US20180202023A1

US20180202023A1 - Hot rolled ferritic stainless steel sheet, hot rolled and annealed ferritic stainless steel sheet and method for manufacturing the same

Info

Publication number: US20180202023A1
Application number: US15/745,001
Authority: US
Inventors: Masataka Yoshino; Mitsuyuki Fujisawa; Chikara Kami
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2015-07-17
Filing date: 2016-07-11
Publication date: 2018-07-19
Also published as: TW201708561A; JPWO2017013850A1; KR20180017177A; WO2017013850A1; JP6112273B1; TWI605134B; CN107835865A; CN107835865B; KR102088341B1

Abstract

Provided are a hot rolled ferritic stainless steel sheet and a hot rolled and annealed ferritic stainless steel sheet and methods for manufacturing these steel sheets. A hot rolled ferritic stainless steel sheet having a chemical composition containing, 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%, and the balance being Fe and inevitable impurities and an absolute value of planar anisotropy in terms of modulus of longitudinal elasticity below of 35 GPa or less.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2016/003286, filed Jul. 11, 2016, which claims priority to Japanese Patent Application No. 2015-142611, filed Jul. 17, 2015, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot rolled ferritic stainless steel sheet and a hot rolled and annealed ferritic stainless steel sheet which have sufficient corrosion resistance and excellent rigidity and methods for manufacturing these steel sheets.

BACKGROUND OF THE INVENTION

Nowadays, since regulations regarding automobile exhaust gas are being strengthened, improvement of fuel efficiency is an urgent task. Therefore, there is a trend toward using an exhaust gas recirculation (EGR) system in which exhaust gas discharged from an automobile engine is reused as the intake gas of the engine. The exhaust gas discharged from an engine is passed through an EGR cooler, which is used for cooling the exhaust gas, and then charged again into the engine. When exhaust gas is recirculated, it is necessary to set a flange between the respective parts of the system in order to prevent the exhaust gas from leaking. In particular, it is necessary that a flange which is used in a connecting portion of a member such as an EGR cooler, which is always subjected to vibration during running of an automobile, have sufficient rigidity in order to prevent gas from leaking from a gap which is formed between the parts due to the bending of the flange resulting from the vibration. Therefore, a flange having a large thickness (for example, a thickness of 6 mm or more) is used as a flange which is fitted to a member such as an EGR cooler, which is always subjected to vibration during running of an automobile.
Conventionally, plain carbon steel is used for such a flange having a large thickness. However, there is a risk of corrosion due to exhaust gas in the case of parts of an EGR system and the like through which exhaust gas is passed. Therefore, consideration is being given to using stainless steel which is superior to plain carbon steel in terms of corrosion resistance, and there is a demand for a hot rolled ferritic stainless steel sheet having a large thickness (for example, a thickness of 6 mm or more) and sufficient rigidity to be used for a flange having a large thickness.
For example, Patent Literature 1 discloses a hot rolled ferritic stainless steel sheet having a chemical composition containing, by massa, C: 0.015% or less, Si: 0.01% to 0.4%, Mn: 0.01% to 0.8%, P: 0.04% or less, S: 0.01% or less, Cr: 14.0% to 18.0% (not inclusive), Ni: 0.05% to 1%, Nb: 0.3% to 0.6%, Ti: 0.05% or less, N: 0.020% or less, Al: 0.10% or less, B: 0.0002% to 0.0020%, and the balance being Fe and inevitable impurities, in which the contents of Nb, C, and N satisfy the relationship Nb/(C+N)≥16, a Charpy impact value at a temperature of 0° C. of 10 J/cm²or more, and a thickness of 5.0 mm to 9.0 mm.
In contrast, nowadays, there is a strong demand for relatively inexpensive stainless steel (such as SUS 430 or 13Cr-stainless steel) in which the contents of chemical elements such as Ti and Nb, which stabilize C and N, are as small as possible.

CITATION LIST

Patent Literature

PTL 1: International Publication No. WO2014/157576

SUMMARY OF THE INVENTION

However, in the case where a conventional hot rolled ferritic stainless steel sheet, in which Ti or Nb is not contained, is formed into, for example, the flange described above, there is a problem in that bending and twist tend to occur, for example, when the flange is subjected to vibration.
An object of aspects of the present invention is, by solving the problems described above, to provide a hot rolled ferritic stainless steel sheet and a hot rolled and annealed ferritic stainless steel sheet which have sufficient corrosion resistance and with which it is possible to inhibit bending and twist from occurring after forming has been performed and methods for manufacturing these steel sheets.
The present inventors conducted close investigations in order to solve the problems and, as a result, found that a steel sheet should have decreased absolute value |ΔE| of planar anisotropy in terms of modulus of longitudinal elasticity, which is expressed by equation (1) below, in order to inhibit deformation such as bending or twist when the steel sheet is used for, for example, a flange and then subjected to vibration. Moreover, it was found that it is possible to sufficiently put the steel sheet into practical use for, for example, a flange in the case where the absolute value of planar anisotropy in terms of modulus of longitudinal elasticity is 35 GPa or less.
|ΔE|=|(E _L−2×E _D +E _C)/2| (1)
Here, E_Ldenotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction, E_Ddenotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction, and E_Cdenotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.
In addition, E_L, E_D, and E_Care respectively defined as the values of modulus of longitudinal elasticity in the rolling direction of a steel sheet, in a direction at an angle of 45° to the rolling direction, and in a direction at a right angle to the rolling direction which are measured at a temperature of 23° C. by using a transverse resonant technique prescribed in JIS Z 2280 (1993).
In addition, it was found that, it is possible to significantly decrease the degree of planar anisotropy in terms of modulus of longitudinal elasticity by appropriately controlling the chemical composition of ferritic stainless steel and, in particular, by appropriately controlling the rolling temperature range and accumulated rolling reduction ratio (=100−(the final thickness/the thickness before rolling in the final 3 passes is performed)×100[%]) of the final 3 passes of a finish hot rolling process composed of multiple passes.
Aspects of the present invention have been completed on the basis of the knowledge described above, and the subject matter of aspects of the present invention is as follows.
[1] A hot rolled ferritic stainless steel sheet having a chemical composition containing, 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%, and the balance being Fe and inevitable impurities and an absolute value |ΔE| of planar anisotropy in terms of modulus of longitudinal elasticity calculated by using equation (1) below of 35 GPa or less.
|ΔE|=|(E _L−2×E _D +E _C)/2| (1)
Here, E_Ldenotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction, E_Ddenotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction, and E_Cdenotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.
[2] The hot rolled ferritic stainless steel sheet according to item [1] above, the steel sheet having the chemical composition further containing, by massa, one, two, or more selected from Cu: 0.1% to 1.0%, Mo: 0.1% to 0.5%, and Co: 0.01% to 0.5%.
[3] The hot rolled ferritic stainless steel sheet according to item [1] or [2] above, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from 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 0.0050%, Ca: 0.0002% to 0.0020%, and REM: 0.01% to 0.10%.
[4] A hot rolled and annealed ferritic stainless steel sheet obtained by performing hot rolled sheet annealing on the hot rolled ferritic stainless steel sheet according to any one of items [1] to [3] above.
[5] A method for manufacturing the hot rolled ferritic stainless steel sheet according to any one of items [1] to [3] above, the method including performing a hot rolling process involving finish rolling composed of 3 passes or more, in which rolling in the final 3 passes of the finish rolling is performed in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more.
[6] A method for manufacturing a hot rolled and annealed ferritic stainless steel sheet, the method including using the method for manufacturing a hot rolled ferritic stainless steel sheet according to item [5] above, and further performing hot rolled sheet annealing at a temperature of 800° C. to 900° C. after the hot rolling process.
According to aspects of the present invention, it is possible to obtain a hot rolled ferritic stainless steel sheet and a hot rolled and annealed ferritic stainless steel sheet which have sufficient corrosion resistance and with which it is possible to inhibit bending and twist from occurring after forming has been performed.
Here, the term “sufficient corrosion resistance” in accordance with aspects of the present invention means a case where a rust area ratio (=the rust area/the total area of a steel sheet×100 [%]) is 25% or less after having performed 8 cycles of a salt spray cyclic corrosion test prescribed in JIS H 8502, where the unit cycle includes salt spraying (35° C., 5-mass %-NaCl, 2-hour spraying), drying (60° C., relative humidity=40%, 4 hours), and wetting (50° C., relative humidity 95%, 2 hours) in this order, on a steel sheet whose surface has been polished by using #600 emery paper and whose end surfaces are sealed.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The hot rolled ferritic stainless steel sheet and the hot rolled and annealed ferritic stainless steel sheet according to aspects of the present invention have a chemical composition containing, 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%, and the balance being Fe and inevitable impurities and an absolute value |ΔE| of planar anisotropy in terms of modulus of longitudinal elasticity calculated by using equation (1) below of 35 GPa or less.
|ΔE|=|(E _L−2×E _D +E _C)/2| (1)
Here, E_Ldenotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction, E_Ddenotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction, and E_Cdenotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.
In addition, E_L, E_D, and E_Care respectively defined as the values of modulus of longitudinal elasticity in the rolling direction of a steel sheet, in a direction at an angle of 45° to the rolling direction, and in a direction at a right angle to the rolling direction which are measured at a temperature of 23° C. by using a transverse resonant technique prescribed in JIS Z 2280 (1993).
Hereafter, aspects of the present invention will be described in detail.
The hot rolled ferritic stainless steel sheet and hot rolled and annealed ferritic stainless steel sheet according to aspects of the present invention are intended to be used mainly for a flange having a large wall thickness which is used for the EGR cooler parts of an automobile. The present inventors used various kinds of hot rolled ferritic stainless steel sheet for a flange having a large thickness for an EGR cooler in order to evaluate its performance in detail. As a result, it was found that large bending and twist tend to occur due to vibration during running of an automobile in the case where a hot rolled ferritic stainless steel sheet having an absolute value of planar anisotropy in terms of the modulus of longitudinal elasticity of more than 35 GPa is used.
Therefore, the present inventors diligently conducted investigations regarding a method for decreasing the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled ferritic stainless steel sheet, in particular, focusing on the rolling temperature and rolling reduction ratio of each of multiple passes of a hot rolling process using multiple rolling stands. As a result, it was found that it is possible to significantly decrease the degree of planar anisotropy in terms of modulus of longitudinal elasticity and to achieve the desired rigidity by performing rolling in the final 3 passes of multiple-pass finish hot rolling composed of 3 passes or more in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more (or preferably 30% or more).
The reasons why the desired degree of planar anisotropy in terms of modulus of longitudinal elasticity is achieved by using the method described above will be described hereafter.
The modulus of longitudinal elasticity of a hot rolled ferritic stainless steel sheet strongly depends on the texture of the steel sheet. Since the texture of a hot rolled steel sheet is formed by repeating the application of processing strain due to rolling and recrystallization, it is possible to control such a texture by adjusting temperature at which rolling work is applied and the amount of strain applied due to rolling.
On the other hand, in the central portion in the thickness direction of a ferritic stainless steel slab which has not yet been subjected to hot rolling, elongated ferrite grains are sequentially distributed in the casting direction. In the case where such a stainless steel slab is subjected to hot rolling by using a conventional method, since there is a decrease in grain boundary area due to an increase in the number of elongated grains in the central portion in the thickness direction, the number of recrystallization sites is less in the central portion in the thickness direction than in the surface layer of the steel sheet.
Moreover, in the case where a steel sheet is rolled, the steel sheet is elongated with deformation starting mainly in the surface layer. Therefore, in the case of a small rolling reduction ratio, the amount of deformation is small in the central portion in the thickness direction, which results in almost no rolling strain being introduced to the central portion in the thickness direction.
Then, in the case of conventional hot rolling, while the application of strain and recrystallization are repeated in the surface layer of a steel sheet, the progress of recrystallization is significantly delayed in the central portion in the thickness direction. Therefore, elongated ferrite grains having similar crystal orientations, which have been formed in a casting process, tend to be retained without being broken, which results in an increase in the degree of planar anisotropy in terms of modulus of longitudinal elasticity after hot rolling has been performed.
As an optimum method for decreasing the degree of planar anisotropy in terms of modulus of longitudinal elasticity, the present inventors devised a method in which rolling in the final 3 passes of finish hot rolling is performed in a temperature range of 900° C. to 1100° C. in which recrystallization actively occurs, with an accumulated rolling reduction ratio of 25% or more, which is a rolling reduction larger than that in conventional art.
Specifically, the present inventors systematically conducted investigations regarding the influences of temperature and rolling reduction ratio at which each rolling pass of finish hot rolling composed of 7 passes was performed on the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled steel sheet manufactured. As a result, it was found that there is a tendency for the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of the steel sheet after hot rolling has been performed to strongly depend on the rolling temperatures and rolling reduction ratios of the final 3 passes while there is almost no influence of the temperatures and rolling reduction ratios of the first 4 passes. Therefore, the present inventors conducted closer investigations regarding the influences of the rolling temperatures and rolling reduction ratios of the final 3 passes and the accumulated rolling reduction ratio of the final 3 passes: As a result, it was found that there is a tendency for the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled steel sheet to significantly decrease in the case where rolling in the final 3 passes is performed in a temperature range of 900° C. to 1100° C. and that the amount of change in the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of the hot rolled steel sheet in this case depends not on the rolling reduction ratio of each of the passes but on the accumulated rolling reduction ratio of the final 3 passes. That is, it was found that it is important to complete finish rolling by performing rolling in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more from the viewpoint of the planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled steel sheet.
The present inventors conducted investigations regarding the reasons why the rolling temperature and rolling reduction ratio of each of the rolling passes prior to the final 3 passes have a small influence on the planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled steel sheet. As a result, it was found that, in the case of the rolling passes prior to the final 3 passes, since the thickness before rolling is performed is large, sufficient rolling strain is not applied to the central portion in the thickness direction even if the rolling reduction ratio is large. In addition, it was found that, since rolling temperature is high, there is an increase in grain size due to an excessive growth of recrystallized crystal grains which are formed after rolling has been performed, which results in a significantly small effect of decreasing the degree of anisotropy in a metallographic structure through the forming of the recrystallized crystal grains compared with the accumulated effect in the case of the final 3 passes.
On the other hand, in the case where the accumulated rolling ratio of the final 3 passes is 25% or more, which is larger than that in conventional art, since rolling strain is effectively applied to the central portion in the thickness direction of a steel sheet due to rolling being performed in the final 3 passes, there is a significant increase in the number of recrystallization sites in the central portion in the thickness direction. By performing such rolling in a temperature range of 900° C. to 1100° C., in which recrystallization actively occurs, since recrystallization is promoted in the central portion in the thickness direction, an elongated-ferrite-grain structure, which was formed in a casting process, is effectively broken, which results in a significant decrease in the degree of planar anisotropy in terms of modulus of longitudinal elasticity after hot rolling has been performed. In addition, by performing rolling at a temperature of 1100° C. or lower, since it is possible to inhibit an increase in the grain size of recrystallized crystal grains, the effect of decreasing the degree of anisotropy in a metallographic structure is sufficiently realized. With this technique, since it is possible to control the absolute value of planar anisotropy in terms of modulus of longitudinal elasticity to be 35 GPa or less, it is possible to inhibit deformation such as large bending and twist when a steel sheet is subjected to vibration after the steel sheet has been formed into, for example, a flange having a large wall thickness.
Moreover, the present inventors found that, in the case where the hot rolled steel sheet according to aspects of the present invention is subjected to hot rolled sheet annealing in a temperature range of 800° C. to 900° C. in order to improve the formability of the hot rolled steel sheet, the effect of decreasing the degree of planar anisotropy in terms of modulus of longitudinal elasticity, which have been obtained through hot rolling, is maintained while the effect of improving formability of the hot rolled and annealed steel sheet is obtained. It was found that this is because the effect of decreasing the degree of planar anisotropy in terms of modulus of longitudinal elasticity in accordance with aspects of the present invention is caused by the breakage of an elongated-ferrite-grain structure in the central portion in the thickness direction and because elongated ferrite grains, which contribute to an increase in the degree of anisotropy in a steel sheet, are not formed in the case where hot rolled sheet annealing is performed in a specified temperature range after hot rolling has been performed.
In addition, although there is no particular limitation on the thickness of the hot rolled ferritic stainless steel sheet and the hot rolled and annealed ferritic stainless steel sheet according to aspects of the present invention, it is preferable that the thickness be 5.0 mm to 15.0 mm, because the steel sheet desirably has a thickness suitable for a flange having a large wall thickness.
Hereafter, the chemical composition of the ferritic stainless steel sheet and the hot rolled and annealed ferritic stainless steel sheet according to aspects of the present invention will be described.
Hereafter, % used when describing a chemical composition means mass %, unless otherwise noted.
C: 0.005% to 0.060%
In the case where the C content is large, there is a deterioration in workability, and there is sensitization and a deterioration in toughness due to the precipitation of Cr-based carbonitrides. Therefore, the upper limit of the C content is set to be 0.060%. On the other hand, there is a significant increase in refining costs in the case where the C content is excessively small. Therefore, the lower limit of the C content is set to be 0.005%, which is at a level at which there is no significant increase in manufacturing costs in a common refining method. It is preferable that the C content be 0.010% to 0.050% from the viewpoint of the stable manufacturability in a steel-making process. The C content is more preferably in a range of 0.020% to 0.045%, even more preferably 0.025% to 0.040%, or even much more preferably 0.030% to 0.040%.
Si: 0.02% to 0.50%
Si is a chemical element which functions as a deoxidizing agent in a process for preparing molten steel. It is necessary that the Si content be 0.02% or more in order to obtain such an effect. However, it is not desirable that the Si content be more than 0.50%, because this results in a deterioration in manufacturability in a hot rolling process due to an increase in rolling load when hot rolling is performed as a result of an increase in the hardness of a steel sheet. Therefore, the Si content is set to be in a range of 0.02% to 0.50%, preferably 0.10% to 0.35%, or more preferably 0.10% to 0.30%.
Mn: 0.01% to 1.00%
It is not desirable that the Mn content be excessively large, because this results in a deterioration in manufacturability in a hot rolling process due to an increase in rolling load when hot rolling is performed as a result of an increase in the hardness of a steel sheet as in the case of Si. In addition, there may be a deterioration in corrosion resistance due to an increase in the amount of MnS. Therefore, the upper limit of the Mn content is set to be 1.00%. The lower limit of the Mn content is set to be 0.01% from the viewpoint of a load placed on a refining process. It is preferable that the Mn content be in a range of 0.10% to 0.90%, or more preferably 0.45% to 0.85%.
P: 0.04% or less
Since P is a chemical element which promotes intergranular fracture due to intergranular segregation, it is desirable that the P content be as small as possible, and the upper limit of the P content is set to be 0.04%. It is preferable that the P content be 0.03% or less, or more preferably 0.01% or less.
S: 0.01% or less
S is a chemical element which deteriorates, for example, ductility and corrosion resistance as a result of existing in the form of sulfide-based inclusions such as MnS, and such negative effects become marked, in particular, in the case where the S content is more than 0.01%. Therefore, it is desirable that the S content be as small as possible, and the upper limit of the S content is set to be 0.01% in accordance with aspects of the present invention. It is preferable that the S content be 0.007% or less, or more preferably 0.005% or less.
Cr: 15.5% to 18.0%
Cr is a chemical element which is effective for improving corrosion resistance by forming a passivation film on the surface of a steel sheet. It is necessary that the Cr content be 15.5% or more in order to obtain such an effect. However, it is not desirable that the Cr content be more than 18.0%, because this results in a significant deterioration in the toughness of a steel sheet. Therefore, the Cr content is set to be in a range of 15.5% to 18.0%, preferably 16.0% to 17.0%, or more preferably 16.0% to 16.5%.
Al: 0.001% to 0.10%
Al is, like Si, a chemical element which functions as a deoxidizing agent. It is necessary that the Al content be 0.001% or more in order to obtain such an effect. However, in the case where the Al content is more than 0.10%, since there is an increase in the amount of Al-based inclusions such as Al₂O₃, there is a tendency for surface quality to deteriorate. Therefore, the Al content is set to be in a range of 0.001% to 0.10%, preferably 0.001% to 0.07%, or more preferably 0.001% to 0.05%.
N: 0.005% to 0.100%
In the case where the N content is large, as in the case of C, there is a deterioration in workability, and there is sensitization and a deterioration in toughness due to the precipitation of Cr-based carbonitrides. Therefore, the upper limit of the N content is set to be 0.100%. On the other hand, there is a significant increase in refining costs in the case where the N content is excessively small as in the case of C. Therefore, the lower limit of the N content is set to be 0.005%, which is at a level at which there is no significant increase in manufacturing costs in a common refining method. It is preferable that the N content be 0.010% to 0.075% from the viewpoint of stable manufacturability in a steel-making process. The N content is more preferably in a range of 0.025% to 0.055%, or even more preferably 0.030% to 0.050%.
Ni: 0.1% to 1.0%
Ni is a chemical element which improves corrosion resistance, and the addition of Ni is effective, in particular, in the case where high corrosion resistance is required. Such an effect becomes marked in the case where the Ni content is 0.1% or more. However, it is not desirable that the Ni content be more than 1.0%, because this results in a deterioration in formability. Therefore, the Ni content is set to be 0.1% to 1.0%. The Ni content is preferably in a range of 0.2% to 0.4%.
The remainder is Fe and inevitable impurities.
Although it is possible to obtain the effects of aspects of the present invention by using the chemical composition described above, the chemical composition may further contain the following chemical elements in order to improve manufacturability or material properties.
One, two, or more selected from Cu: 0.1% to 1.0%, Mo: 0.1% to 0.5%, and Co: 0.01% to 0.5%
Cu: 0.1% to 1.0%
Cu is a chemical element which improves corrosion resistance, and the addition of Cu is effective, in particular, in the case where high corrosion resistance is required. Such an effect becomes marked in the case where the Cu content is 0.1% or more. However, in the case where the Cu content is more than 1.0%, there may be a deterioration in formability. Therefore, in the case where Cu is added, the Cu content is set to be 0.1% to 1.0%. The Cu content is preferably in a range of 0.2% to 0.4%.
Mo: 0.1% to 0.5%
Mo is, like Ni and Cu, a chemical element which improves corrosion resistance, and the addition of Mo is effective, in particular, in the case where high corrosion resistance is required. Such an effect becomes marked in the case where the Mo content is 0.1% or more. However, in the case where the Mo content is more than 0.5%, there may be a deterioration in manufacturability in a hot rolling process due to an increase in rolling load when hot rolling is performed as a result of an increase in the hardness of a steel sheet. Therefore, in the case where Mo is added, the Mo content is set to be 0.1% to 0.5%. The Mo content is preferably in a range of 0.2% to 0.3%.
Co: 0.01% to 0.5%
Co is a chemical element which improves toughness. Such an effect is obtained in the case where the Co content is 0.01% or more. On the other hand, in the case where the Co content is more than 0.5%, there may be a deterioration in formability. Therefore, in the case where Co is added, the Co content is set to be in a range of 0.01% to 0.5%.
One, two, or more selected from V: −0.0l % 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 0.0050%, Ca: 0.0002% to 0.0020%, and REM: 0.01% to 0.10%
V: 0.01% to 0.25%
V is a chemical element which forms carbonitrides more readily than Cr. V is effective for inhibiting sensitization, which is caused by the precipitation of Cr carbonitrides, by precipitating C and N in steel in the form of V-based carbonitrides when hot rolling is performed. It is necessary that the V content be 0.01% or more in order to obtain such an effect. However, in the case where the V content is more than 0.25%, there may be deterioration in workability, and there is an increase in manufacturing costs. Therefore, in the case where V is added, the V content is set to be in a range of 0.01% to 0.25%, or preferably 0.03% to 0.08%.
Ti: 0.001% to 0.015% and Nb: 0.001% to 0.025%
Ti and Nb are, like V, chemical elements which have a high affinity for C and N and which are effective for inhibiting sensitization, which is caused by the precipitation of Cr carbonitrides, by precipitating in the form of carbides and nitrides when hot rolling is performed. In order to obtain such an effect, it is necessary that the Ti content be 0.001% or more or that the Nb content be 0.001% or more. However, in the case where the Ti content is more than 0.015% or in the case where the Nb content is more than 0.030%, there may be a case where it is not possible to achieve good surface quality due to the precipitation of an excessive amount of TiN or NbC. Therefore, the Ti content is set to be in a range of 0.001% to 0.015% in the case where Ti is added, and the Nb content is set to be in a range of 0.001% to 0.025% in the case where Nb is added. It is preferable that the Ti content be in a range of 0.003% to 0.010%. It is preferable that Nb content be in a range of 0.005% to 0.020%, or more preferably 0.010% to 0.015%.
Mg: 0.0002% to 0.0050%
Mg is a chemical element which is effective for improving hot workability. It is necessary that the Mg content be 0.0002% or more in order to obtain such an effect. However, in the case where the Mg content is more than 0.0050%, there may be a deterioration in surface quality. Therefore, in the case where Mg is added, the Mg content is set to be in a range of 0.0002% to 0.0050%, preferably 0.0005% to 0.0035%, or more preferably 0.0005% to 0.0020%.
B: 0.0002% to 0.0050%
B is a chemical element which is effective for preventing secondary cold work embrittlement. It is necessary that the B content be 0.0002% or more in order to obtain such an effect. However, in the case where the B content is more than 0.0050%, there may be a deterioration in hot workability. Therefore, in the case where B is added, the B content is set to be in a range of 0.0002% to 0.0050%, preferably 0.0005% to 0.0035%, or more preferably 0.0005% to 0.0020%.
Ca: 0.0002% to 0.0020%
Ca is a chemical element which is effective for preventing nozzle clogging due to the precipitation of inclusions which tends to occur when continuous casting is performed. It is necessary that the Ca content be 0.0002% or more in order to obtain such an effect. However, in the case where the Ca content is more than 0.0020%, there may be a deterioration in corrosion resistance due to the formation of CaS. Therefore, in the case where Ca is added, the Ca content is set to be in a range of 0.0002% to 0.0020%, preferably 0.0005% to 0.0015%, or more preferably 0.0005% to 0.0010%.
REM: 0.01% to 0.10%
REM (rare earth metals) is a chemical element which improves oxidation resistance and which is effective for improving the corrosion resistance of, in particular, a weld zone by inhibiting the formation of an oxide film in the weld zone. It is necessary that the REM content be 0.01% or more in order to obtain such an effect. However, in the case where the REM content is more than 0.10%, there may be a deterioration in manufacturability such as pickling capability when cold-rolled sheet annealing is performed. In addition, since REM is an expensive chemical element, it is not preferable that the REM content be excessively large, because this results in an increase in manufacturing costs. Therefore, in the case where REM is added, the REM content is set to be in a range of 0.01% to 0.10%, or preferably 0.01% to 0.05%.
Hereafter, the method for manufacturing the ferritic stainless steel sheet and the hot rolled and annealed ferritic stainless steel sheet according to aspects of the present invention will be described.
It is possible to obtain the ferritic stainless steel sheet according to aspects of the present invention by performing a hot rolling process involving rough rolling and finish rolling composed of 3 passes or more on a steel slab having the chemical composition described above, in which rolling in the final 3 passes of finish rolling is performed in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more.
Here, there is no particular limitation on the maximum number of passes of finish rolling from the viewpoint of achieving the specified material properties. However, in the case where the maximum number of passes is more than 15, since there is a tendency for the temperature of a steel sheet to decrease due to an increase in the number of contacts between the sheet and rolling rolls, there may be a deterioration in manufacturability or an increase in manufacturing costs, because, for example, it is necessary to heat the steel sheet from outside in order to maintain the temperature of the steel sheet within the specified temperature range. Therefore, it is preferable that the maximum number of passes be 15 or less, or more preferably 10 or less.
First, molten steel having the chemical composition described above is prepared by using a known method such as one which utilizes, for example, a converter, an electric furnace, or a vacuum melting furnace and made into a steel raw material (slab) by using a continuous casting method or an ingot casting-slabbing method.
This slab is subjected to hot rolling after having been heated at a temperature of 1100° C. to 1250° C. for 1 hour to 24 hours or the slab as cast is directly subjected to hot rolling without having been heated. In accordance with aspects of the present invention, although there is no particular limitation on rough rolling, it is preferable that an accumulated rolling reduction ratio in rough rolling be 65% or more in order to effectively break a cast structure. When finish rolling is subsequently performed in order to obtain a specified thickness, rolling in the final 3 passes of finish rolling is performed in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more.
Rolling temperature range of final 3 passes: 900° C. to 1100° C.
In the final 3 passes of finish rolling, it is necessary to effectively apply rolling strain to the central portion in the thickness direction by a large accumulated rolling reduction ratio and to allow sufficient recrystallization to occur. Therefore, it is necessary that rolling in the final 3 passes of finish rolling be performed in a temperature range of 900° C. to 1100° C., in which sufficient recrystallization occurs. In the case where the rolling temperature of the final 3 passes is lower than 900° C., since there is insufficient recrystallization, it is not possible to achieve the desired degree of planar anisotropy in terms of modulus of longitudinal elasticity. On the other hand, it is not preferable that the rolling temperature of the final 3 passes be higher than 1100° C., because this causes a significant increase in crystal grain size with the result that it is not possible to achieve the specified degree of planar anisotropy in terms of modulus of longitudinal elasticity and with the result that the toughness of a hot rolled steel sheet is deteriorated.
It is preferable that the rolling temperature of the final 3 passes be in a range of 900° C. to 1075° C., or more preferably 930° C. to 1050° C. In addition, in order to prevent an excessive rolling load from being placed in one of the final 3 passes, it is preferable that rolling in the first pass of the final 3 passes be performed in a temperature range of 950° C. to 1100° C., that rolling in the second pass following the first pass be performed in a temperature range of 925° C. to 1075° C., and that rolling in the third pass following the second pass be performed in a temperature range of 900° C. to 1050° C.
Accumulated rolling reduction ratio of final 3 passes: 25% or more
In order to effectively apply rolling strain to the central portion in the thickness direction of a steel sheet, it is necessary that rolling in the final 3 passes of finish rolling be performed with an accumulated rolling reduction ratio of 25% or more. In the case where the accumulated rolling reduction ratio is less than 25%, since recrystallization in the central portion in the thickness direction is delayed due to an insufficient amount of rolling strain applied to the central portion in the thickness direction, it is not possible to achieve the desired degree of planar anisotropy in terms of modulus of longitudinal elasticity. Therefore, it is preferable that the accumulated rolling reduction ratio be 25% or more, more preferably 30% or more, or even more preferably 35% or more. Here, although there is no particular limitation on the upper limit of the accumulated rolling reduction ratio, in the case where the accumulated rolling reduction ratio is excessively large, there is a deterioration in manufacturability due to an increase in rolling load, and there may be a case where rough surface is caused after rolling has been performed. Therefore, it is preferable that the accumulated rolling reduction ratio be 60% or less.
In addition, the accumulated rolling reduction ratio described above is expressed by the formula 100−(the final thickness/the thickness before rolling in the final 3 passes is performed)×100 [%].
In addition, the method for manufacturing the hot rolled ferritic stainless steel sheet according to aspects of the present invention is characterized in that the rolling temperature and accumulated rolling reduction ratio of the final 3 passes of finish rolling are controlled. In the case where the control target is the rolling temperature and accumulated rolling reduction ratio of the final 4 passes or more, since the rolling reduction ratio of each of the passes is too small for applied strain to contribute a decrease in the degree of planar anisotropy in terms of modulus of longitudinal elasticity, it is not possible to sufficiently obtain the effect of decreasing the degree of planar anisotropy in terms of modulus of longitudinal elasticity. In addition, it is not preferable that the control target be the rolling temperature and accumulated rolling reduction ratio of the final 2 passes or less, because this may result in a deterioration in manufacturability due to a significant increase in rolling load as a result of performing high rolling reduction with an accumulated rolling reduction ratio of 25% or more in 2 passes. Therefore, in the method for manufacturing the hot rolled ferritic stainless steel sheet according to aspects of the present invention, the rolling temperature and accumulated rolling reduction ratio of the final 3 passes of finish rolling are controlled.
In addition, in the method for manufacturing the hot rolled ferritic stainless steel sheet according to aspects of the present invention, there is no particular limitation on the number of passes of finish rolling as long as the number is 3 or more so that the rolling temperature and accumulated rolling reduction ratio of the final 3 passes of finish rolling are controlled.
After finish rolling has been performed, the steel sheet is cooled and then subjected to a coiling treatment in order to obtain a hot rolled steel strip. In accordance with aspects of the present invention, although there is no particular limitation on the coiling temperature, in the case where steel having a chemical composition with which an austenite phase is formed during hot rolling is coiled at a coiling temperature of lower than 500° C., since an austenite phase transforms into a martensite phase, there may be a deterioration in formability due to an increase in the hardness of a hot rolled steel sheet. Therefore, it is preferable that a coiling treatment be performed at a temperature of 500° C. or higher.
In accordance with aspects of the present invention, it is possible to achieve the desired corrosion resistance and the desired degree of planar anisotropy in terms of modulus of longitudinal elasticity at the time of the completion of the hot rolling described above, and further a hot rolled and annealed ferritic stainless steel sheet may be manufactured by performing hot rolled sheet annealing on the hot rolled ferritic stainless steel sheet in a temperature range of 800° C. to 900° C. after the hot rolling process in order to improve formability.
Hot rolled sheet annealing temperature: 800° C. to 900° C. In the case where the hot rolled sheet annealing temperature is lower than 800° C., since there is insufficient recrystallization, it is not possible to obtain the effect of improving formability due to deformation microstructure formed by performing hot rolling being retained. On the other hand, in the case where the hot rolled sheet annealing temperature is higher than 900° C., since there is an increase in the degree of planar anisotropy in terms of modulus of longitudinal elasticity due to the formation of an austenite phase when annealing is performed, namely, there may be a case where the specified degree of planar anisotropy in terms of modulus of longitudinal elasticity which has been obtained in the hot rolled steel sheet is lost. In addition, in the case where a cooling rate after hot rolled sheet annealing has been performed at a temperature of higher than 900° C. is large, since there is an increase in the hardness of a steel sheet due to an austenite phase transforming into a martensite phase, there may be conversely a deterioration in formability. Therefore, in the case where hot rolled sheet annealing is performed, it is preferable that the annealing temperature be 800° C. to 900° C. Here, there is no particular limitation on the holding time and method of hot rolled sheet annealing, any one of a box annealing (batch annealing) method and a continuous annealing method may be used.
The obtained hot rolled steel sheet or steel sheet (hot rolled and annealed steel sheet) which has been subjected to hot rolled sheet annealing may be subjected to a descaling treatment such as one which utilizes shot blasting or pickling as needed. Moreover, grinding or polishing may be performed in order to improve surface quality.

EXAMPLES

Hereafter, aspects of the present invention will be described in detail by using examples.
Molten stainless steels having the chemical compositions given in Table 1 were prepared by performing refining which utilized a converter having a capacity of 150 tons and a strong stirring-vacuum oxygen decarburization (SS-VOD) method, and steel slabs having a width of 1000 mm and a thickness of 200 mm were then manufactured by using a continuous casting method. The obtained slabs were heated at a temperature of 1200° C. for one hour and then subjected to hot rolling in which reverse-type rough rolling was performed by using 3 rolling stands in order to obtain steel sheets having a thickness of about 40 mm and in which the final 3 passes (the fifth pass, the sixth path, and the seventh pass) of finish rolling composed of 7 passes were then performed under the conditions given in Table 2 in order to obtain hot rolled steel sheets. In addition, some of the hot rolled steel sheets (Nos. 25, 26, and 38 in Table 2) were subjected to hot rolled sheet annealing in which the hot rolled steel sheets were held under the conditions given in Table 2 for 8 hours after hot rolling had been performed and in which the held steel sheets were subjected furnace cooling in order to obtain hot rolled and annealed steel sheets.
The obtained hot rolled steel sheets and hot rolled and annealed steel sheets were evaluated as described below.
(1) Evaluation of Planar Anisotropy
Test pieces having a length of 60 mm, a width of 10 mm, and a thickness of 2 mm whose longitudinal direction were respectively a direction parallel to the rolling direction, a direction at an angle of 45° to the rolling direction, and a direction at a right angle to the rolling direction were taken from the central portion in the thickness direction within 1 mm on both sides of the center in the thickness direction. The modulus of longitudinal elasticity of each of the obtained test pieces was measured at a temperature of 23° C. by using a transverse resonant technique prescribed in JIS Z 2280 (1993), and the absolute value |ΔE| of planar anisotropy in terms of modulus of longitudinal elasticity was calculated by using equation (1) below.
|ΔE|=|(E _L−2×E _D =E _C)/2| (1)
Here, E_Ldenotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction, E_Ddenotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction, and E_Cdenotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.
A case where the degree of planar anisotropy in terms of modulus of longitudinal elasticity, that is, |ΔE| was 35 GPa or less was judged as a case where it is possible to sufficiently inhibit bending and twist after the steel sheet has been formed into, for example, a flange, that is, judged as satisfactory (◯). A case where the degree of planar anisotropy in terms of modulus of longitudinal elasticity, that is, |ΔE| was more than 35 GPa was judged as unsatisfactory (x).
(2) Evaluation of Corrosion Resistance
A salt spray cyclic corrosion test prescribed in JIS H 8502 was performed on a test piece having a size of 60 mm×100 mm which had been taken from the hot rolled steel sheet, whose surface had been polished by using #600 emery paper, and whose end surfaces were sealed. The salt spray cyclic corrosion test was performed in such a manner that a unit cycle was repeated 8 times, where the unit cycle includes salt spraying (5-mass %-NaCl, 35° C., 2-hour spraying), drying (60° C., 4 hours, relative humidity=40%), and wetting (50° C., 2 hours, relative humidity≥95%).
After having determined the rust area on the surface of the test piece by performing image analysis on a photograph of the surface of the test piece which had been obtained after the 8 cycles of the salt spray cyclic corrosion test, a rust area ratio ((the rust area of the test piece/the total area of the test piece)×100 [%]) was calculated as the ratio of the rust area to the total area of the test piece. A case where the rust area ratio was 10% or less was judged as a case of particularly excellent corrosion resistance, that is, judged as satisfactory (⊙), a case where the rust area ratio was more than 10% and 25% or less was judged as satisfactory (◯), and a case where the rust area ratio was more than 25% was judged as unsatisfactory (x).
The evaluation results are given in Table 2 along with the hot rolling conditions.

TABLE 1

Steel	Chemical Composition (mass %)

Code	C	Si	Mn	P	S	Cr	Al	N	Ni	Other	Note

A	0.043	0.24	0.65	0.03	0.004	16.2	0.002	0.047	0.15		Example
B	0.016	0.16	0.81	0.03	0.004	16.3	0.003	0.033	0.13		Example
C	0.036	0.23	0.68	0.04	0.007	16.2	0.002	0.050	0.52	Cu: 0.4	Example
D	0.044	0.22	0.62	0.01	0.004	16.5	0.003	0.046	0.17	V: 0.03, Ti: 0.01, Nb: 0.02	Example
E	0.027	0.03	0.77	0.02	0.006	16.4	0.005	0.034	0.17		Example
F	0.024	0.48	0.82	0.03	0.004	16.2	0.004	0.031	0.14		Example
G	0.037	0.29	0.03	0.01	0.003	16.4	0.003	0.045	0.12		Example
H	0.036	0.26	0.97	0.02	0.003	16.1	0.005	0.044	0.18		Example
I	0.043	0.16	0.66	0.03	0.004	16.3	0.094	0.039	0.15		Example
J	0.041	0.25	0.67	0.03	0.004	16.2	0.005	0.048	0.11	Mo: 0.3, Co: 0.2	Example
K	0.039	0.24	0.70	0.04	0.006	16.1	0.008	0.052	0.16	Mg: 0.001, B: 0.001, Ca: 0.001	Example
L	0.036	0.21	0.63	0.04	0.005	16.1	0.007	0.049	0.14	REM: 0.04	Example
M	0.044	0.26	0.61	0.03	0.004	14.9	0.004	0.040	0.13		Comparative
											Example
N	0.048	0.28	0.63	0.04	0.006	19.3	0.006	0.043	0.18		Comparative
											Example

•The remainder other than the constituent chemical elements described above is Fe and inevitable impurities.

TABLE 2

		Ending	Starting	Starting	Starting		Ending	Accumulated	Hot rolled
		Thickness	Temper-	Thickness	Temper-	Ending	Thickness	Rolling	sheet						Rust
		of Rough	ature of	of	ature of	Temperature	of 7th	Reduction Ratio	Annealing						Area
	Steel	Rolling	5th Pass	5th Pass	6th Pass	of 7th Pass	Pass	of Final 3 Passes	Temperature	E_L	E_D	E_C	\|ΔE\|	Planar	Ratio	Corrosion
No.	Code	[mm]	[° C.]	[mm]	[° C.]	[° C.]	[mm]	[%]	[° C.]	[GPa]⁽*¹⁾	[GPa]⁽*²⁾	[GPa]⁽*³⁾	[GPa]⁽*⁴⁾	Anisotropy	[%]	Resistance	Note

1	A	40.7	1018	17.8	998	975	13.2	26	Undone	212	192	234	31	◯	12	◯	Example
2	A	40.1	1014	17.5	995	972	12.4	29	Undone	214	194	222	24	◯	12	◯	Example
3	A	39.9	1017	17.8	997	977	11.6	35	Undone	210	205	220	10	◯	20	◯	Example
4	A	40.4	1010	17.8	990	968	10.7	40	Undone	197	206	220	3	◯	14	◯	Example
5	A	40.4	1029	8.8	1014	986	6.4	27	Undone	215	194	232	30	◯	11	◯	Example
6	A	40.0	1023	11.7	1009	982	8.1	31	Undone	213	198	227	22	◯	11	◯	Example
7	A	39.1	1011	7.5	992	967	4.2	44	Undone	201	199	227	15	◯	14	◯	Example
8	A	39.3	962	8.8	942	923	6.3	28	Undone	212	201	217	14	◯	15	◯	Example
9	A	40.9	1097	10.2	1083	1044	6.2	39	Undone	196	204	234	11	◯	12	◯	Example
10	B	40.7	1029	7.9	1009	989	5.9	25	Undone	208	196	230	23	◯	19	◯	Example
11	B	39.9	1015	8.8	1001	972	6.0	32	Undone	207	204	226	12	◯	12	◯	Example
12	B	40.4	1026	9.2	1013	986	5.6	39	Undone	203	205	227	10	◯	14	◯	Example
13	B	39.6	977	12.2	960	920	8.2	33	Undone	193	217	216	13	◯	14	◯	Example
14	C	39.9	1078	9.5	1064	1011	6.1	36	Undone	206	203	230	15	◯	3	⊙	Example
15	C	40.2	996	9.4	979	945	5.8	38	Undone	217	204	236	23	◯	2	⊙	Example
16	C	39.3	1061	8.0	1046	1003	4.3	46	Undone	205	212	233	7	◯	2	⊙	Example
17	C	40.3	1069	16.0	1051	1038	11.7	27	Undone	211	196	221	20	◯	2	⊙	Example
18	D	39.7	1033	10.6	1019	994	7.4	30	Undone	209	202	224	15	◯	16	◯	Example
19	D	39.8	1037	12.3	1021	1000	9.2	25	Undone	204	199	228	17	◯	15	◯	Example
20	D	41.0	962	11.8	946	903	8.5	28	Undone	206	200	221	14	◯	14	◯	Example
21	D	40.5	949	10.3	930	904	6.0	42	Undone	214	205	226	15	◯	11	◯	Example
22	A	40.4	1021	8.2	1002	983	6.4	22	Undone	222	189	234	39	X	18	◯	Comparative
																	Example
23	A	40.7	924	8.3	907	876	5.7	31	Undone	225	186	230	42	X	18	◯	Comparative
																	Example
24	A	39.7	891	9.0	876	852	6.0	33	Undone	226	185	236	46	X	18	◯	Comparative
																	Example
25	B	40.5	1030	17.9	1009	982	12.2	32	889	213	191	226	29	◯	19	◯	Example
26	B	40.8	1042	18.4	1013	977	12.2	34	806	218	206	232	19	◯	15	◯	Example
27	E	40.4	983	18.0	947	922	10.6	41	Undone	201	187	224	26	◯	15	◯	Example
28	F	40.0	974	17.7	951	929	10.8	39	Undone	213	192	205	17	◯	15	◯	Example
29	G	40.3	1006	20.1	977	953	14.7	27	Undone	206	189	231	30	◯	19	◯	Example
30	H	39.7	1111	20.5	984	957	14.9	27	Undone	211	186	229	34	◯	15	◯	Example
31	I	40.6	943	18.8	919	903	13.0	31	Undone	219	193	228	31	◯	20	◯	Example
32	J	40.5	966	19.2	932	909	13.3	31	Undone	208	199	221	16	◯	2	⊙	Example
33	K	40.5	953	19.4	927	905	13.2	32	Undone	206	187	208	20	◯	15	◯	Example
34	L	39.9	962	19.0	931	910	13.3	30	Undone	219	207	231	18	◯	16	◯	Example
35	M	40.1	980	18.4	949	918	13.5	27	Undone	212	186	226	33	◯	38	X	Comparative
																	Example

36	N	It was not possible to perform the evaluations due to fracturing in the hot rolling process.	Comparative
			Example

37	B	40.2	1152	9.4	1134	1109	6.1	35	Undone	222	181	229	45	X	13	◯	Comparative
																	Example
38	B	40.6	1047	18.6	1018	986	12.3	34	936	216	186	235	40	X	13	◯	Comparative
																	Example

⁽*¹⁾E_L: longitudinal elasticity modulus in a direction parallel to the rolling direction
⁽*²⁾E_D: longitudinal elasticity modulus in a direction at an angle of 45° to the rolling direction
⁽*³⁾E_C: longitudinal elasticity modulus in a direction at a right angle to the rolling direction
⁽*⁴⁾\|ΔE\| = \|(E_L− 2 × E_D+ E_C)/2\|
An underlined portion indicates a value out of the range according to the present invention.

Nos. 1 through 21 and Nos. 25 through 34, which satisfied the requirements according to aspects of the present invention regarding the ranges of a chemical composition, hot rolling conditions, and hot rolled sheet annealing conditions, had a small absolute value (|ΔE|) of planar anisotropy in terms of the modulus of longitudinal elasticity of 35 GPa or less, which means these examples had the desired rigidity. Moreover, from the results of the evaluation of corrosion resistance performed on the obtained hot rolled steel sheets and hot rolled and annealed steel sheets, it is clarified that all the steel sheets had a rust area ratio of 25% or less, which means these examples also had sufficient corrosion resistance.
In particular, Nos. 14 through 17 manufactured by using steel C, which contained 0.52 mass % of Ni and 0.4 mass % of Cu, and No. 32 manufactured by using steel J, which contained 0.3 mass % of Mo, had a higher level of excellent corrosion resistance represented by a rust area ratio of 10% or less.
In the case of No. 22 where the accumulated rolling reduction ratio of the final 3 passes was less than the range according to aspects of the present invention, since a large number of elongated grains existed in the central portion in the thickness direction, there was an increase in the degree of planar anisotropy in terms of modulus of longitudinal elasticity so that it was not possible to achieve the specified |ΔE|.
In the case of No. 23 where only the final temperature of the seventh pass among the rolling in the final 3 passes is lower than the range according to aspects of the present invention and in the case of No. 24 where all the rolling temperatures of the final 3 passes were lower than the range according to aspects of the present invention, since there was insufficient recrystallization in the central portion in the thickness direction even though rolling was performed with the specified accumulated rolling reduction ratio, it was not possible to achieve the specified |ΔE|. In addition, in the case of No. 37 where all the rolling temperatures of the final 3 passes were higher than the range according to aspects of the present invention, since there was an increase in crystal grain size, it was not possible to achieve the specified |ΔE|.
In the case of No. 38 where the hot rolled sheet annealing temperature was higher than the range according to aspects of the present invention, since austenite was formed when hot rolled sheet annealing was performed, it was not possible to achieve the specified |ΔE|. In the case where the steel sheets of Nos. 22 through 24, 37, and 38, in which it was not possible to achieve the specified |ΔE|, were used for flanges having a large wall thickness, it was clarified that bending and twist occurred when the flanges were subjected to vibration.
In the case of No. 35 manufactured by using steel M, whose Cr content was less than the range according to aspects of the present invention, since there was an insufficient amount of passivation film formed on the surface of the steel sheet, it was not possible to achieve the desired corrosion resistance.
In the case of No. 36 manufactured by using steel N, whose Cr content was over the range according to aspects of the present invention, since fracturing occurred during the hot rolling process due to a crack formed in the slab when cooling was performed after casting had been performed, it was not possible to perform the specified evaluations.

INDUSTRIAL APPLICABILITY

The hot rolled ferritic stainless steel sheet obtained by using aspects of the present invention can particularly preferably be used for purposes which require satisfactory rigidity and corrosion resistance, for example, for the flange of an EGR cooler.

Claims

1. A hot rolled ferritic stainless steel sheet having a chemical composition containing, 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%, and the balance being Fe and inevitable impurities and

an absolute value |ΔE| of planar anisotropy in terms of modulus of longitudinal elasticity calculated by using equation (1) below of 35 GPa or less:

|ΔE|=|(E _L−2×E _D +E _C)/2| (1),

where, E_Ldenotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction, E_Ddenotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction, and E_Cdenotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.

2. The hot rolled ferritic stainless steel sheet according to claim 1, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from Cu: 0.1% to 1.0%, Mo: 0.1% to 0.5%, and Co: 0.01% to 0.5%.

3. The hot rolled ferritic stainless steel sheet according to claim 1, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from 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 0.0050%, Ca: 0.0002% to 0.0020%, and REM: 0.01% to 0.10%.

4. The hot rolled ferritic stainless steel sheet according to claim 2, the steel sheet having the chemical composition further containing, by mass %, one, two or more selected from 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 0.0050% Ca: 0.0002% to 0.0020%, and REM: 0.01% to 0.10%.

5. A hot rolled and annealed ferritic stainless steel sheet obtained by performing hot rolled sheet annealing on the hot rolled ferritic stainless steel sheet according to claim 1.

6. A hot rolled and annealed ferritic stainless steel sheet obtained by performing hot rolled sheet annealing on the hot rolled ferritic stainless steel sheet according to claim 2.

7. A hot rolled and annealed ferritic stainless steel sheet obtained by performing hot rolled sheet annealing on the hot rolled ferritic stainless steel sheet according to claim 3.

8. A hot rolled and annealed ferritic stainless steel sheet obtained by performing hot rolled sheet annealing on the hot rolled ferritic stainless steel sheet according to claim 4.

9. A method for manufacturing the hot rolled ferritic stainless steel sheet according to claim 1, the method comprising performing a hot rolling process involving finish rolling composed of 3 passes or more, wherein rolling in the final 3 passes of the finish rolling is performed in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more.

10. A method for manufacturing the hot rolled ferritic stainless steel sheet according to claim 2, the method comprising performing a hot rolling process involving finish rolling composed of 3 passes or more, wherein rolling in the final 3 passes of the finish rolling is performed in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more.

11. A method for manufacturing the hot rolled ferritic stainless steel sheet according to claim 3, the method comprising performing a hot rolling process involving finish rolling composed of 3 passes or more, wherein rolling in the final 3 passes of the finish rolling is performed in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more.

12. A method for manufacturing the hot rolled ferritic stainless steel sheet according to claim 4, the method comprising performing a hot rolling process involving finish rolling composed of 3 passes or more, wherein rolling in the final 3 passes of the finish rolling is performed in a temperature range of 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more.

13. A method for manufacturing a hot rolled and annealed ferritic stainless steel sheet, the method comprising using the method for manufacturing a hot rolled ferritic stainless steel sheet according to claim 9, and

further performing hot rolled sheet annealing at a temperature of 800° C. to 900° C. after the hot rolling process.

14. A method for manufacturing a hot rolled and annealed ferritic stainless steel sheet, the method comprising using the method for manufacturing a hot rolled ferritic stainless steel sheet according to claim 10, and

15. A method for manufacturing a hot rolled and annealed ferritic stainless steel sheet, the method comprising using the method for manufacturing a hot rolled ferritic stainless steel sheet according to claim 11, and

16. A method for manufacturing a hot rolled and annealed ferritic stainless steel sheet, the method comprising using the method for manufacturing a hot rolled ferritic stainless steel sheet according to claim 12, and