JPWO2017013850A1 - Ferritic stainless hot-rolled steel sheet, hot-rolled annealed sheet, and methods for producing them - Google Patents

Abstract

To provide a ferritic stainless steel hot-rolled steel sheet and hot-rolled annealed sheet that have sufficient corrosion resistance and can suppress deflection and twisting after forming, and methods for producing the same. By mass%, C: 0.005 to 0.060%, Si: 0.02 to 0.50%, Mn: 0.01 to 1.00%, P: 0.04% or less, S: 0.01 %: Cr: 15.5 to 18.0%, Al: 0.001 to 0.10%, N: 0.005 to 0.100%, Ni: 0.1 to 1.0%, A ferritic stainless hot-rolled steel sheet, the balance of which is Fe and inevitable impurities, and the absolute value of in-plane anisotropy of longitudinal modulus calculated by the following formula (1) is 35 GPa or less. | ΔE | = | (EL−2 × ED + EC) / 2 | (1) where E L is the longitudinal elastic modulus (GPa) in the direction parallel to the rolling direction, and E D is 45 ° with respect to the rolling direction. The longitudinal elastic modulus (GPa) and EC in the direction are the longitudinal elastic modulus (GPa) in the direction perpendicular to the rolling direction.

Description

  The present invention relates to a ferritic stainless hot-rolled steel sheet and hot-rolled annealed sheet having sufficient corrosion resistance and excellent rigidity, and a method for producing them.

  In recent years, regulations on exhaust gas in automobiles have been strengthened, and improvement in fuel efficiency has become an urgent issue. Therefore, an exhaust gas recirculation (EGR) system in which exhaust gas generated from an automobile engine is used again as engine intake air has been applied. Exhaust gas generated from the engine is supplied to the engine again after passing through an EGR cooler for lowering the gas temperature. When circulating the exhaust gas, it is necessary to install a flange between the components in order to prevent gas leakage. Above all, the flange used at the joint with a member such as an EGR cooler, which is subject to vibration all the time when the vehicle is running, has sufficient rigidity to prevent gas leakage due to the generation of gaps between parts due to the deflection of the flange due to vibration. Must be expressed. For this reason, a thick-walled flange (for example, a plate thickness: 6 mm or more) is used as a flange between members to which vibration is applied all the time when the vehicle is traveling, such as an EGR cooler.

  Conventionally, ordinary steel has been used for such a thick flange. However, there is a concern about corrosion caused by exhaust gas in parts through which exhaust gas passes, such as the EGR system. Therefore, the application of stainless steel, which is superior in corrosion resistance compared to ordinary steel, has been studied, and has a sufficient rigidity that can be applied to thick flanges and has a large thickness (for example, a thickness of 6 mm or more). There is a need for stainless hot rolled steel sheets.

For example, Patent Document 1 includes 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%, the balance is Fe and inevitable impurities, and the content of Nb, C and N is Nb / ( A ferritic stainless hot rolled steel sheet satisfying C + N) ≧ 16, a Charpy impact value at 0 ° C. of 10 J / cm ² or more, and a plate thickness of 5.0 to 9.0 mm is disclosed.

  On the other hand, in recent years, there has been a strong demand for relatively inexpensive stainless steel (for example, SUS430, 13Cr stainless steel, etc.) in which the content of C and N stabilizing elements such as Ti and Nb is minimized.

International Publication No. 2014/157576

  However, when a conventional ferritic stainless hot-rolled steel sheet not containing Ti and Nb is formed on the above-described flange or the like, there is a problem that bending or twisting is likely to occur during vibration.

  An object of the present invention is to provide a ferritic stainless steel hot-rolled steel sheet and hot-rolled annealed steel sheet that can solve such problems and have sufficient corrosion resistance and can be prevented from being bent and twisted after forming, and a method for producing the same. And

As a result of detailed studies to solve the problems, the present inventors have found that the following formula (1) is applied to a steel sheet in order to suppress deformation such as deflection and torsion during vibration after application to a flange or the like. It was found that the absolute value | ΔE | of the in-plane anisotropy of the longitudinal elastic modulus represented by Furthermore, it has also been found that by making the absolute value of the in-plane anisotropy of the longitudinal elastic modulus 35 GPa or less, it can be sufficiently put into practical use in a flange or the like.
| ΔE | = | (E _L −2 × E _D + E _C ) / 2 | (1)
Here, E _L is the longitudinal elastic modulus (GPa) in the direction parallel to the rolling direction, E _D is the longitudinal elastic modulus (GPa) in the direction of 45 ° with respect to the rolling direction, and E _C is the longitudinal elasticity in the direction perpendicular to the rolling direction. Elastic modulus (GPa).
In addition, E _L , E _D , and E _C are respectively determined by the transverse resonance method described in JIS Z 2280-1993 under the temperature condition of 23 ° C. in the rolling direction of the steel sheet, the rolling 45 ° direction, and the direction perpendicular to the rolling direction. It can be obtained using the measured longitudinal elastic modulus.

  And, for ferritic stainless steel of an appropriate component, the rolling temperature range and the cumulative reduction ratio (= 100− (final plate thickness / final 3 pass rolling) in the final 3 pass of the finish hot rolling process consisting of multiple passes. It has been found that the in-plane anisotropy of the longitudinal elastic modulus can be significantly reduced by appropriately controlling the plate thickness before start) × 100 [%]).

This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] By mass%, C: 0.005 to 0.060%, Si: 0.02 to 0.50%, Mn: 0.01 to 1.00%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5 to 18.0%, Al: 0.001 to 0.10%, N: 0.005 to 0.100%, Ni: 0.1 to 1.0% Containing, the remainder having a component composition consisting of Fe and inevitable impurities,
A ferritic stainless hot-rolled steel sheet having an in-plane anisotropy absolute value | ΔE | of the longitudinal elastic modulus calculated by the following formula (1) of 35 GPa or less.
| ΔE | = | (E _L −2 × E _D + E _C ) / 2 | (1)
Here, E _L is the longitudinal elastic modulus (GPa) in the direction parallel to the rolling direction, E _D is the longitudinal elastic modulus (GPa) in the direction of 45 ° with respect to the rolling direction, and E _C is the longitudinal elasticity in the direction perpendicular to the rolling direction. Elastic modulus (GPa).
[2] The component composition is, in mass%, further selected from Cu: 0.1 to 1.0%, Mo: 0.1 to 0.5%, and Co: 0.01 to 0.5%. The ferritic stainless hot-rolled steel sheet according to the above [1], containing one or more kinds.
[3] As component composition, in mass%, V: 0.01 to 0.25%, Ti: 0.001 to 0.015%, Nb: 0.001 to 0.025%, Mg: 0.00. One or more selected from 0002 to 0.0050%, B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0020%, REM: 0.01 to 0.10% The ferritic stainless hot-rolled steel sheet according to the above [1] or [2] containing
[4] A ferritic stainless hot-rolled annealed sheet obtained by subjecting the ferritic stainless hot-rolled steel sheet according to any one of [1] to [3] to hot-rolled sheet annealing.
[5] The method for producing a ferritic stainless hot-rolled steel sheet according to any one of [1] to [3] above, wherein in the hot rolling step of performing finish rolling of 3 passes or more, the final 3 passes of finish rolling The manufacturing method of the ferritic stainless steel hot-rolled steel sheet which performs by the temperature range 900-1100 degreeC and the cumulative reduction rate 25% or more.
[6] Using the method for producing a ferritic stainless hot-rolled steel sheet according to [5] above,
The manufacturing method of the ferritic stainless steel hot-rolled annealing board which performs a hot-rolling sheet annealing further at 800-900 degreeC after the said hot rolling process.

  According to the present invention, a ferritic stainless hot-rolled steel sheet and a hot-rolled annealed sheet that have sufficient corrosion resistance and can suppress deflection and twisting after forming can be obtained.

  In addition, sufficient corrosion resistance in the present invention refers to a salt spray cycle test (salt spray (35 ° C., 5% by mass) defined in JIS H8502 on a steel plate whose end face is sealed after polishing the surface with # 600 emery paper. (NaCl, spraying 2 hr) → drying (60 ° C., relative humidity 40%, 4 hr) → wetting (50 ° C., relative humidity ≧ 95%, 2 hr)))) It means that the rusting area ratio (= rusting area / total area of steel plate × 100 [%]) is 25% or less.

The ferritic stainless steel hot-rolled steel sheet and hot-rolled annealed sheet of the present invention are in mass%, C: 0.005 to 0.060%, Si: 0.02 to 0.50%, Mn: 0.01 to 1. 00%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5 to 18.0%, Al: 0.001 to 0.10%, N: 0.005 to 0.100 %, Ni: 0.1 to 1.0%, with the balance being a component composition consisting of Fe and inevitable impurities, the in-plane anisotropy of the longitudinal elastic modulus calculated by the following formula (1) The absolute value | ΔE | is 35 GPa or less.
| ΔE | = | (E _L −2 × E _D + E _C ) / 2 | (1)
Note that, _{E L} is the longitudinal elastic modulus in the direction parallel to the rolling direction (GPa), modulus of longitudinal elasticity in the direction of _{E D} is 45 ° to the rolling direction (GPa), _{E C} is the rolling direction and the vertical direction The longitudinal elastic modulus (GPa).

In addition, E _L , E _D , and E _C are respectively determined by the transverse resonance method described in JIS Z 2280-1993 under the temperature condition of 23 ° C. in the rolling direction of the steel sheet, the rolling 45 ° direction, and the direction perpendicular to the rolling direction. It can be obtained using the measured longitudinal elastic modulus.

  Hereinafter, the present invention will be described in detail.

  The ferritic stainless steel hot-rolled steel sheet and hot-rolled annealed sheet of the present invention are intended to be used mainly for thick-walled flanges used in automobile EGR cooler parts. The present inventors applied various ferritic stainless steel hot-rolled steel sheets to a thick flange for an EGR cooler, and evaluated the performance in detail. As a result, it has been found that when a ferritic stainless hot rolled steel sheet having an in-plane anisotropy of longitudinal elastic modulus exceeding 35 GPa is applied, large deflection and twist are likely to occur due to vibration during vehicle travel.

  Accordingly, the present inventors have developed a method for reducing the in-plane anisotropy of the longitudinal elastic modulus in a ferritic stainless steel hot-rolled steel sheet, in particular, the rolling temperature in each pass of hot rolling consisting of multiple passes using a multi-stage stand. And intensively studied focusing on the reduction ratio. As a result, the longitudinal elastic modulus is obtained by rolling the final three passes in the multipass finish hot rolling consisting of three passes or more in a temperature range of 900 to 1100 ° C. and a cumulative reduction ratio of 25% or more (preferably 30% or more). It was found that the in-plane anisotropy was significantly reduced and the desired rigidity was obtained.

  The reason why the in-plane anisotropy of the desired longitudinal elastic modulus is expressed by the above method will be described below.

  The longitudinal elastic modulus of a ferritic stainless steel hot-rolled steel sheet strongly depends on the texture of the steel sheet. Since the texture of the hot-rolled steel sheet is formed by repeatedly introducing and recrystallizing processing strain due to rolling, the texture can be controlled by the temperature at which the rolling process is performed and the amount of strain.

  On the other hand, expanded ferrite grains are distributed along the casting direction in the center portion of the slab before hot rolling of the ferritic stainless steel. When such a stainless steel slab is hot-rolled by a conventional technique, the center portion of the plate thickness has a large number of stretched grains and a small grain interfacial area, so that there are fewer recrystallization sites than the steel plate surface layer portion.

  Furthermore, when a steel plate is rolled, the steel plate is deformed and stretched mainly from the surface layer portion. For this reason, when the rolling reduction is small, the amount of deformation in the central portion of the plate thickness is small, and almost no rolling strain is introduced into the central portion of the plate thickness.

  In the hot rolling according to the prior art, the introduction of strain and recrystallization are repeated in the steel sheet surface layer portion, while the progress of recrystallization is greatly delayed in the center portion of the plate thickness. Thereby, the expanded ferrite grains having a similar crystal orientation generated during casting are likely to remain without being destroyed, and the in-plane anisotropy of the longitudinal elastic modulus is increased after hot rolling.

  As an optimal method for suppressing the in-plane anisotropy of the longitudinal elastic modulus, the present inventors have a temperature range of 900 to 1100 ° C., which is a temperature range in which recrystallization actively occurs in the final three passes of finish hot rolling. It was devised to apply a greater reduction than the conventional range, with a cumulative reduction rate of 25% or more.

  Specifically, the present inventors systematically affect the effect of the rolling temperature and rolling reduction of each rolling pass on the in-plane anisotropy of the longitudinal elastic modulus of hot-rolled steel sheet produced by 7-pass finishing hot rolling. Investigated. As a result, the in-plane anisotropy of the longitudinal elastic modulus of the steel sheet after hot rolling was hardly affected by the temperature and rolling reduction of the first 4 passes, whereas the rolling temperature and rolling reduction of the final 3 passes were not affected. It was found that there is a tendency to be strongly influenced. Therefore, the present inventors investigated in further detail the influence of the rolling temperature and rolling reduction in the final three passes and the cumulative rolling reduction in the final three passes. As a result, the in-plane anisotropy of the longitudinal elastic modulus of the hot-rolled steel sheet tends to be greatly reduced when the final three-pass rolling is performed in the range of 900 to 1100 ° C., and the longitudinal strength of the hot-rolled steel sheet at that time It has been found that the amount of change in the in-plane anisotropy of the elastic modulus can be arranged not by the rolling reduction rate of each pass but by the cumulative rolling reduction rate of the final three passes. That is, the in-plane anisotropy of the longitudinal elastic modulus in the hot-rolled steel sheet has been found to be important to complete the finish rolling at a temperature range of 900 to 1100 ° C. and a cumulative reduction ratio of 25% or more. .

  The present inventors investigated the reason why the rolling temperature and the rolling reduction of the rolling pass before the final three passes have little influence on the in-plane anisotropy of the longitudinal elastic modulus of the hot-rolled steel sheet. As a result, in the rolling pass before the final three passes, the plate thickness before rolling starts is large, and even if the rolling reduction is increased, the rolling strain is not sufficiently introduced to the central portion of the plate thickness, and the rolling temperature is high. For this reason, excessive growth of recrystallized grains generated after rolling occurs, resulting in coarse grains. Therefore, the effect of eliminating the anisotropy of the metal structure due to the formation of recrystallized grains is significantly larger than the cumulative effect in the final three passes. It was found that this was due to a small amount.

  On the other hand, when the cumulative reduction ratio in the final three passes is increased to 25% or more, the rolling strain is effectively introduced to the plate thickness central portion by rolling in the final three passes. The recrystallized sites in the area greatly increase. By performing such rolling in the range of 900 to 1100 ° C. where recrystallization occurs actively, recrystallization at the center of the plate thickness is promoted, and the expanded ferrite grain structure formed during casting is effectively destroyed, The in-plane anisotropy of the longitudinal elastic modulus after hot rolling is greatly reduced. Moreover, by performing rolling temperature at 1100 degrees C or less, the coarsening of a recrystallized grain is suppressed and the cancellation effect of the metal structure anisotropy fully expresses. With this technology, the absolute value of the in-plane anisotropy of the longitudinal elastic modulus can be 35 GPa or less, and deformation such as large deflection or torsion during vibration can be suppressed after molding into a thick flange or the like.

  Furthermore, the present inventors performed hot-rolled sheet annealing in the range of 800 to 900 ° C. or less to obtain a hot-rolled annealed sheet in order to improve the formability of the hot-rolled steel sheet. It was found that in addition to the effect of improving the property, the effect of reducing the in-plane anisotropy of the longitudinal elastic modulus expressed by hot rolling is maintained. This is because the effect of reducing the in-plane anisotropy of the longitudinal elastic modulus in the present invention is due to the fracture of the stretched ferrite grain structure in the center portion of the sheet thickness, and the hot rolled sheet in a predetermined temperature range after hot rolling It has been found that when annealed, no expanded ferrite grains that promote the anisotropy of the steel sheet are generated.

  Further, the thickness of the ferritic stainless hot rolled steel sheet and ferritic stainless hot rolled annealed sheet of the present invention is not particularly limited, but is preferably a thickness that can be applied to a thick flange. 0.0 mm is preferable.

Next, the component composition of the ferritic stainless steel plate and ferritic stainless steel hot-rolled annealed plate of the present invention will be described.
Hereinafter, unless otherwise specified,% representing the component composition means mass%.

C: 0.005-0.060%
When C is contained in a large amount, the workability is deteriorated, sensitization due to the precipitation of Cr-based carbonitrides, and the toughness are reduced, so the C content is made 0.060% as an upper limit. On the other hand, since extremely reducing the C content causes a significant increase in refining costs, the lower limit of the C content is set to 0.005%, which is a level that does not cause a significant increase in production costs in the refining process. From the viewpoint of stable productivity in the steel making process, the C content is preferably 0.010 to 0.050%. More preferably, the C content is in the range of 0.020 to 0.045%. More preferably, the C content is in the range of 0.025 to 0.040%. Even more preferably, the C content is in the range of 0.030-0.040%.

Si: 0.02 to 0.50%
Si is an element that acts as a deoxidizer during steel melting. In order to obtain this effect, it is necessary to contain 0.02% or more of Si. However, if the Si content exceeds 0.50%, the steel sheet is hardened, the rolling load during hot rolling increases, and the manufacturability in the hot rolling process decreases, which is not preferable. Therefore, Si content is taken as 0.02 to 0.50% of range. Preferably, the Si content is in the range of 0.10 to 0.35%. More preferably, the Si content is in the range of 0.10 to 0.30%.

Mn: 0.01 to 1.00%
If Mn is contained in an excessive amount in the same manner as Si, the steel sheet is hardened, the rolling load during hot rolling increases, and the manufacturability in the hot rolling process decreases, which is not preferable. Moreover, the production amount of MnS may increase and the corrosion resistance may decrease. Therefore, the upper limit of the Mn content is 1.00%. About the minimum of Mn content, it is 0.01% from a viewpoint of the load of a refining process. Preferably, the Mn content is in the range of 0.10-0.90%. More preferably, the Mn content is in the range of 0.45 to 0.85%.

P: 0.04% or less Since P is an element that promotes grain boundary fracture due to grain boundary segregation, it is desirable that P be less. The upper limit of the P content is 0.04%. Preferably, the P content is 0.03% or less. More preferably, the P content is 0.01% or less.

S: 0.01% or less S is an element that exists as sulfide inclusions such as MnS and lowers ductility, corrosion resistance, etc., especially when the S content exceeds 0.01%. The adverse effect of the remarkably occurs. For this reason, the S content is desirably as low as possible. In the present invention, the upper limit of the S content is set to 0.01%. Preferably, the S content is 0.007% or less. More preferably, the S content is 0.005% or less.

Cr: 15.5 to 18.0%
Cr is an element having an effect of improving the corrosion resistance by forming a passive film on the surface of the steel sheet. In order to acquire this effect, it is necessary to make Cr content 15.5% or more. However, if the Cr content exceeds 18.0%, the toughness of the steel sheet is remarkably lowered, which is not preferable. Therefore, the Cr content is in the range of 15.5 to 18.0%. Preferably, the Cr content is in the range of 16.0 to 17.0%. More preferably, the Cr content is in the range of 16.0 to 16.5%.

Al: 0.001 to 0.10%
Al is an element that acts as a deoxidizing agent similarly to Si. In order to obtain this effect, it is necessary to contain 0.001% or more of Al. However, when the Al content exceeds 0.10%, Al inclusions such as Al ₂ O ₃ increase, and the surface properties tend to be lowered. Therefore, the Al content is in the range of 0.001 to 0.10%. Preferably, the Al content is in the range of 0.001 to 0.07%. More preferably, the Al content is in the range of 0.001 to 0.05%.

N: 0.005-0.100%
When N is contained in a large amount, as with C, workability, sensitization due to precipitation of Cr-based carbonitrides, and toughness are reduced. Therefore, the N content is limited to 0.100%. On the other hand, since extremely reducing the N content causes a significant increase in refining costs as in C, the lower limit of the N content is a level that does not cause a significant increase in production costs in the refining of the ordinary method. %. From the viewpoint of stable productivity in the steel making process, the N content is preferably 0.010 to 0.075%. More preferably, the N content is in the range of 0.025 to 0.055%. More preferably, the N content is in the range of 0.030 to 0.050%.

Ni: 0.1 to 1.0%
Ni is an element that improves corrosion resistance, and it is effective to contain it particularly when high corrosion resistance is required. This effect becomes remarkable when the content is 0.1% or more. However, if the content exceeds 1.0%, the moldability is lowered, which is not preferable. Therefore, the Ni content is 0.1 to 1.0%. Preferably, the Ni content is in the range of 0.2-0.4%.

  The balance is Fe and inevitable impurities.

  Although the effects of the present invention can be obtained by the above component composition, the following elements can be contained for the purpose of further improving manufacturability or material characteristics.

One or more selected from Cu: 0.1 to 1.0%, Mo: 0.1 to 0.5%, Co: 0.01 to 0.5% Cu: 0.1 to 1 .0%
Cu is an element that improves corrosion resistance, and it is effective to contain it particularly when high corrosion resistance is required. This effect becomes remarkable when the Cu content is 0.1% or more. However, if the Cu content exceeds 1.0%, the formability may deteriorate. Therefore, when it contains Cu, it is set as 0.1 to 1.0%. Preferably, the Cu content is in the range of 0.2-0.4%.

Mo: 0.1 to 0.5%
Mo is an element that improves the corrosion resistance like Ni and Cu, and it is effective to contain it particularly when high corrosion resistance is required. This effect becomes remarkable when the Mo content is 0.1% or more. However, if the Mo content exceeds 0.5%, the steel sheet becomes hard, the rolling load during hot rolling increases, and the manufacturability in the hot rolling process may decrease. Therefore, when it contains Mo, it is 0.1 to 0.5%. Preferably, the Mo content is in the range of 0.2-0.3%.

Co: 0.01 to 0.5%
Co is an element that improves toughness. This effect is obtained when the content is 0.01% or more. On the other hand, if the content exceeds 0.5%, moldability may be reduced. Therefore, content in the case of containing Co shall be 0.01 to 0.5% of range.

V: 0.01 to 0.25%, Ti: 0.001 to 0.015%, Nb: 0.001 to 0.025%, Mg: 0.0002 to 0.0050%, B: 0.0002 to One or more selected from 0.0050%, Ca: 0.0002 to 0.0020%, REM: 0.01 to 0.10% V: 0.01 to 0.25%
V is an element that forms carbonitride more easily than Cr. V has the effect of suppressing sensitization due to precipitation of Cr carbonitride by precipitating C and N in the steel as V-based carbonitride during hot rolling. In order to acquire this effect, it is necessary to contain V 0.01% or more. However, if the V content exceeds 0.25%, workability may be reduced. Increases manufacturing costs. Therefore, when it contains V, it is set as 0.01 to 0.25% of range. Preferably, the V content is in the range of 0.03 to 0.08%.

Ti: 0.001 to 0.015%, Nb: 0.001 to 0.025%
Ti and Nb, like V, are elements having high affinity with C and N, and precipitate as carbide or nitride during hot rolling, and have the effect of suppressing sensitization due to precipitation of Cr carbonitride. In order to obtain this effect, it is necessary to contain 0.001% or more of Ti or 0.001% or more of Nb. However, if the Ti content exceeds 0.015% or the Nb content exceeds 0.030%, good surface properties may not be obtained due to excessive precipitation of TiN and NbC. Therefore, when Ti is contained, the range is 0.001 to 0.015%, and when Nb is contained, the range is 0.001 to 0.025%. The Ti content is preferably in the range of 0.003 to 0.010%. The Nb content is preferably in the range of 0.005 to 0.020%. More preferably, the Nb content is in the range of 0.010 to 0.015%.

Mg: 0.0002 to 0.0050%
Mg is an element that has an effect of improving hot workability. In order to acquire this effect, 0.0002% or more of Mg needs to be contained. However, when the Mg content exceeds 0.0050%, the surface quality may deteriorate. Therefore, when it contains Mg, it is set as 0.0002 to 0.0050% of range. Preferably, the Mg content is in the range of 0.0005 to 0.0035%. More preferably, the Mg content is in the range of 0.0005 to 0.0020%.

B: 0.0002 to 0.0050%
B is an element effective for preventing embrittlement at low temperature secondary work. In order to obtain this effect, 0.0002% or more of B must be contained. However, when the B content exceeds 0.0050%, the hot workability may decrease. Therefore, when it contains B, it is set as 0.0002 to 0.0050% of range. Preferably, the B content is in the range of 0.0005 to 0.0035%. More preferably, the B content is in the range of 0.0005 to 0.0020%.

Ca: 0.0002 to 0.0020%
Ca is an effective component for preventing clogging of the nozzle due to crystallization of inclusions that are likely to occur during continuous casting. In order to acquire the effect, 0.0002% or more of Ca needs to be contained. However, if the Ca content exceeds 0.0020%, CaS may be generated and the corrosion resistance may be reduced. Therefore, when it contains Ca, it is set as 0.0002 to 0.0020% of range. Preferably, the Ca content is in the range of 0.0005 to 0.0015%. More preferably, the Ca content is in the range of 0.0005 to 0.0010%.

REM: 0.01-0.10%
REM (Rare Earth Metals) is an element that improves oxidation resistance, and is particularly effective in suppressing the formation of an oxide film on the welded portion and improving the corrosion resistance of the welded portion. In order to obtain this effect, it is necessary to contain 0.01% or more of REM. However, if the content of REM exceeds 0.10%, productivity such as pickling at the time of cold rolling annealing may be lowered. Moreover, since REM is an expensive element, excessive inclusion is not preferable because it causes an increase in manufacturing cost. Therefore, when it contains REM, it is set as 0.01 to 0.10% of range. Preferably, the REM content is in the range of 0.01 to 0.05%.

  Next, the manufacturing method of the ferritic stainless steel sheet and ferritic stainless steel hot-rolled annealing plate of the present invention will be described.

  In the ferritic stainless steel sheet of the present invention, the final three pass rolling in the finish rolling is performed in a temperature range of 900 to 1100 ° C. It is obtained by carrying out at a cumulative rolling reduction of 25% or more.

  The maximum number of passes of finish rolling is not particularly limited from the viewpoint of obtaining a predetermined material. However, when the maximum number of passes exceeds 15 passes, the steel sheet temperature is likely to decrease due to an increase in the number of contacts with the rolling roll. Thus, there may be a case where a decrease in manufacturability or an increase in manufacturing cost is caused such that heating from the outside is necessary to maintain the steel plate temperature within a predetermined temperature range. Therefore, it is preferable that the maximum number of paths is 15 paths or less. More preferably, the maximum number of paths is 10 paths or less.

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

  The slab is heated at 1100 to 1250 ° C. for 1 to 24 hours, or directly subjected to hot rolling as cast without heating. In the present invention, the rough rolling is not particularly limited, but it is preferable to set the cumulative rolling reduction in the rough rolling to 65% or more in order to effectively destroy the cast structure. Then, although it rolls to predetermined plate | board thickness by finish rolling, the rolling of the last 3 passes of finish rolling is performed at the temperature range of 900-1100 degreeC with the cumulative reduction rate of 25% or more.

Final 3-pass rolling temperature range: 900-1100 ° C
In the final three-pass finish rolling, it is necessary to effectively introduce rolling strain to the center of the plate thickness and to generate sufficient recrystallization by increasing the cumulative rolling reduction. Therefore, the final three-pass finish rolling needs to be performed in a temperature range of 900 to 1100 ° C. at which recrystallization occurs sufficiently. When the rolling temperature of the final three passes is less than 900 ° C., recrystallization does not occur sufficiently and in-plane anisotropy with a predetermined longitudinal elastic modulus cannot be obtained. On the other hand, if the rolling temperature of the final three passes exceeds 1100 ° C., the crystal grains become extremely coarse, and in-plane anisotropy with a predetermined longitudinal elastic modulus cannot be obtained. Absent. Preferably, the rolling temperature for the final three passes is in the range of 900-1075 ° C. More preferably, the rolling temperature for the final three passes is in the range of 930-1050 ° C. In order to prevent an excessive rolling load from being applied in the specific pass in the final three passes, the rolling temperature range of the first pass in the final three passes is 950 to 1100 ° C., which is performed next to the first pass. The rolling temperature range of the second pass is preferably 925 to 1075 ° C., and the rolling temperature range of the third pass performed after the second pass is preferably 900 to 1050 ° C.

Cumulative rolling reduction of 25% or more in the final three passes In order to effectively impart rolling strain to the center of the plate thickness of the steel sheet, rolling of 25% or more is required as the cumulative rolling reduction for the final three passes of finish rolling. If the cumulative rolling reduction is less than 25%, the introduction of rolling strain into the center of the sheet thickness is insufficient, and recrystallization at the center of the sheet thickness is delayed, and in-plane anisotropy with a predetermined longitudinal elastic modulus cannot be obtained. . Therefore, it is preferable that the cumulative rolling reduction is 25% or more. More preferably, the cumulative rolling reduction is 30% or more. More preferably, the cumulative rolling reduction is 35% or more. The upper limit of the cumulative rolling reduction is not particularly limited, but if the cumulative rolling reduction is excessively increased, the rolling load increases and the productivity decreases, and surface roughness may occur after rolling. It is preferable to do.

  The cumulative rolling reduction is 100− (final plate thickness / plate thickness before starting the final three-pass rolling) × 100 [%].

  In the method for producing a ferritic stainless hot rolled steel sheet according to the present invention, the rolling temperature and cumulative rolling reduction of the final three passes of finish rolling are controlled, and the rolling temperature and cumulative rolling reduction of finish rolling are controlled. If the final four passes or more, since the rolling reduction rate in each pass is small, the introduced strain hardly contributes to the reduction of the anisotropy of the longitudinal elastic modulus, and there is a sufficient effect of reducing the anisotropy of the longitudinal elastic modulus. I can't get it. In addition, when the rolling temperature and the cumulative reduction ratio of the finish rolling are controlled to the final two passes or less, the rolling load is significantly increased and the productivity is lowered because the large reduction with the cumulative reduction ratio of 25% or more is performed in two passes. This is not preferable. Therefore, in the method for producing a ferritic stainless steel hot-rolled steel sheet according to the present invention, the rolling temperature and cumulative rolling reduction of the final three passes of finish rolling are controlled.

  Further, in the method for producing a ferritic stainless hot rolled steel sheet according to the present invention, how many passes of finish rolling are performed as long as the finish rolling is 3 passes or more so as to control the rolling temperature and the cumulative rolling reduction of the final 3 passes. May be.

  After finishing rolling, the steel sheet is cooled, and then the steel sheet is wound to form a hot-rolled steel strip. In the present invention, the coiling temperature is not particularly limited, but in the case of a steel component in which an austenite phase is generated during hot rolling, when the coiling temperature is less than 500 ° C., the austenite phase is transformed into a martensite phase, The rolled steel sheet may become hard and formability may deteriorate. Therefore, the winding process is preferably performed at 500 ° C. or higher.

  In the present invention, when the above hot rolling step is completed, desired corrosion resistance and desired in-plane anisotropy of the longitudinal elastic modulus can be obtained. Further, after the hot rolling step, hot rolled sheet annealing may be performed in the range of 800 to 900 ° C. to obtain a ferritic stainless hot rolled sheet.

Hot-rolled sheet annealing temperature: 800-900 ° C
When the hot-rolled sheet annealing temperature is less than 800 ° C., recrystallization does not occur sufficiently, so that the work structure by hot rolling remains and the effect of improving formability cannot be obtained. On the other hand, when the temperature exceeds 900 ° C., an anustenite phase is generated during annealing, and the anisotropy of the longitudinal elastic modulus increases, that is, the in-plane anisotropy of the predetermined longitudinal elastic modulus that has been developed in the hot-rolled steel sheet disappears. There is. In addition, when the cooling rate after hot-rolled sheet annealing is higher than 900 ° C., the austenite phase is transformed into the martensite phase and the steel sheet is hardened, which may deteriorate the formability. Therefore, when performing hot-rolled sheet annealing, it is preferable that a temperature range shall be 800-900 degreeC. In addition, there is no limitation in particular in the holding | maintenance time and method of hot-rolled sheet annealing, You may implement by either box annealing (batch annealing) or continuous annealing.

  The obtained hot-rolled steel sheet or the steel sheet subjected to hot-rolled sheet annealing (hot-rolled annealed sheet) may be subjected to descaling treatment by shot blasting or pickling as necessary. Furthermore, in order to improve the surface properties, grinding or polishing may be performed.

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

  A molten stainless steel having the chemical composition shown in Table 1 is melted by refining a converter with a capacity of 150 ton and strong stirring and vacuum oxygen decarburization (SS-VOD), and a steel slab having a width of 1000 mm and a thickness of 200 mm by continuous casting. did. The slab was heated at 1200 ° C. for 1 h, and then subjected to reverse rough rolling using a three-stage stand as hot rolling to obtain a steel plate of about 40 mm, and then the final three passes (fifth pass) of 7-pass finish rolling. , 6th pass, 7th pass) were performed under the conditions shown in Table 2 to obtain hot-rolled steel sheets. In addition, some hot-rolled steel sheets (No. 25, 26, and 38 in Table 2) were subjected to hot-rolled sheet annealing that was furnace-cooled after holding for 8 hours under the conditions shown in Table 2 after hot rolling. I got a plate.

  The following evaluation was performed about the obtained hot-rolled steel plate and hot-rolled annealing plate.

(1) Evaluation of in-plane anisotropy Test specimens of 60 mm length × 10 mm width × 2 mm thickness with the parallel direction of rolling, the 45 ° direction of rolling and the direction perpendicular to the rolling as the longitudinal direction were sampled from the position within the plate thickness center ± 1 mm, respectively. did. The sample specimen was measured for longitudinal elastic modulus at 23 ° C. by the transverse resonance method described in JIS Z 2280-1993, and the absolute value (| ΔE |) of in-plane anisotropy of the longitudinal elastic modulus by the following equation (1) Was calculated.
| ΔE | = | (E _L −2 × E _D + E _C ) / 2 | (1)
Here, E _L is the longitudinal elastic modulus (GPa) in the direction parallel to the rolling direction, E _D is the longitudinal elastic modulus (GPa) in the direction of 45 ° with respect to the rolling direction, and E _C is the longitudinal elasticity in the direction perpendicular to the rolling direction. Elastic modulus (GPa).
When the in-plane anisotropy | ΔE | of the longitudinal elastic modulus was 35 GPa or less, it was judged that the bending and twisting after forming on the flange or the like could be sufficiently suppressed, and the result was evaluated as acceptable (◯). A case where the in-plane anisotropy | ΔE | of the longitudinal elastic modulus was more than 35 GPa was regarded as rejected (x).

(2) Evaluation of corrosion resistance A 60 × 100 mm test piece was taken from a hot-rolled steel sheet, and a test piece was prepared by polishing the surface with # 600 emery paper and sealing the end face, and was defined in JIS H8502. Subjected to a salt spray cycle test. In the salt spray cycle test, salt spray (5 mass% NaCl, 35 ° C., spray 2 hr) → dry (60 ° C., 4 hr, relative humidity 40%) → wet (50 ° C., 2 hr, relative humidity ≧ 95%) is one cycle. As a result, 8 cycles were performed.
Photograph the surface of the specimen after 8 cycles of salt spray cycle test, measure the rusting area on the specimen surface by image analysis, and calculate the rusting rate (( Rust area / total area of test piece) × 100 [%]) was calculated. A rusting rate of 10% or less was determined to pass with excellent corrosion resistance (）), more than 10% to 25% or less passed (◯), and more than 25% to reject (x).

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

  Steel components, hot rolling conditions and hot-rolled sheet annealing conditions satisfy the scope of the present invention. 1-21, no. Nos. 25 to 34 have a small absolute value (| ΔE |) of in-plane anisotropy of the longitudinal elastic modulus of 35 GPa or less, and a desired rigidity is obtained. Furthermore, as a result of evaluating the corrosion resistance of the obtained hot-rolled steel sheet or hot-rolled annealed sheet, it was confirmed that all had a rusting rate of 25% or less and sufficient corrosion resistance.

  In particular, No. 1 using steel C containing 0.52% by mass of Ni and 0.4% by mass of Cu. No. 14 to No. 17 using steel J containing 14 to 17 and 0.3% by mass of Mo. In No. 32, a further excellent corrosion resistance with a rusting rate of 10% or less was obtained.

  No. in which the cumulative rolling reduction of the final three passes is below the range of the present invention. In No. 22, since a large amount of expanded grains remained in the central portion of the plate thickness, the in-plane anisotropy of the longitudinal elastic modulus was increased, and a predetermined | ΔE | was not obtained.

  In rolling in the final 3 passes, only the final temperature in the 7th pass falls below the scope of the present invention. No. 23, and the rolling temperatures of the final three passes are all below the scope of the present invention. In No. 24, although the rolling was performed at a predetermined cumulative reduction, recrystallization in the central portion of the plate thickness became insufficient, and a predetermined | ΔE | was not obtained. In addition, No. 3 in which the rolling temperatures in the final three passes all exceed the scope of the present invention. In No. 37, the crystal grains became coarse, and a predetermined | ΔE | was not obtained.

  No. of hot-rolled sheet annealing temperature exceeding the range of the present invention. In No. 38, since austenite at the time of hot-rolled sheet annealing was generated, a predetermined | ΔE | was not obtained. No. which could not obtain the predetermined | ΔE | When steel plates of 22 to 24, 37, and 38 were applied to the thick flange, it was confirmed that deflection and twist occurred during vibration.

  No. using steel M whose Cr content is below the range of the present invention. In No. 35, a passive film could not be sufficiently formed on the steel sheet surface, and the desired corrosion resistance could not be obtained.

  No. using steel N in which the Cr amount exceeds the range of the present invention. In No. 36, breakage occurred during the hot rolling process due to cracks generated in the slab during cooling after casting, and a predetermined evaluation could not be performed.

The ferritic stainless steel hot-rolled steel sheet obtained by the present invention is particularly suitable for applications requiring rigidity and corrosion resistance, for example, application to flanges of EGR coolers.

Claims (6)

By mass%, C: 0.005 to 0.060%, Si: 0.02 to 0.50%, Mn: 0.01 to 1.00%, P: 0.04% or less, S: 0.01 %: Cr: 15.5 to 18.0%, Al: 0.001 to 0.10%, N: 0.005 to 0.100%, Ni: 0.1 to 1.0%, The balance has a component composition consisting of Fe and inevitable impurities,
A ferritic stainless hot-rolled steel sheet having an in-plane anisotropy absolute value | ΔE | of the longitudinal elastic modulus calculated by the following formula (1) of 35 GPa or less.
| ΔE | = | (E _L −2 × E _D + E _C ) / 2 | (1)
Here, E _L is the longitudinal elastic modulus (GPa) in the direction parallel to the rolling direction, E _D is the longitudinal elastic modulus (GPa) in the direction of 45 ° with respect to the rolling direction, and E _C is the longitudinal elasticity in the direction perpendicular to the rolling direction. Elastic modulus (GPa).   As a component composition, it is 1% selected from Cu: 0.1-1.0%, Mo: 0.1-0.5%, Co: 0.01-0.5% by mass%, or The ferritic stainless steel hot-rolled steel sheet according to claim 1 containing two or more kinds.   As component composition, in mass%, V: 0.01-0.25%, Ti: 0.001-0.015%, Nb: 0.001-0.025%, Mg: 0.0002-0 .0050%, B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0020%, REM: One or more selected from 0.01 to 0.10% The ferritic stainless steel hot-rolled steel sheet according to claim 1 or 2.   A ferritic stainless steel hot-rolled annealed sheet obtained by subjecting the ferritic stainless hot-rolled steel sheet according to any one of claims 1 to 3 to hot-rolled sheet annealing.   It is a manufacturing method of the ferritic stainless steel hot-rolled steel sheet in any one of Claims 1-3, Comprising: In the hot rolling process which performs the finish rolling of 3 passes or more, the final 3 passes of finish rolling are temperature range 900-1100. A method for producing a ferritic stainless hot-rolled steel sheet at a temperature of 25 ° C. or more at a reduced temperature of 25 ° C. Using the method for producing a ferritic stainless hot-rolled steel sheet according to claim 5,
The manufacturing method of the ferritic stainless steel hot-rolled annealing board which performs a hot-rolling sheet annealing further at 800-900 degreeC after the said hot rolling process.