JPS59133952A - Production of ferritic stainless steel plate - Google Patents

Abstract

PURPOSE:To make efficient and inexpensive mass production of a titled steel plate having high resistance to ridging by casting continuously a molten steel while adding a specific amt. of metallic powder having a specific grain size to the molten steel flow from a tundish, then subjecting successively the billet to hot rolling, annealing and cold rolling. CONSTITUTION:A molten steel is cast continuously while metallic powder having 5-3,000mu grain size is added at a rate of 0.1-5% by weight of the molten steel to the molten steel flow from a tundish thereby forming a continuously cast billet. The billet is subjected successively to hot rolling, annealing and cold rolling and is made into a steel plate. The stainless steel powder having the same component as the component of an intended ferritic stainless steel plate is preferably used for the metallic powder and the similar effect is obtainable as well by using carbon steel powder of a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a ferritic stainless steel sheet with excellent ridging resistance. In general, ferritic stainless steel sheets represented by 5US430 have good corrosion resistance and can be used for a long period of time. It maintains a beautiful metallic luster, has good workability, and is relatively inexpensive, making it suitable for kitchen utensils, furniture fixtures,
It has a wide range of uses such as household products such as home appliances and automobile parts, but since it is often used for applications that require decorative properties, recently it has been Strict requirements have come to be placed not only on the properties of the surface but also on the beauty of the surface after forming. However, when forming ferritic stainless steel sheets, there is no ridging or roving on the surface of the sheet. Wrinkle 2
It had the troublesome problem of causing ``.

This ridge-like undulation (ridging) that occurs on the steel plate surface is clearly distinguishable from stretcher strain and orange peel (rough skin) because the ridges always occur parallel to the rolling direction. When ridging occurs on a rimless molded product, it impairs its aesthetic appearance, so it must be completely removed by surface polishing, etc., which has caused problems for many users. In addition, if the degree of cracking is severe, this may cause cracking during forming, so when evaluating ferritic stainless steel sheet products for press forming, it is necessary to evaluate the drawability and stretchability. 'ζ, ridging resistance has also come to be recognized as an important factor.

Of course, there have been many reports on methods to reduce the above-mentioned ridging, but all of these methods lead to complication of the steel plate manufacturing process and increase in cost, and they do not completely solve the above-mentioned problem. However, since the occurrence of ridging is more noticeable in continuous cast materials than in steel ingots,
Nowadays, as continuous string casting has progressed, the above problem has come to be perceived as even more serious.

By the way, the characteristics of the ridging phenomenon are: ■ The height of the undulations increases as the working rate increases; ■ The direction is parallel to the rolling direction regardless of the tensile direction; ■ The profile of the undulations follows the surface of the steel sheet. and the back surface correspond to each other, etc. Therefore, [elongated unit regions arranged in parallel along the rolling direction in the steel sheet (regions having the same plastic deformability or plastic deformation anisotropy)] It is thought that this is the generation mechanism of "Kotori song" which shows different deformation behavior in response to plastic working. show behavior゛′
In the case of continuous casting slabs, the formation of "unit areas" is caused by the columnar crystal solidification structure formed in the continuous casting slab existing as a casting structure without undergoing phase transformation during the cooling process, and forming after hot rolling and cold rolling. If we consider that this is done in order to persist without being completely destroyed while forming a unique texture in terms of crystal orientation, this corresponds relatively well with the actual phenomenon.

Therefore, the following four methods can be considered as measures to prevent ridging. That is, ■ Refinement of the solidified structure (equiaxed crystallization, grain refinement) The structure of the slab, which is the material of the steel sheet, is made finer by electromagnetic stirring of the continuously cast slab, low-temperature casting, addition of refinement elements, etc. How to make it.

■ Micro-staining due to recrystallization after hot rolling and cold rolling (strain accumulation due to rolling and promotion of recrystallization due to annealing) Recrystallization during the hot rolling process However, by utilizing this fact, the strain accumulation during hot rolling is increased under the conditions of low temperature heating and low temperature edge rolling, and the residual strain causes recrystallization during the hot rolled sheet annealing process. However, other methods include adding Nb to high-elongation ferritic stainless steel to further retard recrystallization, repeating hot-coat processing and annealing, and continuously casting slabs. It is thought that hot rolling with heat or short interruption of hot rolling is also effective.

[Phase] Refinement by two-phase structure (component control to increase γ potential or high-temperature annealing after hot rolling,
Using the austenite phase that partially exists at high temperatures, as shown in the Fe-Cr phase diagram, the crystal grains are transformed by γ→M transformation during cooling. Possible methods for achieving fineness and randomization include a method of controlling components such as N addition (7) and a method of heating to a temperature in the two-phase region after hot rolling and then rapidly cooling.

■ Refinement by shear deformation during rolling A method of refining the material by applying shear deformation to the material by vertically asymmetric rolling using rolls of different diameters, rolling using striped rolls, etc.

However, even if these methods were adopted, there were many problems in implementation, and in the end, they could not be effective means for completely preventing ridging as described above.

Based on the above-mentioned points, the present inventors have determined that in order to significantly improve the ridging resistance of cold-rolled ferritic stainless steel sheets, refinement of the continuous casting structure (equiaxed crystallization), which is the root cause of ridging, is necessary. Recognizing that this is an essential condition,
In addition to improving the quality of conventional twice-cold-rolled ferritic stainless steel sheets, we will also find a method for manufacturing ferritic stainless steel sheets that can achieve quality equivalent to or better than twice-cold-rolled materials even in once-cold-rolled materials. As a result of continued research, we found that the application of electromagnetic stirring as a technique for refining the solidification structure of conventional continuously cast slabs, the lowering of the casting temperature, and the improvement of Ti, Z
It has been confirmed that the following problems exist in the addition of carbonitride-forming elements such as r + Nb and Aε.

(b) Application of electromagnetic stirring Because stirring is performed after the slab shell has solidified, there is a limit to the equiaxed crystallinity that can be obtained by electromagnetic stirring (at most it is only about 50 parts, and in normal operation it is about 20 to 30 parts). be).

(b) Decrease in casting temperature, which may cause problems such as nozzle clogging during operation, resulting in superheat degree △T (
There is a limit to reducing ΔT - temperature of molten steel poured - solidification temperature), and there is also the problem of surface quality deterioration.

(c) Addition of carbonitride-forming elements There is a risk that the steel composition may change, leading to an increase in costs.

Therefore, while searching for a more effective means to refine the structure of continuous casting, we conducted further research on a method for industrially and efficiently manufacturing ferritic stainless steel sheets with excellent ridging resistance, and as a result, (a) steel ingots were developed. Suspension casting method (known as a measure to improve steel quality such as reducing porosity)
(For example, see Japanese Patent Application Laid-Open No. 53-130234), if applied to the continuous casting of ferritic stainless steel under specific conditions, it is possible to refine the solidification structure of the continuously cast slab with high efficiency; (b) this If the microstructured slab obtained in this way is sequentially hot-rolled, annealed, and cold-rolled using conventional methods, it will be possible to obtain ridging resistance properties equivalent to or better than conventional twice-cold-rolled material even after only one cold-rolling. A ferritic stainless steel plate with
We have come to the knowledge shown in (a) and (b) above that if the steel sheet is cold-rolled twice, the quality of the sample sheet will be further improved.

This invention was made based on the above findings, and involves adding metal powder with a particle size of 5 to 3000 μm to a flow of molten steel of ferritic stainless steel from a tundish by 0.1 of the weight of the molten steel.
Continuous casting is carried out while adding ~5% to obtain a continuously cast piece, and then the piece is sequentially hot-rolled, annealed, and cold-rolled to produce ferrite with excellent ridging resistance. This method is characterized by obtaining a stainless steel sheet.

The details of the mechanism by which the continuous casting slab structure is refined by adding metal powder are not clear, but as the powder acts as a coolant, the degree of superheating △T decreases, and the solidification core It is presumed that the increase in the number of ions greatly contributes to the miniaturization.

As for the type of powder, stainless steel powder having the same composition as the target ferritic stainless steel sheet is preferable, but the same effect can be obtained by using carbon steel powder, which is less expensive.

When using carbon steel powder, it is important not to leave any undissolved powder in order to ensure the corrosion resistance and ductility of the steel plate, and it is also important to calculate the dilution of molten steel components by the carbon steel powder in advance. It is necessary to adjust the composition of molten steel.

Regarding the components of the carbon copper powder, the ash content of the steel powder was determined based on the following considerations in order to prevent unmelting.

That is, in order to lower the melting point of steel powder than the melting point of 5US430 molten steel (1496°C), 1lllj, Si20.2
Weight section, Mn: When assuming 0.6 weight section, formula, TLLC
Liquidus temperature) -1538-155[:C]+80[C]+13. OC Chi Si〕+4-8(Ha Shingetsu(C<0
．． From the formula of Hirai et al. for 5C1 ordinary steel, C nobleness is 0.
It was found that the content must be 41% by weight or more. As a result, high carbon copper powder with C: 0.77 or higher was used.

As a method of adding the powder, for example, using a hollow tool such as the one shown in FIG.
It is good to add it to. Reference numeral 5 in FIG. 1 indicates a mold.

Good results can be obtained if the particle size of the powder is within the range of 5 to 3000 μm, but if the particle size is less than 5 μm, 1. C) Powder particles adhere to each other, making it impossible to ensure a smooth addition state, and on the other hand, if the diameter exceeds 3000 μm, powder will inevitably remain in an undissolved state, which will impair the quality of the product steel sheet, especially the ductility. .

In addition, if the amount of powder added (d) is less than 0.1% relative to the weight of molten steel, it will not be possible to obtain a slab with good bacteria, while if it is added in excess of 5%, it will increase the cost of the steel plate. Therefore, 0.1 to 5% of the weight of molten steel is appropriate.

FIG. 2 shows that the first
FIG. 2 is a diagram showing the relationship between the amount of powder added, the equiaxed muscle modulus of the ingot, and the ridging resistance of the product when casting is performed under the conditions shown in the table. In addition, the non-shaving property is hot rolled 21
Heat) (Tf=760-800°C), annealing: Bell annealing,
Cold rolling: A steel plate under the conditions of one cold rolling, plate thickness, 0-8V, rrt was evaluated.

It can also be seen from FIG. 2 that irrespective of the type of powder, when approximately 3% or more of Table 1 is added, the equiaxed muscle modulus becomes 100, and the ridging resistance shows a high value.

Figure 3 is a macrostructure diagram showing an example of the coagulation structure of a powder-added material and an additive-free material, and the effect of powder addition is clear from Figure 3. In order to obtain good ridging resistance by rolling, the hot rolling conditions are particularly important, and care must be taken not to coarsen the fine-grained solidified structure during the hot rolling process. For this reason, the finishing temperature is 600~8
It is desirable to carry out the entire low-temperature hot rolling at 50°C.

Figure 4 shows the results of using a slab with an equal bacterial content of 100 grass and varying the end temperature by changing the hot rolling start temperature, and being able to play with the effect of the end temperature in the range of 600 to 1000°C. , is a diagram showing the relationship between the ridging resistance and the end hot rolling temperature of the once-cold-rolled material made from the powder-added slab; In order to obtain the above ridging resistance, the hot rolling end temperature Tf must be set to 850
It can be seen that it is preferable to keep the temperature below °C.

Next, the present invention will be specifically explained using Examples and comparing with Comparative Examples.

EXAMPLE When continuously casting 5US430 steel using ordinary continuous casting equipment, by using a hollow stopper as shown in the schematic diagram in Figure 1, S to the molten steel flow from the tundish is controlled.
US 430 powder (average particle size: 45 μm) or carbon steel powder (
0.77% C1 average particle size: 100μ'm) was added,
The equiaxed reinforcement of the obtained continuously cast slab with a thickness of 150 mm was investigated.

Next, the slab' (i: after heating to 1200 ° C., it is hot rolled to a thickness of 4.5 mm, and the finishing temperature is adjusted by adjusting the temperature on a delay table between rough rolling and finishing rolling. Subsequently, the obtained hot rolled coil was cold rolled and annealed under the conditions shown in Fig. 5.Then,
The ridging resistance of the cold-rolled material thus obtained was evaluated. Evaluation of ridging resistance is A, -A', B
, B', C, C', with A being the best and C
' was considered poor.

The results obtained as described above are shown in Table 2 together with the results obtained by the conventional method.

As a result of various tests, in both cases of 5US430 powder and carbon steel powder, powder addition was performed only by one cold rolling process following high-temperature hot rolling, which was equivalent to the conventional two-time cold rolling process (B
It has become clear that steel sheets with ridging resistance properties of
0°C), it is clear that a ferritic stainless steel sheet having the ridging resistance shown in Table 2 can be obtained even with a single cold rolling process as shown in A'.

As mentioned above, according to the present invention, a ferritic stainless steel sheet with excellent ridging resistance that does not lose its beautiful appearance even through forming processing can be produced efficiently through a simpler process than conventional methods. It brings about industrially useful effects such as being able to be mass-produced at low cost and further expanding the field of application of ferritic stainless steel sheets.

[Brief explanation of the drawing]

Figure 1 schematically shows an example of the method of adding powder to the molten steel flow from the tundish. Figure 2 shows the influence of the amount of powder added on the uniform bacterial ratio of slabs. Figure 3 is a photographic diagram showing the macrostructure of the ingot corresponding to the amount of powder added, Figure 4 is a diagram showing the influence of hot rolling end temperature on the ridging resistance of steel plate, and Figure 5 is an example. cold rolling,
It is a process diagram for explaining the entire annealing process. In the drawings: 1...Hollow stopper, 2...Powder. 3... Tandish, 4... Molten steel flow, 5...
·mold. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and 1 other person

Claims (1)

[Claims] From the tundish to the molten steel flow, particle size: 5 to 3000 μm
Continuous casting is carried out while adding metal powder in a ratio of 0.1 to 5 parts of the weight of the molten steel to obtain a continuously cast slab, and then the slab is sequentially subjected to hot rolling, annealing, and cold pressing. A method for manufacturing ferritic stainless steel sheets with excellent ridging resistance.