KR20100077244A - Ferritic stainless steel having excellent surface quality and manufacturing method thereof - Google Patents

Abstract

PURPOSE: A ferritic stainless steel with a superior surface quality and a manufacturing method thereof are provided to improve the surface quality by preventing the deformity of roping and ridging property. CONSTITUTION: A ferritic stainless steel with a superior surface quality is as follows. The nucleus becomes to the liquid in the inside of hot metal complex oxide at a high temperature. The deposition phase is deposited from the complex oxide to the spinal and CaTiO3 of the Mg-Al-O system. Non-uniformity nucleus generates from the deposition phase before the coagulation of TiN. TiN grows to the constant size. The average composition of the oxide for the equiaxed crystal production of slab satisfies (TiO2) +(CaO) +(Al2O3) +(MgO) >=95. The composition of the deposition phase satisfies (CaTiO3) /(MgAl2TiO6) >=1.7 among oxide.

Description

Ferritic stainless steel having excellent surface quality and manufacturing method

The present invention relates to a ferritic stainless steel having excellent surface quality and a method for manufacturing the same, and more particularly, to control the composition of oxide inclusions in molten steel to improve equiaxed crystallization, thereby preventing defects in ridging and roping. The present invention relates to a ferritic stainless steel having excellent quality and a manufacturing method thereof.

In general, ferritic stainless steels have a higher quality and have material properties that require higher elongation as the height of ridding decreases. This is because defects easily occur when forming the exhaust pipe if the ridging property is not good.

Accordingly, in order to secure a material having excellent ridging property, the equiaxed crystallinity must be high among the casting structures in the cast steel produced by continuous casting. In other words, the higher the equiaxed constant in the slab, the lower the ridging height, and as a result, the degree of defects due to wrinkles during machining decreases.

In the production of ferritic stainless steel, the cast steel that does not have an equiaxed crystallinity of 40% or more has a coarse band structure remaining in the hot rolled coil, thereby reducing the deep drawing of the cold rolled steel sheet. In addition, when forming, the surface of the coil is liable to generate irregularities that are parallel and thin in the cold rolling direction, called ridging or roping. Therefore, conventionally, cold rolling re-rolling and hot rolling reduction have been carried out in order to prevent leasing defects, but there is a problem in that manufacturing cost increases and productivity decreases when ferritic stainless steel is manufactured.

The equiaxed crystallization in cast steel can be promoted by various techniques such as low temperature casting method, iron or steel addition method, rare earth element (REM) method, electro magnetic stirring and Al / Ti ratio control. Depending on the technology applied, equiaxed crystals can be produced within the slab in the range of 0% to 100%.

As a result, there are many patent documents for improving the in-piece equiaxed crystallization, for example, the recent patents are as follows.

First, Japanese Patent Application Laid-Open No. 2002-030324 (published on Jan. 1, 2002, Tsumido Metal Industries, Ltd.) adjusts slag basicity (CaO / SiO ₂ ) to 1.2 to 2.4 to improve the ridging property of ferritic stainless steel. And 2) a technique of continuously casting by controlling the ratio of Al / Ti to 0.01 to 0.10. However, when the ratio of Al / Ti is controlled to 0.01 to 0.10, the equiaxed constants in the slab are roughly about 100% and about 30%. In other words, it can be seen that it is difficult to secure a high equiaxed crystal ratio of 80% or more by controlling the ratio of Al / Ti in a real industry.

In addition, Japanese Patent Application Laid-Open No. 2000-160229 (published on June 3, 2000, Ilshin Steel Co., Ltd.) discloses 1) a ratio of CaO / AlO in slag components during VOD vacuum refining to manufacture a dropping ferritic stainless steel. A technique is described in which Al ₂ O ₃ is added so as to be in the range of 0.7 to 2.5, and 2) Ti is added so as to maintain an area ratio of TiN single inclusions at 0.01% or more.

However, when the area ratio of TiN inclusions is 0.01% or more, there is a limit in securing substantially high equiaxed crystallinity. Because TiN has poor contact with molten steel, TiN clusters grow well like grape clusters, and they also easily aggregate and grow with oxides. That is, when nitrides and oxides of a certain size, for example, 15 μm or more are distributed in the cast steel, even if the TiN inclusions satisfy an appropriate condition, they adversely affect nucleation of equiaxed crystals in the cast steel during solidification.

According to Japanese Patent Application Laid-Open No. 1999-323502 (published on November 26, 1999, published by Sumidomo Metal Industry Co., Ltd.), Al-Ti-based composite inclusions having a size of 0.3 to 0.5 µm act as equiaxed crystal nuclei. In addition, Japanese Patent Application Laid-Open No. 2001-049322 (2001.2.10, published by Ilshin Steel Co., Ltd.) reports that equiaxed crystals can be improved by setting the basicity of 0.5 to 3.0 and the superheat of 20 to 70 ° C. . In addition, Japanese Patent Application Laid-Open No. 2002-030395 (published Jan. 1, 2002, Tsumido Metal Industry Co., Ltd.) includes MgO having a diameter of 0.3 to 5 μm when Al is contained in 0.002 to 0.02% and Mg is less than 0.0005%. , It is disclosed that the equiaxed crystal can be secured by generating 30 MgO-Al ₂ O ₃ / m ₃ .

However, in the above patents, there is a problem that a large number of deviations may occur depending on working conditions or conditions because the equiaxed crystal generating mechanism is not clearly identified. In addition, when Ti is added to the ferritic stainless steel and TiN precipitated in molten steel is used as the ferrite coagulation nucleus, the coagulation structure is known to be easy to equiax. However, there are cases where the coagulation structure becomes columnar and equiaxed crystals even under constant conditions of Ti and N content. In addition, in order to improve the equiaxed crystallinity, Ti addition is required, but excessive Ti addition causes defects such as nozzle clogging and surface defects. In view of this, equiaxed crystallization of the coagulation structure by Ti addition is not a fundamental countermeasure to improve equiaxed crystallinity.

Accordingly, an object of the present invention is to control the composition of the oxide inclusions in the molten steel to improve the equiaxed crystallization even under low Ti content conditions, so that the ferrite-based stainless steel with excellent surface quality that can prevent the defects of the ridging and roping property and its manufacturing method To provide.

Ferritic stainless steel having excellent surface quality according to an aspect of the present invention is a mass%, C: 0.02 or less, N: 0.015 or less, Si: 0.3 to 0.7, Ti: 0.1 to 0.3, S: 0.01 or less, Cr: 11.0 95 to 14.0, the balance consisting of Fe and unavoidable impurities, the following formula (1) representing the average composition of the oxide for producing equiaxed crystals of the slab is 95 or more, and the following formula (2) representing the composition of the precipitated phase in the oxide is 1.7 It satisfies the above.

(TiO ₂ ) + (CaO) + (Al ₂ O ₃ ) + (MgO)... … … (One)

(CaTiO ₃ ) / (MgAl ₂ TiO ₆ ). … … (2)

In addition, the nucleus that produces the equiaxed crystal of the cast steel is composed of an oxide and TiN surrounding the oxide.

According to another aspect of the present invention, the method for producing ferritic stainless steel having excellent surface quality is% by mass, C: 0.02 or less, N: 0.015 or less, Si: 0.3 to 0.7, Ti: 0.1 to 0.3, S: 0.01 or less, Cr A method for producing an equiaxed crystal during continuous casting of a ferritic stainless steel containing 11.0 to 14.0, the balance of Fe and inevitable impurities, the method comprising: producing a nucleus in a molten steel at a high temperature in a liquid phase; Precipitating the precipitated phase from the composite oxide to Mg-Al-O-based spinel and CaTiO ₃ ; Generating TiN into a heterogeneous nucleus prior to solidification from the precipitated phase; Growing the TiN to a predetermined size; And forming an equiaxed crystal from the TiN, wherein the average composition of the oxide for producing an equiaxed crystal of the slab is (TiO ₂ ) + (CaO) + (Al ₂ O ₃ ) + (MgO) ≧ 95, among oxides The oxide is controlled in the molten steel so that the composition of the precipitated phase satisfies (CaTiO ₃ ) / (MgAl ₂ TiO ₆ ) ≧ 1.7.

In addition, in the growth of the TiN, the size of the TiN is formed to 3 ~ 10㎛.

As described above, according to the present invention By controlling the composition of the oxide-based inclusions in the molten steel to improve the equiaxed crystal even in a low Ti content condition, it is possible to improve the surface quality by preventing defects of the ridging and roping.

In addition, it is possible to omit the hot rolling annealing process and to use the continuous pickling rolling equipment (TCM) -continuous annealing equipment (CAL) in the cold rolling process, thereby improving productivity.

In addition, it is possible to manufacture a ferritic stainless steel in which the elongation property and workability are improved to prevent ridging defects even after molding.

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

In mass% according to the present invention, C: 0.02 or less, N: 0.015 or less, Si: 0.3 to 0.7, Ti: 0.1 to 0.3, S: 0.01 or less, Cr: 11.0 to 14.0, and the balance is Fe and unavoidable impurities. In order to improve the equiaxed crystallization rate of ferritic stainless steels, the mechanism for generating equiaxed crystals in the steelmaking process and continuous casting process was clarified. This is because the equiaxed crystals in the slab generated in the continuous casting process affect the product as well as in the post rolling process. Accordingly, in the present invention, the average composition of the oxide is (TiO ₂ ) + (CaO) + (Al ₂ O ₃ ) + (MgO) ≥ 95, the composition of the precipitated phase in the oxide (CaTiO ₃ ) The oxide is controlled in molten steel to satisfy / (MgAl ₂ TiO ₆ ) ≧ 1.7. By identifying the equiaxed crystal formation mechanism of the ferritic stainless steel, the ferritic stainless steel can be manufactured without leaching or roping defects.

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

C is 0.02% maximum to minimize the reduction in toughness. However, the smaller the content of C improves the properties of the material, so the minimum is not determined.

When N is excessive, the lower the workability and the corrosion resistance, so the lower the better, the upper limit is limited to 0.015% or less.

Si is added for deoxidation during steelmaking operation. If it is too low, the deoxidation may not be efficient, and the steelmaking defect may increase. If the Si content is too high, pickling may be inferior during the manufacturing process due to the Si component. Limit to%

Ti is an element that forms Ti nitride and acts as a nucleus of equiaxed crystals to increase the equiaxed crystallization of cast steel. For its function, Ti is added at least 0.1%. It is preferable to add 0.3% or less.

Since S forms inclusions such as MnS and inhibits corrosion resistance and hot workability, S is limited to 0.01% or less because it is better to manage as low as possible.

If Cr is low in content, the corrosion resistance is lowered. If the content is too high, the corrosion resistance is improved. However, the content is limited to 11.0 to 14.0% because of high strength and low elongation, which lower workability.

The present inventors have identified mechanisms for generating equiaxed crystals and studied methods for improving isometric crystals based thereon. In general, the production of stainless steel slab is produced in the electric furnace, and then subjected to deoxidation and decarburization in Argon Oxygen Decarburization (AOD) and Vacuum Oxygen Decarburization (VOD), and then transferred to Tundish using a ladle. This molten steel is transferred from the tundish through the mold to the player, the molten steel begins to solidify in the player and the slab is manufactured.

In mass% according to the present invention, C: 0.02 or less, N: 0.015 or less, Si: 0.3 to 0.7, Ti: 0.1 to 0.3, S: 0.01 or less, Cr: 11.0 to 14.0, and the balance is Fe and unavoidable impurities. In the ferritic stainless steel, the average composition of Ti-Ca-Al-Mg-O-based oxides dispersed in steel is represented by the following formula (1), and the precipitated phase of the oxide satisfies the composition range of the following formula (2): Should be. In this case, generation and growth of coarse columnar tablets of the cast slab are suppressed, and an equiaxed crystal can be formed at 80% or more.

(TiO ₂ ) + (CaO) + (Al ₂ O ₃ ) + (MgO)> 95. … … (One)

(CaTiO ₃ ) / (MgAl ₂ TiO ₆ ) ≥ 1.7. … … (2)

1 is a photograph comparing the equiaxed crystals of a ferritic stainless steel cast.

Looking at the equiaxed crystals compared with reference to Figure 1, in the case of the comparative material it can be seen that 20 to 30% of the equiaxed crystal is formed in the center, and in the case of the invention material it can be seen that evenly formed equiaxed crystals. In this way, if the equiaxed crystallinity is improved by controlling the composition of the oxide inclusions in the molten steel, the hot-rolling annealing step can be omitted. It also enables the operation of Tandem cold rolled mill (TCM) -Continuous Annealing Line (CAL) processes and improves productivity. In addition, it is possible to produce a ferritic stainless steel that improves the stretchability and workability and does not generate a ridging or roping defect even after molding.

2 is a graph comparing the size and number of TiNs for each equiaxed crystal in the slab.

As shown in FIG. 2, the size and number of TiNs in the case of 30% equiaxed crystals and 100% equiaxed crystals in the slab were analyzed. It can be seen that when the equiaxed crystal is 100%, the number of TiNs having a size in the range of 3 to 10 μm is greater than the number of TiNs observed at 30% of the equiaxed crystals. That is, in order for TiN to function as a nucleus of equiaxed crystals, it must be more than a certain size, and based on the results of this analysis, it is confirmed that TiN in the range of 3-10 μm is effective for nucleation of equiaxed crystals.

FIG. 3 is a magnified photograph of TiN in a cast steel, and FIG. 4 is an oxide analysis result that is bright in TiN, and FIG. 5 is an oxide analysis result that is dark in TiN.

3 to 5, when the TiN in the slab is enlarged by an electron microscope, black spheres are observed in TiN as shown in FIG. 3. If you look closely at this black sphere through an electron microscope, you can see that it is divided into two kinds of contrast. The black spheres in TiN consist of oxides, with bright and black oxides observed. This contrast difference oxide was identified using an Electron Probe Micro Analyser (EPMA). As a result, the brighter oxide was composed of CaTiO ₃ (FIG. 4), and the blacker oxide was composed of Mg-Al-O-based spinel such as MgAl ₂ TiO ₆ (FIG. 5).

As described above, it can be seen that an equiaxed crystal generating nucleus is generated by an inclusion of an oxide composed of (Ti-Al-Ca-Mg) O and a nucleus composed of TiN.

Hereinafter, a process of forming an equiaxed crystal in a ferritic stainless steel cast will be described.

6 is a schematic diagram illustrating the formation process from oxide to equiaxed solidification.

A method of manufacturing ferritic stainless steel according to the present invention will be described with reference to FIG. 6. In order to suppress the development of coarse columnar crystals, the oxide composition produced as a result of deoxidation is greatly influenced, and the average composition of oxides dispersed in steel satisfies the composition ranges of the following formulas (1) and (2). In this case equiaxed crystals form and grow.

(TiO ₂ ) + (CaO) + (Al ₂ O ₃ ) + (MgO)> 95. … … (One)

(CaTiO ₃ ) / (MgAl ₂ TiO ₆ ) ≥ 1.7. … … (2)

Slag refining lowers the molten oxygen potential, and Mg elutes from the slag and the refractory. Afterwards, the Al-Ti-Ca-Mg composite oxide having low interfacial energy spontaneously generates nuclei in the liquid phase at a temperature range of 1550 to 1650 ° C (step 1). Thereafter, the molten steel is precipitated as precipitated phases of CaTiO ₃ and Mg-Al-O-based spinels from the composite oxide inclusion in the course of cooling to 1500 ° C (step 2). In this precipitation, TiN is formed into a heterogeneous nucleus before solidification (step 3). After TiN is grown to a size of 3 to 10 µm (step 4), equiaxed crystals are formed and grown (step 5). That is, the equiaxed crystal formation nucleus of the slab is composed of an oxide and TiN surrounding the oxide, and the oxide is formed of a spinel of CaTiO _{3 type} and MgAl ₂ TiO ₆ type.

The formation of TiN as an oxide in the third step is influenced by the degree of matching of the crystal lattice of TiN and the oxide. In other words, the larger the crystallinity of TiN and the oxide, the easier TiN is produced. The crystal structure of the precipitated CaTiO ₃ phase in the oxide has a BCC structure, and the crystal structure of TiN also has a BCC structure. As such, the crystal structure of the CaTiO ₃ phase and the crystal structure of TiN coincide with each other in the BCC structure, and thus the matching of the crystal lattice is excellent. Therefore, since CaTiO ₃ precipitated from the oxide may serve as a nucleus for TiN formation, it can be theoretically explained that the higher the CaTiO ₃ content, the more favorable the formation of TiN.

7 is a graph showing the correlation between (CaTiO ₃ ) / (MgAl ₂ TiO ₆ ) and equiaxed crystals in oxides.

Referring to FIG. 7, in the present invention, in order to derive a correlation between CaTiO ₃ and an equiaxed rate, CaTiO ₃ in the oxide is examined under an electron microscope. After quantitatively measuring the fraction occupied by the phase, the correlation between (CaTiO ₃ ) / (MgAl ₂ TiO ₆ ) and equiaxed crystallinity was determined.

When (CaTiO ₃ ) / (MgAl ₂ TiO ₆ ) is 1.7 or more, as shown in FIG. 7, the equiaxed crystals rapidly increase to form about 95% or more, so in the present invention, (CaTiO ₃ ) / (MgAl ₂ TiO) ₆ ) was set to 1.7 or more.

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 photograph comparing the equiaxed crystals of a ferritic stainless steel cast.

Figure 2 is a graph comparing the size and number of TiN for each equiaxed crystal in the slab.

Figure 3 is a close-up photograph of magnified TiN in the cast.

4 shows bright oxide analysis in TiN.

Figure 5 shows the oxide analysis of dark in TiN.

Figure 6 is a schematic diagram of the formation process from oxide to equiaxed solidification.

7 is a graph showing the correlation between (CaTiO ₃ ) / (MgAl ₂ TiO ₆ ) and equiaxed crystals in oxides.

Claims (4)

In mass%, C: 0.02 or less, N: 0.015 or less, Si: 0.3 to 0.7, Ti: 0.1 to 0.3, S: 0.01 or less, Cr: 11.0 to 14.0, and the balance consists of Fe and unavoidable impurities, The ferritic stainless steel having excellent surface quality, characterized in that the following formula (1) representing the average composition of the oxide for producing equiaxed crystals of the cast steel satisfies 1.7 or more, and the following formula (2) representing the composition of the precipitated phase in the oxide . (TiO ₂ ) + (CaO) + (Al ₂ O ₃ ) + (MgO)... … … (One) (CaTiO ₃ ) / (MgAl ₂ TiO ₆ ). … … (2) The method of claim 1, The core for producing an equiaxed crystal of the cast steel is ferritic stainless steel having excellent surface quality, characterized in that consisting of an oxide and TiN surrounding the oxide. Ferrite system containing C: 0.02 or less, N: 0.015 or less, Si: 0.3 to 0.7, Ti: 0.1 to 0.3, S: 0.01 or less, Cr: 11.0 to 14.0 and the balance of Fe by weight and unavoidable impurities In the continuous casting method of stainless steel, Generating a nucleus in the liquid phase at a high temperature of the composite oxide in the molten steel; Precipitating the precipitated phase from the composite oxide to Mg-Al-O-based spinel and CaTiO ₃ ; Generating TiN into a heterogeneous nucleus prior to solidification from the precipitated phase; Growing the TiN to a predetermined size; And Including; forming an equiaxed crystal from the TiN; The average composition of oxides for the formation of equiaxed crystals of cast steel was (TiO ₂ ) + (CaO) + (Al ₂ O ₃ ) + (MgO) ≥ 95, and the composition of the precipitated phase in oxide was (CaTiO ₃ ) / (MgAl ₂ TiO ₆ A method for producing ferritic stainless steel with excellent surface quality, characterized in that the oxide is controlled in molten steel to satisfy? 1.7. The method of claim 3, In the step of growing the TiN, the size of the TiN manufacturing method of the ferritic stainless steel having excellent surface quality, characterized in that 3 ~ 10㎛.

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