WO2013085005A1 - Hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance and manufacturing process therefor - Google Patents

Hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance and manufacturing process therefor Download PDF

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WO2013085005A1
WO2013085005A1 PCT/JP2012/081693 JP2012081693W WO2013085005A1 WO 2013085005 A1 WO2013085005 A1 WO 2013085005A1 JP 2012081693 W JP2012081693 W JP 2012081693W WO 2013085005 A1 WO2013085005 A1 WO 2013085005A1
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hot
steel sheet
less
rolled steel
ferritic stainless
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PCT/JP2012/081693
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French (fr)
Japanese (ja)
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木村　謙
濱田　純一
高橋　淳
祐司 小山
後藤　茂之
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新日鐵住金ステンレス株式会社
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Application filed by 新日鐵住金ステンレス株式会社 filed Critical 新日鐵住金ステンレス株式会社
Priority to JP2013548300A priority Critical patent/JP5866378B2/en
Priority to KR1020147007664A priority patent/KR20140064907A/en
Priority to US14/355,117 priority patent/US20140294660A1/en
Priority to CN201280046386.9A priority patent/CN103857812B/en
Publication of WO2013085005A1 publication Critical patent/WO2013085005A1/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02—Rolling special iron alloys, e.g. stainless steel
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties and a method for producing the same.
  • Ferritic stainless steel is used in a wide range of applications such as home appliances, building materials, and automotive parts.
  • various elements are added to the steel in appropriate amounts according to necessary properties such as corrosion resistance and high temperature properties.
  • corrosion resistance it is known that adding Cr, Mo or Ni is effective.
  • addition of Nb, Al, Si or the like is effective.
  • Cold cracking refers to unrolling a hot-rolled coil (coiled hot-rolled sheet) and then passing the hot-rolled sheet through a continuous pickling line, continuous annealing pickling line, cold rolling line, etc. It is thought that it occurs because the toughness of the hot-rolled sheet is insufficient. In steel types containing many additive elements of ferritic stainless steel, it tends to occur in winter when the temperature is low.
  • Patent Documents 1 and 2 are known as solving means.
  • Patent Document 1 as a technique for improving the toughness value of a hot-rolled sheet made of a steel type to which Cr is added in an amount of 25 to 35% by weight, winding is performed at 400 to 600 ° C. immediately after finishing hot rolling, and immediately A technique for quenching at a cooling rate higher than water cooling is disclosed.
  • Patent Document 2 discloses a method in which a steel sheet is wound at a winding temperature of 550 to 650 ° C. to form a coiled steel strip, and the coiled steel strip is immersed in a water tank within 3 hours after winding.
  • Patent Document 1 As a technique for improving the toughness of the hot-rolled sheet as described above, the techniques of Patent Document 1 and Patent Document 2 are disclosed.
  • the present inventors applied the above-mentioned conventional knowledge to various ferritic stainless steels, it was found that cold cracking may occur and is not necessarily effective in improving toughness. That is, the prior art is not sufficiently effective, and further improvement is required.
  • the present invention has been made in view of the above circumstances, and an object thereof is to provide a ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties and a method for producing the same.
  • the present inventors investigated the relationship between the winding condition of the ferritic stainless steel hot-rolled sheet and the toughness of the hot-rolled sheet.
  • a ferritic stainless steel having different components was hot-rolled to a thickness of 5 mm in a laboratory to obtain a hot-rolled steel sheet.
  • a hot-rolled steel sheet was inserted into a furnace in which the temperature in the furnace was controlled to the coiling temperature, and the coiling process was simulated.
  • the winding temperature temperature in the furnace
  • the time for the winding process was changed in the range of 0.1 h to 100 h.
  • it cooled to room temperature by water cooling, and produced the hot-rolled steel plate.
  • a Charpy test was performed on the obtained hot-rolled steel sheet, and an impact value (toughness) at room temperature (25 ° C.) was evaluated.
  • the metal structure of the hot-rolled steel sheet manufactured under the various conditions described above was examined using an optical microscope and EBSP (Electron Backscattering Analysis Image Method). In the optical microscope, the recrystallization state of the steel sheet was investigated. Furthermore, the presence or absence of subgrain boundaries (subgrain boundaries) in the crystal grains was investigated using EBSP.
  • the measurement in EBSP was performed by the method described in the embodiment described later. Specifically, a measurement sample was collected so as to have a cross section (L cross section) parallel to the rolling direction and perpendicular to the plate surface direction. The L section of the measurement sample was subjected to electrolytic polishing or polishing with colloidal silica. In the L cross section, the measurement range was from 1/4 t to 3/4 t (1/4 to 3/4 of the plate thickness) of the plate thickness t. In this measurement range, the crystal orientation was measured in a measurement step (pitch) of 0.2 ⁇ m in a range of 100 ⁇ m ⁇ 100 ⁇ m. Judgment of the crystal grain boundaries and subgrain boundaries was performed as follows.
  • Charpy impact value of the resulting hot rolled steel sheets were greatly changed in the range of 5 J / cm 2 to about 100 J / cm 2 by the production conditions.
  • FIG. 1 shows the relationship between the toughness value (Charpy impact value) and La / L when the winding conditions (temperature and time) are changed in various ferritic stainless steels. From FIG. 1, when La / L is 0.20 or more, the Charpy impact value is as high as 20 J / cm 2 or more, and when La / L is less than 0.20, it is less than 20 J / cm 2 .
  • a crystal grain boundary indicates an orientation difference between adjacent crystal grains.
  • almost all of the crystal grains on both sides of the crystal grain boundary have an orientation difference of 15 ° or more. That is, in the complete recrystallized structure, there is almost no crystal grain boundary in the range of orientation difference of 1 ° to less than 15 °, so La / L is close to zero.
  • the coiling temperature was 900 ° C.
  • a complete recrystallized structure was obtained for any steel type, and the Charpy impact values were all less than 20 J / cm 2 .
  • the metal structure when the winding temperature is 800 ° C. or lower and the Charpy impact value is 20 J / cm 2 or more appears to have many unrecrystallized grains in the optical microscope structure. As a result of analysis, there were many subgrain boundaries.
  • the present invention has been obtained based on these findings, and the gist of one embodiment of the present invention is as follows.
  • C 0.0150% or less
  • Si 0.01% to 2.00%
  • Mn 0.01% to 2.00%
  • P less than 0.040%
  • S 0 0.010% or less
  • Cr 10.0% to 30.0%
  • Al 0.001% to 3.00%
  • N 0.0200% or less
  • Nb 0.05% to 0.70% or less
  • Ti 0.05% to 0.30% or less
  • Mo 0.1% to 2.5%
  • Ni 0 1% to 1.5%
  • B 0.0001% to 0.0025%
  • Cu 0.1% to 2.0%
  • Sn 0.03% to 0.35%
  • the ferrite having excellent cold cracking properties according to the above (1) which contains at least one species and includes one or both of Nb and Ti so as to satisfy the following formula (1):
  • the element symbol in Formula (1) means content in unit of the mass% of the said element.
  • cold cracking of a hot-rolled steel sheet can be achieved by increasing the proportion of subgrain boundaries that affect the toughness of a ferritic stainless steel hot-rolled steel sheet containing various elements. Can be prevented.
  • cold cracking does not occur even if continuous annealing or pickling is performed after hot rolling.
  • an increase in production yield and an improvement in production efficiency can be achieved by suppressing cold cracking of various ferritic stainless steel hot-rolled steel sheets. As a result, an industrially very useful effect can be exhibited in terms of reduction in manufacturing cost. In addition, energy consumption can be reduced by improving production efficiency, contributing to global environmental conservation.
  • the ferritic stainless steel hot-rolled steel sheet of this embodiment is, in mass%, C: 0.0150% or less, Si: 0.01% to 2.00%, Mn: 0.01% to 2.00%, P: Less than 0.040%, S: 0.010% or less, Cr: 10.0% to 30.0%, Al: 0.001% to 3.00%, and N: 0.0200% or less, respectively
  • the balance L has a steel composition composed of Fe and inevitable impurities, and the length L of all grain boundaries having a misorientation of 1 ° or more and less than 180 ° in the cross section at 1/4 to 3/4 of the plate thickness, The subgrain boundary length La having a difference of 1 ° or more and less than 15 ° satisfies La / L ⁇ 0.20.
  • the C content is preferably 0.0020 to 0.0070%.
  • Si 0.01 to 2.00% Si is an element that improves oxidation resistance. However, if a large amount of Si is added, the workability of the product deteriorates, so the upper limit of the Si amount is 2.00%. On the other hand, since Si is inevitably mixed as a deoxidizer, the lower limit of the Si amount is set to 0.01%. The Si amount is preferably 0.02% to 0.97%.
  • Mn 0.01 to 2.00%
  • Mn is an element that improves the high-temperature strength and oxidation resistance.
  • the addition of a large amount of Mn causes deterioration of the workability of the product as with Si.
  • the upper limit of the amount of Mn is made 2.00%.
  • the lower limit of the amount of Mn is set to 0.01%.
  • the Mn content is preferably 0.02% to 1.95%.
  • P Less than 0.040% P is inevitably mixed from Cr raw materials and the like, so 0.005% or more of P is often mixed, but P decreases ductility and manufacturability. For this reason, the amount of P is preferably as small as possible. However, it is very difficult to dephosphorize excessively, and the manufacturing cost also increases, so the amount of P is made less than 0.04%.
  • the amount of S is preferably small, and the amount of S is 0.010% or less. Further, from the viewpoint of corrosion resistance, the S content is preferably low, and the S content is preferably less than 0.0050%. Since desulfurization technology has been developed in recent years, the lower limit of the amount of S is more preferably 0.0001%. Considering stable manufacturability, the lower limit of the amount of S is more preferably 0.0005%.
  • Cr 10.0-30.0%
  • Cr is a basic element necessary for ensuring corrosion resistance, high-temperature strength, and oxidation resistance, and in order to exert its effects, addition of 10.0% or more of Cr is essential.
  • the addition of a large amount of Cr causes toughness deterioration, so the upper limit of Cr content is made 30.0%.
  • the Cr content is 20.0% or less.
  • Al 0.001 to 3.00% Since Al is used as a deoxidizing element, an appropriate amount of Al is added. Addition of less than 0.001% Al results in insufficient deoxidation capacity, so 0.001% is made the lower limit. On the other hand, with 0.100% Al, the amount of oxygen can be sufficiently reduced, and the deoxidizing ability is almost saturated even when the amount exceeds this. For this reason, when adding Al only for the purpose of deoxidation, the upper limit of Al amount may be 0.100%. In this case, the amount of Al is preferably 0.002% to 0.095%. Al also has the effect of improving high-temperature strength and corrosion resistance.
  • the amount of Al is preferably more than 0.10% to 3.00%, more preferably 0.50% to 2.00%. If a large amount of Al is added, the workability of the product is deteriorated, so the upper limit of the Al amount is Al: 3.00%. The upper limit of the amount of Al is preferably 2.00% or less.
  • N 0.0200% or less
  • the upper limit of the N amount is set to 0.0200%.
  • reducing the amount of N brings about an increase in manufacturing cost such as an increase in refining time.
  • the lower limit of the N amount be 0.0030%.
  • the N content is preferably 0.0050 to 0.0120%.
  • Nb 0.05 to 0.70%
  • Ti 0.05 to 0.30%
  • the element symbol in Formula (1) means content of the said element in the unit of mass%.
  • Nb and Ti have a function of forming precipitates with C and N and reducing solid solution C and N.
  • the high temperature strength and thermal fatigue characteristics of the member are improved by solid solution strengthening at high temperatures.
  • Nb in order to fix C and N, it is necessary to contain 0.05% or more, and it is preferable to contain 0.10% or more.
  • Ti when Ti is contained, it is necessary to contain 0.05% or more in order to fix C and N.
  • the upper limit is Ti: 0.30%. Further, when Nb is added in a large amount, the workability of the product deteriorates. For this reason, the upper limit is more preferably Nb: 0.70% and 0.55% or less.
  • Mo 0.1 to 2.5%
  • Ni 0.1 to 1.5%
  • B 0.0001 to 0.0025%
  • Cu 0.0.
  • Mo, Ni, Cu, and Sn are elements that improve high-temperature strength and corrosion resistance, and may be added as necessary. Ni also has the effect of improving toughness.
  • the increase in the high-temperature strength becomes remarkable because Mo: 0.1% or more, Ni: 0.1% or more, Cu: 0.1% or more, Sn: 0.03% or more, respectively. And In order to further improve the high-temperature strength and corrosion resistance, it is more preferable to set Mo: 0.3% or more, Ni: 0.25% or more, Cu: 0.4% or more, and Sn: 0.10% or more. Addition of a large amount of Mo, Ni, and Cu leads to deterioration of pickling properties and leads to a decrease in productivity. Therefore, the upper limit is set to Mo: 2.5%, Ni: 1.5%, Cu: 2.0%, respectively. And Mo: 2.2% or less, Ni: 1.2% or less, and Cu: 1.4% or less are more preferable. Addition of a large amount of Sn causes toughness deterioration and generation of surface flaws, so the upper limit is made Sn: 0.35%, more preferably 0.20% or less.
  • B is an element that improves secondary workability. When used for applications where secondary workability is required, it may be added as necessary. The effect of improving the secondary workability is manifested when the addition amount of B is 0.0001% or more, and this is the lower limit, and more preferably 0.0003% or more. Moreover, since addition of a large amount of B may reduce the toughness and workability of the hot-rolled sheet, the upper limit of the B amount is set to 0.0025%, and more preferably 0.0015% or less.
  • a ratio of subgrain boundary length La of less than 0 ° satisfies La / L ⁇ 0.20.
  • the ratios of the total grain boundary length L and the subgrain boundary length La are measured by the following method. First, measurement samples are collected from arbitrary 10 locations of the hot-rolled steel sheet. This collection location is not particularly limited.
  • Electrolytic polishing or polishing with colloidal silica is applied to the L cross section of the measurement sample.
  • the measurement range is set to the vicinity of the center of the plate thickness t in the L cross section, that is, a range from 1/4 t to 3/4 t.
  • the grain boundary length is measured using EBSP by the following method.
  • the crystal orientation is measured in a measurement step (pitch) of 0.2 ⁇ m in the measurement range of 100 ⁇ m ⁇ 100 ⁇ m.
  • An interface having an azimuth difference of 1 ° or more and less than 180 ° at adjacent measurement points is regarded as a grain boundary.
  • a grain boundary having an orientation difference of 1 ° or more and less than 15 ° is defined as a sub-grain boundary.
  • the total length of all grain boundaries is calculated as “total grain boundary length L”, and the total length of subgrain boundaries is calculated as “subgrain boundary length La”.
  • the ratio La / L is obtained.
  • the ratio La / L is similarly obtained for 10 measurement samples, and the average value of the 10 La / L values is calculated.
  • La / L When La / L is less than 0.20, the toughness of the hot-rolled steel sheet is as low as less than 20 J / cm 2, so La / L needs to be 0.20 or more. As shown in FIG. 1 described above, the higher the ratio of subgrain grain boundaries (subgrain boundaries), the higher the toughness of the hot-rolled steel sheet. For this reason, it is preferable that La / L is 0.35 or more.
  • the manufacturing method of the ferritic stainless steel hot-rolled steel sheet in this embodiment has the following processes.
  • Ferritic stainless steel having the above composition is cast into a steel piece.
  • ferritic stainless steel having the above steel composition is cast into a steel slab, and this steel slab is hot-rolled to obtain a hot-rolled steel sheet.
  • the hot-rolled steel sheet that has been subjected to hot rolling is cooled to a winding temperature by water cooling, and wound into a coil at the winding temperature.
  • the hot rolling finishing temperature is set to 800 ° C. to 1000 ° C.
  • the winding temperature is set to more than 650 ° C. to 800 ° C.
  • the finishing temperature is less than 800 ° C. or more than 1000 ° C., it becomes very difficult to generate a grain boundary having an orientation difference of 1 ° to less than 15 ° after winding. For this reason, 800 degreeC and 1000 degreeC are made into a minimum and an upper limit, respectively.
  • an austenite phase it is preferable not to generate an austenite phase during hot rolling. Whether or not an austenite phase is generated during hot rolling is determined by the amount of austenite-forming elements in the steel, particularly the amount of C and N having a large austenite-forming ability. In the hot-rolled steel sheet of the present embodiment, the amounts of C and N are both small, and no austenite phase is generated during hot rolling. Even when the coiling temperature is 650 ° C. or less, it is difficult to generate a crystal grain boundary having an orientation difference of 1 ° to less than 15 °. When the winding temperature is higher than 800 ° C., recrystallization progresses at the time of winding, and the ratio of crystal grain boundaries having an orientation difference of 15 ° to less than 180 ° increases, so that the toughness deteriorates.
  • the hot rolled steel sheet wound in a coil shape is immersed in a water tank. This is to suppress the formation of precipitates that degrade toughness in the slow cooling step after winding.
  • the process of forming and coarsening the precipitates described above strongly depends on the temperature and time of the steel sheet after winding. .
  • the time from hot rolling to reaching the winding temperature is within 1 minute, and the cooling rate during this time Is 3 ° C./sec or more. In the case of such a cooling rate condition, precipitates that affect toughness are not generated between the end of finish rolling and the start of winding.
  • the time that is maintained at the above-described winding temperature is an important factor.
  • the time for holding the hot-rolled steel sheet in the water tank after being immersed in the water tank is also an important item.
  • the immersion time for holding the hot-rolled steel sheet in the water tank is preferably 1 hour or longer. When the immersion time of the hot-rolled steel sheet in the water tank is as short as less than 1 hour, cooling becomes insufficient, and precipitates that deteriorate toughness may be generated in the hot-rolled steel sheet due to subsequent reheating or the like.
  • ferritic stainless steel hot-rolled steel sheet according to the present embodiment described above it is possible to control the metal structure that affects the toughness of the hot-rolled steel sheet, as a result of the requirements related to the above components and grain boundaries. It is possible to prevent cold cracking of the hot-rolled steel sheet. Moreover, according to the ferritic stainless steel hot-rolled steel sheet according to the present embodiment, cold cracking does not occur even after continuous annealing or hot pickling after hot rolling.
  • ferritic stainless steel hot-rolled steel sheet according to the present embodiment since cold cracking can be suppressed, it is possible to increase the production yield and improve the production efficiency. As a result, an industrially very useful effect can be exhibited in terms of reduction in manufacturing cost. In addition, by improving production efficiency, energy used in the manufacturing process can be suppressed, which can contribute to global environmental conservation.
  • each steel having the composition shown in Table 1 was melted and cast to obtain a steel ingot (steel piece).
  • the steel ingot was ground to 90 mm thickness.
  • Hot rolling was performed at a finishing temperature (FT) shown in Tables 2 and 3, and the steel ingot was rolled to a plate thickness of 5 mm to obtain a hot-rolled steel plate.
  • FT finishing temperature
  • the steel sheet was cooled to the winding temperature (CT) shown in Tables 2 and 3 by water cooling. The cooling rate at this time was about 20 ° C./sec.
  • a hot-rolled steel sheet was inserted into the furnace in which the temperature in the furnace was controlled to the coiling temperature (CT) shown in Tables 2 and 3, and the winding process was simulated. Thereafter, the hot-rolled steel sheet was immersed in a water tank after the time (t) in Tables 2 and 3 had elapsed. Subsequently, it was kept in the water tank for the immersion time (tx) shown in Tables 2 and 3, and the hot-rolled steel sheet was taken out.
  • CT coiling temperature
  • each of the obtained hot-rolled steel sheets had a ferrite single phase structure. Further, in the same manner as the measurement method described in the embodiment, the grain boundary characteristics (ratio of subgrain boundary length La to total grain boundary length L La / L) were calculated using EBSP. Sub-size Charpy impact test pieces were collected from the hot-rolled steel sheet in accordance with JIS Z 2202, and an impact test was performed on a metal material in accordance with JIS Z 2242 with the vertical direction of the rolling direction as the impact direction. The shock absorption energy was investigated at a test temperature of 25 ° C.
  • the cold cracking property (toughness) of a hot-rolled steel sheet was evaluated by the following method.
  • the hot rolled steel sheet having a Charpy impact value of less than 20 J / cm 2 cold cracking and the like occurred in the subsequent annealing and pickling processes, and the yield decreased.
  • such a cold crack did not occur in a hot-rolled steel sheet having a Charpy impact value of 20 J / cm 2 or more.
  • the cold cracking property of a hot-rolled steel sheet having a Charpy impact value of less than 20 J / cm 2 is evaluated as “bad”, and the cold cracking property of a hot-rolled steel sheet having a Charpy impact value of 20 J / cm 2 or more is “good”. It was evaluated.
  • the Charpy impact value is underlined for values less than 20 J / cm 2 .
  • Tables 2 and 3 FT represents the hot rolling finishing temperature (° C.), and CT represents the coiling temperature (° C.) of the hot-rolled steel sheet.
  • t represents the time (h) from the completion of winding to the start of water cooling (immersion start), and tx represents the time (h) from the start of water cooling to completion (from immersion start to removal).
  • tx represents the time (h) from the start of water cooling to completion (from immersion start to removal).
  • the ferritic stainless hot rolled steel sheet of this embodiment has a Charpy impact value of 20 J / cm 2 or more and is excellent in cold cracking property. For this reason, even if a continuous annealing or a pickling process is performed after hot rolling, cold cracking does not occur. Therefore, the ferritic stainless steel hot-rolled steel sheet of the present embodiment can be suitably applied to manufacturing processes for members such as home appliances, building materials, and automobile parts in which ferritic stainless steel is used.

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Abstract

This hot-rolled ferritic stainless steel sheet has a composition containing, in mass%, up to 0.0150 % of C, 0.01 to 2.00 % of Si, 0.01 to 2.00% of Mn, less than 0.040% of P, up to 0.010% of S, 10.0 to 30.0% of Cr, 0.001 to 0.100% of Al and up to 0.0200% of N with the balance being Fe and unavoidable impurities, and satisfies the relationship: La/L ≥ 0.20 [wherein L is the length of all grain boundaries with misorientation angles of 1 to less than 180° as observed in a section in the range of 1/4 to 3/4 of the sheet thickness and La is the length of subgrain boundaries with misorientation angles of 1 to less than 15° as observed therein].

Description

冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板およびその製造方法Ferritic stainless steel hot-rolled steel sheet with excellent cold cracking property and method for producing the same
　本発明は、冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板およびその製造方法に関する。
　本願は、２０１１年１２月９日に、日本に出願された特願２０１１－２７００９２号に基づき優先権を主張し、その内容をここに援用する。 The present invention relates to a ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties and a method for producing the same.
This application claims priority based on Japanese Patent Application No. 2011-270092 filed in Japan on Dec. 9, 2011, the contents of which are incorporated herein by reference.
　フェライト系ステンレス鋼は、家電、建材、自動車部品等、幅広い用途に用いられている。従来から該鋼には、耐食性や高温特性等の必要特性に応じて種々の元素が適量に添加されている。
　耐食性向上を目的とする場合は、Ｃｒ，ＭｏやＮｉを添加することが有効であることが知られている。また高温特性（強度、耐酸化性）を向上させるためには、Ｎｂ，Ａｌ，Ｓｉ等の添加が有効である。 Ferritic stainless steel is used in a wide range of applications such as home appliances, building materials, and automotive parts. Conventionally, various elements are added to the steel in appropriate amounts according to necessary properties such as corrosion resistance and high temperature properties.
For the purpose of improving corrosion resistance, it is known that adding Cr, Mo or Ni is effective. In order to improve the high temperature characteristics (strength, oxidation resistance), addition of Nb, Al, Si or the like is effective.
　一般的にこれらの添加元素の添加量が多いほど、特性は向上するが、逆に製造性、特に冷間割れ性は低下する。このため、その添加量の上限が決められている。
　冷間割れとは、熱延板のコイル（コイル状に巻かれた熱延板）を巻き解き、次いで、熱延板を連続酸洗ライン、連続焼鈍酸洗ライン、冷間圧延ライン等に通した際に生じる割れを指し、熱延板の靱性が不足しているために生じると考えられている。
　フェライト系ステンレス鋼の多くの添加元素を含有する鋼種において、温度が低い冬季に生じやすい。 In general, the larger the amount of these additional elements, the better the characteristics, but conversely, the manufacturability, particularly the cold cracking property, decreases. For this reason, the upper limit of the addition amount is determined.
Cold cracking refers to unrolling a hot-rolled coil (coiled hot-rolled sheet) and then passing the hot-rolled sheet through a continuous pickling line, continuous annealing pickling line, cold rolling line, etc. It is thought that it occurs because the toughness of the hot-rolled sheet is insufficient.
In steel types containing many additive elements of ferritic stainless steel, it tends to occur in winter when the temperature is low.
　Ｃｒ量の多いフェライト系ステンレス鋼やＡｌが添加されたステンレス鋼からなる熱延板の靱性を向上させるために、解決手段としては特許文献１および特許文献２が公知である。
　特許文献１には、Ｃｒが２５～３５重量％添加された鋼種からなる熱延板の靱性値を向上させる技術として、仕上げ熱間圧延を終了してから、４００～６００℃で巻き取り、直ちに水冷以上の冷却速度で急冷する技術が開示されている。
　特許文献２には、鋼板を５５０～６５０℃の巻き取り温度で巻き取ってコイル状鋼帯とし、巻き取り後３時間以内にコイル状鋼帯を水槽に浸漬する方法が開示されている。 In order to improve the toughness of a hot rolled sheet made of ferritic stainless steel with a large amount of Cr or stainless steel to which Al is added, Patent Documents 1 and 2 are known as solving means.
In Patent Document 1, as a technique for improving the toughness value of a hot-rolled sheet made of a steel type to which Cr is added in an amount of 25 to 35% by weight, winding is performed at 400 to 600 ° C. immediately after finishing hot rolling, and immediately A technique for quenching at a cooling rate higher than water cooling is disclosed.
Patent Document 2 discloses a method in which a steel sheet is wound at a winding temperature of 550 to 650 ° C. to form a coiled steel strip, and the coiled steel strip is immersed in a water tank within 3 hours after winding.
　このように熱延板の靱性を改善する技術として、特許文献１乃び特許文献２の技術が開示されている。しかしながら、本願発明者らが上記従来の知見を、各種フェライト系ステンレス鋼に対して適用したところ、冷間割れが発生する場合があり、必ずしも靭性の改善に対して有効ではないことが分かった。即ち、従来技術は十分に有効ではなく、更なる改善が必要とされるものであった。 As a technique for improving the toughness of the hot-rolled sheet as described above, the techniques of Patent Document 1 and Patent Document 2 are disclosed. However, when the present inventors applied the above-mentioned conventional knowledge to various ferritic stainless steels, it was found that cold cracking may occur and is not necessarily effective in improving toughness. That is, the prior art is not sufficiently effective, and further improvement is required.
特開平５－３２０７６４号公報JP-A-5-320764 特開２００１－２６８２６号公報JP 2001-26826 A
　本発明は、上記の実情に鑑みなされたものであって、冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板及びその製造方法を提供することを目的とする。 The present invention has been made in view of the above circumstances, and an object thereof is to provide a ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties and a method for producing the same.
　本発明者らは、上記の課題を解決すべく、フェライト系ステンレス鋼熱延板の巻き取り条件と熱延板の靱性との関係を調査した。
　まず、成分を変化させたフェライト系ステンレス鋼を実験室で５ｍｍ厚まで熱間圧延して熱延鋼板を得た。次いで、炉内の温度が巻き取り温度に制御された炉の中に熱延鋼板を挿入し、巻き取り処理を模擬した。巻き取り温度（炉内の温度）を５５０℃～９５０℃の範囲で変化させ、かつ巻き取り処理（炉内での加熱）の時間を０．１ｈ～１００ｈの範囲で変化させた。その後に、水冷によって室温まで冷却して熱延鋼板を作製した。 In order to solve the above-mentioned problems, the present inventors investigated the relationship between the winding condition of the ferritic stainless steel hot-rolled sheet and the toughness of the hot-rolled sheet.
First, a ferritic stainless steel having different components was hot-rolled to a thickness of 5 mm in a laboratory to obtain a hot-rolled steel sheet. Next, a hot-rolled steel sheet was inserted into a furnace in which the temperature in the furnace was controlled to the coiling temperature, and the coiling process was simulated. The winding temperature (temperature in the furnace) was changed in the range of 550 ° C. to 950 ° C., and the time for the winding process (heating in the furnace) was changed in the range of 0.1 h to 100 h. Then, it cooled to room temperature by water cooling, and produced the hot-rolled steel plate.
　得られた熱延鋼板に対してシャルピー試験を実施し、室温（２５℃）における衝撃値（靱性）を評価した。
　また、上記種々の条件で製造した熱延鋼板の金属組織を光学顕微鏡並びにＥＢＳＰ（電子後方散乱解析像法）にて調査した。光学顕微鏡では、鋼板の再結晶状態を調査した。さらにＥＢＳＰを用いて、結晶粒内における亜粒界（サブグレイン粒界）の有無を調査した。 A Charpy test was performed on the obtained hot-rolled steel sheet, and an impact value (toughness) at room temperature (25 ° C.) was evaluated.
Moreover, the metal structure of the hot-rolled steel sheet manufactured under the various conditions described above was examined using an optical microscope and EBSP (Electron Backscattering Analysis Image Method). In the optical microscope, the recrystallization state of the steel sheet was investigated. Furthermore, the presence or absence of subgrain boundaries (subgrain boundaries) in the crystal grains was investigated using EBSP.
　ＥＢＳＰにおける測定は、後述する実施形態に記載の方法によって行った。詳細には、圧延方向に平行であり、かつ板面方向に垂直な断面（Ｌ断面）を有するように測定用サンプルを採取した。測定用サンプルのＬ断面に対して、電解研磨又はコロイダルシリカによる研磨を施した。Ｌ断面において、板厚ｔの１／４ｔから３／４ｔ（板厚の１／４～３／４）の範囲を測定範囲とした。この測定範囲のうち、１００μｍ×１００μｍの範囲において、０．２μｍの測定ステップ（ピッチ）で結晶方位を測定した。結晶粒界と亜粒界の判断は、以下のように行った。隣接する測定点での方位差が１°以上１８０°未満の界面を粒界とみなした。このうち方位差が１°以上１５°未満の粒界を亜粒界とした。得られた知見を下記に列挙する。 The measurement in EBSP was performed by the method described in the embodiment described later. Specifically, a measurement sample was collected so as to have a cross section (L cross section) parallel to the rolling direction and perpendicular to the plate surface direction. The L section of the measurement sample was subjected to electrolytic polishing or polishing with colloidal silica. In the L cross section, the measurement range was from 1/4 t to 3/4 t (1/4 to 3/4 of the plate thickness) of the plate thickness t. In this measurement range, the crystal orientation was measured in a measurement step (pitch) of 0.2 μm in a range of 100 μm × 100 μm. Judgment of the crystal grain boundaries and subgrain boundaries was performed as follows. An interface having an azimuth difference of 1 ° or more and less than 180 ° at adjacent measurement points was regarded as a grain boundary. Among these, grain boundaries having an orientation difference of 1 ° or more and less than 15 ° were defined as subgrain boundaries. The findings obtained are listed below.
（１）得られた熱延鋼板のシャルピー衝撃値は、製造条件によって５Ｊ／ｃｍ２から約１００Ｊ／ｃｍ２の範囲で大きく変化した。
（２）得られた熱延鋼板の金属組織を光学顕微鏡で観察したところ、未再結晶組織、完全再結晶組織、及び未再結晶と再結晶の混合組織の３種類の場合が認められた。完全再結晶組織の場合、熱延鋼板のシャルピー衝撃値は２０Ｊ／ｃｍ２未満であった。未再結晶粒の場合、及び未再結晶と再結晶の混合組織の場合、シャルピー衝撃値が２０Ｊ／ｃｍ２以上となる場合が認められた。 (1) Charpy impact value of the resulting hot rolled steel sheets were greatly changed in the range of 5 J / cm 2 to about 100 J / cm 2 by the production conditions.
(2) When the metal structure of the obtained hot-rolled steel sheet was observed with an optical microscope, three types of cases were observed: an unrecrystallized structure, a complete recrystallized structure, and a mixed structure of unrecrystallized and recrystallized. In the case of a complete recrystallized structure, the Charpy impact value of the hot-rolled steel sheet was less than 20 J / cm 2 . In the case of non-recrystallized grains, and in the case of a mixed structure of non-recrystallized and recrystallized, it was recognized that the Charpy impact value was 20 J / cm 2 or more.
（３）ＥＢＳＰによる結晶粒界の調査より、方位差１°以上１８０°未満の結晶粒界の長さの合計（全結晶粒界長さＬ）と、方位差１°以上１５°未満の亜粒界の長さの合計（亜粒界長さＬａ）を求めた。そして、比Ｌａ／Ｌと、シャルピー衝撃値との関係を求めた。
　図１は、種々のフェライト系ステンレス鋼において巻き取り条件（温度及び時間）を変化させたときの靱性値（シャルピー衝撃値）とＬａ／Ｌとの関係を示す。図１より、Ｌａ／Ｌが０．２０以上のときにシャルピー衝撃値は２０Ｊ／ｃｍ２以上と高く、Ｌａ／Ｌが０．２０未満のときに２０Ｊ／ｃｍ２未満となる。 (3) From the investigation of grain boundaries by EBSP, the sum of the lengths of crystal grain boundaries with an orientation difference of 1 ° or more and less than 180 ° (total grain boundary length L) and sub-phases with orientation differences of 1 ° or more and less than 15 ° The total grain boundary length (subgrain boundary length La) was determined. And the relationship between ratio La / L and the Charpy impact value was calculated | required.
FIG. 1 shows the relationship between the toughness value (Charpy impact value) and La / L when the winding conditions (temperature and time) are changed in various ferritic stainless steels. From FIG. 1, when La / L is 0.20 or more, the Charpy impact value is as high as 20 J / cm 2 or more, and when La / L is less than 0.20, it is less than 20 J / cm 2 .
　一般的に、結晶粒界は、隣接する結晶粒間の方位差を示している。完全再結晶組織の場合、結晶粒界を挟んだ両側の結晶粒は、ほぼ全てが１５°以上の方位差を有している。すなわち完全再結晶組織には、方位差１°から１５°未満の範囲の結晶粒界がほとんど存在しないため、Ｌａ／Ｌは０に近くなる。
　本試験においては、巻き取り温度が９００℃の場合においては、いずれの鋼種でも完全再結晶組織が得られ、シャルピー衝撃値はいずれも２０Ｊ／ｃｍ２未満であった。一方、巻き取り温度が８００℃以下であり、シャルピー衝撃値が２０Ｊ／ｃｍ２以上となる場合の金属組織は、いずれも光学顕微鏡組織では未再結晶粒が多く存在しているように見え、ＥＢＳＰでの解析により亜粒界が多く存在していた。 In general, a crystal grain boundary indicates an orientation difference between adjacent crystal grains. In the case of a complete recrystallized structure, almost all of the crystal grains on both sides of the crystal grain boundary have an orientation difference of 15 ° or more. That is, in the complete recrystallized structure, there is almost no crystal grain boundary in the range of orientation difference of 1 ° to less than 15 °, so La / L is close to zero.
In this test, when the coiling temperature was 900 ° C., a complete recrystallized structure was obtained for any steel type, and the Charpy impact values were all less than 20 J / cm 2 . On the other hand, the metal structure when the winding temperature is 800 ° C. or lower and the Charpy impact value is 20 J / cm 2 or more appears to have many unrecrystallized grains in the optical microscope structure. As a result of analysis, there were many subgrain boundaries.
　本発明は、これらの知見に基づいて得られたものであり、本発明の一態様の要旨は、以下の通りである。
（１）質量％で、Ｃ：０．０１５０％以下、Ｓｉ：０．０１％～２．００％、Ｍｎ：０．０１％～２．００％、Ｐ：０．０４０％未満、Ｓ：０．０１０％以下、Ｃｒ：１０．０％～３０．０％、Ａｌ：０．００１％～３．００％、及びＮ：０．０２００％以下をそれぞれ含有し、残部がＦｅおよび不可避的不純物からなる鋼組成を有し、板厚の１／４～３／４における断面において方位差1°以上１８０°未満の全結晶粒界の長さＬと、方位差１°以上１５°未満の亜粒界長さＬａがＬａ／Ｌ≧０．２０を満足する関係にあることを特徴とする冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板。 The present invention has been obtained based on these findings, and the gist of one embodiment of the present invention is as follows.
(1) By mass, C: 0.0150% or less, Si: 0.01% to 2.00%, Mn: 0.01% to 2.00%, P: less than 0.040%, S: 0 0.010% or less, Cr: 10.0% to 30.0%, Al: 0.001% to 3.00%, and N: 0.0200% or less, with the balance being Fe and inevitable impurities The length L of all grain boundaries with an orientation difference of 1 ° or more and less than 180 ° and subgrains with an orientation difference of 1 ° or more and less than 15 ° in the cross section at 1/4 to 3/4 of the plate thickness A ferritic stainless steel hot-rolled steel sheet having excellent cold cracking characteristics, wherein the field length La is in a relationship satisfying La / L ≧ 0.20.
（２）さらに、質量％で、Ｎｂ：０．０５％～０．７０％以下、Ｔｉ：０．０５％～０．３０％以下、Ｍｏ：０．１％～２．５％、Ｎｉ：０．１％～１．５％、Ｂ：０．０００１％～０．００２５％、Ｃｕ：０．１％～２．０％、及びＳｎ：０．０３％～０．３５％から選択される１種以上を含み、Ｎｂ、Ｔｉのいずれか一方または両方を含む場合は、下記式（１）を満足するように含むことを特徴とする前記（１）に記載の冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板。
　Ｎｂ／９３＋Ｔｉ／４８≧Ｃ／１２＋Ｎ／１４　・・・（１）
　但し、式（１）中の元素記号は、当該元素の質量％を単位とする含有量を意味する。
（３）Ａｌ含有量が０．１０％超～３．００％であることを特徴とする前記（１）又は（２）に記載の冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板。 (2) Further, by mass%, Nb: 0.05% to 0.70% or less, Ti: 0.05% to 0.30% or less, Mo: 0.1% to 2.5%, Ni: 0 1% to 1.5%, B: 0.0001% to 0.0025%, Cu: 0.1% to 2.0%, and Sn: 0.03% to 0.35% The ferrite having excellent cold cracking properties according to the above (1), which contains at least one species and includes one or both of Nb and Ti so as to satisfy the following formula (1): Stainless steel hot-rolled steel sheet.
Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (1)
However, the element symbol in Formula (1) means content in unit of the mass% of the said element.
(3) The ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties as described in (1) or (2) above, wherein the Al content is more than 0.10% to 3.00%.
（４）前記（１）乃至（３）の何れか一項に記載のフェライト系ステンレス鋼熱延鋼板を製造する方法であって、前記（１）乃至（３）の何れか一つに記載の鋼組成を有するフェライト系ステンレス鋼を鋳造して鋼片とし、前記鋼片に対して、仕上げ温度が８００℃～１０００℃の条件で熱間圧延を施すことにより熱延鋼板とする工程と、その後、６５０℃超～８００℃で前記熱延鋼板をコイル状に巻き取る工程と、コイル状に巻き取った前記熱延鋼板を、巻き取り後１時間以内に水槽に浸漬させ、水槽内で１時間以上保持し、次いで取り出す工程とを有することを特徴とする冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板の製造方法。 (4) A method for producing a ferritic stainless steel hot-rolled steel sheet according to any one of (1) to (3), wherein the method is any one of (1) to (3). Casting a ferritic stainless steel having a steel composition into a steel slab, and subjecting the steel slab to a hot rolled steel sheet by subjecting the steel slab to hot rolling at a finishing temperature of 800 ° C. to 1000 ° C .; , A step of winding the hot-rolled steel sheet in a coil shape at a temperature higher than 650 ° C. to 800 ° C., and immersing the hot-rolled steel sheet wound in the coil shape in a water tank within one hour after winding, The manufacturing method of the ferritic stainless steel hot-rolled steel plate excellent in the cold cracking characteristic characterized by having the process hold | maintained above and then taking out.
　以上のように、本発明の一態様によれば、種々の元素を含有するフェライト系ステンレス鋼熱延鋼板の靭性に影響を及ぼす亜粒界の割合を高めることにより、熱延鋼板の冷間割れを防ぐことができる。
　また、本発明の一態様に係るフェライト系ステンレス熱延鋼板によれば、熱間圧延後に連続焼鈍あるいは酸洗工程が施されても冷間割れは生じない。
　また、本発明の一態様によれば、各種フェライト系ステンレス熱延鋼板の冷間割れを抑制することで、製造歩留りの増加及び生産効率の向上をもたらすことができる。その結果、製造コストの低減などの面で産業上非常に有用な効果を発揮することができる。また、生産効率の向上により、使用エネルギーを抑制できるため、地球環境保全に貢献できる。 As described above, according to one aspect of the present invention, cold cracking of a hot-rolled steel sheet can be achieved by increasing the proportion of subgrain boundaries that affect the toughness of a ferritic stainless steel hot-rolled steel sheet containing various elements. Can be prevented.
Moreover, according to the ferritic stainless steel hot-rolled steel sheet according to an aspect of the present invention, cold cracking does not occur even if continuous annealing or pickling is performed after hot rolling.
Moreover, according to one aspect of the present invention, an increase in production yield and an improvement in production efficiency can be achieved by suppressing cold cracking of various ferritic stainless steel hot-rolled steel sheets. As a result, an industrially very useful effect can be exhibited in terms of reduction in manufacturing cost. In addition, energy consumption can be reduced by improving production efficiency, contributing to global environmental conservation.
本実施形態におけるフェライト系ステンレス熱延鋼板の全結晶粒界の長さＬと方位差１°以上１５°未満の亜粒界の長さＬａの割合（Ｌａ／Ｌ）と、シャルピー衝撃値との関係を示すグラフである。The ratio (La / L) of the length L of all the grain boundaries of the ferritic stainless steel hot rolled steel sheet in this embodiment and the length La of the subgrain boundaries having an orientation difference of 1 ° or more and less than 15 °, and the Charpy impact value It is a graph which shows a relationship.
　以下に、本実施形態のフェライト系ステンレス熱延鋼板について詳細に説明する。
　本実施形態のフェライト系ステンレス熱延鋼板は、質量％で、Ｃ：０．０１５０％以下、Ｓｉ：０．０１％～２．００％、Ｍｎ：０．０１％～２．００％、Ｐ：０．０４０％未満、Ｓ：０．０１０％以下、Ｃｒ：１０．０％～３０．０％、Ａｌ：０．００１％～３．００％、及び、Ｎ：０．０２００％以下をそれぞれ含有し、残部がＦｅおよび不可避的不純物からなる鋼組成を有し、板厚の１／４～３／４における断面において方位差１°以上１８０°未満の全結晶粒界の長さＬと、方位差１°以上１５°未満の亜粒界長さＬａが、Ｌａ／Ｌ≧０．２０を満足する関係にある。 Below, the ferritic stainless steel hot-rolled steel sheet of this embodiment is demonstrated in detail.
The ferritic stainless steel hot-rolled steel sheet of the present embodiment is, in mass%, C: 0.0150% or less, Si: 0.01% to 2.00%, Mn: 0.01% to 2.00%, P: Less than 0.040%, S: 0.010% or less, Cr: 10.0% to 30.0%, Al: 0.001% to 3.00%, and N: 0.0200% or less, respectively And the balance L has a steel composition composed of Fe and inevitable impurities, and the length L of all grain boundaries having a misorientation of 1 ° or more and less than 180 ° in the cross section at 1/4 to 3/4 of the plate thickness, The subgrain boundary length La having a difference of 1 ° or more and less than 15 ° satisfies La / L ≧ 0.20.
　以下、本実施形態の熱延鋼板の鋼組成を限定した理由について説明する。なお、組成についての％の表記は、特に断りがない場合は質量％を意味する。　 Hereinafter, the reason why the steel composition of the hot-rolled steel sheet of the present embodiment is limited will be described. In addition, the description of% about a composition means the mass% unless there is particular notice. *
Ｃ：０．０１５０％以下
　Ｃが固溶状態で存在すると、溶接部の粒界腐食性が劣化するため、多量のＣの添加は好ましくない。Ｃ量の上限を０．０１５０％とする。また、粒界腐食性の影響を及ぼさないようにＣ量を低減するには、精錬時間の増加等、製造コストの増加をもたらす。このため、Ｃ量の下限を０．００１０％とすることが好ましい。なお、溶接部の粒界腐食性及び製造コストの観点から考えると、Ｃ量は、０．００２０～０．００７０％とすることが好ましい。 C: 0.0150% or less When C exists in a solid solution state, the intergranular corrosion property of the welded portion deteriorates, so that a large amount of C is not preferable. The upper limit of the C amount is 0.0150%. In addition, reducing the amount of C so as not to affect the intergranular corrosivity causes an increase in manufacturing cost such as an increase in refining time. For this reason, it is preferable that the lower limit of the C amount is 0.0010%. In view of the intergranular corrosion property of the weld and the manufacturing cost, the C content is preferably 0.0020 to 0.0070%.
Ｓｉ：０．０１～２．００％
　Ｓｉは、耐酸化性を向上させる元素である。しかし多量のＳｉを添加すると、製品の加工性が劣化するため、Ｓｉ量の上限を２．００％とする。一方、脱酸剤として不可避的にＳｉを混入するため、Ｓｉ量の下限を０．０１％とする。なお、Ｓｉ量は、好ましくは０．０２％～０．９７％である。 Si: 0.01 to 2.00%
Si is an element that improves oxidation resistance. However, if a large amount of Si is added, the workability of the product deteriorates, so the upper limit of the Si amount is 2.00%. On the other hand, since Si is inevitably mixed as a deoxidizer, the lower limit of the Si amount is set to 0.01%. The Si amount is preferably 0.02% to 0.97%.
Ｍｎ：０．０１～２．００％
　Ｍｎは、高温強度、耐酸化性を向上させる元素であるが、多量のＭｎの添加は、Ｓｉと同様に製品の加工性劣化を招く。このため、Ｍｎ量の上限を２．００％とする。また、不可避的に混入する場合があるため、Ｍｎ量の下限を０．０１％とする。なお、Ｍｎ量は、好ましくは０．０２％～１．９５％である。 Mn: 0.01 to 2.00%
Mn is an element that improves the high-temperature strength and oxidation resistance. However, the addition of a large amount of Mn causes deterioration of the workability of the product as with Si. For this reason, the upper limit of the amount of Mn is made 2.00%. Moreover, since it may inevitably be mixed, the lower limit of the amount of Mn is set to 0.01%. The Mn content is preferably 0.02% to 1.95%.
Ｐ：０．０４０％未満
　Ｐは、Ｃｒの原料等から不可避的に混入するため、０．００５％以上のＰが混入する場合が多いが、Ｐは延性や製造性を低下させる。このため、Ｐ量は、可能な限り少ないほうが好ましい。しかし、過度に脱りんを行うことは非常に困難であり、さらには製造コストも増加するため、Ｐ量を０．０４％未満とする。 P: Less than 0.040% P is inevitably mixed from Cr raw materials and the like, so 0.005% or more of P is often mixed, but P decreases ductility and manufacturability. For this reason, the amount of P is preferably as small as possible. However, it is very difficult to dephosphorize excessively, and the manufacturing cost also increases, so the amount of P is made less than 0.04%.
Ｓ：０．０１０％以下
　Ｓは、溶解しやすい化合物をつくり、耐食性を劣化させる場合があるため、Ｓ量は少ない方が好ましく、Ｓ量を０．０１０％以下とする。また、耐食性の観点からはＳ量は低い方が好ましく、Ｓ量は０．００５０％未満とすることが好ましい。近年では脱硫技術が発達しているため、Ｓ量の下限を０．０００１％とするのがより好ましい。安定製造性を考慮すると、Ｓ量の下限は０．０００５％とすることがさらに好ましい。 S: 0.010% or less Since S produces a compound that is easily dissolved and may deteriorate the corrosion resistance, the amount of S is preferably small, and the amount of S is 0.010% or less. Further, from the viewpoint of corrosion resistance, the S content is preferably low, and the S content is preferably less than 0.0050%. Since desulfurization technology has been developed in recent years, the lower limit of the amount of S is more preferably 0.0001%. Considering stable manufacturability, the lower limit of the amount of S is more preferably 0.0005%.
Ｃｒ：１０．０～３０．０％
　Ｃｒは、耐食性、高温強度、及び耐酸化性を確保するために必要な基本元素であり、その効果を発揮するために１０．０％以上のＣｒの添加が必須である。一方、多量のＣｒの添加により、靱性の劣化を招くため、Ｃｒ量の上限を３０．０％とする。なお、Ｃｒ量が多いほど、高強度化し、また「４７５℃脆化」と呼ばれる多量のＣｒを含有する鋼に特有の脆化現象が生じやすくなる。このため、Ｃｒ量は２０．０％以下とすることが好ましい。 Cr: 10.0-30.0%
Cr is a basic element necessary for ensuring corrosion resistance, high-temperature strength, and oxidation resistance, and in order to exert its effects, addition of 10.0% or more of Cr is essential. On the other hand, the addition of a large amount of Cr causes toughness deterioration, so the upper limit of Cr content is made 30.0%. In addition, as the amount of Cr increases, the strength increases and the embrittlement phenomenon peculiar to steel containing a large amount of Cr called “475 ° C. embrittlement” easily occurs. For this reason, it is preferable that the Cr content is 20.0% or less.
Ａｌ：０．００１～３．００％
　Ａｌは、脱酸元素として活用するため、適量のＡｌを添加する。０．００１％未満のＡｌの添加では、脱酸能力が不十分であるため、０．００１％を下限とする。一方、０．１００％のＡｌで、十分に酸素量を低減でき、それを超える添加量でも脱酸能力はほぼ飽和する。このため、脱酸の目的のみでＡｌを添加する場合、Ａｌ量の上限は、０．１００％で良い。この場合、Ａｌ量は、好ましくは、０．００２％～０．０９５％である。
　また、Ａｌは、高温強度や耐食性を向上させる効果も有する。高温強度や耐食性を向上させる目的でＡｌを添加する場合、Ａｌ量は、好ましくは０．１０％超～３．００％であり、更に好ましくは０．５０％～２．００％である。なお、多量のＡｌを添加すると、製品の加工性の劣化を招くため、Ａｌ量の上限をＡｌ：３．００％とする。Ａｌ量の上限は、好ましくは２．００％以下である。 Al: 0.001 to 3.00%
Since Al is used as a deoxidizing element, an appropriate amount of Al is added. Addition of less than 0.001% Al results in insufficient deoxidation capacity, so 0.001% is made the lower limit. On the other hand, with 0.100% Al, the amount of oxygen can be sufficiently reduced, and the deoxidizing ability is almost saturated even when the amount exceeds this. For this reason, when adding Al only for the purpose of deoxidation, the upper limit of Al amount may be 0.100%. In this case, the amount of Al is preferably 0.002% to 0.095%.
Al also has the effect of improving high-temperature strength and corrosion resistance. When Al is added for the purpose of improving the high temperature strength and corrosion resistance, the amount of Al is preferably more than 0.10% to 3.00%, more preferably 0.50% to 2.00%. If a large amount of Al is added, the workability of the product is deteriorated, so the upper limit of the Al amount is Al: 3.00%. The upper limit of the amount of Al is preferably 2.00% or less.
Ｎ：０．０２００％以下
　Ｎは、Ｃと同様、固溶状態で存在すると、溶接部の粒界腐食性が劣化するため、多量のＮの添加は好ましくない。このためＮ量の上限を０．０２００％とする。またＮ量を低減するには、精錬時間の増加等、製造コストの増加をもたらす。このため、Ｎ量の下限を０．００３０％とすることが好ましい。なお、溶接部の粒界腐食性及び製造コストの観点から考えると、Ｎ量を０．００５０～０．０１２０％とすることが好ましい。　 N: 0.0200% or less When N is present in the form of a solid solution, as in C, the intergranular corrosion resistance of the welded portion is deteriorated, so that a large amount of N is not preferable. For this reason, the upper limit of the N amount is set to 0.0200%. Further, reducing the amount of N brings about an increase in manufacturing cost such as an increase in refining time. For this reason, it is preferable that the lower limit of the N amount be 0.0030%. From the viewpoint of intergranular corrosion of the weld and the manufacturing cost, the N content is preferably 0.0050 to 0.0120%.
　また、本実施形態では、上記元素に加えて、Ｎｂ：０．０５～０．７０％、Ｔｉ：０．０５～０．３０％のうちいずれか一方または両方を、下記式（１）を満足するように含むことが好ましい。
　Ｎｂ／９３＋Ｔｉ／４８≧Ｃ／１２＋Ｎ／１４　・・・　（１）
　但し、式（１）中の元素記号は、質量％を単位とする当該元素の含有量を意味する。 In this embodiment, in addition to the above elements, one or both of Nb: 0.05 to 0.70% and Ti: 0.05 to 0.30% satisfy the following formula (1). It is preferable to include.
Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (1)
However, the element symbol in Formula (1) means content of the said element in the unit of mass%.
　Ｎｂ及びＴｉは、ＣやＮと析出物を作り、固溶Ｃ，Ｎを低減する作用がある。加えて、Ｎｂ及びＴｉが固溶状態で存在する場合には、高温においては固溶強化により部材の高温強度、熱疲労特性を向上させる。Ｎｂを含有させる場合、Ｃ，Ｎを固定するためには０．０５％以上含有させる必要があり、０．１０％以上含有させることが好ましい。また、Ｔｉを含有させる場合、Ｃ，Ｎを固定するためには０．０５％以上含有させる必要がある。
　また、鋼中に存在するＣ，Ｎをすべて析出状態とするためには、化学量論的には上記式（１）を満足することが必要である。 Nb and Ti have a function of forming precipitates with C and N and reducing solid solution C and N. In addition, when Nb and Ti are present in a solid solution state, the high temperature strength and thermal fatigue characteristics of the member are improved by solid solution strengthening at high temperatures. When Nb is contained, in order to fix C and N, it is necessary to contain 0.05% or more, and it is preferable to contain 0.10% or more. Further, when Ti is contained, it is necessary to contain 0.05% or more in order to fix C and N.
Further, in order to make all the C and N present in the steel into a precipitated state, it is necessary to satisfy the above formula (1) stoichiometrically.
　一方、Ｔｉを多量に添加し過ぎると、製造途中の靱性の劣化を招き、また表面疵の発生が顕著になる場合がある。このため、上限はＴｉ：０．３０％とする。
　また、Ｎｂの多量添加は、製品の加工性が劣化する。このため、上限をＮｂ：０．７０％とし、０．５５％以下とすることがより好ましい。 On the other hand, if Ti is added in a large amount, the toughness may be deteriorated during the production, and the occurrence of surface flaws may become remarkable. Therefore, the upper limit is Ti: 0.30%.
Further, when Nb is added in a large amount, the workability of the product deteriorates. For this reason, the upper limit is more preferably Nb: 0.70% and 0.55% or less.
　また、本実施形態では、上記元素に加えて、Ｍｏ：０．１～２．５％、Ｎｉ：０．１～１．５％、Ｂ：０．０００１～０．００２５％、Ｃｕ：０．１～２．０％、Ｓｎ：０．０３～０．３５％のうち１種以上を含むことが好ましい。
　Ｍｏ，Ｎｉ，Ｃｕ，及びＳｎは、高温強度や耐食性を向上させる元素であり、必要に応じて添加しても良い。またＮｉは、靱性向上の効果も持つ。 In the present embodiment, in addition to the above elements, Mo: 0.1 to 2.5%, Ni: 0.1 to 1.5%, B: 0.0001 to 0.0025%, Cu: 0.0. It is preferable to contain one or more of 1 to 2.0% and Sn: 0.03 to 0.35%.
Mo, Ni, Cu, and Sn are elements that improve high-temperature strength and corrosion resistance, and may be added as necessary. Ni also has the effect of improving toughness.
　高温強度の増加が顕著になるのは、それぞれＭｏ：０．１％以上、Ｎｉ：０．１％以上、Ｃｕ：０．１％以上、Ｓｎ：０．０３％以上であるため、それを下限とする。高温強度および耐食性をより一層向上させるために、Ｍｏ：０．３％以上、Ｎｉ：０．２５％以上、Ｃｕ：０．４％以上、Ｓｎ：０．１０％以上とすることがより好ましい。
　多量のＭｏ、Ｎｉ、Ｃｕの添加は、酸洗性の劣化を招き、生産性低下につながるため、上限を、それぞれＭｏ：２．５％、Ｎｉ：１．５％、Ｃｕ：２．０％とし、Ｍｏ：２．２％以下、Ｎｉ：１．２％以下、Ｃｕ：１．４％以下とすることがより好ましい。多量のＳｎの添加は、靱性劣化及び表面疵の発生を招くため、上限をＳｎ：０．３５％とし、０．２０％以下とすることがより好ましい。 The increase in the high-temperature strength becomes remarkable because Mo: 0.1% or more, Ni: 0.1% or more, Cu: 0.1% or more, Sn: 0.03% or more, respectively. And In order to further improve the high-temperature strength and corrosion resistance, it is more preferable to set Mo: 0.3% or more, Ni: 0.25% or more, Cu: 0.4% or more, and Sn: 0.10% or more.
Addition of a large amount of Mo, Ni, and Cu leads to deterioration of pickling properties and leads to a decrease in productivity. Therefore, the upper limit is set to Mo: 2.5%, Ni: 1.5%, Cu: 2.0%, respectively. And Mo: 2.2% or less, Ni: 1.2% or less, and Cu: 1.4% or less are more preferable. Addition of a large amount of Sn causes toughness deterioration and generation of surface flaws, so the upper limit is made Sn: 0.35%, more preferably 0.20% or less.
　Ｂは、二次加工性を向上させる元素である。二次加工性が必要とされる用途に用いる場合には、必要に応じて添加しても良い。二次加工性の向上効果は、Ｂの添加量が０．０００１％以上から発現するので、これを下限とし、０．０００３％以上とすることがより好ましい。また、多量のＢの添加は、熱延板の靱性や加工性を低下させる場合があるため、Ｂ量の上限を０．００２５％とし、０．００１５％以下とすることがより好ましい。 B is an element that improves secondary workability. When used for applications where secondary workability is required, it may be added as necessary. The effect of improving the secondary workability is manifested when the addition amount of B is 0.0001% or more, and this is the lower limit, and more preferably 0.0003% or more. Moreover, since addition of a large amount of B may reduce the toughness and workability of the hot-rolled sheet, the upper limit of the B amount is set to 0.0025%, and more preferably 0.0015% or less.
　また、本実施形態の重要な特徴として、板厚の１／４～３／４における断面において、方位差１°以上１８０°未満の全結晶粒界の長さＬと、方位差１°以上１５°未満の亜粒界長さＬａの割合がＬａ／Ｌ≧０．２０を満たす。
　全結晶粒界の長さＬ及び亜粒界長さＬａの割合は、以下の方法により測定される。まず、熱延鋼板の任意の１０箇所から測定用サンプルを採取する。この採取箇所は、特に限定されるものではない。しかし、実際には熱延鋼板をコイルに巻き取る際、巻取り始めの部分（トップ部）と巻取り終了間際の部分（ボトム部）では、巻き取られる温度に差が生じる場合がある。そのため、このような場合は鋼板全体の平均値を得るという意味から熱延鋼板のトップ部、ミドル部、ボトム部などを網羅するように測定用サンプルを採取することが望ましい。熱延鋼板の幅方向に関して、略中央部から測定用サンプルを採取することが望ましい。また、圧延方向に平行であり、かつ板面方向に垂直な断面（Ｌ断面）を有するように測定用サンプルを採取する。
　測定用サンプルのＬ断面に対して、電解研磨又はコロイダルシリカによる研磨を施す。
　表層近傍は、比較的微細な結晶粒が生成し易く、靱性が良好な場合がある。このため、測定範囲を、Ｌ断面のうち、板厚ｔの中心近傍、すなわち１／４ｔから３／４ｔの範囲とする。
　次に、以下の方法によりＥＢＳＰを用いて結晶粒界長さを測定する。上記測定範囲のうち、１００μｍ×１００μｍの範囲において、０．２μｍの測定ステップ（ピッチ）で結晶方位を測定する。そして、隣接する測定点での方位差が１°以上１８０°未満の界面を粒界とみなす。このうち方位差が１°以上１５°未満の粒界を亜粒界とする。
　全ての結晶粒界の長さの合計を「全結晶粒界長さＬ」として算出し、亜粒界の長さの合計を「亜粒界長さＬａ」として算出する。そして、比Ｌａ／Ｌを求める。
　１０個の測定用サンプルについて、同様に比Ｌａ／Ｌを求め、１０個のＬａ／Ｌの値の平均値を算出する。 Further, as an important feature of the present embodiment, the length L of all crystal grain boundaries having a misorientation of 1 ° or more and less than 180 ° and a misorientation of 1 ° or more and 15 or less in a cross section at ¼ to 3/4 of the plate thickness. A ratio of subgrain boundary length La of less than 0 ° satisfies La / L ≧ 0.20.
The ratios of the total grain boundary length L and the subgrain boundary length La are measured by the following method. First, measurement samples are collected from arbitrary 10 locations of the hot-rolled steel sheet. This collection location is not particularly limited. However, in actuality, when the hot-rolled steel sheet is wound around a coil, there may be a difference in the temperature at which the winding is started (top portion) and the portion just before the end of winding (bottom portion). Therefore, in such a case, it is desirable to collect a measurement sample so as to cover the top portion, middle portion, bottom portion, etc. of the hot-rolled steel plate from the viewpoint of obtaining the average value of the entire steel plate. Regarding the width direction of the hot-rolled steel sheet, it is desirable to collect a measurement sample from a substantially central portion. A measurement sample is taken so as to have a cross section (L cross section) that is parallel to the rolling direction and perpendicular to the plate surface direction.
Electrolytic polishing or polishing with colloidal silica is applied to the L cross section of the measurement sample.
In the vicinity of the surface layer, relatively fine crystal grains are likely to be generated, and the toughness may be good. For this reason, the measurement range is set to the vicinity of the center of the plate thickness t in the L cross section, that is, a range from 1/4 t to 3/4 t.
Next, the grain boundary length is measured using EBSP by the following method. The crystal orientation is measured in a measurement step (pitch) of 0.2 μm in the measurement range of 100 μm × 100 μm. An interface having an azimuth difference of 1 ° or more and less than 180 ° at adjacent measurement points is regarded as a grain boundary. Among these, a grain boundary having an orientation difference of 1 ° or more and less than 15 ° is defined as a sub-grain boundary.
The total length of all grain boundaries is calculated as “total grain boundary length L”, and the total length of subgrain boundaries is calculated as “subgrain boundary length La”. Then, the ratio La / L is obtained.
The ratio La / L is similarly obtained for 10 measurement samples, and the average value of the 10 La / L values is calculated.
　Ｌａ／Ｌが０．２０未満の場合には、熱延鋼板の靱性が２０Ｊ／ｃｍ２未満と低くなるため、Ｌａ／Ｌは０．２０以上である必要がある。前述の図１に示すように、サブグレイン粒界（亜粒界）の割合が高いほど、熱延鋼板の靱性は高くなる傾向にある。このため、Ｌａ／Ｌは０．３５以上であることが好ましい。Ｌａ／Ｌの上限は、特に定める必要はないが、全ての粒界の方位差が１°から１５°未満であれば、Ｌａ／Ｌ＝１となる。本発明者らの実験では０．８０以上のＬａ／Ｌは得られていない。 When La / L is less than 0.20, the toughness of the hot-rolled steel sheet is as low as less than 20 J / cm 2, so La / L needs to be 0.20 or more. As shown in FIG. 1 described above, the higher the ratio of subgrain grain boundaries (subgrain boundaries), the higher the toughness of the hot-rolled steel sheet. For this reason, it is preferable that La / L is 0.35 or more. The upper limit of La / L is not particularly required, but if the orientation difference of all grain boundaries is 1 ° to less than 15 °, La / L = 1. In our experiment, La / L of 0.80 or more has not been obtained.
　次に、本実施形態におけるフェライト系ステンレス熱延鋼板の製造方法について説明する。
　本実施形態におけるフェライト系ステンレス熱延鋼板の製造方法は、以下の工程を有する。
（１）上記組成を有するフェライト系ステンレス鋼を鋳造して鋼片とする。次いで前記鋼片に対して、仕上げ温度が８００℃～１０００℃の条件で熱間圧延を施すことにより熱延鋼板（圧延材）とする工程。
（２）熱間圧延後、６５０℃超～８００℃の巻き取り温度で前記熱延鋼板をコイル状に巻き取る工程。
（３）コイル状に巻き取った前記熱延鋼板を、巻き取り後１時間以内に水槽に浸漬させ、水槽内で１時間以上保持し、次いで取り出し、熱延鋼板とする工程。
　以下に、本実施形態におけるフェライト系ステンレス熱延鋼板の製造方法について詳細に説明する。 Next, the manufacturing method of the ferritic stainless steel hot-rolled steel sheet in this embodiment is demonstrated.
The manufacturing method of the ferritic stainless steel hot-rolled steel sheet in this embodiment has the following processes.
(1) Ferritic stainless steel having the above composition is cast into a steel piece. Next, a step of forming a hot-rolled steel sheet (rolled material) by subjecting the steel slab to hot rolling at a finishing temperature of 800 ° C. to 1000 ° C.
(2) A step of winding the hot-rolled steel sheet in a coil shape at a winding temperature of more than 650 ° C. to 800 ° C. after hot rolling.
(3) A step of immersing the hot rolled steel sheet wound in a coil shape in a water tank within 1 hour after winding, holding it in the water tank for 1 hour or more, and then taking it out to form a hot rolled steel sheet.
Below, the manufacturing method of the ferritic stainless steel hot-rolled steel plate in this embodiment is demonstrated in detail.
　まず、上記鋼組成を有するフェライト系ステンレス鋼を鋳造して鋼片とし、この鋼片に対して、熱間圧延を施して熱延鋼板とする。次いで熱間圧延（仕上げ圧延）が施された熱延鋼板を、水冷で巻き取り温度まで冷却し、巻き取り温度にてコイル状に巻き取る。本実施形態においては、熱間圧延の仕上げ温度を８００℃～１０００℃とし、巻き取り温度を６５０℃超～８００℃とする。
　仕上げ温度が８００℃未満あるいは１０００℃超であると、巻き取り後に方位差１°から１５°未満の結晶粒界の生成が非常に困難となる。このため、８００℃及び１０００℃を、それぞれ下限及び上限とする。 First, ferritic stainless steel having the above steel composition is cast into a steel slab, and this steel slab is hot-rolled to obtain a hot-rolled steel sheet. Next, the hot-rolled steel sheet that has been subjected to hot rolling (finish rolling) is cooled to a winding temperature by water cooling, and wound into a coil at the winding temperature. In this embodiment, the hot rolling finishing temperature is set to 800 ° C. to 1000 ° C., and the winding temperature is set to more than 650 ° C. to 800 ° C.
When the finishing temperature is less than 800 ° C. or more than 1000 ° C., it becomes very difficult to generate a grain boundary having an orientation difference of 1 ° to less than 15 ° after winding. For this reason, 800 degreeC and 1000 degreeC are made into a minimum and an upper limit, respectively.
　なお、本実施形態においては、熱間圧延中にオーステナイト相を生成させないことが好ましい。熱間圧延時にオーステナイト相が生成するかどうかは、鋼中のオーステナイト生成元素の量、特にオーステナイト生成能の大きいＣ，Ｎの量によって決定される。本実施形態の熱延鋼板は、Ｃ，Ｎの量は共に少なく、熱間圧延中のオーステナイト相の生成は認められない。
　巻き取り温度が６５０℃以下の場合も、方位差が１°から１５°未満の結晶粒界の生成が困難となる。巻き取り温度が８００℃超の場合は、逆に巻き取り時の再結晶が進行し、方位差が１５°から１８０°未満の結晶粒界の割合が増加するため、靱性は劣化する。 In the present embodiment, it is preferable not to generate an austenite phase during hot rolling. Whether or not an austenite phase is generated during hot rolling is determined by the amount of austenite-forming elements in the steel, particularly the amount of C and N having a large austenite-forming ability. In the hot-rolled steel sheet of the present embodiment, the amounts of C and N are both small, and no austenite phase is generated during hot rolling.
Even when the coiling temperature is 650 ° C. or less, it is difficult to generate a crystal grain boundary having an orientation difference of 1 ° to less than 15 °. When the winding temperature is higher than 800 ° C., recrystallization progresses at the time of winding, and the ratio of crystal grain boundaries having an orientation difference of 15 ° to less than 180 ° increases, so that the toughness deteriorates.
　次に、コイル状に巻き取った熱延鋼板を水槽に浸漬する。これは巻き取り後の緩冷工程において靱性を劣化させる析出物が生成することを抑制するためである。ここで、仕上げ圧延後の水冷により熱延鋼板の温度が巻き取り温度に到達してから、前述の析出物が生成し粗大化する過程は、巻き取り後の鋼板の温度及び時間に強く依存する。なお、通常の条件で熱間圧延を行い、巻き取り温度６５０℃超～８００℃で巻き取る場合、熱間圧延してから巻き取り温度に達するまでの時間は１ｍｉｎ以内であり、この間の冷却速度は３℃／ｓｅｃ以上である。このような冷却速度条件の場合、仕上げ圧延の終了から巻き取りの開始までの間に、靱性に影響を与える析出物が生成することはない。 Next, the hot rolled steel sheet wound in a coil shape is immersed in a water tank. This is to suppress the formation of precipitates that degrade toughness in the slow cooling step after winding. Here, after the temperature of the hot-rolled steel sheet reaches the coiling temperature by water cooling after finish rolling, the process of forming and coarsening the precipitates described above strongly depends on the temperature and time of the steel sheet after winding. . In addition, when hot rolling is performed under normal conditions and winding is performed at a winding temperature of over 650 ° C. to 800 ° C., the time from hot rolling to reaching the winding temperature is within 1 minute, and the cooling rate during this time Is 3 ° C./sec or more. In the case of such a cooling rate condition, precipitates that affect toughness are not generated between the end of finish rolling and the start of winding.
　靱性を劣化させる析出物の生成には、上述した巻き取り温度において保持される時間が重要な因子となる。本実施形態では、巻き取り後１時間以内に熱延鋼板を水槽に浸漬させる必要がある。巻き取りが完了してから水槽への浸漬までの時間が１時間を超えると、巻き取りの完了から水槽への浸漬までの間に析出物が生成し、この生成した析出物によって靱性が劣化する場合がある。
　また、熱延鋼板を水槽に浸漬してから水槽内で保持する時間も重要な項目である。本実施形態において熱延鋼板を水槽内で保持する浸漬時間は、１時間以上であることが好ましい。
　水槽内での熱延鋼板の浸漬時間が１時間未満と短い場合は、冷却が不十分となり、その後の復熱等により熱延鋼板に靱性を劣化させる析出物が生成する場合がある。 In order to generate precipitates that deteriorate toughness, the time that is maintained at the above-described winding temperature is an important factor. In this embodiment, it is necessary to immerse the hot-rolled steel sheet in the water tank within 1 hour after winding. If the time from the completion of winding to immersion in the water tank exceeds 1 hour, precipitates are generated between the completion of winding and immersion in the water tank, and the toughness deteriorates due to the generated precipitates. There is a case.
In addition, the time for holding the hot-rolled steel sheet in the water tank after being immersed in the water tank is also an important item. In this embodiment, the immersion time for holding the hot-rolled steel sheet in the water tank is preferably 1 hour or longer.
When the immersion time of the hot-rolled steel sheet in the water tank is as short as less than 1 hour, cooling becomes insufficient, and precipitates that deteriorate toughness may be generated in the hot-rolled steel sheet due to subsequent reheating or the like.
　以上説明した本実施形態に係るフェライト系ステンレス熱延鋼板によれば、上記成分及び結晶粒界に係る要件により、熱延鋼板の靭性に影響を及ぼす金属組織を制御することが可能となり、その結果、熱延鋼板の冷間割れを防ぐことができる。
　また、本実施形態に係るフェライト系ステンレス熱延鋼板によれば、熱間圧延後の連続焼鈍あるいは酸洗工程を通っても冷間割れは生じない。 According to the ferritic stainless steel hot-rolled steel sheet according to the present embodiment described above, it is possible to control the metal structure that affects the toughness of the hot-rolled steel sheet, as a result of the requirements related to the above components and grain boundaries. It is possible to prevent cold cracking of the hot-rolled steel sheet.
Moreover, according to the ferritic stainless steel hot-rolled steel sheet according to the present embodiment, cold cracking does not occur even after continuous annealing or hot pickling after hot rolling.
　また、本実施形態に係るフェライト系ステンレス熱延鋼板によれば、冷間割れを抑制できるため、製造歩留りの増加、及び生産効率の向上をもたらすことができる。その結果、製造コストの低減などの面で産業上非常に有用な効果を発揮することができる。また、生産効率の向上により、製造工程における使用エネルギーを抑制できるため、地球環境保全に貢献できる。 Moreover, according to the ferritic stainless steel hot-rolled steel sheet according to the present embodiment, since cold cracking can be suppressed, it is possible to increase the production yield and improve the production efficiency. As a result, an industrially very useful effect can be exhibited in terms of reduction in manufacturing cost. In addition, by improving production efficiency, energy used in the manufacturing process can be suppressed, which can contribute to global environmental conservation.
　以下、実施例により本実施形態の効果を説明するが、本実施形態は、以下の実施例で用いた条件に限定されない。 Hereinafter, the effects of the present embodiment will be described by way of examples, but the present embodiment is not limited to the conditions used in the following examples.
　本実施例では、まず、表１に示す組成の各鋼を溶製して鋳造し、鋼塊（鋼片）を得た。
　この鋼塊を９０ｍｍ厚まで研削した。表２，３に示す仕上げ温度（ＦＴ）にて熱間圧延を行い、鋼塊を板厚５ｍｍまで圧延し、熱延鋼板とした。次に、圧延後の鋼板温度を放射温度計でモニターしながら、水冷によって表２，３に示す巻き取り温度（ＣＴ）まで冷却した。なお、この時の冷却速度は約２０℃／ｓｅｃであった。 In this example, first, each steel having the composition shown in Table 1 was melted and cast to obtain a steel ingot (steel piece).
The steel ingot was ground to 90 mm thickness. Hot rolling was performed at a finishing temperature (FT) shown in Tables 2 and 3, and the steel ingot was rolled to a plate thickness of 5 mm to obtain a hot-rolled steel plate. Next, while monitoring the steel plate temperature after rolling with a radiation thermometer, the steel sheet was cooled to the winding temperature (CT) shown in Tables 2 and 3 by water cooling. The cooling rate at this time was about 20 ° C./sec.
　次に、炉内の温度が表２，３の巻き取り温度（ＣＴ）に制御された炉の中に熱延鋼板を挿入し、巻き取り処理を模擬した。その後、表２，３の時間（ｔ）が経過した後に熱延鋼板を水槽に浸漬した。次いで、水槽内に、表２，３に示す浸漬時間（ｔｘ）の間保持し、そして熱延鋼板を取り出した。 Next, a hot-rolled steel sheet was inserted into the furnace in which the temperature in the furnace was controlled to the coiling temperature (CT) shown in Tables 2 and 3, and the winding process was simulated. Thereafter, the hot-rolled steel sheet was immersed in a water tank after the time (t) in Tables 2 and 3 had elapsed. Subsequently, it was kept in the water tank for the immersion time (tx) shown in Tables 2 and 3, and the hot-rolled steel sheet was taken out.
　得られた各熱延鋼板は、全てフェライト単相組織であった。
　また、実施形態に記載の測定方法と同様にして、ＥＢＳＰを用いて結晶粒界特性（全結晶粒界長さＬに対する亜粒界長さＬａの比Ｌａ／Ｌ）を算出した。
　熱延鋼板よりサブサイズシャルピー衝撃試験片をＪＩＳ　Ｚ　２２０２に準拠して採取し、圧延方向の垂直方向を衝撃方向としてＪＩＳ　Ｚ　２２４２に準拠した金属材料の衝撃試験を実施した。試験温度を２５℃として衝撃吸収エネルギーを調査した。 Each of the obtained hot-rolled steel sheets had a ferrite single phase structure.
Further, in the same manner as the measurement method described in the embodiment, the grain boundary characteristics (ratio of subgrain boundary length La to total grain boundary length L La / L) were calculated using EBSP.
Sub-size Charpy impact test pieces were collected from the hot-rolled steel sheet in accordance with JIS Z 2202, and an impact test was performed on a metal material in accordance with JIS Z 2242 with the vertical direction of the rolling direction as the impact direction. The shock absorption energy was investigated at a test temperature of 25 ° C.
　また、得られた結果より、熱延鋼板の冷間割れ性（靭性）を下記の方法により評価した。
　本実施例において、シャルピー衝撃値が２０Ｊ／ｃｍ２未満の熱延鋼板では、その後の工程である、連続焼鈍や酸洗工程において、冷間割れ等が発生し、歩留まりが低下した。これに対して、シャルピー衝撃値が２０Ｊ／ｃｍ２以上の熱延鋼板では、このような冷間割れは発生しなかった。従って、シャルピー衝撃値が２０Ｊ／ｃｍ２未満の熱延鋼板の冷間割れ性を“不良”と評価し、シャルピー衝撃値が２０Ｊ／ｃｍ２以上の熱延鋼板の冷間割れ性を“良好”と評価した。表２，３では、シャルピー衝撃値が２０Ｊ／ｃｍ２未満の値に下線を付した。
　以上の製造条件及び評価結果を表２，３に示す。
　なお、表２，３において、ＦＴは、熱間圧延の仕上げ温度（℃）を示し、ＣＴは、熱延鋼板の巻き取り温度（℃）を示す。ｔは、巻き取りの完了から水冷の開始（浸漬の開始）までの時間（ｈ）を示し、ｔｘは、水冷の開始から完了（浸漬開始から取り出し）までの時間（ｈ）を示す。
　また、表１～３では、本実施形態で規定された範囲外の数値には、下線を付した。 Moreover, from the obtained result, the cold cracking property (toughness) of a hot-rolled steel sheet was evaluated by the following method.
In this example, in the hot rolled steel sheet having a Charpy impact value of less than 20 J / cm 2 , cold cracking and the like occurred in the subsequent annealing and pickling processes, and the yield decreased. On the other hand, such a cold crack did not occur in a hot-rolled steel sheet having a Charpy impact value of 20 J / cm 2 or more. Therefore, the cold cracking property of a hot-rolled steel sheet having a Charpy impact value of less than 20 J / cm 2 is evaluated as “bad”, and the cold cracking property of a hot-rolled steel sheet having a Charpy impact value of 20 J / cm 2 or more is “good”. It was evaluated. In Tables 2 and 3, the Charpy impact value is underlined for values less than 20 J / cm 2 .
The above production conditions and evaluation results are shown in Tables 2 and 3.
In Tables 2 and 3, FT represents the hot rolling finishing temperature (° C.), and CT represents the coiling temperature (° C.) of the hot-rolled steel sheet. t represents the time (h) from the completion of winding to the start of water cooling (immersion start), and tx represents the time (h) from the start of water cooling to completion (from immersion start to removal).
In Tables 1 to 3, numerical values outside the range defined in the present embodiment are underlined.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　表２，３より明らかなように、本実施形態に係る本発明例によれば、シャルピー衝撃値が２０Ｊ／ｃｍ２以上であり、冷間割れ性に優れたフェライト系ステンレス熱延鋼板、すなわち靱性が良好な熱延鋼板を得ることができる。
　一方、本実施形態で規定された範囲外の比較例では、いずれもシャルピー衝撃値が低かった。これにより、比較例における熱延鋼板の冷間割れ性（靭性）が低下してしまったことが分かる。
　これらの結果から、上述した知見を確認することができ、また、上述した各鋼組成及び構成を限定する根拠を裏付けることができた。 As is apparent from Tables 2 and 3, according to the present invention example according to the present embodiment, a ferritic stainless hot-rolled steel sheet having a Charpy impact value of 20 J / cm 2 or more and excellent in cold cracking property, that is, toughness. Can obtain a good hot-rolled steel sheet.
On the other hand, in the comparative examples outside the range defined in the present embodiment, the Charpy impact value was low. Thereby, it turns out that the cold crack property (toughness) of the hot-rolled steel sheet in a comparative example has fallen.
From these results, the above-mentioned findings could be confirmed, and the grounds for limiting the above-described steel compositions and configurations could be supported.
　本実施形態のフェライト系ステンレス熱延鋼板は、２０Ｊ／ｃｍ２以上のシャルピー衝撃値を有し、冷間割れ性に優れる。このため、熱間圧延後に連続焼鈍あるいは酸洗工程が施されても冷間割れは生じない。従って、本実施形態のフェライト系ステンレス熱延鋼板は、フェライト系ステンレス鋼が用いられる家電、建材、自動車部品などの部材の製造工程に好適に適用できる。 The ferritic stainless hot rolled steel sheet of this embodiment has a Charpy impact value of 20 J / cm 2 or more and is excellent in cold cracking property. For this reason, even if a continuous annealing or a pickling process is performed after hot rolling, cold cracking does not occur. Therefore, the ferritic stainless steel hot-rolled steel sheet of the present embodiment can be suitably applied to manufacturing processes for members such as home appliances, building materials, and automobile parts in which ferritic stainless steel is used.

Claims (4)

  1. 　質量％で、
    Ｃ：０．０１５０％以下、
    Ｓｉ：０．０１％～２．００％、
    Ｍｎ：０．０１％～２．００％、
    Ｐ：０．０４０％未満、
    Ｓ：０．０１０％以下、
    Ｃｒ：１０．０％～３０．０％、
    Ａｌ：０．００１％～３．００％、及び
    Ｎ：０．０２００％以下、
    をそれぞれ含有し、
    　残部がＦｅおよび不可避的不純物からなる鋼組成を有し、
    　板厚の１／４～３／４における断面において方位差１°以上１８０°未満の全結晶粒界の長さＬと、方位差１°以上１５°未満の亜粒界長さＬａが、Ｌａ／Ｌ≧０．２０を満足する関係にあることを特徴とする冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板。     % By mass
    C: 0.0150% or less,
    Si: 0.01% to 2.00%
    Mn: 0.01% to 2.00%
    P: less than 0.040%,
    S: 0.010% or less,
    Cr: 10.0% to 30.0%,
    Al: 0.001% to 3.00%, and N: 0.0200% or less,
    Each containing
    The balance has a steel composition consisting of Fe and inevitable impurities,
    The length L of all crystal grain boundaries with an orientation difference of 1 ° or more and less than 180 ° and the subgrain boundary length La with an orientation difference of 1 ° or more and less than 15 ° in a cross section at 1/4 to 3/4 of the plate thickness are La A ferritic stainless steel hot-rolled steel sheet having excellent cold cracking characteristics, which satisfies a relationship satisfying /L≧0.20.
  2. 　さらに、質量％で、
    Ｎｂ：０．０５％～０．７０％以下、
    Ｔｉ：０．０５％～０．３０％以下、
    Ｍｏ：０．１％～２．５％、
    Ｎｉ：０．１％～１．５％、
    Ｂ：０．０００１％～０．００２５％、
    Ｃｕ：０．１％～２．０％、及び
    Ｓｎ：０．０３％～０．３５％
    から選択される１種以上を含み、
    　Ｎｂ、Ｔｉのいずれか一方または両方を含む場合は、下記式（１）を満足することを特徴とする請求項１に記載の冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板。
    　Ｎｂ／９３＋Ｔｉ／４８≧Ｃ／１２＋Ｎ／１４　・・・（１）
    　但し、式（１）中の元素記号は、当該元素の質量％を単位とする含有量を意味する。     Furthermore, in mass%,
    Nb: 0.05% to 0.70% or less,
    Ti: 0.05% to 0.30% or less,
    Mo: 0.1% to 2.5%,
    Ni: 0.1% to 1.5%,
    B: 0.0001% to 0.0025%,
    Cu: 0.1% to 2.0% and Sn: 0.03% to 0.35%
    Including at least one selected from
    2. The ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties according to claim 1, wherein when one or both of Nb and Ti are contained, the following formula (1) is satisfied.
    Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (1)
    However, the element symbol in Formula (1) means content in unit of the mass% of the said element.
  3. 　Ａｌ含有量が０．１０％超～３．００％であることを特徴とする請求項１又は請求項２に記載の冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板。 The ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties according to claim 1 or 2, wherein the Al content is more than 0.10% to 3.00%.
  4. 　請求項１乃至請求項３の何れか一項に記載のフェライト系ステンレス鋼熱延鋼板を製造する方法であって、
    　請求項１乃至請求項３の何れか一項に記載の鋼組成を有するフェライト系ステンレス鋼を鋳造して鋼片とし、前記鋼片に対して、仕上げ温度が８００℃～１０００℃の条件で熱間圧延を施すことにより熱延鋼板とする工程と、
    　その後、６５０℃超～８００℃で前記熱延鋼板をコイル状に巻き取る工程と、
    　コイル状に巻き取った前記熱延鋼板を、巻き取り後１時間以内に水槽に浸漬させ、水槽内で１時間以上保持し、次いで取り出す工程とを有することを特徴とする冷間割れ性に優れたフェライト系ステンレス鋼熱延鋼板の製造方法。     A method for producing a ferritic stainless steel hot-rolled steel sheet according to any one of claims 1 to 3,
    A ferritic stainless steel having the steel composition according to any one of claims 1 to 3 is cast into a steel slab, and the steel slab is heated at a finishing temperature of 800 ° C to 1000 ° C. A process of hot rolling steel sheet by performing hot rolling,
    Thereafter, winding the hot-rolled steel sheet in a coil shape at a temperature exceeding 650 ° C. to 800 ° C.,
    The hot-rolled steel sheet wound in a coil shape is immersed in a water tank within 1 hour after winding, held in the water tank for 1 hour or more, and then taken out, and has excellent cold cracking characteristics. Ferritic stainless steel hot rolled steel sheet manufacturing method.
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