JPS64149B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing ferritic stainless steel for cold rolling, and more particularly to a method for manufacturing ferritic stainless steel by continuous casting. As is well known, when cold-rolled steel sheets made of ferritic stainless steel such as SUS430 are subjected to deformation such as tension or deep drawing, fine ridge-like wrinkles occur along the rolling direction of the cold-rolled steel sheets. A phenomenon called rigging may occur. Ferritic stainless steel is mainly used for decorative parts such as automobile sashes and trims, as well as household appliances, so it is required to have a beautiful appearance, but the above-mentioned ripping may occur. Otherwise, there is a problem that the appearance of the product is damaged and the product value is impaired.
By the way, rigging of ferritic stainless steel cold-rolled steel sheets is significantly more likely to occur in the case of continuously cast material manufactured by continuous casting method than in the case of steel ingot material manufactured by ingot casting method. The reason for this is said to be that in continuous cast materials, the solidified structure in the center of the slab becomes columnar crystals, and as a result, the structure in the center layer of the hot rolled steel sheet tends to become coarse.Therefore, in order to prevent ridging, It is said that it is effective to refine the structure of the center layer of hot rolled steel sheets. Therefore, as a conventional method for preventing rigging of cold-rolled ferritic stainless steel sheets, a method has been adopted in which the austenite potential of the steel is increased and thereby the hot-rolled structure is refined using α-γ transformation. As mentioned above, to increase the austenite potential of ferritic stainless steel, N,
It is sufficient to increase austenite-forming elements such as C, Mn, and Ni. However, if the amount of these austenite-forming elements is increased, the cold-rolled steel sheet will become hard and the yield elongation will increase, resulting in the problem that stretcher strain is likely to occur. This invention was made in view of the above circumstances, and it is an object of continuous casting to produce a ferritic stainless steel for cold rolling that can simultaneously reduce ridging and stretcher strain in cold rolled steel sheets. The purpose is to In other words, this invention was achieved through various experiments and
This was done as a result of studies, and by suppressing the austenite potential to a relatively low value, the occurrence of stretcher strain is prevented,
Furthermore, electromagnetic induction stirring is also used during continuous casting to achieve equiaxed crystallization of the solidified structure, thereby preventing the occurrence of rizzing. Specifically, the method for producing ferritic stainless steel of the present invention involves controlling the contents of the constituent elements C, Si, Mn, Ni, Cr, and N in the ferritic stainless steel as follows:
The austenite potential γpot. defined by equation (1) is adjusted to 65% or less, and when continuously casting the ferritic stainless steel, electromagnetic induction stirring is performed during the slab solidification process. That is. γ _ppt. (%) = 288 [%C] -54 [%Si] +7.5 [%Mn] + 22 [%Ni] -18.75 [%Cr] +350 [%N] + 338.5 ...(1) Further To explain the method of the present invention in detail, in the present invention, when continuously casting ferritic stainless steel, electromagnetic induction stirring is applied during the slab solidification process as described above. In other words, an induction coil is installed in the secondary cooling zone of the continuous casting machine, and a low-frequency current is supplied to this coil to generate an alternating magnetic field generated by the low-frequency current and the molten steel in the unsolidified crater of the slab. The molten steel inside the crater is stirred by the induction effect of the electric current. This prevents the growth of columnar crystals inside the slab and improves the equiaxed crystallinity in the solidified structure of the slab.As a result, the structure in the center of the hot rolled sheet becomes finer, which reduces ridging in the cold rolled sheet. This will reduce the occurrence of this. Although the effect of improving equiaxed crystallinity by electromagnetic induction stirring during the slab solidification stage as described above varies considerably depending on the conditions of electromagnetic induction stirring, the present inventors have found that by selecting appropriate conditions,
Equiaxed crystallinity of 40-75% has been obtained. As mentioned above, in continuous casting of ferritic stainless steel, ridging can be reduced by applying electromagnetic induction stirring. However, the present inventors have developed continuous cast ferritic stainless steel without electromagnetic induction stirring (hereinafter referred to as continuous cast comparative material) and continuous cast ferrite stainless steel with electromagnetic induction stirring (EMS) as described above. Ferritic stainless steel (hereinafter referred to as continuous casting EMS material)
When the austenite potential defined by Equation (1) above was varied and the occurrence of ridging was investigated, it was found that when the austenite potential was high, the situation of ripping occurred between the continuously cast comparative material and the continuously cast EMS material. There was no significant difference in
A significant difference in the occurrence of ridging occurs between the two materials only when the austenite potential is below a certain value.In other words, the effect of reducing ridging by electromagnetic induction stirring is only achieved when the austenite potential is below a certain value. They found that this is the case in the area of . In other words, FIG. 1 is a diagram showing the relationship between austenite potential and ridging score for the continuous cast EMS material, the continuous cast comparison material, and the ferrite stainless steel ingot material obtained by the ingot casting method. , Here, the ridging rating is to observe the degree of ridge-like wrinkles that occur on the surface of the test piece after applying 20% elongation to the test piece. It is graded in stages from 5 to 5. The materials tested here were ferritic stainless steels with various austenite potentials adjusted within the composition range of SUS430, and were continuously cast or manufactured with or without electromagnetic induction stirring as described above. After ingot casting, a cold rolled steel plate with a thickness of 0.7 mm was obtained by processing and treating in the same manner as in the examples described below.
Further, the conditions for electromagnetic induction stirring are also the same as in the examples described later. From FIG. 1, it is clear that in the continuously cast comparative materials, the degree of occurrence of rizzing is strongly dependent on the austenite potential, and the higher the austenite potential, the more the rizzing is improved. On the other hand, in the continuously cast EMS material, the occurrence of ridging is significantly reduced compared to the continuously cast comparison material in the region where the austenite potential is 65% or less, and it is almost the same as in the case of the steel ingot material. In the range higher than %, the ridging properties are almost the same as the continuously cast comparative material. Therefore, it is clear that in a region where the austenite potential exceeds 65%, the effect of reducing ridging by electromagnetic induction stirring is not exhibited, and that the effect of reducing ridging by electromagnetic induction stirring appears only in a region of 65% or less. In addition, as mentioned above, by suppressing the content of austenite-forming elements so that the austenite potential is 65% or less, hardening of the cold rolled steel sheet is prevented, and the elongation at yield point is reduced to prevent the occurrence of stretcher strain. can do. Therefore, in this invention, the austenite potential is set to 65
% or less and combined use of electromagnetic induction stirring during continuous casting, we can reduce the ridding of continuous cast materials of ferritic stainless steel, which was conventionally thought to be prone to ridding, and also improve stretchability. This made it possible to reduce the occurrence of strain. Examples and comparative examples of this invention are described below. Example As shown in sample numbers 1 to 3 in Table 1, the austenite potential is 50%, 55%, and 67%, respectively.
A slab of SUS430 steel whose composition was adjusted so that However, the stirring intensity (current (A) × √ frequency (Hz)) in electromagnetic induction stirring is
1500 (A√Hz). Each slab obtained was hot rolled after heating at 1200°C for 6 hours, annealed at 860°C for 8 hours, pickled, cold rolled,
0.7 through each process of annealing, pickling, and skin pass.
mm cold rolled steel sheets were produced.

[Table] Comparative Example SUS430 steel slabs of sample numbers 1 to 3 in Table 1, whose compositions were adjusted to have austenite potentials of 50%, 55%, and 67%, respectively, as in the above examples were subjected to electromagnetic induction stirring. Continuous casting was performed without Each obtained slab was treated in the same process as in the example to obtain a cold rolled steel plate with a thickness of 0.7 mm. The cold-rolled steel sheets produced in the Examples and Comparative Examples were subjected to 20% elongation in the same manner as described above, and the occurrence of ridging was observed, and the ridging scores were as shown in Table 2.

[Table] From Table 2, it is clear that in areas where the austenite potential is high, the effect of electromagnetic induction stirring on reducing ridging is small, whereas in areas where the austenite potential is low, the effect of electromagnetic induction stirring on reducing riddging is significant. be. Furthermore, when the occurrence of stretcher strain was investigated for the cold-rolled steel sheets of Examples, the results shown in Table 3 were obtained. However, the results in Table 3 indicate that the skin pass elongation rate in the skin pass process of the example
A tensile test was conducted on the material given 0.4%,
This is an observation of the occurrence of stretcher strain.

[Table] From Table 3, it is clear that the occurrence of stretcher strain is prevented in the region where the austenite potential is low. As described above, according to the method of the present invention, ferritic stainless steel that effectively prevents rigging without causing stretcher strain in cold-rolled steel sheets can be manufactured using a highly productive continuous casting method. Therefore, this method is practically useful as a method for producing ferritic stainless steel used in products that require an excellent appearance, such as various decorative items and kitchen equipment.

[Brief explanation of the drawing]

Figure 1 shows continuous casting of ferritic stainless steel.
FIG. 2 is a diagram showing the correlation between the austenite potential of EMS (electromagnetic induction stirring) material, comparative continuous casting material to which no electromagnetic induction stirring is applied, and steel ingot material, and the ridging score of these cold rolled steel materials.

Claims (1)

[Claims] 1 Ferritic stainless steel, γ _ppt. (%) = 288 [%C] - 54 [%Si] + 7.5 [%Mn] + 22 [%Ni] - 18.75 [%Cr] + 350 The composition is adjusted so that the austenite potential (γ _ppt. ) defined by [%N] + 338.5 is 65% or less, and the ferritic stainless steel whose composition has been adjusted is continuously cast. A method for producing ferritic stainless steel for cold rolling, characterized by performing electromagnetic induction stirring during the solidification process of a slab.