JP7038799B2 - Ferritic stainless hot-rolled annealed steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a ferrite-based stainless heat-rolled annealed steel sheet having excellent workability suitable for application to flanges and the like, and a method for producing the same.

In recent years, in order to reduce CO ₂ emissions, which are greenhouse gases, laws and regulations regarding exhaust gas in automobiles have been tightened. Since it is effective to improve fuel efficiency in order to reduce CO ₂ emissions from automobile exhaust gas, studies are underway to raise the combustion temperature of engines.

Exhaust gas generated by the engine is released to the atmosphere through exhaust system components such as an exhaust gas recirculation (EGR) system and a muffler. Each component of such an automobile exhaust system is fastened via a flange to prevent gas leakage. Flange applied to exhaust system parts must have sufficient dimensional accuracy as fastening parts.

Conventionally, ordinary steel has been used for such thick flanges. However, in recent years, due to the demand for further improvement in fuel efficiency of automobiles, the engine combustion temperature and the exhaust gas from the engine have been further increased in temperature. Along with this, flanges are required to have higher high temperature strength and corrosion resistance than before. Against this background, in recent years, stainless steels that are superior in high-temperature strength and corrosion resistance to ordinary steels, especially high-strength ferritic stainless steel plates that have a relatively small thermal expansion rate and are less likely to generate thermal stress (for example, ASTM A240 / 240M-S40975 (for example). The application of thick plates of 11 mass% Cr—Ti—Ni steel (for example, 5 mm or more in plate thickness) is in progress.

However, since the flange used for the exhaust system is thick (often 5 mm or more), there is a problem that cracks may occur during punching when manufacturing the flange, and flange parts may not be manufactured properly. There is a strong demand for thick ferrite stainless steel sheets with excellent punching workability.

In response to such market demands, for example, Patent Document 1 states that, in terms of mass%, C: 0.015% or less, Si: 0.01 to 0.4%, Mn: 0.01 to 0.8%, P: 0.04% or less, S: 0.01% or less, Cr: 14.0 to less than 18.0%, Ni: 0.05 to 1%, Nb: 0.3 to 0.6%, Ti: It contains 0.05% or less, N: 0.020% or less, Al: 0.10% or less, B: 0.0002 to 0.0020%, and the balance is Fe and unavoidable impurities, Nb, C and Disclosed is a ferritic stainless hot-rolled steel sheet having an N content of Nb / (C + N) ≧ 16, a charpy impact value of 10 J / cm ² or more at 0 ° C., and a plate thickness of 5.0 to 9.0 mm. Has been done.

International Publication No. 2014/157576

Using the method disclosed in Patent Document 1, the present inventors have prototyped a ferritic stainless steel sheet having a steel component conforming to ASTM A240 / 240M-S40975 and having a thickness of 10 mm, and forming a flange having a hole of 20 mmφ. It was manufactured by punching with a clearance of 10%. As a result, although cracks did not occur due to punching, the outer peripheral dimensions and / or the hole dimensions at the center of the flange may exceed the allowable tolerance of the part, which is not sufficient for thick-walled flanges. It became clear.

The present invention solves this problem, has sufficient corrosion resistance, and is capable of obtaining predetermined dimensional accuracy without cracking during punching of thick-walled flanges. Ferritic stainless heat with excellent punching workability. It is an object of the present invention to provide an annealed ferritic steel sheet and a method for producing the same.

The present inventors have conducted a detailed study in order to solve the above problems. As a result, in order to obtain a predetermined dimensional accuracy without cracking in the punching process, the metal structure of the steel sheet may be a ferrite single-phase structure and the average crystal grain size thereof may be controlled in the range of 5 to 20 μm. I found that.

Then, hot rolling is performed on the ferritic stainless steel having an appropriate component, and the obtained hot-rolled steel sheet is subjected to appropriate conditions for a ferrite single phase region, specifically, at 600 ° C. or higher and lower than 750 ° C. It was found that the metal structure can be controlled to a ferrite single phase and the average crystal grain size can be controlled in the range of 5 to 20 μm by performing hot-rolled sheet annealing that is held for 1 minute to 24 hours.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] In terms of mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.01 to 0.10%, Cr: 10.0 to 20.0%, Ni: 0.50 to 2.00%, Ti: 0.10 to 0.40%, Ferritic stainless heat, which contains N: 0.001 to 0.020%, has a component composition in which the balance is Fe and unavoidable impurities, and has a metal structure having an average crystal grain size of 5 to 20 μm, which is a ferrite single-phase structure. Annealed ferritic steel plate.
[2] In terms of mass%, Cu: 0.01 to 1.00%, Mo: 0.01 to 2.00%, W: 0.01 to 0.20%, Co: 0.01 to 0. The ferrite-based stainless hot-rolled annealed steel sheet according to the above [1], which contains one or more selected from 20%.
[3] By mass%, V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.20%, REM: 0.001 to 0. The above containing one or more selected from 100%, B: 0.0002 to 0.0025%, Mg: 0.0005 to 0.0030%, Ca: 0.0003 to 0.0030%. The ferritic stainless hot-spread annealed steel sheet according to [1] or [2].
[4] The method for manufacturing a ferrite-based stainless hot-rolled annealed steel sheet according to any one of [1] to [3] above, wherein the hot-rolled steel sheet obtained in the hot rolling step is at 600 ° C. or higher and lower than 750 ° C. A method for manufacturing a ferrite-based stainless hot-rolled annealed steel sheet, which is annealed with a hot-rolled sheet that is held for 1 minute to 24 hours.

According to the present invention, a ferrite-based stainless hot-rolled annealed steel sheet having sufficient corrosion resistance and excellent punching workability can be obtained.

Sufficient corrosion resistance in the present invention is defined as a salt spray cycle test (salt spray (5 mass% NaCl, 35)) specified in JIS H8502 on a steel sheet whose surface is polished and finished with # 600 emery paper and whose end face is sealed. ℃, spray 2hr) → drying (60 ℃, 4hr, relative humidity 40%) → wetting (50 ℃, 2hr, relative humidity ≧ 95%)) in one cycle) on the surface of the steel sheet It means that the rusted area ratio (= rusted area / total steel plate area x 100 [%]) is 25% or less.

As an evaluation of punching workability, first, a test piece of 100 mm × 100 mm is collected from a hot-rolled annealed steel sheet, and then a hole of φ20 mm (tolerance ± 0.1 mm) is formed in the center of the test piece. Five test pieces are produced by punching with a crank press machine equipped with an upper die (punch) having a cylindrical blade for lightening with a diameter of 20 mm and a lower die (die) having a hole with a diameter of 20 mm or more. .. The punching process is performed by selecting the hole diameter on the lower die side according to the thickness of the test piece so that the clearance between the upper die and the lower die is 10%. Here, the above clearance (C) [%], the diameter of the hole of the die (inner diameter of the die) (Dd) [mm], and the diameter of the punch (Dp) [mm] are also the plate thickness (t) [mm]. Including, it is expressed by the relation of the following equation (1).
C = (Dd-Dp) ÷ (2 × t) × 100 ... Equation (1)
The excellent punching workability in the present invention means that the test piece thus obtained has no cracks when the appearance of the test piece is visually observed and the hole diameter at the center of the test piece is measured by a digital caliper, and after punching. It means that the hole diameter of all five test pieces is in the range of 19.9 to 20.1 mm.

Hereinafter, embodiments of the present invention will be described.

The ferrite-based stainless hot-rolled annealed steel sheet of the present invention has C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P in mass%. : 0.04% or less, S: 0.01% or less, Al: 0.01 to 0.10%, Cr: 10.0 to 20.0%, Ni: 0.50 to 2.00%, Ti: It contains 0.10 to 0.40%, N: 0.001 to 0.020%, has a component composition consisting of Fe and unavoidable impurities in the balance, and has a metal structure having an average crystal grain size of 5 to 20 μm. It is a ferritic stainless hot-spread annealed steel sheet having a ferrite single-phase structure.

Hereinafter, the present invention will be described in detail.

The present inventors have ASTM A240 / 240M-S40975 (component composition is mass%, C ≦ 0.03%, Si ≦ 1.00%, Mn ≦ 1.00%, P ≦ 0.040%, S. Contains ≦ 0.030%, Cr: 10.5 to 11.7%, Ni: 0.50 to 1.00%, N ≦ 0.03%, Ti: 6 × (C + N) to 0.74% A flange having a hole of 20 mmφ was produced by punching with a clearance of 10% using various ferrite-based stainless steel plates having a plate thickness of 10 mm and having a hole of 20 mmφ. As a result, it was found that although cracking did not occur due to punching, the outer peripheral dimension and / or the central hole dimension of the flange may exceed the allowable tolerance of the component.

Furthermore, the present inventors have investigated in detail the reason why the dimensional accuracy in the punching process differs greatly depending on the steel sheet. As a result, when the average crystal grain size of the steel sheet subjected to the punching process is less than 5 μm, the component size after the punching process becomes smaller than the allowable tolerance, and the average crystal grain size of the steel sheet exceeds 20 μm. In this case, it was found that the size of the parts after punching tends to be larger than the allowable tolerance. From this, the present inventors cannot stably obtain sufficient dimensional accuracy in punching, because when the average crystal grain size is excessively small, the steel sheet is excessively hard and therefore during punching. It was found that the shear plane ratio of the steel sheet is small, and that if the average crystal grain size is excessively large, large sagging or burrs occur during punching.

Therefore, the present inventors have described a method for obtaining a ferritic stainless steel sheet having a metal structure having a ferrite single-phase structure having an average crystal grain size of 5 to 20 μm from the viewpoint of steel composition, hot rolling method, and hot-rolled sheet annealing method. Diligently examined. As a result, the steel components, especially the contents of Cr and Ni, are controlled to an appropriate range to generate an austenite phase and a ferrite phase in the hot rolling process, and then hot rolling is performed, and then the ferrite single phase temperature range is reached. It was found that it is effective to perform hot-rolled plate annealing in the appropriate temperature range.

Next, the hot-rolled plate annealing step is carried out by holding in an appropriate temperature range in the ferrite single-phase temperature range, specifically, at 600 ° C. or higher and lower than 750 ° C. for 1 minute to 24 hours. As a result, the recrystallization of the ferrite phase and the transformation of the martensite phase into the ferrite phase, which existed in the metal structure after hot rolling, are caused to obtain a ferrite single phase structure. At this time, when the annealing temperature of the hot-rolled sheet is less than 600 ° C., the recrystallization of the ferrite phase and the transformation of the martensite phase into the ferrite phase become insufficient, and punching cracks due to excessive hardening of the steel sheet occur. It is more likely to occur. On the other hand, when the annealing temperature becomes 750 ° C. or higher, the crystal grains become excessively coarse and the average crystal grain size exceeds 20 μm, and large sagging and burrs are likely to occur during the punching process, resulting in a predetermined dimensional accuracy during the punching process. I can't get it. When the holding time is less than 1 minute, the recrystallization of the ferrite phase and the transformation of the martensite phase into the ferrite phase become insufficient, and punching cracks due to excessive hardening of the steel sheet are likely to occur. When the holding time exceeds 24 hours, the crystal grains become excessively coarse and the average crystal grain size exceeds 20 μm, and large sagging and burrs are likely to occur during punching, so that predetermined dimensional accuracy can be obtained during punching. I can't. Therefore, in the present invention, it is necessary to perform hot rolled sheet annealing in a temperature range of 600 ° C. or higher and lower than 750 ° C. for 1 minute to 24 hours.

As described above, in the present invention, the metal structure is a ferrite single-phase structure, and the average crystal grain size of the ferrite single-phase structure is 5 to 20 μm. Preferably, the average crystal grain size is 7 μm or more, and more preferably 10 μm or more. Further, the average crystal grain size is preferably 18 μm or less, and more preferably 15 μm or less.

For the average crystal grain size, a test piece for microstructure observation was collected from the center of the plate width, the cross section in the rolling direction was mirror-polished, and then measured and analyzed in the field including the total thickness using the SEM / EBSD method. A boundary with an orientation difference of 15 ° or more is defined as a grain boundary and can be obtained based on the Area method.

The plate thickness of the ferrite-based stainless hot-rolled annealed steel sheet of the present invention is not particularly limited, but it is preferably 5.0 mm or more because it is desirable that the plate thickness is applicable to a thick flange. , 8.0 mm or more. The plate thickness is preferably 15.0 mm or less, more preferably 13.0 mm or less.

Next, the composition of the ferrite-based stainless hot-rolled annealed steel sheet of the present invention will be described.
Hereinafter, unless otherwise specified, "%", which is a unit of the content of a component, means "mass%".

C: 0.001 to 0.020%
When C is contained in excess of 0.020%, the workability and the corrosion resistance of the welded portion are significantly reduced. The smaller the C content is, the more preferable it is from the viewpoint of corrosion resistance and processability, but in order to reduce the C content to less than 0.001%, refining 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. Further, the C content is preferably 0.015% or less, and more preferably 0.012% or less.

Si: 0.05 to 1.00%
Si has the effect of concentrating on the oxide film formed during welding to improve the corrosion resistance of the welded portion, and is also a useful element as a deoxidizing element in the steelmaking process. These effects are obtained by containing 0.05% or more of Si, and the larger the content, the greater the effect. However, if the content of Si exceeds 1.00%, the rolling load in the hot rolling process increases and remarkable scale is generated, which is not preferable because it induces an increase in surface defects and an increase in manufacturing cost. Therefore, the Si content is set to 0.05 to 1.00%. The Si content is preferably 0.10% or more, more preferably 0.15% or more. Further, the Si content is preferably 0.60% or less, and more preferably 0.40% or less.

Mn: 0.05 to 1.00%
Mn is an austenite-producing element and has the effect of increasing the amount of austenite produced during heating before rolling in the hot rolling process. It also acts as a deoxidizing agent. In order to obtain the effect, it is necessary to contain Mn of 0.05% or more. However, when the Mn content exceeds 1.00%, the precipitation of MnS, which is the starting point of corrosion, is promoted, and the corrosion resistance is lowered. Therefore, the Mn content is set to 0.05 to 1.00%. The Mn content is preferably 0.10% or more, more preferably 0.15% or more. Further, the Mn content is preferably 0.60% or less, and more preferably 0.30% or less.

P: 0.04% or less P is an element inevitably contained in steel, but since it is an element harmful to corrosion resistance and workability, it is preferable to reduce it as much as possible. In particular, when the P content exceeds 0.04%, the workability is remarkably lowered due to the solid solution strengthening. Therefore, the P content is 0.04% or less. Preferably, the P content is 0.03% or less.

S: 0.01% or less S is also an element inevitably contained in steel like P, but since it is an element harmful to corrosion resistance and workability, it is preferable to reduce it as much as possible. In particular, when the S content exceeds 0.01%, the corrosion resistance is remarkably lowered. Therefore, the S content is 0.01% or less. Preferably, the S content is 0.008% or less. More preferably, the S content is 0.003% or less.

Al: 0.01-0.10%
Al is an effective deoxidizing agent. Further, since Al has a stronger affinity with nitrogen than Cr, when nitrogen invades the welded portion, nitrogen is precipitated as Al nitride instead of Cr nitride, which has an effect of suppressing sensitization. These effects are obtained by containing 0.01% or more of Al. However, if Al containing more than 0.10% is contained, the penetration property at the time of welding is lowered and the welding workability is lowered, which is not preferable. Therefore, the Al content is set in the range of 0.01 to 0.10%. The Al content is preferably 0.02% or more, and more preferably 0.03% or more. Further, the Al content is preferably 0.06% or less, and more preferably 0.04% or less.

Cr: 10.0-20.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the content is less than 10.0%, sufficient corrosion resistance cannot be obtained in an automobile exhaust gas atmosphere. On the other hand, if Cr is contained in an amount of more than 20.0%, the amount of austenite phase produced in the hot rolling process is insufficient even if a predetermined amount of Ni is contained, and the metal structure in the hot rolling process is refined. The effect is insufficient and the average crystal grain size after hot-rolled sheet annealing exceeds 20 μm, and a predetermined dimensional accuracy cannot be obtained during punching. Therefore, the Cr content is set in the range of 10.0 to 20.0%. Preferably, the Cr content is in the range of 10.0 to 17.0%. More preferably, the Cr content is 10.5% or more, and even more preferably 11.2% or more. Further, the Cr content is more preferably 12.0% or less, still more preferably 11.7% or less.

Ni: 0.50 to 2.00%
Ni is an austenite-forming element and has the effect of increasing the amount of austenite produced during heating before rolling in the hot rolling process. In the present invention, by controlling the contents of Cr and Ni to predetermined amounts, an austenite phase is generated during heating in the hot rolling step. Due to the formation of this austenite phase, the coarse metal structure formed during casting becomes finer, and the austenite phase undergoes dynamic and / or static recrystallization during hot rolling, so that the metal after hot rolling The structure is further refined, and as a result, it contributes to the miniaturization of the metal structure after hot rolling plate annealing. These effects can be obtained by containing 0.50% or more of Ni. On the other hand, when the Ni content exceeds 2.00%, punching cracks due to excessive hardening of the steel sheet after hot rolling annealing due to excessive solid solution Ni are likely to occur. Therefore, the Ni content is set to 0.50 to 2.00%. The Ni content is preferably 0.60% or more, more preferably 0.70% or more. More preferably, it is 0.75% or more. Further, the Ni content is more preferably 1.50% or less, still more preferably 1.00% or less.

Ti: 0.10 to 0.40%
Ti preferentially binds to C and N to suppress the precipitation of Cr carbonitride, lower the recrystallization temperature, and suppress the deterioration of corrosion resistance due to the sensitization caused by the precipitation of Cr carbonitride. There is. In order to obtain these effects, it is necessary to contain 0.10% or more of Ti. However, if the Ti content exceeds 0.40%, coarse Ti carbonitride is generated in the casting process, the toughness of the steel sheet is significantly lowered, and surface defects are caused, which is not preferable in manufacturing. Therefore, the Ti content is set to 0.10 to 0.40%. The Ti content is preferably 0.15% or more, more preferably 0.20% or more. Further, the Ti content is preferably 0.35% or less, and more preferably the Ti content is 0.30% or less. From the viewpoint of corrosion resistance of the welded portion, it is preferable that the Ti content satisfies the formula: Ti / (C + N) ≧ 8 (Ti, C and N in the formula are the contents (mass%) of each element). ..

N: 0.001 to 0.020%
When the N content exceeds 0.020%, the workability and the corrosion resistance of the welded portion are significantly reduced. From the viewpoint of corrosion resistance, a lower N content is preferable, but it is not preferable to reduce the N content to less than 0.001% because long-term refining is required, which leads to an increase in manufacturing cost and a decrease in productivity. .. Therefore, the N content is in the range of 0.001 to 0.020%. The N content is preferably 0.005% or more, more preferably 0.007% or more. Further, the N content is preferably 0.015% or less, and more preferably the N content is 0.012% or less.

The present invention is a ferritic stainless steel containing the above-mentioned essential components and having a balance composed of Fe and unavoidable impurities. Further, if necessary, one or more selected from Cu, Mo, W and Co, or / further, one selected from V, Nb, Zr, REM, B, Mg and Ca. Alternatively, two or more kinds can be contained in the following range. In addition, since the effect of the present invention is not impaired even if the following elements are contained below the lower limit value in the following range, when the following elements are contained below the lower limit value, the element is regarded as an unavoidable impurity.

Cu: 0.01-1.00%
Cu is an element particularly effective for improving the corrosion resistance of the base metal and the welded portion in an aqueous solution or when weakly acidic water droplets adhere to it. This effect is obtained by the content of 0.01% or more, and the effect becomes higher as the Cu content increases. However, if Cu is contained in an amount of more than 1.00%, the hot workability may be deteriorated and surface defects may be induced. Furthermore, it may be difficult to descale after annealing. Therefore, when Cu is contained, the Cu content is preferably in the range of 0.01 to 1.00%. More preferably, the Cu content is 0.10% or more, and even more preferably 0.30% or more. Further, the Cu content is more preferably 0.60% or less, 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. This effect is obtained by the content of 0.01% or more, and the effect is improved as the content is higher. However, if the Mo content exceeds 2.00%, the rolling load during hot rolling may increase, resulting in a decrease in manufacturability or an excessive increase in steel sheet strength. Moreover, since Mo is an expensive element, its content in a large amount increases the manufacturing cost. Therefore, when Mo is contained, the Mo content is preferably 0.01 to 2.00%. More preferably, the Mo content is 0.10% or more, and even more preferably 0.30% or more. Further, the Mo content is more preferably 1.40% or less, still more preferably 0.90% or less.

W: 0.01 to 0.20%
W has the same effect of improving corrosion resistance as Mo. This effect is obtained by containing 0.01% or more of W. However, if W is contained in excess of 0.20%, the strength increases, which may lead to a decrease in manufacturability due to an increase in rolling load or the like. Therefore, when W is contained, the W content is preferably in the range of 0.01 to 0.20%. More preferably, the W content is 0.05% or more. Further, more preferably, the W content is 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 processability may decrease. Therefore, when Co is contained, the Co content is preferably in the range of 0.01 to 0.20%.

V: 0.01 to 0.20%
V forms a carbonitride with C and N, suppresses sensitization during welding, and improves the corrosion resistance of the welded portion. This effect is obtained when the V content is 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%. More preferably, the V content is 0.02% or more. Further, more preferably, the V content is 0.050% or less.

Nb: 0.01-0.10%
Nb has the effect of increasing the yield strength by 0.2% by refining the crystal grains and precipitating them as fine carbonitrides. These effects are obtained with a Nb content of 0.01% or more. On the other hand, Nb also has the effect of raising the recrystallization temperature, and when the Nb content exceeds 0.10%, the annealing temperature required to generate sufficient recrystallization by hot-rolled sheet annealing becomes excessively high. Therefore, after annealing the hot-rolled plate, a ferrite single-phase structure having an average crystal grain size of 5 to 20 μm required by the present invention may not be obtained. Therefore, when Nb is contained, the Nb content is preferably in the range of 0.01 to 0.10%. More preferably, the Nb content is 0.01 to 0.05%.

Zr: 0.01-0.20%
Zr has the effect of binding to C and 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 excess of 0.20%, the processability may be significantly reduced. Therefore, when Zr is contained, the Zr content is preferably in the range of 0.01 to 0.20%. More preferably, the Zr content is in the range of 0.01 to 0.10%.

REM: 0.001 to 0.100%
REM (Rare Earth Metals) has the effect of improving oxidation resistance, suppresses the formation of an oxide film (welded temper color) in the welded portion, and suppresses the formation of a Cr-deficient region directly under the oxide film. This effect is obtained by containing 0.001% or more of REM. On the other hand, if REM is contained in excess of 0.100%, the hot workability may be deteriorated. Therefore, when REM is contained, the REM content is preferably in the range of 0.001 to 0.100%. More preferably, the REM content is in the range of 0.001 to 0.050%.

B: 0.0002 to 0.0025%
B is an element effective for improving the secondary processing brittleness after deep drawing molding. This effect can be obtained by setting the B content to 0.0002% or more. On the other hand, if B is contained in excess of 0.0025%, processability and toughness may decrease. Therefore, when B is contained, the B content is preferably in the range of 0.0002 to 0.0025%. More preferably, the B content is 0.0003% or more. Further, more preferably, the B content is 0.0006% or less.

Mg: 0.0005-0.0030%
Mg is an element that improves the equiaxed crystal ratio of the slab and is effective in improving workability and toughness. Further, in the steel containing Ti as in the present invention, the toughness decreases when the Ti carbonitride becomes coarse, but Mg also has an effect of suppressing the coarsening of the Ti carbonitride. These effects are 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 Mg content is preferably in the range of 0.0005 to 0.0030%. More preferably, the Mg content is 0.0010% or more. Further, more preferably, the Mg content is 0.0020% or less.

Ca: 0.0003 to 0.0030%
Ca is an effective component for preventing nozzle blockage due to crystallization of Ti-based inclusions that are likely to occur during continuous casting. The effect is obtained by containing 0.0003% or more of Ca. However, if Ca is contained in an amount of more than 0.0030%, the corrosion resistance may decrease due to the formation of CaS. Therefore, when Ca is contained, the Ca content is preferably in the range of 0.0003 to 0.0030%. More preferably, the Ca content is 0.0005% or more. Further, the Ca content is more preferably 0.0015% or less, still more preferably 0.0010% or less.

Next, a method for manufacturing the ferrite-based stainless hot-rolled annealed steel sheet of the present invention will be described.

The ferrite-based stainless hot-rolled annealed steel sheet of the present invention uses a steel slab having the above-mentioned composition to obtain a hot-rolled steel sheet by hot rolling in a conventional method, and further 600 ° C. or higher and lower than 750 ° C. with respect to the hot-rolled steel sheet. It is obtained by performing hot-rolled sheet annealing in which the steel is held for 1 minute to 24 hours.

First, the molten steel having the above-mentioned composition is melted by a known method such as a converter, an electric furnace, a vacuum melting furnace, etc., and a steel material (slab) is obtained by a continuous casting method or a ingot-breaking method.

The slab is heated at 1050 to 1250 ° C. for 1 to 24 hours, or is directly subjected to hot rolling as cast before the cast slab falls below the above temperature range. In the present invention, the method and conditions of hot rolling are not particularly limited, but when the winding process is performed at an excessively low temperature, the steel sheet after hot rolling becomes extremely hard and it is difficult to operate the next process. Therefore, the winding process is preferably performed at 550 ° C. or higher.

Hot-rolled plate annealing: Holds at 600 ° C or higher and lower than 750 ° C for 1 minute to 24 hours In the present invention, hot-rolled plate annealing is performed after the hot rolling step is completed. In hot-rolled sheet annealing, the rolled structure formed in the hot rolling process is recrystallized without excessive coarsening of the metal structure, and the martensite phase generated in the hot rolling process is transformed into a ferrite phase. Let me. In order to obtain this effect, it is necessary to perform hot rolled sheet annealing at 600 ° C. or higher and lower than 750 ° C. If the annealing temperature is less than 600 ° C., recrystallization is insufficient, the hot-rolled structure becomes fine recovery grains, the metal structure becomes excessively fine, and a predetermined dimensional accuracy cannot be obtained during punching. Further, even if the processed structure and the martensite phase remain in the metal structure after annealing of the hot-rolled sheet and the average crystal grain size is within a predetermined range, punching cracks due to excessive hardening of the steel sheet occur. May occur. On the other hand, when the annealing temperature is 750 ° C. or higher, the crystal grains become excessively coarse and exceed the average crystal grain size of 20 μm, and a predetermined dimensional accuracy cannot be obtained during punching. When the holding time is less than 1 minute, the processed structure and martensite phase remain in the metal structure after annealing of the hot-rolled sheet, and even if the average crystal grain size is within a predetermined range, the steel sheet is excessive. Punching cracks due to hardening are likely to occur. If the holding time exceeds 24 hours, the crystal grains become excessively coarse and the average crystal grain size exceeds 20 μm, and the predetermined dimensional accuracy cannot be obtained at the time of punching. Therefore, hot-rolled sheet annealing is performed by holding for 1 minute to 24 hours in a temperature range of 600 ° C. or higher and lower than 750 ° C. The hot rolled sheet annealing temperature is preferably 600 ° C. or higher, more preferably 640 ° C. or higher. Further, preferably, the hot-rolled plate annealing temperature is 700 ° C. or lower. The preferred retention time is 1 hour or longer, more preferably 6 hours or longer. The holding time is preferably 20 hours or less, more preferably 12 hours or less. The method of hot-rolled sheet annealing is not particularly limited, and either box annealing (batch annealing) or continuous annealing may be performed.

The obtained hot-rolled annealed steel sheet may be descaled by shot blasting or pickling, if necessary. Further, in order to improve the surface texture, grinding, polishing or the like may be performed. Further, the hot-rolled annealed steel sheet provided by the present invention may be subsequently cold-rolled and cold-rolled and annealed.

Hereinafter, the present invention will be described in detail with reference to Examples.

A stainless molten steel having the chemical composition shown in Table 1 was melted in a 100 kg vacuum melting furnace. These ingots are heated at 1100 ° C. for 1 hour, hot-rolled to the plate thickness shown in Table 2 (see the plate thickness at the end of hot rolling in Table 2), held at 650 ° C. for 1 hour, and then cooled in a furnace. A hot-rolled steel sheet was obtained by performing a winding simulation process. Then, after holding at the temperature shown in Table 2 (see the hot-rolled sheet annealing temperature in Table 2) for 8 hours, the hot-rolled sheet was annealed by slow cooling to obtain a hot-rolled annealed steel sheet.
The plate thickness of each of the obtained hot-rolled annealed steel sheets was the same as the plate thickness at which hot rolling was completed.
The hot-rolled annealed steel sheet thus obtained was evaluated as follows.

(1) Evaluation of metal structure A test piece for structure observation is collected from the center of the plate width, the cross section in the rolling direction is mirror-polished, and then measured and analyzed in the field including the total thickness using the SEM / EBSD method. The boundary of 15 ° or more was defined as the grain boundary, and the average crystal grain size was determined based on the Area method. Cases with an average crystal grain size of 5 μm or more and 20 μm or less are within the scope of the present invention, and cases of less than 5 μm or more than 20 μm are outside the scope of the present invention, and are underlined in Table 2.

Similarly, a test piece for microstructure observation is collected from the central part of the plate width, the cross section in the rolling direction is mirror-polished, and then corroded for observation with an aqueous solution of picrinic acid and hydrochloric acid to reveal the metal structure, and then the magnification is 500 times. By observing with an optical microscope of No. 1 and distinguishing between a ferrite phase and a martensite phase from the morphology of the metal structure, it was determined whether or not the metal structure of each steel plate has a ferrite single phase structure. Specifically, a region in which a uniform and flat morphology was observed in the crystal grains and a relatively bright contrast was observed was determined to be a ferrite phase. In addition, surface morphology peculiar to the martensite phase such as subgrain boundaries and block boundaries was observed in the crystal grains, and the region exhibiting a darker contrast than the ferrite phase was determined to be the martensite phase. In the table, F indicates that the metal structure was a ferrite single-phase structure.

(2) Evaluation of corrosion resistance A 60 × 100 mm test piece was collected from a hot-rolled annealed steel sheet, the surface was polished with # 600 emery paper, and then a test piece with an end face sealed was prepared and specified in JIS H8502. It was subjected to a salt spray cycle test. The salt spray cycle test consists of one cycle of salt spray (5% by mass NaCl, 35 ° C, spray 2hr) → dry (60 ° C, 4hr, relative humidity 40%) → wet (50 ° C, 2hr, relative humidity ≧ 95%). As a result, 5 cycles were performed. The surface of the test piece after 5 cycles of the salt spray cycle test was photographed, the rusted area on the surface of the test piece was measured by image analysis, and the rusted area ratio ((in the test piece)) was calculated from the ratio to the total area of the test piece. Rusted area / total area of test piece) × 100 [%]) was calculated. A rusted area ratio of 10% or less was passed with particularly excellent corrosion resistance (⊚), a rusted area ratio of more than 10% and 25% or less was passed (◯), and a rusted area ratio of more than 25% was rejected (×).

(3) Evaluation of punching workability After a 100 mm × 100 mm test piece is sampled from a hot-rolled annealed steel sheet, the diameter is 20 mm so that a hole of φ20 mm (tolerance ± 0.1 mm) is formed in the center of the test piece. Punching is performed by a crank press machine equipped with a lower die (die) having a hole appropriately selected so that the clearance between the upper die (punch) having a thinning columnar blade and the upper die is 10%. Five test pieces were prepared by processing. The above clearance (C) [%], the diameter of the hole of the die (inner diameter of the die) (Dd) [mm], and the diameter of the punch (Dp) [mm] are as follows, including the plate thickness (t) [mm]. It is expressed by the relation of the equation (1) of.
C = (Dd-Dp) ÷ (2 × t) × 100 ... Equation (1)
With respect to the test piece thus obtained, the appearance of the test piece was visually observed and the hole diameter at the center of the test piece was measured with a digital caliper. The case where there was no crack and the hole diameter after punching was in the range of 19.9 to 20.1 mm for all five test pieces was regarded as acceptable (◯). If any one of them had a crack, or if the hole diameter was less than 19.9 mm or more than 20.1 mm, it was rejected (x).

The test results are shown in Table 2 together with the hot-rolled sheet annealing conditions.

No. 1 in which the steel composition and the hot-rolled sheet annealing conditions satisfy the scope of the present invention. In Nos. 1 to 36, in addition to the formation of an austenite phase during heating in the hot rolling process, recrystallization occurs without causing excessive coarsening of crystal grains by predetermined hot-rolled sheet annealing, and predetermined average crystal grains occur. As a result of obtaining the diameter, a predetermined punching workability was obtained. Further, as a result of evaluating the corrosion resistance of the obtained hot-rolled annealed plate, it was confirmed that the rusted area ratio was 25% or less and that the hot-rolled annealed plate had sufficient corrosion resistance.

In particular, No. 1 using steel A19 containing Cu. 19. No. 1 using steel A21 containing Cu. 21, No. 20 using steel A20 containing Mo, and No. 20 using steel A22 containing Mo. In No. 22, the rusted area ratio was 10% or less, and more excellent corrosion resistance was obtained.

In addition, No. 1 using steel A3 having a high Cr content of 19.7%. No. 3 using steel A18 having a high Cr content of 19.6%. In No. 18, as a result of the passivation film formed on the surface of the steel sheet being strengthened, a rusted area ratio of 10% or less was obtained, and even better corrosion resistance was obtained.

No. 1 using steel B1 whose Ni content is below the range of the present invention. In 37, as a result of almost no austenite phase being generated during heating in the hot rolling process, the effect of miniaturizing the metal structure was not obtained, and as a result, the average crystal grain size exceeded the range of the present invention, and the predetermined punching workability was achieved. Was not obtained.

No. 1 using steel B2 having a Ni content exceeding the range of the present invention. In No. 38, although a predetermined average crystal grain size was obtained, the steel sheet was excessively hardened due to an excessive amount of solid solution Ni, and as a result, cracks were generated during punching and the steel sheet was processed into a predetermined shape. could not.

No. 1 using steel B3 whose Cr content is below the range of the present invention. In No. 39, as a result of insufficient Cr content, the predetermined corrosion resistance could not be obtained.

No. 1 using steel B4 having a Cr content exceeding the range of the present invention. In No. 40, although a predetermined amount of Ni was contained, the austenite phase generated during heating in the hot rolling step decreased due to the excessive content of Cr. As a result, the miniaturization effect due to the formation of the austenite phase could not be sufficiently obtained in the hot rolling process. As a result, a predetermined average crystal grain size could not be obtained, and a predetermined punching processability could not be obtained.

No. 1 using steel B5 whose Ti content is below the range of the present invention. In No. 41, sensitization occurred due to the precipitation of a large amount of Cr carbonitride during the annealing of the hot-rolled plate, and the predetermined corrosion resistance could not be obtained.

No. 1 whose hot-rolled sheet annealing temperature exceeds the range of the present invention. In No. 43, as a result of significant coarsening of the recrystallized grains produced, a predetermined average crystal grain size could not be obtained, and a predetermined punching processability could not be obtained.

No. Reference numeral 44 is an example in which the steel A14 having a predetermined steel component is annealed at 806 ° C., which exceeds the range of the present invention, and the average crystal grain size is coarsened to 34 μm, which exceeds the range of the present invention. Although it had a predetermined steel component, the crystal grains were excessively coarse, so that significant sagging and burrs occurred during the punching process, and the predetermined punching workability could not be obtained.

The ferrite-based stainless hot-rolled annealed steel sheet obtained in the present invention is particularly suitable for applications where high workability and corrosion resistance are required, for example, a flange having a burring processed portion.

Claims (4)

By mass%,
C: 0.001 to 0.020%,
Si: 0.05 to 0.40%,
Mn: 0.05 to 0.60%,
P: 0.04% or less,
S: 0.01% or less,
Al: 0.01-0.10%,
Cr: 11.2 to 20.0%,
Ni: 0.50 to 0.86 %,
Ti: 0.10 to 0.40%,
N: contains 0.001 to 0.020%, and has a component composition in which the balance is composed of Fe and unavoidable impurities.
A ferritic stainless hot-rolled annealed steel sheet having a ferrite single-phase structure with an average crystal grain size of 5 to 20 μm. By mass%, more
Cu: 0.01-1.00%,
Mo: 0.01-2.00%,
W: 0.01 to 0.20%,
Co: The ferrite-based stainless hot-rolled annealed steel sheet according to claim 1, which contains one or more selected from 0.01 to 0.20%. By mass%, more
V: 0.01-0.20%,
Nb: 0.01-0.10%,
Zr: 0.01-0.20%,
REM: 0.001 to 0.100%,
B: 0.0002 to 0.0025%,
Mg: 0.0005-0.0030%,
Ca: The ferrite-based stainless heat-rolled annealed steel sheet according to claim 1 or 2, which contains one or more selected from 0.0003 to 0.0030%. The method for manufacturing a ferrite-based stainless hot-rolled annealed steel sheet according to any one of claims 1 to 3.
A method for producing a ferritic stainless hot-rolled annealed steel sheet, which is annealed by hot-rolling a hot-rolled steel sheet obtained in a hot-rolling step and held at 600 ° C. or higher and lower than 750 ° C. for 1 minute to 24 hours.