075771 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] When using duplex stainless steel sheets as structural members, good toughness, which is related to fracture characteristics, is important. It is known that toughness is reduced by precipitates in the steel. In the normal manufacturing of duplex stainless steel sheets, solution heat treatment at 900°C to 1100°C is performed after rolling to dissolve chromium nitrides precipitated during rolling and suppress the reduction in toughness. However, when solution heat treatment is performed, residual processing strain is also reduced by recovery, and the desired strength cannot be obtained.

[0009] Therefore, there is a need for a manufacturing method that omits solution heat treatment, leaves sufficient processing strain to achieve high strength, and suppresses the precipitation of chromium nitride, thereby achieving both the high strength and toughness of duplex stainless steel.

[0010] Patent documents 1 and 2 describe a method for producing duplex stainless steel with good surface hardness and weld corrosion resistance by low-temperature finishing. However, these documents do not describe the amount of reduction during low-temperature rolling that is related to the precipitation of chromium nitride, nor do they describe whether the resulting product is free from chromium nitride precipitation and has good toughness.

[0011] The present invention aims to provide a low-alloy ferrite-austenite duplex stainless steel sheet and a method for manufacturing the same, which has toughness equivalent to duplex stainless steel (solution-treated material) that has undergone solution heat treatment without the precipitation of chromium nitrides, and also has good surface hardness. [Means for solving the problem]

[0012] The inventors manufactured steel sheets by varying rolling conditions for steels with various compositions and evaluated the factors affecting the toughness and surface hardness of the products. In particular, focusing on the hardness of the ferrite and austenite phases, they investigated the factors that improve toughness and surface hardness and obtained the following findings. (a) The smaller the increase in hardness of the ferrite phase compared to the solid solution material, the better the toughness. (b) The greater the increase in hardness of the austenite phase compared to the solid solution material, the higher the surface hardness. (c) By controlling the rolling temperature and reduction ratio according to the amount of Mo added to the steel sheet, the ferrite phase can be restored and the precipitation of Cr nitride can be suppressed, thereby reducing the increase in hardness and improving toughness. On the other hand, the restoration of the austenite phase can be suppressed, resulting in a greater increase in hardness and higher surface hardness. This invention is based on the above findings, and its gist is as follows.

[0013] [1] The chemical composition is expressed in mass percent. C: 0 to 0.050%, Si: 0 to 2.00%, Mn: 0.50 to 6.00%, P: 0.050% or less, S: 0.0500% or less, N: 0.08 to 0.30%, Cr: 17.0 to 30.0%, Ni: 0.10 to 8.00%, Mo: 0.10 to 3.50%, Cu: 0 to 3.00% Nb: 0 to 0.10%, Sn: 0 to 1.00%, W: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 0.05%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, <​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​ W: 0.01~1.00%, and, V: 0.01~1.00%, Contains one or more selected from, The ferritic-austenitic duplex stainless steel sheet described above [1]. [3] The aforementioned chemical composition, in mass%, Ti: 0.005~0.05%, and, B: 0.0003~0.0050%, Contains one or more selected from, Ferritic austenitic duplex stainless steel sheet as described in [1] or [2] above. [4] The aforementioned chemical composition, in mass%, Ca: 0.0001~0.0050%, Mg: 0.0001~0.0050%, Al: 0.0030~0.05%, and, REM: 0.005~0.50%, Contains one or more selected from, A ferritic-austenitic duplex stainless steel sheet as described in any one of the above items [1] to [3]. [5] A method for manufacturing a ferritic-austenite duplex stainless steel sheet as described in any one of the above items [1] to [4], A method for manufacturing a ferritic-austenite duplex stainless steel sheet, characterized by hot-rolling a steel having the chemical composition described in any of [1] to [4] above, under conditions where the reduction ratio of the rolling is 1.2 or more when the rolling is performed at a temperature Tc (°C) or lower calculated by the following formula 2, and the final rolling pass temperature is (Tc-100)°C or higher, and then air-cooling for 20 seconds or more after the hot-rolling, followed by cooling in the temperature range of 800°C to 600°C at a cooling rate of 1°C / s or more. Tc(℃)=930+50Mo(℃)...(Formula 2) However, the element symbols in the above formula represent the mass percentage of each element contained in the steel; if an element is not present, substitute 0. The reduction ratio below Tc(°C) is calculated by dividing the thickness of the sheet metal before the first rolling pass at Tc°C or below by the thickness of the steel sheet metal after rolling. [6] A method for manufacturing a ferritic-austenite duplex stainless steel sheet as described in [5], without performing solution heat treatment. [Effects of the Invention]

[0014] According to the present invention, it is possible to obtain a ferrite-austenite duplex stainless steel sheet with excellent toughness and surface hardness without the precipitation of chromium nitrides, in a low-alloy duplex stainless steel sheet. [Modes for carrying out the invention]

[0015] The requirements of this invention will be described in detail below.

[0016] 1. Chemical composition of steel plate The elements in the chemical composition of the steel of this invention will be described below. Unless otherwise specified, in the following description, "%" for the content of each element means "mass%".

[0017] C: 0~0.050% Carbon (C) is an element that increases strength by solid dissolving in the austenite phase. However, if the C content is high, the strength of the steel sheet increases and the workability deteriorates. Also, C promotes the precipitation of chromium carbides, leading to the occurrence of intergranular corrosion. Therefore, the C content should be 0.050% or less. Preferably, the C content should be 0.045% or less, 0.040% or less, 0.035% or less, 0.030% or less, 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less. Furthermore, from the viewpoint of corrosion resistance, it is preferable to have a lower C content, and there is no particular lower limit to the C content. However, in existing steelmaking equipment, reducing the C content would lead to a significant increase in costs, so it is preferable that the C content be 0.002% or more.

[0018] Si: 0~2.00% Si is sometimes used as a deoxidizing element or added to improve oxidation resistance. However, if the Si content is too high, it hardens the steel sheet, degrading its toughness and workability. Therefore, the Si content should be 2.00% or less. Preferably, the Si content should be 1.80% or less, 1.60% or less, 1.50% or less, 1.40% or less, 1.30% or less, 1.20% or less, 1.10% or less, or 1.00% or less. There is no particular lower limit to the Si content. However, reducing the Si content to an extremely small amount would increase the cost during steel refining, so it is preferable that the Si content be 0.03% or more.

[0019] Mn: 0.50~6.00% Mn increases the austenite phase and enhances the solid solubility of nitrogen, suppressing defects such as bubbles during manufacturing. Therefore, the Mn content should be 0.50% or higher. Preferably, the Mn content should be 0.70% or higher, 1.00% or higher, 1.50% or higher, 2.00% or higher, or 2.50% or higher. On the other hand, a large amount of Mn reduces corrosion resistance and hot workability. Therefore, the Mn content should be 6.00% or lower. Preferably, the Mn content should be 5.50% or lower, 5.00% or lower, 4.75% or lower, 4.50% or lower, 4.30% or lower, 4.10% or lower, or 4.00% or lower.

[0020] P:0.050% or less P is an element that is inevitably mixed into steel and is also contained in raw materials such as Cr, but a large amount of P reduces formability. For this reason, the lower the P content, the better, preferably 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0021] S: 0.0500% or less S is an element that is inevitably present in steel, and it can combine with Mn to form inclusions, which can become starting points for rust formation. Furthermore, corrosion resistance improves as the S content decreases, so it is desirable for the S content to be 0.050% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0040% or less, or 0.00300% or less.

[0022] N: 0.08~0.30% N is an element that contributes to alloy reduction by dissolving in the austenite phase, thereby increasing strength and corrosion resistance, and it is desirable to have a concentration of 0.08% or more. More preferably, the N content is 0.10% or more, 0.12% or more, or 0.15% or more. On the other hand, N is also an element that greatly affects the precipitation of chromium nitride, and if it is contained in large quantities, the amount of chromium nitride precipitated will increase, and the difference in corrosion resistance between the product and the solution heat-treated sample will become large, so this should be set as the upper limit. More preferably, it should be 0.23% or less.

[0023] Cr: 17.0~30.0% Cr is an element necessary to ensure corrosion resistance. Therefore, the Cr content should be 17.0%, preferably 18.0% or more, 19.0% or more, 20.0% or more, or 21.0% or more. On the other hand, if a large amount of Cr is included, the amount of chromium nitride precipitate increases, and the risk of hot working cracks also increases. Therefore, the Cr content should be 30.0% or less, preferably 29.5% or less, 29.0% or less, 28.5% or less, 28.0% or less, 27.5% or less, or 27.0% or less.

[0024] Ni: 0.10~8.00% Ni is an austenite-stabilizing element and has the effect of improving corrosion resistance. Therefore, the Ni content is preferably 0.10%, and more preferably 0.30% or more, 0.50% or more, 0.70% or more, or 1.00% or more. On the other hand, including a large amount of Ni leads to an increase in raw material costs. Therefore, the Ni content is preferably 6.00% or less, and more preferably 5.00% or less, 4.00% or less, or 3.00% or less.

[0025] Mo: 0.10~3.50% Mo is an element that suppresses the recovery of the ferrite and austenite phases. However, by utilizing the difference in the degree of influence on the recovery of the two phases (i.e., the difference in the temperature at which recovery becomes difficult during cooling), it is possible to recover the ferrite phase while suppressing the recovery of the austenite phase. Since Mo is more stable in the ferrite phase than in the austenite phase, it becomes concentrated in the ferrite phase. Therefore, if a large amount of Mo is included, the temperature at which recovery of the ferrite and austenite phases becomes difficult approaches, making it difficult to control the rolling process to achieve both the desired toughness and surface hardness. For this reason, the Mo content should be 3.50% or less. Preferably, the Mo content should be 3.00% or less, 2.50% or less, 2.00% or less, or 1.50% or less. On the other hand, Mo has the effect of improving corrosion resistance. For this reason, the Mo content should be 0.10% or more. The Mo content is preferably 0.20% or more, 0.30% or more, 0.40% or more, 0.50% or more, 0.60% or more, 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more.

[0026] In addition to the elements listed above, one or more elements selected from Cu, Nb, Sn, W, and V may be included as needed to improve corrosion resistance. These elements are not required, but their inclusion can provide further benefits. These elements are described below.

[0027] Cu: 0~3.00% Cu is an element that is very effective in improving sulfuric acid resistance, so it may be included as needed. However, including a large amount of Cu will increase raw material costs and worsen hot workability. For this reason, the Cu content is preferably 3.00% or less, and more preferably 1.5% or less. There is no particular lower limit to the Cu content, but to obtain the above effects, the Cu content is more preferably 0.10% or more, or 0.5% or more.

[0028] Nb: 0~0.10% Nb may be included as needed because it has the effect of suppressing the precipitation of chromium nitride by forming compounds with N. However, if a large amount of Nb is included, the workability of the steel sheet will decrease. Therefore, it is preferable to keep the Nb content at 0.10% or less. There is no particular lower limit to the Nb content, but in order to obtain the above effect, it is more preferable that the Nb content be 0.01% or more, or 0.04% or more.

[0029] Sn: 0~1.00% Sn (Sin) is an element that improves corrosion resistance, so it may be included as needed. However, a large amount of Sn will worsen hot workability. Therefore, the Sn content should be 1.00% or less. There is no particular lower limit to the Sn content, but to obtain the above effects, a Sn content of 0.030% or more is preferable.

[0030] W: 0~1.00% Since W is an element that improves corrosion resistance, it may be included as needed. However, including a large amount of W increases the load during rolling, making it easier to generate manufacturing defects. Therefore, the W content should preferably be 1.00% or less, and more preferably 0.80% or less, or 0.50% or less. There is no particular lower limit to the W content, but in order to obtain the above effects, it is preferable that the W content be 0.01% or more.

[0031] V: 0~1.00% V is an element that improves corrosion resistance, so it may be included as needed. However, if a large amount of V is included, the load during rolling will increase, making it easier to generate manufacturing defects. Therefore, the V content should preferably be 1.00% or less, and more preferably 0.80% or less, or 0.50% or less. There is no particular lower limit to the V content, but in order to obtain the above effects, it is preferable that the V content be 0.01% or more.

[0032] Furthermore, in addition to the elements mentioned above, one or more elements selected from Ti and B may be included as needed to improve hot workability and formability. These elements are not required, but their inclusion can yield further benefits. These elements are described below.

[0033] Ti: 0~0.05% Like Nb, Ti may be included as needed because it prevents the coarsening of the heat-affected zone during welding and also has the effect of creating a finely equiaxed solidification structure. However, a large amount of Ti reduces uniform elongation and local elongation. Therefore, the Ti content should be 0.05% or less. There is no particular lower limit to the Ti content, but to obtain the above effects, a Ti content of 0.005% or more is preferable.

[0034] B: 0~0.0050% Since B has the effect of improving hot workability, it may be included as needed. However, if a large amount of B is included, the corrosion resistance deteriorates significantly. Therefore, the B content should preferably be 0.0050% or less, and more preferably 0.040% or less, or 0.030% or less. There is no particular lower limit to the B content, but in order to obtain the above effect, it is preferable that the B content be 0.0003% or more.

[0035] Furthermore, in addition to the elements mentioned above, one or more elements selected from Ca, Mg, Al, and REM may be included as needed, from the viewpoint of deoxidation and desulfurization during refining. These elements are not required to be included, but their inclusion can provide further benefits. These elements are described below.

[0036] Ca: 0~0.0050% Ca may be included as needed for desulfurization and deoxidation. However, if a large amount of Ca is included, hot working cracks are more likely to occur and corrosion resistance will decrease. Therefore, the Ca content should be 0.0050% or less. There is no particular lower limit to the Ca content, but to obtain the above effects, a Ca content of 0.0001% or more is preferable.

[0037] Mg: 0~0.0050% Magnesium (Mg) has the effect of not only deoxidizing but also refining the solidification structure, so it may be included as needed. However, including a large amount of Mg will increase costs in the steelmaking process. Therefore, the Mg content should be 0.0050% or less. There is no particular lower limit to the Mg content, but in order to obtain the above effects, it is preferable that the Mg content be 0.0001% or more.

[0038] Al: 0~0.05% Al may be included as needed for desulfurization and deoxidation. However, including a large amount of Al leads to an increase in manufacturing defects and raw material costs. Therefore, the Al content should be kept below 0.05%. There is no particular lower limit to the Al content, but to obtain the above effects, an Al content of 0.0030% or more is preferable.

[0039] REM: 0~0.50% Rare earth elements (REMs) may be included as needed because they have the effect of improving hot workability. However, including large amounts of REMs will impair manufacturability and increase costs. Therefore, the REM content should be 0.50% or less, 0.40% or less, 0.30% or less, or 0.20% or less. There is no particular lower limit to the REM content, but in order to obtain the above effects, it is preferable that the REM content be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more.

[0040] REM is a collective term for 17 elements, including Sc, Y, and 15 elements from La to Lu (lanthanides). The REM content refers to the total content of these elements. Industrially, lanthanides are added in the form of mischmetal.

[0041] In the chemical composition of the steel sheet of the present invention, the remainder is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of steel due to raw materials such as ore and scrap, and various factors in the manufacturing process, and which are acceptable as long as they do not adversely affect the present invention.

[0042] The steel sheet according to the embodiment of the present invention must have a PREN_Mn value calculated by the following formula within a predetermined range.

[0043] PREN_Mn value: 35.0 or less The PREN_Mn value is a common indicator of the pitting corrosion resistance of stainless steel sheets and is calculated from the chemical composition of the steel sheet using the following formula 1. PREN_Mn value = Cr + 3.3(Mo + 0.5W) + 16N - Mn ... (Equation 1) However, the element symbols in the above formula represent the mass percentage of each element contained in the steel; if an element is not present, substitute 0.

[0044] The PREN_Mn value is an index indicating the pitting corrosion index (PRE) of duplex stainless steel considering the adverse effects of Mn and the effects of W, and indicates that the higher the value, the better the corrosion resistance. On the other hand, although the PREN_Mn value increases with the increase in the contents of Cr and Mo, the sigma phase (σ phase) precipitates. The sigma phase is an intermetallic compound in which Cr, Mo, etc. are concentrated in Fe, and even if it precipitates at about a few percent, in addition to the decrease in hot ductility, the toughness and corrosion resistance of the material extremely decrease. That is, by increasing Cr or Mo, the main factor causing the decrease in toughness changes from the precipitation of chromium nitride to the precipitation of the σ phase, and the effect of suppressing the precipitation of chromium nitride due to the recovery of the ferrite phase becomes small. In addition, the increase in Cr and Mo also causes problems such as an increase in alloy cost, an increase in N content, and the generation of nitrogen bubbles due to a decrease in Mn content. Therefore, the PREN_Mn value should be 35.0 or less. The PREN_Mn value is preferably 33.0 or less, 30.0 or less, or 27.0 or less. Although the lower limit of the PREN_Mn value does not particularly need to be specified, in order to obtain corrosion resistance equivalent to SUS304, it is preferably 18.0 or more, and more preferably 20.0 or more.

[0045] 2. Characteristics of the steel plate surface layer Charpy impact value vE -20℃ is 100 J / cm 2 or more The Charpy impact test is a general index for evaluating the toughness of a steel plate. A sample is taken from the 1 / 4 part of the steel plate thickness (the depth position of 1 / 4 of the plate thickness from the steel plate surface) and evaluated at -20°C in accordance with JIS Z 2242. The Charpy impact value vE at -20°C -20℃ is 100 J / cm 2 or more, then it has sufficient toughness in a normal use environment and can be used as a structural member. Preferably, vE -20℃ is 150 J / cm 2 or more. Since the higher the toughness of the steel material, the more desirable, the upper limit is not particularly limited.

[0046] The Brinell hardness HBW10 / 3000 of the steel plate surface layer is 230 or more Brinell hardness HBW10 / 3000 is a common index for evaluating the hardness of steel plates and is evaluated in accordance with JIS Z 2243. If the HBW10 / 3000 test is difficult due to the size or shape of the test piece, a value obtained by converting a value measured by another evaluation method using an appropriate conversion formula may be used. If the Brinell hardness HBW10 / 3000 is 230 or higher, for example, SUS It is equivalent to 304N2 and can be used for general wear-resistant applications. Preferably, the HBW10 / 3000 should be 250 or higher. Since higher hardness is desirable for wear resistance, there is no particular upper limit.

[0047] The Brinell hardness described above is evaluated on the surface layer of the steel sheet that is exposed to the environment during use. Here, the surface layer of the steel sheet refers to the surface that has been ground 0.1 to 0.2 mm in the thickness direction from the surface of the steel sheet. This is to avoid the evaluation of hardness being affected by surface irregularities, dirt, and residual scale generated during rolling.

[0048] 3. Manufacturing method of steel plates In typical hot-rolled stainless steel, the strain introduced during rolling is recovered and softened by solution heat treatment performed after hot rolling. The steel of the present invention controls the steel sheet temperature and reduction ratio during rolling to achieve both surface hardness suitable for sliding members and good toughness, and omits the solution heat treatment. Specifically, the reduction ratio below the temperature Tc (°C) at which the austenite phase begins to recover slowly during cooling is set to 1.2 to 2.0, and hot rolling is performed with a final rolling pass temperature of (Tc-100)°C or higher. After rolling, the steel is air-cooled for 20 seconds or more, and then cooled in the temperature range of 800°C to 600°C at a cooling rate of 1°C / s or higher, and the solution heat treatment is omitted to produce a steel sheet.

[0049] The reduction ratio at Tc(°C) or below is 1.2 or more and 2.0 or less. To ensure surface hardness and achieve good wear resistance, it is necessary to introduce sufficient strain into the austenite phase. A larger reduction ratio increases the amount of strain introduced into the austenite phase, thereby increasing surface hardness. Therefore, it is desirable to set the reduction ratio below Tc(°C) to 1.2 or higher. Preferably, it should be 1.4 or higher. Furthermore, to suppress the precipitation of chromium nitride and achieve good toughness, it is necessary to restore the strain in the ferrite phase. If the reduction ratio is too high, the amount of strain introduced into the ferrite phase increases, leading to the precipitation of chromium nitride and a decrease in toughness. Therefore, it is desirable to set the reduction ratio below Tc(°C) to 2.0 or lower. Preferably, it should be 1.8 or lower. Here, the reduction ratio below Tc(°C) is the value calculated by dividing the sheet thickness before the first rolling pass at Tc°C or lower by the sheet thickness after hot rolling (after the final rolling pass). Reduction ratio below Tc(°C) = {Steel sheet thickness at the entrance of the first rolling pass where rolling begins below Tc°C} / {Steel sheet thickness at the exit of the final rolling pass during hot rolling}

[0050] The temperature of the final rolling pass is (Tc-100)°C or higher. On the other hand, in order to restore the ferrite phase and suppress the precipitation of chromium nitride, which causes a decrease in toughness, it is necessary to end the rolling process at a temperature above the point at which the ferrite phase begins to become difficult to restore during cooling (Tc-100°C). Therefore, the temperature of the final pass of rolling should be Tc-100°C or higher. Preferably, it should be Tc-50°C or higher.

[0051] Here, Tc is the temperature at which the austenite phase becomes difficult to recover during cooling, and is calculated using Equation 2 below. Since Mo is an element that significantly slows down the recovery of both the ferrite and austenite phases, Tc changes depending on the amount of Mo added. Tc(℃)=930+50Mo(℃)...(Formula 2) However, the element symbol Mo in the above formula represents the percentage of Mo contained in the steel (mass %), and if it is not contained, substitute 0.

[0052] Air-cooled for at least 20 seconds after rolling. In order to restore the ferrite phase and suppress the precipitation of chromium nitride, which causes a decrease in toughness, it is necessary to ensure that the steel sheet is held at a temperature above the temperature at which the ferrite phase begins to become difficult to restore during cooling. Therefore, it is advisable to perform air cooling for 20 seconds or more after rolling. Preferably, the air cooling time after rolling should be 30 seconds or more, 45 seconds or more, or 60 seconds or more.

[0053] A cooling rate of 1°C / s or more in the temperature range of 800°C to 600°C. The chromium nitride deposition nose exists between 800°C and 600°C on the temperature and time chart. To shorten the time the material is held in the temperature range where the chromium nitride deposition rate is high, it is preferable to cool this 800°C to 600°C temperature range at a cooling rate of 1°C / s or more, preferably 3°C / s or more, and more preferably 5°C / s or more. Since deposition decreases as the cooling rate increases with speed, there is no particular upper limit.

[0054] The above manufacturing method makes it possible to obtain a duplex stainless steel sheet with good surface hardness and toughness. Specifically, the ferrite phase can be restored to suppress the precipitation of Cr nitride and improve toughness, while the austenite phase can be suppressed to increase hardness and obtain a steel sheet with enhanced surface hardness. Therefore, the duplex stainless steel according to the present invention does not require solution heat treatment, nor is it necessary.

[0055] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0056] Steel having the chemical composition shown in Table 1 was melted down to form steel billets, which were then hot-rolled to a thickness of 10 mm to 50 mm under the conditions shown in Table 2. Test specimens were taken from the steady-state portion of the obtained steel sheets, and the surface, which was ground 0.1 to 0.2 mm in the thickness direction from the surface of the steel sheet, was evaluated for Brinell hardness HBW10 / 3000. For samples with a thickness of 20 mm or more, a 10 mm thick full-size test specimen was taken from the 1 / 4 of the thickness direction, and for samples with a thickness of less than 20 mm, a 5 mm thick sub-size test specimen was taken from the 1 / 4 of the thickness direction. A V-notch was machined with a notch angle of 45°, a notch depth of 2 mm, and a notch bottom radius of 0.25 mm. A Charpy impact test was performed at -20°C, and the results are shown in Table 2.

[0057] The items in Table 2 are explained below. The reduction ratio is calculated by dividing the sheet thickness before the first rolling pass at Tc°C or below by the sheet thickness after the final rolling pass. The final pass temperature (°C) is the surface temperature at the entry side of the final rolling pass and is measured by a radiation thermometer in front of the rolling mill. The air cooling time (seconds) is the time during which air cooling occurred from the end of rolling until water cooling began. The cooling rate (°C / s) is the average cooling rate in the 800°C to 600°C temperature range. Impact value (J / cm 2 ) is the Charpy impact value at -20°C. HBW is the HBW10 / 3000 value of the product surface.

[0058] Sample numbers with sub-numbers 1 and 2 (e.g., 1-1 and 1-2) for each component number represent embodiments of the present invention, satisfying preferred manufacturing conditions and having a Charpy impact value of 100 J / cm² at -20°C. 2 The above criteria apply, and the HBW is 230 or higher. Samples with a sub-number of 3 in the sample number for each component number (e.g., 1-3) are comparative examples. Those with a reduction ratio of less than 1.2 have an HBW value of less than 230, and samples with a reduction ratio greater than 2.0, or a final pass below Tc-100°C, or an air cooling time of less than 20 seconds, or a cooling rate of less than 1°C / s have a Charpy impact value of 100 J / cm² at -20°C. 2The following conditions must be met: If the reduction ratio is less than 1.2, sufficient strain is not introduced into the austenite phase, and the hardness requirement cannot be met. If the reduction ratio is greater than 2.0, or if the final pass is below Tc-100°C, or if the air cooling time is less than 20 seconds, the ferrite phase does not recover sufficiently, promoting the precipitation of chromium nitride and failing to meet the toughness requirement. If the cooling rate is less than 1°C / s, regardless of the manufacturing conditions, excessive precipitation of chromium nitride occurs, and the toughness requirement cannot be met.

[0059] As described above, the embodiments of the present invention yielded duplex stainless steel sheets that achieved both good surface hardness and toughness. On the other hand, the comparative examples did not satisfy the preferred manufacturing conditions, and either the surface hardness or toughness fell outside the scope of the present invention.

[0060] [Table 1]

[0061] [Table 2] [Industrial applicability]

[0062] According to the present invention, a ferritic-austenite duplex stainless steel sheet with excellent hardness and toughness can be obtained, which is extremely useful in industry. The duplex stainless steel sheet of the present invention can be used in sliding parts and friction-generating areas of structures. For example, it can be used as a gate for sluice gates, or as a lining material for dams and rivers.

Claims

1. The chemical composition is expressed in mass percent. C: 0.002-0.050%, Si: 0-2.00%, Mn: 0.50-6.00%, P: 0.050% or less, S: 0.0500% or less, N: 0.08-0.30%, Cr: 17.0-30.0%, Ni: 0.10-8.00%, Mo: 0.10-1.91%, Cu: 0-3.00%, Nb: 0 to 0.10%, Sn: 0-1.00%, W: 0-1.00%, V: 0-1.00%, Ti: 0 to 0.05%, B: 0 to 0.0050%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, Al: 0-0.05%, REM: 0-0.50%, The remainder consists of Fe and impurities. The PREN_Mn value calculated using the following formula 1 is 35.0 or less. The Charpy impact value vE at -20°C is 100 J / cm² or higher. A ferritic-austenite duplex stainless steel sheet characterized by having a Brinell hardness HBW10 / 3000 of 230 or higher at a position 0.1 to 0.2 mm in the thickness direction from the surface of the steel sheet. PREN_Mn value = Cr + 3.3(Mo + 0.5W) + 16N - Mn ... Equation 1 However, the element symbols in Formula 1 above represent the mass percentage of each element contained in the steel; if an element is not present, substitute 0.

2. In mass percent, Cu: 0.10-3.00%, Nb: 0.01 to 0.10%, Sn: 0.030-1.00%, W: 0.01-1.00%, V: 0.01-1.00%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0050%, Ca: 0.0001-0.0050%, Mg: 0.0001 to 0.0050%, Al: 0.0030–0.05%, and, REM: 0.005-0.50%, Contains one or more selected from, The ferrite-austenite duplex stainless steel sheet according to claim 1.

3. The chemical composition is, in mass%, C: 0.002-0.050%, Si: 0-2.00%, Mn: 0.50-6.00%, P: 0.050% or less, S: 0.0500% or less, N: 0.08-0.30%, Cr: 17.0-30.0%, Ni: 0.10-8.00%, Mo: 0.10-2.50%, Cu: 0-3.00%, Nb: 0 to 0.10%, Sn: 0-1.00%, W: 0-1.00%, V: 0-1.00%, Ti: 0 to 0.05%, B: 0 to 0.0050%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, Al: 0-0.05%, REM: 0-0.50%, The remainder consists of Fe and impurities. Steel whose PREN_Mn value calculated by the following formula 1 is 35.0 or less, The rolling reduction ratio is 1.2 or more and 2.0 or less when the rolling is performed at a temperature Tc (°C) or lower calculated by the following formula 2, and the final rolling pass temperature is (Tc - 100)°C or higher. After the hot rolling, the material is air-cooled for 20 seconds or more, then cooled in the temperature range of 800°C to 600°C at a cooling rate of 1°C / s or higher, and no solution heat treatment is performed. The Charpy impact value vE at -20°C is 100 J / cm² or more. A method for manufacturing a ferritic-austenite duplex stainless steel sheet, wherein the Brinell hardness HBW10 / 3000 at a position 0.1 to 0.2 mm from the surface of the steel sheet in the thickness direction is 230 or higher. PREN_Mn value = Cr + 3.3(Mo + 0.5W) + 16N - Mn ... Equation 1 However, the element symbols in Formula 1 above represent the mass percentage of each element contained in the steel; if an element is not present, substitute 0. Tc (℃) = 930 + 50Mo (℃) ...Formula 2 However, the element symbol Mo in formula 2 above represents the Mo content (mass%) in the steel.

4. The chemical composition is, in mass%, C: 0.002-0.050%, Si: 0-2.00%, Mn: 0.50-6.00%, P: 0.050% or less, S: 0.0500% or less, N: 0.08-0.30%, Cr: 17.0-30.0%, Ni: 0.10–8.00%, and Mo: Contains 0.10-2.50%, and further Cu: 0.10-3.00%, Nb: 0.01 to 0.10%, Sn: 0.030-1.00%, W: 0.01-1.00%, V: 0.01-1.00%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0050%, Ca: 0.0001-0.0050%, Mg: 0.0001 to 0.0050%, Al: 0.0030–0.05%, and, REM: Contains one or more substances selected from the group ranging from 0.005% to 0.50%. The remainder consists of Fe and impurities. Steel whose PREN_Mn value calculated by the following formula 1 is 35.0 or less, The rolling reduction ratio is 1.2 or more and 2.0 or less when the rolling is performed at a temperature Tc (°C) or lower calculated by the following formula 2, and the final rolling pass temperature is (Tc - 100)°C or higher. After the hot rolling, the material is air-cooled for 20 seconds or more, then cooled in the temperature range of 800°C to 600°C at a cooling rate of 1°C / s or higher, and no solution heat treatment is performed. The Charpy impact value vE at -20°C is 100 J / cm² or more. A method for manufacturing a ferritic-austenite duplex stainless steel sheet, wherein the Brinell hardness HBW10 / 3000 at a position 0.1 to 0.2 mm from the surface of the steel sheet in the thickness direction is 230 or higher. PREN_Mn value = Cr + 3.3(Mo + 0.5W) + 16N - Mn ... Equation 1 However, the element symbols in Formula 1 above represent the mass percentage of each element contained in the steel; if an element is not present, substitute 0. Tc (℃) = 930 + 50Mo (℃) ...Formula 2 However, the element symbol Mo in formula 2 above represents the Mo content (mass%) in the steel.

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