DE60213784T2 - BELTS OF FERRITIC STAINLESS STEEL WITH EXCELLENT CONSERVATION OF THE MOLDING SHAPED BY MACHINING - Google Patents

Description

Industrial Field of the invention

The The present invention relates to a sheet of ferritic stainless steel resulting in a product form by compression molding, Bending, roll forming or the like due to the good shape retention or mold-freezability with few dimensional errors, such as rebound and rotation after formation, can be molded, and relates also to a process for its preparation.

background

One Stainless steel sheet is used in many fields been, for example for Interior or exterior elements of buildings, Frame elements of household electrical appliances and kitchen ware, due to its excellent external appearance and its excellent corrosion resistance. The term "a steel sheet" includes one Strip steel in this description.

One Product made from a sheet of stainless steel, comprises often dimensional errors caused by re-deformation because the elastic tension of the sheet of stainless steel is larger as that of a usual one Steel sheet. If, for example, a sheet steel that is easy to a product shape is bent, taken from a molding tool becomes, a bending angle becomes wider than a formed angle due of loosening the elastic tension. Forming is referred to as "springback." Specifically in the case where a product made from a sheet steel by flat drawing is, the elastic stress is not completely dissolved, but remains at one Flange or a stamped bottom, even after the product is out was removed from a mold. The remaining voltage causes Mistakes, such as twisting, and significantly reduces the commercial Value of the product.

One Sheet metal made of relatively soft austenitic stainless steel SUS304 has been used under many types of steel sheets, to the leakage of errors during to inhibit the production. However, austenitic is stainless Steel expensive material due to the high Ni content.

Summary of the invention

The The present invention aims to provide a sheet made of ferritic stainless steel, which is cheaper Material due to the significant reduction in Ni content, but in the form-retention or mold-freezability is improved so to speak the dimensional errors, such as springback and rotation after molding, to inhibit.

The present invention proposes a new sheet of ferritic stainless steel having the alloy composition consisting of up to 0.10 mass% C, up to 1.0 mass% Si, up to 1.0 mass% Mn, up to 0.050 mass% P, up to 0.020 mass% S, up to 2.0 mass% Ni, 8.0 to 22.0 mass% Cr, up to 0.05 mass% N, optionally one or more from 0.01 to 0.50 mass% Ti, 0.01 to 0.50 mass% Nb, 0.01 to 0.30 mass% V, 0.01 to 0.30 mass% Zr and 0.0010 to 0.0100 mass% B, and the balance being Fe, with the proviso that by the formula (1) defined value Fm is set to 0 or less. The ferritic stainless steel sheet has a plane anisotropic degree (r _max -r _min ) of the Lankford value (r) of ≤ 0.80 and an anisotropic grade (σ _max -σ _min ) of the 0.2% yield strength of ≤ 20 N / mm ² on. FM = 420 C - 11.5 Si + 7 Mn + 23 Ni - 3.5 Cr - 12 Mo + 9 Cu - 49 Ti - 50 Nb - 23 V - 52 Al + 470 N + 20 (1)

The stainless steel sheet preferably has a 0.2% proof stress of ≦ 350 N / mm ² along each of a rolling direction (L direction), directions (D direction) crossing the L direction at an angle of 45 degrees, and a transverse direction (direction T) crossing the direction L at a right angle.

The Stainless steel sheet is made by hot rolling a ferritic stainless steel with the specified composition and then hood annealing of the hot rolled Steel sheet for 1 to 24 hours at 700 to 880 ° C produced.

Short description of drawings

1 Fig. 12 is a schematic view for explaining a bending test in which a steel sheet is bent into a box shape, and the corners of the box are measured for the evaluation of the springback.

2 is a graph for explaining the springback angle with respect to a plane anisotropic degree (r _max -r _min ) of the Lankford value (r) and an anisotropic degree (σ _max -σ _min ) of the 0.2% proof stress.

preferred embodiments the invention

properties of ferritic stainless steels depend essentially on the chemical composition and production conditions. The Inventors explored and studied the effects of chemical Composition and manufacturing conditions on the properties, and discovered that the Shape retention or form-freezability (in other words, the oppression the deformation resulting from the springback after shaping) by the combination of specify Alloy composition improved with the production conditions becomes.

There the form-preservation or form-freezability not only by the uniaxial deformation, but also by the multi-axial Deformation during the plastic shaping of a sheet of stainless steel influenced to a product form is, the material properties and the anisotropy along different directions big Effects on shape retention or mold-freezability on. Especially are deviations the Lankford values (r) and the 0.2% proof stress under the directions L, D and T major factors. If the deviations of the Lankford values (r) along the directions L, D and T are smaller, the sheet metal made of stainless steel a lower level anisotropy.

If the Lankford value (r) is different along the L, D and T directions, gives way to the reduction in thickness of a sheet of stainless steel at each part where the same voltage is applied. The deviation Thickness reduction causes irregular distribution of the remaining ones Stresses in the stainless steel sheet resulting in a product shape is formed, resulting in poor shape retention or mold-freezability leads. The deviation of the 0.2% proof stress along the directions L, D and T means that the sheet of stainless Steel several, differing voltages during the plastic shaping of the stainless steel sheet with to be given a certain tension. In this case, the Shape retention or mold-freezability as bad.

In order to improve shape-preservability, a plane anisotropic degree (r _max -r _min ) and an anisotropic degree (σ _max -σ _min ) of 0.2% proof stress are necessarily reduced, where r _max and σ _{max is} the maximum of the Lankford value (r) and the 0.2% proof stress under the directions L, D and T, while r _min and σ _{min is} the minimum of the Lankford value (r) and the 0.2% Yield point under the directions L, D and T.

The planar anisotropic degree (r _max -r _min ) of the Lankford value (r) and the anisotropic degree (σ _max -σ _min ) of the 0.2% proof stress become one by conditioning recrystallized ferrite grains of the stainless steel sheet reduced isotropic state according to the equation of the plane direction. The isotropic recrystallization of the ferrite grains is achieved by precipitating dissolved C and N as fine carbonitride particles uniformly dispersed in a steel matrix. The isotropic recrystallization of the ferrite grains effectively reduces the anisotropic grades (r _max - r _min , σ _max - σ _min ). The effects of uniform dispersion of fine carbonitride particles on the random growth of recrystallized ferrite grains are illustrated as follows:
Carbonitride particles present in a steel matrix act as nuclei for the recrystallization of ferrite grains during final annealing, such as bell annealing or finish annealing, of a stainless steel sheet. Although grain boundaries and deformed zones such as sliding ribbons in a cold-rolled ferritic structure have hitherto been considered as nuclei for recrystallization of ferrite grains, the grain boundaries and deformed zones are elongated by cold rolling. As a result, the grain boundaries and the deformed zones have a specific orientation, and the recrystallized ferrite grains grow, following the orientation. On the other hand, the carbonitride particles are granular and very hard (Vickers hardness above 1000), so that they are not elongated during cold rolling but act as nuclei for the isotropic recrystallization of ferrite grains at boundaries in contact with ferrite grains.

The uniform dispersion of fine carbonitride particles is controlled by appropriately controlling the annealing secured conditions so as to transform a rolling texture, which is produced in a previous 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 oriented by the application of stress in a subsequent cold rolling step, but a majority of the ferrite grains are still homogeneous and isotropic. The uniformly dispersed fine carbonitride particles act as nuclei for recrystallization of ferrite grains from a cold rolling step to an annealing step so as to make the planar orientation of the ferrite grains more uniform. Consequently, the plane anisotropic degree (r _max -r _min ) is reduced, and a stainless steel sheet is molded with good shape-preservability.

The Other features of the present invention are as follows explanation the alloy composition and the manufacturing conditions.

One Ferritic stainless steel according to the present invention contains the following elements as essential components.

Up to 0.10 mass percent C

C turns into carbides by bell annealing and the carbides act as nuclei for the random growth of ferrite grains during the Recrystallization at a final annealing step. However, C is one Element, the strength of a cold-rolled sheet of stainless Steel after annealing unfavorable elevated. An excess C content is also for the tenacity disadvantageous. Therefore, the C content becomes 0.10 mass% or less set.

Up to 1.0 percent by mass Si

Si is an element which acts as a deoxidizer during the Steel production is added, but the solution hardens the steel matrix too much. Since excess Si the hardening and causing the reduction of moldability is considered upper Limit of Si content 1.0 mass percent specified.

Up to 1.0 percent by mass Mn

Mn is an austenite former that has no harmful effects on the steel material because of its small solution-hardening power exerts the for controlling the value FM defined by the formula (1) useful is. However, excess Mn causes the production of vapors while steel production and worsens productivity. In this Meaning, the Mn content is set to 1.0 mass% or less.

Up to 0.050 mass percent P

P is an element that works for the hot workability harmful is. The effect of P becomes less by adjusting the P content suppressed as 0.050 mass percent.

Up to 0.020 mass percent S

S is an element which segregates at the grain boundaries and the hot workability deteriorated. These effects are achieved by adjusting the S content suppressed to less than 0.020 mass%.

Up to 2.0 mass percent Ni

Ni is the same austenite former as Mn and to control the value FM useful. However, the excessive addition increases from Ni over 2.0 percent by mass the steel costs and also cures the steel sheet.

8.0 to 22.0 mass percent Cr

Cr is an essential element for the corrosion resistance. At least 8 mass% Cr is for corrosion resistance necessary as stainless steel. However, the excessive addition deteriorates from Cr over 22.0 percent by mass toughness and moldability of a sheet of stainless steel.

Up to 0.05 mass percent N

N becomes nitrides by bell annealing transformed. The nitrides act as nuclei for the random growth of ferrite grains during the Recrystallization in a final annealing step. However, excess N causes the reduction of toughness, because N is the strength of a annealed cold-rolled steel sheet increased. Therefore, the N content is held at 0.05 mass% or less.

Of the Ferritic stainless steel can also be one or more of additional elements to the above elements.

Up to 0.10 mass percent al

al is an element that acts as a deoxidizer during steelmaking is added. The excessive Al content over 0.10 Mass percent causes an increase in non-metallic inclusions a reduction of toughness and the occurrence of surface defects. Therefore, the Al content is suitably set so that the value FM is set to 0 or less.

Up to 1.0 percent by mass Not a word

Not a word is an element for improving corrosion resistance, but the excessive addition from Mo over 1.0 percent by mass accelerates solution hardening and retards the dynamic recrystallization in a high temperature zone, resulting in Reduction of hot workability leads.

Up to 1.0 percent by mass Cu

Cu is an element made of scrap steel during steelmaking is included. Since excess Cu for the hot processing and corrosion resistance unfavorable is, its upper limit is set to 1.0 percent by mass.

0.01 to 0.50 mass% Ti, 0.01 to 0.50 mass% Nb, 0.01 to 0.30 mass% V and 0.01 to 0.30 mass% Zr

Ti, Nb and V are solved with C in a steel matrix, implemented as carbides and responsible for formability are effective, precipitated. Zr starts dissolved O as an oxide and improves the moldability and toughness of a Sheets made of stainless steel. The effects of these elements are observed at 0.01 wt% or more, respectively the excessive addition is for the productivity disadvantageous. In this sense, the upper limits of these elements on Ti: 0.50 mass%, Nb: 0.50 mass%, V: 0.30 mass% and Zr: 0.30 mass%.

0.0010 to 0.0100 mass percent B

B is an element which transforms the transformed phase into a hot rolled one Steel sheet evenly dispersed and the random one Growth of ferrite grains accelerated in a final structure without generating aggregate structures. An even distribution The transformed phase is typically characterized by the addition of B in a relationship of 0.0010 mass% or more. However, the excessive addition causes from B over 0.0100 mass% the deterioration of the hot workability and weldability.

A value FM of not more than 0

The stainless steel is designed so that a value FM defined by the formula (1) is set to 0 or less, in addition to the specified ratios of the alloying elements for improving the shape-preservability without producing a Austenite phase during the annealing process. FM = 420 C - 11.5 Si + 7 Mn + 23 Ni - 3.5 Cr - 12 Mo + 9 Cu - 49 Ti - 50 Nb - 23 V - 52 Al + 470 N + 20 (1)

The generation of an austenite phase in a high-temperature zone during the bell annealing is inhibited by setting the value of FM to 0 or less. On the other hand, an alloy construction at FM> 0 enables generation of an austenite phase which can dissolve C and N at relatively high ratios, in a ferrite matrix. Since the solubility of C and N between the austenite phase and the ferrite matrix is different, the anisotropic grades (r _max -r _min and σ _max -σ _min ) are increased due to the uneven solubility.

A plane anisotropic degree (r _max - r _min ) of the Lankford value (r) ≤ 0.80

An anisotropic degree (σ _max - σ _min of 0.2% proof stress ≤ 20N / mm ²

When the anisotropic grades (r _max -r _min and σ _max -σ _min ) are smaller, a ferritic stainless steel is press-formed into a product shape with better shape-preservability. The experimental results prove that the shape-retention-or form-Einfrierbarkeit at (r _max - r _min) - is excellent ≤ 20N / mm ² ≤ 0.80 and (σ _min σ _max).

0.2% proof stress ≤ 350 N / mm ²

A complete ferrite structure free of martensite having a 0.2% proof stress of 350 N / mm ² or less is preferred for imparting excellent mold retention to a ferritic stainless steel. A strength exceeding 350 N / mm ² naturally requires applying a large stress to plastic deformation of the stainless steel sheet, resulting in increase of spring-back and deterioration of shape-preservability.

Glow 1 to 24 hours at 700 up to 880 ° C

A sheet of ferritic stainless steel is annealed under the conditions that dissolved C and N are dissolved as fine carbonitride particles uniformly dispersed in a single ferrite matrix to obtain the anisotropic grades (r _max -r _min and σ _max -σ _min ) to reduce. The sufficient precipitation of the carbonitride particles is realized by bell annealing at a temperature of 700 ° C or higher. However, when the stainless steel sheet is bell annealed at a temperature higher than 880 ° C, on the contrary, the stainless steel sheet is converted into an anisotropic structure due to the predominant growth of recrystallized ferrite grains (referred to as "secondary recrystallization").

The The present invention is more apparent from the following examples understandable.

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

From each annealed Steel sheet was sampled to give Lankford (r) and 0.2% proof strength as follows to eat:

Lankford value (r)

After the tensile strain of 15% was applied to a JIS 13B specimen, the Lankford value (r) was measured along each of the L, D and T directions. The difference between the measured maximum and minimum values was calculated and considered to be the plane anisotropic degree (r _max - r _min ) of the Lankford value (r).

0.2% proof stress

After the tensile strain was applied to a JIS 13B specimen at a rate of 3.3 × 10 ^-4 , the 0.2% proof stress was measured along each of the L, D, and T directions. The difference between the measured maximum and minimum values was calculated and considered as the anisotropic degree (σ _max - σ _min ) of the 0.2% proof stress.

Shape retention or Form Einfrierbarkeit

Two specimens, each in the form of an open box (shown in FIG 1 ) comprising a 40 mm ^{2 area} E1, E2 with four elongated areas A1-D1, A2-D2 of 10 mm × 36 mm in size, were made from each annealed steel sheet. One specimen was cut along the direction L (one rolling direction), and the other was cut along the direction D. All sides of the square surfaces E1, E2 were bent at a working speed of 200 mm / minute under a hold-down pressure of 20 tons, and the elongated surfaces A1-D1, A2-D2 were erected by a 200-ton press equipped with a right angle Punch with an outer diameter of 4 mm. A springback angle θ was measured at each measurement point P1-P4 corresponding to the four corners of a bottom of a molded box. The shape-preservability was evaluated by the maximum angle θ _max among the measured values.

Table 2 shows the results of each annealed steel sheet, and 2 shows the distribution of the maximum springback angles θ _max with respect to the anisotropic degrees (r _max -r _min and σ _max -σ _min ).

It goes out 2 show that the steel sheets according to the invention with r _max - r _min ≤ 0.8 and σ _max - σ _min ≤ 20 N / mm ^{2 show} good form-preservability (ie maximum spring back angle θ _max ≤ 3 degrees). On the other hand, comparable steel sheets which did not satisfy r _max -r _min ≦ 0.8 or σ _max -σ _min ≦ 20 N / mm ² showed poor shape retention as by the maximum springback angles θ _max > 3 degrees will be shown.

Table 2: Production conditions and properties of stainless steel sheets

efg. Bsp.

= erfindungsgemäße Beispiele

Vgl.-Bsp.

= Vergleichspeispiele

The underlined numbers are outside the ranges defined by the present invention.

efg. Ex.

= Examples according to the invention

Comp.

= Comparison examples

Industrial applicability

According to the invention, as mentioned above, a ferritic stainless steel sheet in shape-preservability is improved by conditioning the recrystallized ferrite grains into a plane-balanced structure to obtain a plane anisotropic degree (r _max -r _min ) of the Lankford value (r) and an anisotropic degree (σ _max - σ _min ) of the 0.2% proof stress to the lowest possible values. Since the stainless steel sheet is plastically molded into a low spring-back product shape, it is useful in many industrial fields, for example, for parts of electrical or electronic devices such as a sealing member of an organic EL device, molded parts, and building elements.

Claims (3)

Ferritic stainless steel sheet having an alloy composition consisting of up to 0.10 mass% C, up to 1.0 mass% Si, up to 1.0 mass% Mn, up to 0.050 mass% P, up to 0.020 mass% S, to to 2.0 mass% Ni, 8.0 to 22.0 mass pro Cr, up to 0.05 mass percent N, optionally containing one or more of up to 0.10 mass percent Al, up to 1.0 mass percent Mo, up to 1.0 mass percent Cu, 0.01 to 0.50 mass percent Ti , 0.01 to 0.50 mass% Nb, 0.01 to 0.30 mass% V, 0.01 to 0.30 mass% Zr and 0.0010 to 0.0100 mass% B, and the balance being Fe, with Assuming that a value FM defined by the formula (1) is set to 0 or less, and the mechanical properties has a plane anisotropic degree (r _max -r _min ) of the Lankford value (r) and an anisotropic degree ( σ _max - σ _min ) of the 0.2% proof stress can be controlled or regulated to not more than 0.80 or 20 N / mm ² : FM = 420 C - 11.5 Si + 7 Mn + 23 Ni - 3.5 Cr - 12 Mo + 9 Cu - 49 Ti - 50 Nb - 23 V - 52 Al + 470 N + 20 (1). A ferritic stainless steel sheet as defined in claim 1, wherein the 0.2% proof stress is not more than 350 N / mm ² along each of a rolling direction, directions crossing the rolling direction with an angle of 45 degrees, and a direction which crosses the rolling direction at a right angle is. Process for producing a sheet of ferritic stainless steel with good shape retention and mold freezability while plastic forming, comprising the steps of hot rolling a ferritic stainless steel as defined in claim 1 Alloy composition and then the annealing of the hot-rolled steel sheet for 1 to 24 hours at 700 to 800 ° C.