US20140261917A1 - Method for manufacturing austenitic stainless steel - Google Patents

Method for manufacturing austenitic stainless steel Download PDF

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US20140261917A1
US20140261917A1 US14/235,849 US201214235849A US2014261917A1 US 20140261917 A1 US20140261917 A1 US 20140261917A1 US 201214235849 A US201214235849 A US 201214235849A US 2014261917 A1 US2014261917 A1 US 2014261917A1
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stainless steel
temperature
hot
hot rolling
austenitic stainless
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Hayato Kita
Masayuki Shibuya
Shuuji Yoshida
Tomoyuki Sukawa
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
    • B21B1/026—Rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
    • B21B2001/028—Slabs
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D2211/004—Dispersions; Precipitations

Definitions

  • the present invention relates to a method for manufacturing austenitic stainless steel exhibiting corrosion resistance to a concentrated nitric acid. More specifically, the present invention relates to a method for manufacturing high Si-containing austenitic stainless steel usable in a concentrated nitric acid environment at a high temperature.
  • high-Si austenitic stainless steel such as 17Cr-14Ni-4Si (Patent Document 1) and 11Cr-17Ni-6Si (Patent Document 2)
  • high-Si austenitic stainless steel such as 17Cr-14Ni-4Si (Patent Document 1) and 11Cr-17Ni-6Si (Patent Document 2)
  • Si resolved in a trans-passive region due to corrosion is re-oxidized, thereby forming a silicate film, which exhibits excellent corrosion resistance against a nitric acid.
  • One or more types of elements selected from Nb, Ta, Ti, and Zr tend to be added to this high-Si stainless steel.
  • These additive elements have an effect of immobilizing C in steel, and suppressing sensitization. Particularly, they are effective for suppression of sensitization in a welded heat affected zone, and have a significant effect of improving intergranular corrosion resistance in a highly-concentrated nitric acid.
  • a heating temperature of a slab in hot working is preferably as high as possible in view of productivity.
  • high-Si stainless steel there is a problem of cracking caused in the slab during the hot working if upon heated at a higher temperature than a predetermined temperature during the hot working because Si has low solid solubility relative to an austenite phase, and as the Si content becomes more increased, a brittle phase, such as an intermetallic compound and 6 ferrite, is more likely to be generated at a high temperature, which deteriorates high-temperature ductility. Accordingly, in order to stably manufacture high-Si stainless steel industrially, it is necessary to appropriately control a heating temperature in the hot working.
  • Patent Document 3 discloses a method of hot-rolling or hot-forging an ingot of high-Si stainless steel containing Si of 5 to 8% (in this description, unless otherwise specified, percent with respect to chemical composition means mass percent) within a temperature range of at least 900° C. after soaking in a temperature range that satisfies: 1050 to 1100° C.; and T(° C.) ⁇ 1470 ⁇ 35 ⁇ Si ⁇ 5 ⁇ Ni (%).
  • the soaking temperature is determined so as to prevent such cracking.
  • Patent Document 4 discloses that soaks at a temperature of at least 1100° C. and at most 1250° C. for at least two hours a slab of high-Si stainless steel containing Si of 4 to 10% in which S and O are restricted to be at most 30 ppm, and then hot-rolls this steel; finishes the hot rolling at a temperature of at least 950° C.; and subjects this steel to solution heat treatment at a temperature of at least 1000° C. and at most 1200° C.
  • Patent Document 4 discloses that (1) impurity elements such as S and 0, and (2) an intermetallic compound that precipitates during cooling a slab affect high-temperature ductility of high-Si austenitic stainless steel, and also discloses that the intermetallic compound is removed by reduction of S and O, and by soaking of the slab, thereby improving hot workability.
  • a composition of this intermetallic compound is not clearly described, but it is estimated that this is an Ni—Si intermetallic compound having a low melting point as similar to that of Non-Patent Document 1.
  • Patent Document 3 and Patent Document 4 improve hot workability by setting a heating temperature to be not more than a fusing temperature of the Ni—Si intermetallic compound.
  • a heating temperature to be not more than a fusing temperature of the Ni—Si intermetallic compound.
  • Si In high-Si austenitic stainless steel used in a highly-concentrated nitric acid environment at a high-temperature, a great deal of Si thereof deteriorates solid solubility of C, so that sensitization likely occurs, and consequently, intergranular corrosion resistance in the highly-concentrated nitric acid is poor.
  • Each high-Si stainless steel disclosed in Patent Document 1 and Patent Document 2 contains Nb, Ta, Ti, and Zr so as to suppress sensitization, and to greatly improve the nitric acid corrosion resistance, but this brings up another problem of surface defects called as scab that is likely to be generated in the hot rolling process.
  • Patent Document 1 Japanese Patent Publication No. 3237132
  • Patent Document 2 Japanese Patent Publication No. 1119398
  • Patent Document 3 Japanese Patent Laid-Open No. 6-93389
  • Patent Document 4 Japanese Patent Laid-Open No. 5-51633
  • Non Patent Document 1 NKK Technical Report, No. 154, 1996, pp. 14-19
  • An object of the present invention is to ensure manufacturing high-Si containing austenitic stainless steel having corrosion resistance suitable for use in a highly-concentrated nitric acid environment at a high temperature without generating scab in a hot rolling process.
  • the present inventors have conducted studies on conditions to ensure manufacturing high Si-containing austenitic stainless steel (austenitic stainless steel is also referred to simply as “stainless steel”, hereinafter) suitable for use in a highly-concentrated nitric acid environment at a high temperature without generating fractures in a hot rolling process, and as a result, the following (i) to (iii) have been found.
  • an Ni—Si intermetallic compound is produced. As disclosed in Non Patent Document 1, its melting point is estimated to be within a range from 1100 to 1150° C., and this intermetallic compound causes great slab cracking that hinders hot rolling.
  • Ni—Si—X (X ⁇ Nb, Ti, Zr) is produced as the intermetallic compound. Its melting point is within a range of approximately 1150 to 1200° C., and this is approximately 1160° C. for Ni—Si—Nb based on a calculated result of a state diagram calculation thereof, for example. Since Nb, Ta, Ti, Zr and the like are elements that hardly segregate in the steel, an Ni—Si—X ternary system (X ⁇ Nb, Ta, Ti, Zr) intermetallic compound is finely dispersed. Because the Ni—Si—X intermetallic compound is finely dispersed at a high melting point, this causes no slab cracking great enough to hinder the rolling.
  • the scabs in the hot rolling process are generated by the aforementioned Ni—Si—X ternary system intermetallic compound as a starting point, and occur by propagation of the cracking to the surface.
  • Containing Si and X elements is essential for corrosion prevention in the highly-concentrated nitric environment, and thus a method of suppressing the above described propagation of the cracking in the vicinity of the surface have been studied.
  • ductility is deteriorated with excessively high temperature, the cracking likely propagate; therefore, a relation between the composition and the ductility of the steel has been studied. As a result, the following findings have been obtained.
  • Defects (scabs) of the product surface can be prevented by controlling the heating temperature during the hot rolling based on the relation among the contents of Si, Cr, and Ni in the chemical composition of the steel.
  • (B) Sensitization can be suppressed as well as achieving ductility and yield strength by controlling a temperature range and a cooling method of finishing annealing after the rolling.
  • the present invention based on the above findings is a method for manufacturing austenitic stainless steel which heats and hot-rolls a slab of stainless steel at a heating temperature T h during the hot rolling, wherein the slab of stainless steel includes a chemical composition containing C: 0.04% or less; Cr: 7 to 20%, Ni: 10 to 22%, Si: 2.5 to 7%, Mn: 10% or less, sol.
  • the method according to the present invention further includes subjecting the hot-rolled austenitic stainless steel to heat treatment within a temperature range of 1100 to 1160° C., and thereafter, cooling this austenitic stainless steel at cooling rate of 100° C./min. or more.
  • FIG. 1 is a graph showing a test result of a torsion test on a test piece 1 .
  • FIG. 2 is a graph showing a relation between ⁇ T and a scab occurrence ratio in the test piece 1 .
  • FIG. 3 is a graph showing relations of a heat treatment temperature after hot rolling with 0.2% yield strength, and with ductility in the test piece 1 .
  • % relating to the chemical composition of steel means mass%.
  • the remainder of the chemical composition of the steel includes Fe and impurities.
  • the C content is an element for increasing strength of steel, but producing Cr carbide at grain boundaries in a welded heat affected zone, which causes sensitization, and deteriorates corrosion resistance. Accordingly, the C content is controlled to be at most 0.04%. The C content is preferably at most 0.03%, and more preferably at most 0.02%.
  • Cr is a basic element for improving the corrosion resistance of the stainless steel, and the Cr content is controlled to be at least 7% and at most 20%.
  • the Cr content of less than 7% cannot achieve adequate corrosion resistance.
  • the excessive Cr content produces a two-phase structure in which a large amount of ferrite precipitates under the coexistence of Si and Nb, which causes deterioration of workability and impact resistance; and thus the upper limit of the Cr content is controlled to be 20%.
  • the lower limit of the Cr content is preferably 10%, and more preferably 11%.
  • the upper limit of the Cr content is preferably 19%, and more preferably 18%.
  • Ni is a stabilizing element of the austenite phase, and also has an effect of increasing the zero ductility temperature.
  • the Ni content is controlled to be at least 10% and at most 22% or less.
  • the Ni content of less than 10% cannot achieve desired corrosion resistance and toughness.
  • the Ni content of more than 22% causes significant increase in cost.
  • the lower limit of the Ni content is preferably 12%, and more preferably 13%.
  • the upper limit of the Ni content is preferably 20%, and more preferably 16%.
  • Si is contained at a content of at least 2.5% and at most 7% for the purpose of increasing the corrosion resistance in a concentrated nitric acid.
  • Si is contained at a content of at least 2.5% so as to form a silicate film for achieving the corrosion resistance in the nitric acid.
  • An excessive Si content decreases the zero ductility temperature. This excessive content not only increases the cost but also deteriorates weldability; therefore, the upper limit of the Si content is controlled to be 7%.
  • the lower limit of the Si content is preferably 3.0%, and more preferably 3.5%.
  • the upper limit of the Si content is preferably 6%, and more preferably 5%.
  • Mn is an austenite stabilizing element, and is contained as a deoxidizing agent; therefore, the Mn content is controlled to be at most 10%.
  • the Mn content of more than 10% causes deterioration of the corrosion resistance, hot cracking upon welding, as well as deterioration of workability.
  • the upper limit of the Mn content is preferably 6%, and more preferably 4%.
  • the Mn content is preferably at least 0.5%, and more preferably at least 1.0%.
  • Al is contained as a deoxidizing agent in the steel, but produces a toxic inclusion if Al is excessively contained; therefore, the sol. Al content is controlled to be at most 0.03%.
  • P and S are both undesirable elements for the corrosion resistance and the weldability, and each content thereof is preferably as small as possible.
  • the P content is controlled to be at most 0.03%, and the S content is controlled to be at most 0.03%.
  • N has a high affinity to Nb, Ti, Ta, and Zr, and hinders immobility of C by these elements; and thus the N content is preferably as small as possible.
  • the N content is controlled to be at most 0.035%.
  • All of Nb, Ti, Ta, and Zr have an effect of immobilizing C, and suppressing deterioration of intergranular corrosion resistance caused by sensitization, and they are particularly effective for improving the corrosion resistance in the welded heat affected zone.
  • a total amount of these elements of less than 0.05% cannot achieve an effect of improving the intergranular corrosion resistance, and increases hot-working cracking caused by formation of a low melting point Ni—Si based intermetallic compound.
  • the total amount of these elements of more than 0.7% deteriorates the workability. Accordingly, the total amount of one or more types of these elements is controlled to be at least 0.05% and at most 0.7%.
  • each test piece having a parallel portion of 8 mm in diameter and a length of 30 mm was fixed at its one end while being held at a predetermined temperature, and a torsion force was applied to the test piece in one direction with an axial force of 0 kgf at a rotational rate of 300 rpm (strain rate: 4.2 sec ⁇ 1 ) until the test piece was ruptured; and the number of rotations until the test piece was ruptured was defied as torsion cycles of this test piece.
  • FIG. 1 a result of the high-temperature torsion test using a test piece of high-Si stainless steel having a chemical composition indicated as the test piece 1 in Table 1 is shown in FIG. 1 in a relation between the heating temperature and the torsion cycles.
  • Test Piece Mass %, Remainder: Fe and Impurities
  • No. C Si Mn Al P S Cr Ni N Nb + Ta Ti Zr Test Piece 1 0.03 4.12 1.2 0.006 0.014 0.010 17.8 14.05 0.009 0.70
  • Test Piece 2 0.14 4.33 1.0 0.002 0.019 0.004 17.0 13.98 0.017 0.58
  • Test Piece 3 0.12 5.95 0.76 0.011 0.011 0.009 11.1 17.06 0.010 0.70
  • Test Piece 4 0.02 4.16 1.02 0.009 0.014 0.008 17.1 14.12 0.012 0.10 0.51
  • Test Piece 5 0.007 4.03 1.83 0.010 0.016 0.0005 17.7 15.05 0.014 0.11
  • each forged slab After being heated at a predetermined temperature, each forged slab was hot-rolled to have a thickness of 4 mm. Thereafter, scales were removed by pickling, and then the scab occurrence ratio was investigated in the following methods.
  • each steel sheet was segmented into meshes each having a size of 100 ⁇ 100 mm, and a percentage of the number of meshes where scabs occurred relative to total meshes that were investigated was defined as the scab occurrence ratio (%). If the scab occurrence ratio is 5% , only simper treatment may be required prior to the subsequent step.
  • a relation between ⁇ T of the test piece 1 (Table 1) and the scab occurrence ratio is shown in FIG. 2 .
  • the scab occurrence ratio becomes 5% if the heating temperature T h during the hot rolling is set to satisfy ⁇ T 30° C. To the contrary, if ⁇ T is less than 30° C., and as ⁇ T becomes closer to the zero ductility temperature, the scab occurrence ratio becomes abruptly increased.
  • the heating temperature T h during the hot rolling such that ⁇ T is at least 30° C., preferably at least 60° C.
  • Time duration of holding the stainless steel at this heating temperature is not limited to a specific one.
  • setting of the heating temperature is carried out for the purpose of preventing scabs from being generated after the hot rolling; and thus it is only required to control the temperature on the surface of the slab.
  • the heating time duration required for this state depends on the dimension of the slab; and generally, it is preferable to set the heating time duration to be at least 60 minutes.
  • the upper limit of ⁇ T is not limited. In a common hot rolling plant, it is possible to carry out the hot rolling if the hot-rolling finishing temperature is at least 700° C. Preferably, this finishing temperature is set to be at least 950° C.
  • the hot rolling may be performed in a single stage or in multiple stages. In the case of using multiple stages, heating may be applied between roll stands if necessary. At this time, the heating temperature is unnecessary to satisfy ⁇ T of at least 30° C., but it may be preferable to set ⁇ T to be at least 30° C. This process refines grain in the surface during the subsequent hot rolling, so that propagation of the cracking hardly occurs, thereby further suppressing the scab occurrence. After the hot rolling, oxide scales on the surface of the rolled material are removed by pickling with a conventional method.
  • the stainless steel sheet manufactured through the hot rolling can be adjusted in mechanical property (ductility, yield strength) by performing heat treatment for annealing; therefore, it is preferable to subject the stainless steel sheet to heat treatment after the hot rolling.
  • Increase in heat treatment temperature improves the ductility, but reduces the yield strength.
  • Slow cooling rate after the heat treatment allows chrome carbide to precipitate, which causes deterioration of the corrosion resistance. Accordingly, the heat treatment temperature and the subsequent cooling rate should be set so as to achieve both the ductility and the yield strength, as well as to prevent sensitization.
  • FIG. 3 shows relations of the heat treatment temperature with 0.2% yield strength, and with ductility of the test piece 1.
  • black solid circles indicate the 0.2% yield strength (MPa)
  • black solid squares indicate the ductility (%).
  • Si-containing austenitic stainless steel suitable for use in a highly-concentrated nitric acid environment at a high temperature without generating scabs in the hot rolling process.
  • Each test piece 1 to 5 having a corresponding chemical composition shown in Table 1 was melted by a high-frequency electric furnace into an ingot of 10 kg, and a slab produced by forging this ingot was heated at a corresponding predetermined temperature shown in Table 2 for 120 minutes, and thereafter was hot-rolled into a steel sheet having a thickness of 4 mm through a two-stage rolling mill. Each obtained stainless steel sheet was pickled to remove scales therefrom, and thereafter, the scab occurrence ratio on a surface of each steel sheet was investigated in the aforementioned methods. Total results of the investigation are shown in Table 2.

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Abstract

A high Si-containing austenitic stainless steel having corrosion resistance in a nitric acid environment at a high temperature is made by hot-rolling a slab of stainless steel and heat treating the hot-rolled stainless steel at a temperature of 1100 to 1160° C. The steel is cooled at cooling rate of at least 100° C./min. The stainless steel has a chemical composition containing: C: at most 0.04%; Cr: 7 to 20%, Ni: 10 to 22%, Si: 2.5 to 7%, Mn: at most 10%, sol. Al: at most 0.03%, P: at most 0.03%, S: at most 0.03%; N: at most 0.035%, a total of one or more of Nb, Ti, Ta, and Zr being 0.05 to 0.7%; and the remainder Fe and impurities. The heating temperature during the hot rolling is Th in which ΔT of Formula (1): Th =1135−90Si−2.9Cr+40 Ni−ΔT is at least 30□C.

Description

    TECHNICAL FIELD
  • The present invention relates to a method for manufacturing austenitic stainless steel exhibiting corrosion resistance to a concentrated nitric acid. More specifically, the present invention relates to a method for manufacturing high Si-containing austenitic stainless steel usable in a concentrated nitric acid environment at a high temperature.
    • BACKGROUND ART
  • Materials employed at a nitric acid production plant are exposed to a highly-concentrated nitric acid environment at a high temperature. In general, stainless steel is used as a corrosion resistant material for the plant. Stainless steel forms a passive film stable in a nitric acid, and exhibits excellent corrosion resistance. The highly-concentrated nitric acid at a high temperature has so strong oxidizability that trans-passive corrosion occurs in common stainless steel. The trans-passive corrosion results in general corrosion due to dissolution of Cr2O3 that forms the passive film, and results in intergranular corrosion in the vicinity of sensitized grain boundaries (having increased susceptibility to intergranular attack).
  • As a corrosion resistant material having even in such an environment, high-Si austenitic stainless steel, such as 17Cr-14Ni-4Si (Patent Document 1) and 11Cr-17Ni-6Si (Patent Document 2), has been known. In such high-Si stainless steel, Si resolved in a trans-passive region due to corrosion is re-oxidized, thereby forming a silicate film, which exhibits excellent corrosion resistance against a nitric acid. One or more types of elements selected from Nb, Ta, Ti, and Zr tend to be added to this high-Si stainless steel. These additive elements have an effect of immobilizing C in steel, and suppressing sensitization. Particularly, they are effective for suppression of sensitization in a welded heat affected zone, and have a significant effect of improving intergranular corrosion resistance in a highly-concentrated nitric acid.
  • A heating temperature of a slab in hot working is preferably as high as possible in view of productivity. In high-Si stainless steel, however, there is a problem of cracking caused in the slab during the hot working if upon heated at a higher temperature than a predetermined temperature during the hot working because Si has low solid solubility relative to an austenite phase, and as the Si content becomes more increased, a brittle phase, such as an intermetallic compound and 6 ferrite, is more likely to be generated at a high temperature, which deteriorates high-temperature ductility. Accordingly, in order to stably manufacture high-Si stainless steel industrially, it is necessary to appropriately control a heating temperature in the hot working.
  • Patent Document 3 discloses a method of hot-rolling or hot-forging an ingot of high-Si stainless steel containing Si of 5 to 8% (in this description, unless otherwise specified, percent with respect to chemical composition means mass percent) within a temperature range of at least 900° C. after soaking in a temperature range that satisfies: 1050 to 1100° C.; and T(° C.)<1470−35×Si−5×Ni (%). With increase in Si content, an intermetallic compound having a low melting point is generated in casting solidification structure, and the intermetallic compound becomes partially fused at a higher soaking temperature, and thus cracking occurs during the hot working. The soaking temperature is determined so as to prevent such cracking.
  • Non Patent Document 1 reports that, regarding a relation between an intermetallic compound and hot workability in high-Si stainless steel (6.5Si-17Cr-22Ni-0.01Pd), (a) an Si—Ni rich intermetallic compound is crystallized in an interdendritic region of a cast structure, and existence of a large amount of such 2 5 crystallization deteriorates the hot workability, and (b) if an ingot where the intermetallic compound is crystallized is soaked at a temperature of 1000 to 1150° C., the intermetallic compound is partially melted at a temperature of 1150° C., and cracking occurs; but soaking at a temperature of 1100° C. exhibits no fusion of the intermetallic compound, so that the cracking in the hot working can be prevented by dissolving the intermetallic compound.
  • Specifically, in the high-Si stainless steel of Non Patent Document 1, it is estimated that the Ni—Si based intermetallic compound having a low fusion point becomes partially melted at a temperature of more than 1100° C., and propagation of cracking along grain boundaries causes cracking, and thus the heating temperature in the hot working is substantially controlled to be at most 1100° C.
  • In Patent Document 4 discloses that soaks at a temperature of at least 1100° C. and at most 1250° C. for at least two hours a slab of high-Si stainless steel containing Si of 4 to 10% in which S and O are restricted to be at most 30 ppm, and then hot-rolls this steel; finishes the hot rolling at a temperature of at least 950° C.; and subjects this steel to solution heat treatment at a temperature of at least 1000° C. and at most 1200° C. Patent Document 4 discloses that (1) impurity elements such as S and 0, and (2) an intermetallic compound that precipitates during cooling a slab affect high-temperature ductility of high-Si austenitic stainless steel, and also discloses that the intermetallic compound is removed by reduction of S and O, and by soaking of the slab, thereby improving hot workability. A composition of this intermetallic compound is not clearly described, but it is estimated that this is an Ni—Si intermetallic compound having a low melting point as similar to that of Non-Patent Document 1.
  • Techniques disclosed in Patent Document 3 and Patent Document 4 improve hot workability by setting a heating temperature to be not more than a fusing temperature of the Ni—Si intermetallic compound. In high-Si austenitic stainless steel used in a highly-concentrated nitric acid environment at a high-temperature, a great deal of Si thereof deteriorates solid solubility of C, so that sensitization likely occurs, and consequently, intergranular corrosion resistance in the highly-concentrated nitric acid is poor.
  • Each high-Si stainless steel disclosed in Patent Document 1 and Patent Document 2 contains Nb, Ta, Ti, and Zr so as to suppress sensitization, and to greatly improve the nitric acid corrosion resistance, but this brings up another problem of surface defects called as scab that is likely to be generated in the hot rolling process.
  • The cause of this is unclear, and the surface defects tend to be reduced by controlling a heating temperature of a slab lower, but it may be difficult to sufficiently achieve this reduction effect depending on the chemical composition of the steel in some cases; therefore, it is required to heat the slab at an unnecessarily low temperature, or to provide treatment such as cutting for removing the scab after the hot rolling, which causes significant increase in cost.
    • PRIOR ART DOCUMENTS
  • Patent Document 1: Japanese Patent Publication No. 3237132
  • Patent Document 2: Japanese Patent Publication No. 1119398
  • Patent Document 3: Japanese Patent Laid-Open No. 6-93389
  • Patent Document 4: Japanese Patent Laid-Open No. 5-51633
    • Non Patent Document
  • Non Patent Document 1: NKK Technical Report, No. 154, 1996, pp. 14-19
    • SUMMARY OF INVENTION
  • An object of the present invention is to ensure manufacturing high-Si containing austenitic stainless steel having corrosion resistance suitable for use in a highly-concentrated nitric acid environment at a high temperature without generating scab in a hot rolling process.
  • The present inventors have conducted studies on conditions to ensure manufacturing high Si-containing austenitic stainless steel (austenitic stainless steel is also referred to simply as “stainless steel”, hereinafter) suitable for use in a highly-concentrated nitric acid environment at a high temperature without generating fractures in a hot rolling process, and as a result, the following (i) to (iii) have been found.
  • (i) Upon manufacturing stainless steel containing a large amount of Si, an Ni—Si intermetallic compound is produced. As disclosed in Non Patent Document 1, its melting point is estimated to be within a range from 1100 to 1150° C., and this intermetallic compound causes great slab cracking that hinders hot rolling.
  • (ii) If stainless steel having a great content of Si contains Nb, Ta, Ti, Zr, and the like, Ni—Si—X (X═Nb, Ti, Zr) is produced as the intermetallic compound. Its melting point is within a range of approximately 1150 to 1200° C., and this is approximately 1160° C. for Ni—Si—Nb based on a calculated result of a state diagram calculation thereof, for example. Since Nb, Ta, Ti, Zr and the like are elements that hardly segregate in the steel, an Ni—Si—X ternary system (X═Nb, Ta, Ti, Zr) intermetallic compound is finely dispersed. Because the Ni—Si—X intermetallic compound is finely dispersed at a high melting point, this causes no slab cracking great enough to hinder the rolling.
  • (iii) Despite the above, if cracking occurs in the vicinity of the slab surface from the Ni—Si—X intermetallic compound as a starting point, this cracking propagates to the surface, and the inside of the cracking is oxidized, resulting in generation of a number of scabs. Because the Ni—Si—X intermetallic compound is finely dispersed, the amount thereof is so great that a large number of scabs occur.
  • Based on the above result, it has been found that the scabs in the hot rolling process are generated by the aforementioned Ni—Si—X ternary system intermetallic compound as a starting point, and occur by propagation of the cracking to the surface. Containing Si and X elements is essential for corrosion prevention in the highly-concentrated nitric environment, and thus a method of suppressing the above described propagation of the cracking in the vicinity of the surface have been studied. In general, if ductility is deteriorated with excessively high temperature, the cracking likely propagate; therefore, a relation between the composition and the ductility of the steel has been studied. As a result, the following findings have been obtained.
  • (A) Defects (scabs) of the product surface can be prevented by controlling the heating temperature during the hot rolling based on the relation among the contents of Si, Cr, and Ni in the chemical composition of the steel.
  • (B) Sensitization can be suppressed as well as achieving ductility and yield strength by controlling a temperature range and a cooling method of finishing annealing after the rolling. The present invention based on the above findings is a method for manufacturing austenitic stainless steel which heats and hot-rolls a slab of stainless steel at a heating temperature Th during the hot rolling, wherein the slab of stainless steel includes a chemical composition containing C: 0.04% or less; Cr: 7 to 20%, Ni: 10 to 22%, Si: 2.5 to 7%, Mn: 10% or less, sol. Al: 0.03% or less, P: 0.03% or less, S: 0.03% or less; N: 0.035% or less, a sum of one or more types of elements selected from Nb, Ti, Ta, and Zr: 0.05 to 0.7%; and the balance being Fe and impurities, and the heating temperature Th is defined as Th, in which ΔT of Formula (1): Th=1135−90Si−2.9Cr+40 Ni−ΔT is 30° C. or more.
  • In a preferable aspect of the present invention, the method according to the present invention further includes subjecting the hot-rolled austenitic stainless steel to heat treatment within a temperature range of 1100 to 1160° C., and thereafter, cooling this austenitic stainless steel at cooling rate of 100° C./min. or more.
  • According to the present invention, it is possible to securely manufacture high Si-containing austenitic stainless steel suitable for use in a highly-concentrated nitric acid environment at a high temperature without generating scabs in the hot rolling process.
    • BRIEF DESCRIPTION OF DRAWINGS
  • FIG. 1 is a graph showing a test result of a torsion test on a test piece 1.
  • FIG. 2 is a graph showing a relation between ΔT and a scab occurrence ratio in the test piece 1.
  • FIG. 3 is a graph showing relations of a heat treatment temperature after hot rolling with 0.2% yield strength, and with ductility in the test piece 1.
    •  DESCRIPTION OF EMBODIMENT
  • The method for manufacturing austenitic stainless steel according to the present invention will be explained in greater detail while referring to the attached drawings. As described above, “%” relating to the chemical composition of steel means mass%. The remainder of the chemical composition of the steel includes Fe and impurities.
    • [Chemical Composition of Steel]
  • [C: at most 0.04%]
  • C is an element for increasing strength of steel, but producing Cr carbide at grain boundaries in a welded heat affected zone, which causes sensitization, and deteriorates corrosion resistance. Accordingly, the C content is controlled to be at most 0.04%. The C content is preferably at most 0.03%, and more preferably at most 0.02%.
    • [Cr: 7 to 20%]
  • Cr is a basic element for improving the corrosion resistance of the stainless steel, and the Cr content is controlled to be at least 7% and at most 20%. The Cr content of less than 7% cannot achieve adequate corrosion resistance. On the other hand, the excessive Cr content produces a two-phase structure in which a large amount of ferrite precipitates under the coexistence of Si and Nb, which causes deterioration of workability and impact resistance; and thus the upper limit of the Cr content is controlled to be 20%. The lower limit of the Cr content is preferably 10%, and more preferably 11%. The upper limit of the Cr content is preferably 19%, and more preferably 18%.
    • [Ni: 10 to 22%]
  • Ni is a stabilizing element of the austenite phase, and also has an effect of increasing the zero ductility temperature. The Ni content is controlled to be at least 10% and at most 22% or less. The Ni content of less than 10% cannot achieve desired corrosion resistance and toughness. The Ni content of more than 22% causes significant increase in cost. The lower limit of the Ni content is preferably 12%, and more preferably 13%. The upper limit of the Ni content is preferably 20%, and more preferably 16%.
    • [Si: 2.5 to 7%]
  • Si is contained at a content of at least 2.5% and at most 7% for the purpose of increasing the corrosion resistance in a concentrated nitric acid. Si is contained at a content of at least 2.5% so as to form a silicate film for achieving the corrosion resistance in the nitric acid. An excessive Si content decreases the zero ductility temperature. This excessive content not only increases the cost but also deteriorates weldability; therefore, the upper limit of the Si content is controlled to be 7%. The lower limit of the Si content is preferably 3.0%, and more preferably 3.5%. The upper limit of the Si content is preferably 6%, and more preferably 5%.
    • [Mn: at Most 10%]
  • Mn is an austenite stabilizing element, and is contained as a deoxidizing agent; therefore, the Mn content is controlled to be at most 10%. The Mn content of more than 10% causes deterioration of the corrosion resistance, hot cracking upon welding, as well as deterioration of workability. The upper limit of the Mn content is preferably 6%, and more preferably 4%. In order to securely achieve the above effect of Mn, the Mn content is preferably at least 0.5%, and more preferably at least 1.0%.
  • [sol. Al: at Most 0.03%]
  • Al is contained as a deoxidizing agent in the steel, but produces a toxic inclusion if Al is excessively contained; therefore, the sol. Al content is controlled to be at most 0.03%.
    • [P: at Most 0.03%, S: at Most 0.03%]
  • P and S are both undesirable elements for the corrosion resistance and the weldability, and each content thereof is preferably as small as possible. The P content is controlled to be at most 0.03%, and the S content is controlled to be at most 0.03%.
    • [N: at Most 0.035%]
  • N has a high affinity to Nb, Ti, Ta, and Zr, and hinders immobility of C by these elements; and thus the N content is preferably as small as possible. The N content is controlled to be at most 0.035%.
  • [Total Amount of one or More Types of Elements Selected from Nb, Ti, Ta, and Zr: 0.05% to 0.7%]
  • All of Nb, Ti, Ta, and Zr have an effect of immobilizing C, and suppressing deterioration of intergranular corrosion resistance caused by sensitization, and they are particularly effective for improving the corrosion resistance in the welded heat affected zone. A total amount of these elements of less than 0.05% cannot achieve an effect of improving the intergranular corrosion resistance, and increases hot-working cracking caused by formation of a low melting point Ni—Si based intermetallic compound. On the other hand, the total amount of these elements of more than 0.7% deteriorates the workability. Accordingly, the total amount of one or more types of these elements is controlled to be at least 0.05% and at most 0.7%.
    • [Manufacturing Condition]
  • The method for manufacturing the austenitic stainless steel according to the present invention includes: a hot rolling step of heating and hot-rolling a slab of stainless steel having the aforementioned chemical composition at a heating temperature Th during the hot rolling, wherein the heating temperature Th is defined as Th in which ΔT of Formula (1): Th=1135−90Si−2.9Cr+40 Ni−ΔT is at least 30° C.; and a heat treatment step (annealing step) of preferably further subjecting the stainless steel to heat treatment within a temperature range of 1100 to 1160° C., and thereafter, cooling the stainless steel at cooling rate of at least 100° C./min.
    • [Hot Rolling Step]
  • For the purpose of finding an optimum heating temperature range for the hot rolling, a relation between the chemical composition and a high-temperature deformability has been studied through a high-temperature torsion test. Through this test, the zero ductility in the hot rolling can be investigated.
  • In the hot torsion test, each test piece having a parallel portion of 8 mm in diameter and a length of 30 mm was fixed at its one end while being held at a predetermined temperature, and a torsion force was applied to the test piece in one direction with an axial force of 0 kgf at a rotational rate of 300 rpm (strain rate: 4.2 sec−1) until the test piece was ruptured; and the number of rotations until the test piece was ruptured was defied as torsion cycles of this test piece.
  • As an example, a result of the high-temperature torsion test using a test piece of high-Si stainless steel having a chemical composition indicated as the test piece 1 in Table 1 is shown in FIG. 1 in a relation between the heating temperature and the torsion cycles.
  • TABLE 1
    Chemical Composition of Test Piece (Mass %, Remainder: Fe and Impurities)
    No. C Si Mn Al P S Cr Ni N Nb + Ta Ti Zr
    Test Piece 1 0.03 4.12 1.2 0.006 0.014 0.010 17.8 14.05 0.009 0.70
    Test Piece 2 0.14 4.33 1.0 0.002 0.019 0.004 17.0 13.98 0.017 0.58
    Test Piece 3 0.12 5.95 0.76 0.011 0.011 0.009 11.1 17.06 0.010 0.70
    Test Piece 4 0.02 4.16 1.02 0.009 0.014 0.008 17.1 14.12 0.012 0.10 0.51
    Test Piece 5 0.007 4.03 1.83 0.010 0.016 0.0005 17.7 15.05 0.014 0.11
  • In FIG. 1, a maximum value of the torsion cycles appears at approximately 1100° C., the torsion cycles greatly drop at a temperature more than 1100° C., and the test piece was ruptured at the beginning of applying torsion at 1275° C. This shows that the temperature where the high-Si stainless steel shown as the test piece 1 in Table 1 has zero ductility is approximately 1275° C. (referred to as a “zero ductility temperature”, hereinafter).
  • Each of high-Si stainless steel having various chemical composition containing one or more types of elements selected from Nb, Ta, Ti, and Zr was subjected to the above high-temperature torsion test in the same way so as to investigate the zero ductility temperature thereof. As a result, it was found that the zero ductility temperature (T0) can be expressed by the following regression equation (2) as a relation of the zero ductility temperature (T0) with concentrations of Si, Cr, and Ni.

  • T 0=1135−90Si−2.9Cr+40 Ni   (2)
  • The heating temperature (Th) in the hot working is set to be less than the zero ductility temperature (T0) by at least 30° C., that is, the heating temperature (Th) is set to be a temperature in which ΔT of Formula (1): Th=1135−90Si−2.9Cr+40 Ni−ΔT is at least 30° C. so that cracking generated from an Ni—Si—X ternary system (X═Nb, Ta, Ti, Zr) intermetallic compound as a starting point hardly occurs, and scabs are reduced. Reducing scabs provides simpler surface treatment prior to a subsequent step, which is excellent in economy.
  • After being heated at a predetermined temperature, each forged slab was hot-rolled to have a thickness of 4 mm. Thereafter, scales were removed by pickling, and then the scab occurrence ratio was investigated in the following methods.
  • A surface of each steel sheet was segmented into meshes each having a size of 100×100 mm, and a percentage of the number of meshes where scabs occurred relative to total meshes that were investigated was defined as the scab occurrence ratio (%). If the scab occurrence ratio is 5% , only simper treatment may be required prior to the subsequent step. With the chemical composition of the test piece 1 in Table 1, the zero ductility temperature T0=1275° C. was obtained by Formula (2). A relation between ΔT of the test piece 1 (Table 1) and the scab occurrence ratio is shown in FIG. 2.
  • As shown in a graph of FIG. 2, the scab occurrence ratio becomes 5% if the heating temperature Th during the hot rolling is set to satisfy ΔT 30° C. To the contrary, if ΔT is less than 30° C., and as ΔT becomes closer to the zero ductility temperature, the scab occurrence ratio becomes abruptly increased.
  • Accordingly, in order to minimize the scab occurrence, it is preferable to set the heating temperature Th during the hot rolling such that ΔT is at least 30° C., preferably at least 60° C. Time duration of holding the stainless steel at this heating temperature is not limited to a specific one. In the present invention, setting of the heating temperature is carried out for the purpose of preventing scabs from being generated after the hot rolling; and thus it is only required to control the temperature on the surface of the slab. In order to prevent hindrance in the hot rolling, it is preferable to heat the slab until its central portion substantially has a uniform temperature. The heating time duration required for this state depends on the dimension of the slab; and generally, it is preferable to set the heating time duration to be at least 60 minutes.
  • The upper limit of ΔT is not limited. In a common hot rolling plant, it is possible to carry out the hot rolling if the hot-rolling finishing temperature is at least 700° C. Preferably, this finishing temperature is set to be at least 950° C.
  • The hot rolling may be performed in a single stage or in multiple stages. In the case of using multiple stages, heating may be applied between roll stands if necessary. At this time, the heating temperature is unnecessary to satisfy ΔT of at least 30° C., but it may be preferable to set ΔT to be at least 30° C. This process refines grain in the surface during the subsequent hot rolling, so that propagation of the cracking hardly occurs, thereby further suppressing the scab occurrence. After the hot rolling, oxide scales on the surface of the rolled material are removed by pickling with a conventional method.
    • [Heat Treatment Step]
  • The stainless steel sheet manufactured through the hot rolling can be adjusted in mechanical property (ductility, yield strength) by performing heat treatment for annealing; therefore, it is preferable to subject the stainless steel sheet to heat treatment after the hot rolling. Increase in heat treatment temperature improves the ductility, but reduces the yield strength. Slow cooling rate after the heat treatment allows chrome carbide to precipitate, which causes deterioration of the corrosion resistance. Accordingly, the heat treatment temperature and the subsequent cooling rate should be set so as to achieve both the ductility and the yield strength, as well as to prevent sensitization.
  • FIG. 3 shows relations of the heat treatment temperature with 0.2% yield strength, and with ductility of the test piece 1. In a graph of FIG. 3, black solid circles indicate the 0.2% yield strength (MPa), and black solid squares indicate the ductility (%).
  • As shown in FIG. 3, through the heat treatment at a temperature of at least 1100° C. and at least 1160° C., it is possible to achieve the stainless steel having preferable ductility and sufficient yield strength, specifically, ductility of 50 to 53%, and 0.2% yield strength of 325 to 290 MPa.
  • Slow cooling rate after the heat treatment causes sensitization, and increases susceptibility to intergranular corrosion. It is possible to achieve the stainless steel causing no sensitization and exhibiting preferable nitric acid resistance by setting the cooling rate to be at least 100° C./min. According to the present invention, it is possible to surely manufacture high
  • Si-containing austenitic stainless steel suitable for use in a highly-concentrated nitric acid environment at a high temperature without generating scabs in the hot rolling process.
    • EXAMPLE
  • Each test piece 1 to 5 having a corresponding chemical composition shown in Table 1 was melted by a high-frequency electric furnace into an ingot of 10 kg, and a slab produced by forging this ingot was heated at a corresponding predetermined temperature shown in Table 2 for 120 minutes, and thereafter was hot-rolled into a steel sheet having a thickness of 4 mm through a two-stage rolling mill. Each obtained stainless steel sheet was pickled to remove scales therefrom, and thereafter, the scab occurrence ratio on a surface of each steel sheet was investigated in the aforementioned methods. Total results of the investigation are shown in Table 2.
  • TABLE 2
    Heating Temperature
    Test Piece during Hot Rolling (° C.)
    No. T0 (° C.) 1300 1250 1200 1150
    Test Piece 1 1275 X Δ ◯ ◯
    Test Piece 2 1255 X X ◯ ◯
    Test Piece 3 1250 X X ◯ ◯
    Test Piece 4 1276 X Δ ◯ ◯
    Test Piece 5 1323 Δ ◯ ◯ ◯
    Scab Occurrence Ratio
    X: at least 20%,
    Δ: more than 5% to less than 20%,
    ◯: at most 5%
  • As shown in Table 2, all test piece having the heating temperature during the hot rolling less than the zero ductility temperature T0 (° C.), which was calculated based on the chemical composition, by at least 30° C. had a scab occurrence ratio of 5% . To the contrary, every test piece having a heating temperature during the hot rolling more than a temperature less than the zero ductility temperature T0 (° C.), which is calculated based on the chemical composition, by 30° C. had a scab occurrence ratio of more than 5%; therefore, it was not possible to ensure manufacturing high Si-containing austenitic stainless steel suitable for use in a highly-concentrated nitric acid environment at a high temperature without generating scabs in the hot rolling process.

Claims (2)

1. A method for manufacturing austenitic stainless steel comprising
a hot rolling step of heating and hot-rolling a slab of stainless steel at a heating temperature Th during the hot rolling
wherein
the slab of stainless steel has a chemical composition containing: by mass %, C: at most 0.04%; Cr: 7 to 20%, Ni: 10 to 22%, Si: 2.5 to 7%, Mn: at most 10%, sol. Al: at most 0.03%, P: at most 0.03%, S: a at most 0.03%; N: at most 0.035%, a total amount of one or more types of elements selected from Nb, Ti, Ta, and Zr: 0.05 to 0.7%; and the remainder being Fe and impurities,
and
the heating temperature Th is defined as Th in which ΔT of Formula (1): Th=1135−90Si−2.9Cr+40Ni−ΔT is at least 30° C. or more.
2. The method for manufacturing austenitic stainless steel according to claim 1, further comprising
a heat treatment step of subjecting the austenitic stainless steel obtained through the hot rolling to heat treatment within a temperature range of 1100 to 1160° C., and thereafter, cooling the austenitic stainless steel at cooling rate of at least 100° C./min.
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WO2013018628A1 (en) 2013-02-07
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