JP5903881B2 - Ferritic stainless steel with excellent corrosion resistance of welds - Google Patents

Description

  The present invention relates to a ferritic stainless steel that is unlikely to deteriorate in corrosion resistance due to penetration of nitrogen and oxygen from a welding shield gas into a weld bead.

  Ferritic stainless steel has high cost performance that does not need to add a large amount of expensive Ni or Mn to ensure corrosion resistance compared to austenitic stainless steel, and has a good thermal conductivity and a low thermal expansion coefficient. In addition, it has been applied to a wide range of applications such as automobile exhaust systems, building materials such as roofs and fittings, and water-related materials such as kitchens, water storage and hot water storage tanks, etc.

  In producing these structures, a stainless steel plate is often cut and formed into an appropriate shape and then joined by welding. However, since ferritic stainless steel has a small solid solubility limit of C and N, sensitization in which Cr carbonitride is formed in the welded portion due to melting and solidification by welding to form a Cr-deficient layer and corrosion resistance is lowered. The phenomenon called is easy to occur. For this reason, usually, welding is often performed using an inert gas such as Ar as a shielding gas.

  Further, conventionally, a method has been adopted in which Ti or Nb having a higher affinity with carbon nitrogen than Cr is added to suppress the formation of Cr carbonitride and suppress the occurrence of sensitization. For example, Patent Document 1 discloses steel in which intergranular corrosion resistance of ferritic stainless steel is improved by adding Ti and Nb in a composite manner.

  However, in recent years, as the shape of the welded parts has become complicated, it is not possible to perform a sufficient gas shield at the time of welding, and welding under incomplete conditions in which nitrogen and oxygen in the air are mixed in the shield gas Under these welding conditions, the penetration of nitrogen from the shield gas into the weld bead makes the welded portion more susceptible to sensitization. In addition, a thin oxide film is formed in a weld called a temper collar even when oxygen enters. Therefore, the conventional ferritic stainless steel disclosed in Patent Document 1 has a problem that it is difficult to ensure corrosion resistance.

  Examples of ferritic stainless steels having excellent corrosion resistance of welded parts include, for example, Patent Document 2 discloses ferritic stainless steels having excellent corrosion resistance of welded parts, and Patent Document 3 discloses ferritic stainless steels having excellent corrosion resistance of weld gaps. However, Patent Document 4 discloses a ferritic stainless steel excellent in corrosion resistance of a welded portion with austenitic stainless steel. However, even in these ferritic stainless steels, sufficient corrosion resistance cannot always be ensured under welding conditions in which nitrogen and oxygen enter the molten pool from the shielding gas.

JP 51-88413 A JP 2007-270290 A JP 2009-161836 A JP 2010-202916 A Japanese Patent No. 2842787

  In order to solve this problem, it is possible to suppress the occurrence of sensitization by simply increasing Ti or Nb in accordance with the conventional idea, but then, surface defects caused by TiN inclusions called Ti stringers This is not an optimal policy because Nb increases and solute Nb precipitates as coarse Nb precipitates at the weld and causes problems such as weld cracks.

  Therefore, in the present invention, since sufficient gas shielding cannot be performed due to the shape of the welded member in the welding of ferritic stainless steel, nitrogen and oxygen are mixed in the shielding gas, and the nitrogen content of the weld bead increases. Providing ferritic stainless steel with excellent corrosion resistance and good weldability in welding conditions where sensitization occurs and welding conditions in which oxygen is mixed and a temper collar is generated in the weld bead With the goal.

  In order to solve the above-mentioned problems, the present inventors have studied the effects of various elements on the behavior of nitrogen penetration into the weld bead and the suppression of sensitization, and the properties and corrosion resistance of the oxide film formed by oxygen penetration into the weld bead. We conducted earnest research on the relationship.

  First, FIG. Using the ferritic stainless steel shown in Fig. 1, the oxygen concentration of the Ar-based shield gas was varied in the range of 0 to 2 vol%, and TIG welding of a bead-on plate (welding current 90 A, welding speed 60 cm / min, plate thickness 0. 8mm, front shield gas flow rate 15L / min, back shield gas flow rate 10L / min), and after finely polishing the temper color (oxide film thickness) section on the weld bead, the oxide film was observed by high resolution SEM. The result of measuring the thickness is shown.

  When the oxygen is mixed into the surface shield gas, the temper collar thickness of the weld bead becomes almost constant after increasing in proportion to the increase in the oxygen concentration of the shield gas. The temper color of the weld bead showed the same behavior as that of the surface even when oxygen was mixed into the back shield gas. Further, according to the analysis of Auger and the like, this oxide film was an oxide in which enrichment of these elements was observed when a large amount of Al, Si and Ti elements were added to the steel.

  This is because the oxidation of the shield gas due to oxygen mixing does not occur when oxygen enters the molten pool, but occurs at the surface of the molten pool. Therefore, even if oxygen does not diffuse into the molten pool, that is, oxidation occurs even when the bead surface is only in contact with oxygen, oxidation occurs even if oxygen in the back shield gas is in gentle contact. Conceivable. In addition, the oxide film thickness becomes constant at a certain oxygen concentration or higher because the rate-determining step of oxidation is due to the diffusion of oxygen to the weld surface, and various elements in steel move outward in the oxide (from the side of the railway This is thought to be due to the transition to the passing speed (to the reaction surface). From the above, it was found that the temper color thickness of the weld bead grew regardless of the front bead and the back bead. Formation of the temper collar (oxide film) of the welded portion causes Cr deficiency as in the case of sensitization and decreases the corrosion resistance.

  Next, the sensitization, which is the main cause of deterioration of the corrosion resistance of the welded portion, was examined. The effect of nitrogen concentration of shielding gas on nitrogen content of weld bead was investigated. No. 1 in Table 1 The ferritic stainless steel shown in Fig. 1 is used, and the nitrogen concentration of the Ar-based shield gas is varied in the range of 0 to 2 vol%, so that TIG welding of a bead-on plate (welding current 90A, welding speed 60 cm / min, plate thickness 0) 8 mm, front shield gas flow rate 15 L / min, back shield gas flow rate 10 L / min), and the nitrogen content of the weld bead was measured.

  The results are shown in FIG. The nitrogen content of the weld bead increased in proportion to the increase in nitrogen concentration in the shield gas when nitrogen was mixed in the front shield gas, but the nitrogen concentration in the shield gas increased when nitrogen was mixed into the back shield gas. Even so, the nitrogen content of the weld bead hardly changed. This is considered to be due to the fact that the front shield gas is constantly blown from the nozzle toward the molten pool, whereas the back shield gas is only in gentle contact. Sensitization of the weld bead became more pronounced with an increase in nitrogen entering the weld bead. From this, it became clear that the sensitization of the weld bead is strongly influenced by the mixing of nitrogen into the surface gas shield.

  Furthermore, the influence of various elements on the sensitization was evaluated under the welding conditions in which the sensitization of the weld bead was caused by nitrogen intrusion from the shielding gas. Commercially available SUS304 steel with a thickness of 0.8 mm using various ferritic stainless steels based on 18.0 to 19.0% by mass Cr-containing steel using Ar gas having a nitrogen concentration of 2 vol% as the front shield gas ( C concentration: 0.05% by mass, N concentration: 0.07% by mass) After butt TIG welding and polishing the weld bead, the reactivation rate was measured in accordance with JIS G 0580 (2003). .

The results are shown in FIG. Logarithm of the reactivation rate (hereinafter referred to as _{N tr} value. Note that each element symbol in the formula represents the content of each element (mass%)) Nb + 1.3Ti + 0.9V + 0.2Al decreased in proportion to the. The smaller the value of the reactivation rate, the smaller the degree of sensitization. When the reactivation rate is 0.01% or less, it means that there is almost no sensitization. When the N _tr value is 0.6 or more and the reactivation rate is 0.01% or less, the impurities N, C, and N of the shielding gas have a large solid solubility limit. It became clear that the ferritic stainless steel exhibits good corrosion resistance even under welding conditions where the weld bead is sensitized. However, although the relationship between the component and the reactivation rate is maintained up to 18% or more of Cr, the proportional relationship between the logarithm of the N _tr value and the reactivation rate gradually collapses when the Cr concentration is less than 18%. Of Nb, Ti, V, and Al, the reactivation rate did not become 0.01% or less unless Nb was increased.

Furthermore, the effect of various elements on the sensitization under the welding conditions where sensitization occurs and the corrosion resistance of the temper color was evaluated by pitting potential measurement. The 18-19 wt% Cr-containing steel ferritic stainless steels of various _{N tr} values based, nitrogen concentration of 8 vol% in Table shielding gas, commercial plate thickness using 2 vol% of oxygen concentration in the Ar gas 0. Butt TIG welding with 8mm SUS304 steel (C concentration: 0.05 mass%, N concentration 0.07 mass%) without removing the temper collar formed on the front side (torch side) of the weld bead In addition, the pitting potential was measured in a 3.5 mass% NaCl solution at 30 ° C.

The results are shown in FIG. When the N _tr value is 0.43, the pitting potential is −225 to −175 mV regardless of Si, Al, and Ti, and the corrosion resistance is low. Further, pitting corrosion at this time was confirmed in the HAZ part slightly outside the bead of the welded part temper collar part.

On the other hand, when the N _tr value is 0.63, Si + Al + Ti (hereinafter referred to as O _x value. The element symbol in the formula represents the content (% by mass) of each element) in the range of 0.6 to 1.8. Thus, the pitting potential became 0 mV or more, and the corrosion resistance was improved. This is because Si, Al, and Ti are concentrated in the temper collar to form a dense oxide film having good protective properties. In addition, the Cr of the weld bead surface layer is suppressed from being reduced by oxidation, and N _tr It is considered that the main reason is that sensitization of the weld bead portion is suppressed by satisfying the value.

The reduction of Cr by the temper color has a synergistic effect in addition to the reduction of Cr around the Cr carbonitride caused by sensitization by nitrogen intrusion, so that the N _tr value and the O _x value are in appropriate ranges, respectively. It was found that this is necessary to ensure the corrosion resistance of the weld bead under the welding conditions in which nitrogen penetrates from the shielding gas.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] By mass%, C: 0.001 to 0.030%, Si: more than 0.30 to 0.55%, Mn: 0.05 to 0.50%, P: 0.05% or less, S : 0.01% or less, Cr: 18.0 to 19.0%, Ni: 0.05 to less than 0.50%, Mo: 1.0% or less, Al: 0.10 to 1.50%, V : 0.02 to 0.50%, Nb: 0.002 to 0.050%, Ti: 0.05 to 0.50%, Cu: 0.30 to 0.60%, N: 0.001 to 0 A ferritic stainless steel excellent in corrosion resistance of welds, characterized by containing 0.030%, satisfying the following formula (1), and the balance being Fe and inevitable impurities.
0.6 ≦ Si + Al + Ti ≦ 1.8 (1)
In addition, the element symbol in a formula represents content (mass%) of each element.

[2] The ferritic stainless steel excellent in the corrosion resistance of the welded portion according to the above [1], further satisfying the following formula (2).
0.60 ≦ Nb + 1.3Ti + 0.9V + 0.2Al (2)
In addition, the element symbol in a formula represents content (mass%) of each element.

  [3] The ferritic stainless steel having excellent corrosion resistance of the welded portion according to the above [1] or [2], further comprising, by mass%, Sb: 0.05 to 0.30% or less .

  [4] Further, by mass, Zr: 1.0% or less, W: 0.2% or less, REM: 0.1% or less, Co: 0.2% or less, B: 0.1% or less, Mg : Ferritic stainless steel excellent in corrosion resistance of welds according to any one of the above [1] to [3], comprising at least one selected from less than 0.0005%.

  According to the present invention, it has excellent corrosion resistance even under welding conditions in which sensitization occurs due to nitrogen penetration from the shield gas into the weld bead and nitrogen penetration from the welding partner material, and also when oxygen enters from the shield gas. Ferritic stainless steel is obtained. Further, the ferritic stainless steel of the present invention is as good in welding work as the conventional steel.

It is a figure which shows the influence of the oxygen concentration of the shielding gas on the oxide film thickness of a weld bead. It is a figure which shows the influence of the nitrogen concentration of the shielding gas on the nitrogen content of a weld bead. It is a figure which shows the influence of an additive element on the reactivation rate of the weld bead with SUS304 in 18-19% Cr steel. It is a figure which shows the influence of the additive element on the pitting corrosion potential of the weld bead with SUS304 in 18-19% Cr steel.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.001 to 0.030%
C is an element inevitably contained in steel. When the amount of C is large, the strength is improved, and when it is small, workability is improved. Addition of 0.001% or more is necessary to obtain sufficient strength, but if added over 0.030%, the workability deteriorates remarkably, and Cr carbide precipitates to local Cr. It tends to cause a decrease in corrosion resistance due to deficiency. Therefore, the C content is in the range of 0.001 to 0.030%. The lower the amount of C, the better the corrosion resistance, but if it is too low, it takes a long time for refining, so the range is preferably 0.003 to 0.018%.

Si: more than 0.30 to 0.55%
Si is an important element in the present invention. It is an important element that concentrates together with Al and Ti on the temper collar formed by welding to improve the protective property of the oxide film and to improve the corrosion resistance of the welded portion. Under welding conditions in which nitrogen and oxygen penetrate from the shield gas, Al and Ti are likely to be combined with the invaded nitrogen and precipitate, so the concentration to the temper color is reduced. Therefore, in the present invention, Si plays a relatively large role in improving the protection of the temper color. The effect is obtained by adding more than 0.30%. However, if the added amount of Si exceeds 0.55%, the workability is significantly lowered and the molding process becomes difficult. Therefore, the Si amount is in the range of more than 0.30% to 0.55%. Preferably, it is in the range of 0.40% to 0.50%.

Mn: 0.05 to 0.50%
Mn is mixed into stainless steel as an inevitable impurity. However, Mn promotes the precipitation of MnS, which is the starting point of corrosion, and lowers the corrosion resistance. Therefore, the content of 0.50% or less is appropriate. Therefore, the amount of Mn is made 0.05 to 0.50% of range. Preferably, it is 0.05 to 0.40% of range.

P: 0.05% or less P is an element inevitably contained in steel. Excessive content decreases weldability and easily causes intergranular corrosion. This tendency becomes remarkable when the content exceeds 0.05%. Therefore, the P content is 0.05% or less. Preferably it is 0.03% or less.

S: 0.01% or less S is an element inevitably contained in steel. However, the content exceeding 0.01% lowers the corrosion resistance. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less.

Cr: 18.0 to 19.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. If it is less than 18%, sufficient corrosion resistance cannot be obtained at or around the weld bead in which Cr on the surface layer decreases due to oxidation by welding. In particular, sensitization is likely to occur particularly in the case of welding of different steel types with SUS304 or the like due to the penetration of nitrogen during welding. Further, if it is less than 18.0%, the passivation becomes unstable, and the relationship between N _tr and the reactivation rate described later is broken. Moreover, if added over 19.0%, dissolution of the iron into the acid at the time of pickling becomes difficult, and in the high-speed pickling method using the carbon steel line as disclosed in Patent Document 5, In some cases, the film cannot be completely removed and a scale residue may occur at the edge of the steel sheet. Therefore, the Cr content is in the range of 18.0% to 19.0%. Preferably, it is 18.3 to 19.0% of range.

Ni: 0.05 to less than 0.50% Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses the progress of corrosion in a corrosive environment where a passive film cannot be formed and active dissolution occurs. The effect can be obtained by adding 0.05% or more. However, addition of 0.50% or more increases the stress corrosion cracking sensitivity in addition to reducing the workability. Furthermore, since Ni is an expensive element, it causes an increase in cost. Therefore, the amount of Ni is set to a range of 0.05 to less than 0.50%. Preferably, it is in the range of 0.10% to 0.30%.

Mo: 1.0% or less Mo is an element that promotes the repassivation of the passive film and improves the corrosion resistance of stainless steel when the Cr content is 18% or more. It is. However, if the amount of Mo added exceeds 1.0%, the strength increases, and the rolling load increases, so the productivity decreases. Moreover, since it is also an expensive element, addition of a large amount causes an increase in cost.
Therefore, the Mo amount is 1.0% or less. Preferably, it is 0.5% or less.

Al: 0.10 to 1.50%
Al is also an important element for the present invention. Al is an element useful for deoxidation, and in the present invention, it is an element that concentrates in a temper collar formed by welding together with Si and Ti and improves the corrosion resistance of the welded portion. In addition, when nitrogen penetrates into the weld bead from the shielding gas, it is an element that has an effect of suppressing the sensitization that Cr and nitrogen are combined and precipitated. This is presumably because Al, which has a higher affinity with nitrogen than Cr, forms nitrogen and AlN that have entered the weld bead from the shield gas and prevents the formation of Cr nitride. This effect is obtained when the added amount of Al is 0.10% or more. However, when it exceeds 1.50%, workability and manufacturability are lowered. Therefore, the Al content is in the range of 0.10% to 1.50%. Preferably, it is 0.12 to 0.50% of range.

V: 0.02-0.50%
V is an element that improves corrosion resistance and workability. In the present invention, when nitrogen enters the weld bead from the shielding gas, V is an element that suppresses sensitization by combining with nitrogen to become VN. The effect is obtained when the amount of V added is 0.02% or more. However, addition exceeding 0.50% conversely decreases the workability. Therefore, the V amount is in the range of 0.02 to 0.50%. Preferably, it is 0.05 to 0.30%.

Nb: 0.002 to 0.050%
Nb is an element that binds preferentially to C and N and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. Therefore, in this invention, it is an important element in order to suppress the sensitization by nitrogen penetration | invasion from shielding gas, The effect is acquired by 0.002% or more. On the other hand, however, the addition of Nb has the effect of increasing the recrystallization temperature during annealing. Therefore, if added over 0.050%, for example, a so-called functional product (car muffler used in a member that does not place importance on the appearance) Material, exhaust system members, etc.), using a carbon steel line in order to reduce costs, annealing is performed at about 850 to 900 ° C., and high-speed pickling using a carbon steel line as disclosed in Patent Document 5 There is a problem that the method cannot be applied.
Therefore, the Nb content is in the range of 0.002 to 0.050%. Preferably, it is 0.003 to 0.010% of range.

Ti: 0.05 to 0.50%
Ti is also an important element for the present invention. Ti is an element that binds preferentially to C and N and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. In the present invention, it is an important element for suppressing sensitization due to nitrogen intrusion from the shielding gas. Further, it is an element that condenses together with Si and Al in the temper collar of the weld and improves the protective properties of the oxide film. But there is. The effect can be obtained by adding 0.05% or more. However, if added over 0.50%, the workability deteriorates and the Ti carbonitride becomes coarse and causes surface defects. Therefore, the Ti amount is in the range of 0.05 to 0.50%. Preferably, it is 0.10 to 0.35% of range.

Cu: 0.30 to 0.60%
Cu is an element that enhances the corrosion resistance, in particular, in an aqueous solution or when weakly acidic water droplets adhere to any amount of Cr. It is an element that enhances the corrosion resistance of the welded portion, in particular, when an aqueous solution or weakly acidic water droplets adhere. This is presumed that Cu dissolves at a certain electrochemical potential in an aqueous solution or weakly acidic water droplets, and Cu reattaches to the base iron to suppress dissolution resistance. However, when Cu is added in excess of 0.60%, hot workability is deteriorated, and a hot-rolled candy-like oxide called red scale is formed on the hot-rolled slab during hot rolling. It can also be a cause. Therefore, the amount of Cu is set to a range of 0.30 to 0.60%. Preferably, it is 0.30 to 0.50% of range.

N: 0.001 to 0.030%
N, like C, is an element inevitably contained in steel. When the N content is large, the strength is improved, and when it is low, the workability is improved. In order to obtain sufficient strength, the content of 0.001% or more is appropriate. However, if the content exceeds 0.030%, the workability deteriorates significantly, and when Cr nitride is precipitated, In order to reduce the corrosion resistance, the N content is in the range of 0.001 to 0.030%. N is preferably as low as possible for corrosion resistance, but if it is too low, it takes time for refining. Therefore, it is preferably in the range of 0.003 to 0.030%. More preferably, it is 0.003 to 0.015% of range.
Si + Al + Ti (O _x value): 0.6 or more and 1.8 or less In addition, the element symbol in a formula represents content (mass%) of each element.

Si, Al, and Ti all have a strong affinity for oxygen, and when stainless steel is oxidized to form an oxide scale, it is concentrated in the lower layer of the oxide scale (base metal side). When all of these elements are contained in stainless steel, the concentrated layer of Si, Al, and Ti formed by complex oxidation of Si, Al, and Ti becomes a dense and protective oxide film. Therefore, compared with the case where content of these elements is low, it becomes an oxide film excellent in corrosion resistance. The effect is obtained when the O _x value is 0.6 or more. As shown in FIG. 4, it is suggested that the deterioration of the corrosion resistance of the welded portion is slowed under the welding conditions in which nitrogen and oxygen enter the welding bead from the shielding gas. On the other hand, when the O _x value exceeds 1.8, the crystallinity of the oxide film increases, and the effect of suppressing the transmission of metal ions and the like decreases. Therefore, as shown in FIG. 4, when the O _x value exceeds 1.8, the corrosion resistance is lowered again. From the above results, the O _x value is 0.6 or more and 1.8 or less. Preferably they are 0.6 or more and 1.4 or less.

The above is the basic chemical component of the present invention, and the balance consists of Fe and unavoidable impurities. However, the N _tr value may be defined from the viewpoint of preventing sensitization of the weld bead. As an inevitable impurity, Ca: 0.0020% or less is acceptable.

Nb + 1.3Ti + 0.9V + 0.2Al (N _tr value): 0.60 or more In addition, the element symbol in a formula represents content (mass%) of each element.
The sensitization of the weld bead handled in the present invention is mainly that nitrogen entering the weld bead from the shielding gas combines with Cr to form Cr nitride, and a local Cr-depleted region is generated. Responsible. In order to suppress this, it is considered that the addition of an element having a greater affinity for N than Cr is effective. Ti and Nb are well known as stabilizing elements for C and N. However, in welding beads under welding conditions in which nitrogen intrudes from the shielding gas, Al and V have a new C and N stabilizing effect this time. Became clear. When Cr is 18% or more, as shown in FIG. 3, the logarithm of the reactivation rate of the weld bead is proportional to N _tr value = Nb + 1.3Ti + 0.9V + 0.2Al with respect to the mass% of each element. The effect is strong in the order of Ti>Nb>V> Al. When the N _tr value is 0.60 or more, the reactivation rate of the weld bead is 0.01% or less, and sensitization hardly occurs. Therefore, the N _tr value is set to 0.60 or more. When the precipitate of the weld bead was observed using SEM, it was confirmed that Al and V were present in combination with Ti and Nb carbonitride. Thus, it is considered that the precipitation of AlN and VN is promoted by using Ti and Nb carbonitrides as nuclei, so that V and Al can more effectively act as a nitrogen stabilizing element.

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

Sb: 0.05-0.30%
Sb, like Al, has an effect of capturing N mixed in from the atmosphere when the gas shield for TIG welding is insufficient, and is an element that should be added in the case of a structure having a complicated shape. However, when Sb is added too much, non-metallic inclusions are generated at the slab stage, and the surface properties deteriorate. Moreover, the toughness of a hot-rolled sheet is also deteriorated. Therefore, when adding Sb, it is preferable to make Sb into 0.03 to 0.30% of range. More preferably, it is 0.05 to 0.15% of range.

  Furthermore, one or more selected from Zr, W, REM, Co, B, and Mg may be added as a selective element for the purpose of suppressing sensitization and improving corrosion resistance.

Zr: 1.0% or less Zr combines with C and N to suppress sensitization, but the effect can be obtained by adding 0.01% or more. However, addition exceeding 1.0% lowers workability and increases the cost because it is a very high element. Therefore, when adding Zr, the amount of Zr is preferably 1.0% or less.

W: 0.2% or less W, like Mo, has an effect of improving corrosion resistance, but the effect can be obtained by adding 0.01% or more. However, addition over 0.2% increases strength and decreases manufacturability. Therefore, when adding W, it is preferable to make W amount into 0.2% or less.

REM: 0.1% or less REM improves oxidation resistance, suppresses the formation of oxide scale, and suppresses the formation of a Cr-deficient region directly under the weld temper collar. The effect is obtained by adding 0.0001% or more. However, addition over 0.1% reduces the productivity such as pickling properties and increases the cost. Therefore, when REM is added, the REM content is preferably 0.1% or less.

Co: 0.2% or less Co is an element that improves toughness, and the effect can be obtained by adding 0.001% or more. However, addition over 0.2% reduces manufacturability. Therefore, when adding Co, the amount of Co is preferably 0.2% or less.

B: 0.1% or less B is an element that improves secondary work brittleness, and the effect can be obtained by adding 0.0001% or more. However, addition exceeding 0.1% causes a decrease in ductility due to solid solution strengthening. Therefore, when adding B, the amount of B is preferably 0.1% or less.

Mg: less than 0.0005% Mg is an impurity mainly mixed from bricks in the converter. Mg becomes the starting point for a wide variety of inclusions, and even if the amount is mixed, it becomes a nucleation site for other inclusions, and it is difficult to dissolve even if annealing is performed, and deteriorates the surface properties of hot rolled sheets and cold rolled sheets. . Therefore, when adding Mg, the amount of Mg is preferably less than 0.0005%. More preferably, it is 0.0003% or less.

2. Production Conditions Next, a preferred production method for the steel of the present invention will be described. The molten steel having the above-mentioned preferred component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and is made into a steel material (slab) by a continuous casting method or an ingot-bundling method. The steel material is then heated at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-rolled without heating to form a hot-rolled sheet.

  The hot-rolled sheet is usually subjected to hot-rolled sheet annealing at 800 to 1100 ° C. for 1 to 10 minutes, but depending on the application, the hot-rolled sheet annealing may be omitted. Then, after pickling the hot-rolled sheet, it is cold-rolled by cold rolling, and then recrystallized and annealed to obtain a product.

  The rolling reduction of the cold rolling is desirably performed at a rolling reduction of 50% or more in view of stretchability, bendability, press formability, and shape correction. In general, the recrystallization annealing of a cold-rolled sheet is performed according to JIS G 0203 surface finish, No. In the case of a 2B finished product, it is preferable to perform annealing at 800 to 950 ° C. from the viewpoint of obtaining good mechanical properties and pickling properties.

  However, in the case of a functional product, production by an inexpensive process using the high-speed pickling (refer to Patent Document 5) of the above-described carbon steel annealing pickling line using a carbon steel line is most preferable, and the annealing temperature at this time Is most preferably performed at 800 to 900 ° C. In addition, it is effective to perform BA annealing for finishing the member where the luster is desired. Further, as described above, it is disadvantageous in terms of cost to further improve the surface properties after cold rolling and after processing, but there is no problem even if polishing is performed.

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

Stainless steel having the composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These steel ingots were heated to 1150 ° C. in a furnace purged with Ar gas and hot-rolled to obtain 3.5 mm thick hot rolled sheets. Next, these hot-rolled sheets were subjected to hot-rolled sheet annealing at 950 ° C. for 1 minute, and then the surface was subjected to shot blasting treatment of glass beads, and then in a 20% by mass sulfuric acid solution at a temperature of 80 ° C. After dipping for 120 seconds, pickling was performed by dipping in a mixed acid composed of 15% by mass nitric acid and 3% by mass hydrofluoric acid at a temperature of 55 ° C. for 60 seconds, and descaling was performed.
Further, it is cold-rolled to a thickness of 0.8 mm, annealed at 900 ° C. for 1 minute in a weak reducing atmosphere (H ₂ : 5 vol%, N ₂ : 95 vol%, dew point −40 ° C.), and cold-rolled annealed plate Got. This cold-rolled annealed plate was subjected to high-speed descaling in which electrolysis (10 A / dm ² × 2 seconds) was performed twice in a solution comprising a temperature of 50 ° C., 15% by mass nitric acid and 0.5% by mass hydrochloric acid, and cold-rolled acid A washed and annealed plate was obtained.
In Table 1, the O _x value is Si + Al + Ti, and the N _tr value is Nb + 1.3Ti + 0.9V + 0.2Al, which are respectively defined (the element symbol in the formula represents the content (mass%) of each element). .

  Butt TIG welding was performed using the produced cold rolled sheet 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. As the shielding gas, Ar gas containing 8 vol% nitrogen and 2 vol% oxygen was used at a flow rate of 15 L / min. The width of the front side weld bead was approximately 3 mm.

A 20 mm square test piece including the prepared weld bead was collected, covered with a sealing material, leaving a 10 mm square measurement surface, and a hole was formed in a 3.5 mass% NaCl solution at 30 ° C. with a temper collar by welding. Eating potential was measured. The specimen was not polished or passivated. The other measurement methods conformed to JIS G 0577 (2005). The measured pitting potential V ′ _c100 is shown in Table 2. In all of the inventive examples, V ′ _c100 was 0 mV or more, whereas in all of the comparative examples, V ′ _c100 was 0 mV or less, _indicating that the corrosion resistance of the inventive examples is excellent.

  In addition, a 60 × 80 mm test piece including a weld bead was sampled, and a neutral salt spray cycle test of JIS H 8502 (1999) was performed using the front side as a test surface. Salt spray cycle test: 1 cycle of spraying 5% NaCl solution (35 ° C, 2h) → drying (60 ° C, 4h, relative humidity 20-30%) → wet (40 ° C, 2h, relative humidity 95% or more) The number of cycles was 15 cycles. After the test, the weld bead was visually checked for corrosion.

  The results are again shown in Table 2. Corrosion was not confirmed in any of the inventive examples, whereas corrosion was confirmed in any of the comparative examples. It turns out that the corrosion resistance of the weld bead of the example of the present invention is excellent. Further, when Nb exceeds the upper limit, hardening of the material is recognized, and it can be seen that it is difficult in a process of manufacturing with an inexpensive carbon steel line (a method of annealing at less than 900 ° C. and thinning the scale in advance).

According to the present invention, in the welding of ferritic stainless steel, sufficient gas shielding cannot be performed due to the shape of the welded member, etc., so that nitrogen and oxygen are mixed into the shielding gas and the nitrogen content of the weld bead increases. Ferritic stainless steel having excellent corrosion resistance can be obtained under welding conditions where sensitization occurs and welding conditions where an oxide film (weld temper color) is generated. The ferritic stainless steel obtained by the present invention is suitable for applications in which structures are produced by welding, for example, automotive exhaust materials such as mufflers, building materials such as fittings, ventilation openings, ducts, etc. is there.

Claims (3)

By mass%, C: 0.001 to 0.030%, Si: more than 0.30 to 0.55%, Mn: 0.05 to 0.50%, P: 0.05% or less, S: 0.00. 01% or less, Cr: 18.0 to 19.0%, Ni: 0.05 to less than 0.50%, Mo: 1.0% or less, Al: 0.10 to 1.50%, V: 0.00. 02 to 0.50%, Nb: 0.002 to 0.050%, Ti: 0.05 to 0.50%, Cu: 0.30 to 0.60%, N: 0.001 to 0.030% A ferritic stainless steel excellent in the corrosion resistance of the welded portion, characterized in that it contains the following formula (1) and the following formula (2) , and the balance consists of Fe and inevitable impurities.
0.6 ≦ Si + Al + Ti ≦ 1.8 (1)
0.60 ≦ Nb + 1.3Ti + 0.9V + 0.2Al (2)
In addition, the element symbol in a formula represents content (mass%) of each element. Moreover, in mass%, Sb: weld highly corrosion resistant ferritic stainless steel according to claim 1, characterized in that it contains 0.05 to 0.30% or less. Further, in terms of mass%, Zr: 1.0% or less, W: 0.2% or less, REM: 0.1% or less, Co: 0.2% or less, B: 0.1% or less, Mg: 0. ferritic stainless steel excellent in corrosion resistance of the welded portion according to claim 1 or 2, characterized in that it contains one or more selected from among less than 0,005%.

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