CN111295458A - Ferritic stainless steel sheet and method for producing same - Google Patents

Ferritic stainless steel sheet and method for producing same Download PDF

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Publication number
CN111295458A
CN111295458A CN201880070416.7A CN201880070416A CN111295458A CN 111295458 A CN111295458 A CN 111295458A CN 201880070416 A CN201880070416 A CN 201880070416A CN 111295458 A CN111295458 A CN 111295458A
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stainless steel
steel sheet
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井上佳士
川边英尚
吉野正崇
藤泽光幸
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JFE Steel Corp
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JFE Steel Corp
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    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
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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
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C22C38/002—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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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • 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
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/08—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of copper or alloys based thereon
    • 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
    • C21D2211/005—Ferrite

Abstract

The present invention provides a ferritic stainless steel sheet having more excellent toughness and excellent corrosion resistance, and a method for manufacturing the same, the ferritic stainless steel sheet having the following composition: contains, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 0.35%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.300%, Cr: 10.0 to 13.0%, Ni: 0.75 to 1.50%, Ti: 0.05-0.35%, N: 0.001 to 0.020%, and is represented by the following formula (1)Formed gammaI[％]65% or more, the balance of Fe and inevitable impurities, and an average crystal grain diameter of the metal structure of 45 μm or less. Gamma rayI[％]24Ni +12Mn +6 Cu-18 Si-12 Cr-12 Mo +188 (1). In the formula (1), Ni, Mn, Cu, Si, Cr, and Mo represent the content (mass%) of each component, and the component not contained is 0.

Description

Ferritic stainless steel sheet and method for producing same
Technical Field
The present invention relates to a ferritic stainless steel sheet and a method for producing the same, and particularly to a ferritic stainless steel sheet having excellent toughness and excellent corrosion resistance, which is useful for applications to members for flanges, and a method for producing the same.
Background
An exhaust path of an automobile is composed of various components such as an exhaust manifold, a muffler, a catalyst, a flexible pipe, a center pipe, and a head pipe. When these members are connected, a fastening member called a Flange (Flange) is often used. The flange applied to such an exhaust system component needs to have sufficient rigidity. Therefore, a thick flange (for example, a thickness of 5mm or more) is applied to such an exhaust system component.
In addition, the flange is manufactured by a process such as blanking, and general steel may be used.
In recent years, sufficient corrosion resistance is required for flange materials used for parts exposed to high-temperature Exhaust Gas called an EGR (Exhaust Gas Recirculation) system. Therefore, the 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. As a result, ferritic stainless steel sheets having a large plate thickness (for example, a plate thickness of 5mm or more) which can be applied to thick flanges are strongly required.
However, ferritic stainless steel having a large plate thickness has a problem of low-temperature toughness. For example, fracturing in the manufacture of flanges occurs in large quantities during the winter season. Therefore, improvement of toughness of ferritic stainless steel having a large plate thickness is strongly required.
In response to such market demand, for example, patent document 1 discloses toughness (Charpy impact value at-40 ℃ C. is 50J/cm)2Above) is excellent, and is characterized by containing, in mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.005-1.0%, Ni: 0.1 to 1.0%, Mn: 0.1-3.0%, P: 0.04% or less, S: 0.0100% or less, Cr: 10% or more and less than 18%, further contains Ti: 0.05 to 0.30%, Nb: 0.01 to 0.50%, 1 or 2 kinds of Ti and Nb in a total amount of 8(C + N) to 0.75%, and the balance of Fe and inevitable impurities, and gammap70% or more, ferrite grain diameter of 20 μm or less, and martensite generation amount of 70% or less.
Note that γ isp(%) was evaluated using the following formula (i) (expressed as formula (1) in patent document 1).
γp＝420(％C)+470(％N)+23(％Ni)+9(％Cu)+7(％Mn)－11.5(％Cr)－11.5(％Si)－12(％Mo)－23(％V)－47(％Nb)－49(％Ti)－52(％Al)+189(i)
The "% X" represents the mass ratio of each component X.
Documents of the prior art
Patent document
Patent document 1: japanese patent laid-open publication No. 2016-.
Disclosure of Invention
However, the present inventors have tried to use the stainless steel sheet described in patent document 1 to form a thick flange shape having a burring portion, and have found that cracking occurs in the burring portion, and a predetermined flange shape cannot be obtained in some cases, and thus the present inventors have not been able to apply the present invention to a thick flange.
In view of the above circumstances, an object of the present invention is to provide a ferritic stainless steel sheet having further excellent toughness and excellent corrosion resistance, and a method for producing the same.
In the present invention, the term "excellent toughness" means a Charpy impact value of 100J/cm at-50 ℃2The above. In the present invention, the excellent corrosion resistance means that the rust percentage after 3 cycles of the salt spray cycle test specified in JIS H8502 is 25% or less.
The present inventors have conducted detailed studies to solve the above problems. As a result, the following results were obtained.
In order to form a thick flange having a burring without causing cracking, it is effective to refine the metal structure and to have a Charpy impact value of 100J/cm at-50 DEG C2The above. Specifically, by setting the average crystal grain size of the metal structure to 45 μm or less, the occurrence of cracks in the burring part when a thick flange having the burring part is machined can be effectively suppressed, and the method can be sufficiently applied to a thick flange having the burring part.
Further, in order to refine the metal structure and obtain a Charpy impact value of 100J/cm at-50 DEG C2As described above, it is effective means to heat a billet having an appropriate steel composition, specifically, a steel composition in which Si, Mn, Cr, Ni, and the like are controlled in an appropriate range at 1050 to 1250 ℃, then hot-roll the billet, and anneal the hot-rolled plate at an appropriate temperature.
The present invention has been made in view of the above circumstances, and the gist thereof is as follows.
[1]A ferritic stainless steel sheet having the following composition: contains, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 0.35%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.300%, Cr: 10.0 to 13.0%, Ni: 0.75 to 1.50%, Ti: 0.05-0.35%, N: 0.001 to 0.020% of gamma represented by the following formula (1)I[％]65% or more, the remainder being Fe and inevitable impurities, and the average crystal grain size of the metal structure being 45 μm or less.
γI[％]＝24Ni+12Mn+6Cu－18Si－12Cr－12Mo+188 (1)
In the formula (1), Ni, Mn, Cu, Si, Cr, and Mo represent the content (mass%) of each component, and the component not contained is 0.
[2] The ferritic stainless steel sheet according to [1], which comprises, in addition to the above-described composition, Cu: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.01-0.20%, Co: 0.01-0.20% of 1 or more than 2.
[3] The ferritic stainless steel sheet according to [1] or [2], which further comprises, in addition to the above-described component composition, in mass%, V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, Zr: 0.01-0.20% of 1 or more than 2.
[4] The ferritic stainless steel sheet according to any one of the above [1] to [3], which further comprises, in addition to the above-described composition, REM: 0.001-0.100%, B: 0.0002-0.0025%, Mg: 0.0005 to 0.0030%, Ca: 0.0003-0.0030% of 1 or more than 2.
[5] A method for producing a ferritic stainless steel sheet according to any one of the above [1] to [4], comprising a hot rolling step of heating a billet having the above composition at 1050 to 1250 ℃ and then hot rolling the heated billet; and a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet obtained in the hot-rolling step at 750 to 1050 ℃.
According to the present invention, a ferritic stainless steel sheet having further excellent toughness and excellent corrosion resistance is obtained. The ferritic stainless steel sheet of the present invention can be suitably used for applications such as thick flanges.
Detailed Description
The present invention will be described in detail below.
The present inventors have studied in detail the cause of the occurrence of cracks when a flange hole portion of 30mm phi is formed from the surface of a steel sheet in a blank state (after punching) into a flange having a burring portion raised upward by 10mm using various ferritic stainless steel sheets having a thickness of 5.0 mm. As a result, the Charpy impact value at-50 ℃ was 100J/cm2The above steel sheet does not crack, and the summer specific impact value at-50 ℃ of the cracked steel sheet is less than 100J/cm2. It is thus understood that the low toughness is a cause of the fracture.
The present inventors have studied the relationship between the low toughness and the metal structure in detail. As a result, it was found that the larger the average crystal grain size of the steel sheet, the lower the toughness. Therefore, the above-described flange was attempted to be formed using various ferritic stainless steel sheets (5.0 mm in thickness). As a result, it was found that the toughness of the steel sheet having an average crystal grain size of more than 45 μm was lowered and cracking was likely to occur. When the average crystal grain size is 45 μm or less, the toughness is excellent and the punching workability of the steel sheet is good.
As described above, in the present invention, the charpy impact value at-50 ℃ is 100J/cm, with an average crystal particle diameter of 45 μm or less2The above.
The average crystal grain size can be measured by the measurement method of the examples described later. As described later, the charpy impact value is a value measured in accordance with JIS Z2242 (2005).
Next, the composition of the ferritic stainless steel sheet of the present invention will be described.
Hereinafter, unless otherwise specified, the unit "%" as the content of the component means "% by mass".
C：0.001～0.020％
If C is contained in an amount of more than 0.020%, the workability and corrosion resistance are remarkably reduced. From the viewpoint of corrosion resistance and workability, it is preferable that the C content is as small as possible, and refining at a C content of less than 0.001% takes time, 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：0.05～0.35％
Si is an element that has the effect of improving the corrosion resistance of the welded portion by an oxide film formed by concentration during welding and is useful as a deoxidizing element in a steel-making process. These effects are obtained by containing 0.05% or more of Si, and the more the content is, the better the effect is. On the other hand, Si has an effect of promoting the formation of a ferrite phase, and if Si is contained in an amount exceeding 0.35%, a predetermined amount of austenite phase is not sufficiently formed during heating in the hot rolling step, and therefore, even when hot rolling and hot-rolled sheet annealing are performed under the conditions specified in the present invention, a desired metal structure cannot be obtained. Therefore, the Si content is 0.05% to 0.35%. The Si content is preferably 0.10% or more. The Si content is preferably 0.30% or less.
Mn：0.05～1.00％
Mn has an effect of promoting the formation of an austenite phase. 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 corrosion origin, is promoted, and the corrosion resistance is lowered. Therefore, the Mn content is 0.05% to 1.00%. The Mn content is preferably 0.20% or more. The Mn content is preferably 0.80% or less, and more preferably 0.70% or less.
P: less than 0.04%
P is an element inevitably contained in steel and is an element harmful to corrosion resistance and workability, and therefore is preferably reduced as much as possible. 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 is an element that is inevitably contained in steel, as well as P, and is harmful to corrosion resistance and workability, and therefore is preferably reduced as much as possible. 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 0.003% or less.
Al：0.001～0.300％
Al is an effective deoxidizer. Further, since Al has a higher affinity for nitrogen than Cr, when nitrogen intrudes into the weld zone, nitrogen is precipitated as an Al nitride rather than a Cr nitride, and this has the 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 0.300%, the weldability decreases due to a decrease in the penetration during welding, which is not preferable. Therefore, the Al content is in the range of 0.001% to 0.300%. The Al content is preferably 0.010% or more. The Al content is preferably 0.200% or less, more preferably 0.100% or less, and still more preferably 0.050% or less.
Cr：10.0～13.0％
Cr is the most important element for ensuring corrosion resistance. The content thereof is less than 10.0%, and corrosion resistance required for automobile exhaust parts cannot be obtained. On the other hand, if Cr is contained in an amount of more than 13.0%, the steel composition is adjusted to γ represented by the following predetermined formula (1)ISince a predetermined amount of austenite phase is not generated during heating in the hot rolling step, a desired metal structure cannot be obtained even when hot rolling and hot-rolled sheet annealing are performed under the conditions specified in the present invention. Therefore, the Cr content is in the range of 10.0% to 13.0%. The Cr content is preferably 10.5% or more. The Cr content is preferably 12.0% or less, and more preferably 11.7% or less.
Ni：0.75～1.50％
Ni is an austenite forming element, and has an effect of increasing the amount of austenite formed during heating before rolling in the hot rolling step. In the present invention, the steel composition is adjusted so that the billet in the hot rolling step contains a ferrite phase + austenite phase dual phase structure having an austenite phase volume fraction of 70% or more during heating. When the microstructure is a ferrite phase + austenite phase two-phase microstructure, the heterogeneous interface between the ferrite phase and the austenite phase functions as a barrier to grain growth, and therefore the microstructure before hot rolling is refined. Therefore, the work strain which becomes a recrystallization site is accumulated by the predetermined hot rolling, and recrystallization is generated by the hot-rolled sheet annealing in the subsequent step, whereby a fine metal structure is obtained and excellent toughness is exhibited. These effects are obtained by containing 0.75% or more of Ni. On the other hand, if the Ni content exceeds 1.50%, the improvement effect by the refinement of crystal grains is saturated, and the workability is lowered. In addition, stress corrosion cracking is easily generated. Therefore, the Ni content is 0.75% to 1.50%. The Ni content is preferably 0.80% or more. The Ni content is preferably 1.20% or less, and more preferably 1.00% or less.
Ti：0.05～0.35％
Ti preferentially binds to C, N to suppress precipitation of Cr carbonitride, lowers recrystallization temperature, and suppresses deterioration of corrosion resistance due to sensitization caused by precipitation of Cr carbonitride. In order to obtain such an effect, it is necessary to contain 0.05% or more of Ti. On the other hand, if the Ti content exceeds 0.35%, the toughness is significantly reduced by the formation of coarse TiN, and a predetermined toughness cannot be obtained even if the technique of the present invention is applied. Further, the content of Ti exceeding 0.35% is not preferable in terms of production because coarse Ti carbonitride is generated in the casting step, resulting in surface defects. Therefore, the Ti content is 0.05 to 0.35 percent. The Ti content is preferably 0.10% or more. The Ti content is preferably 0.30% or less, and more preferably 0.15% or less.
N：0.001～0.020％
If the N content exceeds 0.020%, the reduction in the workability and corrosion resistance becomes significant. From the viewpoint of workability and corrosion resistance, the lower the N content, the better, and it takes a long time to refine the steel to less than 0.001%, which is not preferable because of the reduction in production cost and productivity. Therefore, the N content is in the range of 0.001% to 0.020%. The N content is preferably 0.005% or more, and more preferably 0.007% or more. The N content is preferably 0.015% or less, and more preferably 0.012% or less.
γI[％]: over 65 percent
When gamma is represented by the following formula (1)IWhen the content is less than 65%, the austenite content of the microstructure is insufficient at the slab heating temperature before the start of hot rolling, and thus a fine microstructure cannot be obtained, and γ is a factorI[％]Is more than 65 percent. Note that γ isI[％]Is obtained by the following formula (1) for evaluating the stability of austenite phase
γI[％]＝24Ni+12Mn+6Cu－18Si－12Cr－12Mo+188 (1)
In the formula (1), Ni, Mn, Cu, Si, Cr, and Mo represent the content (mass%) of each component, and the component not contained is 0.
In the above formula (1), the austenite forming element has a positive coefficient, the ferrite forming element has a negative coefficient, and the respective values are experimentally obtained with reference to the formula of Castro.
In the present invention, the balance other than the above is Fe and inevitable impurities. The inevitable impurities include O (oxygen), and the content of O may be as long as 0.01% or less.
The composition may contain, in addition to the above-mentioned essential components, 1 or 2 or more groups selected from the following groups A to C, as required.
(group A) Cu: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.01-0.20%, Co: 0.01-0.20% of 1 or more than 2
(group B) V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, Zr: 0.01-0.20% of 1 or more than 2
(group C) REM: 0.001-0.100%, B: 0.0002-0.0025%, Mg: 0.0005 to 0.0030%, Ca: 0.0003-0.0030% of 1 or more than 2
Cu：0.01～1.00％
Cu is an element that is particularly effective in improving corrosion resistance in a solution or when weakly acidic water droplets adhere thereto. Further, Cu has an effect of promoting the generation of an austenite phase. This effect is obtained by containing 0.01% or more, and the more the Cu content, the better the effect. However, if Cu is contained in an amount of more than 1.00%, hot rolling workability may be deteriorated, resulting in surface defects. Further, the oxide scale may be difficult to remove after annealing. Therefore, when Cu is contained, the Cu content is in the range of 0.01% to 1.00%. When Cu is contained, the Cu content is preferably 0.10% or more. When Cu is contained, the Cu content is preferably 0.50% or less.
Mo：0.01～1.00％
Mo is an element that significantly improves the corrosion resistance of stainless steel. This effect is obtained by containing 0.01% or more of Mo, and the more the content is, the better the effect is. On the other hand, Mo has an effect of promoting the generation of a ferrite phase, and if the Mo content exceeds 1.00%, a predetermined amount of an austenite phase is not sufficiently generated at the time of heating in the hot rolling step, and thus a desired metal structure cannot be obtained even when hot rolling and hot-rolled sheet annealing are performed under the conditions specified in the present invention. Therefore, when Mo is contained, the Mo content is 0.01 to 1.00%. When Mo is contained, the Mo content is preferably 0.10% or more, and more preferably 0.30% or more. When Mo is contained, the Mo content is preferably 0.80% or less, and more preferably 0.50% 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. On the other hand, 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 in the range of 0.01% to 0.20%. When W is contained, the W content is preferably 0.05% or more. When W is contained, the W content is preferably 0.15% or less.
Co：0.01～0.20％
Co is an element for improving toughness. This effect is obtained by containing 0.01% or more of Co. On the other hand, if the Co content exceeds 0.20%, workability may be reduced. Therefore, when Co is contained, the Co content is in the range of 0.01% to 0.20%.
V：0.01～0.20％
V is a carbon-nitrogen compound with C, N, and inhibits sensitization during welding to improve corrosion resistance of the welded part. This effect is obtained when the V content is 0.01% or more. On the other hand, if the V content exceeds 0.20%, workability and toughness may be significantly reduced. Therefore, when V is contained, the content of V is 0.01% to 0.20%. When V is contained, the content of V is preferably 0.02% or more. When V is contained, the content of V is preferably 0.10% or less.
Nb：0.01～0.10％
Nb has an effect of refining crystal grains. This effect is obtained by containing 0.01% or more of Nb. On the other hand, Nb has an effect of increasing the recrystallization temperature, and when the Nb content exceeds 0.10%, the annealing temperature required for sufficient recrystallization by hot-rolled sheet annealing becomes excessively high, and a microstructure having an average crystal grain size of 45 μm or less may not be obtained. Therefore, when Nb is contained, the Nb content is in the range of 0.01% to 0.10%. When Nb is contained, the Nb content is preferably 0.05% or less.
Zr：0.01～0.20％
Zr has an effect of inhibiting sensitization by binding to C, N. This effect is obtained by containing 0.01% or more of Zr. On the other hand, when Zr is contained in an amount exceeding 0.20%, workability may be significantly reduced. Therefore, when Zr is contained, the Zr content is in the range of 0.01% to 0.20%. When Zr is contained, the Zr content is preferably 0.10% or less.
REM：0.001～0.100％
REM (Rare Earth Metals) has an effect of improving oxidation resistance, and suppresses the formation of an oxide film (weld-tempered color) in the weld zone and the formation of a Cr-deficient region directly below the oxide film. This effect is obtained by containing REM in an amount of 0.001% or more. On the other hand, when REM is contained in an amount exceeding 0.100%, the productivity such as pickling property in cold rolling annealing may be lowered. Therefore, when REM is contained, the REM content is in the range of 0.001% to 0.100%. When REM is contained, the REM content is 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 exceeding 0.0025%, workability and toughness may be deteriorated. Therefore, when B is contained, the B content is in the range of 0.0002% to 0.0025%. When B is contained, the B content is preferably 0.0003% or more. When B is contained, the content of B is preferably 0.0012% or less.
Mg：0.0005～0.0030％
In the Ti-containing steel of the present invention, if the Ti carbonitride compound is coarsened, the toughness may be lowered. In this regard, Mg has an effect of suppressing coarsening of Ti carbonitride. 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 be deteriorated. Therefore, when Mg is contained, the content of Mg is in the range of 0.0005 to 0.0030%. When Mg is contained, the Mg content is preferably 0.0010% or more. When Mg is contained, the Mg content is preferably 0.0020% or less.
Ca：0.0003～0.0030％
Ca is an effective component for effectively preventing nozzle clogging due to crystal precipitation of Ti-based inclusions which are likely to occur during continuous casting. This effect is obtained by containing 0.0003% or more of Ca. On the other hand, if Ca is contained in an amount exceeding 0.0030%, the corrosion resistance may be reduced by CaS formation. Therefore, when Ca is contained, the content of Ca is in the range of 0.0003% to 0.0030%. When Ca is contained, the Ca content is preferably 0.0005% or more. When Ca is contained, the content of Ca is preferably 0.0015% or less, and more preferably 0.0010% or less.
Next, a method for producing a ferritic stainless steel sheet according to the present invention will be described.
As a result of intensive studies on a method for improving toughness in a ferritic stainless steel sheet, the inventors of the present invention have found that a metal structure having an average crystal grain size of 45 μm or less, a Charpy impact value at-50 ℃ of 100J/cm/min of-50 ℃, and the like can be obtained by heating a slab having an appropriate steel component at 1050 to 1250 ℃ and then hot rolling the heated slab in 3 passes or more, and then annealing the hot-rolled sheet at 750 to 1050 ℃ to obtain a hot-rolled sheet2As described above, the toughness is greatly improved. And also to obtain the desired corrosion resistance.
The reason why the hot-rolled annealed steel sheet having a fine metal structure can be obtained by this method will be described below.
The ferritic stainless steel hardly undergoes dynamic recrystallization during hot rolling, and tends to be easily restored to the working strain by rolling. Therefore, in the hot rolling according to the conventional art, the work strain introduced by the rolling is excessively recovered, and the work strain cannot be effectively maintained after the hot rolling. As a result, the recrystallization sites become insufficient, and a fine recrystallized structure cannot be obtained in the hot-rolled sheet annealing in the subsequent step.
Therefore, the present inventors have made extensive studies on both the steel composition and the hot rolling method for a method effective for obtaining a fine structure after annealing a hot rolled sheet. As a result, it has been found that it is effective to control the contents of steel components, particularly Si, Mn, Cr and Ni, within appropriate ranges, and to perform hot rolling by heating the slab at an appropriate temperature in a hot rolling step to form a ferrite phase + austenite phase two-phase structure containing an austenite phase.
When the microstructure is a ferrite phase + austenite phase two-phase structure, coarsening of crystal grains is suppressed at a heterogeneous interface between the ferrite phase existing before heating and the austenite phase generated during heating, and therefore a fine equiaxed structure is obtained at a stage before hot rolling. In addition, by performing the predetermined hot rolling, the work strain which becomes the recrystallization site is sufficiently accumulated in the hot-rolled sheet annealing in the subsequent step, and a fine metal structure can be obtained by the hot-rolled sheet annealing in the subsequent step, and excellent toughness can be exhibited.
Specifically, the following scheme is proposed: a steel having an established formula (1) wherein the contents of Ni and Mn are combined with positive coefficients of Ni and Mn and the contents of Si and Cr are combined with negative coefficients of Si and Cr, respectively, is adjusted so that an austenite phase is formed at a volume fraction of 65% or more during heating before hot rolling, is subjected to slab heating at 1050-1250 ℃, and then hot rolling is performed.
The present inventors have also made intensive studies on preferable conditions for hot-rolled sheet annealing in the subsequent step. The hot-rolled sheet annealing is a step of recrystallizing a worked structure formed by hot rolling. Therefore, annealing at a temperature that produces sufficient recrystallization is required. However, when the hot-rolled sheet is annealed at an excessively high temperature, although recrystallization occurs, the generated recrystallized grains are significantly coarsened, and a predetermined fine structure cannot be obtained.
Therefore, the present inventors investigated in detail the relationship between the grain size of the recrystallized grains and the annealing temperature. As a result, it was found that the generation of coarse recrystallized grains, which cause a decrease in toughness, can be suppressed by suppressing the hot-rolled sheet annealing temperature to 1050 ℃.
Hereinafter, each production condition will be described in detail.
First, molten steel composed of the above-described composition is melted by a known method such as a converter, an electric furnace, or a vacuum melting furnace, and a slab (billet) is formed by a continuous casting method or an ingot-cogging method.
Heating temperature of steel billet: 1050-1250 DEG C
The steel slab is heated at 1050 to 1250 ℃ and subjected to hot rolling. The heating time at the heating temperature is not particularly limited, and is preferably 1 to 24 hours. When the heating temperature is less than 1050 ℃, the formation ratio of the austenite phase becomes low, and a fine metal structure is not obtained, and excellent toughness is not obtained. On the other hand, if the heating temperature is too high, the loss of scale increases with the increase in oxidation quality, and therefore the heating temperature of the billet is 1250 ℃. When hot rolling is performed on a slab, the slab may be directly rolled without heating the slab when the temperature of the cast slab is in a temperature range of 1050 ℃.
The rough rolling conditions are not particularly limited. In the case where the cast structure is effectively destroyed before finish hot rolling, the refining effect by subsequent billet heating is further promoted, and therefore, the cumulative reduction ratio in rough rolling is preferably 65% or more. Thereafter, the steel sheet is rolled to a predetermined thickness by finish hot rolling.
Annealing temperature of hot rolled plate: 750-1050 deg.C
In the present invention, the hot rolled sheet is annealed after the hot rolling. In the hot-rolled sheet annealing, the rolled structure formed in the hot-rolling step is recrystallized. In the present invention, rolling deformation is effectively imparted in the hot rolling step, and the number of recrystallization sites is increased, thereby suppressing the coarsening of recrystallization in the annealing of the hot-rolled sheet. In order to obtain this effect, hot-rolled sheet annealing needs to be performed at a temperature in the range of 750 to 1050 ℃. When the annealing temperature is less than 750 ℃, recrystallization is insufficient, residual stress due to hot rolling deformation remains, and flatness after hot rolling annealing cannot be maintained. On the other hand, when the annealing temperature exceeds 1050 ℃, the recrystallized grains are significantly coarsened, and the desired metal structure cannot be obtained. Therefore, the annealing temperature of the hot-rolled sheet is in the range of 750 to 1050 ℃. The annealing temperature of the hot-rolled sheet is preferably in the range of 750 ℃ to 900 ℃. The holding time and method of hot-rolled sheet annealing are not particularly limited, and can be performed by either box annealing (batch annealing) or continuous annealing.
The ferritic stainless steel sheet obtained as described above may be subjected to descaling treatment by shot blasting or acid pickling, if necessary. In addition, grinding, polishing, etc. may be performed to improve the surface properties. In addition, thereafter, cold rolling and cold-rolled sheet annealing may be performed.
As described above, the ferritic stainless steel sheet of the present invention having excellent toughness and excellent corrosion resistance is produced.
The ferritic stainless steel sheet obtained in the present invention has a ferrite single phase or a ferrite single phase containing 3% or less (volume ratio) of martensite or retained austenite in total, with the balance being a ferrite phase.
The Charpy impact value at-50 ℃ of the ferritic stainless steel sheet of the present invention is 100J/cm2The above. By thus providing excellent low-temperature toughness, the occurrence of cracking in the burring portion during the processing of a thick flange having the burring portion can be effectively suppressed, and the burring portion can be sufficiently applied to a thick flange having the burring portion.
The thickness is not particularly limited, and is preferably 5.0mm or more, and more preferably 8.0mm or more, because it is preferably a thickness that can be applied to thick flanges. The thickness is preferably 15.0mm or less, more preferably 13.0mm or less.
Example 1
The present invention will be described in detail below with reference to examples.
A100 kg steel slab was formed by vacuum induction melting of a molten stainless steel having a composition shown in Table 1. Subsequently, hot rolling was performed under the production conditions shown in table 2 to form hot-rolled steel sheets having the final thicknesses shown in table 2. The hot-rolled steel sheet is subjected to hot-rolled sheet annealing to produce a hot-rolled annealed steel sheet. The hot-rolled sheet annealing was performed at the hot-rolled sheet annealing temperature shown in table 2 for 8 hours.
The hot-rolled annealed steel sheet obtained as described above was evaluated as follows.
(1) Evaluation of average Crystal particle diameter
The average crystal particle size was measured by the EBSD (Electron Back Scattering diffraction) method. The measurement conditions were a measurement magnification of 500 times and a step length of 0.4. mu.m. The obtained data were analyzed by OIM (organization Imaging Microcopy) analysis software of TSL Solutions of K.K., and the circle-equivalent diameter was calculated by defining the difference in azimuth of 15 ℃ or more as the grain boundary. The value calculated from the average value of the obtained circle-equivalent diameters was taken as the average crystal particle diameter.
(2) Evaluation of Charpy impact value
From the center of the width of the hot-rolled annealed steel sheet, a V-notch Charpy test piece according to JIS Z2242 (2005) was taken so that the thickness of the steel sheet was longer in the rolling direction, and the Charpy impact value at-50 ℃ was measured according to JIS Z2242 (2005). The Charpy impact value at-50 ℃ is 100J/cm2The above is qualified, and is less than 100J/cm2The product is rejected.
(3) Evaluation of Corrosion resistance
A60X 80mm test piece is taken from a hot-rolled annealed steel sheet, and a test piece is prepared by polishing and finishing the surface with #600 sandpaper and sealing the end face and the back face, and subjected to a salt spray cycle test specified in JIS H8502, wherein the salt spray cycle test is performed by taking a photograph of the test piece surface after 3 cycles of the salt spray cycle test with salt spray (5 mass% NaCl, 35 ℃ C., spray 2hr) → dry (60 ℃ C., 4hr, relative humidity 40%) → wet (50 ℃ C., 2hr, relative humidity not less than 95%) as 1 cycle, and measuring the rust area of the test piece surface by image analysis, and calculating the rust rate (rust area in the test piece/area of the rust area measuring portion) × 100 [% ] from the ratio thereof to the area of the rust area measuring portion, the rust area measuring portion is a portion of 15mm removed from the outer circumference of the test piece, and the rust area is an area of the rust area of 10% or less as an excellent rust area, and the rust area measuring portion is taken as a portion exceeding 25% of the corrosion resistance (3583) and the rust area is taken as a fail-pass portion.
The test results obtained as described above are shown in table 2 together with the production conditions.
Figure BDA0002471209080000141
TABLE 2
Figure BDA0002471209080000151
Underlining indicates outside the scope of the invention
According to tables 1 and 2, Nos. 1 to 32 and 46 in which the steel composition, hot rolling condition and hot rolled sheet annealing condition satisfy the range of the present invention gave a fine metal structure having an average crystal grain size of 45 μm or less, and a predetermined Charpy impact value. Further, as a result of evaluating the corrosion resistance of the obtained hot-rolled annealed steel sheet, it was found that the rust percentage was 25% or less and the steel sheet had sufficient corrosion resistance. In particular, in steel No.17 using steel a17 containing 0.95% Cu and steel No.18 using steel a18 containing 0.88% Mo, further excellent corrosion resistance was obtained with a rust percentage of 10% or less.
In the present invention examples of nos. 1 to 32 and 46, the thick flange shape having the burring part was attempted to be processed, and as a result, the predetermined flange shape was obtained without causing cracking. The hot-rolled annealed steel sheets according to the examples of the present invention all had a ferrite single-phase structure or a structure in which the total volume fraction of one or both of the martensite and retained austenite phases was 3% or less and the balance was a ferrite phase as a structure observation.
In steel nos. 33 and 34 in which the slab heating temperature is higher than the range of the present invention, steel a1 and steel a2 were used, and although a predetermined amount of austenite phase was generated during heating in the hot rolling step and rolling was performed at a predetermined cumulative reduction ratio, the introduction of recrystallization sites was insufficient due to recovery of work strain caused by an excessively high rolling temperature, and therefore, coarsening of recrystallized grains was likely to occur in the hot-rolled sheet annealing step, and a predetermined charpy impact value was not obtained.
In the steel products of Nos. 35 and 36 in which the hot-rolled sheet annealing temperature was higher than the range of the present invention, the steel products A1 and A2 were used, and the recrystallized grains formed were significantly coarsened, and as a result, the predetermined Charpy impact value could not be obtained.
While satisfying the respective composition ranges of steel, gammaIIn the steels B1, B2 and B3 Nos. 37, 38 and 39 which were less than the range of the present invention, the predetermined hot-rolled and hot-rolled sheet annealing was performed, but the austenite phase was not sufficiently generated at the time of heating in the hot-rolling step, and as a result, the metal was not sufficiently generated in the hot-rolled sheet annealing stepThe structure is made finer and a predetermined Charpy impact value cannot be obtained.
In No.40 using steel B4 having a Cr content greater than the range of the present invention, although the hot rolling and the hot-rolled sheet annealing were performed in a predetermined manner, the austenite phase was not sufficiently generated during the heating in the hot rolling step, and as a result, the microstructure was not sufficiently refined in the hot-rolled sheet annealing step, and the predetermined charpy impact value was not obtained.
In No.41 using steel B5 having an Mn content greater than the range of the present invention, predetermined hot rolling and hot-rolled sheet annealing were performed, but MnS which becomes a corrosion origin excessively precipitated, and as a result, predetermined corrosion resistance was not obtained.
In steel No.42 using steel B6 having a Nb content greater than the range of the present invention, the recrystallization temperature was increased, and therefore, the microstructure was not sufficiently refined, and the predetermined Charpy impact value could not be obtained.
In No.43 using steel B7 having a Si content higher than that of the present invention, the average crystal grain size of the microstructure was higher than 45 μm, and a predetermined Charpy impact value could not be obtained.
In No.44 using steel B8 having a Ti content greater than the range of the present invention, the excessive Ti content resulted in the formation of coarse TiN, and the predetermined Charpy impact value could not be obtained.
In No.45 using steel B9 in which Ti was not added, the recrystallization temperature increased, and therefore the microstructure did not sufficiently become finer, and the predetermined charpy value could not be obtained.
In No.47 using steel B10 having an Ni content less than the range of the present invention, although predetermined hot rolling and hot-rolled sheet annealing were performed, austenite phase was not sufficiently generated during heating in the hot-rolling step, and as a result, the microstructure was not sufficiently refined in the hot-rolled sheet annealing step, and a predetermined charpy impact value was not obtained.
Industrial applicability of the invention
The ferritic stainless steel sheet obtained in the present invention is particularly suitable for applications requiring excellent toughness, for example, for use in a flange or the like.

Claims (5)

1. A ferritic stainless steel sheet comprises the following componentsConsists of the following components: contains, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 0.35%, Mn: 0.05-1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.300%, Cr: 10.0 to 13.0%, Ni: 0.75 to 1.50%, Ti: 0.05-0.35%, N: 0.001 to 0.020% of gamma represented by the following formula (1)I[％]65% or more, the remainder being Fe and inevitable impurities,
and the average crystal grain diameter of the metal structure is 45 μm or less,
γI[％]＝24Ni+12Mn+6Cu－18Si－12Cr－12Mo+188 (1)
in the formula (1), Ni, Mn, Cu, Si, Cr, and Mo represent the content of each component in mass%, and the component not contained is 0.
2. The ferritic stainless steel sheet according to claim 1, further comprising, in mass%, Cu: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.01-0.20%, Co: 0.01-0.20% of 1 or more than 2.
3. The ferritic stainless steel sheet according to claim 1 or 2, further comprising, in mass%, based on the component composition, V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, Zr: 0.01-0.20% of 1 or more than 2.
4. The ferritic stainless steel according to any one of claims 1 to 3, further comprising, in mass%, in addition to the above-described composition, REM: 0.001-0.100%, B: 0.0002-0.0025%, Mg: 0.0005 to 0.0030%, Ca: 0.0003-0.0030% of 1 or more than 2.
5. A method for producing a ferritic stainless steel sheet according to any one of claims 1 to 4, comprising:
a hot rolling step of heating the billet having the above composition at 1050 to 1250 ℃ and then hot rolling the heated billet; and
and a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet obtained in the hot-rolling step at 750 to 1050 ℃.
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