KR20110074217A - The method for manufacturing the ti bearing ferritic stainless steel improved the equiaxed structure ratio - Google Patents

Abstract

PURPOSE: A method for improving the equiaxed structure ratio of Ti-bearing ferritic stainless steel is provided to achieve Ti-bearing ferritic stainless steel with superior surface quality by preventing ridging and roping. CONSTITUTION: A method for improving the equiaxed structure ratio of Ti-bearing ferritic stainless steel is characterized in that, during a refining process in a method for preparing Ti-bearing ferritic stainless steel, the content of Al and Mg in molten metal is increased to raise %(Al2O3)+%(MgO) of slag oxide.

Description

The method for manufacturing the Ti bearing ferritic stainless steel improved the equiaxed structure ratio}

The present invention relates to a method for improving the slab equiaxed ratio in the production of Ti-added ferritic stainless steel, and more particularly, to control the composition and number of oxides so that the oxide inclusions of the slab have a composition favorable for generating equiaxed crystals. The present invention relates to a method for manufacturing ferritic stainless steel having excellent surface quality by preventing the ridging and roping defects by stabilizing the equiaxed crystals of the cast steel by controlling the slag and molten steel components.

Generally, in the manufacture of ferritic stainless steel, cast steels that do not have more than 40% equiaxed crystallization rate are called ridging or roping on the coil surface during deep drawing or forming processing of cold rolled steel due to the coarse band structure remaining in the hot rolled coil. Thin uneven defects tend to occur parallel to the cold rolling direction. Therefore, in order to prevent ridging defects in the past, cold rolling re-rolling and hot rolling reduction are performed, but there is a problem in that manufacturing cost increases and productivity decreases when manufacturing ferritic stainless steel.

For this reason, in order to improve equiaxed crystals, a low temperature casting method, a powdered iron or steel addition method, a rare earth element (REM) addition method, and an electromagnetic stirrer have been proposed and used.

For example, according to Japanese Patent JP1998-127991, it is reported that Al-Ti-based composite inclusions having a size of 0.3 to 0.5 µm act as an equiaxed nucleation seed, and according to Japanese Patent JP1999-222036, the basicity composition is 0.5 to 3.0 and overheated. It is reported that the equiaxed crystal can be improved by making a figure 20-70 degreeC. In addition, the JP2000-220688 patent states that 30 is / m3 of MgO and MgO-Al2O3 having a diameter of 0.3 to 5 μm containing Al and 0.002% to 0.02% and Mg less than 0.0005%, thereby securing equiaxed crystals. However, in these patents, the mechanism for generating equiaxed crystals is not clearly identified, and many deviations may occur depending on the working conditions or ambient conditions.

On the other hand, when Ti is added to ferritic stainless steel and TiN precipitated in molten steel is used as a ferrite solidification nucleus, the solidification structure is known to be easy to equiax. However, even when Ti and N content is constant, the coagulation structure may become equiaxed and crystallized, and in order to improve the equiaxed crystallinity, Ti is required.However, when excessive Ti is added, defects such as nozzle clogging and surface defects may be observed. Since there is a problem that causes it, much research is required.

The present invention aims to improve the above-mentioned conventional problems, and is to identify the conditions for the oxide inclusions of the cast steel to have a composition that is advantageous for generating equiaxed crystals.

In addition, the present invention is to provide a method for improving the equiaxed crystallization rate of the cast through the control of the control factors for each process by finding a control factor for each process that satisfies the identified conditions.

In order to achieve the above technical problem,

In the refining step of the present invention in the manufacturing method of the ferritic stainless steel comprising the molten steel manufacturing step and the refining step and the component adjustment step for the refined molten steel and the continuous casting step to increase the% Al2O3 +% MgO ratio in the composition of the slag oxide It is characterized by increasing the Al component and Mg component of the molten steel.

In addition, in the refining step, the Al component and the Mg component may be increased in a range satisfying the following Equations 3, 4 and 5.

% (CaO) +% (MgO) +% (SiO2) +% (Al2O3) ≥ 85% ····· (3)

% (CaO + MgO) / (SiO2) ≥ 2.3 ···· (4)

% (Al2O3) ≤ 6 ... (5)

In addition, the concentration may be increased to 40 to 100 ppm dissolved Al in the refining step.

In addition, in the component adjustment step, the Ti content may be controlled to 0.185 to 0.230 wt% based on the mild steel Ti component.

In addition, the AOD refining method may be used in the refining step.

Ferritic stainless steel according to the present invention is composed of 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: 9.0 to 15, and other unavoidable impurities such as Fe. And prepared by the above method.

In addition, the ferritic stainless steel according to the present invention may be distributed 10 or more / mm2 Ti-based inclusions satisfying the following formula (1) and (2) contained in the cast.

% (TiO2) +% (CaO) +% (Al2O3) +% (MgO) ≥ 95% ·········· (1)

% (TiO2) / {% (CaO) +% (Al2O3) +% (MgO)} ≤ 4 ... (2)

In addition, the Ti-based inclusions satisfying the above Formulas (1) and (2) may be composed of 40% or more of the total Ti-based inclusions.

From the structural features of the present invention described above,

The origin and cause of the formation of equiaxed crystals could be elucidated. Through this, it was possible to produce casts with high equiaxed crystal by suggesting a factor that can control the equiaxed crystal by each process.

By improving the equiaxed crystallization of the cast steel, it is possible to produce ferritic stainless steel with excellent surface quality by preventing leasing and lofill defects.

Hereinafter, the present invention will be described in detail.

Since equiaxed crystals in the slab produced during the casting process affect not only in the post-rolling process but also to the product, it is important to clarify the mechanism of generating isometric crystals in the steelmaking and continuous casting process in order to improve the equiaxed crystallinity of ferritic stainless steel. It is important.

When comparing the inventive material and the comparative material of FIG. 1, it can be seen that the equiaxed crystals are hardly formed in the center of the cast material in the case of the comparative material. Based on this, the cast steel structure and the similar structural conditions clearly show the difference in casting structure to identify the mechanism of generating equiaxed crystal and proposed a method to improve the equiaxed crystal based on this.

FIG. 2 is a graph analyzing the size and number of TiNs at 0% and 100% equiaxed crystals in the slab. As shown in the graph, when the size of TiN is 3 ~ 10㎛, the number of TiN also increases. As a result of the analysis, it can be seen that the TiN size and number affect the generation of equiaxed crystals.

FIG. 3 shows the results of precise observation by expanding the TiN in the cast steel with an electron microscope. Black spheres are observed in TiN. If you look closely at the 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. Oxides with varying contrast were identified using EMPA. The brighter oxide is composed of CaO-TiO2 system, and the black oxide is present in Al2O3-MgOTiO2 system.

1. Relationship between oxide ratio in tundish and equiaxed rate

In order to study the relationship between the oxide ratio in the tundish and the equiaxed rate, the oxide ratio of the molten steel in the tundish drawn from the cast plate with the high equiaxed crystal rate and the molten steel in the tundish drawn from the cast plate with the low equiaxed crystal rate was investigated.

FIG. 4 shows the oxide composition ratio of the samples taken from each tundish of the slabs with equiaxed rates of 100% and 0%, respectively. In molten steel having an equiaxed crystallization rate (REZ) of 100%, it can be seen that the number of oxide ratios (type 3 and type 5) having the composition as shown in FIG. On the other hand, when the equiaxed crystallinity is 0%, the TiO2 type oxide (type 1) was found to have a much higher ratio. Consequently, the Ti-Ca-Al-Mg-O oxide (type 3) and Ti-Al-Ca- Mg-O oxide (type 5) was found to be a more important factor in determining the equiaxed crystallization than TiO2 type oxide (type 1), and the following equations 1 and 2 were derived.

% (TiO2) +% (CaO) +% (Al2O3) +% (MgO) ≥ 95% ·········· (1)

% (TiO2) / {% (CaO) +% (Al2O3) +% (MgO)} ≤ 4 ... (2)

In addition, as shown in FIG. 4, when the equiaxed crystallinity is 100%, the ratio of the active oxides (type 3 and type 5) among all the oxides is at least 60% or more, and the number should be at least 10 / mm 2 or more.

2. Identify the cause of the formation of effective oxide in tundish

In order to determine the origin of the effective oxide, a study on the composition history of the oxide from the AOD process to the tundish process was conducted. As a result, as shown in FIGS. 5A and 5B, the oxide of the molten steel before Ti input was shown. It has been found that the compositional distribution of plays a critical role in the formation of effective oxides.

In the case where the equiaxed crystallization of the cast steel is 0% and 100%, the oxide compositions before each Ti addition are shown in FIGS. 5A and 5B. When the equiaxed crystallization rate is high, the proportion of oxides with a high% [Al2O3 + MgO] ratio is higher than that of the% [SiO2 + MnO] ratio is about 40%. When decreasing, the equiaxed crystallinity becomes 0%, and as shown in FIG. 5A, it can be seen that the section having a high composition ratio of% [Al 2 O 3 + MgO] is significantly reduced.

On the contrary, as the ratio of% [Al2O3 + MgO] is higher than% [SiO2 + MnO], the higher the composition ratio, the higher the equiaxed crystal yield.

The composition of the oxide changes with Ti input. Since SiO2 and MnO, which are relatively less oxidizing than Ti, are reduced by Ti, most oxides with high% [SiO2 + MnO] transition to TiO2 type oxides. It becomes an invalid oxide. On the other hand, if the high percentage of [Al2O3 + MgO], Ti reacts with dissolved Ti to generate effective oxide.

3. Relationship between the number of active oxides and equiaxed rate

Even if it has an effective oxide composition, if the number is insufficient, it is difficult to secure an equiaxed crystal ratio of more than 80%. Therefore, a criterion for the number of oxides having an effective oxide composition is needed.

In this regard, as a result of analyzing the average number of oxides in the ladle turret (hereinafter referred to as LT), a result of 30 to 50 / mm2 was obtained. In order to obtain an equiaxed crystal ratio of 80% or more,% [Al2O3 + MgO) It was concluded that the proportion of oxides satisfying the condition%> SiO2 + MnO should be at least 30%. Therefore, the number of oxides satisfying the above conditions should be at least 10 / mm 2 or more.

4. Effective Oxide Composition Control Method by Process

According to the above-mentioned conclusions, a method of controlling slag composition during decarburization and deoxidation in a refining furnace in order to produce slabs with high equiaxed crystallinity, and then increasing the production of active oxide by adjusting the Al component immediately before transferring to LT and The control method in the LT process for the effective oxide composition in the tundish will be described in detail below.

(1) Method of controlling slag composition during decarburization and deoxidation

The present inventors have found out that the compositional control of oxides by processes is an essential condition for acting as an effective nucleus of TiN and equiaxed crystals, and furthermore, it is a key factor in determining the equiaxed crystallinity of cast steel. Therefore, the control factors and the control method for the oxide composition control for each process has been explored.

As described above, it was confirmed that molten steel having a higher oxide composition than% [SiO2 + MnO] was developed as an effective oxide after the arrival of the LT process and before the addition of Ti,% [Al2O3 + MgO]. For this reason, in order to increase the% Al2O3 +% MgO ratio in the composition of the oxide that arrived in the LT process, the study was carried out with an interest in the change of equiaxed crystallization rate of the slag composition during the slag deoxidation process. The basic slag composition was based on CaO-SiO2, and the specific component range was 40 to 60% CaO-15 to 30% SiO2-4 to 8% Al2O3-9 to 15% MgO-5 to 15% CaF2.

In order to increase the% Al2O3 +% MgO ratio in the oxide composition in this slag composition, it is possible to increase the Al and Mg components in molten steel, and the basic reaction equation for the reaction between slag and molten steel is inferred. .

2 (Al2O3) + 3Si → 4Al ↑ + 3 (SiO2) (6)

3 (MgO) + 2Al → 3Mg ↑ + (Al2O3) (7)

In the above formula, components in the slag are expressed in parentheses, and others are components in molten steel.

Meanwhile, in order to increase Al and Mg components in molten steel in consideration of Equation 5 to be described later, the ratio of (SiO 2) in slag should be lowered, and at the same time, when MgO in slag is increased, the ratio of Mg may be increased. In addition, the ratio of (CaO), which has the same behavior as (MgO) and has the highest composition ratio among slag and directly affects slag flow, is included in the control factor.

In addition, the (Al 2 O 3) ratio tends to cancel each other, and at the same time, as the (Al 2 O 3) ratio increases, there is a problem that the flowability of slag is lowered and the slag deoxidation efficiency is lowered, so it is necessary to limit it.

Therefore, in the present invention, as a result of investigating the correlation between equiaxed crystallization according to the ratio of (CaO) and (SiO2) and (Al2O3) and (MgO) that affects Al and Mg components of the slag as shown in FIG. Could get a relationship.

When the (CaO + MgO) / (SiO2) ratio was increased, a high equiaxed crystallinity could be obtained, and the ratio should be increased as the ratio of (Al2O3) was increased. Based on the above results, the results show that high equivalence rates of at least 80% can be secured if the following conditions are met.

% (CaO) +% (MgO) +% (SiO2) +% (Al2O3) ≥ 85% ····· (3)

% (CaO + MgO) / (SiO2) ≥ 2.3 ···· (4)

% (Al2O3) ≤ 6 ... (5)

When the conditions of the formulas (3) and (4) were satisfied, effective oxides satisfying the formulas (1) and (2) were obtained from the molten steel in the tundish.

(2) How to increase the production of effective oxide by adjusting Al component before LT transfer

Preferably, if the slag composition is controlled under the above conditions, it is possible to secure a high equiaxed crystallization rate, but when the concentration of Al in the molten steel is not sufficient due to abnormal operation, it is possible to secure the equiaxed crystallization rate by adding additional Al.

It was found that the concentration of dissolved Al in the final slab would be a criterion for determining the slag deoxidation effect and the amount of additional Al input, and the dissolved Al was analyzed by isotropic equivalence.

Figure 7 shows the relationship between the concentration of dissolved Al in the cast steel and the equiaxed crystallinity. The equiaxed crystallization rate and the dissolved Al showed a linear proportional result, and the standard of dissolved Al for securing high isotropic crystallization rate of 80% or more was set to 40 ppm or more, preferably 100 ppm or less. The reason why the upper limit of Al is set to 100 ppm is that if the concentration of Al is 100 ppm or more, the ratio of Al2O3 in the composition of the oxide is increased, resulting in a deviation from the effective oxide composition region. In addition, the high Al2O3 in the oxide composition is converted to MgOAl2O3 type spinel by reaction with dissolved Mg, and this high melting point oxide not only increases the incidence of nozzle clogging but also causes linearity defects during rolling, thus increasing Al concentration. Be careful.

(3) Control method in LT treatment process for effective oxide composition in tundish

In the deoxidation process of the refining furnace, even though the oxide which can be the source of the effective oxide was produced through the control of the Al concentration, if the proper operation was not performed in the Ti and Ca treatment in the later LT treatment process, the effective oxide was increased at a high rate. It is difficult to secure. Ti input is very important in the LT treatment process to produce a final effective oxide composition that is beneficial for isotropic crystallization or for nucleation of TiN.

As mentioned above, the composition of the oxide which arrived at LT process is comprised from the Si-Cr-Al-Mg-Mn-O type complex oxide. Then, when Ti is added, the composition of the oxide is changed by the following reaction formula.

Equation 8: SiO 2 + Ti → TiO 2 + Si

Equation 9: 2MnO + Ti → TiO2 + 2Mn

Equation 10: 2Al2O3 + 3Ti → 3TiO2 + 4Al

Equation 11: 2MgO + Ti → TiO 2 + 2Mg

Here, each reaction rate is determined by the difference in oxidation power between the constituents of the oxide and the Ti component. The order of oxidizing power is high in order of Mg> Al> Ti> Si> Mn, so the MnO and SiO2 components will be reduced to TiO2 and be blown first. Then, the reaction of Al2O3 and MgO to TiO2 will occur. In particular, since the reactions of equations (10) and (11) are thermodynamically unstable, the reaction rate or stabilization will depend on the concentration of Ti.

That is, when the composition is low, the reaction of Formula (10), Formula (11) may or may not occur quite slowly, and when the Ti composition is high, the reaction will proceed quickly.

In this case, the relationship between the composition of the oxide and the equiaxed crystal rate can be predicted as shown in FIG. When the Ti composition is low (region I), the composition of the oxide is changed to Ti-Al-Mg-O by converting MnO and SiO2 from the oxide composition into Ti-Al-Mg-O. It will be enough to be. However, as shown in FIG. 8, the TiN crystallization temperature using the effective oxide as the nucleus is present in a region lower than the liquidus temperature, and as a result, TiN acting as the nucleus of the equiaxed crystal is not produced, so it cannot grow into an equiaxed crystal, Will grow. Afterwards, as the temperature decreases, TiN precipitates between the columnar columnar and the columnar column, so that it cannot finally serve as the nucleus of the equiaxed column. In summary, in the case of region I, the number of effective oxides is sufficient, but the equiaxed crystallinity decreases due to the decrease in TiN crystallization temperature.

On the other hand, when the Ti input amount is excessive (region III), the composition of the oxide is mostly transitioned to the TiO 2 type oxide due to the high concentration of Ti in the liquid phase. As a result, the number of effective oxides that become nucleation sites of TiN is drastically reduced, resulting in a decrease in equiaxed crystallinity.

Therefore, in order to secure a high equiaxed crystallization rate, it is necessary to control so that the Ti input amount is determined in the appropriate region II.

The following results were obtained by investigating the change of equiaxed crystallization according to the Ti component in order to derive the optimum Ti loading amount based on the above basic concept. As shown in Fig. 9, the equiaxed crystallinity of the cast steel showed a parabolic shape by the small steel [Ti] concentration, and the Ti content was limited to a mass ratio within the range of 0.185 to 0.230 in order to secure more than 80% of the equiaxed crystallinity targeted by the present invention. shall.

The above-described control method is briefly summarized as follows.

The oxide composition is Si-Cr-O based on Si deoxidation at the time of decarburization in the AOD process. After adjusting the slag composition and the Al input amount derived from the present invention during the slag deoxidation process, the oxide composition arriving at the subsequent LT process has an oxide ratio of 40% [Al 2 O 3 + MgO] ratio higher than% [SiO 2 + MnO] ratio. More than% will be present, completing a one-step process for transition to active oxides.

After that, the composition of the oxide arriving at the LT process is a Si-Cr-Al-Mg-Mn-O-based oxide, which is determined as an effective oxide and an invalid oxide for isotropic crystal growth by Ti addition. Al-Ti-Mg-O-based oxides identified as effective oxides in the tundish process have an effective oxide composition when the Ti input is adjusted to a specific region, and at the same time, the Ti input affects the TiN crystallization temperature during the solidification process. When controlling the Ti input amount into the present invention, a sufficient number of active oxides are generated, and then, based on the effective oxides in the process of cooling the molten steel to 1500 ° C, TiN is formed as a non-uniform nucleus before solidification, and the TiN size is 3 ~. By growing at 10 mu m, a sufficient number of equiaxed crystals are formed to have an equiaxed crystal ratio of 80% or more.

The long term experiment result for the demonstration of this invention is shown in FIG.

For Ti-added ferritic stainless steel composed of C: 0.02 or less, N: 0.015 or less, Si: 0.3-0.7, Ti: 0.1-0.3, S: 0.01 or less, Cr: 9.0-15, and other unavoidable impurities Cast iron was produced through an electric furnace (EAF) -refining furnace (AOD) -component adjustment (LT) -tundish-continuous casting process, where% (TiO2) / {% (CaO) +% (Al2O3). Slag composition control and Al component control in the AOD process and Ti component control in the LT process in the range of the present invention to produce an effective oxide satisfying (%) +% (MgO)} ≤ 4 It was.

As a result of the experiment, as shown in [Fig. 10], the average cast equiaxed crystal yield was improved from 66.7% to 99.2% compared to the existing technology. The deviation of the equiaxed crystal was significantly improved, and the defective rate was also significantly reduced. As such, tandem mill rolling in the process omission and the cold rolling process according to the improvement of equiaxed crystallinity was enabled, and the productivity was improved.

In conclusion, we derived an effective oxide that acts as an equiaxed crystal nucleus of ferritic stainless steel, and controls the composition and number of oxides that are the source of active oxides in the refining and continuous casting processes to produce such active oxides. It was possible to stably secure the number of effective nuclei to grow positively, and as a result, it was possible to suppress the ridging and roping defects by improving the equiaxed rate.

Although the preferred embodiments of the present invention have been described above, the technical spirit of the present invention is not limited to the above-described preferred embodiments, and various equiaxed crystals are improved in a range that does not depart from the technical spirit of the present invention specified in the claims. It can be implemented by the manufacturing method of the ferritic stainless steel.

1 is a photograph comparing the ferritic stainless steel cast slab equiaxed crystal of the present invention.

Figure 2 is a graph comparing the size and number of TiN according to the equiaxed rate of the slab.

3 is an enlarged photograph of TiN in a continuous cast slab.

4 is a graph comparing oxide composition ratios according to oxide sizes of equiaxed crystals in tundish.

5 is a graph comparing the composition ratio according to the oxide before the Ti addition.

6 is a graph showing the relationship between equiaxed crystallinity and slag composition.

7 is a graph showing the relationship between the concentration of dissolved Al in the cast steel and the equiaxed crystallinity.

8 is a schematic diagram illustrating the relationship between the number of effective oxides and the equiaxed crystallization rate of cast steel according to Ti input amount.

Fig. 9 is a graph showing the relationship between the slab equiaxed crystal and the small steel [Ti] component.

10 is a graph comparing the change of the average equiaxed rate and the defective rate according to the present invention.

Claims (8)

In the manufacturing method of the ferritic stainless steel comprising the molten steel manufacturing step, the refining step, the component adjustment step and the continuous casting step for the refined molten steel, In the refining step, the ferrite-based stainless steel slab crystallization rate improvement method characterized in that to increase the Al component and Mg component in the molten steel to increase the% Al2O3 +% MgO ratio in the composition of the slag oxide. The method of claim 1, In the refining step, the Al component and Mg component is increased in the range of satisfying the following equations 3, 4 and 5, ferritic stainless steel slab isometric enhancement method. % (CaO) +% (MgO) +% (SiO2) +% (Al2O3) ≥ 85% ····· (3) % (CaO + MgO) / (SiO2) ≥ 2.3 ···· (4) % (Al2O3) ≤ 6 ... (5) The method of claim 2, The ferrite-based stainless steel slab equiaxed crystallinity improvement method characterized in that for increasing the concentration so that dissolved Al 40 to 100 ppm in the refining step. The method of claim 2, In the component adjustment step, the ferrite-based stainless steel cast isometric constant improvement method characterized in that the Ti content is controlled to 0.185 to 0.230wt% based on the mild steel Ti component. The method of claim 2, In the refining step, ferrite-based stainless steel slab isometric improvement method, characterized in that using the AOD refining method. It is manufactured by the method of any one of Claims 1-5, C: 0.02 or less, N: 0.015 or less, Si: 0.3-0.7, Ti: 0.1-0.3, S: 0.01 or less, Cr: 9.0-15, and Ferritic stainless steel composed of other unavoidable impurities such as remaining Fe. The method of claim 6, A cast steel made of ferritic stainless steel in which Ti inclusions satisfying the following formulas (1) and (2) interposed on a cast iron are distributed in 10 pieces / mm 2 or more. % (TiO2) +% (CaO) +% (Al2O3) +% (MgO) ≥ 95% ·········· (1) % (TiO2) / {% (CaO) +% (Al2O3) +% (MgO)} ≤ 4 ... (2) The method of claim 7, wherein Ti-based inclusions satisfying the above formulas (1) and (2) are ferritic stainless steels having a composition ratio of 40% or more of all Ti-based inclusions.

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