JP2005220429A - Ferritic stainless steel sheet having excellent corrosion resistance and workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high purity ferritic stainless steel sheet substitutable for SUS304 of which the rust resistance and crevice corrosion resistance can be increased without largely adding Cr or Mo reducing the workability, and of which the workability can be increased as much as possible. <P>SOLUTION: The steel sheet is composed by compositely admixing high purity ferritic stainless steel stabilized by Ti with V and Sn in addition to Cr and Mo. Thus, its rest resistance and crevice corrosion resistance can be remarkably improved, and further, satisfactory elongation, average r value and ridging properties can be secured. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  INDUSTRIAL APPLICABILITY The present invention is equivalent to SUS304, which is a general-purpose austenitic stainless steel, in terms of weathering resistance in an atmospheric environment and in a salt damage environment, and is excellent in workability represented by elongation and average r value. It is related to ferritic stainless steel sheets with excellent corrosion resistance and workability that can be replaced with SUS304 in some applications.

  SUS304, which is a typical austenitic stainless steel, is excellent in corrosion resistance, workability, and weldability, and is most widely used as a stainless steel. SUS304 contains 8% Ni, but the price of Ni is determined by the LME and is subject to speculation. Scrap, which is a raw material for melting SUS304, is also susceptible to Ni price fluctuations. Thus, the raw material cost of SUS304 is unstable, and as a result, the price of the stainless steel plate has to fluctuate. Under such circumstances, substituting SUS304 with ferritic stainless steel whose price is relatively stable as a measure against Ni soaring is meaningful for both stainless steel manufacturers and users.

  SUS430, which is a representative of ferritic stainless steel, is inferior to SUS304 in both corrosion resistance and workability. A so-called high-purity ferritic stainless steel has been developed as a ferritic stainless steel with improved corrosion resistance and workability. High purity ferritic stainless steel is a steel that has been improved by refining C and N, and also stabilized by Ti and Nb, and has improved intergranular corrosion resistance and workability. The pitting corrosion resistance is improved by adjusting the content. However, even in high-purity ferritic stainless steel, the active dissolution rate is higher than that of SUS304, and if the passive state breaks down and active dissolution proceeds as in crevice corrosion, the corrosion resistance is reduced. In order to compensate for such a decrease in corrosion resistance, it is necessary to increase the amount of Cr or Mo as is generally done in SUS445J2 (22Cr-2Mo- (very low C, N)). However, if the amount of Cr and Mo is increased in this way, the steel is solid solution strengthened and the workability is lowered, and it cannot withstand severe processing such as drawing performed in SUS304.

  Therefore, attempts have been made to improve the corrosion resistance of high-purity ferritic stainless steel without containing Cr and Mo as much as possible and without reducing workability. A method for controlling the composition of a product is disclosed. Examples of these include Patent Document 1, Patent Document 2, and Patent Document 3. However, even with these methods, once active dissolution occurs due to crevice corrosion in high-purity ferritic stainless steel, the dissolution rate is reduced to provide the same level of weather resistance and crevice corrosion resistance as SUS304. There is a need for a new alloy design based on high-purity ferritic alloys that cannot be obtained, and can suppress the growth of pits and the growth of pits within the gap. However, if a certain amount of Ni is added to high-purity ferritic stainless steel, improvement in crevice corrosion resistance can be seen. However, in this case, the stress corrosion cracking resistance, which is the superiority of ferritic stainless steel over SUS304, may be impaired. Therefore, in designing the alloy, it must be considered together with maintaining the features of the conventional ferritic stainless steel.

JP-A-6-279951 JP-A-7-216591 JP-A-9-279231

  The problem to be solved by the present invention is to apply high-purity ferritic stainless steel to applications where SUS304 has been used in the past. Without adding, by improving the corrosion resistance and crevice corrosion resistance of high-purity ferritic stainless steel and at the same time improving the workability as much as possible, the corrosion resistance to the extent that SUS304 can be replaced in some applications of SUS304 The object is to provide high purity ferritic stainless steel with improved workability.

  According to the present invention, in a high purity ferritic stainless steel stabilized with Ti, if V and Sn are added in combination with Cr and Mo, the weather resistance and crevice corrosion resistance can be remarkably improved. This combined addition is based on the inventors' knowledge that there is almost no damage to workability and that good workability can be secured.

The point of the present invention is that
(1)
In mass%, C: 0.001 to 0.010%, Si: 0.01 to 0.5%, Mn: 0.01 to 1.00%, P: 0.03% or less, S 0.01% or less , Cr: 16-20%, Mo: 0.3-1.8%, V: 0.3-2.5%, Sn: 0.1-1.5%, Ti: 0.05-0.25 %, N: 0.001 to 0.020%, a ferritic stainless steel sheet having excellent workability comprising the balance Fe and inevitable impurities.
(2)
(1) Ferritic stainless steel plate with excellent corrosion resistance and workability, containing Al: 0.005 to 0.05% by mass.
(3)
Corrosion resistance and workability according to (1) or (2), containing one or more of Cu: 0.05 to 1.0% and Zr: 0.05 to 0.5% by mass% An excellent ferritic stainless steel sheet.
(4)
The ferritic stainless steel sheet having excellent corrosion resistance and workability according to any one of (1) to (3), which contains Mg: 0.0002 to 0.0050% by mass%.
(5)
The ferritic stainless steel sheet having excellent corrosion resistance and workability according to any one of (1) to (4), which contains B: 0.0002 to 0.0050% by mass%.
(6)
A ferritic stainless steel sheet excellent in corrosion resistance and workability according to any one of (1) to (5), which contains Nb: 0.01 to 0.1% by mass%.
(7)
JASO M.J. A ferritic stainless steel sheet excellent in corrosion resistance and workability in which the crevice corrosion depth generated in 50 cycles of the combined cycle test specified in 610 is 350 μm or less.

  The present invention can improve the resistance to cracking and crevice corrosion, and at the same time improve the workability as much as possible without adding a large amount of Cr or Mo which deteriorates the workability of the ferritic stainless steel.

  The present inventors tried to design a new alloy of high-purity ferrite system that can suppress the growth of pits and the growth of pits in the gap, and conducted laboratory melting of some component systems, in addition to Cr and Mo. It has been found that if V and Sn are added in combination, the weather resistance and crevice corrosion resistance can be remarkably improved, and the combined addition of V and Sn has little damage to workability.

  The present inventors melted steels 1 to 13 having chemical components shown in Table 1 in a laboratory vacuum melting furnace into ingots. In the laboratory, hot-rolling, hot-rolled sheet annealing / pickling, cold rolling, annealing, and pickling were performed to produce a steel plate sample having a thickness of 1 mm. The pickling after cold rolling annealing was performed by simulating salt and nitric acid electrolysis in the actual production line. A JIS No. 13 B tensile test piece was produced from the obtained steel sheet in the rolling direction and subjected to a tensile test to determine yield strength, tensile strength, and elongation. Similarly, JIS No. 13 B tensile specimens were produced from the rolling direction, the perpendicular direction of rolling, and the 45 ° direction of rolling, and the average r value (Rankford value) was measured. Furthermore, the unevenness | corrugation of the test piece width direction when producing a JIS5 tension test piece in the rolling direction and giving 16% elongation distortion was measured with the roughness meter, and the maximum ridging height in a measurement range was measured. As for corrosion resistance, weather resistance and crevice corrosion resistance were evaluated by a combined cycle test. The results are shown in Table 2.

  First, the weather resistance was evaluated by a cycle using artificial seawater, which has been shown to well reproduce the generation and development of stainless steel soot in the atmospheric environment. One cycle was repeated (1) artificial seawater spray 35 ° C 4 hours → (2) dry 60 ° C 2 hours → (3) wet relative humidity 95% 50 ° C 2 hours, up to 6 cycles. The specimen was finished by polishing with # 600 emery paper. As a comparative steel, an actual machine manufacturing material of SUS304 2B 1 mm was used. The condition of the surface after the test was evaluated by the stainless steel association rating number (SARN). SARN is evaluated in 10 stages, from 10 (no start) to 1 (most start). In the evaluation of crevice corrosion resistance, two types of strip-shaped test pieces of 50 mm × 100 mm (large) and 30 mm × 70 mm (small) are prepared, and two types of plates are stacked so that the central portions overlap, and 30 mm × 70 mm Crevice corrosion test pieces were prepared by spot welding at three locations of 15 mm, 35 mm, and 55 mm from the upper side of the sample. The cycle of the combined cycle test follows (1) 5% NaCl spray 35 ° C 2 hours → (2) Dry 60 ° C 2 hours → (3) Wet relative humidity 90% 50 ° C 2 hours according to the test method specified by JASO. One cycle was tested for 50 cycles. After the test, spot welded metal was cut out, the large and small specimens were separated, and the pit depth in the gap near the spot weld metal was measured with a laser microscope, and the maximum pit depth in the measured range was defined as the gap corrosion depth. . In this case, the comparative steel was SUS304 2B.

  FIG. 1 shows the influence of Sn on the corrosion resistance and workability of steels 1 to 6 containing 1.0% V. As for the weather resistance, when adding less than 1% of Sn, SARN is 4 and remarkable wrinkle is shown. However, when 0.3% of Sn is added, it is improved to SARN5, and when 0.5% or more of Sn is added, it is improved to SARN6 which is the same as SUS304. The crevice corrosion depth decreases as the Sn content increases, and when it is added to about 1%, it becomes shallower to the same depth as SUS304. On the other hand, the workability maintains an elongation of 37% and an average r value of 2.1 even when Sn is added up to 1%, and the effect of Sn addition on the workability is small.

  FIG. 2 shows the influence of V on the corrosion resistance and workability of steel 3 and 7 to 13 containing 0.5% Sn. As for the weather resistance, when the V addition amount is 0.05%, the SARN is 4, and remarkable wrinkle is shown. However, when V is added to 0.31% and 0.50%, it is improved to SARN5, and when 0.5% or more of Sn is added, it is improved to SARN6 which is the same as SUS304. The crevice corrosion depth also decreases as the V addition amount increases, and when it is added to 2%, it exhibits crevice corrosion resistance equivalent to or better than SUS304.

  On the other hand, the workability maintains an elongation of 37% and an average r value of 2.1 even when V is added up to 2%, and the effect of V addition on the workability is small.

  As can be seen from FIGS. 1 and 2, in a high-purity ferritic stainless steel stabilized with Ti, the addition of V and Sn in addition to Cr and Mo can significantly improve the weather resistance and crevice corrosion resistance. In addition, the combined addition of V and Sn hardly damages the workability and can ensure good workability.

  Next, the reasons for limiting the components in the present invention will be described.

  Reduction of C reduces the strength of high purity ferritic stainless steel and increases elongation, and also improves the average r value. As for corrosion resistance, if C is reduced, the intergranular corrosion resistance of high purity ferritic stainless steel is improved. Therefore, C is preferably as low as possible in the present invention. However, excessive reduction leads to an increase in refining costs, so the C content is set to 0.001 to 0.010% in consideration of the ability of secondary refining necessary for high purification. A preferred range is 0.001 to 0.005%.

  Si is effective as a deoxidizing element, but excessive inclusion increases the solid solution strengthening of ferrite and causes a decrease in ductility. Therefore, it is limited to the minimum necessary for deoxidation and is 0.01 to 0.5%. A preferable range is 0.05 to 0.15%.

  Mn is also effective as a deoxidizing element. However, as with Si, ferrite is strengthened to reduce ductility, and when added excessively, MnS is formed and the weather resistance is inhibited, so 0.01 to 1.00% Add in range. A preferable range is 0.01 to 0.5%.

  P is a harmful impurity that decreases both corrosion resistance and workability, and should be reduced as much as possible. However, since the reduction of P brings about an increase in refining costs including constraints on raw materials, it is limited to 0.03% or less in the present invention. A preferable range is 0.02% or less.

  Since S forms sulfides in steel and becomes a starting point, it is desirable that S be small in the present invention. Considering refining costs, limit to 0.01% or less. A preferred range is 0.002 to 0.006%.

  Cr is a basic element to ensure the necessary corrosion resistance for stainless steel, and it is 16% to ensure corrosion resistance in indoor environments, weather resistance outdoors, and crevice corrosion resistance in salt damage environments. The above addition is performed. However, excessive addition increases the strength of the ferrite and decreases the ductility of the steel, so the upper limit is made 20%. A preferred range is 17-19%.

  Mo is an element effective for improving local corrosion resistance. However, the decrease in ductility due to the addition of Mo is large, and it should be added as much as possible within a range where the necessary corrosion resistance can be obtained. From this viewpoint, in the present invention, Mo is added in the range of 0.3 to 1.8%. A preferable range is 0.5 to 1.5%.

  V and Sn are important additive elements that form the basis of the present invention. By adding V and Sn in addition to Cr and Mo, the corrosion resistance and crevice corrosion resistance, which are weak points of ferritic stainless steel, can be improved, and an appropriate combination can provide corrosion resistance equivalent to or better than SUS304. In addition, if V and Sn are added while minimizing the use of Cr and Mo, elongation and average r value decrease are small, and excellent workability can be secured together with corrosion resistance. In order to obtain such an effect, V and Sn must be added at least 0.3% or more and 0.1% or more, respectively, as shown in FIGS. However, excessive addition of these elements also degrades workability and saturates the effect of improving corrosion resistance, so the upper limit is made V: 2.5% and Sn: 1.5%. A preferable range of V is 0.75 to 2.0%. Moreover, the preferable range of Sn is 0.4 to 1.0%.

  Al remains in the steel as a result of deoxidation and reduction during the refining process. In the present invention, excessive inclusion of Al reduces the cleanliness of the steel due to the inclusions remaining, thereby reducing the elongation, and may reduce the corrosion resistance. Therefore, in the present invention, it is within the range of 0.005 to 0.05%. Contain.

  Ti is a so-called stabilizing element and has the effect of improving intergranular corrosion resistance and increasing the average r value by fixing C or N. A similar effect is also obtained with Nb and Zr. However, since Ti addition is less likely to reduce ductility than these elements, Ti is preferentially used as a stabilizing element in the present invention. In order to obtain such an effect, 0.05% or more of Ti is added. However, if added excessively, ductility is lowered by strengthening the solid solution of ferrite, so the upper limit of Ti is made 0.25%. A preferable range is 0.10 to 0.20%.

  Reduction of N decreases the strength of high purity ferritic stainless steel and increases elongation, and also improves the average r value. As for corrosion resistance, reducing N improves the intergranular corrosion resistance of high-purity ferritic stainless steel. Therefore, N is preferably as low as possible in the present invention. However, excessive reduction leads to an increase in refining costs, so the N content is set to 0.001 to 0.020% in consideration of the ability of secondary refining necessary for high purification. A preferred range is 0.001 to 0.20%.

  In addition to the above elements, in the present invention, one or more of Cu, Zr, Mg, B, and Nb are appropriately selected depending on the purpose in order to further improve corrosion resistance, improve workability, and improve surface characteristics. Added.

  Since Cu has the effect of reducing the dissolution rate at the bottom of the corrosion pit and promoting repassivation, it is effective for improving the weather resistance and the crevice corrosion resistance. Moreover, there exists an effect which average r value improves by addition of a suitable quantity. However, excessive addition increases the strength of the ferrite and lowers the elongation. Therefore, in the present invention, the addition amount of Cu is set to 0.05 to 1.0%. A preferable range is 0.25 to 0.75%.

  Zr is effective in improving weather resistance and crevice corrosion resistance by strengthening the passive film and controlling the composition of inclusions. However, excessive addition causes a decrease in elongation and makes casting difficult in the manufacturing process. Therefore, the amount of Zr added is 0.05 to 0.5%.

  When Mg is used in combination with Ti or Al, it forms a complex oxide that becomes a solidification nucleus of ferrite in the solidification process, and refines the solidification structure. As a result, ridging caused by the solidified structure and a structure harmful to roping are easily destroyed in the manufacturing process, and ridging that tends to be a problem when processing high-purity ferritic stainless steel can be reduced. However, excessive addition leaves a large number of Mg-based inclusions, resulting in a decrease in corrosion resistance. From the above, in the present invention, Mg is added in the range of 0.0002 to 0.0050% where it functions effectively. A preferable range is 0.0002 to 0.0020%.

  B is a grain boundary strengthening element effective for improving the secondary work brittleness of high purity ferritic stainless steel, and 0.0002% or more is added to obtain such an effect. However, excessive addition causes solid solution strengthening of ferrite and causes a decrease in ductility, so the upper limit is made 0.0050%. A preferable range is 0.0005 to 0.0020%.

  Nb is added as a stabilizing element that substitutes a part of Ti when high quality surface quality is required as in BA (bright annealing) products. Nb also has the effect of enhancing the corrosion resistance by strengthening the passive film. However, excessive addition leads to an increase in strength and a decrease in ductility, so in the present invention, it is added in the range of 0.01 to 0.1%.

  In the ferritic stainless steel sheet excellent in corrosion resistance and workability of the present invention, JASO M.C. It is preferable that the crevice corrosion depth generated in 50 cycles of the combined cycle test defined by 610 is 350 μm or less. As shown in FIG. 1 and FIG. 2, the crevice corrosion depth of SUS304 is 350 μm or less, and the ferritic stainless steel of the present invention can obtain corrosivity equivalent to SUS304 by setting the crevice corrosion depth to 350 μm or less. It is.

  Steels 1 to 32 having chemical components shown in Table 1 were melted in a laboratory vacuum melting furnace to form ingots. In the laboratory, hot-rolling, hot-rolled sheet annealing / pickling, cold rolling, annealing, and pickling were performed to produce a steel plate sample having a thickness of 1 mm. The pickling after cold rolling annealing was performed by simulating salt and nitric acid electrolysis in the actual production line. A JIS No. 13 B tensile test piece was produced from the obtained steel sheet in the rolling direction and subjected to a tensile test to determine yield strength, tensile strength, and elongation. Similarly, JIS No. 13 B tensile specimens were produced from the rolling direction, the perpendicular direction of rolling, and the 45 ° direction of rolling, and the average r value (Rankford value) was measured. Furthermore, the unevenness | corrugation of the test piece width direction when producing a JIS5 tension test piece in the rolling direction and giving 16% elongation distortion was measured with the roughness meter, and the maximum ridging height in a measurement range was measured.

  As for corrosion resistance, weather resistance and crevice corrosion resistance were evaluated by a combined cycle test. First, the weather resistance was evaluated by a cycle using artificial seawater, which has been shown to well reproduce the generation and development of stainless steel soot in the atmospheric environment. One cycle was repeated (1) artificial seawater spray 35 ° C 4 hours → (2) dry 60 ° C 2 hours → (3) wet relative humidity 95% 50 ° C 2 hours, up to 6 cycles. The specimen was finished by polishing with # 600 emery paper. As a comparative steel, an actual machine manufacturing material of SUS304 2B 1 mm was used. The condition of the surface after the test was evaluated by the stainless steel association rating number (SARN). SARN is evaluated in 10 stages, from 10 (no start) to 1 (most start).

  In the evaluation of crevice corrosion resistance, two types of strip-shaped test pieces of 50 mm × 100 mm (large) and 30 mm × 70 mm (small) are prepared, and two types of plates are stacked so that the central portions overlap, and 30 mm × 70 mm Crevice corrosion test pieces were prepared by spot welding at three locations of 15 mm, 35 mm, and 55 mm from the upper side of the sample.

  The cycle of the combined cycle test is in accordance with JASO CCT: (1) 5% NaCl spray 35 ° C 2 hours → (2) Dry 60 ° C 2 hours → (3) Wet relative humidity 90% 50 ° C 2 hours as one cycle 50 A cycle test was performed. After the test, spot welded metal was cut out, the large and small specimens were separated, and the pit depth in the gap near the spot weld metal was measured with a laser microscope, and the maximum pit depth in the measured range was defined as the gap corrosion depth. . In this case, the comparative steel was SUS304 2B.

  The criteria for determining good workability were: elongation: 35.0% or more, average r value: 1.8 or more, ridging: 12 μm or less. The criteria for determining good corrosion resistance were weather resistance: SARN 5 or more and crevice corrosion depth: 350 μm or less.

  The results are shown in Table 2. It can be seen that all the steels according to the present invention have excellent workability and corrosion resistance. On the other hand, in Steel 6, Steel 13, and Steel 17, Sn, V, and Mo deviate from the scope of the present invention, respectively, and are inferior in weather resistance and crevice corrosion resistance. Moreover, since steel 18 has too much Mo content, although corrosion resistance is obtained, it is inferior to workability.

The influence of Sn on the corrosion resistance and workability of steel containing 1.0% V is shown. The influence of V on the corrosion resistance and workability of steel containing 0.5% Sn is shown.

Claims (7)

  In mass%, C: 0.001 to 0.010%, Si: 0.01 to 0.5%, Mn: 0.01 to 1.00%, P: 0.03% or less, S: 0.01 % Or less, Cr: 16 to 20%, Mo: 0.3 to 1.8%, V: 0.3 to 2.5%, Sn: 0.1 to 1.5%, Ti: 0.05 to 0 A ferritic stainless steel plate excellent in corrosion resistance and workability, characterized by comprising .25%, N: 0.001 to 0.020%, and the balance being Fe and inevitable impurities.   The ferritic stainless steel sheet having excellent corrosion resistance and workability according to claim 1, wherein Al: 0.005 to 0.05% by mass.   The corrosion resistance according to claim 1 or 2, characterized by containing one or more of Cu: 0.05 to 1.0% and Zr: 0.05 to 0.5% in terms of mass%. Ferritic stainless steel sheet with excellent workability.   The ferritic stainless steel sheet having excellent corrosion resistance and workability according to any one of claims 1 to 3, wherein Mg: 0.0002 to 0.0050% is contained in mass%.   The ferritic stainless steel sheet excellent in corrosion resistance and workability according to any one of claims 1 to 4, characterized by containing, in mass%, B: 0.0002 to 0.0050%.   The ferritic stainless steel sheet having excellent corrosion resistance and workability according to any one of claims 1 to 5, characterized by containing Nb: 0.01 to 0.1% by mass%.   JASO M.J. The ferritic stainless steel sheet having excellent corrosion resistance and workability according to any one of claims 1 to 6, wherein a crevice corrosion depth generated in 50 cycles of the combined cycle test defined by 610 is 350 µm or less.

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