JP5495187B2 - Method for producing continuous cast slab for galvannealed steel sheet - Google Patents

Description

The present invention relates to a method for producing a continuous cast slab that can be used for automobile exterior materials and the like, and that can reduce both the unevenness of the galvanized steel sheet and the wrinkles.
Here, the exterior material of an automobile is a steel plate that constitutes an outer portion such as an automobile body, and is a member that requires a beautiful surface in addition to basic characteristics such as strength and workability.

  Hot dip galvanized steel sheets are manufactured by immersing the steel sheet in a hot dip galvanizing bath and depositing the plating solution, but because the plating thickness is thick, the rust prevention effect is large, and the plating treatment cost is low and suitable for mass production. It is widely used for various steel materials including steel plates. The use of this hot dip galvanized steel sheet covers many fields such as automobile bodies, home appliances, and building materials.

  However, hot dip galvanization has the problem that the uniformity of appearance is inferior to that of electrogalvanization and there are many uneven brightness. Such non-uniform appearance is not particularly problematic in terms of the anticorrosion effect, but cannot be ignored when used as an exterior material.

  In recent years, extremely low carbon Ti-containing IF steel (Interstitial Free steel) excellent in deep drawability has been actively adopted as an exterior material for automobiles. When hot dip galvanizing is applied to such ultra-low carbon Ti-containing IF steel, it is common to perform galvanizing and then alloying to improve slidability, weldability and coating corrosion resistance. Is.

  Alloying treatment refers to heat-treating the steel sheet after hot-dip galvanizing in an alloying furnace. Hereinafter, the hot-dip galvanized steel sheet subjected to the alloying process is referred to as “alloyed hot-dip galvanized steel sheet”. That's it.

The production process for such an alloyed hot-dip galvanized steel sheet can be briefly described as follows.
Cold rolled steel sheet → Line annealing / reduction (780 to 850 ° C.) → Hot dip galvanization (about 460 ° C., 0.5 to 1 second) → Alloying treatment (440 to 520 ° C., 5 to 15 second)

  The alloyed hot-dip galvanized ultra-low carbon Ti-containing IF steel sheet obtained in this way often has streaky irregularities on the plating surface, and the problem has not yet been solved. The feature of this unevenness is that the color tone is extremely blackish or whiter than the ground. Further, as the shape, there are many that extend in a streak shape in the rolling direction, the width of the streaks is 0.5 to 5 mm, and the length is many hundreds of mm. Moreover, the space | interval of a muscle | muscle is 5-50 mm.

  This streak pattern (hereinafter referred to as “strip unevenness”) appears as uneven coating after coating, and the appearance is impaired. However, it is unevenness unique to hot dip galvanizing of ultra-low carbon Ti-containing IF steel. In the case of, it rarely appears.

  Therefore, Patent Document 1 proposes a method of preventing the above-described stripe unevenness by defining the solid solution Ti amount and the shortest diameter of ferrite crystal grains in the vicinity of the plating film interface.

  Moreover, in patent document 2, as a manufacturing method of a high-strength plated steel sheet, a technique for regulating excessive Ti amount, further regulating Mn, preventing uneven color on the plating surface due to Mn, and preventing hot brittleness due to S Has been proposed.

  Moreover, in patent document 3, the method of preventing the streak defect of a steel plate by prescribing | regulating the S content of a steel plate, the care according to S content, and the winding temperature of a hot-rolled steel plate is proposed. .

  Patent Document 4 proposes a method for defining the charging temperature, heating temperature, and finishing temperature during hot rolling in order to prevent streak defects.

  However, any of the methods proposed in Patent Documents 1 to 4 is insufficient as an effect of improving the unevenness of the stripes, and further improvement is necessary.

  One means for preventing the unevenness of the stripes is to adjust the S content. However, when the S content increases, bubbles and inclusions are likely to adhere to the initial solidified shell of the continuous cast slab. (Non-Patent Document 1)

  Bubbles and inclusions attached to these initial solidified shells may not be completely removed even after surface treatment of the slab, and the steel plate has surface defects (hereinafter referred to as pinholes and inclusions on the surface of the slab). Often called "Hege-an".

  That is, in order to prevent streak unevenness, the S content is one of the means of adjustment, but as the S content increases, hesitation wrinkles are likely to occur. There was no established method.

JP-A-2-38550 JP 2001-181786 A JP 2005-2363 A JP 2007-239011 A

`` Effect of sulfur content in steel on adhesion of inclusion and bubble to solidified shell '' CAMP-ISIJ Vol.6 (1993) p.289

  The problem to be solved by the present invention is that, in the prior art, when producing a continuous cast slab for galvannealed steel sheet, a method that can improve both the unevenness of stripes and lashes has not been established. .

  The present invention provides a method for preventing both uneven stripes and lashes generated on the surface when producing a continuous cast slab used for producing an alloyed hot dip galvanized ultra low carbon Ti-containing IF steel sheet. It was made.

The method for producing a continuous cast slab for galvannealed steel sheets according to the present invention is based on the knowledge to be described later by the inventors,
A method for producing a continuous cast slab for galvannealed steel sheets,
When the C content is represented by [mass% C], the S content is represented by [mass% S], the Ti content is represented by [mass% Ti], and the N content is represented by [mass% N],
[Mass% C] ≦ 0.004%,
0.004 mass% ≦ [mass% S] ≦ 0.015 mass%,
0.01% by mass ≦ [% by mass Ti] ≦ 0.10% by mass,
[Mass% N] ≦ 0.004 mass%,
And the molten steel whose solid solution Ti amount represented by the following formula is −0.005 mass% or more and 0.020 mass% or less is electromagnetically stirred in a continuous casting mold, and the size per one slab surface layer within 10 mm is 0.005 mm ^2. The most important feature is that the average existence density of the total number of inclusions and pinholes is 7.0 × 10 ⁻⁴ pieces / mm ³ or less.
[Mass% Ti ^* ] = [mass% Ti] −48 × ([mass% N] / 14 + [mass% S] / 32)

  The density of existence in the present invention is obtained by measuring the total number of all inclusions and pinholes measured within the volume of the measured cross section and dividing by the total volume of the cross section.

In the method for producing a continuous cast slab for an alloyed hot-dip galvanized steel sheet according to the present invention,
Decarburizing the molten steel from the converter or electric furnace in a vacuum degassing facility,
Next, after deoxidizing by adding an Al-containing alloy to the molten steel after decarburization,
When adjusting the Ti-containing alloy, Nb-containing alloy, and S-containing alloy so that the solid solution Ti amount represented by the above formula is −0.005 mass% or more and 0.020 mass% or less, the Ti-containing alloy, Nb-containing alloy, S-containing If the molten steel in which the Al-containing alloy is additionally charged after the alloy is charged is used, the appropriate medium accuracy of the solute Ti amount is greatly improved.

  According to the method of the present invention, it is possible to improve both the hedging and unevenness of the alloyed hot-dip galvanized ultra-low carbon Ti-containing IF steel sheet.

It is the figure which showed the relationship between the amount of solid solution Ti of a galvannealed extremely low carbon Ti containing IF steel plate, and a stripe unevenness generation rate. It is the figure which showed the relationship between S content and the baldness incidence. It is the conceptual diagram which showed the surface quality improvement mechanism of the continuous casting slab when S content is low, (a) is a longitudinal cross-sectional view of a casting_mold | template part, (b) is a figure explaining the detachment | desorption promotion state of a bubble, (c) These are figures explaining the adsorption | suction suppression state of a bubble and an inclusion. It is a figure which shows the product flaw reduction effect at the time of implementing the electromagnetic stirring in a casting_mold | template and improving the quality of a slab surface layer. It is a figure which shows the relationship between the component adjustment method of a vacuum degassing apparatus, and the dispersion | variation in the oxidation loss of Ti.

In the present invention, when producing a continuous cast slab for galvannealed steel sheets, the object is to prevent both uneven stripes and lashes occurring on the surface of the galvannealed ultra-low carbon Ti-containing IF steel sheet. The total number of inclusions and pinholes in which molten steel with a solid solution Ti content of -0.005% by mass or more and 0.020% by mass or less is magnetically stirred and the size per piece within 10 mm of the slab surface layer is 0.005 mm ² or more. This was achieved by setting the average abundance density of 7.0 × 10 ⁻⁴ pieces / mm ³ or less.

The method for producing a continuous cast slab for galvannealed steel sheets of the present invention is as follows:
When the C content is represented by [mass% C], the S content is represented by [mass% S], the Ti content is represented by [mass% Ti], and the N content is represented by [mass% N],
[Mass% C] ≦ 0.004%,
0.004 mass% ≦ [mass% S] ≦ 0.015 mass%,
0.01% by mass ≦ [% by mass Ti] ≦ 0.10% by mass,
[Mass% N] ≦ 0.004 mass%,
This is an alloyed hot-dip galvanized ultra-low carbon Ti-containing IF steel sheet, which tends to cause unevenness and scalp.

And, the so-called solid solution Ti amount calculated by the following formula obtained by subtracting the Ti amount generated as TiN and TiS from the total Ti concentration represented by [mass% Ti ^* ] is -0.005 mass% or more and 0.020 mass% or less. It is what is.
[Mass% Ti ^* ] = [mass% Ti] −48 × ([mass% N] / 14 + [mass% S] / 32)

  This is because when the solid solution Ti amount is −0.005 mass% or more and 0.020 mass% or less, as shown in FIG. 1, the surface unevenness of the surface of the alloyed hot-dip galvanized ultra-low carbon Ti-containing IF steel sheet This is because the frequency of occurrence is greatly reduced.

  In other words, by suppressing the solid solution Ti amount to 0.020% by mass or less, solute Ti unevenness in the alloyed hot-dip galvanized ultra-low carbon Ti-containing IF steel sheet is reduced, so that the crystal grain size variation on the steel sheet surface occurs. This is because it has an effect of preventing the occurrence of uneven plating after cold rolling and annealing.

  This effect increases as the solid solution Ti amount decreases, so it should preferably be 0.010% by mass or less, more preferably 0.006% by mass or less, and still more preferably 0.002% by mass or less. However, since the non-aging required as a characteristic of the alloyed hot-dip galvanized ultra-low carbon Ti-containing IF steel deteriorates if the solid solution Ti amount is too small, the lower limit value at which the bake hardenability (BH) does not appear is − It was set to 0.005% by mass.

  On the other hand, in order to satisfy the solid solution Ti amount (−0.005 mass% or more and 0.020 mass% or less) obtained by the above formula, it is necessary to set the S content high. However, as the S content increases, as shown in FIG. 2, the frequency of occurrence of pinholes and inclusions within the surface layer of the slab increases as shown in FIG. There was a problem that the drop occurrence ratio was high.

  FIG. 3 shows a mechanism for improving the surface quality of a continuously cast slab when the S content is low. In FIG. 3, 1 is mold powder, 2 is molten steel, 3 is a mold, and 4 is an immersion nozzle.

  Unlike the case where the S content shown in FIG. 3 is low, when the S content is high, the tension generated at the interface A between the molten layer 1a of the mold powder 1 and the molten steel 2 becomes small, and bubbles a and inclusions b The action that the withdrawal of is inhibited occurs. Moreover, the tension | tensile_strength which generate | occur | produces in the interface B between the solidification shell 2a and the molten steel 2 becomes small, and the effect | action which becomes easy to adsorb | suck the bubble a and the inclusion b to the solidification shell 2a arises.

  That is, when the S content is high, the frequency of occurrence of pinholes and inclusions within 10 mm of the surface layer of the slab increases due to these complex factors. However, when the S content is low, FIG. As shown, the effect is suppressed.

Therefore, the inventors carried out electromagnetic stirring on the molten steel in the mold at the time of slab casting, even if the S content is 0.007 to 0.010 mass%, which is a disadvantageous condition for the occurrence of whipping (see FIG. 2), and If the average existence density of the total number of inclusions and pinholes with a size of 0.005 mm ² or more within a surface layer of 10 mm or less of the slab surface is 7.0 × 10 ⁻⁴ pieces / mm ³ or less, FIG. As shown, it was found that the ratio of surface flaws generated in the alloyed hot-dip galvanized ultra-low carbon Ti-containing IF steel sheet is greatly reduced.

The method for producing a continuously cast slab for galvannealed steel sheets according to the present invention is based on the above findings of the inventors,
A method for producing a continuous cast slab for galvannealed steel sheets,
When the C content is represented by [mass% C], the S content is represented by [mass% S], the Ti content is represented by [mass% Ti], and the N content is represented by [mass% N],
[Mass% C] ≦ 0.004%,
0.004 mass% ≦ [mass% S] ≦ 0.015 mass%,
0.01% by mass ≦ [% by mass Ti] ≦ 0.10% by mass,
[Mass% N] ≦ 0.004 mass%,
And the molten steel whose solid solution Ti amount represented by the following formula is −0.005 mass% or more and 0.020 mass% or less is electromagnetically stirred in a continuous casting mold, and the size per one slab surface layer within 10 mm is 0.005 mm ^2. The main feature is that the average existence density of the total number of inclusions and bubbles is 7.0 × 10 ⁻⁴ pieces / mm ³ or less.
[Mass% Ti ^* ] = [mass% Ti] −48 × ([mass% N] / 14 + [mass% S] / 32)

  Next, the inventors have obtained the following knowledge regarding a melting method in a vacuum degassing facility in order to improve the medium accuracy of the amount of dissolved Ti represented by the above formula.

  The method of adjusting the components of the molten steel used in the method for producing the continuous cast slab for the galvannealed steel sheet of the present invention with vacuum degassing equipment is vacuum decarburization → deoxidation by addition of Al-containing alloy → Ti-containing alloy, A method of adding an Nb-containing alloy or an S-containing alloy is common. However, since the oxidation loss of Ti varies greatly, it is difficult to adjust the solid solution Ti amount represented by the above formula.

  When the inventors added a small amount of the Al-containing alloy after introducing the Ti-containing alloy, as shown in FIG. 5, the variation in Ti oxidation loss is reduced, and the accuracy of the solid solution Ti amount is greatly improved. I found out. At that time, it was also confirmed that the variation in Ti oxidation loss was further reduced when the Al-containing alloy addition amount after adding the Ti-containing alloy was 0.15 kg / ton or more. It was also found that the timing of charging the Nb-containing alloy and the S-containing alloy was not related to the Ti oxidation loss, and that the order of the Ti-containing alloy and the Al-containing alloy was important.

That is, in the method for producing a continuous cast slab for an alloyed hot-dip galvanized steel sheet according to the present invention,
Decarburizing the molten steel from the converter or electric furnace in a vacuum degassing facility,
Next, after deoxidizing by adding an Al-containing alloy to the molten steel after decarburization,
When adjusting the Ti-containing alloy, Nb-containing alloy, and S-containing alloy so that the solid solution Ti amount represented by the above formula is −0.005 mass% or more and 0.020 mass% or less, the Ti-containing alloy, Nb-containing alloy, S-containing If molten steel in which an Al-containing alloy is additionally charged after the alloy is charged is used, the appropriate medium accuracy of the solid solution Ti amount represented by the above formula is greatly improved.

Hereinafter, examples of the method of the present invention will be described.
Table 1 below shows the steel composition, and Table 2 below shows the main production conditions. In addition, Si, Mn, P, Al, and Nb which are not described in the following Table 1 are Si: 0.025 mass% or less, Mn: 0.05 mass% or more and 0.25 mass% or less, P: 0.020 mass% or less, Al: 0.010 It was set as the mass% or more and 0.060 mass% or less, and Nb: 0.004 mass% or more and 0.025 mass% or less.

  C, S, Ti, N content, solute Ti content, presence / absence of electromagnetic stirring of molten steel in mold during continuous casting, pinholes in slab surface layer, number / density of inclusions, and vacuum degassing device Alloyed hot-dip galvanized steel sheets with different component adjustment conditions were produced by the production steps and production conditions shown in Table 3 below.

  Here, the quality of the slab surface layer part is cut out from the slab surface layer position in the thickness direction 10mm × width direction 3mm × casting direction 50mm samples from the slab width direction 5 locations, the number of inclusions and pinholes for each sample And the average number density was determined.

The number of inclusions and pinholes is measured using a C-scan device that performs high-accuracy ultrasonic flaw detection. The flaw detection frequency of the ultrasonic terminals is 50 MHz, the flaw detection pitch is 0.05 mm, and the size is 0.005 mm ² or more. Identified inclusions and pinholes. The number of inclusions and pinholes was separated and counted according to the difference in the phase of the reflected echo.

Table 1, originating Akirarei 1-3 in Table 2 are invention examples corresponding to claim 1.

The invention Example 1 to 3 and Reference Examples 1-9, Comparative Examples 1-3, perform the comparison to investigate the defect rate, it was evaluated. The defect occurrence rate was quantified by dividing it into those due to balding and those due to streaks.
The results are shown in Table 4 below.

In Reference Examples 1 to 9, the total of the unevenness occurrence rate and the baldness occurrence rate was less than 3%, which was a good result. In particular, Reference Examples 1 to 6 in which the amount of solid solution Ti was in the range of -0.005 to 0.010% obtained good results with both the stripe unevenness occurrence rate and the baldness occurrence rate being less than 1%. Further, in Invention Examples 1 to 3 , which employ a method in which Al is additionally charged after Ti is charged in the vacuum degassing apparatus (see Table 2), the amount of solid solution Ti expressed by the above formula can be accurately and appropriately applied, and the rate of occurrence of streak unevenness In particular, the rate of occurrence of balding was particularly low. Reference Examples 1 and 2 are methods in which no additional Al is added after Ti is added. However, since solid solution Ti has been suitable as a result, the product degradation rate is as low as Reference Examples 7 to 9, but after Ti is added. It is preferable to adopt a method in which Al is additionally added because the accuracy of solute Ti is appropriate.

  On the other hand, in Comparative Examples 1 and 3, the amount of solid solution Ti expressed by the above formula is out of the range defined in the present invention, and the number of pinholes and inclusions in the surface layer portion is not performed on the molten steel in the mold without electromagnetic stirring. Since it was out of the scope of the present invention, the rate of occurrence of lashes was as high as 4.0% in Comparative Example 1 and 3.5% in Comparative Example 3. In Comparative Example 2, the amount of solid solution Ti expressed by the above formula deviated from the range specified in the present invention, and the occurrence rate of streaks was as high as 3.2%.

The present invention is not limited to the above example, if the scope of the technical spirit as set forth in 請 Motomeko, it may of course be changed as appropriate embodiment.

1 Mold Powder 1a Molten Layer 2 Molten Steel 2a Solidified Shell 3 Mold

Claims (1)

A method for producing a continuous cast slab for galvannealed steel sheets,
When the C content is represented by [mass% C], the S content is represented by [mass% S], the Ti content is represented by [mass% Ti], and the N content is represented by [mass% N],
[Mass% C] ≦ 0.004%,
0.004 mass% ≦ [mass% S] ≦ 0.015 mass%,
0.01% by mass ≦ [% by mass Ti] ≦ 0.10% by mass,
[Mass% N] ≦ 0.004 mass%,
And the molten steel whose solid solution Ti amount represented by the following formula is −0.005 mass% or more and 0.020 mass% or less is electromagnetically stirred in a continuous casting mold, and the size per one slab surface layer within 10 mm is 0.005 mm ^2. When producing a continuous cast slab for alloyed hot-dip galvanized steel sheets in which the average abundance of the total number of inclusions and pinholes is 7.0 × 10 ⁻⁴ pieces / mm ³ or less ,
Decarburizing the molten steel from the converter or electric furnace in a vacuum degassing facility,
Next, after deoxidizing by adding an Al-containing alloy to the molten steel after decarburization,
When adjusting the Ti-containing alloy, Nb-containing alloy, and S-containing alloy so that the solid solution Ti amount represented by the following formula is -0.005 mass% or more and 0.020 mass% or less, the Ti-containing alloy, Nb-containing alloy, S-containing A method for producing a continuous cast slab for alloyed hot-dip galvanized steel sheets , wherein an Al-containing alloy is additionally charged after the alloy is charged.
[Mass% Ti ^* ] = [mass% Ti] −48 × ([mass% N] / 14 + [mass% S] / 32)

Images