JP4237183B2 - Stainless steel work hardener - Google Patents

Description

  The present invention relates to a stainless steel work-hardening material that is excellent in strength and bending workability and has a ferrite structure made stronger by work hardening.

As seen in portable notebook personal computers, light-weight members are required for household appliances and office automation equipment such as televisions and personal computers, and the demand for weight reduction is met by reducing the thickness of the members. In order to secure the required strength even if the weight is reduced, 0.2% proof stress in the rolling direction ≧ about 500 N / mm ² or Vickers hardness HV ≧ about 200 is a standard.

  When a metal member is incorporated into a home appliance or OA device as a frame, a protective case or the like, the metal cut plate is processed into a predetermined shape by pressing, bending, or the like. Therefore, as a metal member for home appliances and OA equipment, it is also important that the material has excellent bending workability in addition to strength.

  Moreover, the recent trend is that the need for solid metal materials that do not require plating is high, with an emphasis on environmental conservation and recyclability. Solid metal materials excellent in corrosion resistance include high-strength stainless steels such as martensite type typified by SUS410 and SUS420J2, precipitation hardening type typified by SUS631, and work hardening type austenite type typified by SUS304 and SUS301. .

  Martensitic and precipitation hardened stainless steels are strengthened by processing them into product shapes, followed by quenching, tempering and aging treatments, respectively. However, since the user side needs quenching / tempering treatment and aging treatment, the equipment burden for that is forced. In addition, a pickling or polishing process for removing oxide scale generated on the surface of the heat-treated product and a reworking process for correcting thermal deformation are required.

  Work-hardening austenitic stainless steel is a material that can be tempered at the raw material stage and has good bending workability, so that the heat treatment on the user side can be omitted. However, since it contains a large amount of expensive Ni, the steel material cost increases. Therefore, inexpensive stainless steels having a reduced Ni content have been developed while taking advantage of the work-hardening austenitic stainless steel. For example, (ferrite + martensite) double phase stainless steel (Japanese Unexamined Patent Publication No. 63-169330), (ferrite + martensite) double phase structure or martensite having strength in the martensite phase and workability in the ferrite phase Stainless steel (Japanese Patent Laid-Open No. 11-302791) with improved bending workability by controlling the size and morphology of MnS inclusion particles dispersed in the site single layer structure, and ferritic structure is work-hardened by cold rolling and heat treatment There are omitted stainless steel (Japanese Patent Laid-Open No. 2001-262282) and the like.

  The strength of the (ferrite + martensite) duplex stainless steel disclosed in JP-A-63-169330 increases as the amount of martensite increases. However, when the amount of martensite exceeds 50% by mass, the bending workability decreases remarkably. .

  The stainless steel disclosed in Japanese Patent Laid-Open No. 11-302791 is mainly intended for square steel pipes used for building structures having a relatively large bending radius. However, in home appliance frames, protective cases, casings, and the like that require high dimensional accuracy after bending, the bending radius is significantly smaller than that of square steel pipes for use in building structures. Because of the small bend radius, even if the size and shape of MnS inclusion particles are controlled, the bending process at the level required for the frame, protective case, case, etc. is possible in the case of a multi-phase matrix or a martensite single layer structure. When applied, cracks are likely to occur.

  Moreover, JP-A-11-302791 does not disclose a specific method for controlling the size and form of MnS inclusion particles. Although it is known that the string-like MnS stretched in the rolling direction is harmful to the bending workability, the MnS is further stretched as the cold rolling rate increases, and finally finely dispersed. As a result, MnS of the thin steel plate is rendered harmless by fine dispersion, but MnS cannot be harmed by a relatively thick steel plate that cannot be expected to have sufficient fine dispersion. Furthermore, the required proof strength level varies depending on the application, but the proof strength of martensitic and (ferrite + martensite) multi-phase stainless steel plate without heat treatment on the user side is almost determined by the components, so the components differ. The materials had to meet various levels of required strength.

  Although the method of work hardening the ferrite structure by cold rolling is superior in terms of bending workability compared to increasing the strength by the martensite phase, the stainless steel disclosed in Japanese Patent Laid-Open No. 2001-262282 is a two-wheeled vehicle without bending work. Targeted for disc brake applications. For this reason, if the material to which the method is applied as it is is processed with a small bending tip radius R, cracks are likely to occur and it cannot be used as a material for a frame, a protective case, or the like.

The present invention has been devised to solve such a problem. In the refining process, Al deoxidation and low sulfidation are used in combination to make inclusions fine Al ₂ O ₃ or Al ₂ O ₃ .MgO. By controlling the system and work hardening the ferrite structure by cold working, there is no need for heat treatment on the user side, and there is no need for cracking even when severe bending work is performed, and a high strength stainless steel sheet is provided. For the purpose.

The work hardening material of the present invention is characterized by its component / composition and metal structure.
In terms of components, C: 0.15% by mass or less, Si: 1.0% by mass or less, Mn: 1.0% by mass or less, S: 0.005% by mass or less, Cr: 10-20% by mass, Ni : 0.5% by mass or less, Al: 0.001 to 0.05% by mass, the balance being Fe and inevitable impurities . As needed, Mo: 0.5-2.0 mass%, Cu: 0.5-3.0 mass%, Nb: 0.05-1.0 mass% 1 type or 2 types or more are included. Also good.

The metal structure is a processed ferrite structure in which Al ₂ O ₃ and / or Al ₂ O ₃ .MgO inclusions having a size of 10 μm or less are dispersed with a cleanliness of 0.06 or less. The 2% proof stress is adjusted in the range of 500 to 900 N / mm ² .

  MnS, which is harmful to bending workability, is relatively soft, is stretched in the rolling direction by hot rolling and cold rolling, and is dispersed in the matrix as string-like inclusions. In this state, when the stainless steel plate is bent, stress concentrates on MnS and becomes a starting point of cracking. Prevention of cracking due to string-like MnS inclusions is not sufficient only by low sulfidation, and it is necessary to control the composition, size, and form of the inclusions.

The composition, size and form of inclusions vary depending on the deoxidizer used in the refining process. When silicon is used as a deoxidizer, MnO.SiO ₂ -based or MnO.SiO ₂ .MnS-based inclusions are generated in addition to MnS. When Ti is used as a deoxidizing agent, the formation of string inclusions can be suppressed. However, TiN is also generated in addition to TiO ₂ as a deoxidation product, and individual inclusions aggregate to each other to form a coarse aggregate ( Cluster) and surface flaws are likely to occur on the stainless steel plate. Ti deoxidation is also disadvantageous in that the tundish nozzle tends to be clogged, and requires a reduction in the N content.

As a result of investigating and investigating methods for suppressing the formation of inclusions such as MnS, MnO · SiO ₂ and MnO · SiO ₂ · MnS (oxysulfide) that are free from surface flaws and are harmful to bending workability It has been found that when the inclusions are controlled to Al ₂ O ₃ or Al ₂ O ₃ .MgO by deoxidation, the bending workability of the processed ferrite structure is remarkably improved. Specifically, as will be seen in the examples described later, cleanness: 0.06% or less and size: 10 μm or less Al ₂ O ₃ or Al ₂ O ₃ .MgO by combination of low sulfidation and Al deoxidation When dispersing the system, adverse effects on bending workability are suppressed.

The strength of the stainless steel sheet is imparted by work hardening of the ferrite structure by cold working and does not require a special component design to achieve the required level of 0.2% proof stress. A typical cold working is cold rolling, and the 0.2% proof stress can be adjusted to a range of 500 to 900 N / mm ² (a range of 200 to 300 in the case of Vickers hardness HV) by changing the cold rolling rate. Incidentally, the 0.2% proof stress level of the ferritic stainless steel in the normal annealing state is about 250 to 300 N / mm ² (about 130 to 150 in the case of Vickers hardness HV), which is much lower than the required level.

  In the present invention, ferritic stainless steel in which the composition, size, and form of inclusions are controlled by low sulfidation and Al deoxidation is used. The stainless steel is designed as follows.

[Alloy components]
C: 0.15% by mass or less Although it is an alloy component effective for strengthening the matrix, excessive C content promotes the formation of Cr-based carbides and deteriorates the corrosion resistance. Therefore, the upper limit of the C content is set to 0.15% by mass (preferably 0.08% by mass).
Si: 1.0% by mass or less Si is an alloy component that works as a ferrite forming element and is effective for matrix strengthening. However, when an excessive amount of Si exceeding 1.0% by mass is contained, SiO ₂ -based or MnO.SiO ₂ -based inclusions that are harmful to bending workability are likely to be generated.
Mn: 1.0% by mass or less An austenite forming element that becomes a MnO.SiO ₂ inclusion and deteriorates bending workability. Therefore, the upper limit of the Mn content is set to 1.0% by mass (preferably 0.5% by mass).
S: 0.005% by mass or less Solid solution is formed in MnS, MnO.SiO ₂ inclusions which deteriorate bending workability, and coarse oxysulfide inclusions are formed. In order to suppress the adverse effects caused by S, the upper limit of the S content was set to 0.005% by mass (preferably 0.003% by mass).
Cr: 10 to 20% by mass
It is an alloy component effective for improving corrosion resistance, and 10% by mass or more of Cr is necessary to ensure the corrosion resistance required for stainless steel. However, excessive addition deteriorates toughness, so the upper limit of Cr content was set to 20% by mass. A preferable range of the Cr content is 11 to 18% by mass.
Ni: 0.5% by mass or less Ni is an austenite forming element. When an excessive amount of Ni is contained, the _Ac1 point is lowered, and a martensite phase is easily generated during the cooling process during annealing. Therefore, the Ni content is regulated to 0.5% by mass or less to prevent the formation of martensite.
Al: 0.001 to 0.05 mass%
It is a component added as a deoxidizer, and an Al content of 0.001% by mass is necessary for obtaining a sufficient deoxidation effect. However, when an excessive amount of Al is added, a large amount of Al ₂ O ₃ inclusions are generated, and the inclusions are clustered together to generate defects such as surface defects. Therefore, in order to prevent the occurrence of defects such as surface defects by setting the size of inclusions to 10 μm or less and the cleanliness to 0.06% or less, the upper limit of Al content is specified to 0.05 mass%. Preferably, the Al content is selected in the range of 0.003 to 0.03 mass%.

Mo: 0.5-2.0 mass%, Cu: 0.5-3.0 mass%, Nb: 0.05-1.0 mass%
These are alloy components added as necessary, and all contribute to the improvement of corrosion resistance. The effect of improving corrosion resistance is seen when Mo: 0.5% by mass or more, Cu: 0.5% by mass or more, and Nb: 0.05% by mass or more. However, if the Mo content exceeds 2.0% by mass, the cold workability decreases due to solid solution strengthening and the material cost increases. If the Cu content exceeds 3.0% by mass, the hot workability decreases. Thus, manufacturability is impaired, and if the Nb content exceeds 1.0% by mass, the effect of improving corrosion resistance is saturated and the material cost is increased.

[Processed ferrite structure]
Low sulphide inclusions dispersed in a matrix of Al deoxidation is shape control to _Al 2 _{O 3} system or _Al 2 _O 3 · MgO-based, size inclusions in cold working: 10 [mu] m or less (preferably, 5 [mu] m or less) When dividing into two, the stress concentration on the inclusions that become the starting point of cracking is alleviated. Therefore, even if it is bent into a target shape with a small bending tip radius R, a processed product with greatly reduced cracks can be obtained.

Cold working is effective not only for finely dividing inclusions but also for increasing strength by work hardening. That is, the 0.2% proof stress level of ferritic stainless steel in a normal annealing state is about 250 to 300 N / mm ² (about 130 to 150 in the case of Vickers hardness HV). It is done. Moreover, 0.2% proof stress can be freely adjusted in the range of 500 to 900 N / mm ² and Vickers hardness can be adjusted in the range of HV: 200 to 300 according to the processing rate, so that it is possible to meet the required level without changing the component design. And a material with high strength is provided. When work hardening is performed by cold rolling, the rolling ratio during finish rolling is selected in the range of 15 to 50% (preferably 20 to 35%) in order to improve the strength without impairing the bending workability.

  Next, the present invention will be described specifically by way of examples.

[Example 1]
A stainless steel having the composition shown in Table 1 was prepared by deoxidizing the molten stainless steel. In the table, sample S-1 is a stainless steel plate having a thickness of 1.8 mm that has been recrystallized into a ferrite single layer structure by annealing after hot rolling and then cold rolled at a rolling rate of 25%. Have. Samples S-2 and S-3 were (phases of ferrite and martensite) by holding a stainless steel sheet cold-rolled to 1.8 mm for a short time at a temperature in the austenite + ferrite two-phase region and then air-cooling. It is a stainless steel plate made into a structure. Sample S-2 has a larger amount of martensite than sample S-3.

  JIS 13B test piece was cut out from each sample with different metallographic structure along the rolling direction (L direction) and the direction orthogonal to the rolling direction (C direction), and 0.2% proof stress and elongation were measured by tensile test. did. Table 2 compares the 0.2% yield strength and elongation of each sample. Sample S-1 having a processed ferrite structure has a 0.2% proof stress almost the same as that of sample S-2 having a martensite content of 80% by volume, but the elongation is small.

  For evaluation of bending workability, the V block method (90 degree V bending test according to JIS Z2248) was used. The tip of the punch where the tip R of the punch is changed and the specimen is bent 90 degrees in two directions: a bending axis parallel to the rolling direction (C direction bending) and a bending axis perpendicular to the rolling direction (L direction bending). The bending workability was evaluated by the radius R.

  As can be seen from the test results in Table 3, in the L direction bending, no cracks occurred in each sample even at the minimum tip R = 0.1 mm of V block bending, but in the C direction bending, there was a difference between the samples. It was. That is, the minimum tip R at which cracking occurred was 1.5 mm for sample S-2 with approximately 0.2% proof stress, whereas it was 0.6 mm for sample S-1. Although the elongation was small compared to −2 and S-3, the sample S-1 exhibited excellent bending workability. This result means that the work ferrite structure is more advantageous in bending workability than the structure strengthened by the martensite phase.

In order to investigate the influence of inclusions on the bending workability, sample A-1 was manufactured under the same manufacturing conditions after Al deoxidation of molten stainless steel prepared in the same composition as Si-deoxidized sample S-1. . In Sample A-1, the Al content derived from the deoxidizer was 0.006 % by mass (Table 4).

When inclusions of sample A-1 were identified by component analysis using EPMA, Al ₂ O ₃ system and Al ₂ O ₃ .MgO system were mixed, and inclusion MnO.SiO ₂ system of sample S-1 Or it was very different from the MnO.SiO ₂ .MnS system. Hereinafter, the inclusion mainly composed of SiO ₂ observed in the sample S-1 is referred to as a silicate system, and the inclusion mainly composed of Al ₂ O ₃ observed in the sample A-1 is referred to as an alumina system.

  JIS 13B test pieces were cut out from Samples A-1 and S-1 along the two directions L and C, and 0.2% yield strength and elongation were measured by a tensile test. Samples A-1 and S-1 had almost the same mechanical properties (Table 5). However, in the bending test, the samples A-1 and S-1 were substantially the same proof stress level, but the sample A-1 was clearly superior to the sample S-1 in terms of the bending workability in the C direction ( Table 6).

  The above results mean that excellent bending workability can be ensured regardless of high strength when the inclusions are controlled by low sulfidation and Al deoxidation and adjusted to the processed ferrite structure by cold working. ing.

[Example 2]
Various stainless steels whose components and compositions are shown in Table 7 were melted in a 30 kg vacuum melting furnace and Al deoxidized or Si deoxidized.

  Each stainless steel ingot was forged to a thickness of 55 mm and a width of 100 mm, and adjusted to a thickness of 50 mm by surface grinding. Next, the steel type that was hot-rolled to a thickness of 5 mm and the martensite phase formed by hot rolling was pickled after soft annealing at 850 ° C. for 7 hours. Pickling was performed after annealing.

  About the steel grade which improves strength by work hardening, it cold-rolled to the range of intermediate plate thickness 2.3-2.8mm so that the rolling rate in final plate thickness 1.8mm might be 20-35%. The intermediate rolled stainless steel plate was annealed at 850 ° C. × soaking for 0 minutes, and after pickling, was finished and cold-rolled to a final plate thickness of 1.8 mm.

  For steel grades that improve strength with a martensite single phase or (ferrite + martensite) multiphase structure, cold-roll to an intermediate plate thickness of 3.0 mm after softening annealing, and after annealing and pickling, a final plate thickness of 1.8 mm Finished cold rolled. Subsequently, it was adjusted to a martensite single phase or (ferrite + martensite) multiphase structure by heat treatment of 1000 ° C. × soaking for 1 minute → air cooling.

A specimen was cut out from each manufactured stainless steel plate, and the metal structure, inclusions, and surface defects were observed. Regarding inclusions, the inclusion components are identified by EPMA, the cleanliness is measured by the method defined in JIS G 0555, and the longest diameter of the maximum inclusion observed in the field of view of the cleanliness is evaluated as the size of the inclusion. did. The 0.2% proof stress, elongation, and bending workability were investigated in the same manner as in Example 1.

As can be seen from the results of the investigation in Table 8, test Nos. In which inclusions were controlled in form by Al deoxidation and strengthened with processed ferrite. Nos. 1 to 8 have excellent bending workability because the minimum bending tip radius R is less than 0.1 mm in spite of the high 0.2% proof stress of 700 N / mm ² or more. I understand.

  On the other hand, test no. Although No. 9 had a 0.2% proof stress because it was strengthened in the martensite phase, the minimum bending tip radius R was 2.5 mm, which was extremely inferior in bending workability. Test No. in which the amount of martensite was reduced and the strength was sacrificed. Even in the case of steel No. 10, the minimum bending tip radius R is only 0.6 mm, and it is understood that there is a limit to the improvement of bending workability in the steel type that is strengthened by the martensite phase.

  Test No. in which Al content is insufficient even when Al is deoxidized. No. 11 did not exhibit sufficient bending workability because silicate inclusions remained due to insufficient deoxidation. Conversely, test No. 1 in which Al was deoxidized excessively. In No. 12, the Al concentration in the steel was too high at 0.09% by mass, and although bending workability was improved, wrinkles due to inclusions occurred on the product surface.

  Si deoxidized test no. In Nos. 13 to 15, the inclusions became silicate, and the test No. All were inferior in bending workability compared with 1-8.

The test materials other than Comparative Examples 9, 10, and 13 have a processed ferrite structure obtained by cold working at a finish rolling rate of 20 to 30%. Therefore, the 0.2% proof stress is as high as 700 N / mm ² or more. Since the elongation is 4% or less, it tends to be perceived as having poor ductility, but it has excellent bending workability. The excellent bendability is considered to be the result of being affected by local elongation rather than total elongation. This result suggests that the local ductility is improved on the outer surface of the bent portion by using the processed ferrite structure. Furthermore, since the inclusion form is appropriately controlled, the result of the relaxation of stress concentration at the inclusion / matrix interface appears as an effect of suppressing the occurrence of cracks.

As explained above, the stainless steel sheet whose form is controlled by Al deoxidation and strengthened with the processed ferrite structure has excellent bending workability even at 0.2% proof stress ≧ 700 N / mm ² , and environmental load Can be used as a solid material that does not require plating. Therefore, heat treatment on the user side can be omitted, and since it does not contain a large amount of Ni, it is used for frames, housings, etc. of home appliances, OA equipment, etc., coupled with low steel material costs.

Claims (3)

C: 0.15 mass% or less, Si: 1.0 mass% or less, Mn: 1.0 mass% or less, S: 0.005 mass% or less, Cr: 10-20 mass%, Ni: 0.5 mass % Or less, Al: 0.001 to 0.05% by mass, the balance being composed of Fe and inevitable impurities ,
A work hardening material for a stainless steel sheet, characterized by having a processed ferrite structure in which Al ₂ O ₃ and / or Al ₂ O ₃ .MgO inclusions having a size of 10 μm or less are dispersed with a cleanliness of 0.06 or less.   Furthermore, Mo: 0.5-2.0 mass%, Cu: 0.5-3.0 mass%, Nb: 0.05-1.0 mass% 1 type or 2 types or more are included. Work hardening material. Claim 1 or 2 work hardening material according the 0.2% yield strength is in the range of 500~900N / mm ^2.