KR20000045502A - Method for improving surface quality of austenite stainless steel added with high-titanium - Google Patents

Abstract

PURPOSE: A method for producing austenite stainless steel added with high-titanium is provided to improve the surface quality of a product without grinding a coil by inducing a cause of surface defect generation and a reducing method through various tests. CONSTITUTION: Chemical composition of mold powder is regulated in order to make the basicity(wt% CaO/wt% SiO2) of the mold powder 0.95 to 1.0. After the basicity is secured, the chemical composition of the mold powder is regulated in order to calibrate the viscosity of the initial mold powder by considering the content of TiO2 mixed in a mold slag during cast and the target viscosity.

Description

Improvement of Surface Quality of High Titanium-Austenitic Stainless Steels

The present invention determines the optimal basicity (% CaO /% SiO 2), chemical composition, viscosity of mold powder used in continuous casting of high Ti-added austenitic stainless steel to effectively reduce and remove harmful nonmetallic inclusions. A method for improving the surface quality of high Ti-added austenitic stainless steel products (hot rolled and cold rolled coils).

Generally, in the steelmaking process of stainless steel, molten iron and alloy iron are melted in an electric furnace to produce a molten metal, and then a molten steel is secured in a refining furnace, and the molten steel is added to a ladle and then moved to a continuous casting process. The moved molten steel is supplied to a mold which is cooled through a continuous casting process, that is, a tundish, and when solidified to produce slabs (SLAB, BLOOM, and BILLET), the slabs are finally rolled to produce hot rolled and cold rolled coil products. do. Figure 1 schematically shows the situation in the mold during continuous casting. Molten steel from the tundish is supplied to the mold 2 through the immersion nozzle 1. Since the mold 2 is cooled by the cooling water, the molten steel 3 solidifies from the mold to form the solidification cell 4. On the other hand, 5 denotes a mold powder, which is a mold flux added as a lubricant between the mold and the solidification cell during continuous casting. The mold powder is a mixture of oxides having a composition such as CaO-SiO 2 -Al 2 O 3 -Na 2 O-Na 2 O-F. The mold powder is continuously added to the molten steel in the mold during continuous casting to perform the following four important functions.

1) Cover the surface of molten steel to keep molten steel warm.

2) Prevent molten steel from oxidizing in contact with the atmosphere.

3) The molten mold slag penetrates between the cast steel and the mold to lubricate and enable continuous casting.

4) Absorb and remove non-metallic inclusions in the molten steel.

As such, mold powder plays a very important role in continuous casting, and has a decisive influence on product quality as well as operation.

Meanwhile, the composition of the high Ti-added austenitic stainless steel, which is the object of the present invention, and the composition of the conventional mold powder used when producing the steel species are shown in Tables 1 and 2 below.

Table 1. Composition of high Ti-added austenitic stainless steel (STS 321)

<wt.%> C Si Mn Cr Ni N Ti 0.03 0.5 1.4 17.8 9.3 0.035 0.25

Table 2. Composition of Existing Mold Powder for High Ti-Added Austenitic Stainless Steels

<wt.%> CaO SiO2 Al2O3 MgO Na2O F Li2O *basicity 39.6 33.3 7.3 One 9 8.3 1.5 1.2

* Basicity =% CaO /% SiO2

As a result of producing the target steel with the above mold powder composition, a large amount of surface defects were produced in the hot rolled and cold rolled coil products, and the main factors causing the defects as a result of the defect analysis were nonmetallic inclusions and mold powders of TiO2 and CaO-TiO2 compositions. When these defects are frequent, nonmetallic inclusions accumulate on the surface and surface layers of the cast, and these inclusions cause linear surface defects on the product surface during hot and cold rolling. In the case where a large amount of such defects occur, the conventional solution is to remove the defects by front grinding (CG) the coil surface in which the defects occur, and it can be sold as a product only in this way. Such coil grinding is costly and costly because it is very expensive and takes a long time.

Therefore, the present invention has been studied in this background, and its purpose is to obtain the cause and reduction method of surface defects in the manufacture of high Ti-added austenitic stainless steel through detailed and various tests and to obtain a very expensive coil grinding. To provide a method of improving the surface quality of the product without having to.

1 is a schematic diagram showing a situation in a mold during continuous casting of stainless steel;

2 is a schematic diagram showing a defect generation mechanism of high Ti-added austenitic stainless steel;

3 is a graph showing a ternary state diagram of CaO-SiO2-TiO2 and a CaO-TiO2 crystallization region;

4 is a graph showing the effect of basicity and TiO2 content of mold powder on the crystallization of CaO-TiO2;

5 is a graph showing the effect of the basicity of the mold powder on the glass formation rate during CaO-TiO2 formation and solidification and the range of the optimum basicity;

6 is a graph showing the effect of TiO 2 content on the viscosity of mold powder;

7 is a graph comparing the mold slag CaO-TiO2 crystal phase with XRD as a result of actually applying the existing mold flux (A) and the present invention mold flux (B),

8 is a graph showing a comparison of viscosity changes during casting of the initial mold powder viscosity corrected by the method of the present invention (A) and the conventional mold powder uncorrected viscosity (B),

9 is a graph comparing hot rolled coil surface quality as a result of actually applying the present invention mold flux and the existing invention mold flux.

Explanation of symbols on main parts of drawing

1: Immersion nozzle of continuous casting machine

2: mold of continuous casting machine

3: molten steel

4: solidification cell in mold

5: mold powder

6: CaO-TiO2 Formation Region in CaO-SiO2-TiO2 State Diagram

7: Maximum allowable glass ratio limit line of mold powder to prevent deflection defect of austenitic stainless steel cast

8: Basicity range of the optimum mold powder considering preventing the occurrence of CaO-TiO2 and reducing cast depression

9: target viscosity of mold powder during casting

According to the present invention for achieving the above object, grinding the hot rolled and cold rolled coil products by appropriately adjusting the basicity and viscosity of the mold powder during continuous casting of high Ti-added stainless steel to minimize the incorporation of CaO-TiO2 inclusions and mold slag in the molten steel The following technique is provided as a method of improving the surface quality without the need.

1) Adjust the chemical composition of the mold powder so that the basicity (wt% CaO / wt% SiO2) of the mold powder is 0.95 to 1.0.

2) After securing the basicity above, adjust the chemical composition of the mold powder to correct the viscosity of the initial mold powder (original mold powder) in the following formula in consideration of the content of TiO2 incorporated into the mold slag during casting.

η _o = η + k (% TiO 2)

here,

η _o = corrected viscosity of mold powder (= design viscosity of mold powder, poise at 1300 ° C.),

η = target viscosity of the mold powder during casting,

k = rate of viscosity decrease according to the content of% TiO2 (poise /% TiO2)

= 0.36,

% TiO2 = content (wt.%) Of TiO2 incorporated into mold slag during casting.

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

First, in the present invention, in order to investigate the cause of defects, thermodynamic examination and data investigation were conducted on the cause of formation of each nonmetallic inclusion of TiO2 and CaO-TiO2 composition. As a result, TiO2 was a deoxidation product due to the addition of a large amount of Ti due to the characteristics of the steel grade. As a result of thermodynamic examination, TiO2 was an inevitable inclusion in the manufacturing process. However, the cause of defects caused by CaO-TiO2 and mold powder was investigated by various technical and thesis data, and the defect generation mechanism was as shown in FIG. That is, Ti in the molten steel reacts with SiO2 in the mold slag to form TiO2, and also TiO2 is absorbed into the mold slag as the deoxidation product in the molten steel, and eventually the basicity in the mold powder and TiO2 (because of the SiO2 decrease) are simultaneously. Will increase. Under these conditions, CaO-TiO2 having a high melting point (M.P = 1971 ° C) is formed by reacting with CaO and TiO2 in the mold slag. In addition, the increase in TiO2 in the mold slag decreases the viscosity in the mold slag so that the mold slag is easily incorporated into the mold during casting, thereby increasing inclusions in the slab. CaO-TiO2 and mold slag formed with such a mechanism eventually lead to surface defects of the product.

As a result of the above examination, it can be seen that CaO-TiO2 and mold slag except TiO2 are all defects associated with mold powder. Accordingly, the present inventors have focused on the fact that if a mold powder having an appropriate composition and physical properties is used, product defects can be improved by drastically reducing the incorporation of CaO-TiO 2 and mold slag. First, the present inventors examined the ternary state diagram of CaO-SiO2-TiO2 shown in FIG. 3 to investigate the effect of chemical composition on the formation of CaO-TiO2. Since mold powders contain 80% or more of CaO and SiO2, it is not unreasonable to analyze the system of the entire mold powder. In the figure, the area (6) enclosed by the thick lines shows the area and temperature at which CaO-TiO2 is produced. As can be seen from this figure, the formation of CaO-TiO2 should not be formed below 0.9% basicity (% Cao /% SiO2), and CaO-TiO2 will be the first as the basicity increases in the basicity range (0.8-1.3) used as mold powder. It can be seen that the CaO-TiO2 is easily formed even at a low TiO2 content by decreasing the concentration of TiO2, and more importantly, the crystallization temperature of CaO-TiO2 increases as the basicity increases. That is, it can be seen that the basicity and the content of TiO2 play the most important role in the formation of CaO-TiO2.

Therefore, the present inventors investigated the effects of the basicity of the mold powder and the content of TiO2 on the formation of CaO-TiO2. In the irradiation method, the basicity was changed from 0.8 to 1.3, the content of TiO2 was added to 0-15% in each basic degree, and then melted and cooled to compare CaO-TiO2 contents by XRD (X-ray diffraction). Figure 4 shows the result, even if the basic content of 0.8, 0.9, 1.0 even if the content of TiO2 increased CaO-TiO2 was not formed. However, from the basicity 1.1, CaO-TiO2 was formed. As the basicity increased and the TiO2 increased, CaO-TiO2 increased. From these results, it was found that the basicity of the mold powder not forming CaO-TiO 2 was 1.0 or less.

On the other hand, austenitic stainless steels have a foaming reaction due to their solidification characteristics, so if the basicity of the mold powder is too low, the mold slag becomes glass and heat transfer from the cast to the mold is faster, resulting in uneven solidification and depression. Cast defects called depression occur. Therefore, this should be taken into account when determining the appropriate basicity. The present inventors measured the degree of glazing by basicity using an experimental apparatus for evaluating the degree of glazing of the mold powder. Figure 5 shows the effect of the basicity of the mold powder on the degree of glass and CaO-TiO2 formation at the same time. Here, the mold powder was represented as containing 10% TiO2 (in actual production of high Ti steel grades, the TiO2 increases up to 10% in the mold powder). As shown in the figure, the proper glass ratio is less than 70% to reduce depression defects. It can be seen that an appropriate basicity that satisfies these conditions, that is, an appropriate basicity that does not form CaO-TiO2 and simultaneously prevents cooling in the mold to reduce depression defects is 0.95 to 1.0. Therefore, the basicity of the present invention was 0.95 to 1.0.

Next, to determine the appropriate viscosity of the mold powder to prevent the mixing of the mold slag, in the present invention quantitatively investigated the effect of TiO2 on the viscosity of the mold powder. A rotary viscometer was used for viscosity measurement, and the viscosity was measured by adding 0-10% of TiO2 to the mold powder of basicity 0.9 and 1.0. Figure 6 shows the result, it can be seen that the viscosity decreases with a constant slope as the content of TiO2 increases. Therefore, considering the increase of TiO2 during casting, the initial viscosity of the mold powder (viscosity of the original mold powder) should be designed to be higher than the desired viscosity value, so that defects caused by the inflow of the mold slag into the molten steel due to excessive reduction of the viscosity and excessive It can be seen that defects of cast oscillation marks due to inflow can be prevented at the same time. In order to quantify the effect of TiO 2 on the viscosity, the inventor derives a viscosity correction equation using the data of FIG.

η _o = η + k (% TiO 2)

here,

η _o = corrected viscosity of mold powder (= design viscosity of mold powder, poise at 1300 ° C.),

η = target viscosity of the mold powder during casting,

k = rate of viscosity decrease according to the content of% TiO2 (poise /% TiO2)

= 0.36,

% TiO2 = content (wt.%) Of TiO2 incorporated into mold slag during casting.

The reason for deriving the correction formula using% TiO 2 as a variable is because the content of TiO 2 incorporated into the mold slag during casting is not constant. That is, the content of TiO2 in the mold slag is very different due to differences in steel grades, differences in deoxidation degree, and differences in the ability to remove inclusions before the continuous casting step. Therefore, in the present invention, the viscosity was derived as a function of% TiO 2 so that it can be used under all conditions. In addition, since the target viscosity is also a very subjective value that can vary depending on the desired consumption and type of mold powder, this is also a variable. The focus of the present invention is to determine the initial viscosity of the mold powder in consideration of the viscosity decrease with the increase of TiO2 during casting while the target viscosity is set and the steel grade characteristics know how much the concentration of TiO2 is increased in the mold slag during casting. To derive.

When the effects on the above invention were verified through the examples, the following describes the embodiments of the present invention.

Example 1

Figure 7 is the result of analyzing the presence or absence of CaO-TiO2 in the mold slag collected during casting when using the mold powder of the present invention and the existing mold powder, in the case of the present invention (B) CaO-TiO2 does not occur at all On the other hand, the existing product (A) shows what happens.

Example 2

8 is to obtain the target viscosity during casting as a result of correcting the viscosity of the mold powder by applying the method of the present invention (A) and using the existing method (B) from the initial viscosity to increase the viscosity too much from the initial viscosity to increase the target viscosity It shows that the contrast remains downward.

Example 3

9 is a comparison of the hot rolled coil rejection rate (based on CG%) when the present invention is applied to the case of STS 321 steel casting, which is a representative high Ti austenitic stainless steel, and the defect rate is greatly reduced as a result of applying the present invention. I can see that.

As described in detail above, the present invention determines the optimum basicity and viscosity of the mold powder used during continuous casting of high Ti-added austenitic stainless steel to effectively reduce and remove non-metallic inclusions that adversely affect the product surface quality, thereby adding high Ti. It provides the effect of dramatically improving the surface quality of austenitic stainless steel products.

Claims (1)

As a method of improving product surface quality by minimizing the mixing of CaO-TiO2 inclusions and mold slag in molten steel by controlling the basicity and viscosity of mold powder during continuous casting of high Ti-added stainless steel, The chemical composition of the mold powder is adjusted so that the basicity (wt% CaO / wt% SiO 2) of the mold powder is 0.95 to 1.0, After securing the basicity, the high Ti-added austenitic stainless steel, characterized in that to adjust the chemical composition of the mold powder to correct the viscosity of the initial mold powder in consideration of the content and target viscosity of TiO2 incorporated into the mold slag during casting How to improve surface quality.