KR101082158B1 - Fabricating method for ferritic stainless steel - Google Patents

Abstract

The present invention relates to a method of manufacturing a ferritic stainless steel that can reduce the ridging height by appropriately adjusting the number of intervals affecting the holding time and recrystallization between passes in the rough rolling section during hot rolling. In weight percent according to the invention, C: 0.05 or less (greater than 0), Ti: 0.7 or less (greater than 0), Si: 1.0 or less (greater than 0), Mn: 1.0 or less (greater than 0), P: 0.04 or less (0 ): S: 0.03 or less (greater than 0), Cr: 16.0-18.0, Ni: 0.5 or less (greater than 0), Mo: 1.0 or less (greater than 0), N: 0.05 or less (greater than 0), Cu: 1.0 or less ( Above), Al: 0.15 or less (greater than 0), remaining Fe, and other unavoidable impurities. In the method of manufacturing ferritic stainless steel, the delay action index in the rough rolling section during hot rolling of the stainless steel slab (= 29.5- And 0.89 × ND-0.24 × DT) so that it is greater than 0 and less than or equal to 15. By this configuration, the ridging height of the material having a low equiaxed rate can be reduced to improve the surface quality problem caused by the ridging.

Ferritic Stainless Steel, Rough Rolled, Interval, Leasing, Delay Times, Delay Time

Description

Fabrication method for ferritic stainless steel

The present invention relates to a method for manufacturing ferritic stainless steel, and more particularly, to a method for manufacturing ferritic stainless steel that can reduce the ridging height by appropriately adjusting the number of intervals in the rough rolling section during hot rolling.

In general, ferritic stainless steel is mainly used in automobile exhaust system parts, building materials, kitchen vessels, home appliances, etc., and the molding is one of the important quality characteristics because the mold is manufactured by forming by deep drawing. It is also important to reduce the occurrence of ridging defects formed on the surface after molding. In general, ferritic stainless steel is excellent in corrosion resistance and widely used at low cost as compared to nickel-containing austenitic, but has a disadvantage in that weldability, workability and high temperature strength are poor.

Among these, the representative ferritic stainless steel containing 17 ~ 18Cr is inferior in leasing quality due to the difference in roughness after the final cold rolling. .

Ferritic stainless steel has wrinkled surface defects during molding, which is called ridging. The cause of leasing is primarily due to the development of columnar tablets in the coarse cast structure. That is, when columnar tablets having a certain orientation remain undestructed in the rolling or annealing process, they exhibit deformation behavior in the thickness and width directions different from the surrounding recrystallized structures during tensile processing and are expressed as ridging defects.

This ridging defect not only degrades the appearance of the product but also causes an additional polishing process after molding if the ridging defect occurs severely, which causes a rise in the manufacturing cost of the final product.

Therefore, in order to improve such a problem, the method of improving the isometric constant of the slab is the most fundamental solution, and it is proposed that the equiaxed constant should be at least 60% level in order to obtain a steel plate having excellent ridging property under ordinary rolling conditions. (Sawatani Research Report, Nippon Steel Tech.Report, 21 (1983), p.275).

In addition, as a method of controlling the ridging defect by adjusting the manufacturing method for each process, a method of controlling hot rolling temperature (JP1975-016616, JP2000-256748) and a method of adjusting strain and friction coefficient during hot rolling (2005-0127408) ) May be used. Then, a method (2005-0129406, JP2000-256748) for controlling the finishing rolling reduction rate has been proposed.

The method of improving the ridging property is a method of promoting recrystallization during rolling, that is, dynamic recrystallization, and the approach to static recrystallization generated between rollings is still insufficient, and it is necessary to derive a method to improve the aforementioned problem by changing the operating conditions. Do.

Accordingly, it is an object of the present invention to provide a method for manufacturing ferritic stainless steel that can reduce the ridging height by appropriately adjusting the number of intervals affecting the holding time and recrystallization between passes in the rough rolling section during hot rolling. .

In weight percent according to the invention, C: 0.05 or less (greater than 0), Ti: 0.7 or less (greater than 0), Si: 1.0 or less (greater than 0), Mn: 1.0 or less (greater than 0), P: 0.04 or less (0 ): S: 0.03 or less (greater than 0), Cr: 16.0-18.0, Ni: 0.5 or less (greater than 0), Mo: 1.0 or less (greater than 0), N: 0.05 or less (greater than 0), Cu: 1.0 or less ( In the method of manufacturing a ferritic stainless steel consisting of more than 0), Al: 0.15 or less (greater than 0), the remaining Fe and other unavoidable impurities, during the hot rolling of the stainless steel slab, delaying in the rough rolling section (Delay); It includes.

In addition, the delay action index during the delay is adjusted to be more than 0 to 15 or less.

In addition, the delay action index is according to the following equation (1).

Delay action index = 29.5-0.89 × ND-0.24 × DT. … … (One)

(Number of delay (ND) is the number of delays of the interval between the rough rolling section pass more than 15 seconds, DT (Delay time) is the average delay time)

The method may further include heating the slab to a range of 1150 to 1200 ° C. before the delaying.

In addition, the slab manufactured after the delaying step maintains a ridging height of 15 μm or less.

As described above, according to the present invention, by appropriately adjusting the number of intervals affecting the holding time and recrystallization between passes in the rough rolling section during hot rolling, the leasing height of the material having a low equiaxed rate is reduced, which is caused by leasing. Surface quality problems can be improved.

Hereinafter, a method of manufacturing a ferritic stainless steel according to the present invention will be described in detail with reference to the drawings showing an embodiment of the present invention.

In the present invention, when manufacturing the slab of ferritic stainless steel, the delay action index 29.5-0.89 × ND-0.24 × DT value in the rough rolling section is adjusted to more than 0 to 15 or less, the slab heating temperature is set to 1050 ~ 1200 ℃. As a result, the ridging height of the material having a low equiaxed rate can be reduced, thereby improving the surface quality problem caused by the ridging.

Hereinafter, the composition range of the present invention and the reason for limitation thereof will be described in more detail.

C is an austenite stabilizing element and is bonded to Cr during the annealing process to precipitate in grain boundaries or grains in the form of Cr carbide or Cr carbonitride. C is limited to 0.05% or less (greater than 0), and in the process of decomposing into carbonitride during annealing, it contributes to improving the ridging property by controlling the texture. However, excessive C is limited to 0.05% or less (greater than 0) because the material is hardened due to an increase in Cr carbonitride and workability is lowered.

Si is an element which improves oxidation resistance, but when it exceeds 1.0%, surface defects are likely to occur due to an increase in steelmaking Si inclusions. In addition, since hardness, yield strength, and tensile strength are increased and the elongation is lowered, the workability is limited to 1.0% or less (greater than 0).

Mn is limited to 1.0% or less (greater than 0) because the content of Mn elutes MnS to lower the pitting resistance.

P segregates at grain boundaries and is limited to 0.04% or less (greater than 0) because it is better to manage as low as possible an element that inhibits formability.

S is limited to 0.03% or less (greater than 0) because it forms an inclusion such as MnS and inhibits corrosion resistance and hot workability.

If Cr is low in content, the corrosion resistance is lowered. If the content is too high, the corrosion resistance is improved, but the content is limited to 16-18% because of high strength and low elongation, which lower workability.

Ni is an element which improves corrosion resistance but is an expensive element, and the amount of Ni is limited to 0.5% or less (greater than 0).

Mo is an element that improves the corrosion resistance, the problem that the manufacturing cost of the material increases, and the elongation is reduced by increasing the strength, the workability is poor. Therefore, the Mo content is limited to 1.0% or less (greater than 0) in consideration of corrosion resistance and workability.

N is an austenite stabilizing element that promotes austenite formation during hot rolling. Like C, in the process of decomposition into carbonitride during annealing, it contributes to the improvement of ridging by controlling the texture of the aggregates. However, when excessively added, the upper limit is limited to 0.05% (greater than 0) due to the decrease in processability due to excessive generation of carbonitrides.

Cu is an element that causes scrap contamination during the hot rolling process and is limited to 1.0% or less (over 0).

If Al is added in excess of 0.15%, the inclusions are excessive and the surface quality is degraded. Therefore, Al is limited to 0.15% or less (greater than 0).

Ti is an element added for grain refinement, and excessively brings about deterioration of toughness, so it is preferable to add 0.7% or less (over 0).

The following describes the manufacturing conditions of the present invention and the reason for the limitation thereof.

When the ferritic stainless steel slab according to the present invention is manufactured by hot rolling through a hot rolling process, the slab is heated to a temperature range of 1050 to 1200 ° C. When the heat treatment temperature is high, the AlN phase does not precipitate and exists in a solid solution state, so that the effect of delaying grain boundary growth rapidly decreases, so that the heating temperature is limited to 1200 ° C. or less. In addition, since the conventional slab heating is performed at 1050 ℃ or more, the heating temperature range is limited to 1050 ~ 1200 ℃.

And, by adjusting the delay action index to more than 0 to 15 or less to improve the ridging characteristics. The delay action index is based on the following equation.

Delay Action Index = 29.5-0.89 × ND-0.24 × DT

(Number of delay (ND) is the number of delays of the interval between the rough rolling section pass more than 15 seconds, DT (Delay time) is the average delay time)

Hereinafter, the manufacturing method of the ferritic stainless steel which concerns on this invention is demonstrated.

The present invention promotes static recrystallization in grains in columnar crystals by giving an interval between passes in the rough rolling section during hot rolling. At this time, the given interval can be largely divided into two types. One refers to the number of times more than 15 seconds are given per pass, and the other is the sum of the intervals occurring between all passes in the rough rolling section. Through this, the present invention is characterized by improving the ridging property by promoting the recrystallization in the texture of the {001} <110> orientation formed in the center of the thickness less than t / 4.

The present inventors have found that the leasing characteristic is improved as the sum of the intervals over a certain time and the sum of the total interval times increases in the rough rolling section, and there is a certain correlation between them.

Based on this, the interval given during hot rolling can be defined by Equation (1) below, which is a delay action index, through which the recrystallization rate in the texture changes, thereby changing the ridging property.

Delay action index = 29.5-0.89 × ND-0.24 × DT. … … (One)

Here, the number of delay (ND) is the number of delays in which the interval between the rough rolling section passes is 15 seconds or more, the number of times the interval between the rough rolling section passes is 15 seconds or more, and the delay time (DT) is the average delay time, Total interval time / number of passes in the section], and the delay action index of Equation (1) is applied to the rough rolling section 5 to 8 passes when the rough rolling thickness is 25 to 35 mm. This is the corresponding equation.

1 is a graph showing the ridging height according to the interval between passes in the rough rolling section.

Referring to FIG. 1, it can be seen that the ridging height decreases as the interval between passes in the rough rolling section becomes longer. In other words, if a delay exceeding a predetermined time between passes during rough rolling is applied, dynamic recrystallization is rapidly activated in the collective structure within the {001} <110> direction, thereby improving the ridging property.

2 is a schematic diagram of static recrystallization promotion with increasing average interval.

Referring to FIG. 2, it was confirmed that the ridging height decreases as the average interval time increases in the rough rolling section, and there is a correlation as shown in Equation (1) for obtaining a delay action index therebetween.

In order to secure the surface of the final product excellent in terms of surface quality control, it is difficult for the steel type to not generate any leaching, but the leaching height is required to be maintained at 15 µm or less. If the ridging height value is more than 0 to 15 or less, it means that the condition that satisfies the formula (1).

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

In Table 1 below, the leasing heights of the inventive steels and the comparative steels were evaluated and compared.

Example Crude rolling
Delay times
(Interval≥15 seconds) Rough rolling average delay time
(second) delay
Index of action Hot rolled sheet
thickness
(Mm) Cold rolled plate
thickness
(Mm) Leasing Height
(Μm) Inventive Steel 1 One 10.7 14.9 3.0 0.5 14.3 Inventive Steel 2 3 11.5 13.0 3.2 0.6 13.4 Inventive Steel 3 4 12.9 11.6 3.0 0.6 12.4 Inventive Steel 4 2 10.5 14.1 3.0 0.6 13.5 Inventive Steel 5 0 13.2 14.9 3.2 0.5 14.5 Comparative Steel 1 0 8.1 16.7 3.0 0.6 15.6 Comparative Steel 2 One 9.2 15.4 3.0 0.5 15.5 Comparative Steel 3 0 8.9 16.4 3.2 0.5 16.0 Comparative Steel 4 One 10.3 15.1 3.2 0.6 15.4 Comparative Steel 5 0 11.2 15.6 3.0 0.6 15.8

The steel grade used in the example was made of commercially produced ferritic stainless steel, and the slab of the continuously cast 220 mm thick was maintained at a temperature of 1180 ~ 1200 ° C, and the long time interval recovery and the average time interval between passes were performed in the rough rolling section. It was changed and hot-rolled. In the next step, the hot rolled annealing plate was subjected to cold rolling and cold rolling annealing processes, the specimens of quality characteristics were prepared, and then the ridging height was measured by measuring the surface roughness by a tensile test. As a result of applying and testing as described above, it was possible to create a condition that satisfies the ridging height of 15㎛ or less, which is the standard of the steel sheet having excellent surface quality in the steel grade.

Therefore, in the present invention, by appropriately adjusting the number of intervals affecting the holding time and recrystallization between passes in the rough rolling section during hot rolling, it is possible to improve the surface quality problem due to leasing.

Although the technical spirit of the present invention has been described in detail according to the above-described preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

1 is a graph showing the ridging height according to the interval between passes in the rough rolling section.

Figure 2 is a schematic diagram showing the static recrystallization promotion with increasing average interval.

Claims (5)

By weight%, C: 0.05 or less (greater than 0), Ti: 0.7 or less (greater than 0), Si: 1.0 or less (greater than 0), Mn: 1.0 or less (greater than 0), P: 0.04 or less (greater than 0), S : 0.03 or less (greater than 0), Cr: 16.0-18.0, Ni: 0.5 or less (greater than 0), Mo: 1.0 or less (greater than 0), N: 0.05 or less (greater than 0), Cu: 1.0 or less (greater than 0), In the manufacturing method of ferritic stainless steel of Al: 0.15 or less (greater than 0), remaining Fe and other unavoidable impurities, When hot rolling of the stainless steel slab, Delay in the rough rolling section (Delay); Including, The method of manufacturing a ferritic stainless steel, characterized in that the delay action index according to the following formula (1) is adjusted to be greater than 0 to 15 or less. Delay action index = 29.5-0.89 × ND-0.24 × DT. … … (One) (Number of delay (ND) is the number of delays in which the interval between the rough rolling section pass is 15 seconds or more, and DT (Delay time) is the average delay time (seconds)) delete delete The method of claim 1, Heating the slab in the range of 1150 ~ 1200 ℃ before the delaying step; The method of manufacturing a ferritic stainless steel further comprising. The method of claim 1, The slab manufactured after the delaying step is a manufacturing method of ferritic stainless steel, characterized in that the ridging height is maintained to 15㎛ or less.

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