KR20210064281A - Ferritic stainless steel hot-rolled annealing steel sheet and manufacturing method thereof - Google Patents

Abstract

To provide a ferritic stainless steel hot-rolled annealed steel sheet having sufficient corrosion resistance and excellent punching properties, in which cracks do not occur during punching to a thick flange and predetermined dimensional accuracy is obtained, 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 20.0% , Ni: 0.50 to 2.00%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, the balance has a component composition consisting of Fe and unavoidable impurities, and the metal structure is ferrite having an average grain size of 5 to 20 µm. It should be a single-phase organization.

Description

Ferritic stainless steel hot-rolled annealing steel sheet and manufacturing method thereof

The present invention relates to a ferritic stainless steel hot-rolled annealing steel sheet having excellent workability suitable for application to flanges and the like, and a method for manufacturing the same.

Recently, for the reduction of greenhouse gas emissions of CO _2, the method proceeds strengthening of regulations on an exhaust gas in vehicles. In order to reduce the CO ₂ emissions in the automobile exhaust gas, since the improvement of the fuel efficiency effect, there has been progress review of the heated to high temperature of the combustion temperature in the engine.

Exhaust gas generated by the engine is discharged 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 in order to prevent gas leakage. A flange applied to an exhaust system part needs to have sufficient dimensional accuracy as a fastening part.

Conventionally, ordinary steel has been used for such a thick flange. However, in recent years, in response to the demand for further improving the fuel efficiency of automobiles, further heating of the engine combustion temperature and exhaust gas from the engine is progressing. In connection with this, higher-temperature strength and corrosion resistance than conventional ones are required for flanges. In this context, recently, stainless steel with superior high-temperature strength and corrosion resistance than ordinary steel, in particular, a high-strength ferritic stainless steel sheet with a relatively small coefficient of thermal expansion and low thermal stress (e.g., ASTM A240/240M-S40975 (11 The application of a mass% Cr-Ti-Ni steel) having a thick plate thickness (for example, 5 mm or more in plate thickness) is progressing.

However, since the flange used for the exhaust system has a thick plate thickness (there are many 5 mm or more), cracks occur during punching when manufacturing the flange, and the flange parts cannot be manufactured properly. There is a problem that there is a problem, and there is a large demand for a thick ferritic stainless steel sheet having excellent punchability.

Regarding such a market request, 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 less than 18.0%, 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% and the balance is Fe and unavoidable impurities, the content of Nb, C and N satisfies Nb/(C+N) ≥ 16, the Charpy impact value at 0°C is 10 J/cm 2 or more, and the plate thickness A ferritic stainless hot-rolled steel sheet having a value of 5.0 to 9.0 mm is disclosed.

International Publication No. 2014/157576

The present inventors test-produced a ferritic stainless steel sheet having a sheet thickness of 10 mm having a steel component conforming to ASTM A240/240M-S40975 using the method disclosed in Patent Document 1, and a flange having a hole of 20 mmφ, It produced by punching with a clearance of 10%. As a result, although none of them were cracked by punching, it was clarified that the outer peripheral dimension and/or the central hole dimension of the flange sometimes exceeded the allowable tolerance of the part, which was not sufficient for application to thick flanges.

The present invention solves these problems, has sufficient corrosion resistance, does not generate cracks during punching processing for thick flanges, and obtains predetermined dimensional accuracy, and has excellent punching properties, and manufacturing thereof The purpose is to provide a method.

MEANS TO SOLVE THE PROBLEM The present inventors conducted detailed examination in order to solve the said subject. As a result, it was found that in order to obtain a predetermined dimensional accuracy without cracking in the punching process, the metal structure of the steel sheet should be a ferrite single-phase structure, and the average grain size should be controlled in the range of 5 to 20 µm. .

Then, hot rolling is performed on a ferritic stainless steel of an appropriate component, and the obtained hot-rolled steel sheet is annealed under suitable conditions to become a ferrite single-phase region, specifically, at 600°C or higher and lower than 750°C for 1 minute to 24 hours. It was discovered that the metal structure was a single ferrite phase and that the average grain size could be controlled within a range of 5 to 20 µm.

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

[1] 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 20.0%, Ni: 0.50 to 2.00%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, the balance has a component composition consisting of Fe and unavoidable impurities, and the metal structure has an average grain size of 5 to 20 A ferritic stainless steel hot-rolled annealing steel sheet having a micrometer ferrite single-phase structure.

[2] In mass%, Cu: 0.01 to 1.00%, Mo: 0.01 to 2.00%, W: 0.01 to 0.20%, Co: 0.01 to 0.20% of the above containing one or two or more The ferritic stainless steel hot-rolled annealing steel sheet according to [1].

[3] In mass%, 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: The ferritic stainless steel hot-rolled annealing steel sheet according to the above [1] or [2], containing one or two or more selected from the group consisting of 0.0003 to 0.0030%.

[4] The method for producing a ferritic stainless steel hot-rolled annealing steel sheet according to any one of [1] to [3], wherein the hot-rolled steel sheet obtained in the hot rolling process is maintained at 600° C. or higher and less than 750° C. for 1 minute to 24 hours. A method for producing a ferritic stainless hot-rolled annealing steel sheet in which hot-rolled sheet annealing is performed.

ADVANTAGE OF THE INVENTION According to this invention, while having sufficient corrosion resistance, the ferritic stainless steel hot-rolled annealing steel plate which has the outstanding punching property is obtained.

In addition, sufficient corrosion resistance in this invention means the salt spray cycle test (salt spray (5 mass) prescribed|regulated to JIS H 8502 on the steel plate which sealed the cross section after the surface was polished with #600 emery paper. % NaCl, 35 ° C., spraying 2 hr) → drying (60 ° C., 4 hr, relative humidity 40%) → wet (50 ° C., 2 hr, relative humidity ≥ 95%) 1 cycle test) was performed for 5 cycles It means that the rust occurrence area ratio (= rust occurrence area/steel plate total area x 100 [%]) in the steel plate surface in this case is 25 % or less.

In addition, in evaluation of punching workability, first, a 100 mm × 100 mm test piece is taken 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, the thickness of which is 20 mm in diameter. Five test pieces are produced by punching with a crank press machine provided with an upper die (punch) having a cylindrical cutting edge and a lower die (dice) having a hole having a diameter of 20 mm or more. In addition, punching is performed by adding the hole diameter of the lower die side to the test piece plate|board thickness and selecting so that the clearance of an upper die and a lower die may become 10 %. Here, the above-described clearance (C) [%], the hole diameter of the die (the inner diameter of the die) (Dd) [mm], and the diameter of the punch (Dp) [mm] include the plate thickness (t) [mm] It is represented by the relationship of the following formula (1).

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

The excellent punching property in the present invention means that, for the test piece obtained in this way, when the visual observation of the outer appearance of the test piece and the hole diameter at the center of the test piece are measured with a digital nogis, there is no crack, and the hole diameter after punching is 5 sheets It means to be in the range of 19.9-20.1 mm in all the test pieces.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described.

The ferritic stainless steel hot rolled annealed steel sheet of the present invention, 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 20.0%, Ni: 0.50 to 2.00%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, the balance has a component composition consisting of Fe and unavoidable impurities, and a metal It is a ferritic stainless steel hot-rolled annealing steel plate whose structure is a ferrite single phase structure of 5-20 micrometers with an average grain diameter.

Hereinafter, the present invention will be described in detail.

ASTM A240/240M-S40975 (component composition, in mass%, C ≤ 0.03%, Si ≤ 1.00%, Mn ≤ 1.00%, P ≤ 0.040%, S ≤ 0.030%, Cr: 10.5 to 11.7%, Ni: 0.50 to 1.00%, N ? 0.03%, Ti: 6x(C+N) to 0.74%, the balance consists of Fe and unavoidable impurities.) Various ferritic stainless steels with a plate thickness of 10 mm conforming to) A flange having a hole of 20 mm phi was produced using a steel plate by punching with a clearance of 10%. As a result, it was found that, although none of them were cracked due to punching, the outer peripheral dimension and/or the central hole dimension of the flange may exceed the allowable tolerance of the part.

Furthermore, the present inventors studied in detail the cause of the dimensional accuracy in punching processing differing greatly depending on the steel plate. As a result, when the average grain size of the steel sheet subjected to punching is less than 5 µm, the dimension of the part after punching becomes smaller than the allowable tolerance, and when the average grain diameter of the steel plate exceeds 20 µm, the dimension of the part after punching is within the allowable tolerance It was found that there is a tendency to increase From this, the present inventors believe that the reason that sufficient dimensional accuracy cannot be stably obtained in punching is that, when the average grain size is excessively small, the steel sheet is excessively hard, so the shear surface ratio at the time of punching processing. It was found that this decrease and, when the average grain size was excessively large, resulted from the occurrence of large rollovers or burrs during punching.

Therefore, the present inventors studied diligently from the viewpoint of a steel component, a hot rolling method, and a hot-rolled sheet annealing method about the method of obtaining the ferritic stainless steel sheet in which the metal structure becomes a ferrite single phase structure of 5-20 micrometers with an average grain diameter. As a result, the austenite phase and the ferrite phase are generated in the hot rolling process by controlling the contents of the steel components, particularly Cr and Ni to an appropriate range, and then hot rolling is performed, and then the hot-rolled sheet is in an appropriate temperature range of the ferrite single phase temperature range. It was found that performing annealing is effective.

Next, the hot-rolled sheet annealing step is performed by holding the ferrite single-phase temperature range in an appropriate temperature range, specifically 600°C or higher and lower than 750°C for 1 minute to 24 hours. Thereby, recrystallization of the ferrite phase which existed in the metal structure after hot rolling and transformation into the ferrite phase of a martensite phase are made to arise, and a ferrite single phase structure is obtained. At this time, when the annealing temperature of the hot-rolled sheet is less than 600°C, recrystallization of the ferrite phase and transformation of the martensite phase into a ferrite phase become insufficient, and punching cracks due to excessive hardening of the steel sheet are likely to occur. On the other hand, when the annealing temperature is 750°C or higher, the grains are excessively coarsened and the average grain size exceeds 20 µm, and large rollovers and burrs are likely to occur during punching, and the predetermined dimensional accuracy during punching is reduced. not obtained When the holding time is less than 1 minute, recrystallization of the ferrite phase and transformation of the martensite phase into a ferrite phase become insufficient, and punching cracking due to excessive hardening of the steel sheet tends to occur. When the holding time exceeds 24 hours, the crystal grains become excessively coarse and the average grain size exceeds 20 µm, and large rollovers and burrs are likely to occur during punching, so the predetermined dimensional accuracy during punching is not achieved. not obtained Therefore, in this invention, it is necessary to perform the hot-rolled sheet annealing hold|maintained in the temperature range of 600 degreeC or more and less than 750 degreeC for 1 minute - 24 hours.

As described above, 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 5 to 20 µm. Preferably, this average grain size is 7 micrometers or more, More preferably, it is 10 micrometers or more. Moreover, Preferably, this average grain size is 18 micrometers or less, More preferably, it is 15 micrometers or less.

In addition, with respect to the average grain size, a test piece for tissue observation is taken from the central part of the plate width, and the cross section in the rolling direction is mirror polished, and then measured and analyzed in a field of view including the entire thickness using the SEM / EBSD method. , can be obtained based on the area method by defining a boundary with a difference of 15° or more as a grain boundary.

In addition, the plate thickness of the ferritic stainless hot-rolled annealing steel sheet of the present invention is not particularly limited, but since it is preferably a plate thickness applicable to a thick flange, it is preferably 5.0 mm or more, more preferably 8.0 mm More than that. Moreover, as for plate|board thickness, it is preferable to set it as 15.0 mm or less, More preferably, it is 13.0 mm or less.

Next, the component composition of the ferritic stainless steel hot-rolled annealing steel plate of this invention is demonstrated.

Hereinafter, unless otherwise indicated, "%" which is a unit of content of a component means "mass %".

C: 0.001 to 0.020%

When C is contained exceeding 0.020%, the fall of workability and the fall of corrosion resistance of a weld part will become remarkable. A smaller C content is preferable from the viewpoints of corrosion resistance and workability, 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%. Preferably, the C content is 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 is an element useful also as a deoxidation element in a steelmaking process while having the effect of being concentrated in the oxide film formed at the time of welding and improving the corrosion resistance of a weld part. These effects are obtained by containing 0.05% or more of Si, and the effect becomes large, so that there is much content. However, when Si is contained exceeding 1.00 %, since the increase of the rolling load in a hot rolling process and generation|occurrence|production of a remarkable scale generate|occur|produce, and induces an increase in surface defects, and a raise in manufacturing cost, it is unpreferable. Therefore, Si content shall be 0.05 to 1.00 %. Preferably, the Si content is 0.10% or more, more preferably 0.15% or more. Moreover, Preferably Si content is 0.60 % or less, More preferably, it is 0.40 % or less.

Mn: 0.05 to 1.00%

Mn is an austenite generating element, and has an effect of increasing the amount of austenite generated during heating before rolling in the hot rolling process. Moreover, it also acts as a deoxidizer. In order to obtain the effect, it is necessary to contain 0.05% or more of Mn. However, when Mn content exceeds 1.00 %, precipitation of MnS used as the origin of corrosion is accelerated|stimulated, and corrosion resistance falls. Therefore, Mn content shall be 0.05 to 1.00 %. Preferably, the Mn content is 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

Although P is an element unavoidably contained in steel, since it is a harmful element compared with corrosion resistance and workability, it is preferable to reduce it as much as possible. In particular, when P content exceeds 0.04 %, workability will fall remarkably by solid solution strengthening. Therefore, the P content is made 0.04% or less. Preferably, the P content is 0.03% or less.

S: 0.01% or less

Although S is also an element unavoidably contained in steel like P, since it is a harmful element compared with corrosion resistance and workability, it is preferable to reduce it as much as possible. In particular, when S content exceeds 0.01 %, corrosion resistance will fall remarkably. Therefore, the S content is made 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 deoxidizer. In addition, since Al has a stronger affinity with nitrogen than Cr, when nitrogen penetrates into a welding part, nitrogen is precipitated not as Cr nitride but as Al nitride, and there is an effect of suppressing sensitization. These effects are obtained by containing 0.01% or more of Al. However, when Al exceeding 0.10 % is contained, since the solubility at the time of welding will fall and welding workability will fall, it is unpreferable. Therefore, Al content is made into the range of 0.01 to 0.10%. Preferably, the Al content is 0.02% or more, more preferably 0.03% or more. Moreover, Preferably, Al content is 0.06 % or less, More preferably, it is 0.04 % or less.

Cr: 10.0 to 20.0%

Cr is the most important element in order to secure 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, when Cr is contained in excess of 20.0%, even if a predetermined amount of Ni is contained, the amount of the austenite phase produced in the hot rolling process is insufficient, and the effect of refining the metal structure in the hot rolling process becomes insufficient, and hot rolling becomes insufficient. The average grain size after plate annealing exceeds 20 µm, and predetermined dimensional accuracy is not obtained at the time of punching. Therefore, the Cr content is made into a range of 10.0 to 20.0%. Preferably, the Cr content is in the range of 10.0 to 17.0%. More preferably, Cr content is 10.5 % or more, More preferably, it is 11.2 % or more. Moreover, more preferably, Cr content is 12.0 % or less, More preferably, it is 11.7 % or less.

Ni: 0.50 to 2.00%

Ni is an austenite generating element, and has an effect of increasing the amount of austenite generated during heating before rolling processing in the hot rolling process. In this invention, an austenite phase is produced|generated at the time of heating in a hot rolling process by controlling content of Cr and Ni to predetermined amounts. By the generation of this austenite phase, the coarse metal structure formed at the time of casting is refined, and the austenite phase undergoes dynamic and/or static recrystallization during hot rolling, so that the metal structure after hot rolling is further refined, and as a result, 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 Ni content exceeds 2.00 %, it will become easy to generate|occur|produce the punching crack resulting from excessive hardening of the steel plate after hot-rolling annealing by excess solid solution Ni. Therefore, Ni content shall be 0.50 to 2.00 %. Preferably, the Ni content is 0.60% or more, more preferably 0.70% or more. More preferably, it is 0.75 % or more. Moreover, more preferably, Ni content is 1.50 % or less, More preferably, it is 1.00 % or less.

Ti: 0.10 to 0.40%

Ti binds preferentially with C and N, suppresses precipitation of Cr carbonitride, lowers the recrystallization temperature, and has an effect of suppressing a decrease in corrosion resistance resulting from sensitization by precipitation of Cr carbonitride. In order to obtain these effects, it is necessary to contain 0.10% or more of Ti. However, when Ti content exceeds 0.40%, coarse Ti carbonitride is produced|generated in a casting process, and since the toughness of a steel plate falls remarkably, and also causes surface defects, it is unpreferable in manufacturing. Therefore, Ti content shall be 0.10 to 0.40 %. Preferably, the Ti content is 0.15% or more, and more preferably 0.20% or more. Moreover, Preferably Ti content is 0.35 % or less, More preferably, Ti content is 0.30 % or less. Moreover, from a viewpoint of corrosion resistance of a weld part, it is preferable to set it as Ti content which satisfies Formula: Ti/(C+N) >=8 (Ti, C, and N in the formula are content (mass %) of each element).

N: 0.001 to 0.020%

When N content exceeds 0.020 %, the fall of workability and the fall of corrosion resistance of a weld part will become remarkable. From the viewpoint of corrosion resistance, the lower the N content, the more preferable. However, in order to reduce the N content to less than 0.001%, refining for a long time is required, which is not preferable because it causes an increase in manufacturing cost and a decrease in productivity. Therefore, the N content is in the range of 0.001 to 0.020%. Preferably, the N content is 0.005% or more, more preferably 0.007% or more. Moreover, Preferably, the N content is 0.015% or less, More preferably, the N content is 0.012% or less.

The present invention is a ferritic stainless steel characterized in that it contains the above essential components and the balance consists of Fe and unavoidable impurities. Furthermore, as needed, one or two or more selected from Cu, Mo, W and Co, or/and one or two or more selected from V, Nb, Zr, REM, B, Mg and Ca may be contained in the following range. In addition, since the effect of this invention is not impaired even if it contains the following elements below a lower limit in the following range, when containing the following elements below a lower limit, let that element be an unavoidable impurity.

Cu: 0.01 to 1.00%

Cu is an element particularly effective in improving the corrosion resistance of the base metal and the weld zone in aqueous solution or when weakly acidic water droplets adhere. This effect is obtained by containing 0.01% or more, and the effect becomes high, so that there is much Cu content. However, when Cu is contained exceeding 1.00 %, hot workability may fall and a surface defect may be attracted|attracted. Furthermore, descaling after annealing may become difficult. Therefore, when containing Cu, it is preferable to make Cu content into 0.01 to 1.00% of range. More preferably, Cu content is 0.10 % or more, More preferably, it is 0.30 % or more. Moreover, more preferably, Cu content is 0.60 % or less, More preferably, it is 0.45 % or less.

Mo: 0.01 to 2.00%

Mo is an element that remarkably improves the corrosion resistance of stainless steel. This effect is obtained by containing 0.01% or more, and the effect improves so that there is much content. However, when Mo content exceeds 2.00 %, the rolling load at the time of hot rolling may become large, manufacturability may fall, or an excessive raise of steel plate intensity|strength may generate|occur|produce. Moreover, since Mo is an expensive element, containing a large amount increases manufacturing cost. Therefore, when it contains Mo, it is preferable to make Mo content into 0.01 to 2.00 %. More preferably, Mo content is 0.10 % or more, More preferably, it is 0.30 % or more. Moreover, more preferably, Mo content is 1.40 % or less, More preferably, it is 0.90 % or less.

W: 0.01 to 0.20%

W has an effect of improving corrosion resistance similarly to Mo. This effect is obtained by containing 0.01% or more of W. However, when W is contained exceeding 0.20 %, intensity|strength will rise and the fall of productivity by increase of a rolling load etc. may be caused. Therefore, when containing W, it is preferable to make W content into 0.01 to 0.20% of range. More preferably, the W content is 0.05% or more. Further, more preferably, the W content is 0.15% or less.

Co: 0.01 to 0.20%

Co is an element which improves toughness. This effect is obtained by containing 0.01% or more of Co. On the other hand, when Co content exceeds 0.20 %, workability may fall. Therefore, when it contains Co, it is preferable to make Co content into 0.01 to 0.20% of range.

V: 0.01 to 0.20%

V forms a carbonitride with C and N, suppresses sensitization at the time of welding, and improves the corrosion resistance of a welding part. This effect is obtained when the V content is 0.01% or more. On the other hand, when V content exceeds 0.20 %, workability and toughness may fall remarkably. Therefore, it is preferable to make V content into 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 raising the yield strength by 0.2% since it precipitates as a fine carbonitride while making a crystal grain refinement|miniaturization. These effects are obtained by containing 0.01% or more of Nb. On the other hand, Nb also has an effect of raising the recrystallization temperature, and when the Nb content exceeds 0.10%, the annealing temperature required to cause sufficient recrystallization in hot-rolled sheet annealing becomes excessively high. Therefore, the present invention is necessary after hot-rolled sheet annealing. A ferrite single-phase structure having an average grain size of 5 to 20 µm may not be obtained. Therefore, when containing Nb, it is preferable to make Nb content into 0.01 to 0.10% of range. More preferably, the Nb content is 0.01 to 0.05%.

Zr: 0.01 to 0.20%

Zr has an effect of suppressing sensitization by bonding with C and N. This effect is obtained by containing 0.01% or more of Zr. On the other hand, when Zr is contained exceeding 0.20 %, workability may fall remarkably. Therefore, when Zr is contained, it is preferable to make Zr content into 0.01 to 0.20% of range. 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 an 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 region immediately below the oxide film. This effect is obtained by containing 0.001% or more of REM. On the other hand, when REM is contained exceeding 0.100 %, hot workability may be reduced. Therefore, in the case of containing REM, 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 effective element in order to improve the secondary processing brittleness resistance after deep drawing molding. This effect is obtained by making content of B into 0.0002 % or more. On the other hand, when B is contained exceeding 0.0025 %, workability and toughness may fall. Therefore, when B is contained, it is preferable to make B content into 0.0002 to 0.0025% of range. More preferably, the B content is 0.0003% or more. Moreover, more preferably, B content is 0.0006 % or less.

Mg: 0.0005 to 0.0030%

Mg is an element effective for improving the equiaxed crystal ratio of the slab and improving workability and toughness. Further, in steel containing Ti as in the present invention, toughness decreases when Ti carbonitride is coarsened, but Mg also has an effect of suppressing coarsening of Ti carbonitride. These effects are obtained by containing 0.0005% or more of Mg. On the other hand, when Mg content exceeds 0.0030 %, the surface properties of steel may be deteriorated. Therefore, when containing Mg, it is preferable to make Mg content into 0.0005 to 0.0030 % of range. 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 clogging of nozzles 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, when Ca is contained exceeding 0.0030 %, corrosion resistance may fall by generation|generation of CaS. Accordingly, 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. Moreover, more preferably, Ca content is 0.0015 % or less, More preferably, it is 0.0010 % or less.

Next, the manufacturing method of the ferritic stainless steel hot-rolled annealing steel plate of this invention is demonstrated.

The ferritic stainless steel hot-rolled annealing steel sheet of the present invention uses a steel slab having the above component composition to obtain a hot-rolled steel sheet by hot rolling of a conventional method, and furthermore the hot-rolled steel sheet at 600° C. or more and less than 750° C. for 1 minute It is obtained by performing the hot-rolled sheet annealing hold|maintained for - 24 hours.

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

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

Annealing of hot-rolled sheet: 1 minute to 24 hours at 600 °C or higher and lower than 750 °C

In the present invention, hot-rolled sheet annealing is performed after the completion of the hot rolling process. Annealing of a hot-rolled sheet WHEREIN: While recrystallizing the rolling structure formed in the hot-rolling process, without coarsening a metal structure excessively, the martensite phase produced|generated by the hot-rolling process is transformed into a ferrite phase. In order to acquire this effect, it is necessary to perform hot-rolled sheet annealing at 600 degreeC or more and less than 750 degreeC. When the annealing temperature is less than 600°C, recrystallization becomes insufficient, the hot-rolled structure becomes fine recovered grains, and the metal structure becomes excessively fine, and the desired dimensional accuracy cannot be obtained at the time of punching. Moreover, in the metal structure after hot-rolled sheet annealing, a processed structure and a martensite phase remain, and even if an average grain size is within a predetermined range, the punching crack resulting from excessive hardening of a steel sheet may generate|occur|produce. On the other hand, when the annealing temperature is 750°C or higher, the grains are excessively coarsened and the average grain size exceeds 20 µm, and the predetermined dimensional accuracy cannot be obtained at the time of punching. When the holding time is less than 1 minute, the processed structure or martensite phase remains in the metal structure after the annealing of the hot-rolled sheet, and even if the average grain size is within a predetermined range, punching cracks due to excessive hardening of the steel sheet are not generated. it gets easier When 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 hold|maintaining at the temperature range of 600 degreeC or more and less than 750 degreeC for 1 minute - 24 hours. Preferably, the hot-rolled sheet annealing temperature is 600°C or higher, more preferably 640°C or higher. Moreover, Preferably, the hot-rolled sheet annealing temperature is 700 degrees C or less. Preferred holding time is 1 hour or more, More preferably, it is 6 hours or more. Moreover, the preferable holding time is 20 hours or less, More preferably, it is 12 hours or less. In addition, there is no limitation in particular in the method of hot-rolled sheet annealing, You may implement by either box annealing (batch annealing) and continuous annealing.

You may perform the descaling process by shot blasting or pickling to the obtained hot-rolled annealing steel plate as needed. Furthermore, in order to improve surface properties, you may perform grinding, grinding|polishing, etc. Moreover, the hot-rolled annealing steel sheet provided by this invention may perform cold rolling and cold-rolled sheet annealing after that.

Example

Hereinafter, the present invention will be described in detail by way of Examples.

Molten stainless steel having a 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 is performed to the plate thickness shown in Table 2 (refer to the plate thickness after hot rolling in Table 2), followed by holding at 650 ° C. for 1 h, then performing a furnace-cooling simulation treatment, It was set as the hot-rolled steel plate. Then, after holding|maintenance for 8 hours at the temperature described in Table 2 (refer the hot-rolled sheet annealing temperature in Table 2), the hot-rolled sheet annealing was performed, and the hot-rolled annealing steel sheet was obtained.

In addition, the plate|board thickness of each obtained hot-rolled annealing steel plate was the same as each hot-rolled finished plate|board thickness.

The hot-rolled annealing steel sheet obtained in this way was evaluated below.

(1) Evaluation of metal structure

After taking a test piece for tissue observation from the center of the sheet width, mirror polishing the cross section in the rolling direction, measurement and analysis are performed in a field of view including the entire thickness using the SEM/EBSD method, and a boundary of 15° or more in orientation difference is defined as a grain boundary, The average grain size was calculated based on the area method. The case where the average grain size is 5 µm or more and 20 µm or less is within the scope of the present invention, and the case of less than 5 µm or more than 20 µm is outside the scope of the present invention, and underlined in Table 2.

Also, similarly, a specimen for tissue observation is taken from the central part of the plate width, the cross section in the rolling direction is mirror polished, followed by corrosion for observation with an aqueous solution of picric acid-hydrochloric acid to reveal the metal structure, and then using an optical microscope with a magnification of 500 times. By observing and distinguishing a ferrite phase and a martensite phase from the form of a metal structure, it was determined whether the metal structure of each steel plate was a ferrite single-phase structure. Specifically, a region in which a uniform and flat shape was observed in the crystal grains and exhibiting a relatively bright contrast was determined as a ferrite phase. In addition, in the crystal grains, a surface shape peculiar to a martensite phase such as a granular system and a block boundary was observed, and a region showing a dark contrast compared to the ferrite phase was determined as the martensite phase. In the table, F indicates that the metal structure is a ferrite single-phase structure.

(2) Evaluation of corrosion resistance

A 60 x 100 mm test piece was taken from a hot-rolled annealed steel sheet, the surface was polished and finished with #600 emery paper, and then a test piece in which the cross-section was sealed was produced, and subjected to a salt spray cycle test specified in JIS H 8502. The salt spray cycle test is salt spray (5 mass % NaCl, 35 °C, 2 hr spraying) → dry (60 °C, 4 hr, 40% relative humidity) → wet (50 °C, 2 hr, relative humidity > 95%) 5 cycles were performed as 1 cycle. The surface of the test piece after performing the salt spray cycle test for 5 cycles was photographed, the area of rust occurrence on the surface of the test piece was measured by image analysis, and the rust occurrence area ratio ((Rust occurrence area in the test piece / test piece) from the ratio with the total area of the test piece total area) x 100 [%]) was calculated. 10% or less of rust generation area rate was made into the pass ((circle)) by especially excellent corrosion resistance, 10% and 25% or less were made into pass (circle), and more than 25% were made into rejection (x).

(3) Evaluation of punching workability

After taking a 100 mm x 100 mm test piece from a hot-rolled annealing steel sheet, an upper mold (punch) having a cylindrical blade for thickness reduction with a diameter of 20 mm so that a hole of φ20 mm (tolerance ±0.1 mm) is formed in the center of the test piece (punch) Five test pieces were produced by punching with a crank press machine equipped with a lower die (dice) having a hole appropriately selected so that the clearance between the upper and upper die was 10%. The above-described clearance (C) [%], the diameter of the hole of the die (the inner diameter of the die) (Dd) [mm], and the diameter of the punch (Dp) [mm] include the plate thickness (t) [mm], It is represented by the relationship of the following formula (1).

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

About the test piece obtained in this way, visual observation of the test piece external appearance and the hole diameter of the center part of a test piece were measured with the digital furnace. The case where there was no crack and the hole diameter after punching was in the range of 19.9-20.1 mm in all the test pieces of 5 sheets was set as the pass (circle). The case where there was a crack in any one sheet, or the hole diameter was less than 19.9 mm or more than 20.1 mm was made into rejection (x).

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

In Nos. 1 to 36, in which the steel component and the hot-rolled sheet annealing conditions satisfy the range of the present invention, in addition to the austenite phase generated during heating in the hot rolling process, excessive coarsening of crystal grains by predetermined hot-rolled sheet annealing As a result of recrystallization taking place without causing chatter and a predetermined average grain size was obtained, predetermined punchability was obtained. Further, as a result of evaluating the corrosion resistance of the obtained hot-rolled annealing sheet, it was confirmed that the rust occurrence area ratio was 25% or less, and 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. The corrosion resistance was further excellent with a rust occurrence area ratio of 10% or less.

Moreover, in No. 3 using steel A3 with a high Cr content as 19.7 %, and No. 18 using steel A18 with a high Cr content as 19.6 %, the passivation film formed on the steel plate surface became strong. As a result, the rust occurrence area rate was 10% Further excellent corrosion resistance was obtained below.

In No. 37 using steel B1 whose Ni content was less than the range of the present invention, as a result, almost no austenite phase was produced during heating in the hot rolling process, and as a result, the effect of refining the metal structure was not obtained. As a result, the average grain size was Exceeding the scope of the invention, the predetermined punchability was not obtained.

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

In No. 39 using steel B3 whose Cr content is less than the range of this invention, as a result of the Cr content being insufficient, predetermined corrosion resistance was not acquired.

In No. 40 using steel B4 having a Cr content exceeding the range of the present invention, the austenite phase generated during heating in the hot rolling process is reduced due to the excessive Cr content despite containing a predetermined amount of Ni. did. Thereby, in the hot rolling process, the refinement|miniaturization effect by generation|generation of an austenite phase was not fully acquired. As a result, the predetermined average grain size was not obtained, and the predetermined punchability was not obtained.

In No. 41 using steel B5 whose Ti content is less than the range of this invention, sensitization by Cr carbonitride precipitating abundantly at the time of hot-rolled sheet annealing occurred, and predetermined corrosion resistance was not obtained.

In No. 43 in which the hot-rolled sheet annealing temperature exceeded the range of this invention, as a result of remarkable coarsening of the produced|generated recrystallized grain, a predetermined average grain size was not obtained, but predetermined punching property was not obtained.

No. 44 is an example in which steel A14 having a predetermined steel composition was annealed at 806°C exceeding the range of the present invention, and the average grain size was coarsened to 34 µm exceeding the range of the present invention. Although it had a predetermined steel component, since the crystal grains were excessively coarse, significant rollover and burr|burr occurred at the time of punching, and predetermined punching workability was not obtained.

Industrial applicability

The ferritic stainless hot-rolled annealing steel sheet obtained in the present invention is particularly suitable for applications requiring high workability and corrosion resistance, for example, for flanges having a burring portion.

Claims (4)

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 20.0%,
Ni: 0.50 to 2.00%;
Ti: 0.10 to 0.40%,
N: contains 0.001 to 0.020%, the balance has a component composition consisting of Fe and unavoidable impurities,
A ferritic stainless steel hot-rolled annealing steel sheet in which the metal structure is a ferrite single-phase structure having an average grain size of 5 to 20 µm. The method of claim 1,
In mass %, further,
Cu: 0.01 to 1.00%,
Mo: 0.01 to 2.00%,
W: 0.01 to 0.20%,
Co: A ferritic stainless hot-rolled annealing steel sheet containing one or two or more selected from 0.01 to 0.20%. The method according to claim 1 or 2,
In % by 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 to 0.0030%,
Ca: A ferritic stainless steel hot-rolled annealing steel sheet containing one or two or more selected from 0.0003 to 0.0030%. A method for producing the ferritic stainless steel hot-rolled annealing steel sheet according to any one of claims 1 to 3, comprising:
A method for producing a ferritic stainless steel hot-rolled annealed steel sheet, wherein the hot-rolled steel sheet obtained in the hot-rolling step is annealed at 600°C or higher and lower than 750°C for 1 minute to 24 hours.