JP5458701B2 - Steel ingot casting method - Google Patents

Description

  The present invention relates to a method for steel ingot casting that produces less blowhole surface defects in the epidermis of an ingot.

  For example, as shown in FIG. 2, the ingot casting method is similarly performed from a pouring pipe 2 installed on the upper surface of the surface plate 1 via a runner 1a and a hot water 1b formed inside the surface plate 1. 1 is a method of manufacturing an ingot 4 by injecting molten steel into a mold 3 disposed on the upper surface of 1. In addition, 5 in FIG. 2 shows a feeder sleeve and 6 shows a steel ingot coating agent.

  When the ingot is ingot-made by the above-mentioned ingot casting method, the gap between the ladle and the injection pipe is prevented in order to prevent the molten steel from coming into contact with the atmosphere and prevent the molten steel from oxidizing, nitriding, or picking up hydrogen. Sealed with gas.

  If there is too much argon gas supplied to make this seal more complete, argon gas will be mixed into the molten steel, and during solidification, argon gas bubbles will be generated between the dendrite arms on the solidification front of the ingot surface. May be captured as a defect.

  The blowhole defect generated in the epidermis of such an ingot is a harmful defect remaining as a linear flaw on the outer surface of the product or a void defect immediately below the surface because it is not crimped by rolling or forging. .

  In order to prevent the argon gas from being melted into the molten steel and the absorption of hydrogen, which cause the blowhole defect, a method of casting under reduced pressure while sucking between the injection pipe and the injection pipe brick with a vacuum pump is disclosed. (Patent Document 1).

However, the technique disclosed in Patent Document 1 cannot obtain an improvement effect for blowhole surface defects caused by gas reacted during casting such as H ₂ gas and CO gas reacted on the inner surface of the mold.

JP-A-5-318027

  The problem to be solved by the present invention is that, in the technique of vacuum casting while sucking between the injection pipe and the injection pipe brick with a vacuum pump, there is also an improvement effect on blowhole surface defects caused by the gas reacted during casting. It is a point that cannot be obtained.

The steel ingot casting method of the present invention comprises:
In order to prevent the occurrence of blowhole surface defects caused by the gas reacted during casting such as H ₂ gas and CO gas reacted on the inner surface of the mold,
A steel ingot casting method in which molten steel is injected from the bottom of a mold,
Before pouring the molten steel into the mold, cover the mold and shut off the mold from the atmosphere, and then blow an inert gas or nitrogen gas into the mold to reduce the oxygen concentration in the mold to 1% by mass. Keep it below,
Even then in the molten steel injection into the mold interior to maintain the oxygen concentration in the internal template atmosphere by blowing cast type internal inert gas or nitrogen gas to 1% by mass or less,
In addition, the H ₂ O and CO ₂ contents in the steel ingot coating agent added to the molten steel surface in order to cut off the contact of outside air to the molten steel injected into the mold are mainly set to 1% by mass or less. It is a feature.

  In the present invention, casting is performed with an inert gas or nitrogen gas while maintaining the oxygen concentration in the atmosphere inside the mold at 1% by mass or less. Generation of CO gas due to reaction with C in agent and C in steel can be suppressed.

Further, since the content of each of H ₂ O and CO ₂ uses 1% by mass of the steel ingot coatings, can prevent mixing of the H ₂ gas and CO gas from the steel ingot coating into the molten steel.

  According to the present invention, blowhole surface defects formed in the ingot due to the gas reacted during casting can be prevented, and linear defects on the outer surface of the product originating from the defects and void defects immediately below the surface Can be reduced.

It is a schematic explanatory drawing of the method of this invention. It is a schematic explanatory drawing of the bottom casting ingot method.

In the present invention, the oxygen concentration in the atmosphere inside the mold is kept at 1% by mass or less with an inert gas or nitrogen gas for the purpose of preventing the occurrence of blowhole surface defects caused by the gas reacted during casting. The casting was carried out as it was, and it was realized by using a steel ingot coating material in which each content of H ₂ O and CO ₂ was 1% by mass or less.

Hereinafter, the best mode for carrying out the present invention will be described with reference to FIG. 1 together with the process from the idea of the present invention to the solution of the problem.
After forging the steel ingot at a forging ratio of about 2, the inventor identified the position of the defect by ultrasonic inspection, and in a vacuum defect presumed to exist near the surface of the steel ingot. Holes were drilled with a special drill and the released gas was analyzed. The results are shown in Table 1 below. H ₂ gas and CO ₂ gas were detected.

Of these, the H ₂ gas is thought to be mainly due to the accumulation of H in the steel material due to the diffusion of H in the steel material, but the CO ₂ gas is unlikely to be generated by diffusion and is thought to have been mixed from the outside.

Therefore, we investigated the cause of CO gas and CO ₂ gas mixing from the outside, and the case of mixing with the atmosphere and C, which is the main component of the steel ingot coating, Two factors in the case of contamination by supply from the coating agent were estimated.

[Contamination due to reaction between the atmosphere and C, which is the main component of steel or ingot coating material]
When casting starts and molten steel is poured into the mold, the heat of the molten steel creates an upward flow of air in the center of the mold, and outside air flows in to compensate for it from the outer periphery, resulting in air convection. . By this convection, oxygen is supplied to the surface of the molten steel in the mold, and this oxygen reacts with C, which is the main component of the steel or steel ingot coating material, to generate CO gas.

  It was found that this was entrained in the steel by the hot water surface and existed as a blowhole. Further, not only blowholes but also oxides may be trapped in steel as inclusions.

  From the above examination results, it is necessary to suppress the reaction with the atmosphere to suppress the mixing of CO gas by the former, and a lid is installed on the upper part of the mold to prevent the atmosphere from being caught from the outside due to the upward flow It turned out to be effective.

[Supply from steel ingot coating]
The steel ingot coating is manufactured by mixing SiO ₂ , Al ₂ O ₃ , CaO, Na ₂ O, K ₂ O, CaF ₂ , C as main components and blending raw materials so as to have a predetermined composition, In some cases, raw materials containing H ₂ O and CO ₂ as compounds (for example, fly ash and Portland cement) are used.

Fly ash is mainly composed of silica (SiO ₂ ) and alumina (Al ₂ O ₃ ), and a small amount of ferric oxide (Fe ₂ O ₃ ), magnesium oxide (MgO), calcium oxide (CaO), etc. Is included. In addition, the portland cement, tricalcium silicate (alite, 3CaO · SiO _2), dicalcium silicate (belite, 2CaO · SiO _2), calcium aluminate _{(aluminate, 3CaO · Al 2 O 3)} , It is a mixture of calcium aluminoferrite (ferrite, 4CaO · Al ₂ O ₃ · Fe ₂ O ₃ ) and calcium sulfate (gypsum, CaSO ₄ · 2H ₂ O).

  Of the C that generates CO by reacting with oxygen in the atmosphere, the one with a small particle size reduces the angle of repose due to the effect of controlling the melting rate by covering the base material and lubricity. This has the effect of spreading the steel ingot coating material uniformly. Since these have the effect of significantly improving the steel ingot skin, C must be added appropriately and cannot be excluded from the steel ingot coating.

  From the above, it is possible to suppress the generation of CO gas by lowering C in steel, lowering the C content in steel ingot coating, or reducing oxygen in the atmosphere. This reduction has an effect on material performance and cannot be adopted. Moreover, it is desirable to avoid the reduction | decrease of C in a steel material coating material as much as possible because it causes deterioration of the steel ingot skin.

  Therefore, the first means that can be adopted to suppress the generation of CO gas is to reduce the oxygen in the atmosphere. For this purpose, the inside of the mold is cut off from the atmosphere in order to prevent the air from entering the mold. It has been found effective to replace with inert gas or nitrogen gas.

  Next, as for the gas supply from the steel ingot coating material itself, a method of suspending in a mold and a method of placing on a double surface plate are general.

  In both methods, when the molten steel flows into the mold, the plastic bag and paper bag packing the steel ingot coating agent in double or more are burned by the heat of the molten steel jumping into the mold from the hot water, and the steel ingot coating As the agent spreads on the molten steel and melts, the contact of outside air with the molten steel is interrupted.

If the steel ingot coating material itself contains H ₂ O and CO _{2 components} at this stage of melting, they may be trapped in the molten steel without being released to the outside air side. In the case of a mold that has been used many times, there are melted and recessed portions in the mold. When the steel ingot coating agent is captured at such a site as the molten metal rises, the possibility that H ₂ O and CO ₂ components are trapped in the molten steel is increased without escape to the outside air. Therefore, it is necessary to reduce H ₂ O and CO ₂ in the steel ingot coating as much as possible.

  The steel ingot casting method of the present invention is made on the basis of the above knowledge. For example, the steel is ingoted as follows.

  First, in order to suppress the convection of the atmosphere generated by the inflow of the molten steel 4a, before the injection of the molten steel 4a into the mold 3, as shown in FIG. The lid 7 is placed to block the inside of the mold 3 from the atmosphere. At this time, it is desirable to place a fire-resistant cloth in the gap between the lid 7 and the mold 3 to improve the sealing performance.

  Then, a rubber hose 8 having a diameter of 10 mm for blowing Ar gas from the hole of the lid 7, a rough rope for suspending the steel ingot coating 6, and a diameter for gas suction for oxygen concentration analysis for confirming the effect of the method of the present invention. Insert a 10 mm silica tube.

  Thereafter, Ar gas is blown into the mold 3 to reduce the oxygen concentration in the atmosphere inside the mold 3 to 1% by mass or less.

  This oxygen concentration of 1% by mass is an oxygen concentration at which the change in the oxygen concentration reduction rate becomes around 0% by mass per minute when the passage of time and the change in oxygen concentration are observed in general measurements. This is the oxygen concentration at which the atmosphere (oxygen) in the mold is considered to have been exhausted.

  Whether or not the oxygen concentration in the atmosphere inside the mold is 1% by mass or less, without actually measuring the oxygen concentration, the relationship between the gas input time and the oxygen concentration is investigated in advance and the correlation between the two is grasped. You may judge that oxygen concentration became 1 mass% or less because predetermined time passed.

Next, molten steel 4 a is injected into the mold 3 from the injection tube 2. At that time, even during the injection of the molten steel 4a, Ar gas is blown into the mold 3 as necessary to maintain the oxygen concentration in the atmosphere inside the mold 3 at 1% by mass or less. Moreover, the steel ingot coating material 6 added to the surface of the molten steel 4a in the mold 3 is one having a content of H ₂ O and CO ₂ of 1% by mass or less.

The reason why the steel ingot coating material 6 having a H ₂ O and CO ₂ content of 1% by mass or less is adopted is to suppress the generation of H ₂ gas and CO gas that cause blowhole surface defects. It is.

Also, H ₂ O of about 0.3 wt% is unavoidably included in steel ingot coating carefully selected raw materials. On the other hand, in the case of a steel ingot coating material in which raw materials are not carefully selected, 1 mass% or more of CO ₂ is contained in addition to about 0.5 mass% of adhering water / crystal water. Furthermore, the amount of water attached increases with storage for several months. In the results of moisture-proof packaging, the amount of adhered water increases by about 0.1% by mass per month.

Therefore, the reason why the respective contents of H ₂ O and CO _{2 in} the steel ingot coating material are set to 1% by mass or less is also due to consideration of raw material measures for the steel ingot coating material and storage for several months.

Inventors, H ₂ O contained in the steel ingot coating, of CO _2, the H ₂ O Karl Fischer method, also CO ₂ has been measured by TDS (Atsushi Nobori spectrometry) H ₂ O can also be measured by the TDS method.

  In the present invention described above, for example, Ar gas is blown into a mold cut off from the atmosphere, and casting is performed while maintaining the oxygen concentration in the atmosphere at 1% by mass or less while suppressing the intrusion of the atmosphere. Suppresses the generation of CO gas due to reaction with C in the lump coating and C in the steel.

At that time, a steel ingot coating material having a content of H ₂ O and CO ₂ of 1% by mass or less is used to prevent gas mixture from the steel ingot coating material.
As a result, a steel material that hardly generates blowhole surface defects can be produced.

The results of experiments conducted to confirm the effect of the method of the present invention will be described below.
The mold used for the test was a 10-ton steel ingot, an inscribed circle having a diameter of about 1100 mm, a height of about 2300 mm, and an octagonal horizontal section, and two were cast simultaneously.

  The steel ingot produced by injecting molten steel of the chemical composition shown in Table 2 below is free-forged and finished into a round bar with an outer diameter of 600 mm, and then the surface is peeled to the extent that there is no black skin. Surface flaws were evaluated by ultrasonic inspection of the distribution of void defects. The flaw detection conditions were 15Q10N at a depth of 5 mm from the surface, 10Q10N at a depth of 5-15 mm from the surface, and a 5Q15N ultrasonic probe at a depth of 15-40 mm from the surface.

The test conditions are shown in Table 3 below, and the number of defects generated is shown in Table 4 below. The steel ingot coating material in Table 3 below has the component composition shown in Table 5 below. Type A is carefully selected from H ₂ O and CO ₂ raw materials and analyzed for H ₂ O and CO ₂ in the raw materials. The amount is 1% by mass or less, and the type B is one in which the amount of analysis of H ₂ O and CO ₂ in the raw material exceeds 1% by mass.

  As is clear from Table 4, the method of the present invention (numbers 1, 2, 4, 6, 13, 14, 16, 17) is a comparative example (numbers 3, 5, 7-12, which does not satisfy the requirements of the present invention). Compared with 15), the effect of reducing defects is obvious.

  It goes without saying that the present invention is not limited to the above-described examples, and the embodiments may be appropriately changed within the scope of the technical idea described in each claim.

The present invention described above is not limited to duplex stainless steel, high carbon steel, S45C, and blowhole surface defects caused by the gas reacted during casting of H ₂ gas, CO gas, etc. on the surface of the ingot. If so, it can be applied to ingots of other steel types.

DESCRIPTION OF SYMBOLS 1 Surface plate 2 Injection pipe 3 Mold 4a Molten steel 5 Feeding sleeve 6 Steel lump coating agent 7 Lid 8 Rubber hose

Claims (1)

A steel ingot casting method in which molten steel is injected from the bottom of a mold,
Before pouring the molten steel into the mold, cover the mold and shut off the mold from the atmosphere, and then blow an inert gas or nitrogen gas into the mold to reduce the oxygen concentration in the mold to 1% by mass. Keep it below,
Even then in the molten steel injection into the mold interior to maintain the oxygen concentration in the internal template atmosphere by blowing cast type internal inert gas or nitrogen gas to 1% by mass or less,
And a feature that the content of each of H ₂ O and CO ₂ in the steel ingot coating agent added to the molten steel surface in order to break the contact of the outside air into the molten steel injected into a mold with 1 wt% or less Steel ingot casting method.

Images