CN110225988B - Hot rolled ferritic stainless steel sheet and method for producing same - Google Patents

Hot rolled ferritic stainless steel sheet and method for producing same Download PDF

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CN110225988B
CN110225988B CN201880008313.8A CN201880008313A CN110225988B CN 110225988 B CN110225988 B CN 110225988B CN 201880008313 A CN201880008313 A CN 201880008313A CN 110225988 B CN110225988 B CN 110225988B
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CN110225988A (en
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吉野正崇
藤泽光幸
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JFE Steel Corp
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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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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
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    • 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
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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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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    • C21D8/00—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
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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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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    • 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
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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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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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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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

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  • Heat Treatment Of Steel (AREA)

Abstract

The invention provides a hot-rolled ferritic stainless steel sheet having sufficient corrosion resistance and capable of suppressing cracking during punching into a thick flange, and a method for producing the same. The hot-rolled ferritic stainless steel sheet has a composition containing, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001-0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, Nb: 0.12-0.80%, the balance of Fe and inevitable impurities, and a critical stress intensity factor KICIs 25 MPa.m1/2The above.

Description

Hot rolled ferritic stainless steel sheet and method for producing same
Technical Field
The present invention relates to a hot-rolled ferritic stainless steel sheet having excellent punchability suitable for forming a flange or the like, and a method for producing the same.
Background
In recent years, the legislation relating to automobile exhaust gas has been strengthened, and it is urgent to improve fuel efficiency. Therefore, the application of an Exhaust Gas Recirculation (EGR) system that uses Exhaust Gas generated by an automobile engine as intake air of the engine again is being promoted. Exhaust gas produced by the engine is supplied to the engine again after passing through an EGR cooler for reducing the temperature of the gas. When the exhaust gas is circulated, the exhaust system components are fastened via a flange to prevent gas leakage. The flange plate used in such exhaust system components needs to have sufficient rigidity. Therefore, a thick flange (for example, 5mm or more in thickness) has been used for such an exhaust system member.
Conventionally, a thick-walled flange has been made of a common steel. However, sufficient corrosion resistance is required for a flange used for a member that passes high-temperature exhaust gas, such as an EGR system. Therefore, application of stainless steel having excellent corrosion resistance as compared with ordinary steel, particularly ferritic stainless steel having a small thermal expansion coefficient and hardly generating thermal stress has been studied, and ferritic stainless steel sheets having a large plate thickness (for example, a plate thickness of 5mm or more) which can be used for thick flanges have been strongly required.
In response to such a market demand, for example, patent document 1 discloses an Nb-containing ferritic stainless steel hot-rolled coil having a thickness of 5.0 to 10.0mm, which has a composition consisting of, in mass%, C: 0.030% or less, Si: 2.00% or less, Mn: 2.00% or less, P: 0.050% or less, S: 0.040% or less, Cr: 10.00-25.00%, N: 0.030% or less, Nb: 0.01-0.80%, the balance being Fe and inevitable impurities, the hardness being adjusted to 190HV or less, and the Charpy impact value at 25 ℃ being adjusted to 20J/cm2The above.
Documents of the prior art
Patent document
Patent document 1: japanese laid-open patent publication No. 2012 and 140688
Disclosure of Invention
However, the inventors of the present invention have found that, although the hot rolled coil stock of ferritic stainless steel described in patent document 1 is punched out into a thick flange shape by a crank press, although the charpy impact value is sufficient, cracks are generated in the thickness center portion of the punched portion, and a predetermined flange shape cannot be obtained, and thus the hot rolled coil stock is not sufficiently applicable to a thick flange. In order to obtain the hot-rolled coil disclosed in patent document 1, it is necessary to immerse the coil in water and hold the coil for 15 minutes or more after the completion of the coiling in the hot rolling, and there is also a problem in productivity and productivity.
The invention aims to: the problem is solved, and a ferritic stainless hot-rolled steel sheet having sufficient corrosion resistance and capable of suppressing cracking in punching of a flange formed into a thick wall by a crank press and a method for manufacturing the same are provided.
The present inventors have conducted detailed studies to solve the problems and found that in order to punch a thick flange plate without generating cracks by a machining method with a high machining speed such as a crank press, the critical Stress Intensity Factor (K) K of a steel plate is setICIt is enlarged. Specifically, it was found that by making the critical stress intensity factor KICIs 25 MPa.m1/2As described above, even in a processing method such as a crank press having a high processing speed, it is possible to effectively suppress the occurrence of cracks in the punched end surface portion when the thick flange is punched out, and it is possible to sufficiently apply the method to the thick flange.
The present inventors have conducted detailed studies to solve the problems. As a result, they found that: when a thick steel plate having a plate thickness of more than 5.0mm is punched out into a thick flange without generating cracks by a machining method having a high machining speed such as a crank press, the workability of the steel plate cannot be accurately evaluated by the charpy impact value which has been conventionally used, but the workability is evaluated by a critical Stress Intensity Factor (K) K which is an index for evaluating the toughness in the thick plate fieldICAccurate evaluation can be performed. This is considered to be because, in a thin steel sheet having a thickness of less than 5.0mm, since the plastic deformation region in the vicinity of the punched end face portion at the time of processing is large relative to the sheet thickness, the fracture phenomenon accompanying the forming cannot be completely managed by the processing of fracture mechanics, while, in a thick steel sheet having a thickness of 5.0mm or more, the small-scale yield state in which the plastic deformation region in the vicinity of the punched end face portion at the time of processing becomes sufficiently small relative to the sheet thickness is sufficiently satisfied, and therefore, the fracture phenomenon accompanying the predetermined processing can be processed by the stress intensity factor which is a quantitative index of fracture mechanics, and particularly, the critical stress intensity factor K which is a critical value thereof can be usedICAnd (6) carrying out accurate evaluation.
Thus, the inventors of the present invention have found that the occurrence of cracks and the critical stress intensity factor K are not observed when a flange having a predetermined shape is punched out by a crank pressICThe relationship (2) was investigated in detail. As a result, the following findings were obtained: by making the critical stress intensity factor KICIs 25 MPa.m1/2As described above, the occurrence of cracks in the punched end surface portion when the thick flange is punched out by the crank press can be effectively suppressed, and the present invention can be sufficiently applied to the thick flange.
Further, the following findings are obtained: by appropriately controlling the final 3-pass accumulated reduction ratio (═ 100- (final plate thickness/plate thickness before starting of final 3-pass rolling) × 100 [% ] in the hot finishing (slab rolling) step composed of 3 or more passes on the ferritic stainless steel of an appropriate composition, in particular, in the hot finishing (slab rolling) step]) Thereby making the critical stress intensity factor K of the hot rolled steel sheetICAnd (4) improving.
The present invention has been made based on the above findings, and the gist thereof is as follows.
[1]A hot-rolled ferritic stainless steel sheet having the following composition containing, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001-0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, Nb: 0.12-0.80%, the balance of Fe and inevitable impurities, and a critical stress intensity factor KICIs 25 MPa.m1/2The above.
[2]A hot-rolled ferritic stainless steel sheet having the following composition containing, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001-0.50%, N: 0.001 to 0.020%, Cr: 13.0 to 24.0%, Ni: 0.01 to 0.60%, Nb: 0.12-0.80%, the balance of Fe and inevitable impurities, and a critical stress intensity factor KICIs 25 MPa.m1/2The above.
[3] The hot-rolled ferritic stainless steel sheet according to the above [1] or [2], further comprising, as a component composition, in mass%, a metal selected from the group consisting of Cu: 0.01 to 1.50%, Mo: 0.01-2.00%, W: 0.01-0.20%, Co: 0.01-0.20% of 1 or more than 2.
[4] The hot-rolled ferritic stainless steel sheet according to any one of the above [1] to [3], further comprising, as a component composition, by mass%, a component selected from the group consisting of Ti: 0.01-0.30%, V: 0.01 to 0.20%, Zr: 0.01-0.20%, REM: 0.001-0.100%, B: 0.0002-0.0025%, Mg: 0.0005 to 0.0030%, Ca: 0.0005-0.0030% of 1 or more than 2.
[5] A method for producing a hot-rolled ferritic stainless steel sheet according to any one of the above items [1] to [4], wherein in a hot rolling step of performing a finish rolling of 3 or more passes, the final 3 passes of the finish rolling are performed at a temperature range of 800 to 1100 ℃, and the cumulative reduction of the final 3 passes is 25% or more.
Here, the critical stress intensity factor KICThe stress intensity factor is obtained by taking a CT test piece in accordance with ASTM E399 from the center of the width of the plate so that the fatigue pre-crack is in the rolling orthogonal direction and the stress axis is in the rolling parallel direction, and performing a test in accordance with ASTM E399.
According to the present invention, a hot-rolled ferritic stainless steel sheet having sufficient corrosion resistance and excellent toughness capable of suppressing cracking during punching of a flange formed into a thick wall by a crank press is obtained.
The sufficient corrosion resistance in the present invention means: after the surface to be evaluated was polished with #600 sandpaper, the steel sheet with the end faces sealed was subjected to a salt spray cycle test (test in which (salt spray (5 mass% NaCl, 35 ℃, spray 2hr) → dry (60 ℃, 4hr, relative humidity 40%) → wet (50 ℃, 2hr, relative humidity ≧ 95%)) was set to 1 cycle) prescribed in JIS H8502 for 5 cycles, and the rust area ratio (rust area/total area of steel sheet × 100 [% ] on the evaluation surface of the steel sheet was 25% or less.
Further, the steel plate is excellent in toughness capable of suppressing cracks in punching of a thick flange formed by a crank pressThe method comprises the following steps: a critical stress intensity factor K obtained by sampling a CT test piece according to ASTM E399 from the center of the width of the plate so that the fatigue pre-crack is in the rolling orthogonal direction and the stress axis is in the rolling parallel direction, and testing the CT test piece according to ASTM E399ICIs 25 MPa.m1/2The above.
Detailed Description
The hot-rolled ferritic stainless steel sheet according to the present invention has a composition containing, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001-0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, Nb: 0.12-0.80%, the balance of Fe and inevitable impurities, and a critical stress intensity factor KICIs 25 MPa.m1/2The above.
In a preferred embodiment, the hot-rolled ferritic stainless steel sheet according to the present invention has a composition containing, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001-0.50%, N: 0.001 to 0.020%, Cr: 13.0 to 24.0%, Ni: 0.01 to 0.60%, Nb: 0.12-0.80%, the balance of Fe and inevitable impurities, and a critical stress intensity factor KICIs 25 MPa.m1/2The above.
Critical stress intensity factor KICThe stress intensity factor is obtained by taking a CT test piece in accordance with ASTM E399 from the center of the width of the plate so that the fatigue pre-crack is in the rolling orthogonal direction and the stress axis is in the rolling parallel direction, and performing a test in accordance with ASTM E399.
The present invention will be described in detail below.
The present inventors have conducted detailed studies on the cause of occurrence of cracks when a flange having a shape with a hole portion having a diameter of 30mm is punched out by a crank press using various hot rolled ferritic stainless steel sheets having a thickness of 5.0 mm. As a result, they found that: in the above-described steel sheet in which cracks have occurred, micro cracks are generated in the direction perpendicular to the punching direction in the vicinity of the plate thickness center portion of the punched end face portion, and the cracks develop and are generated.
The present inventors have studied in detail the relationship between the generation and development of the micro cracks and the material characteristics. As a result, it was found that as the critical stress intensity factor of the steel sheet decreases, micro cracks tend to easily develop. Therefore, it was found that the critical stress intensity factor obtained by a predetermined measurement method was 25MPa · m when the flange was punched out using various hot rolled ferritic stainless steel sheets (5.0 mm thick)1/2The above steel sheet does not cause cracks and is less than 25MPa · m1/2The steel sheet of (3) is likely to have cracks.
Therefore, the present inventors have conducted detailed investigations on steel components and hot rolling conditions in order to investigate a method for improving the critical stress intensity factor in a hot-rolled ferritic stainless steel sheet. As a result, they found that the cumulative reduction ratio (100- (final plate thickness/plate thickness before starting rolling of the final 3 passes) × 100 [% ] of the final 3 passes in the temperature range of 800 to 1100 ℃ and the final 3 passes in the hot rolling step of performing finish rolling of multiple passes on ferritic stainless steel having an appropriate composition]) The rolling strain is effectively introduced not only to the surface layer portion but also to the plate thickness center portion by controlling the rolling strain to be 25% or more, and as a result, 25MPa · m is obtained1/2Above critical stress intensity factor KIC。
The thickness of the hot-rolled ferritic stainless steel sheet of the present invention is not particularly limited, but is preferably 5.0mm or more since it is desirable to be applicable to the thickness of a thick flange. The thickness is not particularly limited, but is preferably 15.0mm or less, and more preferably 10.0mm or less.
Hereinafter, the method also effectively applies rolling strain to the central portion of the hot rolled steel sheet and the critical stress intensity factor K of the entire thickness of the steel sheetICThe reason for the rise will be explained.
When a steel sheet is rolled, the steel sheet starts to deform and elongate from the surface layer portion. Therefore, when the reduction ratio is small, the amount of deformation of the plate thickness center portion is small, and rolling strain is hardly applied to the plate thickness center portion. In addition, the ferritic system does notThe steel tends to be easily recovered from the working strain during hot rolling. Therefore, in the hot rolling according to the conventional technique, the reduction ratio is insufficient, and the working strain cannot be effectively applied to the thick central portion of the plate. Further, the applied rolling strain is eliminated and reduced by excessive recovery in hot rolling. As a result, a predetermined critical stress intensity factor K cannot be obtained in hot rolling according to the prior artIC。
Therefore, the present inventors have made intensive studies on both the steel composition and the hot rolling method for a method of effectively and sufficiently imparting rolling strain to the central portion of the thickness of a steel sheet in a hot rolling step.
As a result, it was found that, from the viewpoint of the hot rolling method, the final 3 passes of the hot finish rolling are controlled in an appropriate temperature range, and then rolling is performed at a large cumulative reduction ratio, whereby rolling strain is sufficiently and effectively applied to the central portion of the plate thickness.
However, the following findings were obtained: in ferritic stainless steel containing almost no Nb from the viewpoint of steel composition, recovery during hot rolling is likely to occur, and therefore, even when the hot rolling method proposed by the present inventors or the like is used, a sufficient rolling strain density is not obtained, and a predetermined critical stress intensity factor is not obtained.
On the other hand, the following findings were obtained: in a ferritic stainless steel containing a suitable amount of Nb, fine Nb carbonitrides precipitate during hot rolling, and the fine Nb carbonitrides inhibit dislocation movement, so that a high rolling strain density can be obtained and a predetermined critical stress intensity factor can be obtained in a hot-rolled steel sheet by using the hot rolling method proposed by the present inventors.
The following insights are obtained: in the present invention, the final 3 passes of the hot finish rolling are controlled in an appropriate temperature range in a ferritic stainless steel containing an appropriate amount of Nb, and then rolling is performed at a large cumulative reduction ratio, whereby recovery of rolling strain is suppressed, and rolling strain is sufficiently and effectively imparted to the central portion of the sheet thickness, thereby obtaining a predetermined critical stress intensity factor KIC。
Specifically, the following scheme is proposed: the ferritic stainless steel containing 0.12% or more of Nb is hot-rolled under control so that the final 3 passes of a hot finish rolling step consisting of 3 passes or more are in a temperature range of 800 to 1100 ℃ and the cumulative reduction ratio of the final 3 passes (═ 100- (final plate thickness/plate thickness before starting the final 3 passes) x 100 [% ]) is 25% or more.
Next, the composition of the hot rolled ferritic stainless steel sheet according to the present invention will be described.
Hereinafter, unless otherwise specified,% representing the composition of the components means mass%.
C：0.001～0.020％
If C is contained in an amount exceeding 0.020%, the workability and the corrosion resistance of the weld portion are remarkably reduced. From the viewpoint of corrosion resistance and workability, the smaller the C content, the better, but refining takes time to make the C content less than 0.001%, which is not preferable in terms of production. Therefore, the C content is in the range of 0.001 to 0.020%. The C content is preferably 0.003% or more, more preferably 0.004% or more. The C content is preferably 0.015% or less, and more preferably 0.012% or less.
Si：0.05～1.00％
Si is an element that is useful as a deoxidizing element in a steel-making process, and has an effect of enriching an oxide film formed during welding to improve the corrosion resistance of the welded portion. These effects are obtained by containing 0.05% or more of Si, and the effect is increased as the content is increased. However, if Si is contained in an amount of more than 1.00%, it is not preferable because an increase in rolling load and significant formation of scale occur in the hot rolling step, pickling property is reduced due to formation of an Si-concentrated layer on the surface layer of the steel sheet in the annealing step, surface defects are increased, and production cost is increased. Therefore, the Si content is 0.05 to 1.00%. The Si content is preferably 0.10% or more. The Si content is preferably 0.60% or less, and more preferably 0.40% or less.
Mn：0.05～1.00％
Mn has an effect of improving the strength of steel and also has an effect as a deoxidizer. In order to obtain this effect, 0.05% or more of Mn needs to be contained. However, if the Mn content exceeds 1.00%, precipitation of MnS, which becomes a starting point of corrosion, is promoted, and corrosion resistance is lowered. Therefore, the Mn content is 0.05 to 1.00%. The Mn content is preferably 0.10% or more. The Mn content is preferably 0.50% or less, and more preferably 0.30% or less.
P: less than 0.04%
P is an element that is inevitably contained in steel, but is an element that is detrimental to corrosion resistance and workability, and therefore is preferably reduced as much as possible. In particular, if the P content exceeds 0.04%, the workability is significantly reduced by solid solution strengthening. Therefore, the P content is 0.04% or less. The P content is preferably 0.03% or less.
S: less than 0.01%
S and P are also elements that are inevitably contained in steel, but are preferably reduced as much as possible because they are elements that are detrimental to corrosion resistance and workability. In particular, if the S content exceeds 0.01%, the corrosion resistance is significantly reduced. Therefore, the S content is 0.01% or less. The S content is preferably 0.008% or less. More preferably, the S content is 0.003% or less.
Al：0.001～0.50％
Al is an effective deoxidizer. Further, since Al has a higher affinity for N than Cr, when N enters the welded portion, N is precipitated as an Al nitride rather than a Cr nitride, and this has an effect of suppressing sensitization. These effects are obtained by containing 0.001% or more of Al. However, if Al is contained in an amount exceeding 0.50%, the penetration during welding is lowered, and the welding workability is lowered, which is not preferable. Therefore, the Al content is in the range of 0.001 to 0.50%. The Al content is preferably 0.20% or less, more preferably 0.10% or less.
N：0.001～0.020％
If the N content exceeds 0.020%, the reduction in workability and the reduction in corrosion resistance of the weld portion become significant. From the viewpoint of corrosion resistance, it is preferable that the N content is lower, and in order to reduce the N content to less than 0.001%, long-term refining is required, which is not preferable because the production cost increases and the productivity decreases. Therefore, the N content is in the range of 0.001 to 0.020%. The N content is preferably 0.003% or more, and more preferably 0.005% or more. The N content is preferably 0.015% or less, and more preferably 0.012% or less.
Cr：11.0～24.0％
Cr is an element most important for ensuring corrosion resistance of stainless steel. When the content is less than 11.0%, sufficient corrosion resistance in an automobile exhaust gas atmosphere cannot be obtained. On the other hand, when Cr is contained in an amount exceeding 24.0%, a Sigma (Sigma) phase is formed, which significantly lowers the toughness, and the predetermined critical stress intensity factor cannot be obtained in the present invention. Therefore, the Cr content is in the range of 11.0 to 24.0%. The Cr content is preferably 13.0% or more, more preferably 14.0% or more, further preferably 16.0% or more, and further preferably 17.0% or more. The Cr content is preferably 21.5% or less, more preferably 20.0% or less, and still more preferably 18.5% or less.
Ni：0.01～2.00％
Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses corrosion from proceeding in a corrosive environment in which passive film is not formed and active dissolution occurs. In addition, Ni is a strong austenite forming element, and has the effect of suppressing the formation of ferrite in the welded portion and suppressing sensitization due to the precipitation of Cr carbonitride. This effect is obtained by containing 0.01% or more of Ni, and the effect is higher as the Ni content is higher. However, if the Ni content exceeds 2.00%, not only workability is lowered, but also stress corrosion cracking is likely to occur. Further, since Ni is an expensive element, an increase in the content of Ni is not preferable because it leads to an increase in the manufacturing cost. Therefore, the Ni content is 0.01 to 2.00%. The Ni content is preferably 0.05% or more, and more preferably 0.10% or more. The Ni content is preferably 1.00% or less, more preferably 0.60% or less, still more preferably 0.50% or less, and still more preferably 0.45% or less.
Nb：0.12～0.80％
Nb is bonded to C or N in the hot rolling step and precipitated as Nb carbonitride. The precipitated Nb carbonitride has the effect of pinning the movement of dislocations and suppressing the removal of rolling strain due to recovery from hot rolling. This delays the recovery during hot rolling, and can suppress a decrease in rolling strain density due to the occurrence of excessive recovery. The above-described effect is obtained when 0.12% or more of Nb is contained. However, if the Nb content exceeds 0.80%, Laves phases are formed, which may adversely decrease the toughness and significantly increase the rolling load during hot rolling, making it difficult to apply the hot rolling method provided by the present invention. Therefore, the Nb content is in the range of 0.12 to 0.80%. The Nb content is preferably 0.15% or more, more preferably 0.20% or more. The Nb content is preferably 0.75% or less, and more preferably 0.60% or less.
The present invention is a ferritic stainless steel characterized by containing the above-mentioned essential components and having a remainder made up of Fe and unavoidable impurities. Further, 1 or 2 or more selected from Cu, Mo, W and Co and/or 1 or 2 or more selected from Ti, V, Zr, REM, B, Mg and Ca may be contained in the following ranges as necessary.
Cu：0.01～1.50％
Cu is an element that is particularly effective in improving corrosion resistance of the base material and the welded portion when weakly acidic water droplets are adhered to the base material or the welded portion in an aqueous solution. This effect is obtained by containing 0.01% or more, and the effect is higher as the Cu content is higher. However, if Cu is contained in an amount exceeding 1.50%, hot workability may be deteriorated, and surface defects may be induced. Further, the deoxidized skin after annealing may be difficult. Therefore, when Cu is contained, the Cu content is preferably in the range of 0.01 to 1.50%. The Cu content is more preferably 0.10% or more, and still more preferably 0.30% or more. The Cu content is more preferably 0.60% or less, and still more preferably 0.45% or less.
Mo：0.01～2.00％
Mo is an element that significantly improves the corrosion resistance of stainless steel. The effect is obtained by containing 0.01% or more, and the effect is improved as the content is increased. However, if the Mo content exceeds 2.00%, the rolling load during hot rolling may increase, resulting in a reduction in the manufacturability and an excessive increase in the strength of the steel sheet. Further, Mo is an expensive element, and therefore, a large amount of Mo increases the production cost. Therefore, when Mo is contained, the Mo content is preferably 0.01 to 2.00%. The Mo content is more preferably 0.10% or more. Further, the Mo content is more preferably 1.40% or less. However, since Mo also has an effect of reducing toughness in the steel containing Ti, when 0.15% or more of Ti is contained, the Mo content is preferably 0.30 to 1.40% or less. When 0.15% or more of Ti is contained, the Mo content is more preferably 0.40% or more. When 0.15% or more of Ti is contained, the Mo content is more preferably 0.90% or less.
W：0.01～0.20％
W has an effect of improving corrosion resistance as in Mo. This effect is obtained by containing 0.01% or more of W. However, if W is contained in an amount exceeding 0.20%, the strength may be increased, and the productivity may be decreased due to an increase in rolling load. Therefore, when W is contained, the W content is preferably in the range of 0.01 to 0.20%. The W content is more preferably 0.05% or more. Further, the W content is more preferably 0.15% or less.
Co：0.01～0.20％
Co is an element that improves toughness. This effect is obtained by containing 0.01% or more of Co. On the other hand, if the Co content exceeds 0.20%, the workability may be lowered. Therefore, when Co is contained, the Co content is preferably in the range of 0.01 to 0.20%.
Ti：0.01～0.30％
Ti is an element having higher affinity with C and N than Cr, precipitates as carbide or nitride, and has an effect of suppressing sensitization due to precipitation of Cr carbonitride. In order to obtain this effect, it is necessary to contain 0.01% or more of Ti. However, if the Ti content exceeds 0.30%, good surface properties may not be obtained due to excessive precipitation of TiN. Therefore, when Ti is contained, the Ti content is preferably in the range of 0.01 to 0.30%. The Ti content is more preferably 0.03% or more, and still more preferably 0.10% or more. The Ti content is more preferably 0.20% or less, and still more preferably 0.15% or less.
V：0.01～0.20％
V and C, N form carbonitrides, and inhibit sensitization during welding to improve the corrosion resistance of the welded portion. This effect is obtained by setting the V content to 0.01% or more. On the other hand, if the V content exceeds 0.20%, the workability and toughness may be significantly reduced. Therefore, the V content is preferably 0.01 to 0.20%. The V content is more preferably 0.05% or more. Further, the V content is more preferably 0.15% or less.
Zr：0.01～0.20％
Zr binds to C, N to suppress sensitization. This effect is obtained by containing 0.01% or more of Zr. On the other hand, if Zr is contained in an amount exceeding 0.20%, workability may be significantly deteriorated. Therefore, when Zr is contained, the Zr content is preferably in the range of 0.01 to 0.20%. The Zr content is more preferably 0.10% or less.
REM：0.001～0.100％
REM (Rare Earth Metals) has an effect of improving oxidation resistance, and suppresses formation of an oxide film (weld-tempered color) in a weld zone and formation of a Cr-poor region immediately below the oxide film. This effect is obtained by containing REM in an amount of 0.001% or more. On the other hand, if REM is contained in an amount exceeding 0.100%, the productivity such as pickling property in cold rolling annealing may be deteriorated. Therefore, when REM is contained, the REM content is preferably in the range of 0.001 to 0.100%. The REM content is more preferably 0.050% or less.
B：0.0002～0.0025％
B is an element effective for improving the secondary work embrittlement resistance after deep drawing. This effect is obtained by setting the content of B to 0.0002% or more. On the other hand, if B is contained in an amount of more than 0.0025%, workability and toughness may be deteriorated. Therefore, when B is contained, the content of B is preferably in the range of 0.0002 to 0.0025%. The content of B is more preferably 0.0003% or more. Further, the B content is more preferably 0.0006% or less.
Mg：0.0005～0.0030％
Mg is an element effective for improving the equiaxed crystal ratio of a billet and improving the workability and toughness. This effect is obtained by containing 0.0005% or more of Mg. On the other hand, if the Mg content exceeds 0.0030%, the surface properties of the steel may deteriorate. Therefore, when Mg is contained, the content of Mg is preferably in the range of 0.0005 to 0.0030%. The Mg content is more preferably 0.0010% or more. Further, the Mg content is more preferably 0.0020% or less.
Ca：0.0005～0.0030％
Ca is a component effective in refining inclusions generated during smelting and continuous casting, and particularly effective in preventing clogging of a nozzle in continuous casting. The effect is obtained by containing 0.0005% or more of Ca. However, if Ca is contained in an amount exceeding 0.0030%, CaS may be produced to lower the corrosion resistance. Therefore, when Ca is contained, the content of Ca is preferably in the range of 0.0005 to 0.0030%. The Ca content is more preferably 0.0015% or less, and still more preferably 0.0010% or less.
Critical stress intensity factor KIC：25MPa·m1/2The above
The hot rolled ferritic stainless steel sheet according to the present invention can pass the critical stress intensity factor KICIs 25 MPa.m1/2This suppresses cracking during punching of a thick flange formed by a crank press. Critical stress intensity factor KICPreferably 30MPa m1/2More preferably 35MPa · m or more1/2Above, more preferably 40MPa · m1/2The above. The thick flange is not particularly limited, and examples thereof include flanges having a thickness of 5.0mm or more. The flange is preferably 5.0 to 15.0mm thick, and more preferably 5.0 to 10.0mm thick.
Next, a method for producing a hot-rolled ferritic stainless steel sheet according to the present invention will be described. In the following description, unless otherwise specified, the temperature is the surface temperature of a billet, a hot-rolled steel sheet, or the like measured by a surface thermometer or the like.
The hot-rolled ferritic stainless steel sheet according to the present invention is obtained by using a slab having the above composition, and subjecting the final 3 passes of the finish rolling to a temperature range of 800 to 1100 ℃ and a cumulative reduction of 25% or more in the final 3 passes in a hot rolling process comprising rough rolling and 3 or more passes of finish rolling.
First, molten steel having the above composition is melted by a known method such as a converter, an electric furnace, or a vacuum melting furnace, and a billet (billet) is produced by a continuous casting method or an ingot-cogging method.
The billet is heated at 1100 to 1250 ℃ for 1 to 24 hours, or is cast without heating and then directly subjected to hot rolling. In the present invention, the rough rolling is not particularly limited, but when the cast structure is effectively destroyed before the finish hot rolling, it is expected that the grain size in the finish hot rolling after the finish hot rolling will be significantly reduced, and the toughness will be further improved by reducing the metal structure after the hot rolling, and therefore the cumulative reduction in the rough rolling is preferably 65% or more. Thereafter, the steel sheet is hot finish rolled to a predetermined thickness so that the final 3 passes of the finish rolling are rolled at a temperature of 800 to 1100 ℃ to achieve a cumulative reduction of 25% or more.
Rolling temperature ranges of the final 3 passes of the hot finish rolling: 800-1100 deg.C
Cumulative reduction of final 3 passes of hot finish rolling: over 25 percent
In order to obtain a predetermined critical stress intensity factor after hot rolling, it is necessary to effectively apply rolling strain to the plate thickness center portion while suppressing excessive recovery during rolling by appropriately controlling the temperature and the cumulative reduction ratio of the final 3 passes of the finish hot rolling.
In order to impart sufficient rolling strain to the plate thickness center portion, it is necessary to set the rolling temperature of the final 3 passes of the hot finish rolling to a range of 800 to 1100 ℃, and to set the cumulative reduction ratio of the final 3 passes (100 — (final plate thickness/plate thickness before starting of rolling of the final 3 passes) × 100 [% ]) to 25% or more, thereby preventing the rolling strain imparted by the final 3 passes from being canceled by recovery and effectively imparting rolling strain to the plate thickness center portion.
When the cumulative reduction ratio of the final 3 passes of the finish hot rolling is less than 25%, the central portion of the plate thickness cannot be effectively subjected to rolling strain, and therefore a predetermined critical stress intensity factor cannot be obtained. Therefore, the cumulative reduction ratio of the final 3 passes was 25% or more. The cumulative rolling reduction is preferably 30% or more. The cumulative rolling reduction is more preferably 35% or more. The upper limit of the cumulative reduction ratio is not particularly limited, but if the cumulative reduction ratio is made too large, the rolling load may be increased to lower the productivity, and therefore, it is preferably 60% or less.
When the rolling temperature of the final 3 passes of the finish hot rolling is less than 800 ℃, the rolling load is significantly increased with a decrease in the steel sheet temperature, which is not preferable in terms of production. On the other hand, if the rolling temperature of the final 3 passes exceeds 1100 ℃, the rolling strain imparted by rolling is removed by excessive recovery, and a predetermined critical stress intensity factor cannot be obtained. Therefore, the rolling temperature of the final 3 passes is in the range of 800-1100 ℃. The rolling temperature of the final 3 passes is preferably in the range of 800-1050 ℃. More preferably, the rolling temperature of the final 3 passes is in the range of 850-1000 ℃.
In order to prevent an excessive rolling load from being applied to a specific pass among the final 3 passes of the hot finish rolling, it is preferable that the rolling temperature of the 1 st pass among the final 3 passes is 950 to 1100 ℃, the rolling temperature of the 2 nd pass performed next to the 1 st pass is 925 to 1075 ℃, and the rolling temperature of the 3 rd pass performed next to the 2 nd pass is 875 to 1050 ℃.
Further, a method for producing a hot-rolled ferritic stainless steel sheet according to the present invention is characterized by: after controlling the temperature range in the final 3 passes of the finish hot rolling consisting of 3 or more passes, a large reduction is applied. If rolling with a large reduction is performed in the final 4 or more passes, the reduction is dispersed in each pass even with the same cumulative reduction, so that the application of strain to the center of the plate thickness becomes insufficient, and the cumulative transport time between passes increases, so that recovery during transport between passes is promoted, the effect of the application of strain decreases, and it becomes difficult to obtain a predetermined critical stress intensity factor. On the other hand, if the rolling temperature and the cumulative reduction ratio of the finish rolling are controlled to be the final 2 passes or less, the rolling load may be significantly increased to lower the productivity in order to perform the high reduction with the cumulative reduction ratio of 25% or more in 2 passes, which is not preferable. Therefore, in the method for producing a hot-rolled ferritic stainless steel sheet according to the present invention, the rolling temperature and the cumulative reduction ratio in the final 3 passes of the finish rolling are controlled.
In the method for producing a hot-rolled ferritic stainless steel sheet according to the present invention, it is important to control the rolling temperature and the cumulative reduction ratio of the final 3 passes of the hot finish rolling, and it is possible to perform finish rolling of any pass as long as the finish rolling is performed in 3 or more passes, but if the maximum number of passes is more than 15 passes, the number of times of contact with the rolls increases, and thus the steel sheet temperature is likely to decrease, and in order to maintain the steel sheet temperature within a predetermined temperature range, external heating or the like may be required, which may lead to a decrease in the productivity or an increase in the production cost, and therefore, the maximum number of passes is preferably 15 or less passes. More preferably, the maximum number of passes is 10 passes or less.
After the finish hot rolling, the steel sheet is cooled and then wound to produce a hot rolled steel strip. In the present invention, the winding temperature is not particularly limited, but when the winding temperature is set to more than 450 ℃ and less than 500 ℃, embrittlement may occur due to 475 ℃. Therefore, the coiling temperature is preferably 450 ℃ or lower or 500 ℃ or higher. It is more preferable to perform accelerated cooling such as steam cooling after the final rolling and then perform coiling at 450 ℃ or lower, because the removal of rolling strain due to recovery after coiling can be further suppressed.
The hot-rolled steel sheet obtained by the present invention may be subjected to hot-rolled sheet annealing to produce a hot-rolled annealed steel sheet. The hot-rolled steel sheet provided by the present invention is excellent in toughness, and therefore, can be annealed as a hot-rolled sheet in a continuous annealing line, and there has been a fear that a fracture due to low toughness may be caused and the continuous annealing line may be avoided. Further, the obtained hot-rolled annealed steel sheet may be subjected to cold rolling or cold-rolled sheet annealing.
Examples
The present invention will be described in detail below with reference to examples.
Stainless steel molten steel having a chemical composition shown in table 1 was melted by refining in a converter having a capacity of 150ton and by strong stirring/vacuum oxygen decarburization (SS-VOD), and was continuously cast into a slab having a width of 1000mm and a thickness of 200 mm. The slab was heated at 1200 ℃ for 1hr, and thereafter, as hot rolling, a steel sheet of about 40mm was reversibly rough-rolled using 3 stands, and then, the final 3 passes (5 th, 6 th, and 7 th passes) of finish rolling of 7 passes were performed according to the conditions described in table 2 to produce a hot-rolled steel sheet.
The hot-rolled steel sheet obtained was evaluated as follows.
(1) Critical stress intensity factor KICEvaluation of (2)
A CT test piece in accordance with ASTM E399 was sampled from the center of the plate width so that the fatigue pre-crack was in the rolling orthogonal direction and the stress axis was in the rolling parallel direction. For the test piece, the critical stress intensity factor K was determined from ASTME399IC. Setting the critical stress intensity factor at 25 MPa.m1/2The above evaluation is qualified, and is less than 25 MPa.m1/2The evaluation was failed.
(2) Evaluation of Corrosion resistance
A60X 100mm test piece was taken out of the hot-rolled steel sheet obtained, and the surface to be evaluated was polished with #600 sandpaper to prepare a test piece having sealed end faces, which was subjected to a salt water spray cycle test defined in JIS H8502. The salt spray cycle test was conducted for 5 cycles with 1 cycle of salt spray (5 mass% NaCl, 35 ℃, spray 2hr) → dry (60 ℃, 4hr, relative humidity 40%) → wet (50 ℃, 2hr, relative humidity ≥ 95%). The evaluation surface of the test piece after the 5-cycle salt spray cycle test was photographed, the rust area of the evaluation surface of the test piece was measured by image analysis, and the rust rate ((rust area in the test piece/total area of the test piece) × 100 [% ]) was calculated from the ratio to the total area of the test piece. The rust rate was 10% or less, which was particularly excellent in corrosion resistance and evaluated as "acceptable" (. circinata), "acceptable" (. smallcircle.) was evaluated with a rust rate of more than 10% and 25% or less, and "unacceptable" (. xxxiv) "was evaluated with a rust rate of more than 25%.
The test results and hot rolling conditions are shown in table 2.
Figure BDA0002141768560000151
Figure BDA0002141768560000161
The steel compositions and hot rolling conditions of Nos. 1 to 22 and 29 to 33 satisfying the range of the present invention sufficiently impart rolling strain to the steel sheet by the predetermined hot rolling, and as a result, a predetermined critical stress intensity factor is obtained. In addition, the corrosion resistance of the obtained hot-rolled steel sheet was evaluated, and it was confirmed that the rust percentage was 25% or less and the steel sheet had sufficient corrosion resistance.
In particular, in Nos. 2, 4, 6, 7 and 9 using steel B, D, F, G, I containing Mo, Nos. 5 and 8 using steels E and H containing Cu, and Nos. 13 to 15 using steel M, N and O having a high Cr content, further excellent corrosion resistance was obtained with a rust rate of 10% or less.
In No.23 in which the rolling temperature of the 5 th pass (3 rd pass from the final pass) exceeded the range of the present invention and No.25 in which the rolling temperatures of the 5 th pass and the 6 th pass (2 nd pass from the final pass) exceeded the range of the present invention, rolling was performed at a predetermined cumulative reduction ratio, but excessive recovery of the working strain imparted by rolling occurred because the rolling temperature was too high, and as a result, a predetermined critical stress intensity factor was not obtained after hot rolling. In No.24 in which the cumulative reduction of the final 3 passes was less than the range of the present invention, the application of rolling strain was insufficient, and as a result, the predetermined critical stress intensity factor was not obtained after hot rolling.
In No.26 in which the rolling temperatures of the 5 th pass and the 6 th pass were lower than the range of the present invention, the rolling load was significantly increased because the rolling temperature was too low, and the load exceeded the allowable range of the apparatus when the rolling of the final 7 th pass was carried out, so that the rolling could not be completed, and the predetermined evaluation could not be performed.
In No.27 using steel R having an Nb content exceeding the range of the present invention, significant toughness reduction due to Laves phase precipitation occurred during hot rolling, and a predetermined critical stress intensity factor was not obtained.
In No.28 using steel S having an Nb content lower than the range of the present invention, since a sufficient amount of Nb carbonitride was not precipitated, excessive recovery occurred during hot rolling, and a predetermined critical stress intensity factor was not obtained.
In No.34 using steel Y having a Cr content lower than the range of the present invention, the Cr content was insufficient, and thus the desired corrosion resistance was not obtained.
In No.35 using steel Z having a Cr content exceeding the range of the present invention, the sigma phase precipitated due to excessive Cr content, and therefore, the toughness was significantly reduced, and the predetermined critical stress intensity factor could not be obtained.
Industrial applicability
The hot-rolled ferritic stainless steel sheet obtained by the present invention is particularly excellent in punchability by a crank press, and is particularly suitable for use in thick flanges and the like which are manufactured by punching or the like using a crank press or other methods and require high workability and corrosion resistance.

Claims (4)

1. A hot-rolled ferritic stainless steel sheet having the following composition containing, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001-0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, Nb: 0.12 to 0.60%, the balance being Fe and unavoidable impurities,
critical stress intensity factor KICIs 25 MPa.m1/2In the above-mentioned manner,
the thickness of the plate is more than 5.0mm,
the hot rolled ferritic stainless steel sheet is produced by a production method comprising: in a hot rolling step of performing finish rolling in 3 or more passes, the final 3 passes of the finish rolling are set to a temperature range of 800 to 1100 ℃, and the cumulative reduction ratio of the final 3 passes is set to 25% or more,
the rolling temperature of the 1 st pass in the final 3 passes is set to 950-1100 ℃, the rolling temperature of the 2 nd pass performed after the 1 st pass is set to 925-1075 ℃, and the rolling temperature of the 3 rd pass performed after the 2 nd pass is set to 875-1050 ℃.
2. A hot-rolled ferritic stainless steel sheet having the following composition containing, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001-0.50%, N: 0.001 to 0.020%, Cr: 13.0 to 24.0%, Ni: 0.01 to 0.60%, Nb: 0.12 to 0.60%, the balance being Fe and unavoidable impurities,
critical stress intensity factor KICIs 25 MPa.m1/2In the above-mentioned manner,
the thickness of the plate is more than 5.0mm,
the hot rolled ferritic stainless steel sheet is produced by a production method comprising: in a hot rolling step of performing finish rolling in 3 or more passes, the final 3 passes of the finish rolling are set to a temperature range of 800 to 1100 ℃, and the cumulative reduction ratio of the final 3 passes is set to 25% or more,
the rolling temperature of the 1 st pass in the final 3 passes is set to 950-1100 ℃, the rolling temperature of the 2 nd pass performed after the 1 st pass is set to 925-1075 ℃, and the rolling temperature of the 3 rd pass performed after the 2 nd pass is set to 875-1050 ℃.
3. The hot-rolled ferritic stainless steel sheet according to claim 1 or 2, further comprising one or more selected from the following groups A and B as optional elements in terms of mass% as a component composition,
group A: is selected from Cu: 0.01 to 1.50%, Mo: 0.01-2.00%, W: 0.01-0.20%, Co: 0.01 to 0.20% of 1 or more than 2,
group B: selected from the group consisting of Ti: 0.01-0.30%, V: 0.01 to 0.20%, Zr: 0.01-0.20%, REM: 0.001-0.100%, B: 0.0002-0.0025%, Mg: 0.0005 to 0.0030%, Ca: 0.0005-0.0030% of 1 or more than 2.
4. A method for producing a hot-rolled ferritic stainless steel sheet according to any one of claims 1 to 3, wherein in a hot rolling step of performing a finish rolling of 3 or more passes, the final 3 passes of the finish rolling are performed at a temperature range of 800 to 1100 ℃, and the cumulative reduction of the final 3 passes is 25% or more,
the rolling temperature of the 1 st pass in the final 3 passes is set to 950-1100 ℃, the rolling temperature of the 2 nd pass performed after the 1 st pass is set to 925-1075 ℃, and the rolling temperature of the 3 rd pass performed after the 2 nd pass is set to 875-1050 ℃.
CN201880008313.8A 2017-01-26 2018-01-12 Hot rolled ferritic stainless steel sheet and method for producing same Active CN110225988B (en)

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