FERRITIC STAINLESS-STEEL SHEET AND METHOD FOR MANUFACTURING SAME

Abstract

 The present invention provides a ferritic stainless steel sheet which has more excellent toughness and excellent corrosion resistance, and a method for manufacturing the same. A ferritic stainless steel sheet has a composition containing C: 0.001 to 0.020%, Si: 0.05 to 0.35%, Mn: 0.05 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.300%, Cr: 10.0 to 13.0%, Ni: 0.75 to 1.50%, Ti: 0.05 to 0.35%, and N: 0.001 to 0.020%, with the balance being Fe and inevitable impurities, in which γ_I [%] represented by formula (1) below is 65% or more, and a metal structure has an average crystal grain size of 45 µm or less:

where Ni, Mn, Cu, Si, Cr, and Mo represent contents of the respective elements (percent by mass), and an element not contained represents 0. The ferritic stainless steel sheet is manufactured by subjecting a steel slab having the composition to hot rolling, and performing hot-rolled sheet annealing at 750 to 1,050°C.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel sheet and a method for manufacturing the same, and more particularly relates to a ferritic stainless steel sheet having excellent toughness and excellent corrosion resistance, which is suitable for use as a material for flanges, and a method for manufacturing the same.

Background Art

[0002] An automobile exhaust gas passage is composed of various components, such as an exhaust manifold, a muffler, a catalyst, a flexible tube, a center pipe, and a front pipe. When these components are connected, fastening components called flanges are frequently used. Flanges used for such exhaust system components are required to have sufficient rigidity. Therefore, thick flanges (e.g., with a sheet thickness of 5 mm or more) are used for such exhaust system components.

[0003] Furthermore, flanges are manufactured by press forming and blanking or the like, and plain steel has been used.

[0004] Moreover, in recent years, sufficient corrosion resistance has been required for materials for flanges that are used for components exposed to high-temperature exhaust gas in an exhaust gas recirculation (EGR) system or the like. Accordingly, studies have been conducted on use of stainless steel which has better corrosion resistance than plain steel, in particular, ferritic stainless steel which has a relatively low coefficient of thermal expansion and in which thermal stress is unlikely to occur. Consequently, there has been a strong demand for a ferritic stainless steel sheet having a large thickness (e.g., a sheet thickness of 5 mm or more) that can be used for thick flanges.

[0005] However, a ferritic stainless steel sheet having a large thickness has a problem in low-temperature toughness. For example, breakage during manufacturing of flanges frequently occurs in winter. For this reason, there has been a strong demand for improvement in the toughness of a ferritic stainless steel sheet having a large thickness.

[0006] In response to the market demand, for example, Patent Literature 1 discloses a stainless steel sheet having excellent toughness (with a Charpy impact value of 50 J/cm² or more at -40°C), the stainless steel sheet containing, in percent by mass, C: 0.02% or less, N: 0.02% or less, Si: 0.005 to 1.0%, Ni: 0.1 to 1.0%, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.0100% or less, Cr: 10% or more and less than 18%, and further one or two of Ti: 0.05 to 0.30% and Nb: 0.01 to 0.50%, the sum of Ti and Nb being 8(C+N) to 0.75%, with the balance being Fe and inevitable impurities, in which γ_p is 70% or more, the ferrite grain size is 20 µm or less, and the amount of martensite formation is 70% or less. Note that γ_p (%) is evaluated by using the formula (i) below (in Patent Literature 1, expressed as formula (1)): γ_p = 420(%C) + 470(%N) + 23(%Ni) + 9(%Cu) + 7(%Mn) - 11.5(%Cr) - 11.5(%Si) - 12(%Mo) - 23(%V) - 47(%Nb) - 49(%Ti) - 52(%Al) + 189 (i), where (%X) represents the mass ratio of each element X.

Citation List

Patent Literature

[0007] PTL 1: Japanese Unexamined Patent Application Publication No. 2016-191150

Summary of Invention

Technical Problem

[0008] However, when the present inventors tried to form the stainless steel sheet described in Patent Literature 1 into the shape of a thick flange having a burring portion, in some cases, cracks occurred in the burring portion, and it was not possible to obtain a predetermined flange shape, revealing that the stainless steel sheet was not sufficient to be used for thick flanges.

[0009] The present invention has been made under the circumstances described above, and it is an object of the present invention to provide a ferritic stainless steel sheet which has more excellent toughness and excellent corrosion resistance, and a method for manufacturing the same.

[0010] In the present invention, the term "more excellent toughness" means that the Charpy impact value at -50°C is 100 J/cm² or more. Furthermore, in the present invention, the term "excellent corrosion resistance" means that, after a cyclic salt spray test specified in JIS H 8502 is performed for three cycles, the rusting area ratio is 25% or less.

Solution to Problem

[0011] In order to solve the problem, the present inventors have conducted detailed studies. As a result, the following findings have been obtained.

[0012]  In order to form a steel sheet into a thick flange having a burring portion without occurrence of cracks, it is effective to refine the metal structure and to set the Charpy impact value at -50°C to be 100 J/cm² or more. Specifically, by setting the average crystal grain size of the metal structure to be 45 µm or less, occurrence of cracks in the burring portion can be effectively prevented when worked into a thick flange having a burring portion, and the steel sheet can be satisfactorily put into practical use for a thick flange having a burring portion.

[0013] Furthermore, a method, in which after a slab having a steel composition including appropriate steel elements, specifically, Si, Mn, Cr, Ni, and the like, that are controlled in appropriate ranges is heated at 1,050 to 1,250°C, hot rolling is performed, and hot-rolled sheet annealing is performed at an appropriate temperature, is effective in refining the metal structure and obtaining a Charpy impact value of 100 J/cm² or more at -50°C.

[0014] The present invention has been made on the basis of the findings described above, and the gist of the invention is as follows.

[1] A ferritic stainless steel sheet having a composition containing, in percent by mass, C: 0.001 to 0.020%, Si: 0.05 to 0.35%, Mn: 0.05 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.300%, Cr: 10.0 to 13.0%, Ni: 0.75 to 1.50%, Ti: 0.05 to 0.35%, and N: 0.001 to 0.020%, with the balance being Fe and inevitable impurities, in which γ_I [%] represented by formula (1) below is 65% or more, and a metal structure has an average crystal grain size of 45 µm or less:

where Ni, Mn, Cu, Si, Cr, and Mo represent contents of the respective elements (percent by mass), and an element not contained represents 0.

[2] The ferritic stainless steel sheet according to [1], in which the composition further contains, in percent by mass, one or two or more selected from Cu: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.01 to 0.20%, and Co: 0.01 to 0.20%.

[3] The ferritic stainless steel sheet according to [1] or [2], in which the composition further contains, in percent by mass, one or two or more selected from V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, and Zr: 0.01 to 0.20%.

[4] The ferritic stainless steel sheet according to any one of [1] to [3], in which the composition further contains, in percent by mass, one or two or more selected from REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg: 0.0005 to 0.0030%, and Ca: 0.0003 to 0.0030%.

[5] A method for manufacturing the ferritic stainless steel sheet according to any one of [1] to [4], including a hot rolling process in which a steel slab having the composition is heated at 1,050 to 1,250°C, and then subjected to hot rolling, and a hot-rolled sheet annealing process in which a hot-rolled steel sheet obtained in the hot rolling process is subjected to hot-rolled sheet annealing at 750 to 1,050°C.

Advantageous Effects of Invention

[0015] According to the present invention, it is possible to obtain a ferritic stainless steel sheet having more excellent toughness and excellent corrosion resistance. The ferritic stainless steel sheet of the present invention can be suitably used for thick flanges and the like. Description of Embodiments

[0016] The present invention will be described in detail below.

[0017] The present inventors have investigated in detail the reason for the occurrence of cracks when various ferritic stainless steel sheets with a sheet thickness of 5.0 mm are each formed into a flange having a burring portion in which a flange hole (30 mmφ) is raised by 10 mm from the surface of the steel sheet as blanked. The results have shown that cracks do not occur in steel sheets having a Charpy impact value of 100 J/cm² or more at -50°C, and in steel sheets in which cracks occur, the Charpy impact value at -50°C is less than 100 J/cm². In this way, it has been found that low toughness is a cause for cracks.

[0018] Furthermore, the present inventors have investigated in detail the relationship between the low toughness and the metal structure. As a result, it has been found that as the average crystal grain size of the steel sheet increases, toughness decreases. Accordingly, by using various ferritic stainless steel sheets (sheet thickness: 5.0 mm), forming into the flange has been tried. As a result, it has been found that in steel sheets having an average crystal grain size of more than 45 µm, toughness decreases and cracks are likely to occur, and that when the average crystal grain size is 45 µm or less, toughness is excellent and blanking workability of the steel sheet is good.

[0019] Therefore, in the present invention, the average crystal grain size is set to be 45 µm or less, and the Charpy impact value at -50°C is set to be 100 J/cm² or more. Note that the average crystal grain size can be measured by a measurement method used in examples which will be described later. Furthermore, the Charpy impact value is a value measured in accordance with JIS Z 2242 (2005) as will be described later.

[0020] The composition of the ferritic stainless steel sheet according to the present invention will be described below. Hereinafter, unless otherwise stated, "%", which is the unit of measure for the content of each element, means "percent by mass".

C: 0.001 to 0.020%

[0021] When the C content exceeds 0.020%, deterioration in workability and corrosion resistance becomes conspicuous. A lower C content is more desirable from the viewpoint of corrosion resistance and workability. However, in order to set the C content to be less than 0.001%, it takes a long time to perform refining, which is undesirable in terms of manufacturing. Therefore, the C content is set in a range of 0.001% to 0.020%. The C content is preferably 0.003% or more, and more preferably 0.004% or more. Furthermore, the C content is preferably 0.015% or less, and more preferably 0.012% or less.

Si: 0.05 to 0.35%

[0022] Si is an element that has an effect of improving corrosion resistance of welds by being concentrated in an oxide film formed during welding and is also effective as a deoxidizing element in the steelmaking process. These effects are obtained at a Si content of 0.05% or more and increase with increasing its content. On the other hand, Si has an effect of accelerating ferrite phase formation. When the Si content exceeds 0.35%, a predetermined amount of austenite phase is not formed sufficiently during heating in the hot rolling process. Accordingly, even when hot rolling and hot-rolled sheet annealing are performed under the conditions specified in the present invention, a desired metal structure cannot be obtained. Therefore, the Si content is set to be 0.05% or more and 0.35% or less. The Si content is preferably 0.10% or more. Furthermore, the Si content is preferably 0.30% or less.

Mn: 0.05 to 1.00%

[0023] Mn has an effect of accelerating austenite phase formation. In order to obtain such an effect, a Mn content of 0.05% or more is necessary. However, when the Mn content exceeds 1.00%, precipitation of MnS serving as a starting point of corrosion is accelerated, resulting in deterioration in corrosion resistance. Therefore, the Mn content is set to be 0.05% or more and 1.00% or less. The Mn content is preferably 0.20% or more. Furthermore, the Mn content is preferably 0.80% or less, and more preferably 0.70% or less.

P: 0.04% or less

[0024] P is an element that is inevitably contained in steel. Since P is an element detrimental to corrosion resistance and workability, it is desirable to decrease the amount of P as much as possible. When the P content exceeds 0.04%, workability markedly deteriorates by solid solution strengthening. Therefore, the P content is set to be 0.04% or less. The P content is preferably 0.03% or less.

S: 0.01% or less

[0025] S, similar to P, is an element that is inevitably contained in steel. Since S is an element detrimental to corrosion resistance and workability, it is desirable to decrease the amount of S as much as possible. In particular, when the S content exceeds 0.01%, corrosion resistance markedly deteriorates. Therefore, the S content is set to be 0.01% or less. The S content is preferably 0.008% or less, and more preferably 0.003% or less.

Al: 0.001 to 0.300%

[0026] Al is an effective deoxidizer. Furthermore, since Al has higher affinity for nitrogen than Cr, in the case where nitrogen enters a weld, by precipitating nitrogen as Al nitrides instead of Cr nitrides, Al has an effect of suppressing sensitization. These effects can be obtained at an Al content of 0.001% or more. However, when the Al content exceeds 0.300%, weld penetration deteriorates, resulting in deterioration in weldability, which is undesirable. Therefore, the Al content is set in a range of 0.001% to 0.300%. The Al content is preferably 0.010% or more. Furthermore, the Al content is preferably 0.200% or less, more preferably 0.100% or less, and still more preferably 0.050% or less.

Cr: 10.0 to 13.0%

[0027] Cr is the most important element for securing corrosion resistance. When the Cr content is less than 10.0%, corrosion resistance required for automobile exhaust components cannot be obtained. On the other hand, when the Cr content exceeds 13.0%, even if the steel composition is adjusted so as to satisfy γ_I represented by the predetermined formula (1) which will be described later, a predetermined amount of austenite phase is not formed during heating in the hot rolling process. Consequently, even when hot rolling and hot-rolled sheet annealing are performed under the conditions specified in the present invention, a desired metal structure cannot be obtained. Therefore, the Cr content is set in a range of 10.0% to 13.0%. The Cr content is preferably 10.5% or more. Furthermore, the Cr content is preferably 12.0% or less, and more preferably 11.7% or less.

Ni: 0.75 to 1.50%

[0028] Ni is an austenite-forming element and has an effect of increasing the amount of austenite formed during heating before rolling in the hot rolling process. In the present invention, by adjusting the steel composition, a dual-phase structure of ferrite phase + austenite phase, which includes 70% or more, in volume ratio, of austenite phase, is formed during heating the slab in the hot rolling process. In the case where the metal structure is formed into a dual-phase structure of ferrite phase + austenite phase, the interface between different phases, i.e., between the ferrite phase and the austenite phase, functions as an obstacle to growth of crystal grains, and therefore, the metal structure before hot rolling is refined. Then, working strain acting as recrystallization sites is accumulated by a predetermined hot rolling operation, and recrystallization is caused by hot-rolled sheet annealing in the subsequent process. Thus, a fine metal structure is obtained, and excellent toughness is exhibited. These effects can be obtained at a Ni content of 0.75% or more. On the other hand, when the Ni content exceeds 1.50%, the improvement effect due to refinement of crystal grains is saturated, and workability deteriorates. Moreover, stress corrosion cracking is likely to occur. Therefore, the Ni content is set to be 0.75% or more and 1.50% or less. The Ni content is preferably 0.80% or more. Furthermore, the Ni content is preferably 1.20% or less, and more preferably 1.00% or less.

Ti: 0.05 to 0.35%

[0029] Ti preferentially combines with C and N, suppresses precipitation of Cr carbonitrides, and lowers the recrystallization temperature. Ti also has an effect of suppressing deterioration of corrosion resistance caused by sensitization due to precipitation of Cr carbonitrides. In order to obtain these effects, a Ti content of 0.05% or more is necessary. On the other hand, when the Ti content exceeds 0.35%, formation of coarse TiN causes marked deterioration in toughness, and even if the technique of the present invention is applied, predetermined toughness cannot be obtained. Furthermore, when the Ti content exceeds 0.35%, coarse Ti carbonitrides are formed in the casting process, resulting in surface defects, which is undesirable in terms of manufacturing. Therefore, the Ti content is set to be 0.05% or more and 0.35% or less. The Ti content is preferably 0.10% or more. Furthermore, the Ti content is preferably 0.30% or less, and more preferably 0.15% or less.

N: 0.001 to 0.020%

[0030] When the N content exceeds 0.020%, deterioration in workability and corrosion resistance becomes conspicuous. A lower N content is more desirable from the viewpoint of workability and corrosion resistance. However, in order to decrease the N content to less than 0.001%, it is necessary to perform refining for a long time, resulting in an increase in manufacturing costs and a decrease in productivity, which are undesirable. Therefore, the N content is set in a range of 0.001% to 0.020%. The N content is preferably 0.005% or more, and more preferably 0.007% or more. Furthermore, the N content is preferably 0.015% or less, and more preferably 0.012% or less.

γ_I [%] : 65% or more

[0031] When γ_I represented by formula (1) below is less than 65%, because of an insufficient amount of austenite in the metal structure, a fine metal structure cannot be obtained at a slab heating temperature before starting hot rolling. Therefore, γ_I [%] is set to be 65% or more. Note that γ_I [%] is obtained by using formula (1) below, which evaluates the stability of austenite phase.

where Ni, Mn, Cu, Si, Cr, and Mo represent contents of the respective elements (percent by mass), and an element not contained represents 0. In the formula (1), an austenite-forming element has a positive factor, and a ferrite-forming element has a negative factor. The values were experimentally obtained with reference to the Castro formula.

[0032] In the present invention, the balance other than the above is Fe and inevitable impurities. Examples of the inevitable impurities include oxygen (O), and an O content of 0.01% or less is permissible.

[0033] In addition to the essential elements described above, as necessary, the ferritic stainless steel sheet can further contain one group or two or more groups selected from groups A to C described below.

(Group A) one or two or more selected from Cu: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.01 to 0.20%, and Co: 0.01 to 0.20%

(Group B) one or two or more selected from V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, and Zr: 0.01 to 0.20%

(Group C) one or two or more selected from REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg: 0.0005 to 0.0030%, and Ca: 0.0003 to 0.0030%

Cu: 0.01 to 1.00%

[0034] Cu is a particularly effective element in improving corrosion resistance in an aqueous solution or when weakly acidic water drops adhere to the steel sheet. Furthermore, Cu has an effect of accelerating austenite phase formation. This effect can be obtained at a Cu content of 0.01% or more and increases with increasing Cu content. However, when the Cu content exceeds 1.00%, hot workability deteriorates, which may induce surface defects in some cases. Furthermore, descaling after annealing may become difficult in some cases. Therefore, when Cu is contained, the Cu content is set in a range of 0.01% to 1.00%. When Cu is contained, the Cu content is preferably 0.10% or more. Furthermore, when Cu is contained, the Cu content is preferably 0.50% or less.

Mo: 0.01 to 1.00%

[0035] Mo is an element that markedly improves the corrosion resistance of stainless steel. This effect is obtained at a Mo content of 0.01% or more and improves with increasing content. On the other hand, Mo has an effect of accelerating ferrite phase formation. When the Mo content exceeds 1.00%, a predetermined amount of austenite phase is not formed sufficiently during heating in the hot rolling process. Accordingly, even when hot rolling and hot-rolled sheet annealing are performed under the conditions specified in the present invention, a desired metal structure cannot be obtained. Therefore, when Mo is contained, the Mo content is set to be 0.01% or more and 1.00% or less. When Mo is contained, the Mo content is preferably 0.10% or more, and more preferably 0.30% or more. Furthermore, when Mo is contained, the Mo content is preferably 0.80% or less, and more preferably 0.50% or less.

W: 0.01 to 0.20%

[0036] W, similar to Mo, has an effect of improving corrosion resistance. This effect is obtained at a W content of 0.01% or more. On the other hand, when the W content exceeds 0.20%, strength increases, which may cause deterioration in productivity due to an increase in the rolling load and the like in some cases. Therefore, when W is contained, the W content is set in a range of 0.01% to 0.20%. When W is contained, the W content is preferably 0.05% or more. Furthermore, when W is contained, the W content is preferably 0.15% or less.

Co: 0.01 to 0.20%

[0037] Co is an element that improves toughness. This effect is obtained at a Co content of 0.01% or more. On the other hand, when the Co content exceeds 0.20%, workability may deteriorate in some cases. Therefore, when Co is contained, the Co content is set in a range of 0.01% to 0.20%.

V: 0.01 to 0.20%

[0038] V, together with C and N, forms carbonitrides, and by suppressing sensitization during welding, improves corrosion resistance of welds. This effect is obtained at a V content of 0.01% or more. On the other hand, when the V content exceeds 0.20%, workability and toughness may markedly deteriorate in some cases. Therefore, when V is contained, the V content is set to be 0.01% or more and 0.20% or less. When V is contained, the V content is preferably 0.02% or more. Furthermore, when V is contained, the V content is preferably 0.10% or less.

Nb: 0.01 to 0.10%

[0039] Nb has an effect of refining crystal grains. This effect is obtained at a Nb content of 0.01% or more. On the other hand, Nb also has an effect of increasing the recrystallization temperature. When the Nb content exceeds 0.10%, there may be a case where the annealing temperature required to cause sufficient recrystallization in hot-rolled sheet annealing becomes excessively high, and a metal structure with an average crystal grain size of 45 µm or less cannot be obtained. Therefore, when Nb is contained, the Nb content is set in a range of 0.01% to 0.10%. When Nb is contained, the Nb content is preferably 0.05% or less.

Zr: 0.01 to 0.20%

[0040] Zr has an effect of suppressing sensitization by combining with C and N. This effect is obtained at a Zr content of 0.01% or more. On the other hand, when the Zr content exceeds 0.20%, workability may markedly deteriorate in some cases. Therefore, when Zr is contained, the Zr content is set in a range of 0.01% to 0.20%. When Zr is contained, the Zr content is preferably 0.10% or less.

REM: 0.001 to 0.100%

[0041] Since REM (Rare Earth Metals) has an effect of improving oxidation resistance, it suppresses formation of an oxide film (welding temper color) in welds, and suppresses formation of a Cr-depleted region immediately below the oxide film. This effect is obtained at an REM content of 0.001% or more. On the other hand, when the REM content exceeds 0.100%, productivity, such as picklability during cold-rolled annealing, may deteriorate in some cases. Therefore, when REM is contained, the REM content is set in a range of 0.001% to 0.100%. When REM is contained, the REM content is preferably 0.050% or less.

B: 0.0002 to 0.0025%

[0042] B is an element effective in improving resistance to secondary work embrittlement after deep drawing. This effect is obtained at a B content of 0.0002% or more. On the other hand, when the B content exceeds 0.0025%, workability and toughness may deteriorate in some cases. Therefore, when B is contained, the B content is set in a range of 0.0002% to 0.0025%. When B is contained, the B content is preferably 0.0003% or more. Furthermore, when B is contained, the B content is preferably 0.0012% or less.

Mg: 0.0005 to 0.0030%

[0043] In steel containing Ti as in the present invention, when Ti carbonitrides coarsen, toughness may deteriorate in some cases. In this respect, Mg has an effect of suppressing coarsening of Ti carbonitrides. This effect is obtained at a Mg content of 0.0005% or more. On the other hand, when the Mg content exceeds 0.0030%, surface properties of steel may deteriorate in some cases. Therefore, when Mg is contained, the Mg content is set in a range of 0.0005 to 0.0030%. When Mg is contained, the Mg content is preferably 0.0010% or more. Furthermore, when Mg is contained, the Mg content is preferably 0.0020% or less.

Ca: 0.0003 to 0.0030%

[0044] Ca is an element effective in preventing nozzle blockage due to crystallization of Ti-based inclusions which is likely to occur during continuous casting. This effect is obtained at a Ca content of 0.0003% or more. On the other hand, when the Ca content exceeds 0.0030%, corrosion resistance may deteriorate by formation of CaS in some cases. Therefore, when Ca is contained, the Ca content is set in a range of 0.0003% to 0.0030%. When Ca is contained, the Ca content is preferably 0.0005% or more. Furthermore, when Ca is contained, the Ca content is preferably 0.0015% or less, and more preferably 0.0010% or less.

[0045] A method for manufacturing a ferritic stainless steel sheet according to the present invention will be described below. The present inventors have performed thorough studies on a technique of improving toughness in a ferritic stainless steel sheet. As a result, it has been found that after a steel slab having an appropriate steel composition is heated preferably at 1,050 to 1,250°C, by performing hot rolling preferably with three or more passes, and subjecting the resulting hot-rolled steel sheet to hot-rolled sheet annealing at 750 to 1,050°C, a metal structure with an average crystal grain size of 45 µm or less can be obtained, and toughness is greatly improved to a Charpy impact value of 100 J/cm² or more at -50°C. Furthermore, it has been found that desired corrosion resistance can be obtained.

[0046] The reason why a hot-rolled and annealed steel sheet having a fine metal structure can be obtained by the above technique will be described below. In ferritic stainless steel, dynamic recrystallization hardly occurs during hot rolling, and recovery of working strain due to rolling tends to occur. Therefore, in hot rolling according to existing techniques, excessive recovery of the working strain introduced by rolling occurs, and the working strain cannot be effectively maintained after hot rolling. Consequently, recrystallization sites become insufficient, and a fine recrystallized structure cannot be obtained in the subsequent hot-rolled sheet annealing process.

[0047] Under the circumstances, the present inventors have performed thorough studies on an effective technique for obtaining a fine structure after hot-rolled sheet annealing from the viewpoint of both the steel composition and the hot rolling method. As a result, it has been found that it is effective to control the contents of steel elements, in particular, Si, Mn, Cr, and Ni, in appropriate ranges and to perform hot rolling after performing heating of the slab at an appropriate temperature in the hot rolling process so as to form a dual-phase structure of ferrite phase + austenite phase.

[0048] In the case where the metal structure is formed into a dual-phase structure of ferrite phase + austenite phase, the interface between different phases, i.e., between the ferrite phase existing before heating and the austenite phase formed during heating, suppresses coarsening of crystal grains, and therefore, a fine equiaxed structure can be obtained in the stage before hot rolling. Then, by performing a suitable hot rolling operation, working strain acting as recrystallization sites in the subsequent hot-rolled sheet annealing process is sufficiently accumulated. Thus, a fine metal structure is obtained in the subsequent hot-rolled sheet annealing process, and excellent toughness can be exhibited.

[0049] Specifically, regarding steel which is adjusted so as to satisfy the formula (1) in which the contents of Ni and Mn, i.e., austenite-forming elements, are multiplied by a positive factor for each of Ni and Mn and the contents of Si and Cr, i.e., ferrite-forming elements, are multiplied by a negative factor for each of Si and Cr, so that 65% or more, in volume ratio, of austenite phase is formed during heating before hot rolling, a method has been devised in which the steel, after being heated as a slab at 1,050 to 1,250°C, is subjected to hot rolling.

[0050] Furthermore, the present inventors have performed thorough studies on the suitable conditions for the subsequent hot-rolled sheet annealing process. The hot-rolled sheet annealing process is a process of recrystallizing the worked structure formed by hot rolling. Therefore, it is necessary to perform annealing at a temperature at which sufficient recrystallization occurs. However, when hot-rolled sheet annealing is performed at an excessively high temperature, although recrystallization occurs, recrystallized grains markedly coarsen. Therefore, a desired fine structure cannot be obtained.

[0051] Accordingly, the present inventors have investigated in detail the relationship between the grain size of recrystallized grains and the annealing temperature. As a result, it has been found that by controlling the hot-rolled sheet annealing temperature to 1,050°C or lower, it is possible to suppress formation of recrystallized grains that are coarse to such an extent that toughness deteriorates.

[0052] The manufacturing conditions will be described in detail below.

[0053] First, molten steel having the composition described above is melted by a known method using a converter, an electric furnace, a vacuum melting furnace, or the like and is formed into a steel (slab) by a continuous casting process or an ingot casting-blooming process.

Steel slab heating temperature: 1,050 to 1,250°C

[0054] The steel slab is heated at 1,050 to 1,250°C and subjected to hot rolling. The heating time at the heating temperature is not particularly limited, but preferably, heating is performed for 1 to 24 hours. When the heating temperature is lower than 1,050°C, the austenite phase formation rate decreases, a fine metal structure cannot be obtained, and thus excellent toughness cannot be obtained. On the other hand, when the heating temperature increases excessively, the oxidation mass increases resulting in an increase in scale loss. Therefore, the steel slab heating temperature is set to be 1,250°C or lower. However, when a steel slab is subjected to hot rolling, in the case where the steel slab after casting is in a temperature range of 1,050°C or higher, the steel may be, without being heated, directly subjected to rolling.

[0055] The rough rolling conditions are not particularly limited. In the case where the cast structure is effectively destroyed before finish hot rolling, the refinement effect caused by heating of the slab is further accelerated in subsequent processes. Therefore, the cumulative rolling reduction in rough rolling is preferably set to be 65% or more. Then, finish hot rolling is performed until a predetermined sheet thickness is reached.

Hot-rolled sheet annealing temperature: 750 to 1,050°C

[0056] In the present invention, after the hot rolling is finished, hot-rolled sheet annealing is performed. In hot-rolled sheet annealing, the rolled structure formed in the hot rolling process is recrystallized. In the present invention, by effectively imparting rolling strain in the hot rolling process so that the number of recrystallization sites increases, coarsening of recrystallization grains in hot-rolled sheet annealing is suppressed. In order to obtain this effect, it is necessary to perform hot-rolled sheet annealing at a temperature in a range of 750 to 1,050°C. When the annealing temperature is lower than 750°C, because of insufficient recrystallization, residual stress caused by hot-rolling strain remains, and flatness of the steel sheet after hot-rolling and annealing cannot be maintained. On the other hand, when the annealing temperature exceeds 1,050°C, recrystallized grains markedly coarsen, and a desired metal structure cannot be obtained. Therefore, the hot-rolled sheet annealing temperature is set in a range of 750°C to 1,050°C. Preferably, the hot-rolled sheet annealing temperature is in a range of 750°C to 900°C. Note that the holding time and the technique of hot-rolled sheet annealing are not particularly limited, and either box annealing (batch annealing) or continuous annealing may be performed.

[0057] The ferritic stainless steel sheet thus obtained may be subjected, as necessary, to a descaling treatment by shotblasting or pickling. Furthermore, in order to improve surface properties, the steel sheet may be subjected to grinding, polishing, or the like. Moreover, the steel sheet may be further subjected to cold rolling and cold-rolled sheet annealing.

[0058] In this way, a ferritic stainless steel sheet having excellent toughness and excellent corrosion resistance according to the present invention is manufactured.

[0059] The metal structure of the ferritic stainless steel sheet obtained in the present invention includes a ferrite single phase or includes 3% or less (in volume ratio) in total of one or both of a martensite phase and a retained austenite phase with the balance being a ferrite phase.

[0060] The ferritic stainless steel sheet of the present invention has a Charpy impact value of 100 J/cm² or more at -50°C. Because of such excellent low-temperature toughness, occurrence of cracks in the burring portion can be effectively prevented when worked into a thick flange having a burring portion, and the steel sheet can be satisfactorily put into practical use for a thick flange having a burring portion.

[0061] The sheet thickness is not particularly limited, but is desirably a sheet thickness that can be used for a thick flange. Therefore, the sheet thickness is preferably 5.0 mm or more, and more preferably 8.0 mm or more. Furthermore, the sheet thickness is preferably 15.0 mm or less, and more preferably 13.0 mm or less.

EXAMPLE 1

[0062] The present invention will be described in more detail below on the basis of examples.

[0063] Molten stainless steels having the compositions shown in Table 1 were each formed into a 100-kg steel slab by vacuum induction melting. Subsequently, by performing hot rolling under the manufacturing conditions shown in Table 2, a hot-rolled steel sheet with the finished sheet thickness shown in Table 2 was obtained. By subjecting the hot-rolled steel sheet to hot-rolled sheet annealing, a hot-rolled and annealed steel sheet was obtained. Note that hot-rolled sheet annealing was performed by holding the steel sheet at the hot-rolled sheet annealing temperature shown in Table 2 for 8 hours. The following evaluations were made on the resulting hot-rolled and annealed steel sheet.

(1) Evaluation of average crystal grain size

[0064] The average crystal grain size was measured by an EBSD (Electron Back Scattering Diffraction) method. The measurement conditions were as follows: a magnification, for measurement, of 500 times, with a step size of 0.4 µm. The obtained data were analyzed by OIM (Orientation Imaging Microscopy) analysis software developed by TSL Solutions Ltd., an orientation difference of 15° or more was defined as a grain boundary, and circle equivalent diameters were calculated. A value calculated from the average of the circle equivalent diameters was defined as an average crystal grain size.

(2) Evaluation of Charpy impact value

[0065] A V-notch Charpy specimen according to JIS Z 2242 (2005) was taken from the central part in the sheet width direction of each of the hot-rolled and annealed steel sheets, without changing the thickness of the steel sheet, such that the rolling direction corresponded to the longitudinal direction of the specimen. The specimen was tested in accordance with JIS Z 2242 (2005) to measure a Charpy impact value at -50°C. Specimens with a Charpy impact value of 100 J/cm² or more at -50°C were evaluated as "pass", and specimens with a Charpy impact value of less than 100 J/cm² at -50°C were evaluated as "rejection".

(3) Evaluation of corrosion resistance

[0066] A specimen of 60 × 80 mm was taken from each of the hot-rolled and annealed steel sheets. After a front surface of the specimen was polish-finished with #600 emery paper, end face portions and a back surface of the specimen were sealed. Then, the specimen was subjected to a cyclic salt spray test specified in JIS H 8502. In the cyclic salt spray test, three cycles were performed, each cycle including salt spraying (5% by mass NaCl, 35°C, spraying for 2 hours) → drying (60°C, 4 hours, relative humidity: 40%) → wetting (50°C, 2 hours, relative humidity ≥ 95%). After the cyclic salt spray test was conducted for three cycles, the front surface of the specimen was photographed, and a rusting area in the front surface of the specimen was measured by image analysis. From the ratio of the rusting area to the area of a portion in which the rusting area is measured, the rusting area ratio (rusting area/area of portion in which rusting area is measured in specimen) × 100[%]) was calculated. The portion in which the rusting area is measured refers to a portion excluding an outer peripheral portion with a width of 15 mm of the specimen. Note that the rusting area includes areas of a rusting portion and a portion subjected to flow rust. Specimens with a rusting area ratio of 10% or less were evaluated as "pass" (⊙) with particularly excellent corrosion resistance, specimens with a rusting area ratio of more than 10% and 25% or less were evaluated as "pass" (○), and specimens with a rusting area ratio of more than 25% were evaluated as "rejection" (×).

[0067] The test results thus obtained together with manufacturing conditions are shown in Table 2.

[Table 1]

Steel symbol	Chemical composition (mass%)											γI [%]	Remarks
	C	Si	Mn	P	S	Al	Cr	Ni	Ti	N	Others
A1	0.007	0.26	0.25	0.03	0.002	0.03	11.7	0.85	0.24	0.009	-	66	Example
A2	0.006	0.22	0.38	0.02	0.003	0.03	11.1	1.15	0.21	0.007	-	83	Example
A3	0.008	0.25	0.34	0.03	0.005	0.02	11.4	1.45	0.15	0.011	-	86	Example
A4	0.005	0.17	0.38	0.01	0.002	0.02	10.8	0.77	0.13	0.011	-	78	Example
A5	0.006	0.17	0.98	0.04	0.005	0.04	11.5	0.80	0.19	0.011	-	78	Example
A6	0.008	0.11	0.07	0.03	0.004	0.03	10.9	0.83	0.23	0.011	-	76	Example
A7	0.006	0.33	0.45	0.04	0.005	0.04	11.3	0.82	0.15	0.011	-	72	Example
A8	0.006	0.07	0.20	0.04	0.003	0.03	11.7	0.84	0.24	0.012	-	69	Example
A9	0.009	0.12	0.76	0.03	0.002	0.04	12.4	1.20	0.11	0.011	-	75	Example
A10	0.010	0.21	0.38	0.04	0.005	0.04	10.3	0.79	0.13	0.007	-	84	Example
A11	0.008	0.16	0.40	0.03	0.003	0.02	11.5	1.45	0.33	0.009	-	87	Example
A12	0.005	0.29	0.41	0.03	0.004	0.04	10.9	0.98	0.07	0.007	-	80	Example
A13	0.008	0.15	0.39	0.01	0.004	0.02	10.2	0.76	0.22	0.011	-	86	Example
A14	0.012	0.05	0.97	0.03	0.001	0.03	12.5	0.75	0.25	0.016	Cu:0.15	68	Example
A15	0.009	0.14	0.25	0.03	0.004	0.04	10.5	1.40	0.14	0.011	Mo:0.13, Zr:0.16	94	Example
A16	0.008	0.12	0.44	0.04	0.004	0.02	12.6	1.47	0.23	0.009	-	75	Example
A17	0.007	0.14	0.37	0.01	0.005	0.03	11.0	1.16	0.12	0.008	Cu:0.95	92	Example
A18	0.005	0.17	0.40	0.02	0.002	0.03	10.9	1.00	0.21	0.008	Mo:0.88	72	Example
A19	0.005	0.26	0.42	0.01	0.002	0.03	10.9	1.17	0.11	0.010	W:0.08, Mg:0.0017	85	Example
A20	0.006	0.19	0.45	0.03	0.004	0.03	11.0	0.99	0.09	0.011	Co:0.11	82	Example
A21	0.005	0.11	0.41	0.02	0.003	0.02	11.2	0.91	0.18	0.007	V:0.10	79	Example
A22	0.005	0.10	0.25	0.03	0.006	0.03	11.1	0.81	0.13	0.009	V:0.04, Nb:0.06	75	Example
A23	0.006	0.21	0.42	0.03	0.004	0.04	11.0	0.85	0.26	0.011	Zr:0.06, B:0.0011	78	Example
A24	0.010	0.11	0.42	0.03	0.002	0.04	11.0	0.86	0.13	0.008	REM:0.007	79	Example
A25	0.006	0.17	0.32	0.02	0.004	0.03	10.9	0.88	0.19	0.011	Co:0.013, B:0.0009	79	Example
A26	0.010	0.13	0.30	0.03	0.005	0.02	10.8	0.78	0.11	0.010	W:0.013, Nb:0.02, Ca:0.0008	78	Example
A27	0.007	0.28	0.29	0.04	0.003	0.04	11.6	0.75	0.22	0.011	-	65	Example
B1	0.013	0.28	0.32	0.04	0.003	0.04	11.8	0.77	0.10	0.010	-	64	Comparative Example
B2	0.003	0.34	0.25	0.02	0.003	0.03	12.1	0.84	0.21	0.008	-	60	Comparative Example
B3	0.015	0.33	0.25	0.03	0.005	0.02	12.3	1.08	0.18	0.007	-	63	Comparative Example
B4	0.009	0.08	0.73	0.04	0.004	0.04	13.3	1.25	0.21	0.009	-	66	Comparative Example
B5	0.012	0.29	1.64	0.03	0.009	0.03	10.3	0.81	0.18	0.009	-	98	Comparative Example
B6	0.008	0.13	0.35	0.03	0.004	0.03	11.3	0.81	0.10	0.010	Nb:0.16	74	Comparative Example
B7	0.008	0.45	0.42	0.04	0.002	0.03	10.9	0.83	0.10	0.007	-	74	Comparative Example
B8	0.008	0.16	0.39	0.04	0.003	0.03	11.7	1.30	0.42	0.011	-	81	Comparative Example
B9	0.009	0.19	0.36	0.02	0.002	0.03	11.0	0.76	-	0.010	-	75	Comparative Example
B10	0.008	0.24	0.31	0.02	0.003	0.04	11.3	0.65	0.26	0.008		67	Comparative Example

The balance other than the elements in the chemical composition described above consists of Fe and inevitable impurities. Underlined items are outside the range of the present invention.

[Table 2]

No.	Steel symbol	Steel slab temperature [°C]	Finished sheet thickness [mm]	Hot-rolled sheet annealing temperature [°C]	average crystal grain size [µm]	Charpy impact value (-50°C) [J/cm2]	Corrosion resistance	Remarks
1	A1	1060	5.1	806	43	104	○	Example
2	A2	1071	5.1	820	16	230	○	Example
3	A3	1063	5.1	784	15	233	○	Example
4	A4	1055	5.2	773	37	118	○	Example
5	A5	1094	5.0	824	22	202	○	Example
6	A6	1114	5.2	1020	31	136	○	Example
7	A7	1066	5.1	753	39	108	○	Example
8	A8	1096	5.2	804	42	114	○	Example
9	A9	1074	5.0	803	32	129	○	Example
10	A10	1056	5.1	812	18	209	○	Example
11	A11	1098	5.1	848	15	247	○	Example
12	A12	1084	5.1	842	21	221	○	Example
13	A13	1094	5.1	788	22	198	○	Example
14	A14	1106	5.2	775	43	105	○	Example
15	A15	1102	5.0	775	10	321	○	Example
16	A16	1075	5.1	838	32	139	○	Example
17	A17	1055	5.0	850	11	297	⊙	Example
18	A18	1117	5.1	832	44	110	⊙	Example
19	A19	1114	5.1	846	16	238	○	Example
20	A20	1084	5.2	793	23	202	○	Example
21	A21	1106	5.2	823	24	181	○	Example
22	A22	1132	5.2	808	33	131	○	Example
23	A23	1086	5.1	828	26	165	○	Example
24	A24	1053	5.1	811	24	183	○	Example
25	A25	1087	5.1	793	16	241	○	Example
26	A26	1056	5.1	765	25	175	○	Example
27	A27	1051	5.1	760	45	101	○	Example
28	A1	1077	5.2	772	15	220	○	Example
29	A1	1087	5.0	795	8	341	○	Example
30	A1	1077	5.2	761	11	288	○	Example
31	A1	1148	5.1	764	19	221	○	Example
32	A1	1055	5.1	802	45	102	○	Example
33	A1	1270	5.2	827	85	32	○	Comparative Example
34	A2	1270	5.2	838	60	53	○	Comparative Example
35	A1	1056	5.2	1065	141	16	○	Comparative Example
36	A2	1082	5.1	1062	110	26	○	Comparative Example
37	B1	1133	5.2	898	46	96	○	Comparative Example
38	B2	1098	5.0	810	56	70	○	Comparative Example
39	B3	1115	5.1	843	48	88	○	Comparative Example
40	B4	1095	5.2	846	84	32	○	Comparative Example
41	B5	1120	5.0	847	32	121	×	Comparative Example
42	B6	1147	5.1	807	87	15	○	Comparative Example
43	B7	1148	5.1	843	50	83	○	Comparative Example
44	B8	1103	5.2	827	47	97	○	Comparative Example
45	B9	1086	5.1	817	92	10	○	Comparative Example
46	A1	1096	12.5	765	44	101	○	Example
47	B10	1101	5.2	807	47	95	○	Comparative Example

Underlined items are outside the range of the present invention.

[0068] According to Tables 1 and 2, in Nos. 1 to 32 and No. 46 in which the steel composition, hot rolling conditions, and hot-rolled sheet annealing conditions are within the ranges of the present invention, fine metal structures with an average crystal grain size of 45 µm or less were obtained, and desired Charpy impact values were obtained. Furthermore, as a result of evaluation of corrosion resistance of the resulting hot-rolled and annealed sheets, it was confirmed that the hot-rolled and annealed sheets each have a rusting area ratio of 25% or less, indicating sufficient corrosion resistance. In particular, in No. 17 which used steel A17 with a Cu content of 0.95% and No. 18 which used steel A18 with a Mo content of 0.88%, the rusting area ratio was 10% or less, and thus more excellent corrosion resistance was obtained.

[0069] Furthermore, regarding Nos. 1 to 32 and No. 46 of Examples, when working into the shape of a thick flange having a burring portion was tried, no cracks occurred, and it was possible to obtain a predetermined flange shape. Note that structure observation on the hot-rolled and annealed steel sheets of Examples showed that each of the steel sheets had a ferrite single phase structure or a structure including 3% or less (in volume ratio) in total of one or both of a martensite phase and a retained austenite phase with the balance being a ferrite phase.

[0070] In No. 33 and No. 34 which used steel A1 and steel A2, respectively, and in which the slab heating temperature was higher than the range of the present invention, although a required amount of austenite phase was formed during heating in the hot rolling process and rolling was performed with a required cumulative rolling reduction, since the rolling temperature was excessively high, recovery of working strain occurred, and introduction of recrystallization sites was insufficient. Therefore, in the hot-rolled sheet annealing process, recrystallized grains were likely to be coarsened, and a predetermined Charpy impact value was not obtained.

[0071] In No. 35 and No. 36 which used steel A1 and steel A2, respectively, and in which the hot-rolled sheet annealing temperature was higher than the range of the present invention, formed recrystallized grains markedly coarsened, and consequently, a desired Charpy impact value was not obtained.

[0072] In Nos. 37, 38, and 39 using steel B1, B2, and B3, respectively, in which the steel satisfied the composition ranges, but γ_I was lower than the range of the present invention, although hot rolling and hot-rolled sheet annealing were performed within the ranges of the present invention, as a result of insufficient formation of austenite phase during heating in the hot rolling process, the metal structure was not refined sufficiently in the hot-rolled sheet annealing process, and a predetermined Charpy impact value was not obtained.

[0073] In No. 40 using steel B4 in which the Cr content was higher than the range of the present invention, although hot rolling and hot-rolled sheet annealing were performed within the ranges of the present invention, as a result of insufficient formation of austenite phase during heating in the hot rolling process, the metal structure was not refined sufficiently in the hot-rolled sheet annealing process, and a desired Charpy impact value was not obtained.

[0074] In No. 41 using steel B5 in which the Mn content was higher than the range of the present invention, although hot rolling and hot-rolled sheet annealing were performed within the ranges of the present invention, MnS serving as a starting point of corrosion was excessively precipitated. As a result, predetermined corrosion resistance was not obtained.

[0075]  In No. 42 using steel B6 in which the Nb content was higher than the range of the present invention, since the recrystallization temperature increased, the metal structure was not refined sufficiently, and a desired Charpy impact value was not obtained.

[0076] In No. 43 using steel B7 in which the Si content was higher than the range of the present invention, the average crystal grain size of the metal structure exceeded 45 µm, and a desired Charpy impact value was not obtained.

[0077] In No. 44 using steel B8 in which the Ti content was higher than the range of the present invention, coarse TiN was formed by the excessive Ti content, and a desired Charpy impact value was not obtained.

[0078] In No. 45 using steel B9 in which Ti was not contained, since the recrystallization temperature increased, the metal structure was not refined sufficiently, and a desired Charpy impact value was not obtained.

[0079] In No. 47 using steel B10 in which the Ni content was lower than the range of the present invention, although hot rolling and hot-rolled sheet annealing were performed within the ranges of the present invention, as a result of insufficient formation of austenite phase during heating in the hot rolling process, the metal structure was not refined sufficiently in the hot-rolled sheet annealing process, and a desired Charpy impact value was not obtained.

Industrial Applicability

[0080] The ferritic stainless steel sheet obtained in the present invention is suitable for application requiring excellent toughness, for example, particularly suitable for use in a flange or the like.

Claims

1. A ferritic stainless steel sheet having a composition containing, in percent by mass,
C: 0.001 to 0.020%,
Si: 0.05 to 0.35%,
Mn: 0.05 to 1.00%,
P: 0.04% or less,
S: 0.01% or less,
Al: 0.001 to 0.300%,
Cr: 10.0 to 13.0%,
Ni: 0.75 to 1.50%,
Ti: 0.05 to 0.35%, and
N: 0.001 to 0.020%, with the balance being Fe and inevitable impurities,
wherein γ_I [%] represented by formula (1) below is 65% or more, and a metal structure has an average crystal grain size of 45 µm or less:

where Ni, Mn, Cu, Si, Cr, and Mo represent contents of the respective elements (percent by mass), and an element not contained represents 0.

2. The ferritic stainless steel sheet according to Claim 1, wherein the composition further contains, in percent by mass, one or two or more selected from
Cu: 0.01 to 1.00%,
Mo: 0.01 to 1.00%,
W: 0.01 to 0.20%, and
Co: 0.01 to 0.20%.

3. The ferritic stainless steel sheet according to Claim 1 or 2, wherein the composition further contains, in percent by mass, one or two or more selected from
V: 0.01 to 0.20%,
Nb: 0.01 to 0.10%, and
Zr: 0.01 to 0.20%.

4. The ferritic stainless steel sheet according to any one of Claims 1 to 3, wherein the composition further contains, in percent by mass, one or two or more selected from REM: 0.001 to 0.100%,
B: 0.0002 to 0.0025%,
Mg: 0.0005 to 0.0030%, and
Ca: 0.0003 to 0.0030%.

5. A method for manufacturing the ferritic stainless steel sheet according to any one of Claims 1 to 4, comprising:

a hot rolling process in which a steel slab having the composition is heated at 1,050 to 1,250°C, and then subjected to hot rolling; and

a hot-rolled sheet annealing process in which a hot-rolled steel sheet obtained in the hot rolling process is subjected to hot-rolled sheet annealing at 750 to 1,050°C.