KR101658872B1 - Ferritic stainless steel - Google Patents

Abstract

The present invention provides a ferritic stainless steel excellent in corrosion resistance and welded cracking resistance of a welded portion. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.001 to 0.030% of C, 0.03 to 0.80% of Si, 0.05 to 0.50% of Mn, 0.05 to 0.50% of P, 0.03% or less of P, 0.01% or less of S, 19.0 to 28.0% , Mo: 0.2 to 3.0%, Al: more than 0.15 to 1.2%, V: 0.02 to 0.50%, Cu: less than 0.1%, Ti: 0.05 to 0.50%, and N: 0.001 to 0.030% %, Satisfying the following formula (1), and the balance of Fe and inevitable impurities.
Nb x P? 0.0005 (1)
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 is applicable to applications in which a structure is manufactured by welding, for example, automobile exhaust system materials such as mufflers, hot-water storage tank materials for electric water heaters, fittings, ventilation ducts, , It is difficult to sensitize the welded portion in the construction materials such as the welded portion and the welded portion of the weld bead, To a ferritic stainless steel in which a weld crack is unlikely to occur.

The ferritic stainless steel is superior to austenitic stainless steel in terms of cost performance against corrosion resistance and heat thermal conductivity and has a small coefficient of thermal expansion, And stress corrosion cracking are difficult to occur, it has been widely applied to various materials such as automobile exhaust system members, building materials such as roofs and dry bulbs, wetting spaces such as kitchens and water storage tanks .

In the production of these structures, the steel sheet of the stainless steel is often cut and formed into an appropriate shape and then joined by welding. However, in a ferritic stainless steel, in a double welded portion where welds of the welding beads such as the starting end and the ending end of the welding of the three plates or the circumferential welding are re-welded, . As the shape of the welding member becomes complicated, such a double welding portion increases, and welding cracks have been a problem.

Further, since the double welded portion is not flat and the portion having the scale on the surface is welded again, oxygen and nitrogen are easily taken into the welded bead, so that there is also a problem that the corrosion resistance tends to be deteriorated. However, in the prior art, recognition corresponding to the problem of such a double welded portion is hardly seen.

Patent Document 1 discloses a ferritic stainless steel excellent in corrosion resistance and weldability. This is a ferritic stainless steel in which the corrosion resistance and the weldability are both achieved by adding Mg and optimizing the S content, but there is no mention of cracking and corrosion resistance of the double welded portion. Actually, when the ferritic stainless steel disclosed in Patent Document 1 is welded, cracks sometimes occur in the double welded portion.

Patent Document 2 discloses a ferritic stainless steel excellent in weldability. However, this is a ferritic stainless steel in which the penetration of weldability and workability after welding are improved, and the problem of a double welded portion such as weld crack is not mentioned.

Japanese Patent Application Laid-Open No. 8-246105 Japanese Laid-Open Patent Publication No. 2009-91654

The present invention has been made in view of the above problems in the prior art and it is an object of the present invention to provide a ferritic stainless steel which is less susceptible to sensitization at the time of double welding, And also to provide a ferritic stainless steel in which weld cracking of the weld bead hardly occurs.

In order to solve the above problems, the present invention has been extensively studied on the influence of various elements on welding cracks in double welding. Further, the double welding means welding the same place twice or plural times. The double welded part means, for example, the process of melting and solidification by double welding such as overlapping part of the weld bead at the welding start and end at the welding of the circumferential phase, or overlapping part of the welding bead at the time of cross welding Means a portion repeated at least twice and its periphery.

A portion where weld cracking occurred was cut out by double welding, and the fracture surface was observed by SEM (Scanning Electron Microscope). The precipitation of film-like Nb was confirmed on the fracture surface. As a comparison, if a portion where weld cracks did not occur was taken out and observed by SEM, the film-like precipitate of Nb as shown in the broken section could not be confirmed. It is considered that the precipitation of Nb in film form affects the occurrence of welding cracks.

As a result of investigating the influence of various components on the welding cracks in the double welded portion, it was found that welding cracks did not occur in the steel having a small content of P and Nb. Various ferritic stainless steels were cross-welded with bead-on-plate, and the presence of welding cracks in the double welded part was confirmed with an optical microscope. The results are shown in Fig. In Fig. 1,? Indicates that welding cracks were not confirmed, and? X welding cracks were confirmed. It can be seen that weld cracking does not occur in the range where Nb is less than 0.05%, P is 0.03% or less, and Nb 占 P is 0.0005 or less.

It has become clear that welding cracks can be prevented by reducing the Nb content. However, since Nb is an effective element for inhibiting the sensitization of the weld bead, there is a concern that it is likely to become susceptible to Nb reduction. Further, since the surface of the double welded portion is not flat and the scale is formed, it is easy to inject impurities into the welded bead, which is a welding condition adverse to sensitivity. Therefore, the effect of various elements on the sensitization of weld beads was investigated. As a result, it has become clear that V and Al besides Nb are effective in suppressing the sensitization of the welded portion. This is considered to be because V and Al form VN and AlN, respectively, thereby suppressing the formation of Cr nitride.

In addition, since the oxide bead called a temper color is formed in the weld bead, the Cr deficiency is generated in the same manner as the sensitization, and the corrosion resistance is lowered. Therefore, the influence of various elements on the corrosion resistance of the temper color is evaluated. As a result, Si, Al, and Ti are concentrated to a temper color, thereby forming an oxide film having excellent dense protection. Further, since the amount of oxidation by welding is suppressed and the Cr deficiency due to oxidation is suppressed, it is recognized that the corrosion resistance of the weld bead is improved when the content of Si, Al, and Ti is appropriate.

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

The steel according to any one of [1] to [4], wherein the steel sheet contains, as a% by mass, 0.001 to 0.030% of C, 0.03 to 0.80% of Si, 0.05 to 0.50% of Mn, Of Al, less than 0.10% of Mo, 0.2 to 3.0% of Mo, 0.1 to 1.2% of Al, 0.02 to 0.50% of V, 0.1 to less than 0.1% of Cu, 0.05 to 0.50% of Ti and 0.001 to 0.030% of N, Nb: less than 0.05%, satisfying the following formula (1), and the balance of Fe and inevitable impurities.

Nb x P? 0.0005 (1)

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

[2] The steel sheet according to any one of [1] to [3], further comprising at least one selected from Zr: not more than 1.0%, W: not more than 1.0%, REM: not more than 0.1%, Co: not more than 0.3%, and B: The ferritic stainless steel according to the above [1], which is characterized in that it is a ferritic stainless steel.

According to the present invention, a ferritic stainless steel is obtained that is less susceptible to sensitization of the welded portion when double welding is performed, excellent corrosion resistance of the welded portion's temper color, and less likely to cause weld cracking of the welded bead.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view for explaining the influence of Nb content and P content on weld cracking in a double weld zone. FIG.
2 is a schematic view of a cross weld.

(Mode for carrying out the invention)

The reasons for limiting the constituent elements 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%

C is an element inevitably included in the river. If the amount of C is large, the strength is improved, and if it is small, the workability is improved. In order to obtain sufficient strength, the content is preferably 0.001% or more, but when it exceeds 0.030%, the workability is significantly deteriorated and Cr carbide is precipitated and the corrosion resistance due to the local Cr deficiency tends to decrease. Therefore, the amount of C is in the range of 0.001 to 0.030%. Preferably, it is in the range of 0.002 to 0.018%. And more preferably in the range of 0.002 to 0.010%.

Si: 0.03 to 0.80%

Si is an element useful for deoxidation. In the present invention, it is an important element for enhancing the corrosion resistance of the welded portion by improving the protective property of the oxide film by concentrating it with Al or Ti to a temper color formed by welding. The effect is obtained by adding 0.03%. However, addition of more than 0.80% causes remarkable deterioration of processability, which makes molding processing difficult. Therefore, the amount of Si is set in the range of 0.03 to 0.80%. And more preferably in the range of more than 0.30 to 0.80%. More preferably, it is in the range of 0.33 to 0.50%.

Mn: 0.05 to 0.50%

Mn is an element inevitably included in the steel, and has an effect of increasing strength. The effect is obtained by the addition of 0.05% or more, but addition of more than 0.50% accelerates the precipitation of MnS which is a starting point of corrosion and decreases the corrosion resistance, so the Mn content is set in the range of 0.05 to 0.50%. Preferably, it ranges from 0.08 to 0.40%.

P: not more than 0.03%

P is an element that is inevitably included in the steel, and the excessive content deteriorates the weldability and makes it easy to cause intergranular corrosion. Further, in the present invention, it has been recognized that an increase in P causes weld cracking of the double welded portion. As the coagulation temperature of the ferritic stainless steel is lowered due to the increase of P, the Nb carbonitride precipitates from the liquid phase to form a film, which interferes with the flow of the molten bond in the course of solidification, It is considered that weld cracks tend to occur in the ferritic stainless steel having a large P content. The reason why the tendency of welding cracks becomes more pronounced in double welding is considered to be that Nb is more concentrated and precipitated more easily by repeating the process of melting and solidification. If the content of P exceeds 0.03%, the effect on welding cracks becomes significant. Therefore, the content of P is 0.03% or less. Preferably 0.025% or less.

S: not more than 0.01%

S is inevitably included in the steel, but when it exceeds 0.01%, the formation of water soluble sulfides such as CaS and MnS is promoted and the corrosion resistance is lowered. Therefore, the amount of S should be 0.01% or less. More preferably, it is 0.006% or less. And more preferably 0.003% or less.

Cr: 19.0 to 28.0%

Cr is the most important element for securing the corrosion resistance of stainless steel. When the content is less than 19.0%, weld beads in which Cr in the surface layer is reduced by oxidation due to welding, and sufficient corrosion resistance around the weld bead can not be obtained. On the other hand, if it is added in an amount exceeding 28.0%, the workability and the composition will be lowered. Therefore, the amount of Cr is set in the range of 19.0 to 28.0%. Preferably, it ranges from 21.0 to 26.0%. More preferably, it is 21.0 to 24.0%.

Ni: 0.01 to less than 0.30%

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. The effect is obtained by addition of 0.01% or more. However, in the case of addition of 0.30% or more, not only the workability is lowered but also the cost is increased because it is an expensive element. Therefore, the amount of Ni is set in a range of 0.01 to less than 0.30%. Preferably, it is in the range of 0.03 to 0.24%. And more preferably in a range of 0.03 to less than 0.15%.

Mo: 0.2 to 3.0%

Mo is an element which promotes the passivation of the passive film and improves the corrosion resistance of the stainless steel. The effect is more remarkable when it is contained together with Cr. The corrosion resistance improvement effect by Mo is obtained by adding 0.2% or more. However, when it exceeds 3.0%, the strength is increased and the rolling load is increased, so that the productivity is lowered. Therefore, the amount of Mo is set in the range of 0.2 to 3.0%. , Preferably in the range of 0.6 to 2.4%. And more preferably in the range of 0.6 to 2.0%. More preferably, it is in the range of 0.8 to 1.3%.

Al: more than 0.15 to 1.2%

Al is an element useful for deoxidation. In the present invention, it is an element that enhances the corrosion resistance of a welded portion by concentrating it with a temper color formed by welding together with Si and Ti. In addition, Al, which has a higher affinity with nitrogen than Cr, forms AlN and inhibits the formation of Cr nitride, thereby suppressing the susceptibility of the weld bead. This effect is obtained with an addition of more than 0.15%. However, when it is added in an amount exceeding 1.2%, the crystal grains of the ferrite are increased and the workability and the composition are lowered. Therefore, the Al content is in the range of more than 0.15 to 1.2%. Preferably, it is in the range of 0.17 to 0.8%.

V: 0.02 to 0.50%

V is an element that improves corrosion resistance and workability and is an element that makes it difficult to cause weld cracking. In addition, it is an element that inhibits the sensitization of the welded portion by bonding with nitrogen to form VN. It is known that the composite addition of Nb and Ti is effective for inhibiting sensitization of the welded portion. In the present invention, it is necessary to suppress the content of Nb in order to prevent weld cracking of the double welded portion. However, sufficient addition of Ti alone may not provide a sufficient inhibitory effect on the sensitization. Therefore, the addition of V and Al as substitutes for Nb is effective for suppressing the sensitization of the welded portion. The effect is obtained by adding 0.02% or more. However, the addition of more than 0.50% adversely affects the workability. Therefore, the content of V is in the range of 0.02 to 0.50%. Preferably, it is in the range of 0.03 to 0.40%.

Cu: less than 0.1%

Cu is an impurity inevitably contained. However, in the ferritic stainless steel having the Cr content and the Mo content of the present invention and having excellent corrosion resistance, there is an action of increasing the passivation current to make the passive film unstable and lowering the corrosion resistance. This corrosion resistance lowering effect becomes remarkable when the Cu amount is 0.1% or more. Therefore, the amount of Cu is less than 0.1%.

Ti: 0.05 to 0.50%

Ti is an element that binds preferentially to C and N and suppresses deterioration of corrosion resistance due to precipitation of Cr carbonitride. In the present invention, it is an element which is important for suppressing the sensitization of the welded portion, and is also an element which is enriched with Si and Al in combination with the temper color of the welded portion to improve the protective property of the oxide film. The effect is obtained by adding 0.05% or more. However, in the case of exceeding 0.50%, the workability is lowered and the Ti carbonitride is coarsened, causing surface defects. Therefore, the amount of Ti is set in the range of 0.05 to 0.50%. Preferably, it is in the range of 0.08 to 0.38%. More preferably, it is in the range of 0.25 to 0.35%.

N: 0.001 to 0.030%

N is an element inevitably included in the steel in the same manner as C, and has the effect of increasing the strength of the steel by solid solution strengthening. The effect is obtained at 0.001% or more. However, when Cr nitride is precipitated, the content of Cr is preferably 0.030% or less in order to reduce the corrosion resistance. Therefore, the amount of N is set in the range of 0.001 to 0.030%. Preferably, it is in the range of 0.002 to 0.018%. More preferably, it is in the range of 0.007 to 0.011%.

Nb: less than 0.05%

Generally, Nb is an element which binds preferentially to C and N to suppress deterioration of corrosion resistance due to precipitation of Cr carbonitride. However, by depositing the film in a double welded portion, weld cracks in the double welded portion can be prevented And it is preferable that the addition amount is low. The welding crack becomes remarkable with the addition of 0.05% or more. Therefore, the amount of Nb is less than 0.05%. Preferably, it is less than 0.02%.

Nb x P: not more than 0.0005

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

Nb in the form of film is deposited on the double welded portion, thereby causing weld cracking. The precipitation of Nb mainly depends on the product of the content of Nb and the content of P, and as shown in Fig. 1, when Nb x P is more than 0.0005, welding crack becomes remarkable. Therefore, Nb x P should be 0.0005 or less.

Although the above is the basic chemical component of the present invention and the balance consists of Fe and inevitable impurities, Zr, W, REM, Co, and B may be added as selective elements for the purpose of improving corrosion resistance and toughness good.

Zr: not more than 1.0%

Zr binds with C and N, and has an effect of inhibiting sensitization. The effect is obtained by addition of 0.01% or more. However, the addition of excess causes deterioration of processability and is an extremely expensive element, resulting in an increase in cost. Therefore, when Zr is added, the amount of Zr is preferably 1.0% or less. More preferably, it is 0.2% or less.

W: 1.0% or less

W has an effect of improving the corrosion resistance similarly to Mo. The effect is obtained by addition of 0.01% or more. However, excessive addition raises the strength and lowers the productivity. Therefore, when W is added, the amount of W is preferably 1.0% or less. More preferably, it is 0.5% or less.

REM: Not more than 0.1%

REM (rare earth element) improves oxidation resistance, suppresses the formation of oxide scale, and suppresses the formation of Cr-depleted region immediately below the temper color of the weld. The effect is obtained by adding 0.001% or more. However, excessive addition causes a decrease in the composition such as acid-pickling property and the increase in cost. Therefore, when REM is added, the amount of REM is preferably 0.1% or less.

Co: 0.3% or less

Co is an element for improving toughness. The effect is obtained by adding 0.001% or more. However, excessive addition decreases the productivity. Therefore, when Co is added, the amount of Co is preferably 0.3% or less. More preferably, it is 0.1% or less.

B: not more than 0.1%

B is an element for improving the secondary process brittleness. In order to obtain the effect, the content of B is preferably 0.0001% or more. However, the excessive content causes a decrease in ductility due to solid solution strengthening. Therefore, when B is contained, the amount of B is preferably 0.1% or less. More preferably, it is 0.01% or less.

2. Manufacturing conditions

Next, a suitable manufacturing method of the steel of the present invention will be described. The steel having the above-described composition may be dissolved in a known method such as a converter furnace, an electric furnace or a vacuum melting furnace, and a continuous casting or ingot casting- It is made of steel material (slab slab) by slabbing. The steel material is then heated to 1100 to 1300 캜 and then subjected to hot rolling at a finishing temperature of 700 캜 to 1000 캜 and a coiling temperature of 500 캜 to 850 캜 to obtain a steel sheet having a thickness of 2.0 mm to 5.0 mm strip. The hot rolled strip thus produced is subjected to pickling by annealing at a temperature of 800 ° C to 1200 ° C, followed by cold rolling and annealing at 700 ° C to 1000 ° C for cold- I do. After cold-rolled sheet annealing, pickling is carried out to remove scale. Skin pass rolling may be performed on the cold rolled steel strip on which the scale is removed.

Example 1

Hereinafter, the present invention will be described on the basis of examples.

The stainless steel shown in Table 1 was vacuum-melted and heated to 1200 占 폚, hot-rolled to a plate thickness of 4 mm, annealed in the range of 800 to 1000 占 폚, and scaled by pickling. Further, the sheet was cold-rolled to a sheet thickness of 0.8 mm, annealed at a temperature in the range of 800 ° C to 1000 ° C, and pickled to obtain a specimen.

Cross-welding as shown in Fig. 2 was carried out by TIG welding of the bead-on plate to the prepared sealing material. The welding current was 90 A and the welding speed was 60 cm / min. For the shield gas, 100% Ar gas was used for both the front side (torch side) and the rear side, and the flow rate was 15 L / min for the front side and 10 L / min for the rear side. The width of the front weld bead was approximately 4 mm.

The presence of weld cracks was confirmed by using an optical microscope for the double welded portion of the weld bead. The results are shown in Table 2.

No welding cracks were observed in No.1 to No.15, No.22 and No.23 of Inventive Examples, but Nb and Nb 占 P among No.16 and No.18 to No.21 of Comparative Examples were not within the scope of the invention Welding cracks were confirmed at No. 20, Nb × P where deviation No. 16, P deviated from the scope of the invention, and No. 21 where Nb × P deviated from the scope of the invention. These weld cracks were cut out, and the fracture surface was observed with an SEM. As a result, Nb precipitation in film form was confirmed in all the samples.

Except for No.16, No.20 and No.21 where weld cracks were confirmed, a test specimen of 20 mm square including the double welded portion of the manufactured weld bead was sampled, and the measurement surface of 10 mm square was left, Coated with ash, and the formula potential was measured in a 3.5 mass% NaCl solution at 30 캜 with the temper color by welding. No polishing or passivating treatment of the test piece was performed. Other measuring methods were in accordance with JIS G 0577 (2005). Table 2 shows the measured formula potential V ' _c100 . The invention is YES No.1~No.15, No.22, No.23, 'the _c100 is relative to that with 0mVvs SCE or more, No.18~No.19 comparative example, both the V' both V _c100 is 0mV vs SCE, and it was confirmed that the corrosion resistance of the inventive example was excellent.

Test pieces of 40 x 40 mm including the weld beads of the double welded portion were taken from No. 1 to No. 23 of Table 1, and the front side was used as the test surface and the neutral salt water spray of JIS H 8502 (1999) Cycle test (neutral salt spray cyclic corrosion test). The number of cycles was 3 cycles. After the test, the presence or absence of corrosion of the weld bead was visually confirmed. The results are shown in Table 2. Corrosion was not confirmed in all of the inventive examples No. 1 to No. 15, No. 22, and No. 23, but corrosion was observed in No. 16 and No. 18 to No. 21 of Comparative Examples. It was confirmed that the weld bead of the inventive example was excellent in corrosion resistance.

The ferritic stainless steel of the present invention can be used for applications in which a structural body is manufactured by welding, for example, automobile exhaust system materials such as mufflers, canned materials for storage of electric water heaters, building materials such as dry bulb, ventilating duct, .

Claims (2)

0.001 to 0.030% of C, 0.03 to 0.80% of Si, 0.05 to 0.50% of Mn, 0.03% or less of P, 0.01% or less of S, 19.0 to 28.0% of Cr, 0.01 to 0.30% of Ni, % Of Cu, less than 0.1% of Cu, 0.05 to 0.50% of Ti, 0.001 to 0.030% of N, and a content of Nb : Less than 0.05%, satisfying the following formula (1), and the balance of Fe and inevitable impurities. The ferritic stainless steel is excellent in corrosion resistance and content of contact cracking.
Nb x P? 0.0005 (1)
The symbol of the element in the formula represents the content (mass%) of each element. The method according to claim 1,
Further comprising at least one selected from the group consisting of Zr: not more than 1.0%, W: not more than 1.0%, REM: not more than 0.1%, and Co: not more than 0.3% Ferritic stainless steel excellent in cracking property.

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