TW201512425A - Ferritic stainless steel having excellent weld corrosion resistance - Google Patents

Abstract

Provided is a ferritic stainless steel having excellent weld corrosion resistance. The ferritic stainless steel is characterized by containing, in terms of mass %, 0.001-0.025% of C, 0.05-0.30% of Si, 0.35-2.0% of Mn, 0.05% or less of P, 0.01% or less of S, 0.05-0.80% of Al, 0.001-0.025% of N, 16.0-20.0% of Cr, 0.12-0.50% of Ti, 0.002-0.050% of Nb, 0.30-0.80% of Cu, 0.05% to less than 0.50% of Ni, and 0.01-0.50% of V and satisfying formula (1), with the remainder consisting of Fe and unavoidable impurities. 0.50 < 25 x C + 18 x N + Ni + 0.11 x Mn + 0.46 x Cu (1) Moreover, the chemical symbols in the formula denotes the content (mass %) of the elements in question.

Description

Fermented iron-based stainless steel with excellent corrosion resistance

The present invention relates to a ferrite-based stainless steel which is less likely to cause a decrease in corrosion resistance due to carbon and nitrogen infiltrated into a bead to a bead during welding, or nitrogen which is infiltrated from the atmosphere.

The ferrite-based iron-based stainless steel can be less than the amount of Ni in the Vostian iron-based stainless steel to ensure corrosion resistance. Since Ni is an expensive element, the ferrite-based iron-based stainless steel can be manufactured at a lower cost than the Worthfield iron-based stainless steel. Further, the ferrite-based iron-based stainless steel has excellent characteristics such as higher thermal conductivity, less thermal expansion coefficient, and less stress corrosion cracking than the Worthfield iron-based stainless steel. Therefore, the ferrite-grained stainless steel can be used for a wide range of applications such as building materials for automobiles, roofing materials, building materials, kitchen equipment, water storage tanks, and water storage tanks.

In the production of such structures, the use of the Worthfield iron-based stainless steel, in particular, SUS304 (18%Cr-8%Ni) (JIS G 4305) and the like, and the ferrite-grained stainless steel are used in combination. As a welding method of such a stainless steel, TIG (Tungsten Inert Gas) welding is usually used. In this case as well, it is required that the welded portion has good corrosion resistance similarly to the base material portion.

In order to solve such a problem, it is proposed to fix C and N in the form of Ti or Nb carbonitride by adding Ti or Nb having a higher affinity to Cr and C than N, and suppressing Cr carbonitride. Generate and suppress the generation of sensitization, thus A method of maintaining good corrosion resistance. For example, Patent Document 1 discloses a steel in which the grain boundary corrosion resistance of the ferrite-grained stainless steel is improved by the combined addition of Ti and Nb. Further, Patent Document 2 discloses a steel in which intergranular corrosion resistance is improved by adding Nb.

However, any invention must add 0.3% by mass or more of Mo. Mo is an element that improves the corrosion resistance of the base material. However, since it is a strong ferrite iron-forming element, in the case where 0.3% by mass of Mo is added, the generation of the ferrite-grain iron phase in the welded portion is promoted, and the sensitization of the welded portion is promoted, so that sufficient is not obtained. Corrosion resistance of the welded portion. Further, it is necessary to add 0.1% by mass or more of Nb. However, in the case where a steel containing a large amount of Nb is welded, solid solution Nb is formed in the form of coarse Nb precipitates in the welded portion, and problems such as weld cracking occur. In consideration of manufacturability or practicality, the method of obtaining the corrosion resistance of the welded portion by merely adding a carbonitride generating element such as Nb or Ti is not preferable.

[Previous Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-270290

Patent Document 2: Japanese Patent Laid-Open Publication No. 2010-202916

Accordingly, an object of the present invention is to provide a ferrite-based iron-based stainless steel which is excellent in corrosion resistance of a welded portion which does not require excessive addition of Ti or Nb.

In order to solve the above problems, the inventors of the present invention have made an effort to study a technique capable of further suppressing a decrease in corrosion resistance of a welded portion. First, the inventor and others The degree of sensitization allowed by the corrosion resistance of the sufficient welded portion was investigated using a 16-20 mass% Cr steel system. As a result, it has been found that in the case where the grain boundary coverage ratio of the ferrite-grained iron phase by Cr carbonitride (the measurement method of the grain boundary is referred to in the example) is more than 40%, the fusion portion is caused by sensitization. The reduction in corrosion resistance becomes apparent.

Then, the inventors and the like have studied a method of lowering the grain boundary coverage of the ferrite-grained iron phase by Cr carbonitride. As a result, it has been found that the addition of Mn and Cu, which are iron-forming elements of Worthite, stabilizes the iron phase of the Vostian, and is extremely effective for improving the corrosion resistance of the welded portion.

In other words, it has been found that when the iron phase of the Worth is stabilized by the addition of Mn and Cu, the generation of the iron phase of the ferrite during welding can be suppressed, and the solid solution limit of the iron phase of the ferrite is contained. The C and N are stabilized in a state in which the solid solution is dissolved in the iron phase of the ferrite iron phase which is overwhelmingly large, and the sensitization caused by the formation of Cr carbonitride is prevented, and the welded portion is further improved. Corrosion resistance.

Further, it is also clear that since C and N are dissolved in the iron phase of the Vostian, the amount of C and N is more than that of the prior fixation of C and N by only Ti or Nb. It can prevent sensitization. The inventors have systematically investigated the steel components that can obtain the above effects, and found that the above effects can satisfy the following formula (1) in the content of C, N, Ni, Mn, and Cu in which the iron phase of the Vostian is stabilized. Obtained when the situation arises.

0.50<25×C+18×N+Ni+0.11×Mn+0.46×Cu (1)

Further, the element symbol in the formula means the content (% by mass) of each element.

Further, the present effect can be combined with the immobilization of C and N by adding Ti or Nb as a prior art, so that even if it is not necessary to excessively add Ti or Nb, corrosion resistance of the welded portion which is superior to the prior steel can be obtained. Sex.

The present invention has been completed based on the above knowledge findings, and its subject matter is as follows Said.

[1] A ferrite-based iron-based stainless steel excellent in corrosion resistance of a welded portion, comprising: C: 0.001 to 0.025% by mass, Si: 0.05 to 0.30%, Mn: 0.35 to 2.0%, P: 0.05 % or less, S: 0.01% or less, Al: 0.05 to 0.80%, N: 0.001 to 0.025%, Cr: 16.0 to 20.0%, Ti: 0.12 to 0.50%, Nb: 0.002 to 0.050%, Cu: 0.30 to 0.80% Ni: 0.05% or more and less than 0.50%, V: 0.01 to 0.50%, and satisfying the following formula (1), and satisfying the remainder containing Fe and unavoidable impurities; 0.50<25×C+18×N+Ni+0.11×Mn+0.46×Cu (1)

In addition, the element symbol in the formula means the content (% by mass) of each element.

[2] The ferrite-based iron-based stainless steel having excellent corrosion resistance of the welded portion according to [1], which is further characterized by further comprising, in mass%, selected from the group consisting of Zr: 0.01 to 0.50%, W: 0.01 to 0.20%, and REM: 0.001 to 0.10%, Co: 0.01 to 0.20%, B: 0.0002 to 0.010%, and Sb: 0.05 to 0.30%.

According to the present invention, it is possible to obtain a ferrite-based iron-based stainless steel which has excellent corrosion resistance even under the welding condition in which the material of the welding target material penetrates into the base material, such as C and N.

Hereinafter, the reasons for limiting the respective constituent elements of the present invention will be described.

1. About composition and metal structure

First, the reason for specifying the composition of the steel of the present invention will be explained. Furthermore, the component% refers to the mass%.

C: 0.001~0.025%

Elements that are inevitably contained in the C system. The higher the amount of C, the higher the strength, and the less the amount of C, the higher the processability. In order to obtain sufficient strength, it is necessary to contain 0.001% or more. However, when the content is more than 0.025%, the decrease in workability is conspicuous, and the decrease in corrosion resistance (sensitization) due to localized Cr deficiency due to precipitation of Cr carbide is likely to occur. Therefore, the amount of C is set to be in the range of 0.001 to 0.025%. However, in terms of corrosion resistance and workability, the C amount is preferably as low as possible, but the amount of C is extremely lowered, which takes time in refining, and is not preferable in terms of production. Therefore, it is preferably in the range of 0.003 to 0.018%. More preferably, it is in the range of 0.005 to 0.012%.

Si: 0.05~0.30%

The Si system is an effective element for improving the corrosion resistance of the welded portion. In order to obtain this effect, it is necessary to contain 0.05% or more, and the more the content, the greater the effect. However, when the Si content exceeds 0.30%, the formability and toughness of the welded portion are lowered, which is not preferable. Therefore, the amount of Si is set to be in the range of 0.05 to 0.30%. It is preferably in the range of 0.05 to 0.25%. Further, it is preferably in the range of 0.08 to 0.20%.

Mn: 0.35~2.0%

Mn is an especially important element in the present invention. Mn is effective as an element of a deoxidizer and has an effect of stabilizing the iron phase of Vostian. By containing a certain amount of Mn, it is possible to suppress the generation of the iron phase of the ferrite during welding, and to dissolve the solid solution in the solid solution limit by the C and N contained in the solid solution limit of the iron phase of the ferrite. The state in the iron phase of the Volkswagen phase is stabilized, and the effect of preventing sensitization caused by the formation of Cr carbonitride is exhibited, and the corrosion resistance of the welded portion is improved. In order to obtain such effects, it is necessary to contain 0.35% or more of Mn. However, if When the Mn content is more than 2.0%, the base material is excessively hardened and the ductility is lowered, and the toughness due to the hardening of the welded portion is lowered in the welded portion, which is not preferable. Therefore, the amount of Mn is set to be in the range of 0.35 to 2.0%. It is preferably in the range of 0.50 to 1.5%. Further preferably, it is in the range of 0.75 to 1.25%.

P: 0.05% or less

P is inevitably contained in steel, and excessive inclusion reduces weldability and is likely to cause grain boundary corrosion. This tends to become apparent when it contains more than 0.05%. Therefore, the P amount is set to 0.05% or less. It is preferably 0.03% or less.

S: 0.01% or less

S is also an element which is inevitably contained in steel in the same manner as P, and the corrosion resistance is lowered because it contains more than 0.01%. Therefore, the S amount is set to 0.01% or less. It is preferably 0.008% or less.

Al: 0.05~0.80%

Al is also an element which improves the corrosion resistance of the welded portion in the same manner as Si. Since the affinity between Al and N is stronger than that of Cr, when N is mixed into the welded portion, N is precipitated in the form of Al nitride instead of Cr nitride, and the effect of sensitization is suppressed. Further, Al is also an element for deoxidation in the steel making step. These effects can be obtained by containing 0.05% or more. However, when Al is contained in an amount of more than 0.80%, the ferrite grains are coarsened, and workability or manufacturability is lowered. Therefore, the amount of Al is set to be in the range of 0.05 to 0.80%. It is preferably in the range of 0.10 to 0.60%. Further preferably, it is in the range of 0.15 to 0.50%.

N: 0.001~0.025%

The N system is inevitably contained in steel in the same manner as C. If the N content is high, the strength is increased, and the smaller the N content, the higher the workability. In order to obtain sufficient strength, it is appropriate to contain 0.001% or more. However, if the content exceeds 0.025%, the ductility is remarkably lowered, and the corrosion resistance due to the precipitation of the promoted Cr nitride is lowered, which is not preferable. Therefore, the amount of N is set to be in the range of 0.001 to 0.025%. From the viewpoint of corrosion resistance, the lower the N, the better. However, in order to reduce the amount of N, it is necessary to increase the refining time, resulting in a decrease in manufacturability. Therefore, it is preferably in the range of 0.003 to 0.025%. More preferably, it is in the range of 0.003 to 0.015%. Further, it is preferably in the range of 0.003 to 0.010%.

Cr: 16.0~20.0%

Cr is the most important element to ensure the corrosion resistance of stainless steel. If the Cr content is less than 16.0%, oxidation due to welding may not be sufficient to obtain sufficient corrosion resistance in the bead of the surface layer where Cr is reduced or the periphery thereof. Further, it is further advantageous because it is sensitized by the N of the material to be welded or the N mixed in at the time of welding. On the other hand, when the Cr content exceeds 20.0%, the toughness is lowered or the rust-removing property after annealing is lowered, which is not preferable. Therefore, the amount of Cr is set to be in the range of 16.0% to 20.0%. It is preferably in the range of 16.5% to 19.0%. Further preferably, it is in the range of 17.0 to 18.5%.

Ti: 0.12~0.50%

The Ti system preferentially combines with C and N to suppress an element having a reduced corrosion resistance due to sensitization caused by precipitation of Cr carbonitride. This effect can be obtained by containing 0.12% or more. However, if the content exceeds 0.50%, coarse Ti carbonitride is formed, which causes surface defects and is therefore unsatisfactory. Therefore, the amount of Ti is set to be in the range of 0.12 to 0.50%. It is preferably in the range of 0.15 to 0.40%. Further preferably, it is in the range of 0.20 to 0.35%.

Nb: 0.002~0.050%

The Nb system preferentially combines with C and N to suppress an element having a reduced corrosion resistance due to sensitization caused by precipitation of Cr carbonitride. Further, Nb also has an effect of improving the toughness and flexibility of the welded portion by making the crystal grain size of the welded portion fine. These effects can be obtained by containing 0.002% or more. On the other hand, Nb is also an element which raises the recrystallization temperature, and if it contains more than 0.050%, the annealing temperature required for recrystallization is increased, so annealing does not occur in the annealing step using the high-speed cold-rolled sheet annealing line. Sufficient, it is less preferable because the workability due to the presence of unrecrystallized grains and recrystallized grains is reduced. Therefore, the amount of Nb is set to be in the range of 0.002 to 0.050%. It is preferably in the range of 0.010 to 0.045%. Further preferably, it is in the range of 0.015 to 0.040%.

Cu: 0.30~0.80%

Cu is an element which improves the corrosion resistance, and is an element which is particularly effective for improving the corrosion resistance of the base material and the welded portion in the aqueous solution or when the weak acid water droplets are attached. In addition, the Cu system is a strong Worstian iron-forming element similarly to Ni, and has an effect of suppressing generation of a ferrite-grain iron phase in the welded portion and suppressing sensitization caused by precipitation of Cr carbonitride. These effects can be obtained by containing 0.30% or more. On the other hand, when Cu is contained in an amount of more than 0.80%, hot workability is lowered, which is not preferable. Therefore, the amount of Cu is set to be in the range of 0.30 to 0.80%. It is preferably in the range of 0.30 to 0.60%. Further preferably, it is in the range of 0.35 to 0.50%.

Ni: 0.05% or more and less than 0.50%

Ni is an element which improves the corrosion resistance of stainless steel and is an element which suppresses corrosion in a corrosive environment in which active film is not formed by forming a passive film. Also, Ni is stronger than Voss The iron-forming element has an effect of suppressing the formation of ferrite iron in the welded portion and suppressing sensitization caused by precipitation of Cr carbonitride. These effects can be obtained by containing 0.05% or more. However, when Ni is contained in an amount of 0.50% or more, the workability is lowered, and the stress corrosion cracking sensitivity is enhanced. Further, since Ni is an expensive element, the manufacturing cost is increased, which is not preferable. Therefore, the amount of Ni is set to be 0.05% or less and less than 0.50%. It is preferably in the range of 0.10 to 0.30%. Further preferably, it is in the range of 0.15 to 0.25%.

V: 0.01~0.50%

The V-based element which improves corrosion resistance and workability has the effect of suppressing the formation of Cr nitride by bonding with N when N is mixed in the welded portion, and the effect of sensitization of the welded portion is reduced. This effect can be obtained by containing 0.01% or more. However, if the content exceeds 0.50%, the workability is lowered, which is not preferable. Therefore, the V amount is set to be in the range of 0.01 to 0.50%. It is preferably in the range of 0.05 to 0.30%. Further, it is preferably in the range of 0.08 to 0.20%.

0.50<25×C+18×N+Ni+0.11×Mn+0.46×Cu (1)

Further, the element symbol in the formula means the content (% by mass) of each element.

In order to fix the C and N of the welded portion in the Worthfield iron phase, it is necessary to produce the Worthfield iron phase in the cooled structure after welding. In order to generate the Worthfield iron phase during the cooling process after welding, the above formula (1) must be satisfied. When the right side of the formula (1) is 0.50 or less, the stabilization of the iron phase of the Vostian is insufficient, and it is impossible to effectively immobilize the solid solution C and the solid solution N provided by the present invention in the welded portion. Worthfield iron phase. Therefore, the right side of the formula (1) is set to exceed 0.5. It is preferably 0.60 or more. More preferably, it is 0.70 or more.

The above is the basic chemical composition of the present invention, and the remainder contains Fe and unavoidable impurities. Furthermore, as an unavoidable impurity, Ca: 0.0020% can be tolerated under.

Further, in addition to the above-described basic components, the following elements may be contained for the purpose of suppressing sensitization of the bead and improving corrosion resistance and the like.

Zr: 0.01~0.50%

Zr has an effect of inhibiting sensitization by combining with C and N. This effect can be obtained by containing 0.01% or more. However, if it contains more than 0.50%, workability will fall. Further, since Zr is an expensive element, excessive addition causes an increase in manufacturing cost and is therefore unsatisfactory. Therefore, in the case of containing Zr, it is preferably in the range of 0.01 to 0.50%. More preferably, it is in the range of 0.10 to 0.35%.

W: 0.01~0.20%

The W system has an effect of improving corrosion resistance similarly to Mo. This effect can be obtained by containing 0.01% or more. However, when the content is more than 0.20%, the strength is increased, and the manufacturability due to an increase in rolling load or the like is lowered, which is not preferable. Therefore, in the case where W is contained, it is preferably in the range of 0.01 to 0.20%. More preferably, it is in the range of 0.05 to 0.15%.

REM: 0.001~0.10%

REM has an effect of improving oxidation resistance, and is effective for suppressing the formation of scale and suppressing the formation of a Cr-deficient region directly under the fusion tempering color. In order to obtain this effect, it is necessary to contain 0.001% or more. However, when it contains more than 0.10%, the manufacturability, such as pick-up property, will fall. Further, since REM is an expensive element as Zr, excessive addition causes an increase in manufacturing cost, which is not preferable. Therefore, in the case of containing REM, it is preferably set to 0.001. ~0.10% range. More preferably, it is in the range of 0.010 to 0.08%.

Co: 0.01~0.20%

Co is an element that enhances toughness. This effect can be obtained by containing 0.01% or more. On the other hand, if it contains more than 0.20%, the manufacturability will fall. Therefore, in the case of containing Co, it is preferably in the range of 0.01 to 0.20%. More preferably, it is in the range of 0.05 to 0.15%.

B: 0.0002~0.010%

B is an element which improves the brittleness of secondary processing, and the effect can be obtained by containing 0.0002% or more. However, if it is more than 0.010%, the decrease in ductility due to excessive solid solution strengthening is induced. Therefore, in the case where B is contained, it is preferably in the range of 0.0002 to 0.010%. More preferably, it is in the range of 0.0010 to 0.0075%.

Sb: 0.05~0.30%

Sb is the same as Al, and when the gas shielding of TIG welding is insufficient, it has the effect of capturing N mixed in the atmosphere, and is applied to a structure having a complicated shape in which sufficient gas shielding is difficult. Especially effective element. This effect can be obtained by containing 0.05% or more. However, if it contains more than 0.30%, a non-metal intermediaries are formed in the steel making process, and the surface property is deteriorated. Moreover, the toughness of the hot rolled sheet deteriorates. Therefore, in the case of containing Sb, it is preferably in the range of 0.05 to 0.30%. More preferably, it is in the range of 0.05 to 0.15%.

2. About manufacturing conditions

Next, a preferred method of producing the steel of the present invention will be described. Composition of the above ingredients The molten steel is melted by a known method such as a converter, an electric furnace, or a vacuum melting furnace, and a steel material (steel billet) is produced by a continuous casting method or an ingot-opening method. The slab is heated at 1100 to 1250 ° C for 1 to 24 hours, then hot rolled, or directly hot rolled in a state of being cast without heating, to obtain a hot rolled sheet.

Usually, the hot rolled sheet is annealed at 800 to 1100 ° C for 1 to 10 minutes. Further, the hot rolled sheet annealing may be omitted depending on the application. Then, after the hot-rolled sheet is pickled, the cold-rolled sheet is formed by cold rolling, and then recrystallization annealing and pickling are carried out to obtain a product.

From the viewpoints of elongation, flexibility, press formability, and shape, cold rolling is preferably carried out at a rolling reduction ratio of 50% or more. The recrystallization annealing of the cold-rolled sheet is preferably carried out in the case of the surface finishing of JIS G 0203, the finished product of No. 2B, and the mechanical properties at 800 to 950 ° C in terms of pickling. . Further, it is effective for the member requiring a more lustrous portion to perform BAR annealing (gloss annealing). Further, in order to further improve the surface properties after cold rolling and after the processing, polishing or the like may be performed.

[Example 1]

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

The stainless steel having the chemical composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. The steel blocks were heated at 1,150 ° C for 1 hour, and then hot rolled to obtain a hot rolled sheet having a thickness of 3.5 mm. Then, the hot-rolled sheet was annealed at 950 ° C for 1 minute, and then subjected to bead blasting treatment on the surface, followed by immersion in a 20% by mass sulfuric acid solution at a temperature of 80 ° C for 120 seconds. The mixture was immersed in a mixed acid of 15% by mass of nitric acid and 3% by mass of hydrofluoric acid at a temperature of 55 ° C for 60 seconds to carry out pickling and derusting.

Further, cold rolling was performed until the thickness of the sheet was 0.8 mm, and recrystallization annealing was performed at 900 ° C for 1 minute in a weakly reducing atmosphere (hydrogen: 5 vol%, nitrogen: 95 vol%, dew point: -40 ° C) to obtain cold rolling. Annealed plate. The cold rolled annealed sheet is at a temperature of 50 ° C, The mixed acid solution containing 15% by mass of nitric acid and 0.5% by mass of hydrochloric acid is subjected to electrolytic pickling to thereby perform a rust removing treatment to obtain a cold-rolled pickling annealed sheet.

Furthermore, Table 1-1 and Table 1-2 are a series of consecutive tables.

The cold-rolled sheet produced and the commercially available Vostian iron-based stainless steel SUS304 (C: 0.07 mass%, N: 0.05 mass%, Cr: 18.2 mass%, and Ni: 8.2 mass%) were cooled to a thickness of 0.8 mm. The rolled sheet was subjected to butt TIG welding (the cold rolled sheet of the present invention: base material, welding target material: SUS304). The welding current was set to 90 A, the welding speed was set to 60 cm/min, and the shielding gas system used argon gas containing 8 vol% of nitrogen and 2 vol% of oxygen at 15 L/min. The obtained bead width on the front side was about 3 mm.

The following test was carried out using the prepared test piece containing the bead.

1. Pitting potential test of base metal and welded joint

A test piece of 20 mm square was taken from the test piece after the cold rolling annealing and the test piece after the welding, and a test piece having a measurement surface of 10 mm square and covered with a sheet was produced. For the test piece after welding, a test piece was taken in such a manner as to include a bead, and the tempering color (oxidation film) generated by welding was directly left. The pitting potentials of the base material and the welded portion were measured for the test pieces in a 3.5 mass% NaCl solution at 30 °C. At the time of measurement, the test piece was not subjected to grinding or passivation treatment, and the other measurement methods were based on JIS G 0577 (2005).

The pitting potential of the base material: 150 mV or more, and the pitting potential of the welded portion: 0 mV or more was set as pass.

2. Neutral salt water spray cycle test

A test piece containing a bead of 100 mm square was used in the self-welding test piece, and the surface was subjected to finish grinding using #600 sandpaper, and the end surface portion was sealed to prepare the above test piece, and the neutral salt water specified in JIS H 8502 was prepared. Spray cycle test. Neutral salt spray cycle test Laboratory 5% by mass NaCl solution spray (35 ° C, 2 h) → dry (60 ° C, 4 h, relative humidity 20 ~ 30%) → moist (40 ° C, 2 h, relative humidity 95% or more) for 1 cycle . The case where the test piece was not subjected to corrosion from the base material or the welded portion after 15 cycles was set as a pass.

3. Determination of grain boundary coverage of ferrite grain iron phase caused by Cr carbonitride

A test piece for metal structure observation was taken in the direction perpendicular to the bead of the welded test piece, and after mirror polishing, the metal structure and the precipitate were exposed by etching with a picric acid aqueous solution, and scanning electrons were used. Microscope and energy dispersive X-ray spectroscopy were used to observe the structure and identify the phases of the precipitates, and to measure the grain boundary coverage of the ferrite phase of the bead portion by Cr carbonitride.

The grain boundary coverage ratio is measured by the image analyzer to measure the grain boundary length of the crystal grains in the photographed tissue, and the image analysis device is used to measure the grain boundary parallel to the Cr carbonitride precipitated on the grain boundary. The diameter is calculated according to the grain boundary coverage ratio (%) = (the total of the diameters in the direction parallel to the grain boundary of the Cr carbonitride on the grain boundary) ÷ (the total of the grain boundary lengths of the crystal grains) × 100.

The case where the grain boundary coverage ratio of the ferrite grain iron phase by Cr carbonitride was 40% or less was regarded as acceptable.

4. Evaluation of mechanical properties

A tensile test piece of JIS No. 13B was taken from the cold-rolled annealed sheet produced in parallel with the rolling direction, and a tensile test was carried out in accordance with JIS Z2241, and the elongation at break was measured.

The elongation at break was 25% or more.

5. Surface quality evaluation

After the cold rolling annealing, the surface of the rust-removed steel sheet was observed with the naked eye to confirm whether there were no surface defects such as poor rust removal and linear flaws.

A good surface appearance is obtained by leaving no scale residue or surface defects.

The test results are shown in Table 2.

Steels No. 1 to 20 satisfying the requirements of the present invention have a pitting potential of 150 mV or more, and a pitting potential of the bead of 0 mV or more, and no corrosion occurs even by a neutral salt spray cycle test, even if In the case of welding with the Worthfield iron-based stainless steel, sufficient corrosion resistance can be obtained. Further, in the steel Nos. 1 to 20, the grain boundary coverage ratio of the ferrite-grained iron phase by Cr carbonitride after welding was 40% or less, and a predetermined anti-sensitization effect was obtained. Further, good processing characteristics in which the elongation at break based on the tensile test was 25% or more were obtained, and surface defects were not confirmed.

On the other hand, in Steel No. 21 in which the amount of Cr exceeded the range of the present invention, the toughness of the steel sheet after hot rolling was remarkably lowered, and the subsequent production steps could not be performed, and the characteristics could not be evaluated.

In steel No. 22 in which the amount of Cr is lower than the range of the present invention, sufficient pitting potential cannot be obtained, and in the neutral salt spray cycle test, corrosion occurs from the base material and the welded portion, and the predetermined corrosion resistance is not obtained. .

In Steel No. 23 in which the amount of Mn is less than the range of the present invention, although the base material has obtained sufficient corrosion resistance, the welded portion has not obtained sufficient corrosion resistance.

On the other hand, in Steel Nos. 24 and 25 in which the amount of Mn or the amount of Al exceeds the range of the present invention, although the corrosion resistance of the predetermined base material and the welded portion is obtained, the ductility is lowered due to the hardening of the steel sheet, and the predetermined property is not obtained. Mechanical specials.

In Steel No. 26 in which the amount of Ti exceeds the range of the present invention, although the predetermined corrosion resistance and mechanical properties have been obtained, surface defects caused by the formation of a large amount of coarse Ti-based interpolymer have not been obtained, and a predetermined surface property has not been obtained.

Although the content of each element satisfies the scope of the present invention, in the steel No. 27 to 30 in which the content of the stabilizing element of the Vostian iron is lower than the range of the formula (1), the corrosion resistance and mechanical properties of the predetermined base material have been obtained. . However, it is impossible to obtain the pitting potential and melting of the predetermined bead portion. Corrosion resistance of the joint. When the cross-sectional structure of the welded portion of No. 27 to 30 was investigated, it was confirmed that a large number of Cr carbonitrides having a grain boundary coverage ratio of 50% or more were precipitated at the grain boundary of the ferrite grain. It is considered that in the steel No. 27 to 30, since the stabilizing element of the Worthite iron is insufficient, the iron phase of the ferrite is generated during the cooling after the welding, and the formation of the Cr carbonitride on the grain boundary cannot be suppressed, and the result is conspicuous. Sensitized, but failed to obtain the corrosion resistance of the established weld.

As is apparent from the above results, according to the present invention, it is possible to obtain a ferrite-based iron-based stainless steel having excellent corrosion resistance, mechanical properties, and surface properties without excessive addition of Ti or Nb.

(industrial availability)

The ferrite-based iron-based stainless steel obtained in the present invention is preferably used for a structure in which a structure is produced by welding, such as an automobile exhaust material such as a muffler, a building material, a vent, a building material such as a pipe, and the like.

Claims (2)

A ferrite-based iron-based stainless steel characterized by containing C: 0.001 to 0.025%, Si: 0.05 to 0.30%, Mn: 0.35 to 2.0%, P: 0.05% or less, S: 0.01% or less, and Al by mass%. : 0.05~0.80%, N: 0.001~0.025%, Cr: 16.0~20.0%, Ti: 0.12~0.50%, Nb: 0.002~0.050%, Cu: 0.30~0.80%, Ni: 0.05% or more and less than 0.50 %, V: 0.01 to 0.50%, and satisfy the following formula (1), and the remainder contains Fe and unavoidable impurities; 0.50 < 25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1 Further, the symbol of the element in the formula means the content (% by mass) of each element. The ferrite-based iron-based stainless steel according to the first aspect of the patent application is further selected from the group consisting of Zr: 0.01 to 0.50%, W: 0.01 to 0.20%, REM: 0.001 to 0.10%, and Co: 0.01 to 0.20. % or more of B: 0.0002 to 0.010% and Sb: 0.05 to 0.30%.