Ferritic stainless steel bar

Abstract

To provide a ferritic stainless rod-like steel of Φ16 mm or more with superior oxidation resistance and high-temperature strength, wherein the rod-like steel exhibits superior resistance to bending during the molding process.SOLUTION: A ferritic stainless rod-like steel has a predetermined chemical composition. For the rod-like steel, the amount of Fe+Cr precipitated from extraction residues is 0.20% or less, while the amount of precipitated Nb+Ti+V+W+Zr is 0.05% or more and 0.60% or less. The rod-like steel has a diameter of 16 mm or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel bar. [Background technology]

[0002] In recent years, the need for improved fuel economy and environmental protection has led to the need for materials with better oxidation resistance and high-temperature strength for exhaust system components. For this reason, high-purity ferritic stainless steel, which has reduced carbon and nitrogen content and added stabilizing elements such as niobium and titanium, is being used instead of cast iron.

[0003] To accommodate mass production, the use of forged parts made by combining forging and cutting processes is increasing, rather than using large-diameter steel wire or thick steel plate. Ferritic stainless steel has low-temperature brittleness due to ferrite grains, and it is difficult to ensure toughness, especially in large-diameter steel wire or thick material, because it is difficult to refine the crystal grains. In addition, heating of cast steel and annealing after hot rolling are often performed using LNG burners, which causes thermal decomposition of part of the fuel, creating a hydrogen environment, and sometimes a decrease in ductility is observed, which is thought to be due to hydrogen absorption at high temperatures.

[0004] For this reason, there is room for improvement in the bending resistance during treatment such as straightening and peeling, especially for large-diameter steel wires of Φ16 mm or more.

[0005] Patent Documents 1 and 2 disclose ferritic stainless steel wires and the like that have improved cold forgeability and machinability by defining the number density of inclusions and precipitates that become fracture initiation points. However, they do not disclose cases where the diameter required for forged products is large, exceeding 16 mm.

[0006] Patent Document 3 discloses a ferritic stainless steel sheet with excellent press workability, which has a specified recrystallization rate, grain size, precipitate amount, etc. The examples are a study of a cold-rolled thin material with a thickness of about 2 mm, and does not disclose a hot-rolled steel sheet with a thickness of 5 mm or more, which is necessary for forging applications.

[0007] Patent Document 4 discloses a method for ensuring ductility in high-purity ferritic stainless steel by dehydrogenating it or by controlling the temperature and time course and specifying the hydrogen content. On the other hand, as shown in Non-Patent Document 1, it is stated that as the thickness and diameter of steel increases, the diffusion distance required for dehydrogenation increases, and therefore the diffusible hydrogen that causes embrittlement is desorbed at higher temperatures. Patent Document 4 uses small-diameter steels with a diameter of 5 mm as examples, and does not disclose any studies on large-diameter steels with a diameter of over 16 mm. Furthermore, because it specifies the hydrogen content rather than the control of diffusible hydrogen, it does not represent the state of hydrogen in the steel. Therefore, it is not appropriate to use the hydrogen content to indicate the degree of hydrogen embrittlement. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2014 / 157231 [Patent Document 2] Patent No. 2817266 [Patent Document 3] Patent No. 4519505 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-233251 [Non-patent literature]

[0009] [Non-Patent Document 1] Kobelnicus No.42 OCT.2014 "Evaluation of the effect of trace hydrogen in metals on material properties" Summary of the Invention [Problem to be solved by the invention]

[0010] The possibility of breakage during processing depends on the steel diameter, and is more pronounced in large-diameter steel products of 16 mm or more in diameter. With large-diameter steel products of 16 mm or more in diameter, where the hot rolling reduction rate is small, it is difficult to refine the crystal grains and precipitates, making the problem more apparent. Furthermore, with regard to annealing after hot rolling, while small-diameter steel products can undergo high-temperature, short-time continuous annealing, which is advantageous for recrystallization grain refinement, large-diameter steel products require longer heat input, so batch annealing is often more productive. Therefore, the appropriate manufacturing method varies depending on the wire diameter.

[0011] An object of the present invention is to provide a steel bar having excellent flexural strength when formed into a ferritic stainless steel bar having a diameter of 16 mm or more and excellent oxidation resistance and high-temperature strength. [Means for solving the problem]

[0012] The inventors of the present invention have conducted various studies to solve the above problems, and have found that evaluation of toughness by a three-point bending test with a low strain rate, rather than a Charpy test with a high strain rate, has a good correlation with flexural strength. Based on this finding, they have investigated methods for improving flexural strength and have found that controlling the state of precipitates by rolling or batch annealing is effective in improving toughness.

[0013] Furthermore, it was found that embrittlement and low ductility due to diffusible hydrogen are significant in the low strain rate region, and that reducing diffusible hydrogen is effective in improving fracture toughness. Diffusible hydrogen is easily trapped in specific precipitates, and controlling the state of the precipitates can reduce diffusible hydrogen. It is possible to further reduce diffusible hydrogen by setting annealing conditions that correspond to the steel diameter.

[0014] The present invention has been made to solve the above-mentioned problems, and is summarized as the following ferritic stainless steel bar material.

[0015] (1) Chemical composition, in mass%, is: C: 0.001-0.03%, Si: 0.01-1.0%, Mn: 0.010-1.0%, Ni: 0.05-3.0%, Cr: 10.5-25.0%, Mo: 0-3.0%, Cu: 0.02-1.5%, N: 0.001-0.03%, Nb: 0.1-1.0%, Ti: 0-0.5%, V: 0-1.0%, W: 0-0.50%, Zr: 0-0.50%, B: 0-0.010%, Al: 0-0.50%, Ca: 0-0. A ferritic stainless steel bar material characterized in that the content of precipitated Fe+Cr in the extracted residue is 0.20% or less, and the content of precipitated Nb+Ti+V+W+Zr is 0.05% or more and 0.60% or less, and the diameter is 16 mm or more.

[0016] (2) The ferritic stainless steel bar of (1) above, characterized in that a sample taken from the center of the ferritic stainless steel bar is measured by TDA and the total amount of hydrogen released from the sample at temperatures between 0°C and 400°C is 0.5 ppm or less.

[0017] (3) A ferritic stainless steel bar according to (1) or (2), characterized in that in a three-point bending test, no cracks occur in a plate material that includes the center of the ferritic stainless steel bar and has dimensions of (diameter) x (thickness 5 mm) x (longitudinal direction 240 mm) from 0° to 90°.

[0018] (4) A ferritic stainless steel bar according to any one of (1) to (3), characterized in that the reduction of area of ​​the fracture surface in a round bar tensile test of the ferritic stainless steel bar is 50% or more.

[0019] (5) A method for producing a ferritic stainless steel bar according to any one of (1) to (4), characterized in that the method comprises a step of maintaining the condition of formula (A) in annealing after hot rolling. r<(1.39×t) 0.5×T 1.345 / 1000 ··· (A) However, r is the radius of the steel material (mm), t is the holding time (minutes), T is the holding temperature (°C), and the holding temperature T satisfies 200 < T < 680.

Advantages of the Invention

[0020] According to the present invention, it is possible to obtain a ferritic stainless steel bar-shaped steel material with excellent flexural strength that is not easily damaged even when subjected to straightening or peeling.

Brief Description of the Drawings

[0021] [Figure 1] It is a diagram showing the amount of hydrogen released when TDA measurement is performed on the steel bar of the present invention in the embodiment. [Figure 2] It is a diagram showing the cumulative hydrogen release amount when TDA measurement is performed on the steel bar of the present invention in the embodiment.

Modes for Carrying Out the Invention

[0022] In order to obtain a ferritic stainless steel bar-shaped steel material with excellent flexural strength, the inventors conducted various studies. As a result, the following findings were obtained.

[0023] In a ferritic stainless steel bar-shaped steel material with reduced C and N contents and added Nb, annealing at approximately 700 °C or higher from a state where strain remains significantly increases the Laves phase (Fe2Nb).

[0024] Also, in ferritic stainless steel, the equilibrium hydrogen amount increases as the temperature rises, and the hydrogen absorption amount increases. Therefore, by annealing at a temperature lower than the precipitation temperature of the Laves phase, an increase in trap sites is suppressed, and hydrogen desorption is promoted. That is, precipitation control during annealing and release of diffusible hydrogen contribute to the improvement of ductility and bending toughness.

[0025] The present invention has been made based on the above findings. Preferred embodiments of the present invention will be described in detail. In the following description, the preferred embodiments of the present invention will be described as the present invention. Each requirement of the present invention will be described in detail below.

[0026] <1.Chemical composition>

[0027] The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0028] C: 0.001 to 0.030% Although C has the effect of improving the strength of steel, it is an unavoidable impurity that can degrade mechanical properties. In particular, because the precipitation of chromium carbides on grain boundaries reduces toughness, it is desirable to reduce the C content as much as possible. In this case, if the C content exceeds 0.030%, the Nb or Ti content required to suppress chromium carbides increases, increasing manufacturing costs. Furthermore, the increased precipitation of Nb carbonitrides or Ti carbonitrides may reduce forgeability. Therefore, the C content is set to 0.030% or less. On the other hand, reducing the C content to less than 0.001% increases manufacturing costs, so it is preferable to set the C content to 0.001% or more.

[0029] Si: 0.01 to 1.00% Silicon is an element that has a deoxidizing effect. To adequately deoxidize steel, the silicon content is set to 0.01% or more. However, if the silicon content exceeds 1.0%, hardening may occur and ductility may decrease. For this reason, the silicon content is set to 1.0% or less.

[0030] Mn: 0.01 to 1.00% The Mn content is set to 0.01% or more to ensure sufficient deoxidation. However, if the Mn content exceeds 1.00%, hardening may occur and ductility may decrease. Therefore, the Mn content is set to 1.00% or less.

[0031] Ni: 0.05 to 3.00% Ni has the effect of suppressing brittle cracking of the ferrite phase. Therefore, the Ni content is set to 0.05% or more. However, if the Ni content exceeds 3.00%, the austenite phase appears at high temperatures and begins to harden by quenching, which may significantly reduce ductility and forgeability. Therefore, the Ni content is set to 3.00% or less.

[0032] Cr: 10.5 to 25.0% Cr improves corrosion resistance and intergranular corrosion resistance in weld heat affected zones. Therefore, the Cr content is set to 10.5% or more. The Cr content is preferably set to 12.0% or more, and more preferably set to 15.0% or more. However, if the Cr content exceeds 25.0%, hardening may occur and bending toughness may be significantly reduced. Therefore, the Cr content is set to 25.0% or less. The Cr content is more preferably set to 20.0% or less.

[0033] Mo: 0 to 3.00% Mo is an element that significantly improves corrosion resistance and toughness. However, if the Mo content exceeds 3.00%, Laves phases are likely to precipitate, which may reduce ductility and forgeability. Therefore, the Mo content is set to 3.00% or less. The Mo content is preferably set to 2.00% or less, and more preferably set to 1.00% or less. On the other hand, to achieve the above effects, the Mo content is preferably set to 0.01% or more. The Mo content is preferably set to 0.10% or more, and more preferably set to 0.30% or more.

[0034] Cu: 0.02 to 1.50% Cu is an element that improves corrosion resistance and suppresses deformation twinning, thereby improving toughness. Therefore, the Cu content is set to 0.02% or more. However, if the Cu content exceeds 1.50%, Cu precipitation may harden the steel and reduce ductility. Therefore, the Cu content is set to 1.50% or less. The Cu content is preferably set to 1.00% or less.

[0035] N: 0.001 to 0.030% N is an impurity that is inevitably mixed into steel. N precipitates chromium nitrides during the cooling process, reducing toughness. Reducing the N content as much as possible is effective in improving toughness. In this case, if the N content exceeds 0.030%, the Nb or Ti content required to suppress chromium nitrides increases. As a result, manufacturing costs increase, and the formation of Nb nitrides or Ti carbides may reduce toughness. For this reason, the N content is set to 0.030% or less. It is preferable to reduce N as much as possible, but excessive reduction increases manufacturing costs. For this reason, the N content is preferably set to 0.001% or more.

[0036] Nb: 0.10 to 1.00%, Nb forms carbides or nitrides, which inhibits sensitization of the weld heat-affected zone and improves corrosion resistance. Therefore, the Nb content is set to 0.10% or more. However, if the Nb content exceeds 1.00%, coarse carbonitrides such as NbCN or Laves phases may form, which may significantly reduce toughness and ductility. Therefore, the Nb content is set to 1.0% or less. The Nb content is preferably set to 0.8% or less, and more preferably 0.6% or less.

[0037] The ferritic stainless steel bar according to the present invention may contain, in addition to the above elements, one or more elements selected from Ti, V, Zr and W, as required.

[0038] Ti: 0 to 0.50% V: 0 to 1.00% W: 0 to 0.50% Zr: 0 to 0.50%

[0039] Like Nb, Ti, V, Zr, and W all form carbonitrides and have the effect of suppressing sensitization of welds. Therefore, they may be added as needed. However, excessive addition of Ti, V, Zr, and W may result in the formation of coarse carbonitrides, which may reduce toughness. Therefore, the Ti content is set to 0.50% or less. The V content is set to 1.00% or less. The W content is set to 0.50% or less. The Zr content is set to 0.50% or less. However, to achieve the above effects, the Ti content is preferably set to 0.05% or more. Furthermore, the V content is preferably set to 0.10% or more. Furthermore, the Zr content is preferably set to 0.01% or more. The W content is preferably set to 0.10% or more.

[0040] B: 0 to 0.010% B suppresses P segregation at grain boundaries and improves toughness. Therefore, it may be added as needed. However, if B is added in excess of 0.010%, coarse borides may be formed, which may deteriorate toughness. Therefore, the B content is set to 0.010% or less. On the other hand, to obtain the above effects, the B content is preferably set to 0.0003% or more.

[0041] The ferritic stainless steel bar according to the present invention may contain, in addition to the above elements, one or more elements selected from Al, Ca and Mg, if necessary.

[0042] Al: 0 to 0.500% Ca: 0 to 0.050% Mg: 0 to 0.050%

[0043] Al, Ca, and Mg all have a deoxidizing effect and are added as deoxidizers during melting. Therefore, they may be added as needed. However, excessive addition of Al, Ca, or Mg may cause hardening and reduce ductility. For this reason, the Al content is set to 0.500% or less. The Ca content is set to 0.050% or less. The Mg content is set to 0.050% or less. On the other hand, to obtain the above effects, the Al content is preferably set to 0.001% or more. The Ca content is preferably set to 0.0005% or more. The Mg content is preferably set to 0.0005% or more.

[0044] The ferritic stainless steel bar according to the present invention may contain, in addition to the above elements, one or more elements selected from Co, Ga, Sn, Sb, Ta and REM, if necessary.

[0045] Co: 0 to 0.5% Ga: 0 to 0.05% Sn: 0 to 0.5% Sb: 0 to 0.50% Ta: 0 to 0.50% REM: 0 to 0.10%

[0046] Co: 0 to 0.5% Co has the effect of improving wear resistance. Therefore, it may be contained as needed. However, if the Co content exceeds 0.5%, hardening may occur and ductility may deteriorate. Therefore, the Co content is set to 0.5% or less. On the other hand, in order to obtain the above effect, the Co content is preferably set to 0.01% or more.

[0047] Ga: 0 to 0.05% Ga has the effect of improving corrosion resistance and may be contained as needed. However, excessive Ga content may cause hardening and reduce ductility. Therefore, the Ga content is set to 0.05% or less. On the other hand, to obtain the above effect, the Ga content is preferably set to 0.001% or more.

[0048] Sn: 0 to 0.5% Sn has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if the Sn content exceeds 0.5%, there is a possibility that toughness and forgeability may deteriorate due to grain boundary segregation. Therefore, the Sn content is set to 0.5% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably set to 0.001% or more.

[0049] Sb: 0 to 0.50% Sb has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if the Sb content exceeds 0.50%, the segregation of Sb may deteriorate the toughness and forgeability. Therefore, the Sb content is set to 0.50% or less. On the other hand, in order to obtain the above effect, the Sb content is preferably set to 0.001% or more.

[0050] Ta: 0 to 0.50% Ta has the effect of improving wear resistance and corrosion resistance. Therefore, it may be added as needed. However, if the Ta content exceeds 0.50%, hardening may occur and ductility may deteriorate. Therefore, the Ta content is set to 0.50% or less. On the other hand, to obtain the above effect, the Ta content is preferably set to 0.01% or more.

[0051] REM: 0 to 0.10% REM has a deoxidizing effect and also has the effect of improving hot workability. Therefore, it may be added as needed. However, if the REM content exceeds 0.10%, there is a possibility that the hot workability may be deteriorated. Therefore, the REM content is set to 0.10% or less. On the other hand, to obtain the above effect, the REM content is preferably set to 0.001% or more. Note that REM is a collective term for 17 elements, including 15 lanthanoid elements, Y, and Sc. One or more of these 17 elements can be contained in steel, and the REM content refers to the total content of these elements.

[0052] In the chemical composition of the steel sheet of the present invention, the balance is Fe and unavoidable impurities. Here, "unavoidable impurities" refers to components that are mixed in during industrial production of steel sheet due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. Examples of unavoidable impurities include S, P, O, Zn, Pb, and H. It is desirable to reduce the amount of unavoidable impurities, but if they are present, it is desirable to keep them to 0.01% or less.

[0053] <2. Precipitates>

[0054] In order to ensure sufficient flexural strength, the ferritic stainless steel bar according to the present invention satisfies the requirement that the amount of precipitated Fe+Cr in the extraction residue be 0.25% or less. The amount of precipitated Fe+Cr is preferably 0.20% or less, more preferably 0.15% or less, and even more preferably 0.10% or less. The precipitated Fe+Cr mainly corresponds to Laves phases and Cr nitrides, and by suppressing these precipitates, hydrogen trap sites are reduced and the amount of diffusible hydrogen can be suppressed. Furthermore, since these precipitates tend to precipitate at grain boundaries and significantly reduce toughness, suppressing the amount of precipitated Fe+Cr is important in order to ensure flexural strength.

[0055] To ensure sufficient fracture toughness, the ferritic stainless steel of the present invention requires that the amount of precipitated Nb+Ti+V+W+Zr in the extraction residue satisfy a range of 0.05% to 0.60%. Precipitated Nb, etc., primarily forms MC-MN carbonitrides, thereby suppressing chromium carbonitrides and suppressing sensitization. Therefore, the amount of precipitated Nb+Ti+V+W+Zr is set to 0.05% or more. The amount of precipitated Nb+Ti+V+W+Zr is preferably 0.10% or more. On the other hand, if precipitated Nb, etc., is present at 0.60% or more, the MC-MN carbonitrides may become coarse, reducing toughness or trapping large amounts of diffusible hydrogen, resulting in reduced fracture toughness. Therefore, the amount of precipitated Nb+Ti+V+W+Zr satisfies a range of 0.60% or less. The amount of precipitated Nb+Ti+V+W+Zr is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less.

[0056] The amount of precipitates is investigated using the following procedure. First, a bar-shaped steel material is cut into a plate material with a thickness of 2 to 3 mm, parallel to the longitudinal and radial directions, including the center, and the surface area of ​​the plate is 5 to 20 cm. 2 The longitudinal width is adjusted so that the thickness is 0.4 to 1.0 g. Electrolysis is performed in a non-aqueous electrolyte such as an AA-based solution to separate the precipitate from the cross section of the steel material, and the residue is recovered using a 0.2 μm mesh filter. The recovered residue is then quantitatively analyzed using ICP or other methods.

[0057] <3. Diffusible hydrogen content>

[0058] Ferritic stainless steels are typically produced in combustion furnaces using fuels such as LNG or heavy oil. When the fuel is not fully burned and exposed to high temperatures, it may decompose into hydrogen, which then becomes occluded in the hot steel. In steels with a large specific surface area, such as thin wires or thin plates, hydrogen diffuses sufficiently even at room temperature and desorbs from the surface. On the other hand, in large-diameter steel bars, the distance from the center to the surface is long, so hydrogen does not desorb completely and remains there for a long time. Therefore, even with similar microstructures, large-diameter steel bars exhibit variations in ductility and bending toughness, which can lead to breakage during straightening or peeling. In particular, hydrogen trapping sites exist around precipitate phases, such as carbides and Laves phases, which are difficult to desorb at room temperature and remain there for a long time. Hydrogen trapped in precipitates diffuses when subjected to stress or deformation, resulting in hydrogen embrittlement. In the present invention, controlling the amount of diffusible hydrogen trapped in precipitates is effective in improving bending toughness and ductility.

[0059] To ensure sufficient flexural strength, the ferritic stainless steel bar according to the present invention preferably satisfies a total hydrogen release of 0.5 ppm or less between 100°C and 400°C when measured by TDA on a test specimen including the center of a vertical cross section from the longitudinal direction. The total hydrogen release is more preferably 0.4 ppm or less, and even more preferably 0.3 ppm or less. In this case, samples are preferably collected at a thickness of 1 mm or more to reduce the influence of desorption prior to testing. The test is started at 0°C and heated to 400°C at a heating rate of 100°C / hr. Since hydrogen desorption proceeds from the surface, samples are collected from the center of the steel, where the hydrogen content is greatest. Because hydrogen desorption proceeds immediately after thinning, samples are stored at cryogenic temperatures such as liquid nitrogen or in a freezer after TDA processing. It is preferable to start testing within one week of storage.

[0060] <4.Diameter>

[0061] For ferritic stainless steel bars with large diameters, the grain size and precipitates are coarse, making breakage more pronounced. Therefore, the effects of the present invention are most clearly seen for bars with diameters of 16 mm or more. The upper diameter limit is preferably 100 mm or less. A bar-shaped steel is a steel bar with a uniform cross section in the longitudinal direction. The cross section is not limited to a circular one; it also includes irregularly shaped bars, such as rectangular bars. In this case, the diameter of the steel bar is defined as the straight line that passes through the center of gravity of the vertical cross section and has the shortest length.

[0062] 5. Flexural toughness

[0063] The toughness of the ferritic stainless steel bar according to the present invention is evaluated using the crack initiation angle in a three-point bending test. As described above, the flexural strength can be evaluated by the toughness in a three-point bending test with a low strain rate and by the tensile reduction test described below. Specifically, the evaluation is carried out by the following method.

[0064] Two plate-shaped samples measuring (diameter) x 5 mm thick and 240 mm wide in the longitudinal direction are taken from a vertical cross section of the steel material in the longitudinal direction. A three-point bending test is performed using the push-bending method in accordance with JIS Z 2248. The tip radius of the push-bending tool is 5 mm. The specimen is gradually pushed and bent from 0° to 180° at a stroke speed of 2 mm / min, and the test is stopped when cracks occur; the angle at this point is taken as the crack initiation angle. Materials that do not crack at an angle less than 90° are considered to have high flexural strength. A crack initiation angle of 110° or more is preferable, and 130° or more is even more preferable.

[0065] <6. Tensile drawing> The ductility of the ferritic stainless steel according to the present invention is evaluated using the reduction of area in a tensile test. In accordance with JIS Z 2201, the longitudinal direction of the steel material is aligned with the longitudinal direction of the tensile test piece, and a No. 10 bar-shaped tensile test piece is prepared from the center of the steel material. The test conditions are 10 MPa / s up to the measurement of 0.2% proof stress, and 25 mm / min thereafter. A material with a reduction of area of ​​more than 50% is considered to have good ductility. A reduction of area of ​​60% or more is preferable, and 70% or more is more preferable.

[0066] <7. Manufacturing method> A preferred method for producing the ferritic stainless steel bar according to the present invention will be described below. The ferritic stainless steel bar according to the present invention can obtain the effects as long as it has the above-mentioned configuration, and for example, a ferritic stainless steel bar having the above-mentioned configuration can be produced by the following production method, in which a steel slab is hot-rolled and annealed.

[0067] In the method for producing a ferritic stainless steel bar according to the present invention, the heating temperature and rolling temperature during hot rolling are controlled to 850°C or higher, the steel is coiled at 830°C or higher, and the steel is rapidly cooled by water cooling or the like (for example, immersion, spray, or forced air cooling). By controlling the heating temperature and rolling temperature during hot rolling, the amount of Laves phase precipitation is reduced, improving toughness. Furthermore, by controlling the rolling end temperature and rapidly cooling, the Laves phase can be suppressed while reducing the strain remaining in the steel, thereby improving toughness and ductility.

[0068] The heating temperature and the rolling temperature are preferably 880°C or higher, more preferably 900°C or higher. The finishing temperature of rolling is preferably 850°C or higher, more preferably 880°C or higher. On the other hand, when the heating temperature and the rolling temperature exceed 1250°C, the scale increases significantly and the yield decreases. Therefore, the heating temperature and the rolling temperature are set to 1250°C or lower. The heating temperature and the rolling temperature are preferably 1200°C or lower, more preferably 1150°C or lower. Also, when the finishing temperature of rolling exceeds 1050°C, the recrystallized grains coarsen, and instead, the toughness and ductility decrease. Therefore, the finishing temperature of rolling is set to 1050°C or lower. The finishing temperature of rolling is preferably 1000°C or lower, more preferably 950°C or lower.

[0069] Furthermore, after hot rolling, annealing is performed under the conditions of formula (A), and rapid cooling is carried out by water cooling or the like.

[0070] D < 2 × (1.39 × t) 0.5 × T 1.345 / 1000 ··· (A)

[0071] However, D is the wire diameter of the steel material (mm), t is the holding time (minutes), and T is the holding temperature (°C). The holding temperature T satisfies 200 < T < 680.

[0072] Formula (A) shows that the larger the diameter, the more the annealing temperature and annealing time required for dehydrogenation increase. Formula (A) is a relational expression obtained by performing annealing under various conditions after rolling the wire rod and quantitatively evaluating the reduction of diffusible hydrogen amount and the improvement of ductility.

[0073] If the temperature inside the furnace varies depending on the location, formula (A) is applied at the location where the steel temperature is lowest. There is no particular limit on the holding time t, but considering profitability, it is preferably within 180 minutes, more preferably within 100 minutes, and even more preferably within 60 minutes. The holding temperature is 200°C or higher to efficiently remove diffusible hydrogen trapped in precipitates. Considering dehydrogenation efficiency, it is preferably 250°C or higher, more preferably 300°C or higher. On the other hand, if the holding temperature T exceeds 680°C, Laves phases will precipitate, which will actually reduce toughness and ductility, so it is set to 680°C or lower. It is preferably 650°C or lower, and more preferably 620°C or lower.

[0074] The furnace used during annealing can be either a batch furnace or a continuous annealing furnace. From a cost perspective, the atmosphere is preferably air or combustion gas. When using gas combustion, it is desirable to satisfy an air ratio of 1.05 or more to reduce hydrogen absorption due to incomplete combustion, and from the perspective of thermal efficiency, it is desirable to satisfy an air ratio of 1.30 or less. Furthermore, when using an inert atmosphere, it is desirable to use vacuum, nitrogen, Ar, etc., and not use reducing hydrogen gas. [Example]

[0075] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0076] [Example 1] Steels having the chemical compositions shown in Tables 1 and 2 were melted in a 100 kg vacuum melting furnace and cast into slabs with a diameter of 180 mm. The slabs were then heated to 1100°C and hot rolled at an average temperature of 930°C to a diameter of 65 mm. The rolling was completed at a temperature of 910°C and quickly water-cooled. Steel bars were then produced by batch annealing at 620°C for 100 minutes in an LNG combustion atmosphere with an air ratio of 1.10 and quickly water-cooling.

[0077] (Measurement of precipitate amount) The precipitates in the resulting steel bars were identified using the following method. Plate-shaped test pieces measuring 10 mm in width along the longitudinal direction and 2 mm in thickness were cut from a cross section perpendicular to the longitudinal direction of the steel bars. 0.4 g of each piece was electrolyzed in an AA-based nonaqueous electrolyte, and the extraction residue was recovered using a 0.2 μm mesh filter and subjected to ICP analysis.

[0078] (Measurement of diffusible hydrogen content by TDA) The amount of diffusible hydrogen was measured for the resulting steel bars using the following method. A plate-shaped sample, 20 mm wide in the longitudinal direction and 10 mm long and 2 mm thick, was taken from a cross section perpendicular to the longitudinal direction, passing through the center of the steel material, and including the center. It was quickly washed and dried, and the pre-test weight was measured. The sample was set in a TDA, and the amount of hydrogen desorption was measured at a heating rate of 100°C / hr from 0°C to 400°C. The total amount of hydrogen released between 100°C and 400°C was measured.

[0079] The amount of hydrogen released when TDA was measured on a steel bar manufactured with the composition of steel type No. 1 is shown in Figure 1, and the cumulative amount of hydrogen released is shown in Figure 2.

[0080] (Bending toughness evaluation) Toughness was evaluated using a three-point bending test. Two plate samples measuring 5 mm in diameter x thickness and 240 mm in width were taken from a vertical cross section of the steel material in the longitudinal direction. A three-point bending test was carried out using the push-bending method in accordance with JIS Z 2248. The push-bending test had a tip radius of 5 mm and was gradually pushed and bent from 0° to 180° at a stroke speed of 2 mm / min. The test was stopped when a crack occurred, and the angle at this point was recorded as the crack initiation angle.

[0081] (Drawing ductility evaluation by tensile test) Ductility was evaluated by tensile testing. No. 10 tensile test pieces in accordance with JIS Z 2201 were prepared so that they included the center and the longitudinal direction of the steel material coincided with the longitudinal direction of the tensile test. The test conditions were 10 MPa / s up to the 0.2% proof stress measurement, and 25 mm / min after the proof stress. Reduction of area was measured from the fracture surface.

[0082] [Table 1]

[0083] [Table 2]

[0084] Test Nos. 1 to 35 satisfied the requirements of the present invention and were good in bending toughness and reduction of area, whereas Test Nos. 36 to 50, which did not satisfy the requirements of the present invention, were poor in bending toughness and reduction of area.

[0085] [Example 2] Steel type No. 1 listed in Table 1 was melted in the same manner as in Example 1 to produce a slab with a diameter of 180 mm. Hot rolling was performed under various rolling conditions to produce steel bars with diameters of 16 to 100 mm. The air ratio for badge annealing was 1.10, the same as in Example 1. Measurement of the amount of precipitates, measurement of the total amount of released hydrogen, evaluation of bending toughness, and evaluation of drawing ductility were performed in the same manner as in Example 1. The operating conditions and results are summarized in Tables 3 and 4 below.

[0086] [Table 3]

[0087] [Table 4]

[0088] Test Nos. 51 to 74 satisfied this standard and had good toughness and intergranular corrosion resistance, while Nos. 81 to 89, which did not satisfy this standard, had poor bending toughness and ductility. [Industrial Applicability]

[0089] The ferritic stainless steel bar according to the present invention has good oxidation resistance and high-temperature strength. Furthermore, when the ferritic stainless steel bar according to the present invention is used, it does not break even when peeling or straightening is performed, and it has good toughness and ductility. Therefore, the ferritic stainless steel bar is suitable for automotive structural parts, such as exhaust system flanges, high-pressure fuel pumps, boss materials, injectors, etc.

Claims

1. The chemical composition, in mass%, is C: 0.001-0.03%, Si: 0.01-1.0%, Mn: 0.010-1.0%, Ni: 0.05-3.0%, Cr: 10.5-25.0%, Mo: 0-3.0%, Cu: 0.02 to 1.5%, N: 0.001-0.03%, Nb: 0.1-1.0%, Ti: 0 to 0.5%, V: 0 to 1.0%, W: 0-0.50%, Zr: 0 to 0.50%, B: 0 to 0.010%, Al: 0-0.50%, Ca: 0-0.05%, Mg: 0-0.05%, Co: 0 to 0.50%, Ga: 0-0.05%, Sn: 0 to 0.50%, Sb: 0 to 0.50%, Ta: 0 to 0.50%, REM: 0-0.10%, Remainder: Fe and unavoidable impurities and The amount of precipitated Fe+Cr due to extraction residue is 0.20% or less, and the amount of precipitated Nb+Ti+V+W+Zr is 0.05% or more and 0.60% or less, Diameter 16mm or more A ferritic stainless steel bar material characterized by:

2. 2. The ferritic stainless steel bar according to claim 1, characterized in that a sample taken from the center of the ferritic stainless steel bar is measured by TDA and the total amount of released hydrogen from 0°C to 400°C is 0.5 ppm or less.

3. 3. The ferritic stainless steel bar according to claim 1, wherein in a three-point bending test, no cracks occur in a plate material that includes the center of the ferritic stainless steel bar and has dimensions of (diameter) x (thickness 5 mm) x (longitudinal direction 240 mm) from 0° to 90°.

4. The ferritic stainless steel bar according to any one of claims 1 to 3, characterized in that the reduction of area of ​​the fracture surface in a round bar tensile test of the ferritic stainless steel bar is 50% or more.

5. A method for producing the ferritic stainless steel bar according to any one of claims 1 to 4, Annealing after hot rolling includes a step of maintaining the condition of the following formula (A): A method for producing a ferritic stainless steel bar, comprising: r<(1.39×t) 0.5 ×T 1.345 / 1000 … (A) where r is the radius of the steel (mm), t is the holding time (minutes), and T is the holding temperature (°C), and the holding temperature T satisfies 200<T<680.

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