KR102603113B1 - Ferritic stainless-steel sheet and method for manufacturing same - Google Patents

Abstract

A ferritic stainless steel sheet with superior toughness and corrosion resistance and a method for manufacturing the same are provided. 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: 1.15 Contains ~1.50%, Ti: 0.05-0.35%, N: 0.001-0.020%, and γ _I [%] of the following formula (1) is 65% or more, and the balance consists of Fe and inevitable impurities. It has a chemical composition, and the average crystal grain size of the metal structure is 45 μm or less. It is manufactured by hot rolling a steel slab having the above chemical composition and annealing the hot rolled sheet at 750 to 1050°C. γ _I [%] = 24Ni + 12Mn + 6Cu - 18Si - 12Cr - 12Mo + 188 (1) In addition, Ni, Mn, Cu, Si, Cr and Mo in formula (1) represent the content (mass%) of each component and do not contain In this case, it is set to 0.

Description

Ferritic stainless steel sheet and method of manufacturing same {FERRITIC STAINLESS-STEEL SHEET AND METHOD FOR MANUFACTURING SAME}

The present invention relates to a ferritic stainless steel sheet and a manufacturing method thereof. In particular, it relates to a ferritic stainless steel sheet having excellent toughness and excellent corrosion resistance, which is useful for use as a flange member, and a manufacturing method thereof.

The exhaust gas path of an automobile is composed of various parts such as an exhaust manifold, muffler, catalyst, flexible tube, center pipe, and front pipe. When connecting these parts, fastening parts called flanges are often used. Flanges applied to these exhaust system components need to have sufficient rigidity. In this regard, thick flanges (for example, 5 mm or more in plate thickness) are applied to these exhaust system parts.

In addition, flanges are manufactured by processes such as punching in addition to press forming, and plain steel has been used.

Additionally, in recent years, sufficient corrosion resistance is required for flange materials applied to parts exposed to high-temperature exhaust gas, such as Exhaust Gas Recirculation (EGR) systems. For this reason, the application of stainless steel, which has excellent corrosion resistance compared to ordinary steel, especially ferritic stainless steel, which has a relatively small coefficient of thermal expansion and is less likely to generate thermal stress, is being considered. As a result, there is a strong demand for ferritic stainless steel sheets with a large sheet thickness (for example, 5 mm or more in sheet thickness) that can be applied to thick flanges.

However, ferritic stainless steel with a large plate thickness has problems with low-temperature toughness. For example, press cracks during flange manufacturing occur frequently in the winter. In these respects, there is a strong demand for improvement in the toughness of ferritic stainless steels with large sheet thicknesses.

Regarding this market demand, for example, Patent Document 1 states that, in 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% to 18%, and further containing one or two types of Ti: 0.05 to 0.30% and Nb: 0.01 to 0.50%. The total of Ti and Nb is 8(C＋N) to 0.75%, the balance is made up of Fe and inevitable impurities, γ _p is 70% or more, the ferrite grain size is 20㎛ or less, and the amount of martensite is 70%. A stainless steel sheet excellent in toughness (Charpy impact value at -40°C of 50 J/cm2 or more) is disclosed, which is characterized by the following.

Additionally, γ _p (%) is evaluated using the following formula (i) (indicated as formula (1) in Patent Document 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)

In addition, (%X) represents the mass ratio of each component

Japanese Patent Publication No. 2016-191150

However, when the present inventors attempted to process a thick flange shape with a burring portion using the stainless steel sheet described in Patent Document 1, cracks occurred in the burring portion, and the predetermined flange shape was lost. In some cases, it cannot be obtained, and it has been found that it is not sufficient for application to thick flanges.

In view of these circumstances, the purpose of the present invention is to provide a ferritic stainless steel sheet with superior toughness and excellent corrosion resistance, and a method for manufacturing the same.

In addition, in the present invention, superior toughness means that the Charpy impact value at -50°C is 100 J/cm2 or more. In addition, in the present invention, excellent corrosion resistance means that the rusting area ratio after three cycles of the salt spray cycle test specified in JIS H 8502 is 25% or less.

The present inventors conducted detailed studies to solve the above problems. As a result, the following recognition was obtained.

In order to process a thick flange with a burring process without cracking, it is effective to refine the metal structure and have a Charpy impact value of 100 J/cm2 or more at -50°C. Specifically, by setting the average crystal grain size of the metal structure to 45 ㎛ or less, the occurrence of cracks in the burring portion when processing the flange of the thick material having the burring portion can be effectively suppressed. It can be put to practical use with a flange.

Then, a slab having a steel composition with appropriate steel components, specifically Si, Mn, Cr, Ni, etc. controlled to an appropriate range, is heated at 1050 to 1250°C, hot rolled, and hot rolled sheet annealed at an appropriate temperature. This is an effective means for refining the metal structure and obtaining a Charpy impact value of 100 J/cm2 or more at -50°C.

The present invention has been made based on the above recognition, and has the following as a summary.

[1] In 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 -13.0%, Ni: 1.15-1.50%, Ti: 0.05-0.35%, N: 0.001-0.020%, and γ _I [%] of the following formula (1) is 65% or more, and the remainder is A ferritic stainless steel sheet having a composition consisting of Fe and inevitable impurities, and having an average crystal grain size of the metal structure of 45 μm or less.

γ _I [％]＝24Ni＋12Mn＋6Cu－18Si－12Cr－12Mo＋188 (1)

In addition, Ni, Mn, Cu, Si, Cr, and Mo in formula (1) represent the content (mass %) of each component, and components not contained are set to 0.

[2] In addition to the above component composition, in mass%, it contains one or two or more of 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%. , The ferritic stainless steel sheet described in [1] above.

[3] In addition to the above component composition, the above [1] or [, which contains one or two or more of V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, and Zr: 0.01 to 0.20% in mass%. The ferritic stainless steel sheet described in [2].

[4] In addition to the above component composition, in mass%, it contains one or two or more of 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%. , The ferritic stainless steel sheet according to any one of [1] to [3] above.

[5] A method for manufacturing a ferritic stainless steel sheet according to any one of [1] to [4] above, comprising: a hot rolling process in which a steel slab having the above chemical composition is heated at 1050 to 1250°C and then hot rolled; A method for manufacturing a ferritic stainless steel sheet, comprising a hot-rolled sheet annealing process in which the hot-rolled steel sheet obtained in the hot-rolling process is annealed at 750 to 1050°C.

According to the present invention, a ferritic stainless steel sheet with superior toughness and excellent corrosion resistance is obtained. The ferritic stainless steel sheet of the present invention can be suitably used for applications such as thick flanges.

(Form for carrying out the invention)

Hereinafter, the present invention will be described in detail.

The present inventors used various ferritic stainless steel sheets with a sheet thickness of 5.0 mm, and made a flange with a burring processing part raised 10 mm from the surface of the steel sheet with the flange hole of 30 mm phi blanked (punched). The cause of cracks during molding was examined in detail. As a result, no cracks occurred in steel sheets with a Charpy impact value of 100 J/cm2 or more at -50°C, and in steel sheets where cracks occurred, the Charpy impact value at -50°C was less than 100 J/cm2. In this way, it was found that low toughness was the cause of cracking.

Additionally, the present inventors examined in detail the relationship between this low toughness and metal structure. As a result, it was found that the larger the average crystal grain size of the steel sheet, the lower the toughness. Therefore, an attempt was made to form the above-described flange using various ferritic stainless steel plates (plate thickness 5.0 mm). As a result, it was found that in steel sheets with an average crystal grain size exceeding 45 μm, toughness decreases and cracks are prone to occur. When the average crystal grain size was 45㎛ or less, toughness was excellent and the punching processability of the steel sheet was good.

From the above, in the present invention, the average crystal grain size is 45 μm or less, and the Charpy impact value at -50°C is 100 J/cm2 or more.

In addition, the average crystal grain size can be measured by the measurement method in the Examples described later. In addition, the Charpy impact value is a value measured based on JIS Z 2242 (2005), as will be described later.

Next, the component composition of the ferritic stainless steel sheet of the present invention will be described.

Hereinafter, unless otherwise specified, “%” as a unit of component content means “% by mass.”

C: 0.001 to 0.020%

If C is contained in excess of 0.020%, the processability and corrosion resistance are significantly reduced. A smaller C content is preferable from the viewpoint of corrosion resistance and processability, but setting the C content to less than 0.001% requires time for refining, which is undesirable from a manufacturing perspective. Therefore, the C content is within the range of 0.001% to 0.020%. The C content is preferably 0.003% or more, and more preferably 0.004% or more. Moreover, the C content is preferably 0.015% or less, and more preferably 0.012% or less.

Si: 0.05 to 0.35%

Si has the effect of improving the corrosion resistance of the weld zone by concentrating in the oxide film formed during welding, and is also a useful element as a deoxidizing element in the steelmaking process. These effects are obtained by containing 0.05% or more of Si, and the larger the content, the greater the effect. On the other hand, Si has the effect of promoting the formation of a ferrite phase, and if Si is contained in excess of 0.35%, a predetermined amount of austenite phase is not sufficiently generated during heating in the hot rolling process, so that the Even if hot rolling and hot-rolled sheet annealing are performed under certain conditions, the 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. Additionally, the Si content is preferably 0.30% or less.

Mn: 0.05 to 1.00%

Mn has the effect of promoting the formation of austenite phase. To obtain the effect, Mn content of 0.05% or more is required. However, when the Mn content exceeds 1.00%, precipitation of MnS, which is the starting point of corrosion, is promoted, and corrosion resistance decreases. 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. Additionally, the Mn content is preferably 0.80% or less, and more preferably 0.70% or less.

P: 0.04% or less

P is an element that is inevitably included in steel and is harmful to corrosion resistance and processability, so it is desirable to reduce it as much as possible. When the P content exceeds 0.04%, processability significantly decreases due to solid solution strengthening. Therefore, the P content is set to 0.04% or less. The P content is preferably 0.03% or less.

S: 0.01% or less

Like P, S is an element that is inevitably included in steel and is harmful to corrosion resistance and processability, so it is desirable to reduce it as much as possible. In particular, when the S content exceeds 0.01%, corrosion resistance significantly decreases. Therefore, the S content is set to 0.01% or less. The S content is preferably 0.008% or less, and more preferably 0.003% or less.

Al: 0.001∼0.300%

Al is an effective deoxidizer. In addition, since Al has a stronger affinity for nitrogen than Cr, when nitrogen infiltrates the weld zone, nitrogen is precipitated as Al nitride rather than Cr nitride, which has the effect of suppressing sensitization. These effects are obtained by containing 0.001% or more of Al. However, if Al exceeds 0.300% is contained, penetration during welding decreases and weldability decreases, which is not preferable. Therefore, the Al content is set to be in the range of 0.001% or more and 0.300% or less. The Al content is preferably 0.010% or more. Moreover, 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%

Cr is the most important element to ensure corrosion resistance. If the content is less than 10.0%, the corrosion resistance required for automobile exhaust parts cannot be obtained. On the other hand, if Cr is contained in excess of 13.0%, a predetermined amount of austenite phase is not generated during heating in the hot rolling process even if the steel composition is adjusted to γ _I expressed by the predetermined equation (1) described later. Even if hot rolling and hot-rolled sheet annealing are performed under the conditions stipulated by the present invention, the desired metal structure is not obtained. Therefore, the Cr content is set to be in the range of 10.0% to 13.0%. Cr content is preferably 10.5% or more. Moreover, the Cr content is preferably 12.0% or less, and more preferably 11.7% or less.

Ni: 1.15 to 1.50%

Ni is an austenite forming element and has the effect of increasing the amount of austenite generated during heating before rolling processing in the hot rolling process. In the present invention, by adjusting the steel components, a two-phase structure of a ferrite phase + austenite phase containing an austenite phase of 70% or more by volume is achieved when the slab is heated in the hot rolling process. When the metal structure becomes a two-phase structure of ferrite phase + austenite phase, the two-phase interface between the ferrite phase and austenite phase functions as an obstacle to grain growth, so the metal structure before hot rolling is refined. Afterwards, processing strain that becomes a recrystallization site is accumulated by predetermined hot rolling, and recrystallization occurs by annealing the hot-rolled sheet in the next step, thereby obtaining a fine metal structure and exhibiting excellent toughness. These effects are obtained by containing 1.15% or more of Ni. On the other hand, when the Ni content exceeds 1.50%, the improvement effect due to grain refinement is saturated and workability decreases. Additionally, stress corrosion cracking becomes more likely to occur. Therefore, the Ni content is set to be 1.15% or more and 1.50% or less. Additionally, the Ni content is preferably 1.20% or less.

Ti: 0.05 to 0.35%

Ti combines preferentially with C and N, suppressing the precipitation of Cr carbonitride, lowering the recrystallization temperature, and suppressing the decline in corrosion resistance caused by sensitization due to precipitation of Cr carbonitride. In order to obtain this effect, the inclusion of 0.05% or more of Ti is required. On the other hand, when the Ti content exceeds 0.35%, a significant decrease in toughness occurs due to the formation of coarse TiN, and the desired toughness cannot be obtained even if the technology of the present invention is applied. Additionally, the content of Ti exceeding 0.35% is undesirable in terms of manufacturing because coarse Ti carbonitride is generated during the casting process and causes surface defects. 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. Moreover, the Ti content is preferably 0.30% or less, and more preferably 0.15% or less.

N: 0.001 to 0.020%

If the N content exceeds 0.020%, the processability and corrosion resistance are significantly reduced. From the viewpoint of processability and corrosion resistance, a lower N content is more preferable, but reducing the N content to less than 0.001% requires long-term refining, which is undesirable because it causes an increase in manufacturing cost and a decrease in productivity. Therefore, the N content is within the range of 0.001% to 0.020%. The N content is preferably 0.005% or more, and more preferably 0.007% or more. Moreover, the N content is preferably 0.015% or less, and more preferably 0.012% or less.

γ _I [%]: 65% or more

If γ _I , expressed in the following equation (1), is less than 65%, the austenite content of the metal structure is insufficient at the slab heating temperature before the start of hot rolling, so a fine metal structure cannot be obtained. Therefore, γ _I [%] is set to 65% or more. Additionally, γ _I [%] is obtained using the following equation (1), which evaluates the stability of the austenite phase.

γ _I [％]＝24Ni＋12Mn＋6Cu－18Si－12Cr－12Mo＋188 (1)

In addition, Ni, Mn, Cu, Si, Cr, and Mo in formula (1) represent the content (mass %) of each component, and components not contained are set to 0.

In the above formula (1), the austenite forming element has a positive coefficient, the ferrite forming element has a negative coefficient, and each value was experimentally determined with reference to Castro's equation.

In the present invention, the remainder other than the above is Fe and inevitable impurities. Examples of inevitable impurities include O (oxygen), and the O content is acceptable as long as it is 0.01% or less.

In addition to the above essential ingredients, it may further contain one or two or more groups selected from Groups A to C below, as needed.

(Group A) One or two or more of 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 of 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 of 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%

Cu is a particularly effective element for improving corrosion resistance in an aqueous solution or when weakly acidic water droplets adhere. Additionally, Cu has the effect of promoting the formation of austenite phase. This effect is obtained by containing 0.01% or more of Cu, and the effect becomes higher as the Cu content increases. However, if Cu is contained in excess of 1.00%, hot workability may decrease and surface defects may be induced. Furthermore, descaling after annealing may become difficult. Therefore, when it contains Cu, the Cu content is in the range of 0.01% or more and 1.00% or less. When it contains Cu, the Cu content is preferably 0.10% or more. In addition, when it contains Cu, the Cu content is preferably 0.50% or less.

Mo: 0.01 to 1.00%

Mo is an element that significantly improves the corrosion resistance of stainless steel. This effect is obtained by containing 0.01% or more of Mo, and the effect improves as the content increases. On the other hand, Mo has the effect of promoting the formation of a ferrite phase, and when the Mo content exceeds 1.00%, a predetermined amount of austenite phase is not sufficiently generated during heating in the hot rolling process, so the conditions stipulated by the present invention Even if hot rolling and hot-rolled sheet annealing are performed, the 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. In addition, 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%

W, like Mo, has the effect of improving corrosion resistance. This effect is achieved by containing 0.01% or more of W. On the other hand, if W is contained in excess of 0.20%, the strength increases and may cause a decrease in manufacturability due to an increase in rolling load, etc. Therefore, when it contains W, the W content is in the range of 0.01% or more and 0.20% or less. When it contains W, the W content is preferably 0.05% or more. In addition, when W is contained, the W content is preferably 0.15% or less.

Co: 0.01 to 0.20%

Co is an element that improves toughness. This effect is obtained by containing 0.01% or more of Co. On the other hand, when the Co content exceeds 0.20%, processability may decrease. Therefore, when it contains Co, the Co content is set to be in the range of 0.01% or more and 0.20% or less.

V: 0.01 to 0.20%

V forms carbonitride with C and N, suppresses sensitization during welding, and improves the corrosion resistance of the weld zone. This effect is obtained when the V content is 0.01% or more. On the other hand, when the V content exceeds 0.20%, workability and toughness may decrease significantly. Therefore, when it contains V, the V content is set to be 0.01% or more and 0.20% or less. When it contains V, the V content is preferably 0.02% or more. In addition, when it contains V, the V content is preferably 0.10% or less.

Nb: 0.01 to 0.10%

Nb has the effect of refining crystal grains. This effect is achieved by containing 0.01% or more of Nb. On the other hand, Nb has the effect of increasing the recrystallization temperature, and when the Nb content exceeds 0.10%, the annealing temperature required to generate sufficient recrystallization by annealing the hot-rolled sheet becomes excessively high, resulting in a metal structure with an average crystal grain size of 45㎛ or less. There are cases where it cannot be obtained. Therefore, when containing Nb, the Nb content is set to be in the range of 0.01% or more and 0.10% or less. When containing Nb, the Nb content is preferably 0.05% or less.

Zr: 0.01 to 0.20%

Zr has the effect of suppressing sensitization by combining with C and N. This effect is achieved by containing 0.01% or more of Zr. On the other hand, if Zr is contained in excess of 0.20%, processability may be significantly reduced. Therefore, when containing Zr, the Zr content is in the range of 0.01% or more and 0.20% or less. When containing Zr, the Zr content is preferably 0.10% or less.

REM: 0.001 to 0.100%

REM (Rare Earth Metals) has the effect of improving oxidation resistance and suppresses the formation of an oxide film (welding temper color) in the weld zone, preventing the formation of a Cr-depleted area just below the oxide film. Suppress. This effect is obtained by containing 0.001% or more of REM. On the other hand, if REM is contained in excess of 0.100%, manufacturability, such as acid washability during cold rolling annealing, may be reduced. Therefore, when REM is contained, the REM content is in the range of 0.001% or more and 0.100% or less. When containing REM, the REM content is preferably 0.050% or less.

B: 0.0002 to 0.0025%

B is an effective element for improving secondary processing brittleness after deep drawing forming. This effect is obtained by setting the B content to 0.0002% or more. On the other hand, if B is contained in excess of 0.0025%, workability and toughness may decrease. Therefore, when B is contained, the B content is in the range of 0.0002% or more and 0.0025% or less. When it contains B, the B content is preferably 0.0003% or more. In addition, when B is contained, the B content is preferably 0.0012% or less.

Mg: 0.0005 to 0.0030%

In steel containing Ti as in the present invention, toughness may decrease when Ti carbonitride becomes coarse. In this regard, Mg has the effect of suppressing the coarsening of Ti carbonitride. This effect is obtained by containing 0.0005% or more of Mg. On the other hand, when the Mg content exceeds 0.0030%, the surface properties of the steel may deteriorate. Therefore, when Mg is contained, the Mg content is set to be in the range of 0.0005 to 0.0030%. When it contains Mg, the Mg content is preferably 0.0010% or more. In addition, when it contains Mg, the Mg content is preferably 0.0020% or less.

Ca: 0.0003 to 0.0030%

Ca is an effective ingredient in preventing blockage of the nozzle due to crystallization of Ti-based inclusions, which tend to occur during continuous casting. The effect is obtained by containing 0.0003% or more of Ca. On the other hand, if Ca is contained in excess of 0.0030%, corrosion resistance may decrease due to the formation of CaS. Therefore, when it contains Ca, the Ca content is in the range of 0.0003% or more and 0.0030% or less. When it contains Ca, the Ca content is preferably 0.0005% or more. Moreover, when it contains Ca, the Ca content is preferably 0.0015% or less, and more preferably 0.0010% or less.

Next, the manufacturing method of the ferritic stainless steel sheet of the present invention will be described. As a result of careful study of methods for improving the toughness of ferritic stainless steel sheets, the present inventors have found that a steel slab having an appropriate steel composition is preferably heated at 1050 to 1250°C and then hot rolled, preferably in three or more passes. Then, by performing hot-rolled steel sheet annealing at 750 to 1050°C on the obtained hot-rolled steel sheet, a metal structure with an average crystal grain size of 45 ㎛ or less is obtained, and the Charpy impact value at -50°C is 100 J/cm2 or more, showing toughness. I found that there was a significant improvement. Additionally, it was found that the desired corrosion resistance was also obtained.

The reason why a hot-rolled annealed steel sheet with a fine metal structure can be obtained as a result of the above will be explained below.

Ferritic stainless steel rarely undergoes dynamic recrystallization during hot rolling, and recovery of processing strain by rolling tends to occur easily. Therefore, in hot rolling according to the prior art, excessive recovery of the processing strain introduced by rolling occurs, and the processing strain cannot be effectively maintained until after hot rolling. As a result, the recrystallization sites become insufficient, making it impossible to obtain a fine recrystallization structure in the next process of annealing the hot-rolled sheet.

Therefore, the present inventors carefully studied effective methods for obtaining a fine structure after annealing a hot-rolled sheet from both the steel composition and the hot rolling method. As a result, the content of steel components, especially Si, Mn, Cr and Ni, was controlled to an appropriate range, and the slab was heated at an appropriate temperature in the hot rolling process to achieve a two-phase structure of a ferrite phase containing an austenite phase + an austenite phase. It was found that hot rolling is effective.

When the metal structure becomes a two-phase structure of ferrite phase + austenite phase, the abnormal interface between the ferrite phase existing before heating and the austenite phase generated during heating suppresses the coarsening of crystal grains, so that fine equiaxed particles are formed in the stage before hot rolling. organization is obtained. Afterwards, by performing a predetermined hot rolling, processing strain that becomes a recrystallization site is sufficiently accumulated in the hot-rolled sheet annealing in the next step, and a fine metal structure is obtained by the hot-rolled sheet annealing in the next step, enabling excellent toughness to be exhibited. .

Specifically, the content of Ni and Mn, which are austenite forming elements, and positive coefficients for each of Ni and Mn, so that an austenite phase of 65% or more by volume is generated during heating before hot rolling, and a positive coefficient for each of Ni and Mn, which are ferrite forming elements. For steel adjusted so that the above equation (1), which combines the contents of Si and Cr and the negative coefficients for each of Si and Cr, holds, slab heating is performed at 1050 to 1250 ° C, and then hot rolling is performed. designed something

In addition, the present inventors also carefully studied suitable conditions for hot-rolled sheet annealing in the next process. Hot-rolled sheet annealing is a process that recrystallizes the processed structure formed by hot rolling. Therefore, it is necessary to perform annealing at a temperature at which sufficient recrystallization occurs. However, when annealing a hot-rolled sheet is performed at an excessively high temperature, although recrystallization occurs, the recrystallized grains become significantly coarsened, so that the desired fine structure cannot be obtained.

Therefore, the present inventors investigated in detail the relationship between the grain size of recrystallized grains and annealing temperature. As a result, it was discovered that by suppressing the hot-rolled sheet annealing temperature to 1050°C or lower, the formation of coarse recrystallized grains to the extent of lowering toughness could be suppressed.

Hereinafter, each manufacturing condition will be described in detail.

First, molten steel with the above-mentioned composition is melted by a known method such as a converter, electric furnace, or vacuum melting furnace, and then subjected to a continuous casting method or an ingot casting-blooming process. It is made of steel material (slab).

Heating temperature of steel slab: 1050∼1250℃

The steel slab is heated at 1050 to 1250°C and subjected to hot rolling. The heating time at the above heating temperature is not particularly limited, but is preferably heated for 1 to 24 hours. If the heating temperature is less than 1050°C, the production rate of the austenite phase decreases, a fine metal structure cannot be obtained, and excellent toughness cannot be obtained. On the other hand, if the heating temperature is too high, it leads to an increase in scale loss accompanying an increase in oxidized mass, so the heating temperature of the steel slab is set to 1250°C or lower. However, when performing hot rolling on a steel slab, if the steel slab after casting is in a temperature range of 1050°C or higher, direct rolling may be performed without heating the steel material.

There is no particular limitation on rough rolling conditions. When the casting structure is effectively destroyed before finishing hot rolling, the refinement effect during subsequent slab heating is further promoted, so it is desirable to set the cumulative reduction ratio in rough rolling to 65% or more. After that, the sheet is rolled to a predetermined thickness by finishing hot rolling.

Hot rolled sheet annealing temperature: 750∼1050℃

In the present invention, annealing of the hot rolled sheet is performed after completion of the hot rolling. In hot-rolled sheet annealing, the rolled structure formed in the hot rolling process is recrystallized. In the present invention, the coarsening of recrystallization during hot-rolled sheet annealing is suppressed by effectively applying rolling strain in the hot rolling process and increasing the number of recrystallization sites. In order to obtain this effect, hot-rolled sheet annealing needs to be performed in the range of 750 to 1050°C. If the annealing temperature is less than 750°C, recrystallization is insufficient, residual stress resulting from hot rolling deformation remains, and flatness after hot rolling annealing cannot be maintained. On the other hand, if the annealing temperature exceeds 1050°C, the recrystallized grains become significantly coarsened, and the desired metal structure cannot be obtained. Therefore, the hot-rolled sheet annealing temperature is set in the range of 750°C or more and 1050°C or less. Preferably, the hot rolled sheet annealing temperature is in the range of 750°C or more and 900°C or less. In addition, there is no particular limitation on the holding time and method of hot-rolled sheet annealing, and it may be performed by either box annealing (batch annealing) or continuous annealing.

The ferritic stainless steel sheet obtained above may be subjected to descaling treatment by shot blasting or acid cleaning as needed. Additionally, in order to improve the surface properties, grinding or polishing may be performed. Additionally, cold rolling and annealing of the cold rolled sheet may be performed thereafter.

As a result of the above, the ferritic stainless steel sheet of the present invention is manufactured with excellent toughness and also excellent corrosion resistance.

The metal structure of the ferritic stainless steel sheet obtained in the present invention contains a total of 3% or less (volume ratio) of a ferrite single phase, or one or both martensite and retained austenite phases, and the remainder is the ferrite phase.

The ferritic stainless steel sheet of the present invention has a Charpy impact value of 100 J/cm2 or more at -50°C. With such excellent low-temperature toughness, the occurrence of cracks in the burring portion can be effectively suppressed when processing into a thick flange with a burring portion, and the flange can be sufficiently put into practical use as a thick flange with a burring portion.

The plate thickness is not particularly limited, but is preferably a plate thickness that can be applied to a thick flange, so 5.0 mm or more is preferable, and 8.0 mm or more is more preferable. Additionally, the plate thickness is preferably 15.0 mm or less, and more preferably 13.0 mm or less.

Example 1

Hereinafter, the present invention will be described in detail by examples.

Stainless steel having the component composition shown in Table 1 was made into a 100 kg steel slab by vacuum induction melting. Next, hot rolling was performed under the manufacturing conditions shown in Table 2 to obtain a hot rolled steel sheet with a finished thickness shown in Table 2. Hot-rolled sheet annealing was performed on this hot-rolled steel sheet to obtain a hot-rolled annealed steel sheet. In addition, hot-rolled sheet annealing was performed by maintaining the hot-rolled sheet annealing temperature shown in Table 2 for 8 hours.

The following evaluation was performed on the hot rolled annealed steel sheet obtained as above.

(1) Evaluation of average grain size

The average crystal grain size was measured by the EBSD (Electron Back Scattering Diffraction) method. The measurement conditions were 500 times the measurement magnification and a step of 0.4 μm. For the obtained data, an orientation difference of 15° or more was defined as a grain boundary using OIM (Orientation Imaging Microscopy) analysis software from TSL Solutions Co., Ltd., and the equivalent circle diameter was calculated. The value calculated from the average value of the obtained equivalent circle diameters was taken as the average grain size.

(2) Evaluation of Charpy impact value

A V-notch Charpy test piece conforming to JIS Z 2242 (2005) is taken from the central portion of the hot rolled annealed steel sheet so that the rolling direction is on the longer side while maintaining the sheet thickness of the steel sheet, and the test piece is subjected to JIS Z 2242 (2005). Based on this, the Charpy impact value at -50°C was measured. Those with a Charpy impact value at -50°C of 100 J/cm2 or more were considered acceptable, and those with a Charpy impact value of less than 100 J/cm2 were considered rejected.

(3) Evaluation of corrosion resistance

A test piece measuring 60 Provided for spray cycle testing. The salt spray cycle test is one cycle of salt spray (5% by mass NaCl, 35°C, spray 2hr) → dry (60°C, 4hr, relative humidity 40%) → wet (50°C, 2hr, relative humidity ≥95%). So, 3 cycles were performed. The surface of the test piece after three cycles of the salt spray cycle test was photographed, the rusting area of the surface of the test piece was measured through image analysis, and the rusting rate (rusting area in the test piece/rusting area measurement) was calculated from the ratio of the rusting area to the area of the measured portion. Area of the part) × 100 [%]) was calculated. The rusting area measurement portion is the portion excluding the 15 mm outer circumference of the test piece. In addition, the rusting area was defined as the area of the rusting portion and the area of the whey portion subjected to flow rust. A rusting rate of 10% or less was considered to be a pass (◎) with particularly excellent corrosion resistance, a rate of more than 10% and 25% or less was considered a pass (○), and a rate of more than 25% was considered a failure (×).

The test results obtained above are shown in Table 2 along with the manufacturing conditions.

According to Table 1 and Table 2, Nos. 2, 3, 9, 11, 15 to 17 and 19, whose steel composition, hot rolling conditions and hot rolled sheet annealing conditions meet the range of the present invention, have an average crystal grain size of 45㎛. The following fine metal structure was obtained, and a predetermined Charpy impact value was obtained. Additionally, as a result of evaluating the corrosion resistance of the obtained hot rolled annealed steel sheets, it was confirmed that all had a rusting rate of 25% or less and sufficient corrosion resistance. In particular, in No. 17 using steel A17 containing 0.95% Cu, superior corrosion resistance was obtained with a rusting rate of 10% or less.

In addition, regarding the example of the present invention, when processing into a flange shape of thick material having a burring processing portion was attempted, cracks did not occur and a predetermined flange shape was obtained. In addition, as a result of structural observation of the hot-rolled annealed steel sheet of the example of the present invention, it was found that all of them had a ferrite single-phase structure, or that the total of one or both martensite and retained austenite phases was 3% or less by volume and the remainder was a ferrite phase. I had it.

In No. 34, where steel A2 is used and the slab heating temperature exceeds the range of the present invention, a predetermined amount of austenite phase is generated during heating in the hot rolling process, and rolling at a predetermined cumulative reduction rate is difficult. However, because the rolling temperature was excessively high, recovery of processing strain occurred and introduction of recrystallization sites was insufficient, so coarsening of recrystallized grains was likely to occur in the hot-rolled sheet annealing process, and a predetermined Charpy impact value was not obtained. Didn't lose.

In No. 36, where steel A2 was used and the hot-rolled sheet annealing temperature exceeded the range of the present invention, the generated recrystallized grains were significantly coarsened, and as a result, the prescribed Charpy impact value was not obtained.

In No. 37, No. 38, and No. 39 using steels B1, B2, and B3, which meet the respective component ranges of steel but whose γ _I is below the range of the present invention, predetermined hot rolling and hot-rolled sheet annealing were performed. However, as a result of the austenite phase not being sufficiently generated during heating in the hot rolling process, the metal structure was not sufficiently refined in the hot rolled sheet annealing process, and the desired Charpy impact value was not obtained.

In No. 40 using steel B4 whose Cr content exceeds the range of the present invention, prescribed hot rolling and hot-rolled sheet annealing were performed, but as a result, the austenite phase was not sufficiently generated during heating in the hot rolling process, and hot-rolled sheet annealing was performed. During the process, the metal structure was not sufficiently refined, and the desired Charpy impact value was not obtained.

In No. 41 using steel B5 whose Mn content exceeds the range of the present invention, the prescribed hot rolling and hot-rolled sheet annealing were performed, but as a result of excessive precipitation of MnS, which is the starting point of corrosion, the prescribed corrosion resistance was not obtained. didn't

In No. 42 using steel B6 with an Nb content exceeding the range of the present invention, the recrystallization temperature was increased, so the metal structure was not sufficiently refined, and the desired Charpy impact value was not obtained.

In No. 43 using steel B7 with a Si content exceeding that of the present invention, the average grain size of the metal structure exceeded 45 μm, and the prescribed Charpy impact value was not obtained.

In No. 44 using steel B8 with a Ti content exceeding the range of the present invention, coarse TiN was generated due to excessive Ti content, and the desired Charpy impact value was not obtained.

In No. 45 using steel B9 without Ti added, the recrystallization temperature was increased, so the metal structure was not sufficiently refined, and the desired Charpy impact value was not obtained.

In No. 47 using steel B10 whose Ni content is below the range of the present invention, prescribed hot rolling and hot-rolled sheet annealing were performed, but as a result, the austenite phase was not sufficiently generated during heating in the hot-rolling process, and hot-rolled sheet annealing was performed. During the process, the metal structure was not sufficiently refined, and the desired Charpy impact value was not obtained.

The ferritic stainless steel sheet obtained in the present invention is particularly suitable for applications requiring excellent toughness, for example, flanges, etc.

Claims (7)

In mass%,
C: 0.001 to 0.020%,
Si: 0.05 to 0.35%,
Mn: 0.05 to 1.00%,
P: More than 0% and less than or equal to 0.04%,
S: More than 0% and less than or equal to 0.01%,
Al: 0.001 to 0.300%,
Cr: 10.0 to 13.0%,
Ni: 1.15 to 1.50%,
Ti: 0.05 to 0.35%,
N: 0.001 to 0.020%
It contains and has a component composition in which γ _I [%] of the following formula (1) is 65% or more, and the balance consists of Fe and inevitable impurities,
It has a metal structure containing, in terms of volume fraction, a total of more than 0% and 3% or less of one or both martensite and retained austenite phases, with the remainder being a ferrite phase,
The average crystal grain size of the metal structure is 45㎛ or less, the plate thickness is 5mm or more,
A ferritic stainless steel sheet for flange processing with a plate thickness of 5 mm or more having a burring portion and having a Charpy impact value of 100 J/cm2 or more at -50°C measured in accordance with JIS Z 2242.
γ _I [％]＝24Ni＋12Mn＋6Cu－18Si－12Cr－12Mo＋188 (1)
In addition, Ni, Mn, Cu, Si, Cr, and Mo in formula (1) represent the content (mass %) of each component, and components not contained are set to 0. According to paragraph 1,
In addition to the above component composition, in mass%,
Cu: 0.01 to 1.00%,
Mo: 0.01 to 1.00%,
W: 0.01 to 0.20%,
Co: A ferritic stainless steel sheet for flange processing with a plate thickness of 5 mm or more and having a burring portion, containing 0.01 to 0.20% of one type or two or more types. According to paragraph 1,
In addition to the above component composition, in mass%,
V: 0.01 to 0.20%,
Nb: 0.01 to 0.10%,
Zr: A ferritic stainless steel sheet for flange processing with a plate thickness of 5 mm or more and having a burring portion, containing 0.01 to 0.20% of one type or two or more types. According to paragraph 2,
In addition to the above component composition, in mass%,
V: 0.01 to 0.20%,
Nb: 0.01 to 0.10%,
Zr: A ferritic stainless steel sheet for flange processing with a plate thickness of 5 mm or more and having a burring portion, containing 0.01 to 0.20% of one type or two or more types. According to any one of claims 1 to 4,
In addition to the above component composition, in mass%,
REM: 0.001 to 0.100%,
B: 0.0002 to 0.0025%,
Mg: 0.0005 to 0.0030%,
A ferritic stainless steel sheet for flange processing with a plate thickness of 5 mm or more having a burring portion, containing one or two or more kinds of Ca: 0.0003 to 0.0030%. A method for manufacturing a ferritic stainless steel sheet according to any one of claims 1 to 4, comprising:
A hot rolling process of performing hot rolling on a steel slab having the above chemical composition after heating at 1050 to 1250°C;
A method of manufacturing a ferritic stainless steel sheet for flange processing with a thickness of 5 mm or more having a burring portion, comprising a hot-rolled sheet annealing process of hot-rolled steel sheet obtained in the hot rolling process at 750 to 1050°C. A method for manufacturing the ferritic stainless steel sheet according to claim 5, comprising:
A hot rolling process of performing hot rolling on a steel slab having the above chemical composition after heating at 1050 to 1250°C;
A method of manufacturing a ferritic stainless steel sheet for flange processing with a thickness of 5 mm or more having a burring portion, comprising a hot-rolled sheet annealing process of hot-rolled steel sheet obtained in the hot rolling process at 750 to 1050°C.