JP6399666B1 - Ferritic stainless steel and welded structures - Google Patents

Abstract

The present invention provides a ferritic stainless steel that can be manufactured at a relatively low cost, can ensure good corrosion resistance and strength, and has excellent weldability. SOLUTION: In mass%, C: 0.030% or less, Si: 1.0% or more and 2.0% or less, Mn: 1.0% or less, Ni: 0.5% or more and 1.0% or less, Cr: 18.0% to 21.0%, Mo: 0.4% to 1.1%, Cu: 2.0% or less, P: 0.050% or less, S: 0.020% or less, N: 0.030% or less, and Al: 0.15% or less, at least one of Ti and Nb is contained in a total of 10 (C + N)% or more and 0.7% or less, with the balance being Fe and Consists of inevitable impurities. In the range of the content of each element of such an alloy composition, Mn + 2Ni + 0.5Mo ≧ 1.5 is satisfied, 5Si + Cu ≧ 5.5 is satisfied, and Cr + Ni + 3Mo + 0.75Si ≧ 22 is satisfied. [Selection figure] None

Description

  The present invention relates to a ferritic stainless steel and a welded structure having good corrosion resistance.

  For example, an electric water heater such as Ecocute (registered trademark) uses a hot water can for storing hot water.

  The water temperature in the hot water can body is about 80 ° C. at the maximum, and since there is a maximum of 200 ppm of chloride ions in the tap water, it becomes a corrosive environment. Therefore, stainless steel is used for the hot water can body.

  In addition, the hot water can body is manufactured by forming and welding a stainless steel plate, but the corrosion resistance of the weld heat affected zone is lower than that of the material before welding due to oxidation.

  Therefore, high corrosion resistance ferritic stainless steel is widely used as a material for the hot water can body.

  As a ferritic stainless steel used for the material of this type of hot water can body, for example, SUS445J1 (22Cr-1Mo), which is a ferritic stainless steel with high corrosion resistance described in Patent Document 1 or the like, is known.

  In addition, the materials for hot water cans have been studied from the viewpoint of cost reduction and weight reduction by thinning. For example, as described in Patent Document 2 and the like, the cost is reduced by reducing Cr. Ferritic stainless steel is known.

  Furthermore, as a ferritic stainless steel effective for reducing the weight of a hot water can body, as described in Patent Document 3 and the like, a ferritic stainless steel whose strength has been increased by adding Si is known.

Japanese Patent Laid-Open No. 5-271880 JP 2007-270226 A JP 2010-229470 A

  Here, in general, ferritic stainless steel has high thermal conductivity and welding heat is easily diffused, so the weldability is low, and it is particularly difficult to adjust the appropriate welding range as the hot water can body becomes thinner. Is assumed.

  In Patent Documents 1 to 3 described above, the corrosion resistance including the welded portion and the improvement of the strength are studied, but the weldability is not sufficiently studied.

  Therefore, a ferritic stainless steel not only having good corrosion resistance and strength but also excellent weldability has been demanded as a material for hot water cans that can be manufactured at a relatively low cost.

  The present invention has been made in view of these points, and an object thereof is to provide a ferritic stainless steel and a welded structure that can not only ensure good corrosion resistance and strength but also have excellent weldability.

The ferritic stainless steel described in claim 1 has C: 0.030% by mass or less, Si: 1.0% by mass to 2.0% by mass, Mn: 1.0% by mass or less, Ni: 0.00%. 5 mass% to 1.0 mass%, Cr: 18.0 mass% to 21.0 mass%, Mo: 0.4 mass% to 1.1 mass%, Cu: 0.03 mass% to 2 0.0 mass% or less, P: 0.050 mass% or less, S: 0.020 mass% or less, N: 0.030 mass% or less, and Al: 0.15 mass% or less, Ti and Nb At least one of a total of 10 (C + N) mass% or more and 0.7 mass% or less, with the balance being Fe and inevitable impurities, satisfying Mn + 2Ni + 0.5Mo ≧ 1.5, and 5Si + Cu ≧ 5.5 And satisfy Cr + Ni + 3Mo + 0.75Si ≧ 22 It is a plus.

  The ferritic stainless steel according to claim 2 is the ferritic stainless steel according to claim 1 and contains at least one of V, W, and Co in a total amount of 1.0% by mass or less.

  The ferritic stainless steel according to claim 3 is the ferritic stainless steel according to claim 1 or 2, and contains at least one of REM and Ca in a total amount of 0.10% by mass or less.

  A welded structure described in claim 4 is formed by welding the ferritic stainless steel according to any one of claims 1 to 3.

  According to the present invention, Mn + 2Ni + 0.5Mo ≧ 1.5, 5Si + Cu ≧ 5.5, Cr + Ni + 3Mo + 0.75Si ≧ 22 in the alloy composition defined within a predetermined range, so that good corrosion resistance and strength can be ensured. In addition, the weldability can be improved.

It is a top view which shows typically the test piece shape of the test which evaluates corrosion resistance.

  Hereinafter, the configuration of an embodiment of the present invention will be described in detail.

The ferritic stainless steel according to one embodiment of the present invention has C (carbon): 0.030% by mass or less, Si (silicon): 1.0% by mass to 2.0% by mass, Mn (manganese): 1.0 mass% or less, Ni (nickel): 0.5 mass% or more and 1.0 mass% or less, Cr (chromium): 18.0 mass% or more and 21.0 mass% or less, Mo (molybdenum): 0.0. 4 mass% or more and 1.1 mass% or less, Cu (copper): 0.03 mass% or more and 2.0 mass% or less, P (phosphorus): 0.050 mass% or less, S (sulfur): 0.020 mass % Or less, N (nitrogen): 0.030% by mass or less, and Al (aluminum): 0.15% by mass or less, and a total of at least one of Ti (titanium) and Nb (niobium) is 10 ( C + N) contained by mass% to 0.7 mass% with the balance being Fe (iron) And an alloy composition comprising inevitable impurities.

  Moreover, you may contain 1.0 mass% or less in total of at least 1 sort (s) of V (vanadium), W (tungsten), and Co (cobalt) as needed.

  Furthermore, you may contain 0.10 mass% or less in total of at least 1 sort (s) of REM (rare earth element) and Ca (calcium) as needed.

  And in the range of the content of each element of the alloy composition, the formula (1) of Mn + 2Ni + 0.5Mo ≧ 1.5 based on the content of Mn, Ni and Mo is satisfied, and the content of Si and Cu is The formula (2) of 5Si + Cu ≧ 5.5 is satisfied, and the formula (3) of Cr + Ni + 3Mo + 0.75Si ≧ 22 based on the contents of Cr, Ni, Mo and Si is satisfied.

  In addition, each element symbol of (1) type | formula, (2) type | formula, and (3) type | formula shows content of each element which the ferritic stainless steel contains, and the value (mass%) of the content is 0 is substituted for the non-added one.

  C is an element that is inevitably contained in steel and reduces intergranular corrosion resistance (sensitization-inhibiting action) and workability. Therefore, it is preferable to suppress the content, but the content is excessively reduced. Doing so will raise the refining cost more than necessary. Therefore, the C content is 0.030% by mass or less.

  Si is an element effective for increasing the strength, suppressing insufficient penetration, and improving the corrosion resistance (particularly the heat-affected zone). In order to achieve these effects, it is necessary to contain 1.0% by mass or more. On the other hand, when Si is contained exceeding 2.0 mass%, workability and weld toughness may be deteriorated. Therefore, the Si content is set to 1.0% by mass or more and 2.0% by mass or less.

  Mn is an element effective for suppressing burn-off and suppressing welding hot cracking. However, if it is contained in an amount exceeding 1.0% by mass, it tends to generate MnS as a starting point of corrosion, and the ferrite phase is unstable. There is a possibility to make it. Therefore, the Mn content is 1.0% by mass or less.

  Ni is an element effective for suppressing the burn-through, improving the toughness of the welded portion, and improving the corrosion resistance. On the other hand, if Ni is contained in an amount exceeding 1.0 mass%, the ferrite phase may be destabilized and the material cost will be increased more than necessary. Therefore, the Ni content is 0.5 mass% or more and 1.0 mass% or less.

  Cr is an important element for securing the corrosion resistance of the stainless steel including the welded portion and the heat affected zone, and it is necessary to contain 18.0% by mass or more in order to achieve this effect. On the other hand, when Cr is contained in excess of 21.0% by mass, the melting point is likely to cause burnout, and the weld bead shape may be deteriorated, resulting in a decrease in strength of the welded portion. The cost will rise more than necessary. Therefore, the Cr content is 18.0% by mass or more and 21.0% by mass or less.

  Mo is an element effective for improving the corrosion resistance of the stainless steel including the welded portion and the heat affected zone, and in order to exhibit this action, it is necessary to contain 0.4% by mass or more. On the other hand, if Mo is contained in an amount exceeding 1.1 mass%, the workability may be lowered and the material cost will be increased more than necessary. Therefore, the Mo content is set to 0.4% by mass or more and 1.1% by mass or less.

If Cu is contained in an amount exceeding 2.0 mass%, the ferrite phase may be destabilized and the material cost will increase more than necessary. Therefore, the Cu content is set to 0.03% by mass or more and 2.0% by mass or less.

  P is an element that decreases weldability, weld toughness, and workability. Therefore, it is preferable to suppress the content, but if it is excessively reduced, the refining cost increases more than necessary. Therefore, the P content is 0.050 mass% or less.

  S is an element that lowers the toughness of the welded portion and also generates MnS that is a starting point of corrosion. Therefore, it is preferable to suppress the content, but if it is excessively reduced, the refining cost increases more than necessary. Therefore, the S content is 0.020% by mass or less, preferably 0.010% by mass or less.

  N, like C, is an element that reduces intergranular corrosion resistance (sensitization-inhibiting action) and workability. Therefore, it is preferable to suppress the content, but if it is excessively reduced, the refining cost is more than necessary. It will rise. Therefore, the N content is 0.030% by mass or less.

  Al acts as a deoxidizing element, but if it is contained in an amount exceeding 0.15% by mass, it may cause a reduction in the surface quality of the product due to generation of surface defects. Therefore, the Al content is 0.15 mass% or less.

  Ti and Nb are elements that suppress the sensitization and improve the intergranular corrosion resistance. To achieve this effect, Ti and Nb are obtained from the relationship with the contents of C and N that reduce the intergranular corrosion resistance. And Nb must be contained in a total of 10 (C + N) mass% or more. On the other hand, if the total content of Ti and Nb exceeds 0.7 mass%, the workability may be reduced. Therefore, the total content of Ti and Nb is 10 (C + N) mass% or more and 0.7 mass% or less. If Ti is contained in an amount exceeding 0.3% by mass, workability and surface quality may be deteriorated. If Nb is contained in an amount exceeding 0.4% by mass, the workability and toughness are increased. Since it may be lowered, the Ti content is preferably 0.3% by mass or less, and the Nb content is preferably 0.4% by mass or less.

  When the total content of V, W and Co exceeds 1.0% by mass, the workability and toughness may be lowered, and the material cost will be increased more than necessary. Therefore, when it contains at least one of V, W, and Co, the total content of V, W, and Co is 1.0 mass% or less.

  When the total content of REM and Ca exceeds 0.10% by mass, the material cost increases more than necessary. Therefore, when it contains either REM or Ca, the total content of REM and Ca is 0.10% by mass or less.

  Here, with respect to SUS445J1 (22Cr-1Mo), which is a conventional steel, in order to improve the corrosion resistance of the weld heat-affected zone while reducing the Cr content in order to reduce the cost, the action of suppressing oxidation by adding Si In addition, the action of suppressing the progress of corrosion due to the addition of Ni is important. From these viewpoints, in the above alloy composition, the alloy components are adjusted so as to satisfy the formula (3) represented by Cr + Ni + 3Mo + 0.75Si ≧ 22 based on the contents of Si, Ni, Cr and Mo.

  To improve weldability, it is effective to enable welding at a low current by making it difficult for insufficient penetration to occur, and to enable welding at a high current by making it difficult for melting to occur. It is. That is, the appropriate welding range can be expanded and weldability can be improved by simultaneously suppressing the lack of penetration and the burn-through.

  Then, expansion of the appropriate welding range was examined. First, regarding the contents of Ni, Mn, and Mo, melt-down can be suppressed by adjusting the alloy components so as to satisfy the formula (1) represented by Mn + 2Ni + 0.5Mo ≧ 1.5. Moreover, about the content of Si and Cu, the lack of penetration can be suppressed by adjusting the alloy components so as to satisfy the formula (2) represented by 5Si + Cu ≧ 5.5. Therefore, a wide appropriate welding range can be secured by simultaneously satisfying the expressions (1) and (2).

  And welding structures, such as a hot water can of an electric water heater, are formed by welding the ferritic stainless steel in which the component adjustment was carried out in this way.

  Next, the operation and effect of the one embodiment will be described.

  According to the ferritic stainless steel and the welded structure, by specifying each element in the above range, the cost can be reduced and good strength can be secured.

  By adjusting the alloy components so as to satisfy the formula (1) expressed by Mn + 2Ni + 0.5Mo ≧ 1.5, it is possible to suppress the burn-through and widen the range of the appropriate welding current and improve the weldability. .

  By adjusting the alloy components so as to satisfy the formula (2) shown by 5Si + Cu ≧ 5.5, the current value capable of TIG welding is reduced, so that insufficient penetration is less likely to occur and the range of the appropriate welding current is widened. And weldability can be improved.

  By adjusting the alloy components so as to satisfy the formula (3) expressed by Cr + Ni + 3Mo + 0.75Si ≧ 22, the corrosion resistance including the welded portion is improved by utilizing the oxidation suppressing action by Si and the corrosion progress suppressing action by Ni. it can.

  Therefore, ferritic stainless steel satisfying the above alloy composition and each formula can be manufactured at a relatively low cost as compared with conventional steels such as SUS445J1, and can not only ensure good corrosion resistance and strength, but also has excellent weldability.

  Further, such ferritic stainless steel and welded structure excellent in corrosion resistance, strength and weldability are suitable for applications such as automotive parts, household appliance parts, and hot water cans of electric water heaters.

  Hereinafter, this example and a comparative example will be described.

  First, stainless steels having the respective alloy compositions shown in Table 1 are melted by laboratory melting, and hot rolled, annealed, cold rolled, annealed and pickled to produce a 0.6 mm annealed pickled plate. It was subjected to tests for evaluating weldability, mechanical properties and corrosion resistance.

  In Table 1, no. Nos. 1 to 11 are the present examples that satisfy the conditions of the above-described embodiment. 12-23 are comparative examples.

  In the evaluation of weldability, each stainless steel shown in Table 1 was subjected to TIG at a welding speed of 600 mm / min, a gas flow rate of 10 L / min, a back gas flow rate of 5 L / min, an electrode diameter of 1.6 mm, and an interelectrode distance of 0.6 mm. Tanning welding was performed.

  Then, by changing the welding current, the range of the welding current where the penetration is insufficient and the melting does not occur is set as an appropriate welding current range (ΔA), and when ΔA ≧ 100, the weldability is evaluated as good. ○ indicates.

  In the evaluation of the mechanical properties, a No. 13B test piece defined in JIS Z2241, which was welded at the center of the test piece in the direction perpendicular to the tensile direction, was used. The welding conditions were the same as in the evaluation of the weldability, and the current was adjusted so that the weld bead width was 3 mm. Then, a tensile test was performed in accordance with JIS Z2291, and the case where the tensile strength was 500 MPa or more was evaluated as having good mechanical properties (strength).

  In the evaluation of corrosion resistance, as shown in FIG. 1, a welded portion 11 is obtained by performing welding perpendicularly to the longitudinal direction on the central portion in the longitudinal direction of a rectangular test piece having a longitudinal length of 40 mm and a widthwise length of 15 mm. A constant potential immersion test was performed using the test piece. The welding conditions were the same as in the evaluation of the weldability, and the current was adjusted so that the weld bead width was 3 mm.

  In the constant potential immersion test, the test piece was immersed in a solution having a chloride ion concentration of 200 ppm (assuming the upper limit of tap water standard) and 80 ° C. (assuming the upper limit temperature of use of the electric water heater) to obtain an Ar deaeration atmosphere. . Further, the counter electrode (Pt) and the reference electrode (Ag / AgCl) were immersed, the applied potential was set to 0.3 (assuming the worst 0.25V in the actual environment), and held for 48 hours. When the current value after 48 hours was 1 μA or less, it was evaluated that corrosion did not occur and the corrosion resistance was good.

  Table 1 shows the alloy compositions, weldability evaluation results, mechanical property evaluation results, and corrosion resistance evaluation results of the present examples and comparative examples.

As shown in Table 1, No. 1 in this example. In all of 1 to 9, 11, weldability, mechanical properties, and corrosion resistance were all good.

  On the other hand, No. 1, which is a comparative example that does not satisfy at least one of the relationships among the above-mentioned alloy component regulations, formulas (1), (2), and (3). Nos. 12 to 20 did not satisfy at least any of the criteria of weldability, mechanical properties, and corrosion resistance.

  (2) No. which is a comparative example that does not satisfy the relationship of the formula. No. 21 had an appropriate welding current range of less than 100 and did not satisfy the standard of weldability.

  (1) No. which is a comparative example not satisfying the relationship of the formula. No. 22 had an appropriate welding current range of less than 100 and did not satisfy the standard of weldability.

  (3) No. which is a comparative example which does not satisfy the relationship of the formula. No. 23 did not satisfy the corrosion resistance standard by the constant potential immersion test.

Claims (4)

C: 0.030% by mass or less, Si: 1.0% by mass to 2.0% by mass, Mn: 1.0% by mass or less, Ni: 0.5% by mass to 1.0% by mass, Cr: 18.0% to 21.0% by mass, Mo: 0.4% to 1.1% by mass, Cu: 0.03% to 2.0% by mass, P: 0.050% by mass Hereinafter, S: 0.020% by mass or less, N: 0.030% by mass or less, and Al: 0.15% by mass or less, and a total of 10 (C + N) mass% of at least one of Ti and Nb More than 0.7% by mass, the balance consists of Fe and inevitable impurities,
Mn + 2Ni + 0.5Mo ≧ 1.5 is satisfied,
5Si + Cu ≧ 5.5 is satisfied,
A ferritic stainless steel characterized by satisfying Cr + Ni + 3Mo + 0.75Si ≧ 22. 2. The ferritic stainless steel according to claim 1, wherein the ferritic stainless steel contains at least one of V, W, and Co in a total amount of 1.0 mass% or less. The ferritic stainless steel according to claim 1 or 2, characterized by containing at least one of REM and Ca in a total amount of 0.10% by mass or less. A welded structure formed by welding the ferritic stainless steel according to any one of claims 1 to 3.

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