JP2013209706A - Ferritic stainless steel having excellent corrosion resistance in weld zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel having excellent corrosion resistance in a weld zone even under welding conditions where air is intruded into a shielding gas and the sensitization of a weld zone is easy to occur.SOLUTION: A ferritic stainless steel having excellent corrosion resistance in a weld zone includes, by mass, 0.001 to 0.030% C, >0.30 to 0.80% Si, 0.05 to 0.50% Mn, ≤0.05% P, ≤0.01% S, 19.0 to 28.0% Cr, 0.01 to <0.30% Ni, 0.2 to 3.0% Mo, >0.08 to 1.2% Al, 0.05 to 0.50% Ti, 0.0001 to 0.1% REM and 0.001 to 0.030% N, and in which the content of O inevitably included is ≤0.010%, and the balance Fe with inevitable impurities, and satisfies inequality (1): Al+8×REM-10×O≥0.10; wherein, the atomic symbols in the inequality denote the contents (mass%) of the respective elements.

Description

  The present invention is used in applications in which a structure is produced by welding, such as automobile exhaust system materials such as mufflers, hot water storage can materials for electric water heaters, building materials such as fittings, ventilation openings, ducts, etc. The present invention relates to a ferritic stainless steel that is excellent in corrosion resistance of the welded part even under welding conditions in which air is mixed into the shield gas and the welded part is apt to be sensitized due to insufficient gas shielding.

  Ferritic stainless steel has various thermal properties such as good thermal conductivity, low thermal expansion coefficient, and resistance to stress corrosion cracking, so it can be used for automobile exhaust parts, roofing and building materials, kitchens, water storage and hot water storage tanks. It has been applied to a wide range of applications such as water-related materials.

  In producing these structures, a stainless steel plate is often cut and formed into an appropriate shape and then joined by welding. Ferritic stainless steel is often welded by TIG welding, but if sufficient gas shielding cannot be achieved due to the shape of the weld, etc., air is entrained in the shielding gas and N penetrates into the weld bead and becomes sensitized. A phenomenon called occurs. When sensitization occurs, the corrosion resistance of the welded portion decreases, and preventing this becomes an essential technique in welding ferritic stainless steel.

Examples of ferritic stainless steel having excellent corrosion resistance of the welded portion include ferritic stainless steel having excellent corrosion resistance of the welded portion in Patent Document 1, and ferritic stainless steel having excellent corrosion resistance of the weld gap in Patent Document 2. However, Patent Document 3 discloses ferritic stainless steels each having excellent corrosion resistance of a welded portion with austenitic stainless steel.
However, even with these ferritic stainless steels, sufficient corrosion resistance cannot always be ensured under welding conditions in which N penetrates the welding bead from the shielding gas.

JP 2007-270290 A JP 2009-161836 A JP 2010-202916 A

  In view of the above-mentioned problems of the prior art, the present invention is a weld of ferritic stainless steel, and even in welding conditions in which air is mixed into the shielding gas and the weld is likely to be sensitized, the corrosion resistance of the weld is It aims at providing the ferritic stainless steel which is excellent in.

  In the present invention, in order to solve the above problems, 1 vol% of air is mixed into the shielding gas to weld various ferritic stainless steels, and the welds and the oxides formed in the welds are analyzed, so that they are sensitive. We have conducted intensive research on the effects of various elements on crystallization.

  First, 1 vol% oxygen was mixed into the shielding gas, and bead-on-plate TIG welding was performed on SUS444 to analyze the weld bead. Concentration of N and O was recognized in the components of the weld bead as compared with those before welding. This is probably because N and O entered the weld bead from the shielding gas. When the cross section of the weld bead was made to appear by oxalic acid electrolytic etching according to JIS G 0571 (2003) and observed with an optical microscope, clear sensitization was observed at the grain boundaries. It is considered that N and O that have entered the weld bead react with Cr in the steel to form precipitates, and a Cr-deficient layer is formed (= sensitized). Since sensitization lowers corrosion resistance, it is necessary to suppress the penetration of N and O from the shield gas into the weld bead.

  Therefore, 0.01% C-0.4% Si-0.2% Mn-25% Cr-0.8% Mo-0.4% Ti-0.01% N ferritic stainless steel as a standard Using ferritic stainless steel with various elements adjusted, welding was performed in which 1 vol% air was mixed in the shielding gas, and the influence of various elements on the penetration of N and O was investigated.

  As a result, as shown in FIG. 1 and FIG. 2, it became clear that the addition of Al and REM is effective in preventing the intrusion of N and O. When the components of the weld bead surface of the steel in which the penetration of N and O was suppressed were analyzed in the depth direction by GDS, REM oxides such as Al and La coexisted in the outermost layer. It is thought that these oxides suppressed the penetration of N and O into the weld bead by covering the surface of the molten pool during welding.

  Further, the effect of suppressing the penetration of N and O by Al and REM tended to be more remarkable when the content of O contained as an inevitable impurity in the steel is small. Since Al and REM easily bond with O, when the impurity of O increases in the steel, Al and REM exist in the steel in an oxide state. Al and REM oxides have a high melting point and are present in the weld pool without melting. Therefore, when Al and REM are dispersed in the steel in the oxide state, they react with the shield gas O on the weld pool surface compared to the case where they are present in the solid solution state. It becomes difficult to cover the molten pool as an oxide film. Therefore, in order to cover the molten pool surface with an oxide and suppress the intrusion of N and O, Al and REM need to be in a solid solution state, and it is considered that a more remarkable effect appears when the amount of impurity O is small. .

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

[1] By mass%, C: 0.001 to 0.030%, Si: more than 0.30% to 0.80% or less, Mn: 0.05 to 0.50%, P: 0.05% or less, S: 0.01% or less, Cr: 19.0 to 28.0%, Ni: 0.01% or more and less than 0.30%, Mo: 0.2 to 3.0%, Al: more than 0.08% 1.2% or less, Ti: 0.05 to 0.50%, REM: 0.0001 to 0.1%, N: 0.001 to 0.030%, inevitably O is 0 0.010% or less, the balance being Fe and inevitable impurities, satisfying the following formula (1), a ferritic stainless steel excellent in corrosion resistance of a welded portion.
Al + 8 × REM-10 × O ≧ 0.10 (1)
In addition, the element symbol in a formula represents content (mass%) of each element.

  [2] The ferritic stainless steel having excellent corrosion resistance of the welded portion according to the above [1], further comprising, by mass%, Cu: less than 0.1%.

  [3] Further, by mass%, Nb: 1.0% or less, V: 1.0% or less, Zr: 1.0% or less, W: 1.0% or less, Co: 0.3% or less, B : Ferritic stainless steel excellent in corrosion resistance of the welded portion according to the above [1] or [2], characterized by containing any one or more of 0.1% or less.

  According to the present invention, a ferritic stainless steel having excellent corrosion resistance in a welded part can be obtained even in welding conditions in which air is mixed into the shield gas and the welded part is easily sensitized by welding of ferritic stainless steel.

It is a figure explaining the influence of Al content which acts on N concentration and O concentration of a weld bead. It is a figure explaining the influence of the REM content which gives to N concentration and O concentration of a weld bead. It is a figure explaining the influence of Al + 8REM content and O density | concentration which have on the sensitization of a weld bead.

  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. In order to obtain sufficient strength, the content of 0.001% or more is appropriate. However, if it exceeds 0.030%, the workability deteriorates remarkably, and Cr carbide is precipitated to cause local Cr deficiency. It tends to cause a decrease in corrosion resistance. Therefore, the C content is in the range of 0.001 to 0.030%. Preferably, it is 0.002 to 0.018% of range.

Si: more than 0.30% and less than 0.80% Si is an element useful for deoxidation, but in the present invention, it is concentrated in a temper collar formed by welding to improve the protective property of the oxide film, It is an element that makes the corrosion resistance of the material good. The effect is obtained when the content exceeds 0.30%. However, if the amount of Si exceeds 0.80%, the workability deteriorates significantly, and the molding process becomes difficult. Therefore, the Si amount is in the range of more than 0.30% and 0.80% or less. Preferably, it is 0.33% to 0.50% of range.

Mn: 0.05 to 0.50%
Mn is an element inevitably contained in steel and has an effect of increasing strength. The effect is obtained with a content of 0.05% or more. However, a content exceeding 0.50% promotes the precipitation of MnS, which is a starting point of corrosion, and lowers the corrosion resistance. The range is 50%. Preferably, it is 0.08% 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. The tendency becomes remarkable when it 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, if it exceeds 0.01%, formation of water-soluble sulfides such as CaS and MnS is promoted and corrosion resistance is lowered. Therefore, the S content is 0.01% or less.

Cr: 19.0 to 28.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. If it is less than 19.0%, sufficient corrosion resistance cannot be obtained in the weld bead in which Cr on the surface layer is reduced by oxidation due to welding or in the vicinity thereof. On the other hand, if it exceeds 28.0%, workability and manufacturability are lowered, so the Cr content is in the range of 19.0 to 28.0%. Preferably, it is 21.0 to 26.0% of range.

Ni: 0.01 or more and less than 0.30% 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 is acquired by 0.01% or more of containing. However, if the content is 0.30% or more, in addition to lowering workability, it is an expensive element, which causes an increase in cost. Therefore, the Ni content is 0.01 or more and less than 0.30%. Preferably, it is 0.03% to 0.24% of range.

Mo: 0.2-3.0%
Mo is an element that promotes repassivation of the passive film and improves the corrosion resistance of stainless steel. The effect becomes more remarkable by containing with Cr. The effect of improving the corrosion resistance by Mo is obtained with a content of 0.2% or more. However, if it exceeds 3.0%, the strength increases, and the rolling load increases, so the productivity decreases. Therefore, the Mo amount is in the range of 0.2 to 3.0%. Preferably, it is 0.6 to 2.4% of range.

Al: more than 0.08% and 1.2% or less Al is an element useful for deoxidation, but in the present invention, an oxide is formed on the weld pool surface of welding, and N and O from the shielding gas to the welding bead are formed. It is an important element that suppresses intrusion. This effect is obtained when the Al content exceeds 0.08%. However, if the Al content exceeds 1.2%, the ferrite crystal grain size increases, and the workability and manufacturability deteriorate. Therefore, the Al content is set to a range of more than 0.08% and 1.2% or less. Preferably, it is 0.17 to 0.8% of range.

Ti: 0.05 to 0.50%
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 the sensitization of the welded portion by combining with N entering from the shielding gas. The effect is obtained when the Ti content is 0.05% or more. However, when the amount of Ti exceeds 0.50%, workability is reduced and Ti carbonitrides are coarsened to cause surface defects. Therefore, the Ti amount is set to a range of 0.05 to 0.50%. Preferably, it is 0.08 to 0.38% of range.

N: 0.001 to 0.030%
N is an element that is inevitably contained in steel like C, and has the effect of increasing the strength of the steel by solid solution strengthening. The effect is obtained when the N content is 0.001% or more. However, when Cr nitride is deposited, the content of 0.030% or less is appropriate in order to reduce the corrosion resistance. Therefore, the N amount is set in the range of 0.001 to 0.030%. Preferably, it is 0.002 to 0.018% of range.

REM: 0.0001 to 0.1%
REM is an element having atomic numbers 57 to 71 such as La, Nd, and Sm, and the content is the total amount of these elements. The REM of the weld pool has a strong affinity for oxygen contained in the shield gas and tends to align with the surface of the weld pool. The REM aligned on the surface forms a film-like oxide, suppresses the penetration of N from the shielding gas into the weld bead, and prevents the corrosion resistance from being lowered due to sensitization. The Al oxide is formed so as to reinforce the REM oxide formed in this film shape, and more effectively suppresses the penetration of N from the shield gas. The effect of REM can be obtained with a content of 0.0001% or more. In addition, since the presence of oxides of La and Ce is confirmed by X-ray diffraction on the surface of the weld bead, it is considered that La and Ce particularly contribute to the suppression of N penetration from the shield gas. However, if the amount of REM exceeds 0.1%, the productivity such as pickling properties is lowered and the cost is increased. Therefore, the REM amount is set in the range of 0.0001 to 0.1%. Preferably, it is 0.0001 to 0.07% of range.

O: 0.010% or less O is an element inevitably contained in steel, and is an element that increases the penetration of welding. In the present invention, however, it forms an oxide with Al or REM to form a solid solution. It has been clarified that it is an element that reduces Al and REM and hinders the effect of suppressing the penetration of N and O into the weld bead. Therefore, it is desirable that the content is small, and the content of O is 0.010% or less. Preferably it is 0.008% or less.

Al + 8 × REM-10 × O: 0.10 or more In order for Al or REM to cover the surface of the molten pool with an oxide, it is necessary to exist in the steel in a solid solution state. Since the impurity O contained in the steel precipitates Al and REM, it is desirable that the content be less. 0.01% C-0.4% Si-0.2% Mn-25% Cr-0.8% Mo-0.4% Ti-0.01% N based on ferritic stainless steel , REM, and steel with various contents of O were welded under welding conditions in which 1 vol% of air was mixed in the shielding gas, and the sensitization of the weld bead was compliant with JIS G 0571 (2003). Judged by oxalic acid electrolytic etch. The results are shown in FIG. The case where no sensitization occurred was indicated by ○, and the case where sensitization occurred was indicated by ×. Sensitization of the weld bead was confirmed when the oxygen concentration was 0.010% or more and Al + 8 × REM-10 × O was less than 0.10. Therefore, Al + 8 * REM-10 * O shall be 0.10 or more.

  The above is the basic chemical component of the present invention, and the balance consists of Fe and unavoidable impurities, but the following elements can be further contained as required.

Cu: Less than 0.1% Cu is a ferritic stainless steel having excellent Cr and Mo contents according to the present invention, which increases the passive maintenance current, destabilizes the passive film, and lowers the corrosion resistance. There is an effect. Therefore, when Cu is contained, the amount of Cu is preferably less than 0.1%.

Nb: 1.0% or less Nb is an element that preferentially bonds with C and N and suppresses a decrease in corrosion resistance due to the precipitation of Cr carbonitride. In the present invention, N entering from the shield gas is made harmless by Ti having a stronger binding force with N, so Nb content is not essential, but 1.0% or less that can ensure moderate workability in production. You may make it contain. Therefore, when Nb is contained, the Nb content is preferably 1.0% or less. More preferably, it is 0.02 to 0.50% of range.

V: 1.0% or less V is an element that improves corrosion resistance and workability, and is an element that hardly causes weld cracking. The effect is obtained when the V content is 0.01% or more. However, if it exceeds 1.0%, the workability is conversely reduced. Therefore, when V is contained, the V content is preferably 1.0% or less. More preferably, it is 0.02 to 0.50% of range.

Zr: 1.0% or less Zr combines with C and N and has an effect of suppressing sensitization. The effect is acquired by 0.01% or more of containing. However, excessive content lowers workability and increases the cost because it is a very high element. Therefore, when Zr is contained, the amount of Zr is preferably 1.0% or less.

W: 1.0% or less W, like Mo, has an effect of improving corrosion resistance. The effect is acquired by 0.01% or more of containing. However, excessive content increases strength and decreases manufacturability. Therefore, when it contains W, it is preferable that W amount shall be 1.0% or less.

Co: 0.3% or less Co is an element that improves toughness. The effect is acquired by 0.001% or more of containing. However, excessive content reduces manufacturability. Therefore, when it contains Co, it is preferable to make Co amount into 0.3% or less.

B: 0.1% or less B is an element that improves the secondary work brittleness. In order to obtain the effect, the content of 0.0001% or more is appropriate. However, excessive addition causes a decrease in ductility due to solid solution strengthening. Therefore, when it contains B, it is preferable to make B amount into 0.1% or less.

2. Production conditions Next, a preferred production method of the steel of the present invention will be described. Stainless steel having the above-described component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and a steel material (slab) is obtained by a continuous casting method or an ingot-bundling method. The steel material is heated to 1100 ° C. to 1300 ° C., and then hot rolled to a plate thickness of 2.0 mm to 5.0 mm at a finishing temperature of 700 ° C. to 1000 ° C. and a winding temperature of 500 ° C. to 850 ° C. The hot-rolled steel strip thus produced is annealed at a temperature of 800 ° C. to 1000 ° C. and pickled, and then cold-rolled and cold-rolled sheet annealed at a temperature of 700 ° C. to 1000 ° C. After cold-rolled sheet annealing, pickling is performed to remove scale. Skin pass rolling may be performed on the cold-rolled steel strip from which the scale has been removed.

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

  Stainless steel shown in Table 1 was vacuum-melted and heated to 1200 ° C., then hot-rolled to a thickness of 4 mm, annealed in the range of 800 to 1000 ° C., and the scale was removed by pickling. Furthermore, it cold-rolled to plate | board thickness 0.8mm, annealed in the range of 800 to 900 degreeC, pickled, and it was set as the test material.

TIG welding of the bead-on-plate was performed on the prepared test material. The welding current was 90 A and the welding speed was 60 cm / min. The shielding gas used was 1 vol% air-99 vol% Ar on the front side (torch side), 100 vol% Ar gas on the back side, and the flow rate was 15 L / min on the front side and 10 L / min on the back side. The width of the front side weld bead was approximately 4 mm.
The cross section of the produced weld bead was made to appear by oxalic acid electrolytic etching based on JIS G 0571 (2003) and observed using an optical microscope. The results are shown in Table 2.

  No. deviating from the scope of the present invention. 12-No. No. 15 confirmed sensitization. Further, the weld bead was cut out, and the N concentration of the weld bead was measured by a JIS G 1228 (1997) iron and steel-nitrogen determination method. No. which is an example of the present invention. 1-No. In No. 11, the N concentration of the weld bead was 0.014 to 0.018%, which was relatively small. On the other hand, No. which is a comparative example. 12-No. In No. 15, the N concentration was 0.024 to 0.028%, which was larger than the example of the present invention. In the example of the present invention, it is considered that the N concentration of the weld bead has decreased due to the effect of suppressing N penetration by Al and REM.

  Inventive example No. 1-No. No. 11 has a component value within the range of the invention, so the N concentration in the weld is low and no sensitization has occurred.

  On the other hand, No. which is a comparative example. No. 12 is Al and Al + 8 × REM-10 × O does not satisfy the scope of the invention. No. 13 is oxygen because the oxygen does not meet the scope of the invention. No. 14 is because REM does not meet the scope of the invention. No. 15 has Al + 8 × REM-10 × O not satisfying the scope of the invention, so that the N concentration in the welded portion is high and sensitization occurs.

  The ferritic stainless steel obtained in the present invention is excellent in corrosion resistance of the welded part even under welding conditions in which air is mixed into the shield gas and the welded part is likely to be sensitized. For example, it is suitable for application to automobile exhaust system materials such as mufflers, hot water storage can bodies for electric water heaters, building materials such as fittings, ventilation openings, and ducts.

Claims (3)

In mass%, C: 0.001 to 0.030%, Si: more than 0.30% to 0.80% or less, Mn: 0.05 to 0.50%, P: 0.05% or less, S: 0 0.01% or less, Cr: 19.0 to 28.0%, Ni: 0.01% or more and less than 0.30%, Mo: 0.2 to 3.0%, Al: more than 0.08% 1.2 %: Ti: 0.05 to 0.50%, REM: 0.0001 to 0.1%, N: 0.001 to 0.030%, O inevitably included is 0.010% A ferritic stainless steel having excellent corrosion resistance of a welded portion, characterized in that the balance is Fe and inevitable impurities and the following formula (1) is satisfied.
Al + 8 × REM-10 × O ≧ 0.10 (1)
In addition, the element symbol in a formula represents content (mass%) of each element.   Furthermore, the ferritic stainless steel excellent in the corrosion resistance of the welded part according to claim 1, characterized in that, in mass%, Cu: less than 0.1%.   Further, by mass%, Nb: 1.0% or less, V: 1.0% or less, Zr: 1.0% or less, W: 1.0% or less, Co: 0.3% or less, B: 0.0. The ferritic stainless steel excellent in corrosion resistance of the welded portion according to claim 1, wherein the ferritic stainless steel contains one or more of 1% or less.

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