JP2002332549A - Ferritic stainless steel strip having excellent shape fixability on forming and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel strip having excellent shape fixability in which the defects of shape such as springback and twist appearing in a formed product can be reduced by using an inexpensive material having a low Ni content, and to provide a production method therefor. SOLUTION: The stainless steel strip is obtained by subjecting ferritic stainless steel having a composition containing, by mass, <=0.10% C, <=1.0% Si, <=1.0% Mn, <=0.050% P, <=0.020% S, <=2.0% Ni, 8.0 to 22.0% Cr and <=0.05% N, and, if required, further containing one or more kinds selected from <=0.10% Al, <=1.0% Mo, <=1.0% Cu, 0.01 to 0.50% Ti, 0.01 to 0.50% Nb, 0.01 to 0.30% V, 0.01 to 0.30% Zr and 0.0010 to 0.0100% B, and the balance Fe with inevitable impurities to hot rolling, and thereafter subjecting the steel sheet to batch annealing at 700 to 880 deg.C for 1 to 24 hrs.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has an excellent shape freezing property which can reduce shape defects such as springback and torsion after processing of a molded product subjected to processing such as press molding, bending and roll forming. The present invention relates to a stainless steel strip and a method for producing the same.

[0002]

2. Description of the Related Art A stainless steel strip (hereinafter referred to as a steel strip including a thin steel sheet) is a steel sheet having excellent design and corrosion resistance, and is used for various purposes such as interior and exterior materials of buildings, frames of home electric appliances, kitchen members, and the like. It is used for However, compared to ordinary steel, since the elastic strain amount is large, a shape defect due to elastic recovery occurs in the product. For example, in the case of a simple bent product, the elastic angle is released when the product separates from the mold after molding, so that the product angle is larger than the design angle. This phenomenon is generally called springback, and is one of the problems during molding.

[0003] In the case of a shallow drawn product, the elastic strain is not completely released even after the product is separated from the mold, and remains on the flange or the bottom of the punch, causing problems such as twisting of the product. To cover these stainless steel deficiencies,
Until now, soft SUS304 has been used in stainless steel.

[0004]

However, since austenitic stainless steel contains a large amount of Ni, the material cost is naturally high. Therefore, the present invention has been devised to solve such a problem. By using an inexpensive material having a low Ni content, it is possible to reduce shape defects such as springback and torsion after processing of a molded product. It is an object of the present invention to provide a stainless steel strip excellent in shape freezing property and a method for producing the same.

In order to achieve the object, the ferritic stainless steel strip of the present invention having excellent shape freezing properties has a C content of 0.10% or less and a Si content of 0.1% by mass.
1.0% or less, Mn: 1.0% or less, P: 0.050%
Below, S: 0.020% or less, Ni: 2.0% or less, C
r: 8.0 to 22.0%, N: 0.05% or less, and in some cases, Al: 0.10% or less, Mo: 1.0%
Hereinafter, Cu: 1.0% or less, Ti: 0.01 to 0.50
%, Nb: 0.01-0.50%, V: 0.01-0.
30%, Zr: 0.01 to 0.30% or B: 0.0
One or more of 010 to 0.0100% is contained in such a ratio that the FM value defined by the following formula (1) becomes 0 or less,
The balance has a composition consisting of Fe and unavoidable impurities,
The in-plane anisotropy r _max -r _min is 0.80 or less, and the 0.2% proof stress anisotropy σ _max -σ _min is 20 N / mm ² or less. (1) FM = 420C-11.5Si + 7Mn + 23Ni-3.5Cr-12Mo + 9Cu-49Ti-
50Nb-23V-52Al + 470N + 20 In terms of strength, the 0.2% proof stress in the rolling direction, 45 ° direction to the rolling direction and perpendicular to the rolling direction is 350N.
/ Mm ² or less. After hot-rolling a ferritic stainless steel having the above-mentioned composition, 700
It is obtained by performing batch annealing at ８880 ° C. for 1 to 24 hours.

[0005]

The present inventors have studied various means for obtaining a ferritic stainless steel strip having a small shape change such as springback during processing, that is, excellent in shape freezing property during forming. It has been found that this can be achieved by optimization. In particular, hitherto by combining two parameters planar anisotropy r _max -r _min and 0.2% proof stress, which has not been noted different Katado sigma _max - [sigma] _min, excellent ferritic stainless steel strip It has been found that shape freezing can be imparted.

[0006] When molding an actual processed product, it is necessary to consider not only the deformation in the uniaxial direction but also the deformation in the multiaxial direction. It was found that the value and furthermore the anisotropy in each direction greatly affected the shape freezing property. In particular, the important factor is the difference between the r value and the 0.2% proof stress in the rolling direction (L direction), the direction at 45 ° to the rolling direction (D direction) and the direction perpendicular to the rolling direction (T direction). It is. If the Rankford value r is different in each direction, the thickness reduction when the same strain is applied differs depending on the location, and the strain distribution is alternated in commercialization, resulting in poor shape freezing property. Further, the difference in the 0.2% proof stress in the direction means that the strain applied varies depending on the location when molding with a constant stress at the time of processing, which adversely affects the shape freezing property.

Accordingly, in order to improve the shape fixability at the time of processing, reduce the two parameters planar anisotropy r _max -r _{m in} a 0.2% yield different Katado sigma _max - [sigma] _min It is effective to do. (Note that max and min are the maximum and minimum values in the L, D, and T directions.) To reduce such anisotropy and anisotropy, the ferrite stainless steel It is necessary to eliminate anisotropy in the recrystallized grains and make the plane orientation uniform. In order to reduce the in-plane anisotropy r _max -r _min and the 0.2% proof stress anisotropy σ _max -σ _min , C and N in solid solution are converted into carbonitrides in a ferrite single phase matrix. It is necessary to uniformly disperse and disperse.

If the carbonitrides are uniformly and finely precipitated and dispersed, the fine carbonitrides can be used as nuclei for recrystallization of ferrite generated in the process of cold rolling and annealing thereafter.
The plane orientation can be made uniform, and as a result, the in-plane anisotropy r _max -r _min and the 0.2% proof stress anisotropy σ _max -σ _min can be reduced. In the present invention, by selecting batch annealing conditions, it is possible to uniformly precipitate and disperse carbonitrides during annealing to obtain a ferritic stainless steel strip in which the plane orientation of recrystallized grains is made uniform. Can improve the shape freezing property.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The alloy components, contents, production conditions and the like contained in the stainless steel of the present invention will be described below. C: 0.10% or less C forms carbides during batch annealing, which acts as randomized recrystallization nuclei of recrystallized ferrite in final annealing. However, C is an element that increases the strength after cold-rolling annealing, and if it is too high, it causes a decrease in toughness.
It was as follows.

Si: 1.0% or less Si is usually used for deoxidation. However, Si has a high solid solution strengthening ability, and if its content is excessive, the material is hardened and the ductility is reduced. 0.0% was made the upper limit. Mn: 1.0% or less Mn is an austenite-forming element, can be effectively used for controlling the FM value, and has a small solid solution strengthening ability and has little adverse effect on the material. However, when the content is large, M
Since the manufacturability deteriorates due to generation of n fume, the upper limit is set to 1.0%.

P: 0.050% or less P is an element harmful to hot workability. Especially 0.050
%, The effect becomes significant. S: 0.020% or less S is an element that easily segregates at crystal grain boundaries and promotes reduction in hot workability due to grain boundary embrittlement. When S exceeds 0.020%, the effect becomes significant. .

Ni: 2.0% or less Ni is an austenite-forming element like Mn.
It is an element that can be effectively used for controlling the FM value. But,
Since addition exceeding 2.0 causes hardening and cost increase, the upper limit is 2.0%. Cr: 8.0 to 22.0% Cr must be at least 8% to exhibit the corrosion resistance of stainless steel. On the other hand, a large amount causes reduction in toughness and workability, so the upper limit was set to 22.0%.

N: 0.05% or less N forms a nitride by batch annealing like C, and it acts as a recrystallization nucleus for randomizing the crystal orientation of recrystallized ferrite in final annealing. However, N is an element that increases the strength of the cold-rolled annealed material. If N is too high, the toughness is reduced.

The following elements are added as needed. Al: 0.10% or less Al is an element effective for deoxidation, but excessive addition causes a decrease in toughness and surface defects due to the presence of nonmetallic inclusions. Therefore, it is selectively added while keeping the following FM value balance, but the upper limit is set to 0.10%. Mo: 1.0% or less Mo is an element effective for improving corrosion resistance. However, excessive addition causes reduction in hot workability due to solid solution strengthening at high temperatures and delay of dynamic recrystallization. Therefore, when it is contained, the content is made 1.0% or less. Cu: 1.0% or less Cu is inevitably contained in many cases, such as mixing from scrap at the time of melting, but excessive content lowers hot workability and corrosion resistance, so 1.0% or less. To

Ti: 0.01 to 0.50% Nb: 0.01 to 0.50% V: 0.01 to 0.30% Zr: 0.01 to 0.30% Ti, Nb and V are solid Zr is an effective element from the viewpoint of improving workability and toughness by capturing O in steel as an oxide by the effect of precipitating dissolved C as carbide. However, if a large amount is added, the productivity is reduced. Therefore, when adding, a preferable addition amount is Ti, Nb.
Is 0.01 to 0.50%, and V and Zr are 0.01 to 0.3%.
0%.

B: 0.0010 to 0.0100% B uniformly disperses the transformed phase distribution of the hot-rolled sheet, and this effectively works for randomization without forming a texture of a ferrite structure as a final structure. . An effect of less than 0.0010% is not sufficient. On the other hand, if the content exceeds 0.0100%, a decrease in hot workability and a decrease in weldability are caused. Therefore, a preferable content when the content is 0.0010 to 0.0100%.
It is.

FM value ≦ 0 In addition to the above-mentioned individual component ranges, in order to avoid the formation of an austenite phase and improve the shape freezing property in the batch annealing in the steel strip manufacturing process, the following formula (1) is used. ) Were defined. By reducing this value to 0 or less, a device was devised so that austenite was not formed at the high temperature in the batch annealing step. When the FM value becomes 0 or more, an austenite phase is partially formed in the ferrite phase. The austenite phase can dissolve more C and N than the ferrite phase. Therefore, when the austenite phase is partially formed in the ferrite phase, the solid solubility of C and N changes in both phases, and the non-uniformity of the solid solubility causes in-plane anisotropy and proof anisotropy. Will be caused.

(1) FM = 420C-11.5Si + 7Mn + 23Ni-3.5Cr-12
Mo + 9Cu-49Ti-50Nb-23V-52Al + 470N + 20

[0019]In-plane anisotropy r _max −r _min ≦ 0.80 0.2% proof stress anisotropy σ _max -σ _min 20 N / mm ²  As described above, the in-plane anisotropy r_max-R_minand
0.2% proof stress anisotropy σ _max−σm_minTwo parameters
Is preferably smaller. The present inventors, FM value 0 or less
The above two parameters and shapes after forming various steel strips
After examining the relationship between the freezing properties, the in-plane anisotropy r_max−
r_minIs 0.8 or less, and 0.2% proof stress anisotropy σ_max−
σ_minIs 20 N / mm^TwoIf below, excellent shape freezing property
And found.

In order to obtain a ferritic stainless steel excellent in shape freezing property, 0.2% proof stress ≦ 350 N / mm ² , a complete ferrite structure in which martensite is not formed, and 0.2% proof stress is 350 N / mm ² It is preferable to set the following. If the 0.2% proof stress is higher than this value, the applied stress for inevitably plastic deformation increases, springback and the like increase, and the shape freezing property deteriorates.

Annealing conditions: (700 to 880) ° C. × (1 to
24) making the Hr ferritic stainless steel, in-plane anisotropy as described above r _max -r _{m in} and 0.2% proof stress different Katado σ
To reduce _max- σ _min , C and N
Must be uniformly precipitated and dispersed as a carbonitride in a ferrite single-phase matrix. Batch temperature is 7
If the temperature is lower than 00 ° C., carbonitrides cannot be sufficiently precipitated, and
If the temperature exceeds 80 ° C. and is too high, preferential growth of recrystallized ferrite (secondary recrystallization) occurs, and conversely, the anisotropy increases. At least one hour of soaking is required to sufficiently precipitate carbonitrides, but heating is not required for a long time, and the heating is performed within 24 hours from an economical viewpoint.

[0022]

EXAMPLE Steel having the chemical composition values (mass%) and FM values shown in Table 1 was melted in a vacuum melting furnace, then forged and hot-rolled to a sheet thickness of 3.0 mm. Intermediate annealing was performed. Further, after pickling, the plate was cold-rolled to a sheet thickness of 0.5 mm, and subjected to finish annealing at 880 ° C. × 1 minute soaking and air cooling to perform pickling. The obtained steel sheet was used as a test material, and the Rankford value and 0.2% proof stress were measured by the following methods.

After applying 15% tensile strain using a r-value JIS13B test piece, the rolling direction (L direction), the direction at 45 ° to the rolling direction (D direction), and the direction perpendicular to the rolling direction (T direction) ) Was determined. Among the values obtained in the above three directions,
The difference between the maximum value and the minimum value was defined as in-plane anisotropy r _max -r _min .

A 0.2% proof stress JIS No. 13 test piece was used, and the strain rate was 3.3 × 10 −4.
Then, 0.2% proof stress in the rolling direction (L direction), the 45 ° direction (D direction) with respect to the rolling direction, and the direction perpendicular to the rolling direction (T direction) was determined. Among the values obtained in the above three directions, the difference between the maximum value and the minimum value was defined as 0.2% proof stress anisotropy σ _max −σ _min .

Shape Freezing Properties Shapes (A to D: 1 to A) shown in FIG.
(0 mm x 36 mm, E: □ 40 mm), and sampled by a square punch, 90 ° on each side of E with a square punch
Bending was performed (R / t = 8 R: punch tip diameter 4 mm,
t: test plate thickness 0.5 mm). The molding conditions were as follows: 200 ton mechanical press, plate holding pressure 20 ton, molding speed 20
The measurement was performed at 0 mm / min, and the springback angle after bending the rectangular cylinder was measured. The spring angles Δθ at eight locations indicated by arrows (1) to (8) in FIG. 1 are measured, and the maximum value is defined as Δθ _max.
And

In-plane anisotropy r _max -r _min after each heat treatment,
Table 2 also shows the 0.2% proof stress anisotropy σ _max -σ _min and the maximum spring angle Δθ _max . FIG. 2 shows the maximum spring angle distribution arranged in a relationship of in-plane anisotropy r _max -r _min and 0.2% proof stress anisotropy σ _max -σ _min .

[0027]

[0028]

As can be seen from the results shown in FIG. 2, when the in-plane anisotropy r _max -r _min is 0.8 or less, the 0.2% proof stress anisotropy σ
_The examples of the present invention satisfying both _max- σ _min of 20 N / mm ² or less have a maximum spring angle of 3 ° or less and are excellent in shape freezing property. On the other hand, in the case of the comparative example in which the above two parameters are not within the predetermined numerical ranges, the maximum spring angle exceeds 3 °, and the shape freezing property is poor.

[0030]

As described above, the plane orientation of ferrite recrystallized grains of ferritic stainless steel is made uniform, and the in-plane anisotropy of the r value and the anisotropy of 0.2% proof stress are minimized. By doing this, it is possible to improve the shape freezing property at the time of molding, and to use ferritic stainless steel strips suitable for IT related parts such as insulating sealing members for organic EL elements, etc., various precision pressed products, building materials, etc., which have strict dimensional accuracy. It became possible to provide.

[Brief description of the drawings]

FIG. 1 is a diagram showing the concept of a square tube bending test and shape measurement performed for evaluating shape freezing properties.

FIG. 2 Influence of in-plane anisotropy r on maximum spring angle
_max -r _min and diagrams showing the relationship between 0.2% yield different Katado σ _max -σ _min.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Kenichi Morimoto 4976 Nomura Minamicho, Shinnanyo-shi, Yamaguchi Prefecture Inside Nisshin Steel Co., Ltd. Nisshin Steel Corporation Stainless Steel Business Unit (72) Inventor Naoto Hiramatsu No. 4976 Nomura Minamimachi, Shinnanyo City, Yamaguchi Prefecture Nisshin Steel Corporation Stainless Steel Business Unit

Claims (4)

[Claims] 1. A mass% of C: 0.10% or less,
Si: 1.0% or less, Mn: 1.0% or less, P: 0.0
FM defined by the following formula (1): 50% or less, S: 0.020% or less, Ni: 2.0% or less, Cr: 8.0 to 22.0%, N: 0.05% or less The content is such that the value is 0 or less, the balance has a composition consisting of Fe and unavoidable impurities, and the in-plane anisotropy r _max -r _min is 0.80.
Below, 0.2% proof stress anisotropy σ _max -σ _min is 20 N / m
Excellent ferritic stainless steel strip in shape fixability during molding, characterized in that at m ² or less. (1) FM = 420C-11.5Si + 7Mn + 23Ni-3.5Cr-12Mo + 9Cu-49Ti-
50Nb-23V-52Al + 470N + 20 2. In addition to the components described in claim 1, Al: 0.10% or less and Mo: 1.0 in mass%.
%, Cu: 1.0% or less, Ti: 0.01 to 0.5
0%, Nb: 0.01 to 0.50%, V: 0.01 to
0.30%, Zr: 0.01 to 0.30% or B:
0.0010 to 0.0100% of one or more kinds are contained in such a ratio that the FM value defined by the following formula (1) becomes 0 or less, and the balance has a composition consisting of Fe and unavoidable impurities; The in-plane anisotropy r _max -r _min is 0.80 or less,
A ferritic stainless steel strip having excellent shape freezing property at the time of forming, characterized in that the 0.2% proof stress anisotropy σ _max -σ _min is 20 N / mm ² or less. (1) FM = 420C-11.5Si + 7Mn + 23Ni-3.5Cr-12Mo + 9Cu-49Ti-
50Nb-23V-52Al + 470N + 20 3. A 0.2% proof stress value of 350 in the rolling direction, a direction at 45 ° to the rolling direction and a direction perpendicular to the rolling direction.
Excellent ferritic stainless steel strip in shape fixability during molding according to claim 1 or 2 is N / mm ² or less. 4. After hot-rolling a ferritic stainless steel having the component composition according to claim 1 or 2,
The method for producing a ferritic stainless steel strip having excellent shape freezing property during forming according to claim 1 or 2, wherein batch annealing is performed at 880 ° C for 1 to 24 hours.