JP6390594B2 - Ferritic stainless steel - Google Patents

Description

  The present invention relates to a ferritic stainless steel excellent in removability of oxide scale formed on a surface by a heating process in an oxidizing environment such as welding or heat treatment.

  Compared with austenitic stainless steel, ferritic stainless steel has various excellent characteristics such as high cost performance with respect to corrosion resistance, good thermal conductivity, low thermal expansion coefficient, and resistance to stress corrosion cracking. For this reason, ferritic stainless steel is applied to a wide range of uses such as automobile exhaust system members, building materials such as roofs and fittings, and water-related materials such as kitchens and water / hot water storage tanks.

  In producing these structures, an oxide scale may be formed on the surface by welding or heat treatment. When the oxide scale is formed on the surface of the stainless steel, the corrosion resistance of the stainless steel is lowered. Therefore, the formed oxide scale may be removed in applications that require excellent corrosion resistance.

  On the other hand, the corrosion resistance required for stainless steel is becoming stricter year by year, and high alloying with a corrosion resistance improving element such as Cr and Mo is progressing. While these elements improve the corrosion resistance, the removal of oxide scale by acid treatment or electrolytic treatment is reduced, so that removal of the oxide scale becomes difficult as the alloy becomes higher. Therefore, a ferritic stainless steel that has excellent corrosion resistance and excellent oxide scale removability is desired.

Patent Document 1 discloses a ferritic stainless steel excellent in oxide scale removability. The invention described in Patent Document 1 is an invention that improves the removability of the weld temper color by adding a large amount of Ti to reveal 30 / mm ² or more of TiN on the surface. However, in the present invention, since a large amount of Ti is contained and a large amount of TiN is precipitated, the toughness at low temperatures is low, and there is a problem in use in cold regions.

JP 2014-84482 A

  In view of the above-mentioned problems of the prior art, the present invention is a ferritic stainless steel having improved corrosion scale and low temperature toughness, and improved ferritic stainless steel with improved removal of oxide scale formed on the surface of the stainless steel. The purpose is to provide.

  In the present invention, in order to solve the above-mentioned problems, the effects of various additive elements and inclusions on the removability of oxide scale have been intensively studied. As a result, the following findings were found.

  It is well known that the oxide scale is enriched with easily oxidizable elements having strong affinity for oxygen such as Si and Al. When these oxidizable elements form a dense oxide film, it becomes difficult to remove the oxide scale. In the range of the contents of Si and Al in ordinary stainless steel, Si concentrates relatively outside and Al relatively inside in the oxide scale, and each forms a dense oxide film layer. These oxide films are difficult to change because they are hardly changed by the acid used for removing the oxide scale or electrolytic treatment.

  It has been found that when the Si content is reduced as much as possible and the Al content is large, the structure of the oxide film changes and a coarse, relatively thick oxide scale is formed. Almost no Si concentration was observed in this oxide scale, and Al was concentrated in a wide range in the thickness direction of the oxide scale. Since this oxide scale has a rough structure, compared to the oxide scale formed on conventional stainless steel, the penetration of acid and electrolyte is easier. Therefore, the oxide scale can be easily removed. Furthermore, it was found that by allowing coarse Al oxide to be scattered on the surface of the steel sheet, the penetration of the electrolyte solution can be promoted starting from the oxide scale defects near the oxide.

  The present invention has been completed based on the above findings. The gist of the present invention is as follows.

[1] C: 0.001 to 0.030% by mass%, Si: 0.01 to 0.15%, Mn: 0.01 to 0.50%, P: 0.05% or less, S: 0 0.01% or less, Cr: 18.0 to 24.0%, Ni: 0.01 to less than 0.20%, Mo: 0.1 to 3.0%, Al: more than 0.15% to 1.2 %, V: 0.01 to 0.50%, Nb: 0.01 to 0.50%, Ti: 0.01 to 0.30%, N: 0.001 to 0.030%, the balance Is a ferritic stainless steel characterized by comprising Fe and unavoidable impurities, and Al ₂ O ₃ having a particle size of 1 μm or more is present on the surface at a density of 10 pieces / mm ² or more.

  [2] The component composition is mass%, and Zr: 1.0% or less, W: 1.0% or less, REM: 0.1% or less, Co: 0.3% or less, B: 0.00. 1 type or less of any 1 type or 2 types or more are contained, Ferritic stainless steel as described in [1] characterized by the above-mentioned.

  According to the present invention, a ferritic stainless steel having excellent oxide scale removability can be obtained.

  According to the present invention, the removal property of oxide scale can be enhanced while enhancing the corrosion resistance with Cr, Mo or the like.

  According to the present invention, the removability of oxide scale can be enhanced while suppressing the Ti content. Therefore, unlike the case where the Ti content is high, there is no problem that the low temperature toughness is greatly reduced.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

  The component composition of the ferritic stainless steel of the present invention is C: 0.001 to 0.030% in mass%, Si: 0.01 to 0.15%, Mn: 0.01 to 0.50%, P: 0.05% or less, S: 0.01% or less, Cr: 18.0 to 24.0%, Ni: 0.01% to less than 0.20%, Mo: 0.1 to 3.0%, Al : More than 0.15% to 1.2%, V: 0.01 to 0.50%, Nb: 0.01 to 0.50%, Ti: 0.01 to 0.30%, N: 0.001 -0.030% is contained, and the balance consists of Fe and inevitable impurities.

  Further, the above component composition is mass%, and Zr: 1.0% or less, W: 1.0% or less, REM: 0.1% or less, Co: 0.3% or less, B: 0.1 Any 1 type or 2 types or less of% or less may be contained.

  Hereinafter, each component will be described. In the following description, “%” indicating the content of each element is “% by mass” unless otherwise specified.

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 it is low, the workability is improved. In order to obtain sufficient strength, it is appropriate that the C content is 0.001% or more. On the other hand, when the C content exceeds 0.030%, the workability is remarkably lowered, and Cr carbide is precipitated, and the corrosion resistance is likely to be lowered due to local Cr deficiency. Therefore, the C content is set to 0.001 to 0.030%. More preferably, it is 0.002 to 0.018%.

Si: 0.01 to 0.15%
Si is an element useful for deoxidation. The effect is acquired by making Si content 0.01% or more. On the other hand, Si is also an element that concentrates on the oxide scale, strengthens the oxide scale, and makes it difficult to remove the oxide scale by acid or electrolysis. In particular, in completing the present invention, it has been clarified that the inclusion of a large amount of Si promotes the concentration of Al to a narrow range in the oxide scale and promotes the formation of a dense oxide scale. Therefore, suppression of the Si content is one of the important factors for improving the removability of oxide scale for the present invention. When the Si content exceeds 0.15%, densification of the Al oxide scale becomes significant. Therefore, the Si content is set to 0.01 to 0.15%. More preferably, it is 0.03 to 0.12%.

Mn: 0.01 to 0.50%
Mn is an element inevitably contained in steel and has an effect of increasing strength. The effect is acquired by making Mn content 0.01% or more. On the other hand, excessive addition of Mn promotes the precipitation of MnS, which is the starting point of corrosion, and lowers the corrosion resistance. For this reason, the Mn content is suitably 0.50% or less. Therefore, the Mn content is set to 0.01 to 0.50%. More preferably, it is 0.05 to 0.40%.

P: 0.05% or less P is an element inevitably contained in steel. Excessive content decreases weldability and easily causes intergranular corrosion. This tendency becomes remarkable when the content exceeds 0.05%. Therefore, the P content is set to 0.05% or less. More preferably, it is 0.04% or less.

S: 0.01% or less S is an element inevitably contained in steel. If the S content exceeds 0.01%, the formation of MnS, which is the starting point of corrosion, is promoted to lower the corrosion resistance. Therefore, the S content is set to 0.01% or less.

Cr: 18.0 to 24.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. The effect is acquired by making Cr content 18.0% or more. On the other hand, Cr is an element that concentrates on an oxide scale to reduce its removability. If the Cr content exceeds 24.0%, formation of dense Cr oxide scale becomes remarkable, and it becomes more difficult to remove the oxide scale. Therefore, the Cr content is 18.0 to 24.0%. More preferably, it is 19.0-22.0%, More preferably, it is 20.1-21.5%.

Ni: 0.01% to less than 0.20% Ni is an element that suppresses active dissolution of stainless steel and suppresses the progress of corrosion in an environment where a passive film cannot be maintained. The effect is acquired by making Ni content 0.01% or more. On the other hand, when the Ni content is 0.20% or more, the dissolution of the stainless steel immediately below the oxide scale is suppressed, making it difficult to remove the oxide scale. Therefore, the Ni content is set to 0.01% to less than 0.20%. More preferably, it is 0.02 to 0.16%.

Mo: 0.1-3.0%
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 0.1% or more. However, if the Mo content exceeds 3.0%, densification of the oxide scale becomes remarkable and its removal becomes difficult. Therefore, the Mo content is set to 0.1 to 3.0%. More preferably, it is 0.4 to 1.8%.

Al: more than 0.15% to 1.2% or less Al is an important element for forming a sparse oxide scale in the present invention. Al is an element that normally concentrates on the oxide scale and prevents its dissolution. However, as a result of detailed studies by the present inventors, scale growth is promoted and a sparse oxide scale is formed when a large amount of Al exceeding 0.15% is contained under conditions where the Si content is low. Has been found to be easier to do. That is, when the Al content exceeds 0.15%, Al ₂ O ₃ having a particle size of 1 μm or more, which will be described later, can be formed. On the other hand, if the Al content exceeds 1.2%, the oxide scale becomes too thick and it becomes difficult to remove the scale. Therefore, the content of Al is set to more than 0.15% to 1.2% or less. More preferably, it is more than 0.15% to 1.0% or less.

V: 0.01 to 0.50%
V is an element that combines with N in steel to suppress a decrease in corrosion resistance due to sensitization. The effect is acquired by making V content 0.01% or more. However, if the V content exceeds 0.50%, the workability deteriorates, which is not preferable. Therefore, the content of V is set to 0.01 to 0.50%. More preferably, it is 0.02 to 0.40%.

Nb: 0.01 to 0.50%
Nb is an element that binds preferentially to C and N and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. The effect is acquired by making Nb content 0.01% or more. However, if the Nb content exceeds 0.50%, the hot strength increases, the hot rolling load increases, and the productivity decreases. Further, it is liable to precipitate at the crystal grain boundary of the weld and cause a weld crack. Therefore, the Nb content is set to 0.01 to 0.50%. More preferably, it is 0.07 to 0.38%.

Ti: 0.01 to 0.30%
Ti is an element that binds preferentially to C and N and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. The effect is obtained when the Ti content is 0.01% or more. However, when the Ti content exceeds 0.30%, the workability deteriorates, and the Ti carbonitride becomes coarse and causes surface defects. Therefore, the Ti content is set to 0.01 to 0.30%. More preferably, it is 0.07 to 0.30%.

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 content of N is set to 0.001 to 0.030%. More preferably, it is 0.002 to 0.018%.

  As described above, the composition of the ferritic stainless steel of the present invention may include the following components.

Zr: 1.0% or less Zr combines with C and N and has an effect of suppressing sensitization. The effect is acquired by making Zr content 0.01% or more. However, excessive Zr content decreases workability and Zr is an extremely expensive element, and excessive content of Zr causes an increase in cost. Therefore, the content of Zr is set to 1.0% or less.

W: 1.0% or less W, like Mo, has an effect of improving corrosion resistance. The effect is obtained when the W content is 0.01% or more. However, excessive W content increases strength and decreases manufacturability. Therefore, the content of W is set to 1.0% or less.

REM: 0.1% or less REM has an effect of improving oxidation resistance, suppressing the formation of oxide scale, and suppressing the formation of a Cr-deficient region directly under the temper collar of welding. The effect is obtained when the REM content is 0.0001% or more. However, the excessive content of REM reduces the productivity such as pickling properties and increases the cost. Therefore, the REM content is set to 0.1% or less.

Co: 0.3% or less Co is an element that improves toughness. The effect is obtained when the Co content is 0.001% or more. However, excessive Co content reduces manufacturability. Therefore, the content of Co is set to 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, it is appropriate that the B content is 0.0001% or more. However, excessive addition causes a decrease in ductility due to solid solution strengthening. Therefore, the B content is set to 0.1% or less.

  The balance other than the above is Fe and inevitable impurities. Inevitable impurities include Zn: 0.03% or less, Sn: 0.3% or less, Cu: less than 0.10%, and the like. In addition, in the ferritic stainless steel having excellent corrosion resistance having the Cr content and the Mo content according to the present invention, Cu has the effect of increasing the passive state maintaining current to make the passive film unstable and lowering the corrosion resistance. From this viewpoint, it is better not to contain Cu. When Cu is contained, its content is suitably less than 0.10%. Therefore, the content of Cu as an impurity is set to less than 0.10%.

In the ferritic stainless steel of the present invention, Al ₂ O ₃ having a particle size of 1 μm or more is present on the surface at a density of 10 pieces / mm ² or more.

Al ₂ O ₃ having a particle size of 1 μm or more is present on the surface at a density of 10 pieces / mm ² or more. The removal of the oxidized scale of stainless steel is generally performed by electrolysis in an acid or an electrolytic solution. However, if the oxide scale is sufficiently dense, it is difficult to remove the oxide scale because the acid and the electrolyte do not permeate the base iron just below the scale. At this time, the inventors have found that the presence of coarse inclusions on the surface facilitates the penetration of the acid. In the present invention, the Al content is increased, and a large number of Al ₂ O ₃ having a particle diameter of 1 μm or more is generated. After removal of the oxide scale, it was revealed that the removal property of the oxide scale is improved when Al ₂ O ₃ is present on the surface at a density of 10 pieces / mm ² or more. Therefore, in the present invention, Al ₂ O ₃ having a particle diameter of 1 μm or more is present on the surface at a density of 10 pieces / mm ² or more. The density of Al ₂ O ₃ present on the surface can be adjusted by adjusting the amount of pickling reduced by pickling immediately before becoming a product. On the other hand, when a large amount of Al ₂ O ₃ is present on the surface, the agglomerated Al ₂ O ₃ forms a surface defect, so that Al ₂ O ₃ having a particle diameter of 1 μm or more is 1.0 × 10 ⁵ pieces / mm on the surface. It is preferably present at a density of ² or less.

  Then, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.

  The ferritic stainless steel of the present invention may be manufactured using any manufacturing method, but an example of a preferable manufacturing method is shown below.

After the steel material having the above composition is heated to 1100 to 1300 ° C., hot rolling is performed so that the finishing temperature is 700 to 1000 ° C. and the coiling temperature is 500 to 850 ° C. and the plate thickness is 2.0 to 5.0 mm. . The hot-rolled steel strip thus produced is annealed at a temperature of 800 to 1200 ° C. and pickled, then cold-rolled and cold-rolled sheet annealed at a temperature of 700 to 1100 ° C. After cold-rolled sheet annealing, pickling is performed, the pickling loss is 0.5 g / m ² or more, the steel sheet surface is removed by 0.05 μm or more on both sides, and Al ₂ O ₃ appears on the surface. By this pickling, Al ₂ O ₃ having a particle size of 1 μm or more present on the surface is made 10 pieces / mm ² or more. Acid pickling methods include acid pickling such as sulfuric acid pickling, nitric acid pickling, and nitric hydrofluoric acid pickling, and / or electrolytic pickling such as neutral salt electrolytic pickling and nitric hydrochloric acid electrolytic pickling. These pickling methods may be combined. It may also be made to appear the Al ₂ O ₃ on the surface by a method other than pickling. 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 plate thickness of 4 mm, annealed in the range of 850 to 1100 ° C., and the scale was removed by pickling. Furthermore, it cold-rolled to plate thickness 0.8mm, and annealed in the range of 800-1000 degreeC. Then, electrolytic pickling was performed at 70 to 150 C / dm ² in a mixed acid of 15 mass% nitric acid and 10 mass% hydrochloric acid to obtain a test material. No. In all cases except 19, the pickling weight loss was 0.5 g / m ² or more. No. No. 19 had a pickling weight loss of 0.33 g / m ² .

The surface of the prepared test material was observed by SEM, and the distribution density of Al ₂ O ₃ present on the surface was determined. SEM was used to observe 10 fields of view of an arbitrary 100 μm × 100 μm range (excluding the end) on the surface of the test material, and the precipitates on the surface were observed. Among the observed precipitates, a spherical precipitate having a particle size of 1 μm or more and a ratio of major axis to minor axis (major axis / minor axis) of 1.2 or less was regarded as Al ₂ O ₃ . The number of Al ₂ O _{3 in} 10 fields was counted and averaged, and the number of Al ₂ O ₃ per 1 mm ² was calculated. The particle diameter was measured by measuring the major axis and minor axis of Al ₂ O ₃ observed by SEM, and taking the average as the particle diameter. The measurement results are shown in Table 2.

In order to evaluate the removability of the oxide scale, a test piece of 50 × 80 mm was taken from the prepared test material and subjected to heat treatment at 800 ° C. for 1 min to generate an oxide scale on the surface. The atmosphere was air and the dew point was 25 ° C. The test piece on which the oxide scale was generated was subjected to an electrolytic treatment of 50 C / dm ² in each of the anode and the negative electrode in a 20% Na ₂ SO ₄ solution at 80 ° C.

  After the electrolytic treatment, the element distribution in the depth direction of the weld was measured by GDS (Glow Discharge Spectroscopy). It is determined that the temper color remaining is present when the element that concentrates in a temper color such as Si or Al has a maximum GDS signal intensity of 1.2 times or more of the GDS maximum signal intensity in the surface layer. “X” is marked in the item “Presence / absence of remaining oxide scale”.

  In all of the inventive examples, no concentration of Si, Al or the like was observed on the surface layer by GDS, and the oxide scale removal was good. On the other hand, no. No. 14, Si, No. 15 is Cr, No. 16, Ni, No. In Nos. 17 and 18, Al was out of the scope of the present invention, and in both cases, the scale residue was confirmed by GDS after the electrolytic treatment, and the oxide scale removability was insufficient.

  According to the present invention, a ferritic stainless steel having excellent oxide scale removability can be obtained. Thereby, when removing the oxide scale formed by heat treatment or welding, a ferritic stainless steel from which the oxide scale can be more easily removed by acid treatment or electrolytic treatment is obtained. In other words, it is suitable for applications in which a structure is produced by welding and oxide scale is removed by acid treatment after welding, for example, a can body material for hot water storage of an electric water heater.

Claims (2)

C: 0.001 to 0.030% by mass%, Si: 0.01 to 0.15%, Mn: 0.01 to 0.50%, P: 0.05% or less, S: 0.01% Hereinafter, Cr: 18.0 to 24.0%, Ni: 0.01% to less than 0.20%, Mo: 0.1 to 3.0%, Al: more than 0.15% to 1.2%, V: 0.01 to 0.50%, Nb: 0.01 to 0.50%, Ti: 0.01 to 0.30%, N: 0.001 to 0.030%, the balance being Fe and has a Ru component composition Na unavoidable impurities,
A ferritic stainless steel characterized in that Al ₂ O ₃ having a particle diameter of 1 μm or more is present on the surface at a density of 10 pieces / mm ² or more.   The component composition is in mass%, and Zr: 1.0% or less, W: 1.0% or less, REM: 0.1% or less, Co: 0.3% or less, B: 0.1% or less 2. The ferritic stainless steel according to claim 1, wherein the ferritic stainless steel contains at least one of the above.