JP6645816B2 - Ferritic stainless steel - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a ferritic stainless steel excellent in shape and corrosion resistance of a weld portion, which is suitably used for manufacturing a structure to be joined by welding after deep drawing.

  Conventionally, ferritic stainless steel is inferior in press formability as compared with austenitic stainless steel and high-tensile steel sheet, and its use is limited to applications that require excellent press formability. Have been.

  However, the press formability of ferritic stainless steel in recent years, especially the deep drawability, has been remarkably improved, and ferrite stainless steel is used for applications where severe press processing is performed, such as kitchen materials, electrical equipment parts, and automotive parts. The application of steel is progressing.

  Patent Literature 1 discloses a ferritic stainless steel sheet having excellent deep drawability. In this steel sheet, the composition of the steel and the manufacturing conditions are controlled within appropriate ranges, the average r value of the steel sheet after finish annealing is 2.0 or more, the average crystal grain size is 50 μm or less, and (tensile strength (MPa) × By setting the (average r value) / (crystal grain size (μm))) to 20 or more, the deep drawability is improved.

  Patent Document 2 discloses a ferritic stainless steel cold-rolled steel sheet having excellent press formability. In this steel sheet, the precipitation of AlN is prevented and the precipitation strengthening due to the fine AlN is reduced, the local elongation is increased by reducing the ferrite grain size to less than 10 μm, and the average of Cr carbonitride in the ferrite grains is further increased. By setting the particle size to 0.6 μm or more, uniform elongation is improved, and press formability is improved.

  Patent Document 3 discloses a ferritic stainless steel sheet having excellent deep drawability. In this steel sheet, by adjusting the hot rolling conditions, the average ferrite crystal grain size is 40 μm or less, and the azimuth difference between {111} // ND occupying the cross section composed of the rolling direction and the thickness direction is within 10 °. When the ratio of ferrite crystal grains is 20% or more, the deep drawability is improved.

  However, even if these ferritic stainless steel sheets having excellent press-formability are used, it is not always possible to sufficiently suppress the occurrence of vertical cracks that occur when severe press-forming is performed.

  Patent Document 4 discloses a ferritic stainless steel sheet excellent in deep drawability, secondary work brittleness and corrosion resistance in order to suppress this vertical crack. In this steel sheet, in addition to the proper addition of Nb and / or Ti and B and V, the steel sheet after finish annealing, pickling or further skin pass rolling has an average crystal grain size of 40 μm or less and a surface roughness of not more than 40 μm. By setting Ra to 0.30 μm or less, both the deep drawability and the resistance to secondary working brittleness are achieved.

JP 2003-138349 A JP 2007-119847 A JP 2009-299116 A JP-A-2003-201547

  However, even if the ferritic stainless steel sheet of Patent Document 4 is used, cracks in the vicinity of the welded portion, which occur particularly when welding is performed after press forming, cannot be completely prevented.

  In view of the above problems of the prior art, the present invention, when performing welding after deep drawing, cracks near the weld due to expansion and contraction due to the heat effect of welding and stress due to deformation, An object of the present invention is to provide a ferritic stainless steel that is difficult to be formed and has excellent corrosion resistance in the vicinity of a weld.

  The present inventors have investigated the correlation between the component composition of ferritic stainless steel and cracking and corrosion resistance in the vicinity of a weld in order to solve the above problems, and obtained the following findings (1) to (3). Was.

  (1) When welding is performed on a region in which the strength of the crystal grain boundary has been reduced by deep drawing, cracks are generated near the welded portion due to expansion and shrinkage stress generated near the welded portion due to welding heat.

  (2) Since the addition of Co decreases the coefficient of thermal expansion, expansion and contraction due to welding heat are reduced, and deformation of the weld and stress near the weld are reduced. As a result, cracking in the vicinity of the weld is less likely to occur due to the addition of Co.

  (3) Since the addition of B suppresses a decrease in the strength of the crystal grain boundaries due to the deep drawing, cracks are less likely to occur even if thermal stress occurs near the weld after deep drawing.

  The present invention is configured based on the above results. That is, the present invention has the following structure.

[1] In mass%, C: 0.001 to 0.020%, Si: 0.01 to 0.30%, Mn: 0.01 to 0.50%, P: 0.04% or less, S: 0.01% or less, Cr: 18.0 to 22.0%, Ni: 0.01 to 0.40%, Mo: 0.30 to 3.0%, Al: 0.01 to 0.15%, Ti: 0.01 to 0.50%, Nb: 0.01 to 0.50%, V: 0.01 to 0.50%, Co: 0.01 to 6.00%, B: 0.0003 to A ferritic stainless steel containing 0.0050% and N: 0.001 to 0.020%, satisfying the following expression (1), and the balance being Fe and an inevitable impurity. .
0.30 ≦ Ti + Nb + V ≦ 0.60% (1)
The element symbol in the formula (1) means the content (% by mass) of each element.
[2] The component composition further contains, by mass%, one or more of Zr: 0.5% or less, W: 1.0% or less, and REM: 0.1% or less. The ferritic stainless steel according to [1], characterized in that:

  If the ferritic stainless steel of the present invention is used for manufacturing a structure in which joining is performed by welding after deep drawing, cracks near the weld due to expansion and shrinkage due to the heat effect of welding and stress due to deformation are generated. A structure which does not easily occur and has excellent corrosion resistance near the weld is obtained.

  In addition, since it is difficult for cracks to occur in the vicinity of the welded portion, it can be said that the above-described structure has excellent welded portion shape.

  Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments.

The component composition of the ferritic stainless steel of the present invention is as follows: C: 0.001 to 0.020%, Si: 0.01 to 0.30%, Mn: 0.01 to 0.50%, P: 0.04% or less, S: 0.01% or less, Cr: 18.0 to 22.0%, Ni: 0.01 to 0.40%, Mo: 0.30 to 3.0%, Al: 0 0.01 to 0.15%, Ti: 0.01 to 0.50%, Nb: 0.01 to 0.50%, V: 0.01 to 0.50%, Co: 0.01 to 6.00 %, B: 0.0003 to 0.0050%, and N: 0.001 to 0.020%, satisfying the following formula (1), with the balance being Fe and inevitable impurities.
0.30% ≦ Ti + Nb + V ≦ 0.60% (1)
The element symbol in the formula (1) means the content (% by mass) of each element.

  Further, the component composition of the ferritic stainless steel of the present invention further includes, by mass%, any one or more of Zr: 0.5% or less, W: 1.0% or less, and REM: 0.1% or less. It may contain more than one species.

  Hereinafter, the component composition of the ferritic stainless steel of the present invention will be described in detail. In addition,% indicating the content of each element is represented by mass% unless otherwise specified.

C: 0.001 to 0.020%
When the content of C is large, the strength is improved, and when the content is small, the workability is improved. In order to obtain an appropriate strength, the content of 0.001% or more is appropriate. However, when the C content exceeds 0.020%, the workability is significantly reduced, and is not suitable for deep drawing. Therefore, the C content is set to 0.001 to 0.020%. More preferably, it is 0.002 to 0.018%. When the C content is in the above range with only C inevitably contained, there is no need to actively add C.

Si: 0.01 to 0.30%
Si is an element useful for deoxidation. The effect is obtained when the content is 0.01% or more. However, when the Si content exceeds 0.30%, the workability is significantly reduced, which is not suitable for deep drawing. Therefore, the content of Si is set to 0.01% to 0.30%. More preferably, it is 0.05% to 0.20%.

Mn: 0.01 to 0.50%
Mn has the effect of increasing the strength. The effect is obtained when the content is 0.01% or more. On the other hand, when Mn is excessively contained, the workability is remarkably reduced, and is not suitable for deep drawing. Therefore, the Mn content is suitably 0.50% or less. Therefore, the content of Mn is set to 0.01 to 0.50%. More preferably, it is 0.03% to 0.40%. Since Mn is unavoidably contained in steel, it is not necessary to add Mn as long as the unavoidable Mn content is within the above range.

P: 0.04% or less P is an element inevitably contained in steel, segregates at the crystal grain boundary after deep drawing, reduces the strength of the crystal grain boundary, and easily causes grain boundary cracking. Element. Therefore, the P content is preferably as small as possible, and is set to 0.04% or less. It is more preferably at most 0.03%.

S: 0.01% or less S is an element inevitably contained in steel. If the S content exceeds 0.01%, the formation of water-soluble sulfides such as CaS and MnS is promoted, and the corrosion resistance is reduced. Therefore, the S content is set to 0.01% or less.

Cr: 18.0 to 22.0%
Cr is the most important element that determines the corrosion resistance of stainless steel. If the Cr content is less than 18.0%, sufficient corrosion resistance cannot be obtained as stainless steel. In particular, the corrosion resistance at the welded portion becomes insufficient. On the other hand, if Cr is contained excessively, the workability is reduced, and it is not suitable for deep drawing. Therefore, the Cr content is suitably 22.0% or less. Therefore, the Cr content is set to 18.0 to 22.0%. More preferably, it is 18.5 to 21.5%.

Ni: 0.01 to 0.40%
Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses the progress of corrosion in a corrosive environment in which a passive film cannot be formed and active dissolution occurs. The effect can be obtained by setting the Ni content to 0.01% or more. However, when the Ni content is 0.40% or more, the workability is deteriorated, so that it is not suitable for deep drawing. Therefore, the content of Ni is set to 0.01 to 0.40%. More preferably, it is 0.03 to 0.18%.

Mo: 0.30 to 3.0%
Mo is an element that promotes the passivation of the passivation film and improves the corrosion resistance of stainless steel. The effect becomes more remarkable by containing together with Cr. The effect of improving the corrosion resistance by Mo is obtained when the content is 0.30% or more. However, if the Mo content exceeds 3.0%, the high-temperature strength increases, and the rolling load increases, thereby reducing the manufacturability. Therefore, the Mo content was set to 0.30 to 3.0%. More preferably, it is 0.40 to 2.0%.

Al: 0.01 to 0.15%
Al is an element useful for deoxidation, and the effect is obtained when the Al content is 0.01% or more. However, when the Al content exceeds 0.15%, the ferrite crystal grain size tends to increase, and cracks near the welded portion tend to occur. Therefore, the Al content is set to 0.01 to 0.15%. More preferably, it is 0.02 to 0.10%.

Ti: 0.01 to 0.50%
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, if the Ti content exceeds 0.50%, C and N dissolved as solid solutions are excessively reduced, the strength of the crystal grain boundary after deep drawing becomes insufficient, and cracks are likely to occur near the welded portion. Therefore, the Ti content is set to 0.01 to 0.50%. More preferably, it is 0.15 to 0.40%. In the present specification, carbonitride includes carbide and nitride.

Nb: 0.01 to 0.50%
Nb is an element that preferentially bonds to C and N and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. The effect is obtained when the Nb content is 0.01% or more. However, if the Nb content exceeds 0.50%, C and N dissolved as solid solutions are excessively reduced, the strength of the crystal grain boundary after deep drawing becomes insufficient, and cracks are likely to occur near the welded portion. Therefore, the content of Nb is set to 0.01 to 0.50%. More preferably, it is 0.05 to 0.40%.

V: 0.01 to 0.50%
V is an element that suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. The effect is obtained when the V content is 0.01% or more. However, an excessive content exceeding 0.50% lowers workability and is not suitable for deep drawing. Therefore, the V content is set to 0.01 to 0.50%. More preferably, it is 0.02 to 0.30%.

0.30% ≦ Ti + Nb + V ≦ 0.60%
As described above, Ti, Nb, and V are all elements that suppress the generation of Cr carbonitride and improve the corrosion resistance of the weld. In order to suppress the sensitization due to the precipitation of Cr carbonitride and to make the corrosion resistance of the weld sufficient, it is necessary that the total of the Ti content, the Nb content, and the V content be 0.30% or more. is there. It is more preferably at least 0.35%. In addition, since the cooling rate of the weld is usually very fast, the addition of any one or only two of Ti, Nb, and V rapidly passes through the temperature range in which the carbonitride of each element is likely to precipitate. In some cases, C and N cannot be completely rendered harmless. Therefore, it is necessary to contain 0.01% or more of any of the elements Ti, Nb and V.

  On the other hand, if the total content of Ti, Nb, and V exceeds 0.60%, the workability is reduced, and thus the material is not suitable for deep drawing. Therefore, the total of the Ti content, the Nb content, and the V content is set to 0.60% or less. More preferably, it is 0.55% or less.

Co: 0.01 to 6.00%
Co is an important element for the present invention. The addition of Co changes the electronic state of the ferritic stainless steel and lowers the coefficient of thermal expansion. This reduction in the coefficient of thermal expansion mitigates the expansion and deformation of the weld caused by the heat of the weld. In the vicinity of the weld after deep drawing, cracking may occur due to stress generated by thermal expansion and deformation due to welding. The decrease in the coefficient of thermal expansion due to the addition of Co alleviates the stress load applied to the vicinity of the weld due to the thermal effects and deformation of welding, and suppresses the occurrence of cracks. The effect is obtained when the Co content is 0.01% or more. On the other hand, if the Co content exceeds 6.00%, the workability deteriorates, so that it is not suitable for deep drawing. Therefore, the Co content is set to 0.01 to 6.00%. More preferably, it is 0.03 to 3.00%.

B: 0.0003-0.0050%
B is an important element for the present invention. In high-purity ferritic stainless steel, P segregates at the crystal grain boundary on the wall surface portion of the deep drawing by deep drawing, and the crystal grain boundary becomes brittle. Therefore, after excessive deep drawing is performed, cracks may occur along the deep drawing direction. In particular, the tendency is remarkable in a component in which C and N dissolved in Ti and Nb are reduced. In a crystal grain boundary where cracks are easily generated by deep drawing, a stress load due to the heat effect of welding may cause cracks. The addition of B suppresses segregation of P due to deep drawing, strengthens the crystal grain boundaries, and suppresses the occurrence of such cracks. This effect can be obtained by containing B in an amount of 0.0003% or more. On the other hand, if the B content exceeds 0.0050%, the workability is reduced, so that it is not suitable for deep drawing. Therefore, the B content is set to 0.0003 to 0.0050%. More preferably, it is 0.0004 to 0.0020%.

N: 0.001 to 0.020%
N has the effect of increasing the strength of steel by solid solution strengthening. The effect is obtained when the N content is 0.001% or more. However, when the N content exceeds 0.020%, the workability significantly decreases, and is not suitable for deep drawing. Therefore, the N content is set to 0.001 to 0.020%. More preferably, it is 0.002 to 0.018%.

  Further, the ferritic stainless steel of the present invention may contain the following components.

Zr: 0.5% or less Zr has an effect of binding to C and N to suppress sensitization. The effect can be obtained by setting the Zr content to 0.01% or more. (When the Zr content is less than 0.01%, the effect is not great, but the effect of the present invention is not impaired. There is no problem if the amount is less than 0.01%). However, excessive Zr content lowers processability. In addition, since Zr is a very expensive element, excessive Zr content causes an increase in cost. Therefore, the content of Zr is set to 0.5% or less.

W: 1.0% or less W has the effect of improving the corrosion resistance like Mo. The effect can be obtained by setting the W content to 0.01% or more. (When the W content is less than 0.01%, the effect is not great, but the effect of the present invention is not impaired. Is less than 0.01%.) However, an excessive W content increases the strength and decreases the manufacturability. Therefore, the W content is set to 1.0% or less.

REM: 0.1% or less REM improves oxidation resistance, suppresses formation of oxide scale, and improves corrosion resistance of a welded portion. The effect is obtained by setting the REM content to 0.0001% or more. (When the REM content is less than 0.0001%, the above effect is not great, but the effect of the present invention is not impaired. Is less than 0.0001%). However, when REM is excessively contained, the productivity such as pickling properties is reduced, and the cost is increased. Therefore, the content of REM 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 0.1%, and the like. In addition, in the ferritic stainless steel having the Cr content and the Mo content of the present invention and having excellent corrosion resistance, Cu has the effect of increasing the passivation maintaining current, destabilizing the passivation film, and decreasing the corrosion resistance. From this viewpoint, it is better not to include Cu. When Cu is contained, its content is suitably less than 0.1%. Therefore, the content of Cu as an impurity is set to less than 0.1% as described above.

  The method for producing the ferritic stainless steel of the present invention is not particularly limited. An example of a suitable manufacturing method is shown below.

  After heating the stainless steel having the above component composition to 1100 to 1300 ° C, the finishing temperature is set to 700 to 1000 ° C, the winding temperature is set to 400 to 800 ° C, and hot rolling is performed so that the sheet thickness becomes 2.0 to 5.0 mm. . The hot-rolled steel strip thus produced is annealed at a temperature of 800 to 1100 ° C. and pickled. Next, cold rolling is performed to a sheet thickness of 0.5 to 2.0 mm, and cold-rolled sheet annealing is performed at a temperature of 700 to 1050 ° C. After the cold rolled sheet annealing, pickling is performed to remove scale. The cold-rolled steel strip from which the scale has been removed may be subjected to skin pass rolling.

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

  The stainless steel shown in Table 1 was vacuum-melted, heated to 1200 ° C, hot-rolled to a thickness of 4 mm, annealed in the range of 800 to 1000 ° C, and scale was removed by pickling. Further, it was cold-rolled to a sheet thickness of 0.8 mm, annealed in the range of 800 to 950 ° C., and pickled to obtain a test material.

  A disk of φ72 mm is sampled from the prepared test material, and deep-drawing is performed in four stages using punches of φ49 mm, φ35 mm, φ26 mm, and φ22 mm in order, and the ears are cut so that the height after processing is 50 mm. Then, a hole of φ5 mm was made in the center of the deep drawing bottom, and a cylindrical deep drawing test piece was prepared. Thereafter, a φ23 mm disk was joined by TIG welding so as to cover the φ22 mm opening of the test piece. The welding conditions were a welding current of 100 A and a welding speed of 60 cm / min. Ar was used as the shielding gas, and the flow rate was 20 L / min. After 24 hours from the welding, the inside of the test piece was filled with water, and a pressure of 10 atm was applied to check the presence or absence of cracks. Thereafter, the vicinity of the welded portion (position of 2 to 5 mm from the fusion line) on the cylindrical deep drawing wall surface was observed at a magnification of 200 times using an optical microscope, and the length of the crack was confirmed. The results are shown in Table 2 as "x" when there was a crack having a length of 0.5 mm or more and "と し て" when there was no crack.

  In Table 2, no cracks could be confirmed in the vicinity of the weld in any of the examples of the present invention. On the other hand, in Comparative Example No. In No. 12, cracking occurred because Co was not added. No. In No. 13, cracking occurred because B was not added. No. In No. 14, cracks occurred because Al was excessively added. No. In No. 15, cracks occurred due to excessive addition of Cr.

  Subsequently, the corrosion resistance of the as-welded weld was evaluated using a test piece for which the presence or absence of cracks was confirmed. Five cycles of a neutral salt spray cycle test in accordance with JIS H8502 were performed, and the presence or absence of corrosion near the welded portion (range of 5 mm from the weld bead center to the fusion line) was visually confirmed. Table 2 shows that the sample having corrosion having a major axis of 1 mm or more in the vicinity of the welded portion in the five-cycle test was marked as “x”, and that no corrosion occurred as “「 ”in Table 2.

  In all of No. 12, No. 13, and No. 14, corrosion occurred from cracks near the weld. No. 16 did not satisfy the expression (1), so corrosion occurred from the weld bead. In No. 17, the amount of Cr was small and corrosion occurred from the weld bead and the tempered portion. In No. 18, corrosion was generated from the weld bead because the Nb content was small. In No. 19, since the amount of Ti was small, corrosion occurred from the weld bead. In No. 20, corrosion was generated from the weld bead because V was not added.

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel excellent in the shape of a welding part and corrosion resistance suitable for using for the structure joined by welding after deep drawing is obtained. The ferritic stainless steel obtained by the present invention is suitable for applications in which a structure is produced by welding after deep drawing, for example, application to electronic device parts such as a battery case, and automotive parts such as a converter.

Claims (1)

In mass%, C: 0.001 to 0.020%, Si: 0.01 to 0.30%, Mn: 0.01 to 0.50%, P: 0.04% or less, S: 0.01 %: Cr: 18.3 to 22.0%, Ni: 0.01 to 0.40%, Mo: 0.30 to 3.0%, Al: 0.01 to 0.15%, Ti: 0 0.01 to 0.50%, Nb: 0.01 to 0.50%, V: 0.01 to 0.50 %, Co: 0.010 to 5.327 %, B: 0.0004 to 0.0046 %, N: 0.001 to 0.020%, satisfying the following formula (1), the balance being Fe and a component composition that is an inevitable impurity.
0.30% ≦ Ti + Nb + V ≦ 0.60% (1)
The element symbol in the formula (1) means the content (% by mass) of each element.