KR102274976B1 - Ferritic stainless hot-rolled steel sheet and manufacturing method thereof - Google Patents

Abstract

A ferritic stainless steel hot-rolled steel sheet having sufficient corrosion resistance and capable of suppressing cracking during punching to 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: 0.001 to 0.50%, N: 0.001 to 0.020% , Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, Nb: 0.12 to 0.80%, the balance has a component composition consisting of Fe and unavoidable impurities, and the limiting stress intensity coefficient K _IC is 25 MPa·m ^{1 /2} or more ferritic stainless hot-rolled steel sheet.

Description

Ferritic stainless hot-rolled steel sheet and manufacturing method thereof

The present invention relates to a ferritic stainless steel hot-rolled steel sheet having excellent punchability suitable for application to a flange, etc.

In recent years, the strengthening of laws and regulations regarding exhaust gas in automobiles is progressing, and improvement of fuel efficiency has become an urgent priority. Therefore, the application of an exhaust gas recirculation (EGR) system in which exhaust gas generated from an automobile engine is used again as intake air of the engine is being applied. The exhaust gas generated from the engine is supplied to the engine again after passing through the EGR cooler for lowering the gas temperature. In circulating exhaust gas, each exhaust system component is fastened through a flange to prevent gas leakage. Flanges applied to such exhaust system components need to have sufficient rigidity. From this point of view, a thick flange (for example, 5 mm or more in plate thickness) is applied to such exhaust system parts.

Conventionally, plain steel has been used for thick flanges. However, sufficient corrosion resistance is required for flanges applied to parts through which high-temperature exhaust gases such as EGR systems pass. Therefore, the application of stainless steel, which has excellent corrosion resistance compared to ordinary steel, especially ferritic stainless steel, which has a relatively small coefficient of thermal expansion and does not easily generate thermal stress, is being considered, and the plate thickness applicable to thick flanges is large (e.g., 5 mm or more in plate thickness) A ferritic stainless steel plate is strongly requested|required.

With respect to such market demand, for example, in Patent Document 1, in mass%, C: 0.030% or less, Si: 2.00% or less, Mn: 2.00% or less, P: 0.050% or less, S: 0.040% or less, Cr: 10.00 to 25.00%, N: 0.030% or less, Nb: 0.01 to 0.80%, the balance has a composition consisting of Fe and unavoidable impurities, and has a hardness of 190 HV or less and a Charpy impact value at 25°C of 20 J/cm 2 The Nb-containing ferritic stainless steel hot-rolled coil having a plate thickness of 5.0 to 10.0 mm adjusted as described above is disclosed.

Japanese Patent Laid-Open No. 2012-140688

However, as a result of punching using a crank press in a thick flange shape using the ferritic stainless steel hot-rolled coil described in Patent Document 1 by the present inventors, the punched part despite having a sufficient Charpy impact value Significant cracks occurred in the central portion of the plate thickness, and a predetermined flange shape could not be obtained in some cases, which was found to be insufficient for application to thick flanges. Moreover, in order to obtain the hot-rolled coil disclosed by patent document 1, after completion|finish of winding of hot rolling, it is necessary to immerse the coil in water and hold it for 15 minutes or more, and there exists a subject also in manufacturability and productivity.

An object of the present invention is to provide a ferritic stainless hot-rolled steel sheet capable of solving such a problem and having sufficient corrosion resistance and capable of suppressing cracking when punching a thick flange with a crank press, and a method for manufacturing the same. do it with

As a result of detailed studies to solve the problem, the present inventors have conducted a detailed study to solve the problem, and as a result, in order to punch a thick flange without generating cracks by a processing method with a relatively high processing speed such as a crank press, the limiting stress expansion coefficient of the steel sheet ( Threshold Stress Intensity Factor) It was found that it is necessary to increase the _{K IC.} Specifically, by setting the critical stress magnification coefficient K _IC to 25 MPa·m ^1/2 or more, even in a machining method with a large machining speed such as a crank press, when punching with a thick flange, It was found that the occurrence of cracks can be effectively suppressed and that it can be sufficiently put to practical use in thick flanges.

MEANS TO SOLVE THE PROBLEM The present inventors performed detailed examination in order to solve the subject. As a result, when a thick steel sheet having a plate thickness exceeding 5.0 mm is punched into a thick flange without cracking by a machining method with a high processing speed such as a crank press, the workability has been conventionally used. Although it is not possible to accurately evaluate the Charpy impact value, it was found that it can be accurately evaluated with the Threshold Stress Intensity Factor K _{IC, which} is a toughness evaluation index in the field of heavy plates. This is because, in the case of a thin steel sheet having a sheet thickness of less than 5.0 mm, the plastic deformation region near the punched end surface during processing is large relative to the sheet thickness, so the fracture phenomenon accompanying molding cannot be uniquely summarized as fracture mechanic handling. , in a thick steel plate having a plate thickness of 5.0 mm or more, the small-scale yield state in which the plastic deformation region in the vicinity of the punched cross-section at the time of processing is sufficiently small with respect to the plate thickness is sufficiently satisfied. It is considered that this is because it can be treated as a stress intensity factor, which is a quantitative index, and can be accurately evaluated with _{its limit value, that is, the critical stress intensity factor K IC .}

In view of the above, the present inventors have investigated in detail the relationship between _{the occurrence of cracks and the critical stress magnification coefficient K IC in} the case of punching into a flange of a predetermined shape by a crank press. As a result, by making the critical stress magnification coefficient K _IC at least 25 MPa·m ^1/2 , it is possible to effectively suppress the occurrence of cracks in the punched end face when punching into a thick flange by crank press, and It was discovered that it can fully be put to practical use for a flange.

And, with respect to the ferritic stainless steel of an appropriate component, the cumulative rolling reduction ratio of the last three passes (= 100 - (final plate thickness/before the start of rolling of the last three passes) in the finish hot rolling process consisting of three or more passes in particular It was found that by appropriately controlling the sheet thickness) × 100 [%]), the critical stress intensity coefficient K _IC of the hot-rolled steel sheet is improved.

The present invention has been made based on the above findings, and has the following as its gist.

[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: 0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, Nb: 0.12 to 0.80%, the balance has a component composition consisting of Fe and unavoidable impurities, and the limiting stress intensity coefficient K _IC is 25 MPa ·M ^1/2 or more ferritic stainless hot-rolled steel sheet.

[2] 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: 0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 13.0 to 24.0%, Ni: 0.01 to 0.60%, Nb: 0.12 to 0.80%, the balance has a component composition composed of Fe and unavoidable impurities, and the limiting stress intensity coefficient K _IC is 25 MPa ·M ^1/2 or more ferritic stainless hot-rolled steel sheet.

[3] As a component composition, in mass %, Cu: 0.01 to 1.50%, Mo: 0.01 to 2.00%, W: 0.01 to 0.20%, Co: 1 type or 2 or more types selected from 0.01 to 0.20% The ferritic stainless hot-rolled steel sheet according to the above [1] or [2], comprising:

[4] As a component composition, in mass%, Ti: 0.01 to 0.30%, V: 0.01 to 0.20%, Zr: 0.01 to 0.20%, REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg The ferritic stainless steel hot-rolled steel sheet according to any one of [1] to [3], containing one or two or more selected from: 0.0005 to 0.0030% and Ca: 0.0005 to 0.0030%.

[5] The method for producing a ferritic stainless steel hot-rolled steel sheet according to any one of [1] to [4], wherein in the hot rolling step of performing finish rolling in three or more passes, the last three passes of finish rolling are performed in a temperature range of 800 to A method for producing a ferritic stainless hot-rolled steel sheet at 1100°C and in which the cumulative reduction ratio of the last three passes is 25% or more.

Here, the limiting stress intensity coefficient K _IC is obtained by taking a CT test piece conforming to ASTM E399 from the center of the sheet width so that the fatigue precracking direction is perpendicular to the rolling direction and the stress axis is in the rolling parallel direction, and testing according to ASTM E399. refers to the stress magnification factor.

ADVANTAGE OF THE INVENTION According to this invention, while having sufficient corrosion resistance, the ferritic stainless steel hot-rolled steel plate excellent in toughness which can suppress the crack at the time of performing punching to a thick flange by a crank press is obtained.

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

In addition, the excellent toughness capable of suppressing cracking when punching a thick flange is performed by a crank press indicates that a CT test piece conforming to ASTM E399 from the center of the plate width was subjected to fatigue precracking in the direction perpendicular to rolling, stress axis is taken parallel to the rolling direction, and indicates that the threshold stress intensity factor K _IC obtained by testing in accordance with ASTM E399 is greater than or equal to 25 ㎫ · m ^1/2.

The ferritic stainless hot rolled 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: 0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, Nb: 0.12 to 0.80%, the balance has a component composition consisting of Fe and unavoidable impurities, limiting stress Magnification coefficient K _IC is 25 MPa·m ^1/2 or more.

In a preferred embodiment, the ferritic stainless hot-rolled 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: 0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 13.0 to 24.0%, Ni: 0.01 to 0.60%, Nb: 0.12 to 0.80%, the balance being Fe and unavoidable impurities. It has, and the critical stress magnification coefficient K _IC is 25 MPa·m ^1/2 or more.

The limiting stress magnification coefficient K _IC is obtained by taking a CT test piece conforming to ASTM E399 from the center of the sheet width so that the fatigue precracking direction is perpendicular to the rolling direction and the stress axis in the rolling direction parallel to the rolling direction, and testing according to ASTM E399. indicates the count.

Hereinafter, the present invention will be described in detail.

The present inventors studied in detail the cause of cracks generated when punched by a crank press using various ferritic stainless hot-rolled steel sheets with a sheet thickness of 5.0 mm, with a flange having a hole portion having a diameter of 30 mm. As a result, it was found that, in the above-mentioned steel sheet in which cracks occurred, microcracks occurred in the direction orthogonal to the punching direction in the vicinity of the plate thickness central portion of the punched end face, and cracks were generated by the propagation.

The present inventors studied in detail the relationship between the occurrence and propagation of these microcracks and material properties. As a result, it was found that the propagation of microcracks tends to occur more easily as the limiting stress expansion coefficient of the steel sheet is small. Therefore, using various ferritic stainless hot-rolled steel sheets (plate thickness 5.0 mm) and punching the flanges, as a result, the limiting stress magnification coefficient obtained by a predetermined measurement method is 25 MPa·m ^1/2 or more. This did not generate|occur|produce, but it discovered that it is easy to generate|occur|produce in the steel plate which was less than 25 Mpa*m ^1/2.

Then, in order to examine the method of improving the critical stress intensity factor in a ferritic stainless steel hot-rolled steel sheet, the present inventors conducted detailed investigation of a steel component and hot rolling conditions. As a result, the final 3 passes of the hot rolling process in which multi-pass finish rolling is performed on ferritic stainless steel of appropriate components is performed in a temperature range of 800 to 1100° C., and the cumulative reduction ratio of the last 3 passes (= By appropriately controlling 100 − (final plate thickness/thickness before the start of rolling in the last three passes) × 100 [%]) to be 25% or more, rolling deformation is effectively introduced not only in the surface layer portion but also in the central portion of the plate thickness, 25 It was discovered that the critical stress intensity coefficient K _IC of MPa·m ^{1/2 or more was obtained.}

In addition, although the plate thickness of the ferritic stainless steel hot-rolled steel plate of this invention is not specifically limited, Since it is preferable that it is a plate thickness applicable to a thick flange, 5.0 mm or more is preferable. Moreover, although the said board|board thickness is although it does not specifically limit, 15.0 mm or less is preferable and 10.0 mm or less is more preferable.

The rolling strain is effectively given to the plate thickness center of the steel plate after hot rolling by the above method will be described below as to why the threshold stress intensity factor K _IC is increased in the entire thickness of the steel sheet.

When the steel sheet is rolled, the steel sheet is deformed and elongated from the surface layer portion. Therefore, when the reduction ratio is small, the deformation amount of the central portion of the plate thickness is small, and the rolling deformation is hardly applied to the central portion of the plate thickness. In addition to this, the ferritic stainless steel tends to easily recover from work strain during hot rolling. Therefore, in the hot rolling by the prior art, the processing strain cannot be effectively applied to the central portion of the plate thickness due to the lack of the reduction ratio. In addition, the applied rolling strain is relieved and reduced by excessive recovery during hot rolling. As a result, in the hot rolling by the prior art, the predetermined critical stress intensification coefficient K _IC is not obtained.

Then, the present inventors earnestly studied from both the steel component and the hot-rolling method about the method of providing a rolling deformation effectively and sufficiently to the plate-thick center part of a steel plate in a hot-rolling process.

As a result, from the viewpoint of the hot rolling method, it was found that rolling deformation is sufficiently and effectively imparted to the central portion of the plate thickness by performing rolling at a large cumulative reduction ratio after managing the last three passes of finish hot rolling in an appropriate temperature range. did.

However, from the viewpoint of the steel component, in ferritic stainless steel containing little Nb, recovery during hot rolling tends to occur, so even if the hot rolling method proposed by the present inventors is used, sufficient rolling strain density cannot be obtained. , found that the predetermined limiting stress intensity factor was not obtained.

On the other hand, in ferritic stainless steel containing an appropriate amount of Nb, fine Nb carbonitride is precipitated during hot rolling, and since this fine Nb carbonitride inhibits the movement of dislocations, by using the hot rolling method proposed by the present inventors While it became possible to obtain a high rolling strain density, it discovered that the predetermined|prescribed limiting stress expansion coefficient was obtained in a hot-rolled steel sheet.

That is, in the present invention, in the ferritic stainless steel containing an appropriate amount of Nb, the final three passes of finish hot rolling are managed in an appropriate temperature range, and then rolling is performed at a large cumulative reduction ratio, while suppressing recovery of rolling strain, It was found that the rolling strain was sufficiently and effectively applied to the central portion of the plate thickness, and a predetermined limiting stress magnification coefficient K _IC was obtained.

Specifically, in a ferritic stainless steel containing 0.12% or more of Nb, the final 3 passes of the finish hot rolling process consisting of 3 passes or more are performed in a temperature range of 800 to 1100° C., and the cumulative reduction ratio of the last 3 passes (= It was devised to perform hot rolling under appropriate control so that 100 − (the final plate thickness/thickness before the rolling start of the last 3 passes) × 100 [%]) is 25% or more.

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

Hereinafter, unless otherwise indicated, % showing a component composition 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%. C content becomes like this. Preferably it is 0.003 % or more, More preferably, it is 0.004 % or more. Moreover, C content becomes like this. Preferably it is 0.015 % or less, More preferably, it is 0.012 % or less.

Si: 0.05 to 1.00%

Si is concentrated in the oxide film formed at the time of welding, has an effect of improving the corrosion resistance of a welding part, and is an element useful also as a deoxidation element in a steelmaking process. 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 in excess of 1.00%, the increase in the rolling load in the hot rolling process, the formation of significant scale, and the formation of the Si-enriched layer in the steel sheet surface layer in the annealing process cause a decrease in pickling properties, respectively. , which is undesirable because it induces an increase in surface defects or an increase in manufacturing cost. Therefore, Si content shall be 0.05 to 1.00 %. Si content becomes like this. Preferably it is 0.10 % or more. Moreover, Si content becomes like this. Preferably it is 0.60 % or less, More preferably, it is 0.40 % or less.

Mn: 0.05 to 1.00%

Mn has an effect of increasing the strength of steel, and 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 will be accelerated|stimulated, and corrosion resistance will fall. Therefore, Mn content shall be 0.05 to 1.00 %. The Mn content is preferably 0.10% or more. Moreover, Mn content becomes like this. Preferably it is 0.50 % or less, More preferably, it is 0.30 % or less.

P: 0.04% or less

P is an element unavoidably contained in steel, and since it is an element harmful to 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, it is an element harmful to corrosion resistance and workability, so 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: 0.001 to 0.50%

Al is an effective deoxidizer. Moreover, since Al has a stronger affinity with N than Cr, when N penetrates into a welding part, N is precipitated as Al nitride instead of Cr nitride, and there exists an effect of suppressing sensitization. These effects are obtained by containing 0.001% or more of Al. However, when Al exceeding 0.50 % is contained, since the penetration property at the time of welding will fall and welding workability will fall, it is unpreferable. Therefore, Al content is made into the range of 0.001 to 0.50%. Al content becomes like this. Preferably it is 0.20 % or less, More preferably, it is 0.10 % or less.

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 is, 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%. N content becomes like this. Preferably it is 0.003 % or more, More preferably, it is 0.005 % or more. Moreover, N content becomes like this. Preferably it is 0.015 % or less, More preferably, it is 0.012 % or less.

Cr: 11.0 to 24.0%

Cr is the most important element to secure the corrosion resistance of stainless steel. If the content is less than 11.0%, sufficient corrosion resistance cannot be obtained in an automobile exhaust gas atmosphere. On the other hand, when Cr is contained exceeding 24.0 %, toughness will fall remarkably by generation|generation of the sigma (sigma) phase, and in this invention, the predetermined|prescribed limiting stress intensity|strengthening factor cannot be obtained. Therefore, the Cr content is in the range of 11.0 to 24.0%. Cr content becomes like this. Preferably it is 13.0 % or more, More preferably, it is 14.0 % or more, More preferably, it is 16.0 % or more, More preferably, it is 17.0 % or more. Moreover, Cr content becomes like this. Preferably it is 21.5 % or less, More preferably, it is 20.0 % or less, More preferably, it is 18.5 % or less.

Ni: 0.01 to 2.00%

Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses the progress of corrosion in a corrosive environment in which a passivation film is not formed and active dissolution occurs. Moreover, Ni is a strong austenite generating element, and has the effect of suppressing the ferrite formation in a welding part, and suppressing the sensitization by precipitation of Cr carbonitride. This effect is obtained by containing 0.01% or more of Ni, and it becomes high, so that there is much content of Ni. However, when Ni content exceeds 2.00 %, in addition to workability falling, it will become easy to generate|occur|produce stress corrosion cracking. Furthermore, since Ni is an expensive element, since increase in content of Ni causes increase in manufacturing cost, it is unpreferable. Therefore, Ni content shall be 0.01 to 2.00 %. Ni content becomes like this. Preferably it is 0.05 % or more, More preferably, it is 0.10 % or more. Moreover, Ni content becomes like this. Preferably it is 1.00 % or less, More preferably, it is 0.60 % or less, More preferably, it is 0.50 % or less, More preferably, it is 0.45 % or less.

Nb: 0.12 to 0.80%

Nb combines with C or N in a hot rolling process to precipitate as Nb carbonitride. The precipitated Nb carbonitride has an effect of pin fixing the movement of dislocations and suppressing the rolling strain imparted by hot rolling from being resolved by recovery. Thereby, the recovery during hot rolling is delayed, and it is possible to suppress a decrease in the rolling strain density due to excessive recovery occurring. The said effect is obtained when 0.12 % or more of Nb is contained. However, if the Nb content exceeds 0.80%, the toughness may be rather reduced due to the generation of the Laves phase, and the rolling load in hot rolling increases remarkably. Therefore, the hot rolling method provided by the present invention is applied. it becomes difficult to do Therefore, the Nb content is in the range of 0.12 to 0.80%. Nb content becomes like this. Preferably it is 0.15 % or more, More preferably, it is 0.20 % or more. Moreover, Nb content becomes like this. Preferably it is 0.75 % or less, More preferably, it is 0.60 % or less.

The present invention is a ferritic stainless steel comprising the above essential components and the remainder being Fe and unavoidable impurities. In addition, if necessary, one or two or more selected from Cu, Mo, W and Co, or/ additionally one or two or more selected from Ti, V, Zr, REM, B, Mg and Ca , may be contained in the following range.

Cu: 0.01 to 1.50%

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 so high that there is much Cu content. However, when Cu is contained exceeding 1.50 %, 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 1.50 % of range. Cu content becomes like this. More preferably, it is 0.10 % or more, More preferably, it is 0.30 % or more. Moreover, Cu content becomes like this. More preferably, it 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 %. Mo content becomes like this. More preferably, it is 0.10 % or more. Moreover, Mo content becomes like this. More preferably, it is 1.40 % or less. However, since Mo also has the effect of reducing toughness in Ti containing steel, when Ti is contained 0.15 % or more, it is preferable to make Mo content into 0.30 to 1.40 % or less. When 0.15 % or more of Ti is contained, 0.40 % or more of Mo content is more preferable. Moreover, when 0.15 % or more of Ti is contained, as for Mo content, 0.90 % or less is more preferable.

W: 0.01 to 0.20%

W has the same effect as Mo to improve corrosion resistance. 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. The W content is more preferably 0.05% or more. Moreover, W content becomes like this. More preferably, it 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 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.

Ti: 0.01 to 0.30%

Ti is an element having a higher affinity for C and N than Cr, and is precipitated as a carbide or nitride, and has an effect of suppressing sensitization by precipitation of Cr carbonitride. In order to acquire this effect, it is necessary to contain 0.01% or more of Ti. However, when Ti content exceeds 0.30 %, favorable surface properties may not be acquired by excessive precipitation of TiN. Therefore, when containing Ti, it is preferable to make Ti content into 0.01 to 0.30% of range. Ti content becomes like this. More preferably, it is 0.03 % or more, More preferably, it is 0.10 % or more. Moreover, Ti content becomes like this. More preferably, it is 0.20 % or less, More preferably, it is 0.15 % or less.

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 %. The V content is more preferably 0.05% or more. Moreover, V content becomes like this. More preferably, it is 0.15 % or less.

Zr: 0.01 to 0.20%

Zr has the effect of suppressing sensitization by binding 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. The Zr content is more preferably 0.10% or less.

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 %, productivity, such as pickling property at the time of cold rolling annealing, may be reduced. Therefore, in the case of containing REM, the REM content is preferably in the range of 0.001 to 0.100%. The REM content is more preferably 0.050% or less.

B: 0.0002 to 0.0025%

B is an effective element for improving secondary processing brittleness after deep drawing forming. 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. The B content is more preferably 0.0003% or more. Moreover, B content becomes like this. More preferably, it is 0.0006 % or less.

Mg: 0.0005 to 0.0030%

Mg is an element effective in improving the equiaxed crystal ratio of the slab and improving workability and toughness. This effect is obtained by containing 0.0005% or more of Mg. On the other hand, when Mg content exceeds 0.0030 %, the surface property of steel may deteriorate. Therefore, when it contains Mg, it is preferable to make Mg content into 0.0005 to 0.0030 % of range. The Mg content is more preferably 0.0010% or more. Moreover, Mg content becomes like this. More preferably, it is 0.0020 % or less.

Ca: 0.0005 to 0.0030%

Ca has an effect of refining the inclusions generated during smelting and continuous casting, and is an effective component for preventing clogging of nozzles in particularly continuous casting. The effect is obtained by containing 0.0005% or more of Ca. However, when Ca is contained exceeding 0.0030 %, corrosion resistance may fall by generation|generation of CaS. Therefore, when Ca is contained, it is preferable to make Ca content into the range of 0.0005 to 0.0030%. Ca content becomes like this. More preferably, it is 0.0015 % or less, More preferably, it is 0.0010 % or less.

Limit stress magnification factor K _IC : 25 MPa m ^1/2 or more

The ferritic stainless hot-rolled steel sheet of the present invention has a critical stress magnification coefficient K _IC of 25 MPa·m ^1/2 or more, so that cracking can be suppressed when punching a thick flange with a crank press. The critical stress intensity coefficient K _IC is preferably 30 MPa·m ^1/2 or more, more preferably 35 MPa·m ^1/2 or more, still more preferably 40 MPa·m ^1/2 or more. In addition, the thick flange is although it does not specifically limit, For example, the flange of 5.0 mm or more of plate|board thickness is mentioned. As said flange, the flange of 5.0-15.0 mm of plate|board thickness is preferable, for example, and the flange of 5.0-10.0 mm of plate|board thickness is more preferable.

Next, the manufacturing method of the ferritic stainless steel hot-rolled steel sheet of this invention is demonstrated. In the following description, unless otherwise specified, the temperature is the surface temperature of a steel slab, hot-rolled steel sheet, etc. measured with a surface thermometer or the like.

The ferritic stainless hot-rolled steel sheet of the present invention uses a steel slab having the above component composition, and in hot rolling consisting of rough rolling and three or more passes of finish rolling, the final three passes of finish rolling are performed in a temperature range of 800 to 1100°C. , obtained by setting the cumulative reduction ratio of the last 3 passes to 25% or more.

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 1100 to 1250°C for 1 to 24 hours or directly subjected to hot rolling in the state of casting without heating. In the present invention, there is no particular limitation on the rough rolling, but when the cast structure is effectively destroyed before the finish hot rolling, it predominantly acts on the refinement of the crystal grains in the finish hot rolling after that, and the metal structure after the hot rolling. Since further improvement in toughness by this miniaturization can be expected, it is preferable to set the cumulative reduction ratio in rough rolling to 65% or more. Then, although rolling to a predetermined|prescribed plate|board thickness by finish hot rolling is carried out, the rolling of the last 3 passes of finish rolling is made into the temperature range of 800-1100 degreeC, and the cumulative reduction ratio is 25 % or more.

The rolling temperature range of the last 3 passes of finish hot rolling: 800 to 1100 ° C.

Cumulative reduction ratio of the last 3 passes of finish hot rolling: 25% or more

In order to obtain a predetermined critical stress expansion coefficient after hot rolling, by appropriately controlling the rolling temperature and cumulative rolling reduction in the last three passes of finish hot rolling, excessive recovery during rolling is suppressed, and rolling strain is also effectively applied to the central portion of the plate thickness. Needs to be.

In order to provide sufficient rolling deformation also to the central portion of the plate thickness, the rolling temperature of the last three passes of the finish hot rolling is set in the range of 800 to 1100° C., and the cumulative reduction ratio of the last three passes (= 100 − (final plate thickness/final plate thickness/final By setting the plate thickness before the start of rolling in three passes) × 100 [%]) to 25% or more, while preventing the rolling deformation imparted by the last three passes from being resolved by recovery, the rolling deformation is also effectively imparted to the center of the plate thickness need to go

When the cumulative reduction ratio of the last three passes of finish hot rolling is less than 25%, since rolling strain is not effectively imparted to the central portion of the plate thickness, a predetermined critical stress expansion coefficient cannot be obtained. Therefore, the cumulative reduction ratio of the last three passes is made 25% or more. Preferably, the cumulative reduction ratio is 30% or more. More preferably, the cumulative reduction ratio is 35% or more. In addition, the upper limit of the cumulative reduction ratio is not particularly limited, but if the cumulative reduction ratio is made excessively large, the rolling load may increase and the productivity may decrease. Therefore, it is preferably set to 60% or less.

When the rolling temperature of the last 3 passes of finish hot rolling is less than 800 degreeC, since a rolling load rises remarkably with the fall of the steel plate temperature, it is unpreferable in manufacturing. On the other hand, when the rolling temperature of the last three passes exceeds 1100 degreeC, the rolling distortion given by rolling will be cancelled|resolved by excessive recovery, and the predetermined|prescribed limiting stress expansion coefficient cannot be obtained. Therefore, the rolling temperature of the last three passes is made into the range of 800-1100 degreeC. Preferably, the rolling temperature of the last 3 passes is made into the range of 800-1050 degreeC. More preferably, the rolling temperature of the last three passes is made into the range of 850-1000 degreeC.

In addition, in order to prevent an excessive rolling load from being applied in a specific pass in the last three passes of finish hot rolling, the rolling temperature range of the first pass among the last three passes is set to 950 to 1100° C. It is preferable that the rolling temperature range of the 2nd pass performed next shall be 925-1075 degreeC, and it shall be 875-1050 degreeC as the rolling temperature range of the 3rd pass implemented after this 2nd pass.

Moreover, in the manufacturing method of the ferritic stainless steel hot-rolled steel plate of this invention, it is characterized by the large rolling reduction after controlling a temperature range in the last 3 passes of finish hot rolling which consists of 3 passes or more. If rolling with a large rolling reduction is performed over the last 4 passes or more, even if the cumulative reduction ratio is the same, since the reduction ratio is dispersed in each pass, the deformation imparted to the center of the plate thickness becomes insufficient, and the cumulative conveyance time between passes is reduced. Since it increases, recovery while conveying between each path|pass is encouraged, the effect of strain provision falls, and it becomes difficult to obtain a predetermined|prescribed limit stress expansion coefficient. On the other hand, if the control of the rolling temperature and the cumulative reduction ratio of the finish rolling is made to be less than or equal to the final two passes, the rolling load is remarkably increased to perform a large reduction in the cumulative reduction ratio of 25% or more in two passes, resulting in a decrease in manufacturability It is not preferable because there is Therefore, in the manufacturing method of the ferritic stainless steel hot-rolled steel sheet of this invention, the rolling temperature and cumulative reduction ratio of the last three passes of finish rolling are controlled.

In addition, in the method for manufacturing a ferritic stainless steel hot rolled steel sheet of the present invention, it is important to control the rolling temperature and the cumulative reduction ratio of the last three passes of finish hot rolling, and in the case of finish rolling of three or more passes, several passes of finish rolling are performed. However, when the maximum number of passes is greater than 15 passes, a decrease in the steel sheet temperature due to an increase in the number of contact with the rolling rolls tends to occur, and external heating is required to maintain the steel sheet temperature within a predetermined temperature range. It is preferable that the maximum number of passes be 15 passes or less, in order to cause a decrease in the productivity of such as, or an increase in manufacturing cost. More preferably, the maximum number of passes is 10 passes or less.

After finish hot rolling, the steel sheet is cooled, and then the steel sheet is wound up to obtain a hot rolled steel strip. Although the coiling temperature is not specifically limited in this invention, When the coiling temperature is made into more than 450 degreeC - less than 500 degreeC, embrittlement resulting from 475 degreeC embrittlement may generate|occur|produce. Therefore, the coiling temperature is preferably 450°C or lower or 500°C or higher. After final rolling, after performing accelerated cooling, such as brackish water cooling, when a winding process is performed at 450 degrees C or less, since elimination of rolling distortion by recovery|restoration after winding can be suppressed further, it is more preferable.

In addition, the hot-rolled steel sheet obtained by this invention is good also as a hot-rolled annealing steel sheet by performing hot-rolled sheet annealing. Since the hot-rolled steel sheet provided by the present invention has excellent toughness, hot-rolled sheet annealing can be performed in a continuous annealing line, which is conventionally avoided for fear of fracture due to low toughness. Moreover, you may perform cold rolling and cold-rolled sheet annealing after that for the obtained hot-rolled annealing steel plate.

Example

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

Molten stainless steel having the chemical composition shown in Table 1 was smelted by a converter with a capacity of 150 ton and refining by steel stirring and vacuum oxygen decarburization (SS-VOD), and continuous casting was performed to form a steel slab having a width of 1000 mm and a thickness of 200 mm. . After heating the slab at 1200° C. for 1 hr, reverse rough rolling using three stages of stand is performed as hot rolling to obtain a steel sheet of about 40 mm, and then the final 3 passes (5 passes) of finish rolling consisting of 7 passes. 2nd, 6th, and 7th pass) were performed under the conditions shown in Table 2 to obtain a hot-rolled steel sheet.

The following evaluation was performed about the obtained hot-rolled steel sheet.

(1) Evaluation of limiting stress magnification factor K _IC

A CT test piece conforming to ASTM E399 was sampled from the center of the sheet width so that the fatigue precracking direction was perpendicular to the rolling direction and the stress axis was in the rolling direction parallel to the rolling direction. For the test piece, in accordance with ASTM E399 it was determined the threshold stress intensity factor K _IC. The limiting stress magnification coefficient made 25 MPa·m ^1/2 or more pass, and ^less than 25 MPa·m 1/2 was rejected.

(2) Evaluation of corrosion resistance

From the obtained hot-rolled steel sheet, a 60 × 100 mm test piece is taken, the surface to be evaluated is polished and finished with #600 emery paper, and then a test piece in which the cross-section is sealed is prepared, and subjected to the salt spray cycle test specified in JIS H 8502 did. 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. After 5 cycles of salt spray cycle test, the evaluation surface of the test piece was photographed, and the rust generation area of the evaluation surface of the test piece was measured by image analysis, and the rust occurrence rate ((rust generation area in the test piece) from the ratio with the total area of the test piece / test piece total area) x 100 [%]) was computed. Corrosion resistance especially excellent in 10% or less of rust occurrence rate was made into pass ((circle)), more than 10% and 25% or less were made into pass (circle), and more than 25% were made into rejection (x).

The test results are shown in Table 2 together with the hot rolling conditions.

In Nos. 1 to 22, 29 to 33, in which the steel component and hot rolling conditions satisfy the range of the present invention, as a result of sufficiently imparting rolling deformation in the steel sheet by a predetermined hot rolling, a predetermined limiting stress expansion coefficient was obtained. . In addition, as a result of evaluating the corrosion resistance of the obtained hot-rolled steel sheet, it was confirmed that all of them had sufficient corrosion resistance as the rust occurrence rate was 25% or less.

In particular, No. 2, 4, 6, 7, 9 using Mo-containing steels B, D, F, G, I, and No. 5 and 8 using Cu-containing steels E and H, and Cr content In Nos. 13 to 15 using high steel M, N, and O, corrosion resistance further excellent was obtained with a rust occurrence rate of 10 % or less.

No. 23 in which the rolling temperature of the 5th pass (the 3rd pass from the last pass) exceeds the range of the present invention, and the rolling temperatures of the 5th and 6th passes (the 2nd pass from the last pass) are within the range of the present invention In No. 25 exceeding , rolling was performed at a predetermined cumulative reduction ratio, but because the rolling temperature was excessively high, excessive recovery of the processing strain imparted by rolling occurred. As a result, a predetermined limiting stress magnification factor was obtained after hot rolling. did not support In No. 24 in which the cumulative rolling reduction of the last three passes was less than the range of this invention, as a result of the provision of rolling strain becoming inadequate, the predetermined|prescribed critical stress expansion coefficient was not obtained after hot rolling.

In No. 26 in which the rolling temperature of the 5th pass and the 6th pass was less than the range of this invention, since the rolling temperature was too low, the rolling load rose remarkably, and the load was applied at the time of rolling implementation of the last 7th pass. Since it exceeded the allowable range, rolling could not be completed and predetermined|prescribed evaluation could not be implemented.

In No. 27 using steel R whose Nb content exceeds the range of this invention, the remarkable toughness fall resulting from precipitation of the Laves phase during hot rolling occurred, and the predetermined|prescribed limiting stress expansion coefficient was not obtained.

In No. 28 using steel S whose Nb content was less than the range of this invention, since a sufficient amount of Nb carbonitride did not precipitate, excessive recovery during hot rolling occurred, and the predetermined|prescribed critical stress expansion coefficient was not obtained.

In No. 34 using the steel Y whose Cr content is less than the range of this invention, since Cr content ran short, desired corrosion resistance was not acquired.

In No. 35 using steel Z with a Cr content exceeding the range of the present invention, since σ phase was precipitated due to excessive Cr content, a significant decrease in toughness occurred, and a predetermined critical stress magnification coefficient could not be obtained.

Industrial Applicability

The ferritic stainless hot-rolled steel sheet obtained in the present invention is particularly excellent in punching workability by crank press, and is manufactured by punching using crank press or other methods, and is suitable for application to thick flanges requiring high workability and corrosion resistance. Especially preferred.

Claims (5)

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: 0.001 to 0.50%;
N: 0.001 to 0.020%,
Cr: 11.0 to 24.0%,
Ni: 0.01 to 2.00%,
Nb: 0.12 to 0.60%
contains, and the balance has a component composition consisting of Fe and unavoidable impurities,
The limiting stress magnification coefficient K _IC is 25 MPa·m ^1/2 or more,
A ferritic stainless hot-rolled steel sheet with a sheet thickness of 5.0 mm or more. 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: 0.001 to 0.50%;
N: 0.001 to 0.020%,
Cr: 13.0 to 24.0%,
Ni: 0.01 to 0.60%,
Nb: 0.12 to 0.60%
contains, and the balance has a component composition consisting of Fe and unavoidable impurities,
The limiting stress magnification coefficient K _IC is 25 MPa·m ^1/2 or more,
A ferritic stainless hot-rolled steel sheet with a sheet thickness of 5.0 mm or more. The method of claim 1,
A ferritic stainless hot-rolled steel sheet, characterized in that it further contains at least one of the following groups A to B in terms of mass% as a component composition.
Group A:
Cu: 0.01 to 1.50%,
Mo: 0.01 to 2.00%,
W: 0.01 to 0.20%,
Co: 0.01 to 0.20%
1 type or 2 or more types selected from
Group B:
Ti: 0.01 to 0.30%,
V: 0.01 to 0.20%,
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: 0.0005 to 0.0030%
1 type or 2 or more types selected from 3. The method of claim 2,
A ferritic stainless hot-rolled steel sheet, characterized in that it further contains at least one of the following groups A to B in terms of mass% as a component composition.
Group A:
Cu: 0.01 to 1.50%,
Mo: 0.01 to 2.00%,
W: 0.01 to 0.20%,
Co: 0.01 to 0.20%
1 type or 2 or more types selected from
Group B:
Ti: 0.01 to 0.30%,
V: 0.01 to 0.20%,
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: 0.0005 to 0.0030%
1 type or 2 or more types selected from A method for manufacturing a ferritic stainless steel hot-rolled steel sheet according to any one of claims 1 to 4, wherein, in the hot rolling process in which three or more passes of finish rolling are performed, the final three passes of finish rolling are performed in a temperature range of 800 to 1100°C; Further, the method for producing a ferritic stainless steel hot-rolled steel sheet, wherein the cumulative reduction ratio of the last three passes is 25% or more.