JP2013209705A - Ferritic stainless steel having excellent corrosion resistance and low temperature toughness in weld zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel combining high corrosion resistance and low temperature toughness in a weld zone and usable even in a cold district.SOLUTION: A ferritic stainless steel 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, >20.0 to 28.0% Cr, 0.01 to <0.30% Ni, 0.2 to 3.0% Mo, 0.02 to 1.2% Al, 0.05 to 0.35% Ti, 0.02 to 0.5% V, 0.001 to 0.3% Co and 0.001 to 0.030% N, and the balance Fe with inevitable impurities.

Description

  The present invention is a ferritic stainless steel excellent in corrosion resistance and low-temperature toughness of a welded part in applications where the structure is manufactured by welding and used outdoors in cold regions, for example, hot water storage cans for electric water heaters. About.

  A hot water storage can of an electric water heater is required to have high corrosion resistance that does not cause corrosion even when immersed in high-temperature tap water for a long time. Although a stainless steel is mentioned as a highly corrosion resistant material, a general austenitic stainless steel is not suitable for use because stress corrosion cracking occurs in this environment. For this reason, a high corrosion resistance ferritic stainless steel having a low sensitivity to stress corrosion cracking has been used.

  The can of the water heater has a vertically long cylindrical shape with rounded ends, and is often manufactured by TIG welding a bowl-shaped member called a mirror and a cylindrical member called a body plate. In addition, although a SUS316L pipe is used for water supply and drainage to the can body, a joint (SUS316L) connecting the pipe and the can body is also joined to the can body by TIG welding.

  Ferritic stainless steel used for water heater cans has a low impurity concentration and coarse crystal grains in the welded portion, resulting in lower low-temperature toughness than the base metal. In addition, in welding with SUS316L, there is a problem that Cr carbonitride is generated in the welded portion and the corrosion resistance is lowered. For this reason, in severe cold districts, the corrosion resistance and toughness of the welded portion are insufficient, and weld cracks may occur.

  As a method for improving the toughness of the welded portion, for example, Patent Document 1 discloses a ferritic stainless steel having an improved toughness of the welded heat affected zone in which the toughness of the welded heat affected zone is improved by adding Nb and V. Yes.

  Patent Document 2 discloses a ferritic stainless steel for fusion welding that is excellent in toughness of a welded portion in which a martensite phase of 7% or more is generated in a weld bead.

  Patent Document 3 discloses a high corrosion resistance low strength stainless steel excellent in workability and toughness of a welded portion and a welded joint thereof.

Japanese Examined Patent Publication No. 63-66378 Japanese Patent No. 2733786 Japanese Patent No. 3975882

  The ferritic stainless steel with excellent toughness of the weld heat affected zone disclosed in Patent Document 1 is obtained by improving the toughness of the weld heat affected zone by adding Nb and V. There is a problem that the toughness of the welded portion of the weld not using the above is not mentioned.

  The ferritic stainless steel for fusion welding that is excellent in the toughness of the welded portion disclosed in Patent Document 2 secures the toughness of the welded portion by generating a martensite phase of 7% or more in the weld bead. If formed into a two-phase structure with ferrite, macrocells are formed and the corrosion resistance is lowered, so that there is a problem that it cannot be applied to a hot water can body having a severe corrosive environment.

  High corrosion resistance low strength stainless steel with excellent workability and toughness of the welded portion disclosed in Patent Document 3 has low corrosion resistance due to low Cr, and there is no mention of sensitization by welding with SUS316L. There is a problem that it is not suitable for use in a water heater can.

  An object of the present invention is to provide a ferritic stainless steel having both high corrosion resistance and low temperature toughness of a welded part that can be used even in a cold region, in view of the above-described problems of the prior art.

  In the present invention, in order to solve the above-mentioned problems, TIG welding of various ferritic stainless steels and SUS316L was performed, and the corrosion resistance and toughness of the welded portion were investigated, and the following knowledge was obtained.

  The addition of V and Co improves the low temperature toughness of the weld.

  The addition of Ti refined the crystal grains of the weld bead and improved the low temperature toughness of the weld, but excessive addition decreased the low temperature toughness. A large amount of coarse TiN is observed in the weld bead whose low-temperature toughness is reduced by adding excessive Ti.

The addition of Ti or Nb was effective for improving the corrosion resistance of the weld zone. However, when Nb is added exceeding Nb ≦ Ti, the crystal grain of the weld bead becomes coarse and the low temperature toughness decreases.
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 0.80% or less, Mn: 0.05 to 0.50%, P: 0.05% or less, S : 0.01% or less, Cr: more than 20.0% and 28.0% or less, Ni: 0.01% or more and less than 0.30%, Mo: 0.2-3.0%, Al: 0.02- Contains 1.2%, Ti: 0.05 to 0.35%, V: 0.02 to 0.5%, Co: 0.001 to 0.3%, N: 0.001 to 0.030% And ferritic stainless steel excellent in corrosion resistance and low-temperature toughness of the welded portion, characterized in that the balance consists of Fe and inevitable impurities.

  [2] The ferritic stainless steel having excellent corrosion resistance and low temperature toughness of the welded portion according to the above [1], wherein Nb: 0.01% or more and Nb ≦ Ti in mass%.

  [3] The ferritic stainless steel excellent in corrosion resistance and low temperature toughness of the welded portion according to [1] or [2], further comprising Cu: less than 0.1% by mass.

  [4] Further, by mass%, one or more selected from Zr: 1.0% or less, W: 1.0% or less, REM: 0.1% or less, B: 0.1% or less The ferritic stainless steel excellent in corrosion resistance and low temperature toughness of the welded portion according to any one of the above [1] to [3].

  [5] The ferritic stainless steel according to any one of [1] to [4] above, wherein an average crystal grain size of a weld bead portion of TIG welding is 100 μm or less, and TiN contained in the weld bead. The ferritic stainless steel excellent in corrosion resistance and low temperature toughness of the welded portion according to any one of the above [1] to [4], wherein the average particle size is 3.0 μm or less.

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel which was excellent in the corrosion resistance and low temperature toughness of the weld part which has the high corrosion resistance and low temperature toughness of the weld part which can be used also in a cold region is obtained.

  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 C content is large, the strength is improved, and when the C content 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 remarkably, and Cr carbide is precipitated to cause 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%. 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, and it is concentrated in the temper color formed by welding to improve the protection of the oxide film and to improve the corrosion resistance of the weld. The element to be considered. 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, if it exceeds 0.50%, the precipitation of MnS, which is the starting point of corrosion, is promoted and the corrosion resistance is lowered. Therefore, the amount of Mn is 0.05 to 0.50. % Range. 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. 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, but the content exceeding 0.01% promotes the formation of water-soluble sulfides such as CaS and MnS and lowers the corrosion resistance. Therefore, the S content is 0.01% or less.

Cr: more than 20.0% and not more than 28.0% Cr is the most important element for securing the corrosion resistance of stainless steel. If it is 20.0% or less, 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. On the other hand, if it exceeds 28.0%, workability and manufacturability are deteriorated, so the Cr content is in the range of more than 20.0% and 28.0% or less. 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 it is 0.30% or more, in addition to lowering the workability, it is an expensive element, which increases the 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: 0.02 to 1.2%
Al is an element useful for deoxidation. In the present invention, Al is an element necessary for forming AlN to suppress coarsening of TiN and obtaining good low temperature toughness. This effect is obtained when the Al content is 0.02% or more. However, if it exceeds 1.2%, the ferrite crystal grain size increases and the low temperature toughness of the welded portion decreases. Therefore, the Al content is set to a range of 0.02 to 1.2%. Preferably, it is 0.05 to 0.8% in range.

Ti: 0.05 to 0.35%
Ti is an element that binds preferentially to C and N and suppresses a decrease in corrosion resistance due to the precipitation of Cr carbonitride, and is an element necessary for obtaining the required corrosion resistance in the welded portion with SUS316L. In the present invention, Ti (C, N) precipitated in the welded portion also has an effect of suppressing coarsening of crystal grains and improving the low temperature toughness of the welded portion. The effect is obtained when the content is 0.05% or more. However, if it exceeds 0.35%, coarse TiN precipitates on the weld bead, and the low-temperature toughness of the welded portion decreases. Therefore, the Ti content is in the range of 0.05 to 0.35%. Preferably, it is 0.08 to 0.28% of range.

V: 0.02-0.5%
V is an important element that improves the low temperature toughness of the weld. In addition, by forming VN, the amount of N bonded to Ti is reduced and formation of coarse TiN is suppressed. The effect is obtained when the V content is 0.02% or more. However, if it exceeds 0.5%, the workability is reduced. Therefore, the V amount is in the range of 0.02 to 0.5%. Preferably, it is 0.03 to 0.3% of range.

Co: 0.001 to 0.3%
Co is an important element that improves the low temperature toughness of the weld. The effect is obtained with a content of 0.001% or more, but if it exceeds 0.3%, the productivity is lowered. Therefore, the amount of Co is in the range of 0.001 to 0.3%. Preferably, it is 0.01 to 0.25% 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 content is 0.001% or more. However, when Cr nitride is precipitated, in addition to lowering the corrosion resistance, excessive inclusion promotes coarsening of TiN and lowers the low temperature toughness of the welded portion, so the content is 0.030% or less. Is appropriate. 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.

  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.

Nb: 0.01% or more, Nb ≦ Ti
Nb is an element that preferentially binds to C and N, suppresses a decrease in corrosion resistance due to the precipitation of Cr carbonitride, and improves the corrosion resistance of the TIG welded portion with SUS316L. The effect is acquired by 0.01% or more of containing. In addition, in the present invention, when Nb satisfying Nb> Ti is contained, Ti (C, N), which suppresses the coarsening of crystal grains, is hardly formed in the weld bead. It was revealed that the low temperature toughness of the welded portion was lowered. Therefore, the Nb content is preferably 0.01% or more and Nb ≦ Ti. More preferably, the Nb content is 0.03% or more and Nb ≦ Ti.

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 Cu content is preferably less than 0.1%.

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, if the content exceeds 1.0%, the workability is lowered, and the cost is increased because it is a very high element. Therefore, the Zr content 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, the content exceeding 1.0% increases the strength and decreases the productivity. Therefore, the W amount is preferably 1.0% or less.

REM: 0.1% or less REM is an element having atomic number 57 to 71 such as La, Nd, Sm, and the content is the total amount of these elements. REM improves oxidation resistance, suppresses the formation of oxide scale, and suppresses the formation of a Cr-deficient region immediately below the temper collar of the weld. The effect is obtained when the content is 0.0001% or more. However, if the content exceeds 0.1%, the productivity such as pickling properties is lowered and the cost is increased. Therefore, the REM content is preferably 0.1% or less.

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

2. About a weld bead part Average crystal grain diameter: 100 micrometers or less Since a weld bead becomes very high temperature, low temperature toughness falls by coarsening of a crystal grain. This tendency becomes remarkable when the average crystal grain size exceeds 100 μm. Therefore, it is preferable that the average crystal grain size is 100 μm or less.

Average particle size of TiN: 3.0 μm or less Since TiN precipitates from the liquid phase in the weld bead, if TiN is a relatively fine precipitate having an average particle size of 3.0 μm or less, an increase in crystal particle size is suppressed. On the other hand, when TiN exceeds the average particle size of 3.0 μm and becomes a coarse precipitate, it becomes a starting point for brittle fracture, so that the low temperature toughness of the weld bead decreases. Therefore, it is preferable that the average particle diameter of TiN is 3.0 μm or less.

3. Production Conditions Next, a preferred production method for 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 thickness 0.8mm, annealed in the range of 800 degreeC-950 degreeC, pickled, and it was set as the test material.

  The produced specimen was butt welded with SUS316L. The welding is TIG welding, the welding current is 90 A, the welding speed is 60 cm / min, and no welding wire is used. The welding position was adjusted so that the penetration ratio of SUS316L into the weld bead was approximately 50%. As the shielding gas, 100% Ar gas was used for both the front side (torch side) and 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 to 5 mm.

  The cross section of the produced weld bead was revealed by aqua regia etching and observed using an optical microscope. A cross section of the weld bead was photographed, and five straight lines were arbitrarily drawn perpendicular to the thickness direction, and the number of intersections with the grain boundaries was measured. The total number of intersections with the obtained crystal grain boundaries was divided by 5 times the plate thickness (0.8 mm) to obtain the average crystal grain size in the plate thickness direction. The measurement results are shown in Table 2.

  Furthermore, the cross section of the weld bead was electrolyzed using a 10% by mass acetylacetone-1% by mass tetramethylammonium chloride-methanol electrolytic solution, and a precipitate was revealed and observed by SEM. The observed precipitate component was analyzed by EDX, and the precipitate in which both Ti and N were detected was determined to be TiN. The particle diameters of all the TiN particles observed in an arbitrarily selected range of 100 × 100 μm were measured and averaged to obtain the average particle diameter of TiN. The particle size of TiN was the average of the length of the major axis and the length of the minor axis. The results are shown in Table 2.

A Charpy impact test piece having a 2 mm V-notch in the center of the weld bead of the manufactured weld was prepared, and a Charpy impact test in accordance with JIS Z 2242 was performed. The test temperature was −15 ° C. The results are shown in Table 2. The Charpy impact value at −15 ° C. was 10 J / cm ² or more, and it was determined as good low temperature toughness.

  No. which is an example of the present invention. 1-No. 8 and Comparative Example No. No. 9 showed good low temperature toughness. No. 10-No. 16, the average crystal grain size in the plate thickness direction of the weld bead is 100 μm or more, the average grain size of TiN is 3 μm or more, or the content of V or Co is out of the scope of the present invention. Low temperature toughness was below standard.

  A 60 × 80 mm test piece including the prepared welded portion was collected, the unevenness of the scale and weld bead formed by welding was removed by polishing, and finish-polished using # 600 emery polishing paper. The corrosion resistance of the weld was evaluated by a compliant neutral salt spray cycle test. Table 2 shows the case where corrosion occurred in the 10-cycle test as x and the case where corrosion did not occur as ◯. No. 9, no. No. 11 was corroded and failed. No. In No. 9, Cr is no. In No. 11, it was considered that sufficient corrosion resistance was not obtained because Ti was out of the scope of the present invention.

  In Table 1, No. 9 is Cr. 10 is Al. 11, no. 12 is Ti, No. 12; 13, no. 14 is V, no. 15 is Co, No. In No. 16, the ratio of the content of Nb and Ti was out of the scope of the present invention, and the low temperature toughness of the weld and / or the corrosion resistance of the weld became insufficient.

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel which was excellent in the corrosion resistance and low temperature toughness of the weld part which has the high corrosion resistance and low temperature toughness of the weld part which can be used also in a cold region is obtained. The ferritic stainless steel obtained by the present invention is suitable for application to an outdoor use, for example, a hot water storage can material for an electric water heater, etc., in which a structure is produced by welding.

Claims (5)

  C: 0.001 to 0.030% by mass%, Si: more than 0.30% and 0.80% or less, Mn: 0.05 to 0.50%, P: 0.05% or less, S: 0.00. 01% or less, Cr: more than 20.0% and 28.0% or less, Ni: 0.01% or more and less than 0.30%, Mo: 0.2 to 3.0%, Al: 0.02 to 1.2 %, Ti: 0.05-0.35%, V: 0.02-0.5%, Co: 0.001-0.3%, N: 0.001-0.030%, the balance A ferritic stainless steel excellent in corrosion resistance and low temperature toughness of a welded portion, characterized in that is composed of Fe and inevitable impurities.   Furthermore, ferritic stainless steel excellent in corrosion resistance and low temperature toughness of the welded portion according to claim 1, wherein Nb: 0.01% or more and Nb ≦ Ti in mass%. The ferritic stainless steel excellent in corrosion resistance and low temperature toughness of the welded portion according to claim 1 or 2, further comprising Cu: less than 0.1% by mass.   Furthermore, it contains at least one selected from the group consisting of Zr: 1.0% or less, W: 1.0% or less, REM: 0.1% or less, and B: 0.1% or less in terms of mass%. The ferritic stainless steel excellent in corrosion resistance and low temperature toughness of the welded portion according to any one of claims 1 to 3.   5. The ferritic stainless steel according to claim 1, wherein an average crystal grain size of a weld bead portion of TIG welding is 100 μm or less, and an average grain size of TiN contained in the weld bead is 3. The ferritic stainless steel excellent in corrosion resistance and low-temperature toughness of the welded portion according to any one of claims 1 to 4, wherein the ferritic stainless steel is 0 µm or less.