JPH02165853A - High-speed continuous casting method for steel - Google Patents

Abstract

PURPOSE:To prevent the generation of defects in steel and to allow direct rolling by subjecting the steel to high-speed continuous casting under specific conditions by using a molten metal surface protecting material which is specified in the components contained therein and is specified in surface tension, break point and viscosity. CONSTITUTION:The continuous casting of the steel is executed at >=600m ingot width and >=1.2m/min casting speed by using the molten metal surface protecting material which consists essentially of CaO, SiO2, and Al2O3, contains the CaO/ SiO2 (by weight) in a 0.5 to 0.95 range, contains one or >=2 kinds of oxides, carbonates and fluorides of alkaline metals, alkaline earth metals, and other metals, and further a carbon-component as a melting speed adjusting agent, has >=290dyne/cm surface tension at 1250 deg.C and <=1000 deg.C break point, and has the viscosity eta (poise) at 1300 deg.C satisfying the relation 3.5<=etaV<=6.0 with the casting speed V (m/min). The generation of the restraining breakout at the time of the high-speed casting and the generation of the inclusion defects by the intrusion of the molten metal surface protecting material are prevented in this way and the direct rolling is enabled.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a high-speed continuous casting method for steel, particularly a continuous casting method operating at a casting speed of 1.2 m/min or higher.

[Prior Art] During continuous casting of steel, a molten metal level protectant is added to the mold. The role of this molten metal surface protectant is: 1) By adding it to the mold, it coats the surface of the molten steel to keep it warm, and prevents the surface of the molten steel from solidifying and oxidizing.

■To prevent inclusion defects on the surface of slabs by absorbing non-metallic inclusions floating up from molten steel.

■ Uniform cooling of the mold by uniformly inflowing lubrication between the mold and the solidified steel shell to prevent cracks on the slab surface and restraint breakout.

It is.

In order to ensure the above characteristics, the hot water surface protectant is
．． Δ'120. ．． The base material is oxides such as Cab, Fe2O3, MgO, etc. In addition, oxides, carbonates, or fluorides of alkali metals and alkaline earth metals are used as physical property adjusting agents for the hot water surface protective agent. Contains carbon as a melting rate regulator. As the carbon component, powder or alkali of coke, carbon black, natural graphite, artificial graphite, coal, etc. is used.

The ingredients of a general hot water surface protectant are 5i02: 20
~4596 (wt%, same below), l;ao:20~
4596°At, 03+ 5~10!6. MgO:l
~20! ni, Na, O: l ~20! k.

F-=2~20! It is composed of (::]H or less, and if necessary, Tie, MnO, SrO, Li2O
and B2O3 are also used. Analytically, fluorides are expressed as oxides and F-, and carbonates are expressed as oxides and C. In addition, as for physical properties, the viscosity at 1300°C is 0.5 to 7.
poise, which has a melting point of 800 to 120'0°C, is used.

Incidentally, in recent years, there has been an increasing demand for cost reduction through energy saving, resource saving, and improvement in yield and unit consumption. Therefore, in addition to speeding up continuous casting, direct conveyance rolling (CG-DI), in which slabs produced by high-speed continuous casting are sent directly to the rolling process without cooling and reheating, is being actively adopted. . In order to carry out direct rolling, it is necessary to eliminate all defects in the slab produced by high-speed continuous casting (pinholes, inclusion defects caused by entrainment of molten metal surface protective agent, cracks, etc.), or to eliminate defects that would impede direct rolling. It must be kept to an extremely low level.

However, the surface protection agent used at the conventional casting speed (around 1 m/min on average) was used at a casting speed of 1.2 m/min.
When used for high-speed casting of n or more, the consumption of the surface protectant decreases, resulting in poor lubrication between the mold and the slab, which tends to cause restrictive breakout. Conventionally, as a preventive measure, the viscosity of the surface protectant was increased to 13% due to high-speed casting.
By lowering the viscosity to 0.5 to 1.5 poise at 00°C, the consumption of the hot water surface protective agent was increased and the lubricity was maintained (see JP-A-61-150752).

However, when the viscosity of the surface protectant is lowered, the molten surface protectant gets caught up in the molten steel flow due to the intense injection flow due to the increased speed, resulting in defects in the slab (pinholes, entrainment of the surface protectant, etc.). Inclusion defects, etc.) have increased significantly, and this has become a new major problem in implementing high-speed casting and direct rolling.

In addition, the present applicant has already published JP-A-61-14055, JP-A-60-180655, JP-A-59-209465 as a surface protective agent having good lubricity to suppress slab defects. JP-A-62-23805:1
Applications have been filed for the products disclosed in publications such as No. 1, and the desired effects have been achieved. However, in both cases, there are still issues regarding the proper usage conditions for the surface protection agent for high-speed casting, such as the conditions for preventing the occurrence of breakouts and inclusion defects. It left me unsatisfied.

[Problems to be Solved by the Invention] The present invention prevents the occurrence of restrictive breakout during high-speed casting and the occurrence of inclusion defects due to entrainment of the molten metal surface protective agent, which are problems with the above-mentioned conventional technology. , provides a continuous casting method that can enable direct rolling.

[Means for Solving the Problems] Each structure of the present invention for achieving the above objects will be explained in detail below.

As stated above, the biggest problem with the low viscosity associated with high-speed casting is the entrainment of the molten layer of the surface protectant into the slab by the molten steel flow. The present inventors conducted various studies in order to prevent this entrainment and ensure lubricity. First, regarding prevention of entrainment, it is presumed that the cause of entrainment is entrainment of the molten layer of the molten metal surface protectant into the molten steel due to disturbance of the molten steel surface due to high-speed inflow during high-speed casting. We focused on surface tension as a physical property and measured the surface tension of commercially available products. Surface tension was measured using the dipping cylinder method shown in Figure 1.
d). First, about 60 gr of a hot water surface protective agent that has been decarburized for 1 hour in a Matsufuru furnace at 600°C is placed in a graphite crucible, and melted for 10 minutes in a Matsufuru furnace at 1400°C. This molten sample was placed in platinum crucible 3 and heated to 1250°C.
It was then inserted into the electric furnace 2 of the measuring device (FIG. 1), which was maintained at a temperature of 100 mL and replaced with Ar gas. After the temperature stabilizes, the platinum cylinder 4 connected to the upper balance 1 is immersed in the melting temperature to balance the balance. Next, the liquid level is lowered from the platinum cylinder 4, and weights are added until the balance reaches equilibrium.

Repeat this operation until the platinum cylinder leaves the liquid surface and record the maximum weight at the moment.

This measurement is carried out six times, and the average value excluding the maximum and minimum values is taken as W□(g). The calculation formula is g - wmaX
It is expressed as ×v 0 = 4πR. Here 0 is surface tension (dyne/cm)
, g is the acceleration of gravity (980cm/5ec2), W
,,,aX is the maximum pull (gr) exerted on the platinum cylinder,
R is the radius of the cylinder (cm), and v is a correction coefficient, which is determined by the cylinder used and the density of the melt.

The surface tension measurement results of the conventional products investigated were 240 to 320 dyne/am at 1250°C, and the viscosity was 1300 dyne/am.
0.5 to 7.0 poise at °C, and no correlation was observed between viscosity and surface tension.

Next, a casting test was conducted by changing the value of surface tension, and the relationship with the quality of the slab was investigated. The results are shown in FIG.

As for the casting conditions, (a) is a casting speed of 1.5m1m1n.
, the viscosity of the hot water surface protective agent is 0.6 potse at 1300°C, and (b) is the casting speed of 1.5 m/min and the viscosity at 1300°C.
It was performed at 3.2 poise.

As is clear from Figure 2, there is a tendency for inclusion defects to decrease as the surface tension increases;
A significant decrease in inclusions can be seen from the point 0. this is,
Compared to the degree of surface turbulence caused by the casting conditions, a high surface tension indicates that the surface bonding force of the molten metal surface protective agent is strong, making it difficult to mix with the molten steel and become less likely to be entangled. Under high speed casting, 1250
It has been found that when the temperature is 290 dyne/cm or higher, almost no entrainment occurs.

Next, we investigated lubricity (breakout incidence,
As a result of conducting surveys and research on means to guarantee the incidence rate (hereinafter referred to as 8.0 incidence rate), we have obtained findings. In other words, the viscosity of the surface protectant is generally measured at 1300°C, but this is because the average temperature of the inflow film of the surface protectant directly below the meniscus of molten steel in the mold is approximately 1300°C. This is because the measured values can be obtained relatively stably. The present inventors melted 200 gr of various hot water surface protective agents after decarburization at 1400°C for 10 minutes using a rotor rotation method, and
The viscosity was measured after holding at 0°C, then the temperature was lowered by 20°C, and the viscosity was measured after holding at each temperature. I created a relationship graph of 0K). During this measurement, due to the temperature drop, crystals crystallized in the sample, and the sample ceased to be a Newtonian fluid.
A high temperature was defined as a breakpoint. Figure 3 shows an example of the measurement. As is clear from FIG. 3, even if the viscosity at 1300°C is an approximate value, there are hot water surface protective agents with significantly different break points.

That is, for the product names A and B shown in Figure 3, 13Of
The viscosity at 7℃ is 7.6 poise and 7.2 poise
ise, the breakpoints are 980℃ and 1180℃, respectively, and the viscosity of products with product names C and D is 5.
．． 5 and 5.0 poise, the breakpoints are significantly different at 1000°C and 1080°C, respectively.

Therefore, I created a prototype sample with changed breakpoints,
An actual machine casting test was conducted to investigate the effect of engraving on the 8.0 occurrence rate. The steel type and casting speed are the same as before, and the viscosity is 13.
It was set to 3.2 poise at 00°C. Figure 4 shows the results. As is clear from the figure, there is a large correlation between the 8.0 generation rate and the breakpoint, and by lowering the breakpoint, it is possible to lower the B and O generation rate, and the temperature can be kept below 1000℃. It has been found that the 8.0 incidence rate can be almost eliminated by

This point will be explained using a lubrication model inside the mold. As shown in FIG. 5(b), the conventional scene protectant 7 has a liquid molten metal surface protectant 8 only in the meniscus part of the molten steel, and the other lower parts are covered with a solid scene protectant 9. Therefore, good lubricity cannot be maintained. Furthermore, since the break point of the scene protective agent 10 of the present invention is below 1000°C, the surface of the slab at the lower end of the mold is generally at 1000°C, as shown in FIG. 5(a).
Therefore, it is considered that the molten liquid scene protective agent 11 exists even at the lower end of the mold, creating an ideal lubrication state. It is considered that as a result, the occurrence of restrictive breakout almost disappeared. In the figure, 12 is a solid hot water surface protective agent, 13 is a mold, and 14 is molten steel.

Next, in order to find the appropriate viscosity range, the surface tension was set at 1250℃.
290 dyne/cm or more, break point 1
We prototyped a mold surface protectant whose viscosity was varied in the range of 000°C or less, and checked the quality of slabs by casting on an actual machine at different casting speeds. The results are shown in FIG. The O mark indicates defects below the target value, and the x mark indicates defects above the target value. From the figure, it was found that there is an appropriate viscosity range for the casting speed.

In other words, for the casting speed ■, the viscosity η at 1300℃ is 3.5 ≦η■≦6.0 It turns out that it is necessary to design within the range of .

Next, the composition of the hot water surface protective agent used may be the composition of commonly used hot water surface protective agents, and the surface tension at 1250°C is 2.
90 dyne/c+++ or higher, breakpoint 10
Physical properties below 00°C are 0.5 to 0 for GaO/5i02
．． 95. If Cab/5i02 is less than 0.5, a large amount of flux components (Na20, Li2O, F-, etc.) are required to adjust the physical properties, especially the viscosity.
The immersion nozzle melt tMffi becomes large and the cost increases, making it impractical.
In the case of M, the liquid properties are lost due to denaturation in the mold (AM.03 inclusion absorption, reduction of SiO□ in the molten metal surface protective agent by AQ in the steel), and crystal precipitation occurs, resulting in poor slip resistance. Sexuality becomes poor. When CaO/Sin□ is 0.95 or less, Sin□ has a chain structure in which crystal precipitation is difficult in terms of ionic structure, and the break point is also low. For this reason, Cab15jO□ was set in the range of 0.5 to 0.95.

For slight variations in the breakpoint or surface tension of the scene protectant, Na2O, 1j20. F, B20
．． ,.

Ail'3. Na5AQF3. MgF2. TiO
2, it can be adjusted with MnO, SrO, etc.

In addition, the reason why the slab width was set to 600 mm or more was because 600
If the length is less than 600 mm, it is easy to form a relatively uniform shell core with a short long side length, and surface defects are difficult to occur.If the length is more than 600 mm, the shell core becomes uneven and surface defects are likely to occur. The slab width is 600++un
That's all.

Based on the above results, the structure of the present invention can be summarized as follows. That is, the present invention was obtained by considering the relationship between the most suitable physical properties of the hot water surface protective agent and the casting speed obtained from the above experimental results, and its composition is as follows: Cab, 5in2. The main component is 82203, Ca
b/StO□ (wt% ratio) is in the range of 0.5 to 0.95, and one or more oxides, carbonates, and fluorides of alkali and alkaline earth metals and other metals, and Contains carbon as a melting rate regulator, has a surface tension of 290 dyne/cm or more at 1250°C, a break point of 1000°C or less, and a viscosity η (poise) at 1300°C equal to the casting speed v (m/min). High-speed continuous casting of steel, characterized by using a scene protectant that satisfies the range of the relational expression %, and operating at a casting speed of V≧1.2 m/min with a slab width of 600 mm or more. It is the law.

In this way, in the present invention, by newly limiting the surface tension and break point characteristics of the scene protective agent, and by specifying the most appropriate usage conditions of viscosity and casting speed, Even in the above high-speed casting, the mold
It is possible to prevent the occurrence of restrictive breakout by sufficiently flowing the surface protective agent between the slabs, and to produce slabs with extremely few slab defects that can be directly rolled.

[Example] Table 1 shows the components and physical properties of the hot water surface protective agent used in the present invention, the comparative example, and the conventional hot water surface protective agent. Casting was carried out using a continuous slab caster using these scene protective agents. The casting conditions are mold size 250 thickness x 1600 width (+
nm), and the casting speed was 1.2 to 2.0 m/min. The casting results at that time are shown in Table 2.

Symbols A to F in the table are examples of the present invention, and the 0.0 occurrence rate index was 0, and the slab quality showed good results.

G to J are comparative examples. In G, the break point and viscosity were within the range of the present invention, or the surface tension was outside the range of the present invention.]) Although the occurrence rate of 0 was low, inclusion defects occurred in the slab. H is an example in which the surface tension and viscosity are within the range of the present invention, but the break point is outside the range of the present invention, and the occurrence rate of [1,0] is high and cracks occur. In addition, although the surface tension and break point of I were within the range of the present invention, the viscosity was too high for the casting speed, and as a result, the occurrence rate of B and O was high, and defects occurred. On the contrary, J is an example in which the viscosity is too low, and although the occurrence rate of 8.0 is low, inclusion defects and cracks occur. K is a conventional example, and although the occurrence rate of 8.0 was low, there were many scratches caused by entrainment of the molten metal surface protective agent on the slab.

No. table Can be done.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the surface tension measurement device, Figure 2 is a graph showing the relationship between surface tension and slab surface defect index, Figure 3 is a graph of an example of measuring viscosity, and Figure 4 is a graph of surface tension. Graph showing the relationship between breakout occurrence index and breakout occurrence index, Figure 5 (a)
5(b) is a lubrication model in the mold of the present invention, and FIG. 5(b) is a lubrication model in the conventional mold. FIG. 6 is a graph showing the relationship between casting speed, viscosity, and surface tension in slab quality.

Claims (1)

[Claims] 1. CaO, SiO_2, Al_2O_3 are the main components, and CaO/SiO_2 (weight % ratio) is 0.5 to 0.95.
One or more oxides, carbonates, and fluorides of alkali metals, alkaline earth metals, and other metals within the range of
Furthermore, it contains carbon as a melting rate regulator, and the temperature is 1250℃.
The surface tension at is 290 dyne/cm or more, the break point is 1000°C or less, and the viscosity η (poise) at 1300°C is in the range of the relational expression 3.5≦ηv≦6.0 with the casting speed v (m/min). A high-speed continuous casting method for steel, characterized in that the process uses a surface protective agent that satisfies the following, and operates at a casting speed of v≧1.2 m/min with a slab width of 600 mm or more.