The present invention relates to a ferritic stainless steel sheet, which can be formed to a product shape by press-forming, bending, roll-forming or the like due to good shape-freezability with less dimensional defects such as spring-back and torsion after forming, and also relates to a method of manufacturing thereof.

A stainless steel sheet has been used in various fields, e.g. interior or exterior members of buildings, frame members of electric home appliances and kitchen goods, due to its excellent external appearance and corrosion-resistance. The wording of "a steel sheet" involves a steel strip in this specification.

A product formed from a stainless steel sheet often involves dimensional defects originated in elastic recovery, since elastic strain of the stainless steel sheet is bigger than a common steel sheet. For instance, when a steel sheet, which is simply bent to a product shape, is detached from a shaping die, an angle of bent becomes broader than a designed angle due to release of elastic strain. The reformation is so-called as "spring-back". Especially in the case where a product is manufactured from a steel sheet by shallow drawing, elastic stain is not completely released but remains at a flange or a punched bottom even after the product is detached from a shaping die. The residual strain causes defects such as torsion and significantly reduces commercial value of the product.

A relatively soft austenitic stainless steel sheet such as SUS304 has been used among various kinds of stainless steels, in order to inhibit occurrence of defects during fabrication. However, austenitic stainless steel is expensive material due to high Ni content.

The present invention aims at provision of a ferritic stainless steel sheet, which is cheaper material due to remarkable decrease of Ni content but is improved in shape-freezability so as to inhibit dimensional defects such as spring-back and torsion after forming.

The present invention proposes a new ferritic stainless steel sheet, which has the alloying composition consisting of C up to 0.10 mass %, Si up to 1.0 mass %, Mn up to 1.0 mass %, P up to 0.050 mass %, S up to 0.020 mass %, Ni up to 2.0 mass %, 8.0-22.0 mass % of Cr, N up to 0.05 mass %, optionally one or more of 0.01-0.50 mass % of Ti, 0.01-0.50 mass % of Nb, 0.01-0.30 mass % of V, 0.01-0.30 mass % of Zr and 0.0010-0.0100 mass % of B, and the balance being Fe, with the provision that a value FM defined by the formula (1) is adjusted to 0 or less. The ferritic stainless steel sheet has an in-plane anisotropic degree (r_max-r_min) of Lankford value (r) ≤0.80 and an anisotropic degree (σ_max-σ_min) of 0.2%-yield strength ≤20 N/mm² $$\mathrm{FM}=420\mathrm{C}-11.5\mathrm{Si}+7\mathrm{Mn}+23\mathrm{Ni}-3.5\mathrm{Cr}-12\mathrm{Mo}+\mathrm{Cu}-49\mathrm{Ti}-50\mathrm{Nb}-23\mathrm{V}52\mathrm{Al}+470\mathrm{N}+20$$

The stainless steel sheet preferably has 0.2%-yield strength ≤350 N/mm² along any of a rolling direction (Direction-L), directions (Direction-D) crossing Direction-L with an angle of 45 degrees and a traverse direction (Direction-T) crossing Direction-L with a right angle.

The stainless steel sheet is manufactured by hot-rolling a ferritic stainless steel having the specified composition and then batch-annealing the hot-rolled steel sheet 1-24 hours at 700-880 °C.

• Fig. 1 is a schematic view for explaining a bending test, whereby a steel sheet is bent to a box shape, and corners of the box are measured for evaluation of spring-back.
• Fig. 2 is a graph for explaining a spring-back angle in relation with a plane anisotropic degree (r_max-r_min) of Lankford value (r) and an anisotropic degree (σ_max-σ_min) of 0.2%-yield strength.

Properties of ferritic stainless steels substantially depend on chemical composition and manufacturing conditions. The inventors have researched and examined effects of the chemical composition and manufacturing conditions on the properties, and discovered that shape-freezability (in other words, suppression of deformation derived from spring-back after forming) is improved by combination of a specified alloying composition with manufacturing conditions.

Since the shape-freezability is influenced not only by uniaxial deformation but also multi-axial deformation during plastically forming a stainless steel sheet to a product shape, materialistic properties and anisotropy along various directions put big effects on the shape-freezability. Especially, deviations of Lankford values (r) and 0.2%-yield strength among Directions-L, -D and -T are main factors. As the deviations of Lankford values (r) along directions-L, -D and -T are smaller, the stainless steel sheet has less plane anisotropy.

If Lankford value (r) is different from each other along Directions-L, - D and -T, thickness reduction of a stainless steel sheet is deviated at every part to which the same stress is applied. Deviation of thickness reduction causes irregular distribution of residual strains in the stainless steel sheet formed to a product shape, resulting in poor shape-freezability. Deviation of 0.2%-yield strength from each other along Directions-L, -D and -T means that various strains different from each other are given to the stainless steel sheet during plastically forming the stainless steel sheet with a certain stress. In this case, the shape-freezability is also poor.

In order to improve shape-freezability, a plane anisotropic degree (r_max-r_min) and an anisotropic degree (σ_max-σ_min) of 0.2%-yield strength are necessarily decreased, wherein r_max and σ_max are maximum of Lankford value (r) and 0.2%-yield strength among Directions-L, -D and -T, while r_min and σ_min are minimum of Lankford value (r) and 0.2%-yield strength among Directions-L, -D and -T.

The plane anisotropic degree (r_max-r_min) of Lankford value (r) and the anisotropic degree (σ_max-σ_min) of 0.2%-yield strength are decreased by conditioning re-crystallized ferrite grains of the stainless steel sheet to an isotropic state with equation of planar orientation. Isotropic re-crystallization of ferrite grains is attained by precipitating dissolved C and N as fine carbonitride particles uniformly dispersed in a steel matrix. Isotropic re-crystallization of ferrite grains effectively reduces the anisotropic degrees (r_max-r_min, σ_max-σ_min). Effects of uniform dispersion of fine carbonitride particles on random growth of re-crystallized ferrite grains are explained as follows:

Carbonitride particles present in a steel matrix act as seeds for re-crystallization of ferrite grains during final annealing, e.g. batch-annealing or finish-annealing, of a stainless steel sheet. Although grain boundaries and deformed zones such as slip bands in a cold-rolled ferritic structure have been heretofore regarded as seeds for re-crystallization of ferrite grains, the grain boundaries and the deformed zones are elongated by cold-rolling. As a result, the grain boundaries and the deformed zones have specified orientation, and re-crystallized ferrite grains grow while succeeding to the orientation. On the other hand, carbonitride particles are granular and very hard (Vickers hardness above 1000), so that they are not elongated during cold-rolling but act as seeds for isotropic re-crystallization of ferrite grains at boundaries in contact with ferrite grains.

Uniform dispersion of fine carbonitride particles is assured by properly controlling annealing conditions, so as to reform a rolling texture generated in a former hot-rolling step to an isotropic ferrite structure. The isotropic structure is maintained even in a cold-rolled state. That is, each ferrite grain is orientated due to application of stress in a following cold-rolling step, but a whole of the ferrite grains is still homogeneous and isotropic. The uniformly dispersed fine carbonitride particles act as seeds for re-crystallization of ferrite grains from a cold-rolling step to an annealing step, so as to further uniform planar orientation of ferrite grains. Consequently, an in-plane anisotropic degree (r_max-r_min) is reduced, and a stainless steel sheet is press-formed with good shape-freezability.

The other features of the present invention will be apparent from the following explanation on an alloying composition and manufacturing conditions.

A ferritic stainless steel according to the present invention contains the following elements as essential components.

C is converted to carbides by batch-annealing, and the carbides act as seeds for random growth of ferrite grains during re-crystallization at a final-annealing step. However, C is an element which unfavorably raises strength of a cold-rolled stainless steel sheet after annealing. Excess C content is also disadvantage for toughness. Therefore, C content is controlled to 0.10 mass % or less.

Si is an element, which is added as a deoxidizing agent during steel-making, but solution-hardens a steel matrix too much. Since excess Si causes hardening and decrease of ductility, an upper limit of Si content is determined to 1.0 mass %.

Mn is an austenite former, which does not put harmful effects on steel material due to its small solution-hardening power, useful for controlling the value-FM defined by the formula (1). However, excess Mn causes generation of fumes during steel-making and worsens productivity. In this sense, Mn content is controlled to 1.0 mass % or less.

P is an element harmful on hot-workability. The effect of P is suppressed by controlling P content less than 0.050 mass %.

S is an element, which segregates at grain boundaries and worsens hot-workability Such effects are suppressed by controlling S content less than 0.020 mass %.

Ni is the same austenite former as Mn and useful for controlling the value-FM. However, excess addition of Ni above 2.0 mass % raises a steel cost and also hardens a steel sheet.

Cr is an essential element for corrosion resistance. At least 8 mass % of Cr is necessary for corrosion resistance as stainless steel. However, excess addition of Cr above 22.0 mass % worsens toughness and formability of a stainless steel sheet.

N is converted to nitrides by batch-annealing. The nitrides act as seeds for random growth of ferrite grains during re-crystallization in a final-annealing step. However, excess N causes decrease of toughness, since N raises strength of an annealed cold-rolled steel sheet. Therefore, N content is controlled to 0.05 mass % or less.

The ferritic stainless steel may further contain one or more of the following elements in addition to the above-mentioned elements.

Al is an element, which is added as a deoxidizing agent during steel-making. Excess Al content above 0.10 mass % causes increase of non-metallic inclusions, decrease of toughness and occurrence of surface defects. Therefore, Al content is properly determined so as to control a value-FM to 0 or less.

Mo is an element for improvement of corrosion resistance, but excess addition of Mo above 1.0 mass % promotes solution-hardening and retards dynamic re-crystallization in a high-temperature zone, resulting in decrease of hot-workability.

Cu is an element included in steel from scraps during steel-making. Since excess Cu is unfavorable for hot-workability and corrosion-resistance, its upper limit is determined to 1.0 mass %.
0.01-0.50 mass % of Ti, 0.01-0.50 mass % of Nb,
0.01-0.30 mass % of V and 0.01-0.30 mass % of Zr

Ti, Nb and V are reacted with C dissolved in a steel matrix and precipitated as carbides effective for formability. Zr captures dissolved O as oxide and improves formability and toughness of a stainless steel sheet. Effects of these elements are noted at every 0.01 mass % or more, but excess addition is disadvantageous for productivity. In this sense, upper limits of these elements are determined to Ti: 0.50 mass %, Nb: 0.50 mass %, V: 0.30 mass % and Zr: 0.30 mass %.
0.0010-0.0100 mass % of B

B is an element, which uniformly disperses transformed phase in a hot-rolled steel sheet and promotes random growth of ferrite grains in a final structure without generation of aggregate structure. Uniform distribution of the transformed phase is typically noted by addition of B at a ratio of 0.0010 mass % or more. However, excess addition of B above 0.0100 mass % causes degradation of hot-workability and weldability.

The stainless steel is designed so as to adjust a value-FM defined by the formula (1) to 0 or less in addition to the specified ratios of the alloying elements, for improvement of shape-freezability without generation of an austenite phase during batch-annealing. $$\mathrm{F}\mathrm{M}=420\mathrm{C}-11.5\mathrm{Si}+7\mathrm{Mn}+23\mathrm{Ni}-3.5\mathrm{Cr}-12\mathrm{Mo}+9\mathrm{Cu}-49\mathrm{Ti}-50\mathrm{Nb}-23\mathrm{V}-52\mathrm{Al}+470\mathrm{N}+20$$

Generation of an austenite phase in a high-temperature zone during batch-annealing is inhibited by controlling the value-FM to 0 or less. On the other hand, an alloying design at FM>0 allows generation of an austenite phase, which can dissolve C and N at relatively high ratios, in a ferrite matrix. Since solubility of C and N is different between the austenite phase and the ferrite matrix, the anisotropic degrees (r_max-r_min and σ_max-σ_min) are raised due to the uneven solubility. An in-plane anisotropic degree (r_max-r_min) of Lankford value (r) ≤0.80 An anisotropic degree (σ_max-σ_min) of 0.2%-yield strength ≤20N/mm²

As the anisotropic degrees (r_max-r_min and σ_max-σ_min) are smaller, a ferritic stainless steel is press-formed to a product shape with better shape-freezability. Experimental results prove that the shape-freezability is excellent at (r_max-r_min) ≤0.80 and (σ_max- σ_min) ≤ 20N/mm².

A complete ferrite structure free from martensite with 0.2%-yield strength of 350N/mm² or less is preferable in order to impart excellent shape-freezability to a ferritic stainless steel. Strength above 350N/mm² naturally requires application of a big stress for plastic deformation of the stainless steel sheet, resulting in increase of spring-back and degradation of shape-freezability.

A ferritic stainless steel sheet is annealed under the conditions that dissolved C and N are precipitated as fine carbonitride particles uniformly dispersed in a single ferrite matrix, in order to reduce the anisotropic degrees (r_max-r_min and σ_max-σ_min). Sufficient precipitation of carbonitride particles is realized by batch-annealing at a temperature of 700 °C or higher. However, when the stainless steel sheet is batch-annealed at a temperature higher than 880 °C, the stainless steel sheet is rendered to an anisotropic structure on the contrary due to predominant growth of re-crystallized ferrite grains (so-called as "secondary re-crystallization").

The present invention will be more clearly understood by the following examples.

Several stainless steels shown in Table 1 were melted in a vacuum furnace, cast, forged and then hot-rolled to thickness of 3.0 mm. Each hot-rolled steel sheet was batch-annealed or intermediate-annealed under conditions shown in Table 2, pickled and then cold-rolled to thickness of 0.5 mm. The cold-rolled steel sheet was finish-annealed 1 minute at 880 °C, cooled in the open air and then pickled again.

Each annealed steel sheet was sampled for measurement of Lankford value (r) and 0.2%-yield strength as follows:

After tensile strain of 15% was applied to a test piece JIS 13B, Lankford value (r) was measured along each of Directions-L, -D and -T. A difference between measured maximum and minimum values was calculated and evaluated as an in-plane anisotropic degree (r_max-r_min) of Lankford value (r).

After tensile strain was applied to a test piece JIS 13B at a rate of 3.3 × 10^-4, 0.2%-yield strength was measured along each of Directions-L, -D and

-T. A difference between measured maximum and minimum values was calculated and evaluated as an anisotropic degree (σ_max-σ_min) of 0.2%-yield strength.

Two test pieces, each of which had a developed box-shape (shown in Fig. 1) comprising a 40 mm-square area E₁, E₂ with four oblong areas A₁-D₁, A₂-D₂ of 10 mm x 36 mm in size, were prepared from each annealed steel sheet. One test piece was cut along Direction-L (a rolling direction), and the other was cut along Direction-D. All sides of the square areas E₁, E₂ were bent at a working speed of 200 mm/minute under a hold-down pressure of 20 ton, and the oblong areas A₁-D₁, A₂-D₂ were raised upright, by a 200-ton press equipped with a rectangular punch having a tip diameter of 4mm. A spring-back angle θ was measured at every measurement point P1-P4 corresponding to four corners of a bottom of a formed box. Shape-freezability was evaluated by a maximum angle θ_max among the measurement values.

Table 2 shows results of each annealed steel sheet, and Fig. 2 shows distribution of maximum spring-back angles θ_max in relation with anisotropic degrees (r_max-r_min and σ_max-σ_min).

It is understood from Fig. 2 that the inventive steel sheets with rmax-rmin ≤0.8 and σ_max-σ_min ≤20N/mm² were good of shape freezability (i.e. maximum spring-back angles θmax ≤3 degrees). On the other hand, comparative steel sheets, which did not satisfy either one of r_max-r_min ≤0.8 and σ_max-σ_min ≤20N/mm², were poor of shape-freezability, as noted by maximum spring-back angles θ_max >3 degrees Test No. Steel Kind Batch-annealing or Intermediate annealing In-plane anisotropy r_max-r_min 0.2%-yield strength σ_max(N/mm²) Anisotropic degree of 0.2%-yield strength σ_max-σ_min(N/mm²) Maximun sping back angle θ_min(degrees) Note °C Period 1 A 720 12 hrs. 0.53 256 11 1.7 Inventive Examples 2 A 770 8 hrs 0.65 276 15 2.2 3 A 835 20 hrs. 0.24 234 12 2.6 4 A 750 60 sec. 1.07 276 23 5.2 Comparative Examples 5 A 930 10 hrs. 0.86 241 15 4.6 6 C 775 10 hrs. 0.32 203 8 1.8 Inventive Examples 7 C 845 20 hrs. 0.29 199 5 0.9 8 C 670 20 hrs. 0.96 219 12 3.9 Comparative Examples 9 C 1000 60 sec. 1.02 232 25 4.3 10 B 890 8 hrs. 0.23 322 7 1.6 Inventive Examples 11 D 790 10 hrs. 0.74 289 18 2.3 12 E 835 18 hrs. 0.22 215 12 2.8 13 F 850 8 hrs. 0.48 221 19 1.7 14 G 765 22 hrs. 0.54 331 13 1.6 15 H 750 8 hrs. 1.21 222 21 4.7 Comparative Examples 16 I 750 12 hrs. 1.11 312 12 6.5 17 J 830 20 hrs. 0.87 254 23 7.3 18 K 850 15 hrs. 1.23 392 35 8.5

The underlines figures are out of the ranges defined by the present invention.

According to the present invention as above-mentioned, a ferritic stainless steel sheet is improved in shape-freezability by conditioning re-crystallized ferrite grains to a structure with equalized planar orientation so as to reduce a plane anisotropic degree (r_max-r_min) of Lankford value (r) and an anisotropic degree (σ_max-σ_min) of 0.2%-yield strength to possible lowest values. Since the stainless steel sheet is plastically formed to a product shape with less spring-back, it is useful in various industrial fields, e.g. parts of electric or electronic devices such as a sealing member of an organic EL device, precise pressed parts, and building members.