KR20150060919A

KR20150060919A - Ferritic stainless steel

Info

Publication number: KR20150060919A
Application number: KR1020157010761A
Authority: KR
Inventors: 마사타카 요시노; 타카시 사무카와; 토모히로 이시이; 쿠니오 후쿠다; 히로키 오타; 히로유키 오가타
Original assignee: 제이에프이 스틸 가부시키가이샤
Priority date: 2012-09-25
Filing date: 2013-09-17
Publication date: 2015-06-03
Also published as: WO2014050011A1; JP5534119B1; TW201420781A; CN104662187A; MY155937A; JPWO2014050011A1; TWI516614B

Abstract

A ferritic stainless steel having excellent corrosion resistance even under welding conditions in which sufficient gas shielding can not be performed is provided. 0.001 to 0.030% of C, 0.05 to 0.30% of Si, 0.05 to 0.50% of Mn, 0.05 to 0.50% of P, 0.05 or less of P, 0.01 to 0.01% of S, 18.0 to 19.0% of Cr, % Of Al, 0.10 to 1.50% of Al, 0.05 to 0.50% of Ti, 0.002 to 0.050% of Nb, and 0.01 to 0.50% of V, A ferritic stainless steel characterized by satisfying the expressions (1) and (2) and the remainder being Fe and unavoidable impurities.
0.40? Si + 1.5? A1 + 1.2? Ti? 2.4 (1)
0.60? 1.2 Nb + 1.7 Ti + V + 2.2 Al (2)
The symbol of the element in the formula represents the content (mass%) of each element.

Description

Ferritic stainless steel {FERRITIC STAINLESS STEEL}

The present invention relates to a ferritic stainless steel which is less prone to deterioration in corrosion resistance even in a welding condition in which oxygen or nitrogen from the atmosphere enters the weld bead from nitrogen or carbon from the welded mating member will be.

The ferritic stainless steel can secure corrosion resistance with a small amount of Ni as compared with the austenitic stainless steel. Since Ni is an expensive element, ferritic stainless steels can be manufactured at a lower cost than austenitic stainless steels. In addition, ferritic stainless steels have excellent properties such as high thermal conductivity, low coefficient of thermal expansion, and difficulty of stress corrosion cracking compared with austenitic stainless steels. In this respect, ferritic stainless steels are applicable to a wide range of applications such as automobile exhaust system members, construction materials such as roofs and sashes, and equipment for use in kitchens and water such as water storage tanks Has come.

These are often produced by assembling parts made of a stainless steel sheet by shearing or press working, etc. by welding. TIG welding (tungsten inert gas welding) is widely used for the welding method. In the case of welding, it is required that the weld portion has good corrosion resistance as in the base material portion.

However, when TIG welding austenitic stainless steels, especially steel types such as SUS304 (18% Cr-8% Ni) (JIS (Japanese Industrial Standards) G 4305) and ferritic stainless steels, The corrosion resistance of the welded portion may be lower than that of the base metal due to the development. Cr is precipitated in the grain boundaries as Cr carbide (Cr ₂₃ C ₆ or the like) or Cr nitride (Cr ₂ N or the like) by Cr bonding in the steel due to thermal history at the time of welding, A Cr depletion layer having a Cr concentration lower than that of the base material is generated, thereby lowering the corrosion resistance in the grain boundaries. Austenitic stainless steels such as SUS304 and the like may be susceptible to generation of susceptibility when ferritic stainless steels are welded with C and N contents higher than those of ferritic stainless steels.

When TIG welding is performed, an inert gas such as argon gas is generally used as a shielding gas to remove oxygen or nitrogen from the atmosphere into a weld metal pool (a portion in which the metal is melted at the time of welding) Suppress invasion. However, recently, as the structure of the welding member is complicated, sufficient gas shielding can not be performed at the time of welding, and welding under incomplete conditions such as mixing oxygen and nitrogen in the atmosphere into the melting paper is increasing. Nitrogen penetrated into the melting zone from the atmosphere promotes sensitization of the welded portion, resulting in deterioration of corrosion resistance.

In addition, oxygen generates a Cr-based oxide film called a "temper color" in the welded portion, and the Cr concentration of the welded portion is lowered by this growth, and the corrosion resistance is lowered. Therefore, ferritic stainless steels applicable to such applications are required to be a steel component capable of securing the corrosion resistance of the welded portion even when the penetration of nitrogen or oxygen from the atmosphere by the shielding gas can not be suppressed.

On the other hand, in recent years, in addition to conventional glossy products such as No. 2B finishing and BA finishing, so-called functional products (hereinafter referred to as " functional products " functional products are increasing in demand. In order to suppress the manufacturing cost, the functional product is annealed at about 850 to 900 DEG C by using an annealing line of carbon steel, and then manufactured using a high-speed pickling method as disclosed in, for example, Patent Document 1. [ Therefore, in order to manufacture not only the No. 2B finish and the BA finish but also the functional products, a steel component having a recrystallization temperature capable of being annealed in the carbon steel annealing line and capable of high-speed acid pickling is required.

To solve these problems, there is disclosed a method of inhibiting the generation of sensitization by inhibiting the formation of Cr carbonitride by adding Ti or Nb having a higher affinity to carbon and nitrogen than Cr. For example, Patent Document 2 discloses a steel in which the intergranular corrosion resistance of ferritic stainless steels is improved by additionally adding Ti and Nb. However, ferritic stainless steels disclosed in Patent Document 2 require addition of Mo in an amount of 1.5% or more. Mo is an element for improving the corrosion resistance of a base material. However, when 1.5% of Mo is added, a ferrite phase is generated in the welded portion to cause sensitization and sufficient corrosion resistance of the welded portion can not be obtained because it is a strong ferrite generating element.

As a ferritic stainless steel excellent in corrosion resistance of a welded portion, for example, Patent Document 3 discloses a ferritic stainless steel excellent in corrosion resistance, and Patent Document 4 discloses a ferritic stainless steel excellent in corrosion resistance of a weld portion with an austenitic stainless steel. Respectively. All of these starting examples require the addition of 0.1% or more Nb, and the recrystallization temperature is high. For this reason, there is a problem that a functional product can not be manufactured at low cost by using an annealing line of a general carbon steel.

Japanese Patent Publication No. 2842787 (JP-A-8-10823) Japanese Laid-Open Patent Publication No. 51-88413 Japanese Patent Application Laid-Open No. 2007-270290 Japanese Laid-Open Patent Publication No. 2010-202916

When the occurrence of sensitization is suppressed by simply increasing Ti or Nb according to the conventional art, increase of surface defects due to TiN inclusions and increase of Nb which is solid solution form coarse Nb precipitates at welds So that problems such as weld crack are generated.

Therefore, in the present invention, it is impossible to perform sufficient gas shielding due to the shape of the welding member and the like due to the welding of the ferritic stainless steel. Therefore, welding conditions in which oxygen penetrates into the melting paper and a temper color (oxidation film) It is an object of the present invention to provide a ferritic stainless steel having excellent corrosion resistance in a welding condition such that nitrogen is invaded to cause sensitization and a welding condition in which nitrogen enters the weld bead from a welded mating member.

In order to solve the above problems, the inventors of the present invention have found that the effect of various additive elements on the relationship between the generation of temper color due to oxygen penetration in TIG welding and the corrosion resistance and the relationship between the occurrence of sensitization due to nitrogen penetration and corrosion resistance Exemplary researches were conducted using a steel in which Si, Al, Ti, Nb and V were widely changed based on 18.0 to 19.0 mass% Cr-0.15 mass% Mn-0.1 mass% Ni-0.35 mass% Cu.

As a result, the relationship between the change of the corrosion resistance due to the occurrence of the temper color due to oxygen penetration and the various additive elements is expressed by Si + 1.5Al + 1.2Ti (hereinafter referred to as O _X value. Mass%)), and it was found that the corrosion resistance was improved when the O _X value was 0.40 or more. If Si, Al and Ti are added so as to have an O _X value of 0.40 or more, the temper color is not an oxide film mainly composed of a Cr-based oxide produced in a conventional steel, but a dense and protective This is because the lowering of the base Cr concentration due to the growth of the temper color is suppressed. However, it has also become apparent that the addition of Si, Al, and Ti to the extent that the O _x value exceeds 2.4 causes an increase in the crystallinity of the oxide film, deteriorates the effect of inhibiting permeation of ions and the like, and degrades the corrosion resistance again.

The relationship between the behavior of sensitization by nitrogen intrusion and the various additive elements was investigated using a steel having an O _X value of 0.65 to 0.70. As a result, the reactivation rate defined by JIS G0580 (1986) 1.2 Nb + 1.7 Ti + V + 2.2 Al (hereinafter referred to as N _tr value), and the symbol of the element is the content of each element ( Mass%)), and found that when the N _tr value is 0.60 or more, the reactivation rate becomes 0.01% or less. That is, by setting the N _tr value to be not less than 0.60, nitrogen penetration from the atmosphere, or nitrogen penetration into a welded portion by welding with an austenitic stainless steel having a large nitrogen content, Good corrosion resistance can be obtained even under the welding conditions to be generated.

It is also assumed that the coefficient of each element for obtaining the values of O _x and N _tr is proportional to the affinity between the element and oxygen or nitrogen.

The present invention has been made based on the above-described recognition, and the gist of the present invention is as follows.

[1] A ferritic stainless steel comprising, by mass%, 0.001 to 0.030% of C, 0.05 to 0.30% of Si, 0.05 to 0.50% of Mn, 0.05 to 0.5% of P, 0.01 to 0.01% of S, 18.0 to 19.0% , Ti: 0.05 to 0.50%, Nb: 0.002 to 0.05%, V: 0.01 to 0.50% , And the following formulas (1) and (2) are satisfied, and the balance of Fe and unavoidable impurities.

0.40? Si + 1.5? A1 + 1.2? Ti? 2.4 (1)

0.60? 1.2 Nb + 1.7 Ti + V + 2.2 Al (2)

The symbol of the element in the formula represents the content (mass%) of each element.

The steel sheet according to any one of the above items [1] to [4], further comprising, by mass%, Zr: 0.01 to 0.50%, W: 0.01 to 0.20%, REM: 0.001 to 0.10%, Co: 0.01 to 0.20%, B: 0.0002 to 0.010% % Based on the total weight of the ferritic stainless steel according to [1].

[3] The ferritic stainless steel according to [1] or [2], further containing 0.05 to 0.30% of Sb by mass%.

According to the present invention, a ferritic stainless steel having excellent corrosion resistance can be obtained even in a welding condition in which oxygen or nitrogen from the atmosphere and nitrogen or carbon from the welded member enter the weld bead.

(Mode for carrying out the invention)

The reasons for limiting each constituent requirement of the present invention will be described below.

1. About composition

First, the reason why the steel composition composition of the present invention is defined will be described. The term "%" means "% by mass".

C: 0.001 to 0.030%

The greater the amount of C, the higher the strength, and the smaller the C, the better the workability. In order to obtain sufficient strength, it is necessary to contain 0.001% or more. However, if it exceeds 0.030%, deterioration of workability becomes remarkable and corrosion resistance due to local Cr deficiency due to precipitation of Cr carbide tends to decrease. Therefore, the amount of C is set in the range of 0.001 to 0.030%. However, the lower C content is preferable from the viewpoint of corrosion resistance and workability, but the C content is extremely lowered because it takes time to refine and is not preferable for production, and therefore it is preferably in the range of 0.003 to 0.018%. And more preferably 0.005 to 0.012%.

Mn: 0.05 to 0.50%

Mn is an element inevitably included. If the Mn content exceeds 0.50%, precipitation of MnS as a starting point of corrosion is promoted, and corrosion resistance is lowered. Therefore, the amount of Mn should be 0.50% or less. On the other hand, if the amount of Mn is reduced to less than 0.05%, the production cost is remarkably increased. Therefore, the amount of Mn is set in the range of 0.05 to 0.50%. And preferably 0.05 to 0.40%. And more preferably in the range of 0.05 to 0.35%.

P: not more than 0.05%

P is an element which is inevitably included in the steel, and an excessive content thereof deteriorates the weldability and makes it easy to cause grain boundary corrosion. This tendency is remarkable with the content exceeding 0.05%. Therefore, the P amount is set to 0.05% or less. And preferably 0.03% or less.

S: not more than 0.01%

S is an element inevitably contained in steel as in P, and corrosion resistance is lowered by the content exceeding 0.01%. Therefore, the amount of S should be 0.01% or less. It is preferably 0.008% or less.

Cr: 18.0 to 19.0%

Cr is the most important element to ensure corrosion resistance. If it is less than 18.0%, sufficient corrosion resistance can not be obtained at the weld bead and the periphery thereof where Cr of the surface layer is reduced due to oxidation by welding. Especially, in the case of welding different kinds of steel with an austenitic stainless steel such as SUS304, the sensitization is further promoted by the penetration of nitrogen. In the case of less than 18.0%, the passivation becomes unstable, and the relationship between the N _tr value and the reactivation rate, which will be described later, collapses and corrosion resistance is reduced due to sensitization. On the other hand, if the content is more than 19.0%, the dissolution rate of the substrate is lowered at pickling time. Therefore, in the high-speed pickling method using an acid pickling line of carbon steel as disclosed in Patent Document 1, A scale residue may be generated at the end of the substrate. Therefore, the amount of Cr is set in the range of 18.0 to 19.0%. Preferably, it ranges from 18.0 to 18.7%. And more preferably in the range of 18.3 to 18.7%.

Ni: not less than 0.05% and not more than 0.50%

Ni is an element that improves the corrosion resistance of stainless steel and is an element that inhibits the progress of corrosion in a corrosive environment where active dissolution occurs because a passive film can not be formed. Further, Ni is a strong austenite generating element and has the effect of suppressing the ferrite formation in the welded portion and suppressing the sensitization due to precipitation of Cr carbonitride. These effects are obtained by the content of 0.05% or more. However, when the content is 0.50% or more, the susceptibility to stress corrosion cracking is increased in addition to the deterioration of workability. Further, because Ni is an expensive element, it causes an increase in manufacturing cost. Therefore, the amount of Ni is set in a range of 0.05% or more and less than 0.50%. And preferably in the range of 0.10 to 0.30%. And more preferably in the range of 0.15 to 0.25%.

Cu: 0.30 to 0.60%

Cu is an element which improves the corrosion resistance, and is an element particularly effective for improving the corrosion resistance of a base material and a welded portion in the case of adhering a droplet in an aqueous solution or a weak acid. Further, Cu is an austenite-generating element strong as Ni, and has the effect of suppressing the ferrite formation in the welded portion and suppressing the sensitization due to precipitation of Cr carbonitride. These effects are obtained with a content of 0.30% or more. On the other hand, when the content is more than 0.60%, the hot workability is deteriorated and an oxide of Cu origin called red scale at the time of hot rolling is generated on the surface of the steel sheet to cause surface defects. Therefore, the amount of Cu is set in the range of 0.30 to 0.60%. And preferably in the range of 0.30 to 0.50%. And more preferably in the range of 0.35 to 0.45%.

N: 0.001 to 0.030%

When the N content is high, the strength is improved, and the smaller the N content, the better the workability. In order to obtain sufficient strength, the content is preferably 0.001% or more, but when the content exceeds 0.030%, the elongation is remarkably lowered and the corrosion resistance is promoted by promoting the precipitation of Cr nitride, which is not preferable . Therefore, the amount of N is set in the range of 0.001 to 0.030%. From the viewpoint of corrosion resistance, N is preferably as low as possible. However, in order to reduce the amount of N, it is necessary to increase the refining time and increase the production cost and lower the productivity. Therefore, the N content is preferably in the range of 0.003 to 0.030% to be. And more preferably in the range of 0.003 to 0.015%. And more preferably 0.005 to 0.010%.

Si + 1.5Al + 1.2Ti (O _X value): 0.40 to 2.4

The symbol of the element in the formula represents the content of each element.

Si, Al and Ti are very important elements in the present invention. All of these three elements have strong affinity with oxygen. Therefore, when the stainless steel to which these elements are added is oxidized, a coating of an oxide mainly composed of Si, Al, and Ti is formed on the surface of the steel sheet. Since the film of this oxide is dense and highly protective, the deterioration of the corrosion resistance caused by the Cr concentration in the base material due to the oxidation of Cr is suppressed. This effect is obtained when the O _X value is 0.40 or more. However, when the value of O _X exceeds 2.4, the crystallinity of the oxide film becomes high, and the effect of suppressing the permeation of metal ions and the like is lowered, so that the corrosion resistance is lowered again. Therefore, the value of O _X is in the range of 0.40 to 2.4. Preferably in the range of 0.40 to 1.8. More preferably in the range of 0.50 to 1.5.

1.2 Nb + 1.7 Ti + V + 2.2 Al (N _tr value): 0.60 or more

The sensitization in the welded part is caused by the fact that nitrogen entering from the atmosphere into the melting paper or nitrogen entering from the welding counterpart forms nitride and precipitates out of Cr and local Cr deficient area is generated. When Nb, Ti, V, and Al having a larger affinity with N than that of Cr are added, nitrogen is precipitated as a nitride of these four elements other than Cr. Therefore, generation of Cr-depleted regions can be suppressed and the corrosion resistance of the welded portion can be improved. This effect is obtained when the N _tr value is greater than or equal to 0.60. More preferably, it is 0.80 or more.

When the N _tr value exceeds 4.00, surface defects due to Ti-based or Al-based inclusions are generated. Therefore, the upper limit value is set to 4.00. Preferably 2.50 or less.

Si, Al, Ti, Nb, and V are added so as to satisfy the above-mentioned appropriate range of the O _X value and the N _tr value. In the present invention, the addition amount of each element is further defined for the following reason.

Si: 0.05 to 0.30%

As described above, Si is an element that enhances the corrosion resistance of the welded portion by enhancing the protection of the oxide film by being concentrated with Al or Ti in a temper color formed by welding. This effect can be obtained by the content of 0.05% or more. However, when Si is contained in an amount exceeding 0.30%, the rolling load in the hot rolling step increases and in the annealing step, the deterioration of the acid cleaning property due to the formation of the Si-enriched layer in the surface layer of the steel sheet occurs, And an increase in the manufacturing cost. Therefore, the amount of Si is set in the range of 0.05 to 0.30%. And preferably in the range of 0.05 to 0.25%. And more preferably in the range of 0.08 to 0.20%.

Al: 0.10 to 1.50%

Al is an element which, like Si, is enriched in temper color formed by welding together with Si and Ti, thereby improving the corrosion resistance of the welded portion. Further, since Al has stronger affinity than nitrogen than Cr, Al has an effect of suppressing sensitization by precipitating nitrogen as Al nitride instead of Cr nitride when nitrogen is incorporated into the welded portion. Al is also an element useful for deoxidation in the steelmaking process. These effects are obtained with a content of 0.10% or more. However, when Al is contained in excess of 1.50%, the ferrite crystal grains are coarse, and the workability and manufacturability are lowered. Therefore, the Al content is in the range of 0.10 to 1.50%. Preferably, it ranges from 0.12 to 0.80%. And more preferably in the range of 0.15 to 0.50%.

Ti: 0.05 to 0.50%

Ti, like Si and Al, is an element that is concentrated to a temper color formed by welding and improves the protection of the oxide film. Ti is also an element that binds preferentially to C and N to suppress deterioration of corrosion resistance due to sensitization due to precipitation of Cr carbonitride. These effects are obtained by adding 0.05% or more. However, if it is added in an amount exceeding 0.50%, a coarse Ti carbonitride is produced, which causes surface defects, which is not preferable. Therefore, the amount of Ti is set in the range of 0.05 to 0.50%. Preferably, it ranges from 0.10 to 0.40%. And more preferably in the range of 0.15 to 0.35%.

Nb: 0.002 to 0.050%

Nb is an element that binds preferentially to C and N and suppresses deterioration of corrosion resistance due to sensitization due to precipitation of Cr carbonitride. This effect is obtained with a content of 0.002% or more. On the other hand, Nb is an element for raising the recrystallization temperature. If the content exceeds about 0.050%, the annealing temperature required for recrystallization becomes high, and therefore it is difficult to produce the functional product by annealing using acidic annealing line and acid cleaning It becomes. Therefore, the amount of Nb is set in the range of 0.002 to 0.050%. And preferably 0.010 to 0.045%. And more preferably 0.015 to 0.040%.

V: 0.01 to 0.50%

V is an element that improves corrosion resistance and processability. When nitrogen enters the welded portion, nitrogen is precipitated as VN to inhibit sensitization. This effect can be obtained with a content of 0.01% or more. However, if it exceeds 0.50%, the workability is lowered. Therefore, the amount of V is set in the range of 0.01 to 0.50%. Preferably, it is in the range of 0.05 to 0.30%. And more preferably in the range of 0.08 to 0.20%.

The remainder is composed of Fe and inevitable impurities. However, as the inevitable impurities, Ca: 0.0020% or less can be allowed.

Further, the following elements may be contained for the purpose of suppressing the sensitization of the weld bead and improving the corrosion resistance.

Zr: 0.01 to 0.50%

Zr has an effect of inhibiting sensitization by binding with C and N. This effect is obtained by the content of 0.01% or more, but if it exceeds 0.50%, the workability is deteriorated. Further, since Zr is an expensive element, excessive addition causes undesirable increase in production cost. Therefore, in the case of containing Zr, it is preferable that the content is in the range of 0.01 to 0.50%. And more preferably in the range of 0.05 to 0.35%.

W: 0.01 to 0.20%

W has the same effect as Mo for improving corrosion resistance. This effect is obtained by the content of 0.01% or more, but if it exceeds 0.20%, the strength is increased and the productivity is lowered due to an increase in the rolling load and the like, which is not preferable. Therefore, in the case of containing W, the content is preferably in the range of 0.01 to 0.20%. And more preferably in the range of 0.05 to 0.15%.

REM: 0.001 to 0.10%

The REM has an effect of improving the oxidation resistance and is effective in suppressing the formation of the Cr-depleted region immediately below the temper color of the welded portion by suppressing the growth rate of the oxidized scale. In order to obtain this effect, a content of not less than 0.001% is required. However, if it is contained in an amount exceeding 0.10%, the composition such as acid cleaning property is lowered. Further, since REM is an expensive element, excessive incorporation is not preferable because it causes an increase in manufacturing cost. Therefore, the content of REM is preferably in the range of 0.001 to 0.10%. And more preferably 0.03 to 0.08%.

Co: 0.01-0.20%

Co is an element that improves toughness. This effect is obtained by the content of 0.01% or more. On the other hand, if it is contained in an amount exceeding 0.20%, the productivity is lowered. Therefore, in the case of containing Co, the amount is preferably in the range of 0.01 to 0.20%. And more preferably in the range of 0.05 to 0.15%.

B: 0.0002 to 0.010%

B is an element which improves secondary working embrittlement. Its effect is obtained by the content of 0.0002% or more. However, if it exceeds 0.010%, excessive ductility is caused by solid solution strengthening. Therefore, in the case of containing B, it is preferable that the content is in the range of 0.0002 to 0.010%. And more preferably in the range of 0.0002 to 0.007%. And more preferably in the range of 0.0003 to 0.003%.

Mo: 0.01 to 1.0%

Mo is an element that improves the corrosion resistance of stainless steel by promoting repassivation of the passive film when the Cr content is 18% or more. This effect is obtained by the content of 0.01% or more. However, if the content is more than 1.0%, the rolling load becomes large, the composition is reduced, and the steel sheet strength excessively increases. Also, since Mo is an expensive element, the incorporation of a large amount increases the manufacturing cost. Therefore, in the case where the Mo content is contained, it is preferable to be in the range of 0.01 to 1.0%. And more preferably in the range of 0.05 to 0.5%.

Further, Sb may be contained as a selective element for the purpose of stabilizing nitrogen.

Sb: 0.05 to 0.30%

Sb, like Al, has an effect of capturing N contained in the atmosphere when the gas shield of TIG welding is insufficient, and particularly when applied to a structure having a complicated shape in which it is difficult to perform sufficient gas shielding, It is an element. This effect is obtained by the content of 0.05% or more, but if it exceeds 0.30%, nonmetallic inclusions are produced in the steelmaking process, and the surface quality is deteriorated. Further, toughness of the hot-rolled sheet is deteriorated. Therefore, in the case of containing Sb, it is preferable that the content is in the range of 0.05 to 0.30%. And more preferably in the range of 0.05 to 0.15%.

2. Manufacturing conditions

Next, a suitable manufacturing method of the steel of the present invention will be described. The molten steel of the above-mentioned suitable composition may be dissolved by a known method such as a converter, an electric furnace or a vacuum melting furnace, and the molten steel may be subjected to continuous casting or ingoting, (Slab). The slab is hot-rolled directly from the cast steel at a temperature of 1100 to 1250 占 폚 for 1 to 24 hours, without heating, to obtain a hot-rolled steel sheet.

Normally, the hot-rolled sheet is annealed at 800 to 1100 ° C for 1 to 10 minutes, but annealing of the hot-rolled sheet may be omitted depending on the application. Subsequently, the hot-rolled sheet is washed with acid, cold-rolled to obtain a cold-rolled sheet, and subjected to recrystallization annealing and pickling to obtain a product.

The cold rolling is preferably carried out at a reduction ratio of 50% or more in order to secure elongation properties, bending properties, press formability and good shape. The recrystallization annealing of the cold-rolled sheet is preferably performed at 800 to 950 占 폚 in order to obtain good mechanical properties in the case of surface finishing of JIS G 0203 and Finish No. 2B in general, and good surface properties by pickling .

However, in the case of a so-called functional product (tandem cold rolling-steel sheet produced by the continuous annealing process), for example, a low-cost product using a continuous annealing acid cleaning line of carbon steel as disclosed in Patent Document 1 Most preferably, the annealing temperature is 800 to 900 占 폚. In addition, it is effective to perform BA annealing (bright annealing) for finishing the member of a portion requiring more gloss. Further, in order to further improve the surface property after cold rolling and after processing, polishing or the like may be performed.

Example 1

Hereinafter, the present invention will be described in more detail based on examples.

Stainless steels having the chemical compositions shown in Tables 1-1 to 1-4 were dissolved in a 50 kg small vacuum melting furnace. The values of O _X and N _{tr in} Tables 1-2 and 1-4 are defined as Si + 1.5Al + 1.2Ti and 1.2Nb + 1.7Ti + V + 2.2Al, respectively (note that the symbol for the element is the content Mass%)). These slabs were heated to 1150 占 폚 in a purged furnace by Ar gas, and then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 3.5 mm. Subsequently, the hot-rolled sheet was annealed at 950 ° C for 1 minute, and then the surface thereof was subjected to shot-blast treatment using glass beads. Then, the sheet was subjected to shot-blast treatment at a temperature of 80 ° C for 120 seconds After the immersion, the substrate was immersed in a mixed acid consisting of 15 mass% acetic acid and 3 mass% hydrofluoric acid at a temperature of 55 deg. C for 60 seconds to carry out pickling and descaling.

The sheet was subjected to cold rolling to a sheet thickness of 0.8 mm and recrystallization annealing at 900 캜 for 1 minute in a weak reducing atmosphere (hydrogen: 5 vol%, nitrogen: 95 vol%, dew point: -40 캜) &Lt; / RTI > This cold annealing plate was subjected to a high-speed descaling process in which electrolysis (2 seconds at 10 A / dm 2) was performed twice in a mixed acid solution composed of 15 mass% acetic acid and 0.5 mass% hydrochloric acid at a temperature of 50 deg. C, &Lt; / RTI > Tensile strength (TS) and elongation after fracture of 0.2% strength (YS), tensile strength (TS) and elongation after fracture were measured in accordance with JIS Z2201 and tensile test was carried out in accordance with JIS Z2201, (El) were measured.

[Table 1-1]

[Table 1-2]

[Table 1-3]

[Table 1-4]

Butt TIG welding was performed on the cold-rolled annealing pickling top plate produced and a commercially available cold-rolled sheet of SUS304 (C: 0.07 mass%, N: 0.05 mass%). The welding current was 90 A and the welding speed was 60 cm / min. The shield gas was assumed to be an atmosphere of nitrogen or oxygen, and argon gas containing 8 vol% of nitrogen and 2 vol% of oxygen was used at 15 L / min. The width of the obtained weld bead on the measurement side was approximately 3 mm.

A 20 mm square test piece containing the manufactured weld bead was collected and covered with a sealing material to leave a measurement surface of 10 mm square and a 3.5 mass% NaCl solution at 30 占 폚 The pitting potential was measured. No polishing or passivating treatment of the test piece was carried out, but the other measurement methods were in accordance with JIS G 0577 (2005).

Further, a test piece of 60 x 80 mm was obtained by positioning the weld bead parallel to the long side at the center of the short side. The surface of the test piece was polished with 600 abrasive paper and subjected to a neutral salt spray cyclic corrosion test for 5 cycles to examine the corrosion resistance. The salt water spraying cycle test was carried out in accordance with JIS H 8502 and sprayed with 5% NaCl (35 ° C, 2 hr) → drying (60 ° C, 4hr, relative humidity 20-30%) → wet (40 ° C, 2hr, ) As one cycle.

These evaluations were carried out, and it was found that there was no occurrence of corrosion by the formal electric potential of the base material of 150 mV vs SCE, the welding electric potential of the weld bead of 0 mV vs SCE or more, the neutral salt spray cycle test, the elongation at break of 25% It was judged that the predetermined material provided by the present invention was obtained when the surface property was good.

The evaluation results are shown in Tables 2-1 and 2-2.

[Table 2-1]

[Table 2-2]

The welded beads of Nos. 1 to 22 satisfying the requirements of the present invention are all 0 mV vs SCE or more, corrosion is not caused by the neutral salt spray cycle test, and the welds with the austenitic stainless steels have sufficient corrosion resistance Respectively. Further, the elongation at break by the tensile test was all at least 25%, and no surface defects were found.

However, the contents of the individual elements, but is within the scope of the invention, _X O value, which of the N _tr value, or the No.23, 24, 31 and 32, the value of _X O on both sides is less than the range of the present invention In No. 25, which exceeded the scope of the invention, the formal potential of the base material of 150 mV vs SCE or more was obtained, but since the formal potential of the weld bead is less than 0 mV vs SCE, corrosion of the weld bead during the neutral salt spray test So that a predetermined welded portion corrosion resistance was not obtained.

In No. 26 in which the Si content exceeds the range of the present invention, an oxide scale residue is generated after the pickling process due to the formation of a strong oxide scale layer in which Si is concentrated in the surface layer portion of the steel sheet during annealing, During the test corrosion occurred with the oxidation scale as the starting point. Similarly, in No. 27 in which Al was less than the range of the present invention and No. 28 in which Cu was outside the range of the present invention, corrosion occurred during the neutral salt spray cycle test despite the predetermined O _X value and N _tr value did.

In Nos. 29 to 31 where the content of Al, Ti or Cu exceeded the range of the present invention, a large amount of surface defects occurred after hot rolling or cold rolling, and proper surface properties could not be obtained.

From these results, it was confirmed that it is necessary to adjust not only the content of each element but also the O _x value and the N _tr value within the range of the present invention, in order to obtain the predetermined material characteristics provided by the present invention without surface defects.

The ferritic stainless steel obtained in the present invention is suitable for application to the construction of the structure by welding, for example, automotive exhaust system materials such as mufflers, construction materials such as dryers and ventilators, and ducts.

Claims

0.001 to 0.030% of C, 0.05 to 0.30% of Si, 0.05 to 0.50% of Mn, 0.05 to 0.50% of P, 0.05 or less of P, 0.01 to 0.01% of S, 18.0 to 19.0% of Cr, % Of Al, 0.10 to 1.50% of Al, 0.05 to 0.50% of Ti, 0.002 to 0.050% of Nb, and 0.01 to 0.50% of V, A ferritic stainless steel characterized by satisfying the expressions (1) and (2) and the remainder being Fe and unavoidable impurities.
0.40? Si + 1.5? A1 + 1.2? Ti? 2.4 ... (One)
0.60? 1.2 Nb + 1.7 Ti + V + 2.2 Al ... (2)
The symbol of the element in the formula represents the content (mass%) of each element.

The method according to claim 1,
In addition, it is preferable that Zr is 0.01 to 0.50%, W is 0.01 to 0.20%, REM is 0.001 to 0.10%, Co is 0.01 to 0.20%, B is 0.0002 to 0.010%, and Mo is 0.01 to 1.0% Wherein the ferritic stainless steel contains at least one selected from the group consisting of iron and iron.

3. The method according to claim 1 or 2,
The ferritic stainless steel further contains, by mass%, Sb: 0.05 to 0.30%.