Al-containing precipitation hardening stainless steel, steel strip and method for manufacturing said strip

Abstract

To provide a stainless steel strip that can be used to produce an Al-containing precipitation hardened product with excellent fatigue properties, a stainless steel that can be used to produce such a steel strip by hot working, and a method for producing the steel strip. [Solution] This is an Al-containing precipitation hardening stainless steel strip having a rolled dual-phase structure with ferrite particles dispersed in a matrix of austenite. The predetermined chemical composition satisfies 65<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<340. The Al-containing precipitation hardening stainless steel that is rolled to give a strip has a dual-phase structure with ferrite particles dispersed in a matrix of austenite, and the shape of the ferrite particles in a cross section perpendicular to the longitudinal direction satisfies (circularity)=4π×S1 / L. 2 The simple average circularity defined by the formula (1) is 0.10 or more. The manufacturing method involves heating a slab made of steel having the same predetermined component composition and a dual-phase structure in which the shape of ferrite phase particles in a cross section perpendicular to the longitudinal direction has a simple average circularity of 0.10 or more to a temperature between 1100°C and 1270°C, and rolling it.

Description

[Technical Field]

[0001] The present invention relates to a steel strip before aging treatment that gives an Al-containing precipitation hardening stainless steel product with excellent fatigue properties, a stainless steel obtained by hot working the steel strip, and a method for producing the steel strip. [Background technology]

[0002] Precipitation-hardened stainless steel containing Al is a stainless steel that has high mechanical strength and excellent corrosion resistance due to the precipitation of intermetallic compounds consisting of Ni and Al by heat treatment. Utilizing these characteristics, it is used in steel belts for food conveyance, etc. However, in such mechanical applications, belts are often subjected to repeated bending processes in the drive parts, etc., and high durability that allows for long-term use without fracture, i.e., excellent fatigue properties, is required.

[0003] Non-metallic inclusions present inside the base material of stainless steel act as starting points for fatigue cracks and reduce fatigue properties. Therefore, methods have been widely proposed to improve fatigue properties by reducing the size and number of non-metallic inclusions.

[0004] For example, Patent Document 1 discloses a precipitation-hardened martensitic stainless steel with excellent fatigue properties, in which the size of nonmetallic inclusions with different compositions, such as nitrides and oxides, contained in the steel is reduced overall, and the number density of large-sized nonmetallic inclusions present in the matrix is ​​limited. Among the nonmetallic inclusions, there are no nonmetallic inclusions with an equivalent circular diameter of 10 μm or more, or, even if nonmetallic inclusions with an equivalent circular diameter of 10 μm or more are present, their number density is limited to 0.100 pieces / mm 2 It will be controlled as follows: [Prior art documents] [Patent documents]

[0005] [Patent Document 1] WO2022 / 138194 International Publication Summary of the Invention [Problem to be solved by the invention]

[0006] It has been found that when aluminum-containing precipitation hardening stainless steels, particularly stainless steels with a specific composition in which the two-phase structure consisting mainly of austenite and ferrite is subjected to aging and martensite treatment to produce products, sufficient fatigue properties cannot be obtained by simply controlling the precipitation of nonmetallic inclusions as described above. Therefore, there has been a demand for improving the fatigue properties of such aluminum-containing precipitation hardening stainless steel products.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel strip that can be used to produce a product made of Al-containing precipitation hardening stainless steel with excellent fatigue properties, an Al-containing precipitation hardening stainless steel obtained by hot working such a steel strip, and a method for producing the steel strip. [Means for solving the problem]

[0008] The present inventors have discovered that in stainless steel with a specific component composition in which the primary metal structure is a two-phase structure consisting of austenite and ferrite, the shape of the ferrite phase particles dispersed in the austenite matrix can be controlled to be spherical by adjusting the amounts of trace elements such as P, S, Sn, and O, and that by hot rolling such steel into a hot steel strip, fine cracks caused by the hot rolling into the strip can be eliminated after subsequent shaping, solution treatment, aging treatment, and martensitization treatment to give a product with excellent fatigue properties.

[0009] That is, the present invention provides an Al-containing precipitation hardening stainless steel strip, which comprises, in mass%, Cr: 15.0 to 18.0%, Ni: 6.0 to 8.0%, Al: 0.90 to 1.50%, Mo: 0.10 to 0.50%, C: 0.010 to 0.200%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.50%, P: 0.010 to 0.050%, S: 0.0001 to 0.0050%, Cu: 0.05 to 0.50%, Ti: 0.001 to 0.050%, W: 0.001 to 0.040%, Nb: The composition of the alloy is characterized by having a rolled dual-phase structure in which ferrite phase particles are dispersed in an austenite matrix, the composition comprising 0.001 to 0.030%, Sn: 0.0003 to 0.0200%, N: 0.001 to 0.040%, O: 0.0001 to 0.0050%, and the balance being Fe and unavoidable impurities, and the composition satisfies 65<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<340, where [M] is the mass% of element M, and the alloy has an austenite matrix.

[0010] According to these characteristics, it is possible to obtain a stainless steel strip that can provide an Al-containing precipitation hardening stainless steel product with excellent fatigue properties.

[0011] In the above invention, the number of cracks with a width of 3 μm or more in optical observation in a cross section perpendicular to the hot rolling direction is 100 mm 2 The present invention may be characterized in that the number of defects is 18 or less among the total defects. According to this characteristic, it is possible to obtain a stainless steel strip that can provide an Al-containing precipitation hardening stainless steel product having excellent fatigue properties.

[0012] In the above invention, in the component composition, δ cal δ defined as =4.3×(1.3×[Si]+[Cr]+[Mo]+2.2×[Al]+[Nb]+0.3×[Sn]+1.5×[W])-3.9×(30×[C]+30×[N]+[Ni]+0.8×[Mn]+0.3×[Cu])-31.5 cal may be in the range of 5 to 30. According to this feature, it is possible to control the volume fraction of the ferrite phase and obtain a stainless steel strip that can provide an Al-containing precipitation hardening stainless steel product with excellent fatigue properties.

[0013] The present invention also provides an Al-containing precipitation hardening stainless steel that is hot-rolled to give a strip material, the stainless steel comprising, by mass%, 15.0 to 18.0% Cr, 6.0 to 8.0% Ni, 0.90 to 1.50% Al, 0.10 to 0.50% Mo, 0.010 to 0.200% C, 0.01 to 0.90% Si, 0.30 to 1.50% Mn, 0.010 to 0.050% P, 0.0001 to 0.0050% S, 0.0001 to 0.0050% Cu, 0.05 to 0.50% Ti, 0.001 to 0.050% W, and 0.001 to 0.030% Nb. %, Sn: 0.0003 to 0.0200%, N: 0.001 to 0.040%, O: 0.0001 to 0.0050%, with the balance being Fe and unavoidable impurities, and the composition satisfies 65<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<340, where [M] is the mass% of element M, and has a two-phase structure in which ferrite phase particles are dispersed in a matrix made of austenite phase, and in the shape of the ferrite phase particles in a cross section perpendicular to the longitudinal direction, S1: area (mm 2 ), L: perimeter of ferrite phase particle (mm), (circularity) = 4π × S1 / L 2 The simple average of the circularity defined by is 0.10 or more.

[0014] According to these characteristics, high fatigue properties can be obtained in Al-containing precipitation hardening stainless steel products that are obtained by hot rolling into a steel strip and then appropriately performing shaping, solution treatment, aging treatment, martensitic treatment, etc.

[0015] In the above invention, in the component composition, δ cal δ defined as =4.3×(1.3×[Si]+[Cr]+[Mo]+2.2×[Al]+[Nb]+0.3×[Sn]+1.5×[W])-3.9×(30×[C]+30×[N]+[Ni]+0.8×[Mn]+0.3×[Cu])-31.5 calmay be in the range of 5 to 30. According to this feature, the volume fraction of the ferrite phase can be controlled, and high fatigue properties can be obtained in an Al-containing precipitation hardening stainless steel product.

[0016] Furthermore, the present invention relates to a method for producing an Al-containing precipitation hardening stainless steel strip, the method comprising the steps of, by mass%, Cr: 15.0 to 18.0%, Ni: 6.0 to 8.0%, Al: 0.90 to 1.50%, Mo: 0.10 to 0.50%, C: 0.010 to 0.200%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.50%, P: 0.010 to 0.050%, S: 0.0001 to 0.0050%, Cu: 0.05 to 0.50%, Ti: 0.001 to 0.050%, W: 0.001 to 0.040%, Nb: 0.001 to 0.030%, Sn The composition comprises 0.0003 to 0.0200% of P, 0.001 to 0.040% of N, 0.0001 to 0.0050% of O, with the balance being Fe and unavoidable impurities, and the composition satisfies 65<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<340 when the mass% of element M is [M], and has a two-phase structure in which ferrite phase particles are dispersed in a matrix made of an austenite phase, and in the shape of the ferrite phase particles in a cross section perpendicular to the longitudinal direction, S1: area (mm 2 ), L: perimeter of ferrite phase particle (mm), (circularity) = 4π × S1 / L 2 The method is characterized in that a slab made of steel having a two-phase structure in which ferrite phase particles are dispersed in a matrix made of an austenite phase, with a simple average circularity defined as 0.10 or more, is heated to a temperature between 1100°C and 1270°C and rolled.

[0017] According to this feature, it is possible to produce a stainless steel strip that provides an Al-containing precipitation hardening stainless steel product with excellent fatigue properties.

[0018] In the above invention, the number of cracks with a width of 3 μm or more in optical observation in a cross section perpendicular to the hot rolling direction is 100 mm 2The present invention may be characterized in that the number of defects is 18 or less among the total defects. According to this characteristic, it is possible to produce a stainless steel strip that provides an Al-containing precipitation hardening stainless steel product having excellent fatigue properties.

[0019] In the above invention, in the component composition, δ cal δ defined as =4.3×(1.3×[Si]+[Cr]+[Mo]+2.2×[Al]+[Nb]+0.3×[Sn]+1.5×[W])-3.9×(30×[C]+30×[N]+[Ni]+0.8×[Mn]+0.3×[Cu])-31.5 cal may be in the range of 5 to 30. According to this feature, it is possible to control the volume fraction of the ferrite phase and produce a stainless steel strip that provides an Al-containing precipitation hardening stainless steel product with excellent fatigue properties. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a scatter diagram showing the relationship between the circularity of ferrite phase particles in the cross section of a slab before hot rolling and the number of cracks in the cross section of a hot strip steel. [Figure 2] FIG. 2 is a schematic diagram showing the relationship between the degree of circularity of ferrite phase particles and cracks caused by rolling. [Figure 3] FIG. 1 is a scatter diagram showing the relationship between the PSSnO value and the circularity of ferrite phase particles. [Figure 4] 1 is a list of chemical compositions of steels used in manufacturing tests. [Figure 5] 1 is a table showing the results of a manufacturing test. DETAILED DESCRIPTION OF THE INVENTION

[0021] In considering the provision of various products using Al-containing precipitation hardening stainless steel, the inventors of the present application conducted extensive research, taking into consideration that if steel strip material contains fine cracks resulting from hot rolling into strip material before undergoing shaping, solution treatment, aging treatment, martensite treatment, etc., as a product, these traces will remain in the final product, ultimately affecting fatigue strength.

[0022] First, we conducted detailed investigations of samples taken from the area around the fracture and unfractured areas of a 0.5 mm thick steel belt made of aluminum-containing precipitation-hardened stainless steel that had fractured shortly after the start of use. The surfaces of these samples were mirror-polished and then examined in detail using optical and electron microscopes. As a result, we discovered that microcracks existed not only around the fracture but also within the unfractured areas. Furthermore, to identify the time point at which these microcracks developed, we took samples from several lots of aluminum-containing precipitation-hardened stainless steel strips, from 1.0 mm thick cold-rolled material (cold strip), 3 mm thick hot-rolled material (hot strip), and 150 mm thick slabs. We then examined the cross sections of the samples using optical and electron microscopes. Microcracks were observed in the cold and hot strips, but not in the slabs. This finding suggests that the microcracks originated during hot rolling.

[0023] Furthermore, to identify the cause of microcracks during hot rolling, we mirror-polished the cross-sections of samples taken from hot strips and electrolytically etched them in a 10% potassium hydroxide solution to reveal the microstructure. The microstructure of the observation surface was then observed using optical and electron microscopes. We found that the microcracks propagated along the ferrite / austenite phase boundary. Furthermore, no nonmetallic inclusions or other impurities that could cause cracks were observed around the fracture site. The Al-containing precipitation-hardening stainless steel studied here is transformed into a martensite-rich structure through heat treatments, including aging, after cold rolling for use in final products such as steel belts. However, the slab (billet), hot strip, and cold strip before hot rolling have a two-phase structure consisting primarily of austenite and ferrite. Based on these findings, we conducted further intensive research into the relationship between the morphology of the ferrite / austenite phase boundary and cracking.

[0024] This study focused on the shape of ferrite phase particles in slabs before hot rolling. Samples were cut from multiple lots of Al-containing precipitation-hardened stainless steel slabs before hot rolling, and the observation surface was a vertical cross section in the longitudinal direction, which was the casting direction. The samples were embedded in resin, the observation surface was mirror-polished, and then electrolytically etched in a 10% potassium hydroxide etchant to reveal the structure. The structure of the observation surface was observed using an optical microscope. During the microstructural observation, the shape of the ferrite phase particles was focused on, and the particle area and perimeter of more than 100 ferrite phase particles were measured using image analysis software. The degree of sphericity of the ferrite phase particles was then evaluated by calculating the circularity, defined by the following formula: (Circularity) = 4π × S1 / L 2 In the formula, S1 is the area of ​​the ferrite phase particles (mm 2 ), L is the particle perimeter (mm).

[0025] Figure 1 shows the relationship between the circularity of ferrite particles measured in the slab before hot rolling and the number of cracks observed in the cross section of the hot strip steel. Here, the number of cracks was measured in a 100 mm section perpendicular to the rolling direction of a 4.5 mm thick hot-rolled plate material. 2 Cracks with a width of 3 μm or more were counted. The thickness of the plate material is preferably 3.0 to 6.0 mm. According to the figure, it was found that the greater the circularity, that is, the closer the shape of the ferrite phase particles is to a sphere, the fewer the number of cracks. In particular, by setting the circularity to 0.10 or more, the occurrence of fine cracks during hot rolling can be sufficiently suppressed.

[0026] The relationship between the circularity of ferrite phase particles and defects caused by cracks is shown schematically in Figure 2. As mentioned above, in order to obtain good fatigue properties in Al-containing precipitation hardening stainless steels, it is important to suppress the propagation of cracks along the austenite / ferrite phase interface.

[0027] First, as shown in Fig. 1(a), a case will be described in which the circularity of ferrite phase particles 12 in the matrix of austenite phase 11 in a slab 1 before hot rolling is low, i.e., the ferrite phase particles 12 have a thin, elongated shape. As shown in Fig. 1(b), when obtaining a hot strip steel 2, if fine cracks are generated during hot rolling, the cracks 15 will propagate along the austenite / ferrite phase interface, becoming larger cracks 15 and leaving many cracks inside the resulting steel strip.

[0028] In contrast, as shown in Figure 1(c), if the ferrite phase particles 12 in the slab 1 before hot rolling have a high circularity, i.e., if the ferrite phase particles 12 have a shape close to a sphere, then even if fine cracks 15 occur during hot rolling, these cracks 15 will not propagate, and the number of cracks remaining inside the steel strip can be reduced, as shown in Figure 1(d).

[0029] Cracks that occur in the hot strip steel 2 after hot rolling can be removed by grinding with a grinder if they are near the surface. However, if fine cracks are present inside, even if the surface cracks are removed with a grinder, fine cracks will remain inside, and when used as a steel belt, the fine cracks will grow due to repeated bending, leading to the breakage of the steel belt.

[0030] Next, the inventors determined the relationship between each component of the Al-containing precipitation hardening stainless steel and the above-mentioned circularity by sorting through a large amount of data.

[0031] As a result, it was found that there is a relationship between the value calculated from the content of P, S, Sn, and O and the circularity of ferrite phase particles, as shown in Figure 3. In particular, it was concluded that by controlling the content of these elements to a range of more than 65 and less than 340 (PSSnO value), the circularity of ferrite phase particles in the slab can be made 0.10 or more, and the fatigue strength of Al-containing precipitation hardened stainless steel products can be improved. PSSnO value = ([P] x 5000 + [S] x 500 + [Sn] x 5300 + [O] x 2500) Here, [M] is the mass % content of element M in the steel composition. That is, each element symbol is substituted with the mass % content of the corresponding element.

[0032] There are still many unknowns about the mechanism by which ferrite phase particles become spheroidized (the circularity of ferrite phase particles in cross section increases) when the component composition satisfies the above-mentioned PSSnO value of more than 65 and less than 340. However, it is thought that the interfacial activation elements S, Sn, and O promote the spheroidization of ferrite phase particles by acting on the ferrite / austenite phase interface. In addition, P, which has the effect of lowering the melting point, is thought to suppress the formation of a network due to the bonding of ferrite particles by forming a residual liquid phase during solidification.

[0033] As described above, it was found that by precisely controlling the contents of P, S, Sn, and O and controlling the PSSnO value to be more than 65 and less than 340, it is possible to spheroidize ferrite phase particles and reduce the microcracks inherent in Al-containing precipitation hardened stainless steel strips.

[0034] Therefore, in this example, the alloys are selected from the group consisting of, in mass%, Cr: 15.0 to 18.0%, Ni: 6.0 to 8.0%, Al: 0.90 to 1.50%, Mo: 0.10 to 0.50%, C: 0.010 to 0.200%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.50%, P: 0.010 to 0.050%, and S: 0.0 The present invention defines a chemical composition including: 0.001-0.0050% Cu, 0.05-0.50% Cu, 0.001-0.050% Ti, 0.001-0.040% W, 0.001-0.030% Nb, 0.0003-0.0200% Sn, 0.001-0.040% N, and 0.0001-0.0050% O. Furthermore, the chemical composition ensures that the PSSnO value described above is greater than 65 and less than 340. The Al-containing precipitation hardened stainless steel strip in this example has a rolled dual-phase structure in which ferrite phase particles are dispersed in an austenite matrix. This allows for the production of an Al-containing precipitation hardened stainless steel strip with minimal inherent cracking, resulting in an Al-containing precipitation hardened stainless steel product with excellent fatigue properties.

[0035] Furthermore, it is also preferable that the shape of the ferrite phase particles in a cross section perpendicular to the longitudinal direction (casting direction) of the slab before hot rolling has a simple average circularity of 0.10 or more. By having such a microstructure in the slab, it is possible to suppress the propagation of cracks during hot rolling as described above, and to reduce the number of inherent cracks in the resulting Al-containing precipitation hardened stainless steel strip.

[0036] In the above-mentioned composition, δ defined by the following formula cal It is also preferable that δ is in the range of 5 to 30. cal The value of is a calculation formula that corresponds to the volume fraction of the ferrite phase in Al-containing stainless steel. cal If the value of δ is small, the volume fraction of the ferrite phase will be low, and the amount of segregated elements such as P and S at the grain boundaries during solidification will increase, making cracks more likely to occur during hot rolling. cal If the value of δ is large, the volume fraction of the ferrite phase increases, which increases the area fraction of the ferrite / austenite phase interface, which is the starting point for cracks during hot rolling, and causes fine cracks to be present and propagate in the resulting Al-containing precipitation hardening stainless steel strip, resulting in a decrease in the fatigue properties of the Al-containing precipitation hardening stainless steel product. cal It is preferable to set the value of within the above range. In the formula, [M] is the mass % of the content of element M in the chemical composition of the steel. δ cal =4.3×(1.3×[Si]+[Cr]+[Mo]+2.2×[Al]+[Nb]+0.3×[Sn]+1.5×[W])-3.9×(30×[C]+30×[N]+[Ni]+0.8×[Mn]+0.3×[Cu])-31.5

[0037] In addition, in the hot strip steel obtained after hot rolling, the number of cracks with a width of 3 μm or more in optical observation of the cross section perpendicular to the hot rolling direction is 100 mm 2 It is also preferable that the number of cracks is 18 or less. This also reduces the number of microcracks inherent in the hot strip steel, which can impart excellent fatigue properties to the resulting Al-containing precipitation hardening stainless steel product.

[0038] <Manufacturing Test> Next, a description will be given of the results of producing a duplex stainless steel sheet by manufacturing slabs having the multiple component compositions shown in FIG. 4 using the manufacturing method described below and then hot rolling the slabs under the conditions shown in FIG.

[0039] First, raw materials such as stainless steel scrap were melted in a 60-ton electric furnace. Subsequently, oxygen refining (oxidation refining) was performed to remove unnecessary carbon using the electric furnace and an argon-oxygen smelting furnace (AOD). Limestone and fluorite were then added to produce a CaO-SiO2-AlO3-MgO-F slag. Furthermore, pure Si and / or Al were added to perform Cr reduction for deoxidation, and Ar was added to desulfurize the slag and adjust the Al and other components. The slag was then poured into a ladle, where the temperature was adjusted, and the P, S, Sn, and other components were adjusted. Finally, a 150-mm-thick slab was produced by continuous casting. Samples for component analysis were collected from the tundish during continuous casting.

[0040] Here, P, S, Sn, and O have a significant effect on the circularity of ferrite phase particles in the slab before hot rolling, so their contents must be precisely controlled. To precisely control the trace amounts of P, S, and Sn, we first investigated the raw materials. As a result, we identified that the main source of P contamination was deposits on the inner surface of pipe-shaped scrap in the raw materials. We also identified that the main source of S contamination was scrap of S-containing steels, such as free-cutting steel. Furthermore, we identified that the main source of Sn contamination was certain scrap, such as that with plating. Therefore, we selected the scrap used as raw materials to prevent unexpected contamination of P, S, and Sn.

[0041] Furthermore, when the P concentration is low during the refining process of the steelmaking process, an Fe-20 wt% P alloy is added to prevent P evaporation loss and distribution loss to the slag, making it possible to precisely control the P content.

[0042] Furthermore, the S content was precisely controlled by desulfurization using slag with a composition in which the mass ratio of CaO to SiO2 during the refining process was 1.5 to 22. In this desulfurization using slag, the O concentration in the molten metal is important as well as the slag composition, and as a result, the O content can be precisely controlled. Furthermore, the S content can be precisely controlled by controlling the concentrations of the deoxidizing agents Al, Si, and Mn in the molten metal. When the S concentration is low during the refining process of steelmaking, the addition of an Fe-25 wt% S alloy prevents distribution loss to the slag when S is added, making it possible to precisely control the S content.

[0043] Furthermore, when the Sn concentration is low during the refining process of the steelmaking process, Ni-40 wt% Sn alloy is added to prevent evaporation loss of Sn, making it possible to precisely control the Sn content.

[0044] Samples were taken from the obtained slabs to measure the circularity of the ferrite phase particles. Each slab was then heated at the temperature and for the heating time described above and hot-rolled to obtain hot steel strips with the thicknesses (3.0 to 6.0 mm) shown in the figure. The hot steel strips were then annealed and pickled to remove surface scale, and cold-rolled to obtain cold steel strips. The strip material obtained by cold-rolling is the Al-containing precipitation-hardened stainless steel strip targeted in this example, and has a rolled dual-phase structure in which ferrite phase particles are dispersed in a matrix of austenite phase.

[0045] The hot rolling conditions are determined as follows. Slabs of aluminum-containing precipitation-hardened stainless steel cast using a continuous casting machine are heated in a heating furnace before hot rolling. Low heating temperatures result in insufficient ductility during hot rolling, leading to surface cracks. High heating temperatures exceed the ductility recovery temperature (nil temperature: the temperature at which embrittlement occurs as the temperature increases) due to the segregation of elements such as phosphorus and sulfur to grain boundaries. This reduces ductility and leads to brittle cracks at grain boundaries, deteriorating the fatigue properties of aluminum-containing precipitation-hardened stainless steel products. Furthermore, short heating times before hot rolling result in uneven temperatures throughout the slab, resulting in uneven ductility across the thickness during hot rolling, leading to surface and internal cracks. Long heating times result in thick scale formation on the slab surface. Even with a water spray to remove the scale during hot rolling, the scale remains, resulting in fine cracks and poor surface quality. Furthermore, cracks also occur at the interface between the scale and the Al-containing precipitation hardening stainless steel. Therefore, the heating temperature before hot rolling is preferably 1100°C to 1270°C and the heating time is preferably 2.0 to 8.0 hours. More preferably, the heating temperature before hot rolling is 1150°C to 1250°C and the heating time is 3.0 to 6.0 hours.

[0046] The steel strip was then subjected to solution heat treatment, martensite heat treatment, and precipitation hardening heat treatment to form a 1.0 mm-thick strip simulating an Al-containing precipitation hardened stainless steel product such as a steel belt. Thin plate samples for plane bending fatigue tests were taken from this thin strip. The fatigue properties of the obtained thin plate samples were evaluated in accordance with the "Plane bending fatigue test method (JIS Z2275-1978)."

[0047] The chemical composition of the Al-containing precipitation hardening stainless steel was determined by quantitative analysis using an X-ray fluorescence analyzer for the above-mentioned sample for composition analysis. The oxygen content was determined by quantitative analysis using an inert gas impulse fusion infrared absorption method.

[0048] The circularity of the ferrite phase particles was determined as follows. Test pieces were prepared so that the observation surface was a cross section perpendicular to the longitudinal direction, which is the casting direction of the slab before hot rolling. The sampling position and size of the test piece were not particularly limited. The observation surface of the prepared test piece was mirror-polished. The mirror-polished observation surface was electrolytically etched in a 10% potassium hydroxide etching solution to reveal the structure. The observation surface with the revealed structure was observed in 10 fields of view using an optical microscope. The measurement field of view was not particularly limited, but in this case, it was 1.00 mm 2 The area was observed at 100x magnification. The ferrite phase was identified from the contrast in each field of view, and the particle area and perimeter of each identified ferrite phase particle were measured, and the circularity of each particle was calculated. The circularity was measured for a total of 100 or more particles in 10 fields of view, and the simple average was calculated and recorded. Image analysis software was used to measure the particle area and perimeter.

[0049] In addition, the number of cracks in the cross section of the hot steel strip, which is the strip material after hot rolling, was measured. 2 The cracks with a width of 3 μm or more were counted and recorded as the number of cracks.

[0050] Fatigue properties were evaluated by conducting fatigue tests in accordance with the "Plane bending fatigue test method (JIS Z2275-1978)" using 1.0 mm thick thin plate samples obtained from thin plates that had been subjected to martensite treatment, etc., simulating the manufacturing process, as described above. The measurement conditions were all the same, with a repeated stress of 600 N / mm 2 The bending speed was 1500 times / min, the stress ratio R = -1 (reversed), and the test temperature was room temperature. In each of the examples and comparative examples, the number of repetitions until breakage of five thin plate samples was measured, and the average value was recorded as the "number of repetitions until breakage." The measurement was performed until breakage or when the number of repetitions was 1.00 x 10 7 The fatigue properties were evaluated by measuring the number of cycles to failure, which is the average number of cycles, until the number of cycles reached 1.00 × 10 7 In the case of no fracture until the end of the test, it is evaluated as the best and is given a "◎" rating, with a score of 1.00 x 10 6 times or more ~ ​​1.00 x 107 If it is less than 1.00 times, it is evaluated as excellent and marked with a "○". 5 times or more ~ ​​1.00 x 10 6 If it is less than 1.00 times, it is evaluated as good and marked with "△". 4 times or more ~ ​​1.00 x 10 5 If it is less than 1.00 times, evaluate it as acceptable and mark it with a square. 4 If the number of times was less than this, it was rated as unacceptable and recorded as "X."

[0051] 5, the ribbons made of Al-containing precipitation hardened stainless steel obtained in Examples 1 to 18 had good fatigue properties, with many cycles required for fracture in the fatigue test or no fracture at all. Among these, Examples 1 to 7 were able to obtain particularly good fatigue properties, with the fatigue properties being evaluated as "excellent" or better.

[0052] 4 again, in Examples 8, 9, and 12, the contents of P, S, Sn, and O were relatively high, and the PSSnO values ​​were relatively large at 269, 274, and 286, although they were within the range of more than 65 and less than 340. As a result, the circularity of the ferrite phase particles was relatively low, and fine cracks were observed at the ferrite / austenite phase interface, which were thought to have occurred during hot rolling, and the fatigue properties were evaluated as "good."

[0053] In Examples 10 and 11, the contents of P, S, Sn, and O were relatively low, and the PSSnO values ​​were within the range of more than 65 and less than 340, but were relatively small at 83 and 84. As a result, the circularity of the ferrite phase particles was relatively low, and cracks were observed at the ferrite / austenite phase interface, which were thought to have occurred during hot rolling, and the fatigue properties were evaluated as "good."

[0054] For Example 13, δ cal The value of σ was large at 33. It is thought that the volume fraction of the ferrite phase increased, increasing the area ratio of the ferrite / austenite phase interface, which is the starting point for cracks. As a result, cracks were observed at the ferrite / austenite phase interface, and the fatigue properties were evaluated as "fair."

[0055] For Example 14, δ cal The value of 3 was small. It is thought that the volume fraction of the ferrite phase was low, and the amount of elements such as P and S that segregated at the grain boundaries during solidification increased, making it easier for fine cracks to occur during hot rolling. As a result, the fatigue properties were evaluated as "Fair."

[0056] In Example 15, the heating temperature before hot rolling was relatively high at 1285°C. Therefore, it is thought that the segregation at the grain boundaries that occurred during solidification in continuous casting exceeded the nil temperature, causing a lack of ductility and resulting in brittle cracks at the grain boundaries. As a result, the fatigue properties were evaluated as "fair."

[0057] In Example 16, the heating temperature before hot rolling was relatively low at 1085°C. Therefore, it is believed that the lack of ductility during hot rolling caused fine cracks to occur. As a result, the fatigue properties were evaluated as "Fair."

[0058] In Example 17, the heating time before hot rolling was relatively short at 1.8 hours. Therefore, the soaking time for the slab was insufficient, and hot rolling was performed while the center of the slab was kept at a low temperature. This is thought to have caused fine cracks to form inside the slab due to uneven ductility in the thickness direction. As a result, the fatigue properties were evaluated as "Fair."

[0059] For Example 18, the heating time before hot rolling was relatively long at 8.2 hours. This resulted in the formation of a thick layer of scale on the slab surface. Although the scale could be removed by water spraying during hot rolling or by pickling during the cold rolling process, it is believed that this left thread-like internal oxide layers along the grain boundaries, which became the initiation points for fracture during fatigue testing. As a result, the fatigue properties were evaluated as "Fair."

[0060] On the other hand, the fatigue properties of all of Comparative Examples 1 to 8 were evaluated as "unacceptable." Each example will be described below.

[0061] In Comparative Example 1, although the individual contents of P, S, Sn, and O were not particularly high, the overall contents were high, resulting in a large PSSnO value of 359. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.02. This is thought to have caused frequent cracking at the ferrite / austenite phase interface during hot rolling. As a result, the fatigue properties were evaluated as "unacceptable."

[0062] In Comparative Example 2, although the individual contents of P, S, Sn, and O were not particularly low, the overall content was low, resulting in a small PSSnO value of 57. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.06. This is thought to have caused frequent cracking at the ferrite / austenite phase interface during hot rolling. As a result, the fatigue properties were evaluated as "unacceptable."

[0063] In Comparative Examples 3 and 4, the contents of two of the individual elements P, S, Sn, and O were particularly low, resulting in a small PSSnO value of 61 in both cases, and the circularity of the ferrite phase particles in the slabs before hot rolling was also small at 0.07 in both cases. This is thought to have caused frequent cracking at the ferrite / austenite phase interface during hot rolling. As a result, the fatigue properties were evaluated as "unacceptable."

[0064] In Comparative Examples 5 and 6, the P, S, Sn, and O contents were particularly high, with two and one of them being particularly high, resulting in large PSSnO values ​​of 347 and 343, respectively. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.03 and 0.04, respectively. This is thought to have resulted in frequent cracking at the ferrite / austenite phase interface during hot rolling. As a result, the fatigue properties were evaluated as "unacceptable."

[0065] In Comparative Example 7, the P, S, and Sn contents were relatively high, and the O content was particularly high, resulting in a large PSSnO value of 343. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.04. It is believed that this caused frequent cracking at the ferrite / austenite phase interface during hot rolling. In particular, the high O content is believed to have caused the inclusion of numerous nonmetallic inclusions that could serve as crack initiation sites during fatigue testing. As a result, the fatigue properties were evaluated as "unacceptable."

[0066] In Comparative Example 8, the individual contents of P, S, Sn, and O were all high, resulting in a large PSSnO value of 378. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.01. This is thought to have caused frequent cracking at the ferrite / austenite phase interface during hot rolling. As a result, the fatigue properties were evaluated as "unacceptable."

[0067] As described above, by using the steel strips made of the steels of Examples 1 to 18, it is possible to obtain various products using Al-containing precipitation hardening stainless steel having high fatigue strength.

[0068] The ranges of the composition of the steels of the present invention, including Examples 1 to 18, are determined as follows.

[0069] C is necessary to ensure the required mechanical strength. However, excessive C content causes precipitation of carbides such as Cr carbide, which leads to embrittlement. Taking these factors into consideration, the C content is set within the range of 0.010 to 0.200%, preferably 0.050 to 0.150%, and more preferably 0.072 to 0.120%, by mass.

[0070] Si is an effective element for deoxidation and is necessary to obtain a sufficient deoxidation effect. However, excessive Si content can form a σ phase, which reduces corrosion resistance and can cause embrittlement. In consideration of these factors, the Si content is set within the range of 0.01 to 0.90%, preferably 0.10 to 0.80%, and more preferably 0.20 to 0.48%, by mass.

[0071] Mn is an element that is useful for deoxidation and is necessary. However, excessive Mn content reduces corrosion resistance. Taking these factors into consideration, the Mn content is set within the range of 0.30 to 1.50%, preferably 0.50 to 1.30%, and more preferably 0.70 to 1.10%, by mass.

[0072] P is an important element because it has the effect of lowering the melting point of steel and suppresses the network-like connections of the ferrite phase by forming a residual liquid phase during solidification, making the ferrite phase particles uniform and spherical. However, if it is contained in excess, it segregates at grain boundaries, deteriorating hot workability and causing cracks during hot rolling. Taking these factors into consideration, the P content is set within the range of 0.010 to 0.050%, preferably 0.012 to 0.040%, and more preferably 0.015 to 0.038%, by mass.

[0073] S is an interface-activating element and is one of the important elements that acts on the ferrite / austenite phase boundary to spheroidize ferrite phase particles. However, excessive S content causes segregation at grain boundaries, deteriorating hot workability and causing cracks during hot rolling. In consideration of these factors, the S content is set within the range of 0.0001 to 0.0050%, preferably 0.0002 to 0.0040%, and more preferably 0.0003 to 0.0030%, by mass.

[0074] Ni is necessary to ensure the toughness of the resulting duplex stainless steel sheet. However, excessive Ni content increases the material cost. Taking these factors into consideration, the Ni content is set within the range of 6.0 to 8.0%, preferably 6.2 to 7.8%, and more preferably 6.5 to 7.5%, by mass.

[0075] Cr forms a dense passive film on the surface of duplex stainless steel sheet and is essential for maintaining corrosion resistance. However, excessive Cr content forms carbides, reducing corrosion resistance. Taking these factors into consideration, the Cr content is 15.0 to 18.0%, preferably 15.5 to 17.5%, and more preferably 16.0 to 17.0%, by mass.

[0076] Mo has the effect of improving corrosion resistance and is therefore necessary. However, excessive Mo content increases the cost of the material. Taking these factors into consideration, the Mo content is set within the range of 0.10 to 0.50%, preferably 0.15 to 0.40%, and more preferably 0.18 to 0.35%, by mass.

[0077] Cu is necessary to ensure toughness. However, excessive Cu content will form carbides, adversely affecting corrosion resistance. Taking these factors into consideration, the Cu content is set within the range of 0.05 to 0.50%, preferably 0.08 to 0.48%, and more preferably 0.10 to 0.45%, by mass.

[0078] Al is an important element because it is necessary to impart high mechanical strength by precipitating an intermetallic compound with Ni during heat treatment. However, excessive Al content can lead to reduced toughness and cracking of welded joints. In consideration of these factors, the Al content is set within the range of 0.90 to 1.50%, preferably 0.95 to 1.20%, and more preferably 0.98 to 1.10%, by mass.

[0079] Ti has the effect of precipitating an intermetallic compound with Ni, thereby imparting high mechanical strength. However, excessive Ti content can form carbides, causing embrittlement. In consideration of these factors, the Ti content is set within the range of 0.001 to 0.050%, preferably 0.002 to 0.040%, and more preferably 0.003 to 0.009%, by mass.

[0080] W has the effect of improving mechanical strength. However, excessive W content forms carbides and a δ phase, which causes embrittlement and reduces corrosion resistance. In consideration of these factors, the W content is set within the range of 0.001 to 0.040%, preferably 0.005 to 0.030%, and more preferably 0.010 to 0.025%, by mass.

[0081] Nb has the effect of precipitating an intermetallic compound with Ni, thereby improving mechanical strength. However, excessive Nb content can form carbides, causing embrittlement. In consideration of these factors, the Nb content is set within the range of 0.001 to 0.030%, preferably 0.002 to 0.025%, and more preferably 0.003 to 0.020%, by mass.

[0082] Sn is an interface-activating element that acts on the ferrite / austenite phase boundary to spheroidize ferrite phase particles, making it an important element that must be precisely controlled, even in trace amounts. However, excessive Sn content causes segregation at grain boundaries, degrading hot workability and causing cracks during hot rolling. Taking these factors into consideration, the Sn content is set within the range of 0.0003 to 0.0200%, preferably 0.0005 to 0.0160%, and more preferably 0.0010 to 0.0100%, by mass.

[0083] N is necessary to ensure the required mechanical strength. However, excessive N content forms nitrides with Al and precipitates intermetallic compounds with Ni, reducing the Al content of the matrix and lowering mechanical strength. Furthermore, N forms Cr nitrides, which adversely affect corrosion resistance. Taking these factors into consideration, the N content is set within the range of 0.001 to 0.040%, preferably 0.003 to 0.035%, and more preferably 0.005 to 0.030%, by mass.

[0084] O is an interface-activating element and is one of the important elements that acts on the ferrite / austenite phase boundary to spheroidize ferrite phase particles. However, excessive O content increases the number of nonmetallic inclusions that cause defects. As described above, controlling the O concentration in molten steel also controls desulfurization during the refining process, which allows for precise control of the S concentration. Taking these factors into consideration, O is limited to a range of 0.0001 to 0.0050%, preferably 0.0003 to 0.0030%, and more preferably 0.0005 to 0.0020%, by mass.

[0085] The PSSnO value is an index for sufficient spheroidization of ferrite phase particles. Although the mechanism for spheroidization of ferrite phase particles remains largely unknown, the interfacially activating elements S, Sn, and O promote spheroidization of ferrite phase particles by acting on the ferrite / austenite phase interface. Furthermore, P, which lowers the melting point, is thought to contribute to spheroidization of ferrite phase particles by forming a residual liquid phase during solidification, thereby suppressing the formation of a network due to the bonding of ferrite particles. On the other hand, excessive S, Sn, and O content excessively reduces the energy of the ferrite / austenite phase interface, which in turn leads to coarsening of ferrite phase particles. Furthermore, excessive P content is thought to result in an excessive amount of residual liquid phase during solidification, leading to a heterogeneous structure. Taking these factors into consideration, the PSSnO value is set to a range of greater than 65 and less than 340, preferably greater than 90 and less than 230, and more preferably greater than 120 and less than 190.

[0086] δ cal The value of δ is an index of the volume fraction of the ferrite phase in Al-containing precipitation hardening stainless steel. cal If the value of δ is small, the volume fraction of the ferrite phase is reduced, which increases the segregation of segregating elements such as P and S at the grain boundaries during solidification, making cracks more likely to occur during hot rolling. cal A large value of increases the volume fraction of the ferrite phase, which increases the area ratio of the ferrite / austenite phase interface, which is the starting point of cracks, and increases the number of inherent microcracks. calThe value of is preferably in the range of 5 to 30, more preferably in the range of 7 to 20.

[0087] While typical embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited to these, and a person skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims.

Claims

1. An Al-containing precipitation hardening stainless steel strip, In mass%, Cr: 15.0-18.0%, Ni: 6.0 to 8.0%, Al: 0.90-1.50%, Mo: 0.10-0.50%, C: 0.010-0.200%, Si: 0.01-0.90%, Mn: 0.30 to 1.50%, P: 0.010-0.050%, S: 0.0001-0.0050%, Cu: 0.05-0.50%, Ti: 0.001 to 0.050%, W: 0.001-0.040%, Nb: 0.001-0.030%, Sn: 0.0003 to 0.0200%, N: 0.001-0.040%, O: 0.0001 to 0.0050%, and the balance being Fe and unavoidable impurities, The component composition is as follows, where the mass % of element M is [M]: 65<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<340 Fulfilling An Al-containing precipitation hardening stainless steel strip characterized by having a rolled two-phase structure in which ferrite phase particles are dispersed in a matrix of an austenite phase.

2. The number of cracks with a width of 3 μm or more in optical observation of the cross section perpendicular to the hot rolling direction is 100 mm ２ 2. The Al-containing precipitation hardening stainless steel strip according to claim 1, wherein the number of particles is 18 or less.

3. In the above-mentioned composition, when the mass % of the element M is [M], δ ｃａｌ 4.3 × (1.3 × 31)+31)+71)+2 0.2×21++080+0.3×130+1.5× 3.9×(30×30×30×18)+30 i]+0.8×⁻⭭nk+0.3×⁄3⁄4)-31.5 δ defined as ｃａｌ The Al-containing precipitation hardening stainless steel strip according to claim 1 or 2, characterized in that the value of β-alloy is in the range of 5 to 30.

4. An Al-containing precipitation hardening stainless steel that is hot rolled to provide a strip, In mass%, Cr: 15.0-18.0%, Ni: 6.0 to 8.0%, Al: 0.90-1.50%, Mo: 0.10-0.50%, C: 0.010-0.200%, Si: 0.01-0.90%, Mn: 0.30 to 1.50%, P: 0.010-0.050%, S: 0.0001-0.0050%, Cu: 0.05-0.50%, Ti: 0.001 to 0.050%, W: 0.001-0.040%, Nb: 0.001-0.030%, Sn: 0.0003 to 0.0200%, N: 0.001-0.040%, O: 0.0001 to 0.0050%, and the balance being Fe and unavoidable impurities, The component composition is as follows, where the mass % of element M is [M]: 65<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<340 Fulfilling It has a two-phase structure in which ferrite phase particles are dispersed in a matrix made of austenite phase, In the shape of the ferrite phase grains in a longitudinal cross section, S １ : Area of ​​ferrite phase particles (mm ２ ), L: perimeter length of ferrite phase particle (mm), (circularity) = 4π × S １ / L ２ The aluminum-containing precipitation hardening stainless steel has a simple average circularity defined by the formula (1) of 0.10 or more.

5. In the above-mentioned composition, when the mass % of the element M is [M], δ ｃａｌ 4.3 × (1.3 × 31)+31)+71)+2 0.2×21++080+0.3×130+1.5× 3.9×(30×30×30×18)+30 i]+0.8×⁻⭭nk+0.3×⁄3⁄4)-31.5 δ defined as ｃａｌ 5. The Al-containing precipitation hardening stainless steel according to claim 4, wherein the value of Cr is in the range of 5 to 30.

6. A method for producing an Al-containing precipitation hardening stainless steel strip, comprising: In mass%, Cr: 15.0-18.0%, Ni: 6.0 to 8.0%, Al: 0.90-1.50%, Mo: 0.10-0.50%, C: 0.010-0.200%, Si: 0.01-0.90%, Mn: 0.30 to 1.50%, P: 0.010-0.050%, S: 0.0001-0.0050%, Cu: 0.05-0.50%, Ti: 0.001 to 0.050%, W: 0.001-0.040%, Nb: 0.001-0.030%, Sn: 0.0003 to 0.0200%, N: 0.001-0.040%, O: 0.0001 to 0.0050%, and the balance being Fe and unavoidable impurities, The component composition is as follows, where the mass % of element M is [M]: 65<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<340 Fulfilling It has a two-phase structure in which ferrite phase particles are dispersed in a matrix made of austenite phase, In the shape of the ferrite phase grains in a longitudinal cross section, S １ : Area of ​​ferrite phase particles (mm ２ ), L: perimeter length of ferrite phase particle (mm), (circularity) = 4π × S １ A method for producing an Al-containing precipitation hardening stainless steel strip, characterized by heating a slab made of steel having a two-phase structure in which ferrite phase particles are dispersed in a substrate made of an austenite phase, with a simple average circularity defined as σ / L2 of 0.10 or more, to a temperature between 1100°C and 1270°C, and rolling it.

7. After hot rolling, the number of cracks with a width of 3 μm or more in optical observation of the cross section perpendicular to the hot rolling direction is 100 mm ２ 7. The method for producing an Al-containing precipitation hardening stainless steel strip according to claim 6, wherein the number of particles is 18 or less.

8. In the above-mentioned composition, when the mass % of the element M is [M], δ ｃａｌ 4.3 × (1.3 × 31)+31)+71)+2 0.2×21++080+0.3×130+1.5× 3.9×(30×30×30×18)+30 i]+0.8×⁻⭭nk+0.3×⁄3⁄4)-31.5 δ defined as ｃａｌ 8. The method for producing an Al-containing precipitation hardening stainless steel strip according to claim 6, wherein the range of is 5 to 30.

Images