KR101082297B1 - A method for manufacturing ferritic stainless steel having improved equiaxed crystals in slab - Google Patents

Abstract

The present invention removes C and N by injecting oxygen and Ar gas in an AOD (Argon Oxygen De carburization) refining furnace in the production of ferritic stainless steel for exhaust system. The present invention relates to a method for improving casting structure by improving molten steel oxygen by controlling molten steel using a ladle treatment process after removing slag.

The present invention is a ferritic stainless steel manufactured by melting the molten metal in the electric furnace through the AOD refining process and the casting ladle treatment process (LT), by adding Ti in the casting ladle treatment process (LT) to the final component system Cr: 10.0 ~ 12wt%, C: 0.02wt% or less (greater than 0), N: 0.015wt% or less (greater than 0), Si: 0.3 ~ 0.7wt%, S: 0.01wt% or less (greater than 0), Ti : Provides a method for producing ferritic stainless steel with improved equiaxed crystallization rate of 0.1-0.3wt% and the balance of Fe and unavoidable impurities.

AOD, equiaxed crystal, ferrite, stainless steel

Description

A method for manufacturing ferritic stainless steel having improved equiaxed crystals in slab}             

BRIEF DESCRIPTION OF THE DRAWINGS The figure which compares the cast iron equiaxed crystal of this invention material and a comparative material.

Figure 2 is a photographic view of the immersion nozzle clogging phenomenon and the attachment.

3 is a diagram showing a relationship between surface quality according to Ti and N concentrations.

4 is a view showing a manufacturing process of a ferritic stainless steel (409L steel) for a general exhaust system.

5 is a view showing a change in the Ti real rate according to the Al input amount of the refinery.

6 is a view showing the change in Ti real rate according to the slag basicity of the refinery.

7 is a view showing a change in the Ti real rate according to the concentration of [Si] before the input of Ti.

The present invention relates to a method for manufacturing ferritic stainless steel for automobile exhaust systems, and more particularly, to a ferritic stainless steel for automobile exhaust systems and a method for manufacturing the same, which have improved the casting structure by improving the equiaxed crystal of cast steel.

In general, ferritic stainless steels for automobile exhaust systems have a chromium concentration of 10 to 12%, C and N concentrations of impurity elements of 0.020% or less, 0.015% or less, and Ti / C + N 6.0 or more. There should be no ridging in the product by securing the isotropic rating of cast iron over the ratio. Leasing refers to a phenomenon in which striped surface irregularities occur in the rolling direction after rolling. In particular, when used as an automobile exhaust system, processing cracks occur due to stress concentration at the stripe irregularities in the processing step. If the equiaxed crystallization rate of the cast steel is low, the material must be reheated to refine and homogenize the tissue, thereby increasing the manufacturing cost.

Cast constant equiaxed rate in this invention is a value defined as follows.

Cast isometric equivalence (%) = thickness direction equiaxed length / casting thickness × 100

The general manufacturing process of ferritic stainless steel, especially 409L stainless steel for exhaust systems, consists of (1) electric furnace → AOD → VOD → ladle refining → continuous casting or (2) molten iron → converter → VOD → ladle refining → continuous casting. . In the above manufacturing process, both are manufactured using a vacuum refining apparatus called VOD (Vacuum Oxygen Decarburization). This is because the C and N levels in the steel should be lower than 150ppm, respectively, and vacuum refining is much easier to lower C and N in steels containing Cr. However, VOD is advantageous for the production of high purity, high clean steel with low C, N reduction and low oxygen concentration, but has high manufacturing cost. In addition, VOD is disadvantageous to TiN generation in molten steel which is related to the formation of equiaxed crystals, and low temperature casting and the use of an electric stirrer (EMS) are essential to increase the equiaxed crystallization.

On the other hand, it may be manufactured only through the Argon Oxygen Decarburization (AOD) process without going through such a VOD process. In the case of producing only AOD process, the manufacturing process is simple and advantageous to increase the equiaxed crystallization rate of the cast steel.

As a method for improving the equiaxed crystallinity of conventional stainless steel, there are JP 2001-58243, 2002-30324, JP 2000-160299 and EP 924313. In JP 2001-58243, the method of adding superheat of 30 ° C or less and 0.5 to 2.0% B is added, but it is not practical to control the degree of superheat of 30 ° C or less, and B is not preferable because it lowers the corrosion resistance of stainless steel. Can not do it. In JP 2002-30324 and JP 2000-160299, it is possible to increase the equiaxed crystallization rate by controlling molten steel component.In particular, JP 2000-160299 claims that TiC can be precipitated to secure more than 60% equiaxed crystallization rate. There is a limit to improving isotropic crystallization with control alone, and TiC cannot be thermodynamically generated at concentrations of 0.2% Ti and 0.01% C. In EP 924313, the equiaxed crystallization rate can be increased by controlling molten steel components, and in particular, it is possible to secure more than 50% equiaxed crystals by controlling the concentration of Ti × N / Al ≥ 0.14, [Ti] ≥ 0.10%, and [N], [Al] concentration. Claims that it can. However, the equiaxed crystals produced during solidification of cast steels depend on the amount and size of TiN or TiO ₂ fine precipitates in molten steel, and the amount and size of these precipitates affect the deoxidation state of molten steel and the method of treatment of molten steel after Al and Ti addition. As a result, the equiaxed crystallinity of ferritic stainless steel slabs manufactured by conventional methods that specify only molten steel components will show a large deviation.

The present invention has been made to solve the above problems, omit the VOD refining furnace process after tapping in AOD, and by going through the electric furnace → AOD → LT (ladle refining) → continuous casting process, while simplifying the manufacturing process In order to improve convenience equiaxed crystallization, on the other hand, in order to properly adjust the cleanliness reduction and the large amount of TiO ₂ generation due to VOD omission, the molten steel component, slag composition and Ti input method in LT process are improved. An object of the present invention is to provide a method for manufacturing ferritic stainless steel that can stably increase the equiaxed crystallinity of ferritic stainless steel by controlling the size and amount of TiN and TiO ₂ in molten steel.

First, the present invention is a ferritic stainless steel produced by melting the molten metal in the electric furnace through the AOD refining process and the casting ladle treatment process (LT), by adding Ti in the casting ladle treatment process (LT) to the final component system Cr: 10.0-12 wt%, C: 0.02 wt% or less (greater than 0), N: 0.015 wt% or less (greater than 0), Si: 0.3-0.7 wt%, S: 0.01 wt% or less (greater than 0), Ti : Provides a method for producing ferritic stainless steel with improved equiaxed crystallization rate of 0.1-0.3wt% and the balance of Fe and unavoidable impurities.

In the present invention, the molten metal melted in the electric furnace is Cr: 11wt%, C: 2.0wt%, Si: 0.15wt%, N: 0.04wt%, S: 0.03wt% and the balance is composed of Fe and inevitable impurities It is done.

In addition, in the present invention, in the AOD refining process, the AOD slag basicity is adjusted to 2.2 to 2.6 and then deoxidized to Si, and after deoxidation, Al ₂ O ₃ concentration is adjusted to 4 to 8% by adding Al to improve isotropic crystallinity. Characterized in that.

In the present invention, Si in the molten steel is preferably 0.4 to 0.7wt%.

In the present invention, it is characterized in that to remove the slag slashed out through the casting ladle treatment process (LT) to 200kg / ch or less, and to put the powder CaO 200 ~ 500kg / ch.

In addition, in the present invention, the ladle molten steel withdrawn through the casting ladle treatment process (LT) is bubbling at an Ar flow rate of 4 to 6 Nl / min / t before Ti is injected, and after Ti is injected, 2 to Bubbling is carried out at a low Ar flow rate of 3Nl / min / t.

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

In the present invention, the molten metal dissolved in the electric furnace is blown with oxygen and argon gas in an AOD refining furnace to remove C and N concentrations, and then, Cr oxide is reduced, Si is added as a molten steel deoxidizer, and molten steel is stirred with Ar to make slag. The molten steel pulled together with the AOD refining process and the AOD slag to complete refining is adjusted to the component system shown in Table 1 by adding Ti in the casting ladle treatment process (hereinafter referred to as LT process). Figure 4 shows the above manufacturing process.

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

In the present invention, the molten metal dissolved in the electric furnace is blown with oxygen and argon gas in an AOD refining furnace to remove C and N concentrations, and then, Cr oxide is reduced, Si is added as a molten steel deoxidizer, and molten steel is stirred with Ar to make slag. The molten steel pulled together with the AOD refining process and the AOD slag to complete refining is adjusted to the component system shown in Table 1 by adding Ti in the casting ladle treatment process (hereinafter referred to as LT process). Figure 4 shows the above manufacturing process.

409L molten steel component condition (unit: wt.%) Cr C N Si S Ti 10.0-12.0 0.020 or less 0.015 or less 0.3-0.7 0.010 or less 0.10 to 0.30

At this time, the size and amount of TiN and TiO ₂ in the molten steel are caused depending on the treatment method, and thus the equiaxed crystallization cannot be stably increased by the conventional molten steel component condition alone. Therefore, the present invention can stably increase the equiaxed crystallinity of ferritic stainless steel by controlling the size and amount of TiN and TiO ₂ in the molten steel by improving the molten steel component, slag composition, and Ti input method in the LT process in the AOD-LT process.

Then, first, the TiO ₂ and TiN forming mechanism will be described.

<TiO ₂ and TiN forming mechanism>

Non-metallic inclusions of TiO ₂ , TiN, and Al ₂ O ₃ present in the steel may cause clogged nozzles and surface defects, as shown in FIG. 2, so that the N concentration is lowered to prevent the formation of these nonmetallic inclusions, and deoxidizing power is greater than Ti. It deoxidizes with Al which is this big element.

In the step of removing C and N by blowing oxygen and Ar gas in the AOD refining furnace, a large amount of oxygen is dissolved in the steel and Si is added as a deoxidizer to remove it.

The chromium oxide produced during decarburization is reduced by Si as in reaction (1), and SiO ₂ generated in reaction (1) is reduced in reaction (2) when Ti is added.

Cr ₂ O ₃ + Si → Cr + SiO ₂ ---------- (1)

SiO ₂ + Ti → Si + TiO ₂ ---------- (2)

That is, Cr ₂ O ₃ is reduced by Si, and the produced SiO ₂ is reduced again by Ti injection, thereby generating a large amount of TiO ₂ . Most of these TiO ₂ inclusions are removed into the slag, but remaining in molten steel causes nozzle clogging, as shown in Figure 2.

Therefore, if the Ti yield is low in Ti-added steel, it means that TiO _{2 was} generated due to lack of deoxidation before Ti addition, and thus the Ti real rate can be used as a measure of TiO ₂ production or as a measure of molten steel deoxidation.

Ti real percentage (%) = [% Ti] × molten steel (kg) / total injected Ti (kg) × 100 in molten steel

Meanwhile, TiN in molten steel is determined when the concentration of N and Ti in molten steel is higher than the concentration of Ti and Ti, and in order to obtain excellent surface quality, TiN precipitation temperature, which is a measure of TiN generation, should be controlled to 1530 ° C. or less.

TiN precipitation temperature is computed using the following formula.

Therefore, in order to prevent the clogging of the nozzle and to increase the equiaxed crystal ratio, it can be seen that the proper production of TiN and the improvement of the Ti yield are possible.

The following briefly explains the reason for limiting the composition range of the present invention.

409L steel for automobile exhaust system is mainly used in high temperature and corrosive environment, so corrosion resistance is very important and high formability such as stamping, bending and expansion processing is required. Therefore, the C and N components should be maintained at 0.02% or less and 0.015% or less, respectively, and the Cr component is the minimum component for securing corrosion resistance, and if too high, the elongation is inferior to formability. Si is no problem when Al deoxidation in the steel grade to be injected Ti, but in the present invention, since the deoxidizer in the AOD-LT refining process is used Si, a minimum concentration is required to lower the molten oxygen before Ti, In addition, it is known that an appropriate amount must be contained in order to control the structure satisfying the heat treatment condition after hot rolling, and it should be adjusted according to the C and N concentrations. It is known that the moldability increases due to the increase of the hardness of the material when excessively added. S is a harmful element which is regarded as an impurity in stainless steel and should be minimized. Preferred composition ranges are Cr: 11.0-11.3wt%, C: 0.003-0.02wt%, N: 0.003-0.015wt%, Si: 0.3-0.7wt%, S: 0.01wt% or less, Ti: 0.1-0.3wt% to be.

Hereinafter, the present invention will be described in more detail with reference to the examples in Table 2.                     

The present invention improves the deoxidation and slag preparation criteria after the AOD decarburizer in the AOD-LT process and the Ti input conditions of the LT process, and has excellent equiaxed crystallization of 10.0-12% Cr, 0.3-0.7% Si, and 0.02% C or less (0 Over), 0.015% N or less (greater than 0), and 0.1 to 0.3% Ti, and a method for producing the equiaxed crystallization rate of the ferritic stainless steel for exhaust system having a component condition of 80% or more.

For this purpose, after removing carbon and nitrogen from AOD refining furnace, slag basicity (CaO / SiO ₂ ratio) is adjusted to 2.2 ~ 2.6 by adding ferro silicon, CaO, CaF ₂ , and dolomite subsidiary materials. added at 25 ~ 35kg / t-slag by 58 ~ 62% CaO-22 ~ 26% SiO 2 - 4 ~ 6% Al 2 O 3 -8 ~ the slag and the molten steel [Si] concentration of the preparation by 10% MgO 0.4 ~ After tapping to the casting ladle by adjusting to 0.7%, remove the upper ladle slag to 200kg or less and add the powder CaOO 200 ~ 500kg / Ch thereon.

In addition, the LT process adjusts the bubbling time before the Ti opening for the initial opening of the Ar purging plug, the temperature adjustment, and the addition of the ferroalloy before the Ti injection, and the fine bubbling time after the Ti injection to minimize molten steel. (After the bubbling time) is characterized in that to adjust.

Hereinafter, the reason for limiting the refining conditions of the above-described AOD-LT process will be described.

The reason for limiting the basicity to 2.2 to 2.6 in the present invention is that when the basicity is lower than 2.2, the content of SiO _{2 in} the slag is high, so that the SiO ₂ in the remaining slag is reduced by Ti, resulting in lower Ti yield, and the basicity than 2.6. When high, the slag fluidity is lowered, and the CaF ₂ input is increased, and the refractory erosion may be accelerated by the increased slag amount.

In addition, Al is added to reduce SiO ₂ in slag, and the reason for adding Al at 25 ~ 35kg / t-slag is that when the content is over 35kg / t-slag, the content of Al ₂ O ₃ in slag increases. The residual Al ₂ O ₃ in the molten steel increases, causing the plugging of the immersion nozzle. In addition, when the Al input amount is less than 25kg / t-slag there is a problem that the SiO ₂ content in the slag is increased to decrease the Ti yield.

here

slag amount (kg) = input of subsidiary materials (quick lime + fluorspar + dolomite) + SiO ₂ generation

SiO ₂ generation (kg) = {FeSi injection amount * Si grade-molten steel * [% Si]} * 2.14

5 and 6 show the Ti real ratio according to the Al input amount and the slag basicity, respectively.

In addition, the reason for limiting the [Si] concentration of the refining furnace to 0.4 to 0.7% is explained.

Figure 7 shows the relationship between the [Si] concentration and the Ti real ratio before the addition of Ti, in particular, in the present invention, when the final deoxidation component in the refining furnace is out of the 0.4 ~ 0.7% range is added to the ladle during tapping to further perform Si deoxidation It is done. In other words, if the molten steel [Si] concentration after the final refining of the refining furnace is lower than 0.4%, as shown in Fig. 7, the oxygen concentration is increased and the Ti yield is lowered. When the concentration is higher than 0.7%, the elongation is lowered. Will escape.

Next, Ti should be added while suppressing reoxidation of the molten steel refined to AOD as much as possible.To do this, completely remove the upper slag of the ladle with which the slag is removed. Bubbling flow should be minimized in order to suppress as much as possible and Ti realization rate should be secured by bubbling for a certain time after Ti input.

After slamming the final refined molten steel with slag in the AOD refining furnace, slag on the upper part of the ladle is completely removed, and 200 ~ 500kg / Ch of warming powder CaO is added. When Ti is added without completely removing slag, TiO ₂ is generated by reacting with SiO ₂ and Cr ₂ O ₃ in the slag. In addition, the reason for adding 200 ~ 500kg / Ch of powdered CaO is that if the input amount is less than 200kg / Ch, the upper part of the ladle cannot be sufficiently covered, so it is difficult to prevent reoxidation of molten steel during the input of ferroalloy and Ti, and the input amount is more than 500kg / Ch. This is because the addition of ferrous alloy becomes difficult.

Figure 1 is a photograph showing the macrostructure of the cast steel of 409L steel, the equiaxed crystallinity of the conventional material is about 20 ~ 30%, the equiaxed crystallinity of the present footing material is 100%. As described above, according to the present invention, the equiaxed crystallinity is remarkably improved to obtain a desired tissue component.

In addition, the present invention has the effect that can be manufactured in low cost and high quality that can produce ferritic stainless steel with improved equiaxed crystallization while omitting VOD.

Claims (6)

The molten molten metal is ferritic stainless steel manufactured through AOD refining process and casting ladle treatment process (LT). The final component system is Cr: 10.0 ~ by adding Ti in the casting ladle treatment process (LT). 12wt%, C: 0.02wt% or less (more than 0), N: 0.015wt% or less (more than 0), Si: 0.3 ~ 0.7wt%, S: 0.01wt% or less (more than 0), Ti: 0.1-0.3wt A method for producing a ferritic stainless steel having an improved equiaxed crystallization rate of% and balance consisting of Fe and unavoidable impurities. The method of claim 1, The molten metal melted in the electric furnace was ferrite system with improved equiaxed crystallization rate of Cr: 11wt%, C: 2.0wt%, Si: 0.15wt%, N: 0.04wt%, S: 0.03wt% and the balance of Fe and unavoidable impurities. Method of manufacturing stainless steel. The method according to claim 1 or 2, In the AOD refining process, the AOD slag basicity was adjusted to 2.2 to 2.6, and then deoxidized to Si, and after deoxidation, Al was added to adjust the concentration of Al ₂ O ₃ in the slag to 4 to 8%. Method for producing stainless steels. The method of claim 3, Si in the molten steel is a method for producing a ferritic stainless steel having an equiaxed crystallinity of 0.4 ~ 0.7wt%. 5. The method of claim 4, Removing the slag slashed to the 200kg / ch or less through the casting ladle treatment process (LT), 200 ~ 500kg / ch injection method for producing ferritic stainless steel with improved equiaxed crystal. The method of claim 5, The ladle molten steel cast through the casting ladle treatment process (LT) is bubbling at an Ar flow rate of 4 to 6 Nl / min / t before Ti is added, and 2 to 3 Nl / min / t after Ti is injected. A method for producing ferritic stainless steel having improved isotropic crystallization by bubbling at low Ar flow rate.

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