KR20230142630A - Ferrite stainless hot-rolled-and-annealed steel sheet and production method for same - Google Patents

Abstract

To provide a ferritic stainless steel hot-rolled annealed steel sheet with sufficient corrosion resistance and excellent punching properties that do not cause cracks and achieve a given dimensional accuracy when punching into a thick flange, and a method for manufacturing the same. In 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 10.0% 20.0% , Ni: 0.50 to 2.00%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, and has a component composition where the balance consists of Fe and inevitable impurities, and the metal structure is ferrite with an average grain size of 5 to 20 μm. Make sure it is a single-phase organization.

Description

Ferritic stainless steel hot rolled annealed steel sheet and method of manufacturing the same {FERRITE STAINLESS HOT-ROLLED-AND-ANNEALED STEEL SHEET AND PRODUCTION METHOD FOR SAME}

The present invention relates to a ferritic stainless steel hot-rolled annealed steel sheet with excellent processability suitable for application to flanges, etc., and a method of manufacturing the same.

Recently, in order to reduce _CO2 emissions, which are greenhouse gases, laws and regulations regarding exhaust gases from automobiles are being strengthened. In order to reduce CO ₂ emissions from automobile exhaust gases, improvements in fuel efficiency are effective, and therefore, increasing the combustion temperature in engines is being studied.

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

Conventionally, ordinary steel has been used for such thick flanges. However, in recent years, in response to demands to further improve automobile fuel efficiency, efforts have been made to further increase engine combustion temperature and exhaust gas from the engine. In line with this, high-temperature strength and corrosion resistance of flanges that exceed those of the past have been required. Against this background, recently, stainless steels have been developed that have better high-temperature strength and corrosion resistance than ordinary steel, especially high-strength ferritic stainless steel sheets that have a relatively small coefficient of thermal expansion and do not easily generate thermal stress (e.g., ASTM A240/240M-S40975 (11 Mass% Cr-Ti-Ni steel) with a thick plate (for example, 5 mm or more in plate thickness) is being applied.

However, because the flanges used in exhaust systems are thick (often 5 mm or more), cracks occur during punching when manufacturing the flanges, making it impossible to manufacture the flange parts properly. There is a problem, and there is a great demand for thick ferritic stainless steel sheets with excellent punching properties.

Regarding such market demands, for example, in Patent Document 1, in 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 18.0% or less, Ni: 0.05 to 1%, Nb: 0.3 to 0.6%, Ti: 0.05% or less, N: 0.020% or less, Al: 0.10% or less, B: 0.0002 to 0.0020% to The balance is Fe and inevitable impurities, the Nb, C and N contents satisfy Nb/(C + N) ≥ 16, the Charpy impact value at 0°C is 10 J/cm2 or more, and the plate thickness is A ferritic stainless steel hot-rolled steel sheet with a thickness of 5.0 to 9.0 mm is disclosed.

International Publication No. 2014/157576

Using the method disclosed in Patent Document 1, the present inventors test-manufactured a ferritic stainless steel plate with a steel composition conforming to ASTM A240/240M-S40975 with a thickness of 10 mm, and a flange with a hole of 20 mmϕ, It was produced by punching with a clearance of 10%. As a result, although no cracks occurred due to punching, it became clear that the outer circumferential dimension of the flange and/or the central hole dimension sometimes exceeded the allowable tolerance of the part and were not sufficient for application to thick flanges.

The present invention solves these problems and provides a ferritic stainless steel hot-rolled annealed steel sheet with sufficient corrosion resistance and excellent punching properties that do not cause cracks and achieve a predetermined dimensional accuracy during punching into a thick flange, and the production thereof. The purpose is to provide a method.

The present inventors conducted detailed studies to solve the above problems. As a result, it was discovered that in order to obtain the desired dimensional accuracy without cracks occurring during punching, the metal structure of the steel sheet should be a ferrite single-phase structure, and the average grain size should be controlled within the range of 5 to 20 μm. .

Then, hot rolling is performed on a ferritic stainless steel with an appropriate composition, and the obtained hot rolled steel sheet is annealed under appropriate conditions that bring it into the ferrite single phase range, specifically at a temperature of 600°C or more and less than 750°C for 1 minute to 24 hours. By doing this, it was found that the metal structure was a single phase of ferrite and that the average grain size could be controlled in the range of 5 to 20 μm.

The present invention has been made based on the above knowledge, and has the following as a summary.

[1] C 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 -20.0%, Ni: 0.50-2.00%, Ti: 0.10-0.40%, N: 0.001-0.020%, the balance is Fe and inevitable impurities, and the metal structure has an average grain size of 5-20. A ferritic stainless steel hot-rolled annealed steel sheet with a ㎛ ferrite single-phase structure.

[2] In mass%, the above product further contains one or two or more selected from among Cu: 0.01 to 1.00%, Mo: 0.01 to 2.00%, W: 0.01 to 0.20%, and Co: 0.01 to 0.20%. The ferritic stainless steel hot rolled annealed steel sheet described in [1].

[3] In mass%, further: V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.20%, REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg: 0 .0005 ∼ 0.0030 %, Ca: 0.0003 to 0.0030%. The ferritic stainless steel hot-rolled annealed steel sheet according to [1] or [2] above, containing one or more types selected from the group consisting of 0.0003 to 0.0030%.

[4] A method for manufacturing a ferritic stainless steel hot-rolled annealed steel sheet according to any one of [1] to [3] above, comprising maintaining the hot-rolled steel sheet obtained in the hot rolling process at a temperature of 600°C or more and less than 750°C for 1 minute to 24 hours. A method of manufacturing a ferritic stainless steel hot-rolled annealing steel sheet that performs hot-rolled annealing.

According to the present invention, a ferritic stainless steel hot-rolled annealed steel sheet having sufficient corrosion resistance and excellent punching properties is obtained.

In addition, sufficient corrosion resistance in the present invention refers to a salt spray cycle test (salt spray (5 mass % NaCl, 35 ℃, spraying 2 hr) → drying (60 ℃, 4 hr, relative humidity 40%) → wetting (50 ℃, 2 hr, relative humidity ≥ 95%) as 1 cycle) was performed for 5 cycles. This means that the rust occurrence area rate (= rust occurrence area/total area of the steel plate x 100 [%]) on the surface of the steel sheet in this case is 25% or less.

In addition, to evaluate the punching properties, first, a test piece of 100 mm Five test pieces were produced by punching using a crank press equipped with an upper mold (punch) having a cylindrical blade for cutting and a lower mold (die) having a hole with a diameter of 20 mm or more. In addition, the punching process is performed by selecting the hole diameter on the lower mold side by adding it to the test piece thickness so that the clearance between the upper mold and the lower mold is 10%. Here, the clearance (C) [%], the hole diameter of the die (inner diameter of the die) (Dd) [mm], and the diameter of the punch (Dp) [mm] include the plate thickness (t) [mm]. It is expressed by the relationship in equation (1) below.

C = (Dd - Dp) ÷ (2 × t) × 100 ···Equation (1)

The excellent punching properties in the present invention mean that, for the test piece obtained in this way, when the external appearance of the test piece is visually observed and the hole diameter in the center of the test piece is measured with a digital nozzle, there are no cracks, and the hole diameter after punching is 5 pieces. This means that it is in the range of 19.9 to 20.1 mm for all test pieces.

Hereinafter, embodiments of the present invention will be described.

The ferritic stainless steel hot-rolled annealed steel sheet of the present invention contains, in 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: It contains 0.01 to 0.10%, Cr: 10.0 to 20.0%, Ni: 0.50 to 2.00%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, and has a chemical composition with the remainder being Fe and inevitable impurities. It is a ferritic stainless steel hot-rolled annealed steel sheet whose structure is a single-phase ferrite structure with an average grain size of 5 to 20 μm.

Hereinafter, the present invention will be described in detail.

The inventors are ASTM A240/240m-s40975 (component composition is mass %, C ≤ 0.03 %, Si ≤ 1.00 %, Mn ≤ 1.00 %, p ≤ 0.040 %, S ≤ 0.030 %, CR: 10.5 to 11.7 %, Contains Ni: 0.50 to 1.00%, N ≤ 0.03%, Ti: 6 A flange with a hole of 20 mmϕ was produced using a steel plate by punching with a clearance of 10%. As a result, although cracks due to punching did not occur in any of them, it was found that the outer circumferential dimension of the flange and/or the hole dimension at the center sometimes exceeded the allowable tolerance of the part.

Additionally, the present inventors studied in detail the reasons why dimensional accuracy during punching varies greatly depending on the steel sheet. As a result, when the average grain size of the steel sheet provided for punching is less than 5 ㎛, the part size after punching becomes smaller than the allowable tolerance, and when the average grain size of the steel sheet is more than 20 ㎛, the part size after punching becomes less than the allowable tolerance. It has been observed that there is a tendency for it to become larger. From this, the present inventors found that the reason why sufficient dimensional accuracy cannot be stably obtained during punching is that when the average grain size is excessively small, the steel sheet is excessively hard, and the shear surface ratio during punching is It was found that this decrease was due to the occurrence of large rollover or burrs during punching when the average grain size was excessively large.

Therefore, the present inventors carefully studied the method of obtaining a ferritic stainless steel sheet whose metal structure is a ferrite single-phase structure with an average grain size of 5 to 20 μm from the viewpoint of steel composition, hot rolling method, and hot-rolled sheet annealing method. As a result, the content of steel components, especially Cr and Ni, is 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 a hot rolled sheet is produced at an appropriate temperature range of the ferrite single phase temperature range. It was found that annealing is effective.

Next, the hot-rolled sheet annealing process is performed by holding the sheet at an appropriate temperature range in the ferrite single phase temperature range, specifically, 600°C or more and less than 750°C for 1 minute to 24 hours. In this way, recrystallization of the ferrite phase existing in the metal structure after hot rolling and transformation of the martensite phase into the ferrite phase are caused to occur, thereby obtaining a ferrite single-phase structure. At this time, if the hot-rolled sheet annealing temperature is set to less than 600°C, the recrystallization of the ferrite phase and the transformation of the martensite phase into the ferrite phase become insufficient, making it easy for punching cracks to occur due to excessive hardening of the steel sheet. On the other hand, when the annealing temperature is 750°C or higher, the crystal grains become excessively coarse, the average grain size exceeds 20 μm, large rollover and burrs are likely to occur during punching, and the predetermined dimensional accuracy during punching is lost. not obtained If 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, making it easy for punching cracks to occur due to excessive hardening of the steel sheet. If the holding time exceeds 24 hours, the crystal grains become excessively coarse, the average grain size exceeds 20 μm, and large rollover or burrs are likely to occur during punching, so the predetermined dimensional accuracy during punching is lost. not obtained Therefore, in the present invention, it is necessary to perform hot-rolled sheet annealing maintained at a temperature range of 600°C or more and less than 750°C for 1 minute to 24 hours.

In this way, in the present invention, the metal structure is a ferrite single-phase structure, and the average grain size of the ferrite single-phase structure is set to 5 to 20 μm. Preferably, this average grain size is 7 ㎛ or more, more preferably 10 ㎛ or more. Also, the average grain size is preferably 18 μm or less, and more preferably 15 μm or less.

In addition, for the average grain size, a test piece for structure observation was taken from the center of the sheet width, the cross section in the rolling direction was mirror polished, and measurement and analysis were performed using the SEM/EBSD method in a field of view covering the entire thickness. , the boundary with an azimuth difference of 15° or more is defined as a grain boundary and can be obtained based on the area method.

In addition, the sheet thickness of the ferritic stainless steel hot rolled annealed steel sheet of the present invention is not particularly limited, but since it is desirable to have a sheet thickness that can be applied to thick flanges, it is preferably 5.0 mm or more, and more preferably 8.0 mm. That's it. Additionally, the plate thickness is preferably 15.0 mm or less, and more preferably 13.0 mm or less.

Next, the component composition of the ferritic stainless steel hot-rolled annealed steel sheet of the present invention will be explained.

Hereinafter, unless otherwise specified, “%” as a unit of component content means “% by mass.”

C: 0.001 to 0.020%

If C is contained in excess of 0.020%, the processability decreases and the corrosion resistance of the weld zone decreases significantly. A smaller C content is preferable from the viewpoint of corrosion resistance and processability, but setting the C content to less than 0.001% requires time for refining, which is undesirable from a manufacturing perspective. Therefore, the C content is set in the range of 0.001 to 0.020%. Preferably the C content is 0.003% or more, and more preferably 0.004% or more. Also, 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 improving the corrosion resistance of the weld zone by concentrating in the oxide film formed during welding, 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 Si is contained in excess of 1.00%, the rolling load in the hot rolling process increases and significant scale formation occurs, which leads to an increase in surface defects and an increase in manufacturing cost, which is undesirable. Therefore, the Si content is set to 0.05 to 1.00%. Preferably the Si content is 0.10% or more, and more preferably 0.15% or more. Also, 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 forming element and has the effect of increasing the amount of austenite generated during heating before rolling processing in the hot rolling process. Additionally, it also acts as a deoxidizer. In order to obtain the effect, Mn content of 0.05% or more is required. However, when the Mn content exceeds 1.00%, precipitation of MnS, which becomes the starting point of corrosion, is promoted, and corrosion resistance decreases. Therefore, the Mn content is set to 0.05 to 1.00%. Preferably the Mn content is 0.10% or more, and more preferably 0.15% or more. Also, 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 that is inevitably contained in steel, but it is a harmful element compared to corrosion resistance and workability, so it is desirable to reduce it as much as possible. In particular, when the P content exceeds 0.04%, processability significantly deteriorates due to solid solution strengthening. Therefore, the P content is set to 0.04% or less. Preferably the P content is 0.03% or less.

S: 0.01% or less

Like P, S is an element that is inevitably contained in steel, but it is a harmful element compared to corrosion resistance and workability, so it is desirable to reduce it as much as possible. In particular, when the S content exceeds 0.01%, corrosion resistance significantly decreases. Therefore, the S content is set to 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 to 0.10%

Al is an effective deoxidizing agent. Additionally, since Al has a stronger affinity for nitrogen than Cr, when nitrogen enters the weld zone, nitrogen is precipitated as Al nitride rather than Cr nitride, which has the effect of suppressing sensitization. These effects are obtained by containing 0.01% or more of Al. However, containing Al exceeding 0.10% is not preferable because solubility during welding decreases and welding workability decreases. Therefore, the Al content is set in the range of 0.01 to 0.10%. Preferably the Al content is 0.02% or more, and more preferably 0.03% or more. Also, the Al content is preferably 0.06% or less, and more preferably 0.04% or less.

Cr: 10.0 to 20.0%

Cr is the most important element to ensure 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 excess of 20.0%, even if a predetermined amount of Ni is contained, the amount of austenite phase generated in the hot rolling process is insufficient, and the effect of refining the metal structure in the hot rolling process becomes insufficient, resulting in hot rolling. The average grain size after plate annealing exceeds 20 μm, so the desired 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. Moreover, the Cr content is more preferably 12.0% or less, and even 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 generated during heating before rolling processing in the hot rolling process. In the present invention, the austenite phase is generated during heating in the hot rolling process by controlling the Cr and Ni contents to a predetermined amount. Due to the generation of this austenite phase, the coarse metal structure formed during casting is refined, and the austenite phase undergoes dynamic and/or static recrystallization during hot rolling, so the metal structure after hot rolling is further refined, resulting in hot rolling. It contributes to the refinement of the metal structure after plate annealing. These effects are obtained by containing 0.50% or more of Ni. On the other hand, when the Ni content exceeds 2.00%, punching cracks resulting from excessive hardening of the steel sheet after hot rolling annealing due to excessive dissolved Ni are likely to occur. Therefore, the Ni content is set to 0.50 to 2.00%. Preferably the Ni content is 0.60% or more, and more preferably 0.70% or more. More preferably, it is 0.75% or more. Moreover, the Ni content is more preferably 1.50% or less, and even more preferably 1.00% or less.

Ti: 0.10 to 0.40%

Ti combines preferentially with C and N, suppressing the precipitation of Cr carbonitride, lowering the recrystallization temperature, and suppressing the decline in corrosion resistance caused by sensitization due to precipitation of Cr carbonitride. In order to obtain these effects, the inclusion of 0.10% or more Ti is required. However, if the Ti content exceeds 0.40%, coarse Ti carbonitride is generated during the casting process, which not only significantly reduces the toughness of the steel sheet but also causes surface defects, which is not preferable for manufacturing. Therefore, the Ti content is set to 0.10 to 0.40%. Preferably the Ti content is 0.15% or more, and more preferably 0.20% or more. Also, the Ti content is preferably 0.35% or less, and more preferably, the Ti content is 0.30% or less. Furthermore, from the viewpoint of weld zone corrosion resistance, it is preferable to set the Ti content to satisfy the formula: Ti/(C + N) ≥ 8 (wherein Ti, C, and N are the content (mass%) of each element).

N: 0.001 to 0.020%

When the N content exceeds 0.020%, the workability decreases and the corrosion resistance of the weld zone decreases significantly. From the viewpoint of corrosion resistance, a lower N content is more preferable, but reducing the N content to less than 0.001% requires long-term refining, which is undesirable because it causes an increase in manufacturing cost and a decrease in productivity. Therefore, the N content is set in the range of 0.001 to 0.020%. Preferably the N content is 0.005% or more, and more preferably 0.007% or more. Also, 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 characterized by containing the above essential components and the balance consisting of Fe and inevitable impurities. Additionally, if necessary, one or two or more types selected from Cu, Mo, W and Co, or/additionally one or two or more types selected from V, Nb, Zr, REM, B, Mg and Ca. It may be contained in the following range. In addition, since the effect of the present invention is not impaired even if the elements below are contained below the lower limit in the range below, when the elements below are contained below the lower limit, the elements are regarded as inevitable impurities.

Cu: 0.01 to 1.00%

Cu is an element that is particularly effective in improving the corrosion resistance of the base material and weld zone in an aqueous solution or when weakly acidic water droplets adhere. This effect is obtained by containing 0.01% or more of Cu, and the effect becomes higher as the Cu content increases. However, if Cu is contained in excess of 1.00%, hot workability may decrease and surface defects may be induced. Furthermore, descaling after annealing may become difficult. Therefore, when it contains Cu, it is preferable that the Cu content is 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. Moreover, the Cu content is more preferably 0.60% or less, and even more preferably 0.45% or less.

Mo: 0.01 to 2.00%

Mo is an element that significantly improves the corrosion resistance of stainless steel. This effect is obtained by containing 0.01% or more, and the effect improves as the content increases. However, when the Mo content exceeds 2.00%, the rolling load during hot rolling increases, which may reduce manufacturability or cause an excessive increase in the strength of the steel sheet. Additionally, since Mo is an expensive element, its inclusion in large amounts increases manufacturing costs. Therefore, when it contains Mo, it is preferable that the Mo content is 0.01 to 2.00%. More preferably, the Mo content is 0.10% or more, and even more preferably 0.30% or more. Moreover, the Mo content is more preferably 1.40% or less, and even more preferably 0.90% or less.

W: 0.01 to 0.20%

W, like Mo, has the effect of improving corrosion resistance. 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, etc. Therefore, when it contains W, it is preferable that the W content is in the range of 0.01 to 0.20%. More preferably, the W content is 0.05% or more. Moreover, more preferably, the W content is 0.15% or less.

Co: 0.01 to 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, when the Co content exceeds 0.20%, processability may decrease. Therefore, when it contains Co, it is preferable that the Co content is in the range of 0.01 to 0.20%.

V: 0.01 to 0.20%

V forms carbonitride with C and N, suppresses sensitization during welding, and improves the corrosion resistance of the weld zone. This effect is obtained when the V content is 0.01% or more. On the other hand, when the V content exceeds 0.20%, workability and toughness may decrease significantly. Therefore, it is preferable that the V content is 0.01 to 0.20%. More preferably, the V content is 0.02% or more. Moreover, more preferably, the V content is 0.050% or less.

Nb: 0.01 to 0.10%

Nb has the effect of refining the crystal grains and increasing the yield strength by 0.2% because it precipitates as fine carbonitride. These effects are achieved by containing 0.01% or more of Nb. On the other hand, Nb also has the effect of increasing the recrystallization temperature. When the Nb content exceeds 0.10%, the annealing temperature required to cause sufficient recrystallization in hot-rolled sheet annealing becomes excessively high, so the present invention is necessary after annealing the hot-rolled sheet. There are cases where a ferrite single phase structure with an average grain size of 5 to 20 μm cannot be obtained. Therefore, when containing Nb, it is preferable that the Nb content is in the range of 0.01 to 0.10%. More preferably, the Nb content is 0.01 to 0.05%.

Zr: 0.01 to 0.20%

Zr has the effect of suppressing sensitization by combining with C and N. 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%, processability may be significantly reduced. Therefore, when containing Zr, 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 (weld temper color) in the weld zone, and suppresses the formation of a Cr-depleted area immediately beneath 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%, hot workability may be reduced. Therefore, when containing REM, it is preferable that the REM content is 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 secondary processing brittleness resistance after deep drawing molding. This effect is obtained by setting the B content to 0.0002% or more. On the other hand, if B is contained in excess of 0.0025%, workability and toughness may decrease. Therefore, when it contains B, it is preferable that the B content is in the range of 0.0002 to 0.0025%. More preferably, the B content is 0.0003% or more. Moreover, more preferably, the B content is 0.0006% or less.

Mg: 0.0005 to 0.0030%

Mg is an element that is effective in improving the equiaxed crystallinity of the slab and improving workability and toughness. Additionally, in steel containing Ti as in the present invention, toughness decreases when Ti carbonitride coarsens, but Mg also has the effect of suppressing coarsening of Ti carbonitride. These effects are obtained by containing 0.0005% or more of Mg. On the other hand, when the Mg content exceeds 0.0030%, the surface properties of the steel may deteriorate. 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. Moreover, more preferably, the Mg content is 0.0020% or less.

Ca: 0.0003 to 0.0030%

Ca is an effective component in preventing blockage of the nozzle due to crystallization of Ti-based inclusions, which tend to occur during continuous casting. The effect is obtained by containing 0.0003% or more of Ca. However, if Ca is contained in excess of 0.0030%, corrosion resistance may decrease due to the formation of CaS. Therefore, when it contains Ca, it is preferable that the Ca content is in the range of 0.0003 to 0.0030%. More preferably, the Ca content is 0.0005% or more. Moreover, the Ca content is more preferably 0.0015% or less, and even more preferably 0.0010% or less.

Next, the manufacturing method of the ferritic stainless steel hot rolled annealed steel sheet of the present invention will be described.

The ferritic stainless steel hot-rolled annealed steel sheet of the present invention uses a steel slab having the above chemical composition to obtain a hot-rolled steel sheet by hot rolling in a conventional manner, and the hot-rolled steel sheet is further heated at 600°C or higher and lower than 750°C for 1 minute. It is obtained by performing hot-rolled sheet annealing for ~24 hours.

First, molten steel with the above-mentioned composition is melted by a known method such as a converter, electric furnace, or vacuum melting furnace, and then made into a steel material (slab) by continuous casting or ingot-disintegration method.

This slab is heated at 1050 to 1250°C for 1 to 24 hours, or is directly subjected to hot rolling as cast before the slab after casting falls below the above temperature range. In the present invention, there are no particular restrictions on the hot rolling method and conditions, but if the coiling treatment is performed at an excessively low temperature, the steel sheet after hot rolling may become significantly hardened, making operation of the next process difficult. , the winding treatment is preferably performed at 550°C or higher.

Hot rolled sheet annealing: Maintained above 600℃ but below 750℃ for 1 minute to 24 hours.

In the present invention, annealing of the hot rolled sheet is performed after completion of the hot rolling process. In hot-rolled sheet annealing, the rolled structure formed in the hot rolling process is recrystallized without excessively coarsening the metal structure, and the martensite phase generated in the hot rolling process is transformed into a ferrite phase. In order to obtain this effect, it is necessary to perform hot-rolled sheet annealing at a temperature of 600°C or more and less than 750°C. If the annealing temperature is less than 600°C, recrystallization becomes insufficient, the hot-rolled structure becomes fine recovered grains, the metal structure becomes excessively fine, and the desired dimensional accuracy cannot be obtained during punching. In addition, in the metal structure after annealing the hot-rolled sheet, a processed structure or martensite phase remains, and even if the average grain size is within a predetermined range, there are cases where punching cracks occur due to excessive hardening of the steel sheet. On the other hand, when the annealing temperature is 750°C or higher, the crystal grains become excessively coarse and the average grain size exceeds 20 μm, making it impossible to obtain the desired dimensional accuracy during punching. If the holding time is less than 1 minute, the processed structure or martensite phase remains in the metal structure after annealing the hot-rolled sheet, and even if the average grain size is within a predetermined range, punching cracks may occur due to excessive hardening of the steel sheet. It gets easier. If the holding time exceeds 24 hours, the crystal grains become excessively coarse and the average grain size exceeds 20 μm, making it impossible to obtain the desired dimensional accuracy during punching. Therefore, hot-rolled sheet annealing is performed by maintaining the temperature in the range of 600°C or more and less than 750°C for 1 minute to 24 hours. Preferably, the hot-rolled sheet annealing temperature is 600°C or higher, and more preferably 640°C or higher. Also, preferably, the annealing temperature of the hot rolled sheet is 700°C or lower. A preferable holding time is 1 hour or more, and more preferably 6 hours or more. Moreover, the preferable holding time is 20 hours or less, and more preferably 12 hours or less. Additionally, there is no particular limitation on the method of annealing the hot rolled sheet, and it may be performed by either box annealing (batch annealing) or continuous annealing.

If necessary, the obtained hot rolled annealed steel sheet may be subjected to descaling treatment by shot blasting or pickling. Additionally, in order to improve the surface properties, grinding or polishing may be performed. In addition, the hot rolled annealed steel sheet provided by the present invention may be subsequently subjected to cold rolling and cold rolled annealing.

Example

Hereinafter, the present invention will be described in detail by examples.

Molten stainless steel having the chemical composition shown in Table 1 was melted in a 100 kg vacuum melting furnace. After heating these steel ingots at 1100°C for 1 hour, hot rolling was performed to the plate thickness shown in Table 2 (refer to the plate thickness at the end of hot rolling in Table 2), and then held at 650°C for 1 h, followed by a rolling simulation treatment of yellow cooling. It was made of hot rolled steel sheet. Next, after holding for 8 hours at the temperature shown in Table 2 (refer to the hot-rolled sheet annealing temperature in Table 2), annealing of the hot-rolled sheet with slow cooling was performed to obtain a hot-rolled annealed steel sheet.

In addition, the sheet thickness of each obtained hot rolled annealed steel sheet was the same as each hot rolled finished sheet thickness.

The following evaluation was performed on the hot rolled annealed steel sheet obtained in this way.

(1) Evaluation of metal structure

A test piece for tissue observation is collected from the center of the sheet width, the cross section in the rolling direction is mirror polished, and measurement and analysis are performed in a field of view covering the entire thickness using the SEM/EBSD method, and the boundary with an azimuth difference of 15° or more is defined as a grain boundary. The average grain size was obtained based on the area method. Cases where the average grain size is 5 ㎛ or more and 20 ㎛ or less are considered to be within the scope of the present invention, and cases where the average grain size is less than 5 ㎛ or more than 20 ㎛ are considered to be outside the scope of the present invention, and are underlined in Table 2.

In addition, similarly, a test piece for structure observation was taken from the center of the sheet width, the cross section in the rolling direction was mirror polished, and then corrosion was performed for observation with an aqueous picric acid-hydrochloric acid solution to expose the metal structure, and then an optical microscope with a magnification of 500x was used. By observing and distinguishing between the ferrite phase and the martensite phase based on the shape of the metal structure, it was determined whether the metal structure of each steel sheet was a ferrite single-phase structure. Specifically, a region in which a constant and flat shape was observed within the crystal grains and a relatively bright contrast was determined to be a ferrite phase. In addition, surface forms unique to the martensite phase, such as sub-grain systems and block boundaries, were observed within the crystal grains, and regions showing a darker contrast compared to the ferrite phase were determined to be martensite phases. In the table, F indicates that the metal structure is a ferrite single phase structure.

(2) Evaluation of corrosion resistance

A test piece measuring 60 The salt spray cycle test is salt spray (5% by mass NaCl, 35°C, spray 2 hr) → dry (60°C, 4 hr, relative humidity 40%) → wet (50°C, 2 hr, relative humidity ≥ 95%). 5 cycles were performed as 1 cycle. The surface of the test piece was photographed after 5 cycles of the salt spray cycle test, the rust area on the surface of the test piece was measured through image analysis, and the rust area rate ((rust area in the test piece/test piece) was determined from the ratio to the total area of the test piece. Total area) × 100 [%]) was calculated. A rust occurrence area rate of 10% or less was considered to be a pass (◎) with particularly excellent corrosion resistance, a rate of more than 10% and 25% or less was considered a pass (○), and a rate of more than 25% was considered a failure (×).

(3) Evaluation of punching processability

After taking a test piece of 100 mm Five test pieces were produced by punching using a crank press machine equipped with a lower mold (die) having holes appropriately selected so that the clearance between the upper and upper molds was 10%. The clearance (C) [%], the diameter of the die hole (inner diameter of the die) (Dd) [mm], and the diameter of the punch (Dp) [mm] include the plate thickness (t) [mm], It is expressed by the relationship in equation (1) below.

C = (Dd - Dp) ÷ (2 × t) × 100 ···Equation (1)

For the test piece obtained in this way, the external appearance of the test piece was visually observed and the hole diameter in the center of the test piece was measured using a digital nozzle. The case where there were no cracks and the hole diameter after punching was in the range of 19.9 to 20.1 mm for all five test pieces was evaluated as passing (○). If any one sheet had a crack or a hole diameter of less than 19.9 mm or more than 20.1 mm, it was rated as rejected (×).

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

Nos. 1 to 36, whose steel composition and hot-rolled sheet annealing conditions satisfy the scope of the present invention, in addition to the formation of an austenite phase during heating in the hot rolling process, excessive coarsening of crystal grains due to a predetermined hot-rolled sheet annealing As a result of recrystallization occurring without causing cracking and a predetermined average grain size being obtained, the predetermined punching processability was obtained. Additionally, as a result of evaluating the corrosion resistance of the obtained hot-rolled annealed sheets, it was confirmed that the rust occurrence area ratios were all 25% or less and that they also had sufficient corrosion resistance.

In particular, No.19 using steel A19 containing Cu, No.21 using steel A21 containing Cu, No.20 using steel A20 containing Mo, and No.22 using steel A22 containing Mo. In , more excellent corrosion resistance was obtained with a rust occurrence area ratio of 10% or less.

In addition, in No.3 using steel A3 with a high Cr content of 19.7% and No.18 using steel A18 with a high Cr content of 19.6%, the passive film formed on the surface of the steel sheet became stronger, resulting in a rust occurrence area rate of 10%. Further excellent corrosion resistance was obtained as follows.

In No. 37 using steel B1 whose Ni content is below the range of the present invention, almost no austenite phase was generated during heating in the hot rolling process, and as a result, the effect of refining the metal structure was not obtained, and the average grain size was as follows. As the scope of the invention was exceeded, the desired punching properties could not be obtained.

In No. 38, which used steel B2 with a Ni content exceeding the range of the present invention, a predetermined average grain size was obtained, but because the amount of dissolved Ni was excessive, the steel sheet was excessively hardened and cracks occurred during punching. occurred and could not be processed into the desired shape.

In No. 39 using steel B3 whose Cr content was below the range of the present invention, the desired corrosion resistance was not obtained as a result of the insufficient Cr content.

In No. 40 using steel B4 whose Cr content exceeds the range of the present invention, despite containing a predetermined amount of Ni, the austenite phase generated during heating in the hot rolling process decreases due to the excessive Cr content. did. As a result, the refinement effect due to the formation of the austenite phase in the hot rolling process was not sufficiently obtained. As a result, the prescribed average grain size was not obtained, and the prescribed punching properties were not obtained.

In No. 41 using steel B5 with a Ti content below the range of the present invention, sensitization occurred due to a large amount of Cr carbonitride precipitating during annealing of the hot-rolled sheet, and the desired corrosion resistance could not be obtained.

In No. 43, where the hot-rolled sheet annealing temperature exceeded the range of the present invention, the generated recrystallized grains were significantly coarsened, and as a result, the desired average grain size was not obtained, and the desired punching properties were not obtained.

No. 44 is an example in which steel A14 with a predetermined steel component was annealed at 806°C, which is above the range of the present invention, and the average grain size was coarsened to 34 μm, which is above the range of the present invention. Although it had a specified steel component, the crystal grains were excessively coarse, and significant rollover and burrs occurred during punching, and the desired punching properties were not obtained.

The ferritic stainless steel hot-rolled annealed steel sheet obtained in the present invention is particularly suitable for applications requiring high processability and corrosion resistance, for example, flanges having burring processing portions, etc.

Claims (5)

In mass%,
C: 0.001 to 0.015%,
Si: 0.10 to 1.00%,
Mn: 0.05 to 0.60%,
P: 0.04% or less,
S: 0.01% or less,
Al: 0.01 to 0.10%,
Cr: 10.5 to 20.0%,
Ni: 0.50 to 0.86%,
Ti: 0.20 to 0.40%,
N: Contains 0.001 to 0.015%, and has a component composition where the balance consists of Fe and inevitable impurities,
The metal structure is a single-phase ferrite structure with an average grain size of 5 to 20 μm,
After taking a test piece of 100 mm A test piece is produced by punching using a crank press machine equipped with a lower mold so that the clearance between the upper mold and the lower mold is 10%, there are no cracks, and the hole diameter after punching is in the range of 19.9 to 20.1 mm. ,
After polishing the surface with #600 emery paper, a steel plate with a sealed cross section was subjected to a salt spray cycle test specified in JIS H 8502 (salt spray (5% by mass NaCl, 35°C, spray 2 hr) → dry (60°C, 4 hr, relative humidity 40%) → Wetting (50°C, 2 hr, relative humidity ≥ 95%) as 1 cycle) is performed for 5 cycles (= rust occurrence area on the surface of the steel sheet) /Total area of the steel sheet × 100 [%]) is 25% or less. A ferritic stainless steel hot rolled annealed steel sheet. According to claim 1,
In mass%, additionally,
Cu: 0.01 to 0.60%,
Mo: 0.01 to 2.00%,
W: 0.01 to 0.20%,
Co: A ferritic stainless steel hot-rolled annealed steel sheet containing one or two or more types selected from 0.01 to 0.20%. The method of claim 1 or 2,
In mass%, additionally,
V: 0.01 to 0.20%,
Nb: 0.01 to 0.10%,
Zr: 0.01 to 0.20%,
REM: 0.001 to 0.100%,
B: 0.0002 to 0.0025%,
Mg: 0.0005 to 0.0030%,
Ca: 0.0003 to 0.0030% A ferritic stainless steel hot-rolled annealed steel sheet containing one or two or more types selected from among. A method for manufacturing the ferritic stainless steel hot rolled annealed steel sheet according to claim 1 or 2,
A method for producing a ferritic stainless steel hot-rolled annealed steel sheet, which involves performing hot-rolled sheet annealing at a temperature of 600°C or higher but lower than 750°C for 1 minute to 24 hours on the hot-rolled steel plate obtained in the hot rolling process. A method for manufacturing the ferritic stainless steel hot rolled annealed steel sheet according to claim 3, comprising:
A method for producing a ferritic stainless steel hot-rolled annealed steel sheet, which involves performing hot-rolled sheet annealing at a temperature of 600°C or higher but lower than 750°C for 1 minute to 24 hours on the hot-rolled steel plate obtained in the hot rolling process.