KR101938588B1 - Manufacturing method of ferritic stainless steel having excellent ridging property - Google Patents

Abstract

Disclosed is a method for manufacturing ferritic stainless steel excellent in final product ridging property as a result of a change in the microstructure of the center of a cross section resulting from additional cold rolling preceding hot annealing heat treatment. The method for manufacturing ferritic stainless steel having an excellent ridging property according to an embodiment of the present invention includes a step of manufacturing a hot-rolled plate through a strip casting process so as to contain 0.005 to 0.1 wt% of C, 0.01 to 2.0 wt% of Si, 0.01 to 1.5 wt% of Mn, 0.05 wt% or less of P, 0.005 wt% or less of S, 10 to 30 wt% of Cr, 0.005 to 0.1 wt% of N, 0.005 to 0.2 wt% of Al, remaining Fe, and other unavoidable impurities and satisfy a γ_max of 20 wt% to less than 50 wt%; and a step of performing cold rolling before hot annealing heat treatment of the hot-rolled plate.

Description

TECHNICAL FIELD [0001] The present invention relates to a ferritic stainless steel having excellent ridging properties,

The present invention relates to a method for producing a ferritic stainless steel having excellent ridging properties, and more particularly, to a method for manufacturing a ferritic stainless steel excellent in ridging property, And a method for producing a ferritic stainless steel having improved surface quality.

Generally, stainless steel is classified according to chemical composition or metal structure. According to the metal structure, the stainless steel is classified into an austenitic system, a ferritic system, a martensitic system and an ideal system. Among these stainless steels, ferritic stainless steels are mainly used for various kitchen appliances, automobile exhaust system parts, building materials, household appliances and the like because they are excellent in corrosion resistance while adding a small amount of expensive alloying elements. Required steel grade.

However, ferritic stainless steels have a problem of ridging defects which are wrinkled surface defects parallel to the rolling direction during forming processing such as deep drawing. The ridging defect not only deteriorates the appearance of the product, but also causes a problem that when the ridging is severe, a polishing process is added after molding to increase the manufacturing time and increase the manufacturing cost. For this reason, in order to expand the use of ferritic stainless steels, it is necessary to improve ridging characteristics and ensure excellent surface quality.

The cause of ridging is rooted in the development of columnar structure in casting. That is, when the main phase having a constant orientation remains without being broken in the rolling or annealing process, it is expressed as a ridging defect due to the width and thickness direction deformation behavior different from the surrounding recrystallized structure at the time of tensile processing. Various attempts have been made to remove the tissue causing ridging to overcome such ridging defects. By improving the equilibrium constant, it is possible to improve the ridging property by reducing the fraction of the main phase, or by controlling the process parameters such as hot rolling temperature, hot rolling reduction ratio and annealing temperature control during the manufacturing process.

However, there is little attempt to improve the texture of a thin-walled hot-rolled sheet produced by the strip casting process by subjecting the hot-rolled sheet to a symmetrical rolling or asymmetric rolling and then annealing annealing successively.

Korean Patent Publication No. 10-2008-0061863 (published on Mar. 3, 2008) Korean Patent Publication No. 10-2014-0080348 (published on June 30, 2014)

The present invention provides a method for manufacturing a ferritic stainless steel excellent in ridging characteristics of a final product by changing the microstructure of a cross section of the ferritic stainless steel hot-rolled sheet by further performing cold rolling before hot-annealing the hot- I want to.

According to an embodiment of the present invention, there is provided a ferritic stainless steel producing method which is excellent in ridging properties, comprising: 0.005 to 0.1% of C, 0.01 to 2.0% of Si, 0.01 to 1.5% of Mn, S: 0.005% or less, Cr: 10 to 30%, N: 0.005 to 0.1%, Al: 0.005 to 0.2%, the balance of Fe and other γ _max contained inevitable impurities, and represented by the following formula (1) 20% By weight of a hot rolled sheet satisfying at least 50% by a strip casting process; And cold rolling the hot-rolled sheet before subjecting the hot-rolled sheet to a heat-annealing heat treatment.

(1) 420 × C + 470 × N + 10 × Mn + 180-11.5 × Cr-11.5 × Si-52.0 × Al

According to an embodiment of the present invention, the cold rolling step may be carried out under a rolling condition in which the rolling reduction factor exceeds 40% and the rolling shape factor (l / d) represented by the following formula (2) is 2.0 or more .

(2)

According to an embodiment of the present invention, the cold rolling step may be performed by asymmetric cold rolling.

Further, according to an embodiment of the present invention, the asymmetric cold rolling can be performed with a reduction ratio of 30% or more.

According to an embodiment of the present invention, the asymmetric cold rolling may be performed such that the rolling speed ratio (V _h / V ₁ ) of the upper and lower rolling rolls is 1.25 or more and the rolling form factor l / d ) Is 2.0 or more.

(2)

According to an embodiment of the present invention, the method may further include a step of performing a hot rolling annealing after the cold rolling step.

According to an embodiment of the present invention, the heat-annealing step may be performed within a temperature range of 550 to 950 캜 within 60 minutes.

The method of producing ferritic stainless steel according to the embodiment of the present invention can suppress the occurrence of ridging defects on the surface of the product by controlling the band structure aspect ratio of the thickness central portion structure of the cross section of the steel sheet through cold rolling before hot rolling annealing.

1 is a microstructure photograph of a cross-section of a hot-rolled material obtained by annealing a hot-rolled sheet produced by a strip casting process without cold rolling.
Fig. 2 is a microstructure photograph of a cross section of a hot rolled material obtained by annealing a hot rolled sheet produced by ordinary hot rolling without cold rolling.
Fig. 3 is a microstructure photograph of a cross-section of a hot-rolled material obtained by symmetrically cold-rolling a hot-rolled sheet produced by the strip casting process of Fig. 1 and then annealing.
FIG. 4 is a microstructure photograph of a hot rolled material cross section subjected to asymmetric cold rolling and annealing heat treatment of the hot rolled sheet produced by the strip casting process of FIG. 1;
Fig. 5 is a cross-sectional microstructure photograph of the hot-rolled annealed material of Fig. 3 after cold-rolling and cold-annealing annealing.
Fig. 6 is a cross-sectional microstructure photograph of the hot-rolled annealed material of Fig. 4 after cold-rolling and cold-annealing annealing.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

A ferritic stainless steel producing method having excellent ridging properties according to an embodiment of the present invention comprises 0.005 to 0.1% of C, 0.01 to 2.0% of Si, 0.01 to 1.5% of Mn, 0.05% or less of P , 0.005% or less of S, 10 to 30% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, and the balance of Fe and other unavoidable impurities, satisfying γ _max of 20% or more and less than 50% .

The role and content of each component included in the ferritic stainless steel manufacturing method according to the present invention will be described below. % ≪ / RTI > by weight refers to weight percent.

The content of C is 0.005% or more and 0.1% or less.

C is an element which greatly affects the strength of the steel. If the content is excessive, the strength of the steel is excessively increased to deteriorate the ductility, which is limited to 0.1% or less. However, if the content is low, the strength required for the steel can not be satisfied, so add 0.005% or more.

The content of Si is 0.01% or more and 2.0% or less.

Si is an element added for deoxidation of molten steel during steelmaking and stabilization of ferrite. In the present invention, 0.01% or more Si is added. However, if the content is excessive, the material will be hardened and the ductility of the steel will be lowered to 2.0% or less.

The content of Mn is 0.01% or more and 1.5% or less.

Mn is an element effective for improving the corrosion resistance. In the present invention, 0.01% or more is added, and more preferably, 0.2% or more is added. However, if the content is excessive, the occurrence of Mn-based fumes is increased so that the weldability is deteriorated and the ductility of the steel is deteriorated due to the formation of excessive MnS precipitates, which is limited to 1.5% or less, more preferably 1.0% do.

The content of P is 0 or more and 0.05% or less.

P is an impurity inevitably contained in the steel, and is an element that causes grain boundary corrosion during pickling or hinders hot workability. Therefore, it is preferable to control the content as low as possible. In the present invention, the upper limit of the P content is controlled to 0.05%.

The content of S is 0 or more and 0.005% or less.

S is an impurity inevitably contained in steel, and is an element that segregates in grain boundaries and is a main cause of inhibiting hot workability. Therefore, it is preferable to control the content to be as low as possible. In the present invention, the upper limit of the S content is controlled to 0.005%.

The Cr content is 10% or more and 30% or less.

Cr is an element effective for improving the corrosion resistance of steel. In the present invention, Cr is added in an amount of 10% or more. However, if the content is excessive, there is a problem that the manufacturing cost increases sharply, so that it is limited to 30% or less.

The content of N is 0.005% or more and 0.03% or less.

N is an element which forms nitride and is present intrinsically. Therefore, if N is excessively contained, impact toughness and moldability are deteriorated, and it is limited to 0.03% or less.

The content of Al is 0.005% or more and 0.2% or less.

Al is a strong deoxidizing agent and serves to lower the content of oxygen in the molten steel, so it is added in an amount of 0.005% or more in the present invention. However, if the content is excessive, the sleeve defect of the cold-rolled strip occurs due to the increase of nonmetallic inclusions, and the weldability is deteriorated. The content is limited to 0.2% or less, more preferably 0.15% or less.

γ _max is a well-known index of austenite stability that corresponds to the maximum amount of austenite at high temperature. ? _max is calculated by the following equation (1). In the present invention, the gamma _max value satisfies 20% or more and less than 50%.

(1) 420 × C + 470 × N + 10 × Mn + 180-11.5 × Cr-11.5 × Si-52.0 × Al

If? _max is less than 20%, sufficient deformation of the ferrite phase due to the austenite phase is not accumulated during the hot rolling, and recrystallization of the ferrite band is not promoted and improvement in ridging properties can not be obtained. On the other hand, it is possible to control the contents of austenite forming elements such as C, N, Mn and Ni to be high in order to increase? _Max , but since they cause hardening of steel and increase in cost,? _Max is required to be less than 50% .

In the case of ferritic stainless steels satisfying the above-mentioned component system and gamma _max range, strain energy accumulation for recrystallization before hot-annealing is sufficient, so that an aggregate structure favorable to ridging property and formability can be formed.

In order to improve the ridging property and the surface quality of the ferritic stainless steel, it is necessary to remove the coarse band structure having {001} < 110 > crystal orientation which promotes the formation of aggregate structure favorable for moldability and induces ridging. It is important to accelerate the recrystallization during annealing of the hot-rolled sheet for the formation of aggregate structure and band structure, and it is necessary to sufficiently accumulate strain energy before annealing.

Fig. 1 is a microstructure photograph of a hot rolled sheet produced by a strip casting process, and Fig. 2 is a cross-sectional view of a hot rolled sheet obtained by annealing a hot rolled sheet produced by ordinary hot rolling without cold rolling.

Referring to FIG. 1, a coarse texture is formed in the center layer of the cross section, and the aspect ratio of the center layer structure by hot rolling is high as shown in FIG. As shown in FIG. 1, the hot-rolled sheet produced by the strip casting process according to the present invention has a coarse texture in the center central layer, and it is necessary to accumulate strain energy for recrystallization before hot-annealing.

Conventionally, attempts have been made to lower the hot rolling finishing temperature in order to accumulate strain energy on the hot rolled steel sheet, but this is insufficient for accumulation of strain energy. Accordingly, in order to accelerate recrystallization according to accumulation of strain energy, cold rolling is performed before hot annealing to form aggregate structure favorable for moldability.

In general, the deformation state during rolling deformation of a plate can be represented by two factors, shear deformation and plane deformation. In the conventional symmetric rolling, the shear deformation acts on the surface layer of the plate material, and the shear strain is reduced due to the symmetry inherent in the center layer, so that the shear strain at the center layer of the plate is always zero. That is, plane deformation always acts on the central layer of the plate. In the present invention, by applying asymmetric rolling, shear deformation can be applied to the center of thickness of the plate material. There are many rolling parameters when applying asymmetric rolling. Optimization of these parameters can reduce the ridging height, which is important for the surface quality of the final cold rolled product, by changing the microstructure by activating recrystallization by operating the appropriate shear strain in all thickness layers .

A method for producing ferritic stainless steel excellent in ridging property according to an embodiment of the present invention is characterized by comprising 0.005 to 0.1% of C, 0.01 to 2.0% of Si, 0.01 to 1.5% of Mn, 0.05% or less of P 0.005% or less of S, 10 to 30% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, and the balance of Fe and other unavoidable impurities, wherein γ _max satisfies 20% or more and less than 50% Producing a hot rolled sheet by a strip casting process; And cold rolling the hot-rolled sheet before subjecting the hot-rolled sheet to a heat-annealing heat treatment.

The strip casting method is a technique for producing molten steel with a thin hot rolled steel sheet (hot coils) having a thickness of 2 to 6 mm. This enables the continuous casting, reheating and hot rolling steps to be omitted, thereby improving productivity.

The hot-rolled sheet produced by the strip casting process can be further subjected to cold rolling before hot-annealing to accumulate strain energy for promoting recrystallization.

The cold rolling may be carried out under rolling conditions in which the reduction rate is more than 40% and the rolling shape factor (l / d) is 2.0 or more.

Along with a reduction of more than 40%, the rolling form factor (l / d) is also required to be 2.0 or more to cause shear deformation to the center of the thickness. Below that, shear deformation may not be imparted to the center of the thickness. The rolling shape factor associated with the size and the reduction rate of the rolling roll is a measure imposing a shear strain upon rolling, and is defined by the following formula (2).

(2)

Where d is the average thickness of the sheet material (d = (h ₀ + h) / 2), r is the rolling roll radius, h ₀ is the initial thickness of the sheet material Thickness, h means the final thickness of the plate.

Fig. 3 is a microstructure photograph of a cross-section of a hot-rolled material obtained by symmetrically cold-rolling a hot-rolled sheet produced by a strip casting process and then annealing. Compared with FIG. 1, shear deformation is applied to the cross-sectional central layer structure through normal cold rolling according to the above conditions, and the hot-annealed heat treatment is performed by the accumulated strain energy to promote recrystallization and grain refinement.

Meanwhile, in the method of manufacturing ferritic stainless steel according to an embodiment of the present invention, the cold rolled sheet may be subjected to asymmetric cold rolling in a step of cold rolling the hot rolled sheet before the heat treatment.

As described above, in the present invention, asymmetric rolling can be applied to cause shear deformation at the center of the thickness of the plate material. Appropriate shear deformation acts on the center of the thickness to activate the recrystallization and change the microstructure, thereby lowering the ridging height which is important for the surface quality of the final cold rolled product.

Asymmetric cold rolling can be carried out under a rolling condition where the reduction ratio is 30% or more, the speed ratio (V _h / V ₁ ) of the upper and lower rolling rolls is 1.25 or more, and the rolling shape factor (l / d) is 2.0 or more.

In order to cause shear deformation from asymmetric cold rolling to the center of the thickness, the rolling roll speed ratio (V _h / V ₁ ) should be 1.25 or more. If it is less than 1.25, shear deformation may not be given to the center of the thickness. Here, V _h means the speed of the fast roll and V _l means the slow roll speed.

The rolled shape factor (l / d) is also required to be 2.0 or more to cause shear deformation to the thickness center. Below that, shear deformation may not be imparted to the center of the thickness.

Fig. 4 is a microstructure photograph of a cross-section of a hot-rolled material obtained by asymmetric cold-rolling and annealing of a hot-rolled sheet produced by a strip casting process. Compared with FIG. 1, shear deformation is applied to asymmetric cold-rolling through the asymmetric cold rolling according to the above conditions, and the hot-annealed heat treatment is performed by the accumulated deformation energy to promote recrystallization and grain refinement.

As a result of investigating the correlation between the rolling parameters during symmetric rolling or asymmetric rolling and the liquefaction property in the cold rolling before the hot rolling annealing heat treatment, the rolling reduction ratio, the rolling ratio of the upper and lower rolling rolls, and the rolling shape factor (l / d) Thereby improving ridging properties.

The hot rolled sheet subjected to cold rolling or asymmetric cold rolling may be subjected to hot rolling annealing. The hot-annealing heat treatment can be performed within a temperature range of 550 to 950 캜 within 60 minutes. The hot-annealing heat treatment is a step carried out to further improve the ductility of the hot-rolled hot-rolled steel sheet, thereby leading to the precipitation and recrystallization of the carbonitride. For this purpose, it is necessary to carry out the annealing at an annealing temperature of 550 ° C or higher. However, when the annealing temperature exceeds 950 ° C or the annealing time exceeds 60 minutes, the crystal grains become coarse, which may lower moldability and ridging characteristics. On the other hand, although the lower limit of the annealing time is not particularly defined, it is preferable to conduct the annealing for 30 seconds or more in order to obtain a sufficient effect.

For example, a ferritic stainless steel manufactured according to an embodiment of the present invention may have a ridging height of 12 탆 or less.

Fig. 5 is a photograph of a cross-section microstructure after the hot-rolled annealed material subjected to the symmetric cold rolling before the annealing heat treatment, and Fig. 6 is a photograph of the cross-sectional microstructure after the cold rolling and the cold- And a cross-sectional microstructure photograph.

5 and 6, by performing the symmetric or asymmetric cold rolling according to the present invention before the hot-annealing process, the coarse structure existing in the central layer at the end face can be changed into a fine structure with a low aspect ratio.

Conditions other than the case where the method for producing ferritic stainless steels excellent in ridging property according to the present invention are controlled as described above can be carried out in accordance with a usual ferritic stainless steel producing method. It goes without saying that the hot-rolled annealed material can also be produced as a cold-rolled steel sheet by performing a cold rolling and a cold-rolling annealing.

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

Example

A hot-rolled steel sheet having a thickness of 2.8 mm was prepared by strip casting the molten steel having the composition shown in Table 1 below. Thereafter, cold rolling was performed before the hot-annealing heat treatment.

C Si Mn P S Cr Ni Al N content
(weight%) 0.049 0.18 0.58 0.02 0.002 16.18 0.25 0.094 0.028

Cold rolling before hot rolling annealing usually involves cold rolling at a reduction ratio of 20 to 50% and asymmetric cold rolling at a reduction ratio of 20 to 40%. Thereafter, hot - rolled annealing and pickling were performed, followed by cold rolling. Specimens were produced through cold - annealing annealing and pickling. The specimens were subjected to a tensile test at 15 ° in a 0 ° direction with respect to the rolling direction, and the surface roughness was measured to evaluate the ridging height at the Wt value.

Table 2 below shows the ridging heights of Examples and Comparative Examples according to the rolling roll speed ratio, rolling reduction ratio and rolling shape factor.

division Cold rolling
Kinds Up / down rolling roll speed ratio
(V _h / V _l) Reduction rate
(%) Rolled shape factor
(l / d) Rising height
(탆) Comparative Example 1 Normal
Cold rolling 1.0 20 1.7 16.0 Comparative Example 2 30 2.2 15.0 Comparative Example 3 40 2.6 13.0 Example 1 50 3.0 11.9 Comparative Example 4 Asymmetry
Cold rolling 1.25 20 1.7 14.3 Example 2 30 2.2 11.8 Example 3 40 2.6 10.6 Comparative Example 5 1.5 20 1.7 13.1 Example 4 30 2.2 11.5 Example 5 40 2.6 9.0

In Comparative Examples 1, 2 and 3 in which the hot-rolled sheet was subjected to the symmetric cold rolling at a reduction ratio of 20 to 40% before the hot-annealing, the ridging height was as high as 13 탆 or more. In Example 1 in which the cold rolling was performed at a reduction ratio of 50% which is a reduction ratio of more than 40%, the rolling shape factor was also 3.0 to 2.0 or more and the ridging height was 11.9 占 퐉.

In the case of Comparative Examples 4 and 5 in which the asymmetric cold rolling was performed before the hot-rolled annealing of the hot-rolled sheets, the reduction ratio was less than 30% and the ridging height was as high as 13 탆 or more. On the other hand, when asymmetric cold rolling was performed at a reduction ratio of 30% or more before the hot-annealing process as in Examples 2 to 5, a ridging height of 12 탆 or less was obtained and it was difficult to observe with naked eyes, The ridging height of 12 mu m or less can be achieved.

Compared with symmetric rolling, it can be seen that the ridging height decreases by about 20% or more when cold rolling is performed by asymmetric rolling. This means that the rough texture can be sufficiently miniaturized by shear deformation during asymmetric rolling than symmetrical rolling, and it can be seen that the ridging characteristic can be improved.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will recognize that other embodiments may occur to those skilled in the art without departing from the scope and spirit of the following claims. It will be understood that various changes and modifications may be made.

Claims (7)

0.005 to 0.1% of Cr, 0.01 to 2.0% of Si, 0.01 to 1.5% of Mn, 0.05% or less of P, 0.005% or less of S, 10 to 30% (Al): 0.005 to 0.2%, the balance Fe and other unavoidable impurities, and satisfying a gamma _max of 20% or more and less than 50% expressed by the following formula (1) by a strip casting process ;
Asymmetric cold rolling at a reduction ratio of 30% or more before the hot-rolled annealing process; And
Annealing the asymmetric cold-rolled steel sheet in a temperature range of 550 to 950 DEG C within 60 minutes;
The asymmetric cold rolling is carried out under a rolling condition in which the speed ratio (V _h / V ₁ ) of the upper and lower rolling rolls is not less than 1.25 and the rolling shape factor (l / d) A method for producing a ferritic stainless steel.
(1) 420 × C + 470 × N + 10 × Mn + 180-11.5 × Cr-11.5 × Si-52.0 × Al
(2)

Here, C, N, Mn, Cr, Si, and Al represent the content (weight%) of each element,
V _h : Fast roll speed
V _l : slow roll speed
l: length in which the contact arc of the roll and the plate in the rolling roll bite is projected
d: average thickness of the sheet material d = (h ₀ + h) / 2
r: Rolled roll radius
h ₀ : Initial thickness of plate
h: Final thickness of plate delete delete delete delete delete delete

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