JP4612271B2 - Continuous casting method of ultra-low carbon steel - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a continuous casting method of ultra-low carbon steel and a steel material in which the occurrence of surface defects on a steel sheet, particularly surface defects due to oxide scale, is extremely small.
[0002]
[Prior art]
Carbon and nitrogen concentrations in steel are removed as much as possible by vacuum degassing treatment, and these are fixed as carbides and nitrides with Ti, Nb, Al, etc. It is used for various purposes as a steel plate. However, various defects are likely to occur and the product yield is low at present. The cause is the deoxidation product generated in the molten steel stage, the inactive gas and powder inevitably used in the continuous casting mold suspended in the molten steel and trapped in the slab, and the heating furnace Examples include scales that are not completely removed in the atmosphere but remain locally on the product surface.
[0003]
Among these, it is reported in, for example, a paper by Seze et al. (Iron and Steel, vol. 87 (2001), No. 2, P85-92) that defects caused by scale are particularly likely to occur in extremely low carbon steel. . Extremely low carbon steel has an extremely low carbon concentration of about 30 ppm or less, and therefore, extremely low carbon steel contains elements such as Al and Ti that are less likely to cause a decarburization reaction when heated and are preferentially oxidized over iron. For this reason, it has been reported that these oxides are easily formed as surface defects when they are formed during heating and deteriorate the workability of the surface layer.
Examples of measures against defects due to scale include optimization of heating temperature and heating atmosphere in a heating furnace, optimization of descaling conditions with high-pressure water, and the like.
[0004]
[Problems to be solved by the invention]
However, once grain boundary oxidation occurs on the side of the iron / scale interface, it is difficult to remove the scale by descaling, so the scale-induced defects can reduce the grain boundary oxidation depth. Countermeasure technology was needed.
Accordingly, an object of the present invention is to provide an ultra-low carbon steel and a continuous casting method in which surface defects of a steel sheet, in particular, surface defects caused by oxide scale are extremely small.
[0005]
[Means for Solving the Problems]
An ultra-low carbon steel in which the concentration (mass%) of Si, Nb, and Ti in the steel satisfies the following relational formula and does not contain Al, and the C concentration is 0.0030 mass% or less as a mold The present invention relates to a continuous casting method for ultra-low carbon steel, characterized in that casting is performed using a mold provided with a moving magnetic field forming device that forms a swirling flow in a horizontal section.
11.5 ×% Si + 47 ×% Nb + 49 ×% Ti ≦ 4
[0006]
DETAILED DESCRIPTION OF THE INVENTION
The inventors cut out samples from slabs of ultra-low carbon steel obtained by casting molten steel with various concentrations of solute elements, conducted heating experiments on these samples, The relationship between solute elements and concentration was investigated and analyzed. Here, the ultra-low carbon steel means one having a carbon concentration of about 30 ppm (mass base) or less, and using a vacuum melting furnace, various alloy elements are added to the electrolytic iron to melt the molten steel, and then the mold By casting and solidifying the slab, slabs having different solute element concentrations were produced. The heating conditions were a heating temperature of 1200 ° C., a heating time of 2 hours, and a heating atmosphere of 3 volume% O ₂ , 8 volume% CO ₂ , 16 volume% H ₂ O, and 73 volume% N ₂ atmosphere.
[0007]
As a result, it has been found that there is a clear correlation between the solute concentrations (mass%) of the ferrite forming elements Si 2 , Nb and Ti and the average grain boundary oxidation depth of the ultra-low carbon steel. Here, the average grain boundary oxidation depth is a notch-shaped notch seen on the side of the scale / base metal interface after heating, as shown in FIG. That means Moreover, the measuring method of the average grain boundary oxidation depth was measured by observing the scale / base metal interface of the sample after heating with a microscope.
[0008]
As an index for comprehensively evaluating the ferrite forming ability (hereinafter referred to as F value) of the solute concentrations (mass%) of the ferrite forming elements Si , Nb, and Ti, the following equation (1) was derived from the experimental results. Therefore, FIG. 2 shows the result of investigating the relationship between the F value and the average grain boundary oxidation depth. The lower the F value, the smaller the average grain boundary oxidation depth, and it is expressed by the equation (1). It can be seen that when the F value exceeds 4, the average grain boundary oxidation becomes significant.
F value = 11.5 ×% Si + 47 ×% Nb + 49 ×% T i ≦ 4 (1)
Therefore, by setting the F value ≦ 4, the average grain boundary oxidation depth can be reduced.
[0009]
Here, the effect of reducing the grain boundary oxidation depth will be described. The scale is formed not only on the ground iron but also on the grain boundary oxidation part. The scale formed in the grain boundary oxidation part cannot be completely removed by descaling because the form of grain boundary oxidation is complicated. Therefore, at the time of rolling, this scale is left as an oxide inside the steel plate. In other words, an oxide is newly introduced near the surface of the steel plate during heating. In addition, the deeper the oxidation depth, the more difficult it is to remove the scale on the ground iron by descaling. Therefore, it is preferable that the oxidation depth is as shallow as possible.
[0010]
Next, each sample of ultra-low carbon steel was heated under the same conditions as above, followed by a descaling experiment, and the relationship between the average grain boundary oxidation depth of the surface iron and scale interface on the surface of these samples and the residual ratio of scale. investigated. The descaling experiment was performed under the conditions of a descaling temperature of 1100 ° C., a back pressure of 15 MPa, and a flow rate of 110 liters / minute. The scale residual ratio is the area of the area where the scale remains / the surface area of the sample × 100, and the area measurement method visually observes whether the scale remains on the sample surface after descaling. Evaluation was made by measuring the remaining area and the total surface area of the sample.
[0011]
The results are shown in FIG. 3, and it can be seen that when the average grain boundary oxidation depth exceeds 50 μm, the scale residual ratio after descaling increases remarkably. The scale remaining after descaling is left inside the steel plate during hot rolling as shown in FIG. 4, and remains as a defect because it remains even if the steel plate surface is pickled. Therefore, in the ultra-low carbon steel after the heat treatment performed before hot rolling, it is left on the steel plate surface and inside the steel material that the average grain boundary oxidation depth of the steel surface and the scale interface is 50 μm or less. From the viewpoint of reducing the scale as much as possible.
[0012]
As a method for producing the ultra-low carbon steel, first, molten steel having a C concentration of 0.003% or less is produced by refining in a converter and treatment in a reflux type vacuum degassing apparatus. After various alloys are added to the molten steel so that the F value ≦ 4, a slab having a predetermined shape is manufactured using a continuous casting apparatus. The average grain boundary oxidation depth of the base metal and scale interface on the steel surface of the ultra-low carbon steel thus obtained is 50 μm or less.
[0013]
By the way, if such a slab is to be manufactured by continuous casting, the component concentration of Si , Nb and Ti will be lowered in order to achieve F value ≦ 4. As a result, free oxygen in the molten steel The concentration will increase. Since C and O react to generate CO gas, depending on the concentration product of CO, the generation of CO bubbles becomes significant, making casting difficult. Since it is basically an extremely low carbon steel, the CO concentration product is low, and the generation of CO bubbles is almost no problem. Therefore, the slab described in the present invention can be cast by various casting methods regardless of the slab size and shape.
[0014]
However, there may be a possibility that the free oxygen concentration in the molten steel injected into the mold is increased by re-oxidation in the tundish or the like from the pan to the tundish. Therefore, when producing a slab by continuous casting using the molten steel of the component of the present invention, in order to reliably prevent the generation of CO bubbles at the front of the solidified shell particularly in the vicinity of the meniscus where the molten steel has a low static pressure, If casting is performed with a swirling flow in a horizontal section using a mold equipped with a moving magnetic field forming device that can impart a flow velocity to the steel, it is possible to stably cast even ultra-low carbon steel with a low F value component composition ,preferable.
[0015]
【Example】
Hereinafter, the present invention will be described in detail with reference to examples and comparative examples.
Example 1
300 t of molten steel with a C concentration of 0.0018% by mass was produced by refining in a converter and treatment in a reflux-type vacuum degassing apparatus. An alloy was added to the molten steel, and 0.01 mass% Si, 0.15 mass% Mn, and 0.035 mass% Nb were not added at all. In this case, the F value is 1.76. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. The cast slab was cut to a length of 8500 mm to make one coil unit. Moreover, before carrying out hot rolling of some cast pieces, the grain boundary oxidation depth was measured.
As a result, the average value of the grain boundary oxidation depth was 20 microns. The other slabs were hot-rolled and cold-rolled, and finally made a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1800 mm. Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects were observed.
[0016]
(Example 2)
300 t of molten steel with a C concentration of 0.0015% by mass was produced by refining in a converter and treatment in a reflux-type vacuum degassing apparatus. An alloy was added to this molten steel, and no Al was added as 0.01 mass% Si, 0.15 mass% Mn, 0.035 mass% Nb, and 0.035 mass% Ti. In this case, the F value is 3.5. The molten steel was cast with electromagnetic stirring at an average flow rate of 40 cm / s using a continuous casting machine having an in-mold electromagnetic stirrer to obtain a slab having a thickness of 250 mm and a width of 1800 mm. The cast slab was cut to a length of 8500 mm to make one coil unit. Moreover, before carrying out hot rolling of some cast pieces, the grain boundary oxidation depth was measured. As a result, the average value of the grain boundary oxidation depth was 40 microns. The other slabs were hot-rolled and cold-rolled, and finally made a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1800 mm. Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects were observed.
[0017]
(Comparative example)
The molten steel with a C concentration of 0.0015% by mass was deoxidized with Al by refining in a converter and treatment with a reflux-type vacuum degassing apparatus to obtain an Al concentration of 0.04% by mass. An alloy was added to the molten steel to obtain 0.01 mass% Si, 0.15 mass% Mn, and 0.05 mass% Ti. In this case, the F value is 4.6. The molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm. The cast slab was cut to a length of 8500 mm to make one coil unit. Moreover, before carrying out hot rolling of some cast pieces, the grain boundary oxidation depth was measured.
As a result, the average grain boundary oxidation depth was 70 microns. The other slabs were hot-rolled and cold-rolled, and finally made a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1800 mm. Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, 10 to 20 surface defects were observed per coil.
[0018]
【The invention's effect】
By using the method described in the present invention, it is possible to reliably prevent surface defects caused by the scale formed when the slab is heated.
[Brief description of the drawings]
FIG. 1 is a diagram showing a method for measuring grain boundary oxidation depth.
FIG. 2 is a diagram showing a relationship between an F value determined by Si, Al, Nb, and Ti and an average depth of grain boundary oxidation.
FIG. 3 is a graph showing the relationship between the grain boundary oxidation depth and the scale residual ratio after the Deske experiment.
FIG. 4 is a diagram schematically showing a situation in which the scale of the grain boundary oxidized portion is left inside the steel plate during rolling.

Claims (1)

Concentration (mass%) in steel of Si, Nb, and Ti satisfies the following relational expression, and does not contain Al, and an extremely low carbon steel having a C concentration of 0.0030 mass% or less in the mold. continuous casting how the ultra low carbon steel, characterized in that the cast using a mold having a moving magnetic field forming device for forming a swirl flow in horizontal cross section.
11.5 ×% Si + 47 ×% Nb + 49 ×% Ti ≦ 4

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