KR100482225B1 - Method for continuous casting of high carbon alloy steel - Google Patents

Abstract

The present invention relates to a continuous casting method of high carbon (0.70% by weight or more) alloy steel used as a material for tool steels, and an object thereof is to provide a continuous casting method of high carbon alloy steel having excellent cast surface and internal quality.

The present invention for achieving the above object,

In the method of continuously casting high carbon steel of 0.7% by weight or more of carbon,

Injecting molten steel with a N content of 50 ppm or less into a tundish;

The superheat degree of molten steel taken in tundish is injected into the mold while managing it at 15-20 ℃, and the upper portion of the molten steel in the mold is CaO: 22-33%, SiO ₂ : 22-33%, B ₂ O ₃ : 5-7 Injecting a mold powder including%;

The method relates to the continuous casting method of high carbon alloy tool steel, including the step of casting at a rate of 1.0-1.2m / min while secondary cooling with a non-amount of 0.4-0.5ℓ / kg to the cast from the mold Make a point.

Description

Method for continuous casting of high carbon alloy steel

The present invention relates to a continuous casting method of high carbon (0.70% by weight or more) alloy steel used as a material for tool steel, and more particularly, to a continuous casting method of high carbon alloy steel having excellent cast surface and internal quality.

In general, high carbon alloy steel containing more than 0.7% by weight of carbon is used as a tool material for tape measure, saw blade, spring, etc. in hot rolled state. The high-carbon alloy steel is mainly produced in converters and players of 100 tons of converters, but as demand increases recently, production in 350 tons of converters and players of large capacity is required.

High carbon steel with carbon content of more than 0.70% by weight is super-vacuum steel, and the solidification characteristics are not easy to solidify because the temperature of solid and liquid coexisting zone is about 100 ° C or more during the solidification degree compared to general steel with 0.70% by weight or less have. If the coagulation delay occurs, the thickness of the coagulation shell is weakened, resulting in damage to the equipment and productivity due to the occurrence of restrictive brake-out. In addition, mechanical stress is applied to the slab by bending and bending of the slab during the solidification process. In addition, due to the high carbon content, the uneven component dispersion, or segregation, in the casting structure is intensified, and it is sensitive to the occurrence of internal crack defects occurring at high temperatures during continuous casting solidification. It is recognized as difficult.

Therefore, in the related art, in order to stably obtain the internal quality of the high carbon alloy steel as much as possible, the cooling conditions were applied in the secondary cooling zone, but the surface quality deteriorated accordingly.

Accordingly, the present invention provides a continuous casting method of hot-rolled carbon alloy steel with almost no surface defects as well as the inside of the cast by appropriately controlling the overall N component and the continuous casting conditions of the target material.

Continuous casting method of the present invention for achieving the above object,

Injecting molten steel of the vacuum degassing ladle so that the content of N is 50 ppm or less into a tundish;

Superheat degree of molten steel taken in tundish is injected into the mold while managing it at 15-20 ℃, and in the molten steel upper part in the mold by weight% (hereinafter simply referred to as%) CaO: 27-33%, SiO ₂ : 22-33 %, B ₂ O ₃ : a step of introducing a mold powder comprising 5-7%;

Casting at a rate of 1.0-1.2 m / min while secondary cooling with a non-aqueous quantity of 0.4-0.5 L / kg on the cast slab from the mold.

Hereinafter, the present invention will be described in detail.

The present invention is to control the N component to minimize the degradation of the cast steel quality, to control the weak cooling conditions in the secondary cooling stand, and also to improve the casting quality, mold flux to improve the inside and surface quality of the cast, There is a characteristic. As described above, in the present invention, the conditions of (1) [N] component, (2) superheat degree, (3) mold powder, (4) secondary cooling condition, and (5) casting speed are controlled collectively. If we break down the condition of:

(1) [N] component control

In order to minimize the deterioration of cast steel quality, it is preferable to use molten steel having a content of [N] component of 50 ppm or less as the continuous casting material.

The present inventors, as shown in Figure 1, in order to confirm the solidification characteristics according to the [N] content in the solidification mechanism of the high carbon alloy steel, and produced a high carbon steel test specimen and reheated to 900 ℃ to investigate the reduction rate through the tensile test at high temperature, The higher the content of [N] in the chemical composition, the lower the reduction of area (Reduction of area) was found to promote the crack generation of the cast.

Therefore, in the present invention, the steel content [N] is managed to 50 ppm or less to prevent cracking of the cast steel. For this purpose, the [N] component of the molten steel in the RH process is controlled to 50 ppm or less, which can be obtained by sufficiently degassing at least 15 minutes with a low vacuum.

(2) degree of superheat

In the present invention, the temperature at which casting can be stably performed during continuous casting, that is, the degree of superheat (theoretical coagulation temperature-casting temperature) is set to 10 to 20 ° C.

A stable temperature is required during the casting time (typically 40-50 minutes) to complete a stable casting process (overheating) .As a low temperature, the molten steel will stop casting due to the clogging of the nozzle, and if the temperature is above a certain temperature, the solidification delay will occur. As a result, break-out occurs due to internal quality instability and weakening of the solidification shell. Due to the low-speed casting, high carbon steel requires more than 10 minutes of casting time compared to conventional materials, so proper temperature management is very difficult.

In consideration of this aspect, the present invention casts at a temperature (superheat) of 10-20 ° C. higher than the theoretical solidification temperature. If the superheat is less than 10 ℃, the temperature drops during a long casting time and successful casting is impossible.If the superheat is over 20 ℃, the temperature does not drop during casting, but the quality is improved due to weakening of the solidification shell and cracking of the cast steel. I get nervous.

(3) mold powder

As shown in Table 1 below, high carbon steel has a very low casting temperature (1475 to 1485 ° C) compared to general steel.

division High carbon steel General Carbon Steel Low carbon steel C: 0.7% or more C: 0.2% C: 0.008% (Mn: 0.3%, Si: 0.34%, P: 0.02%) Liquid line temperature (℃) 1470 1507 1520 Solidus temperature (℃) 1360 1440 1493 Solid liquid coexistence section (℃) 110 67 27

Therefore, in the case of high carbon steel, it is preferable to use a mold powder having sufficient lubricating ability even at a low temperature, and having a low viscosity and slightly cooling effect in the mold. Considering this aspect, mold powder has sufficient heat transfer function even at low temperature and contains 5-7% of B ₂ O ₃ of flux component to increase the thickness of solidification shell and to obtain the necessary consumption for lubrication performance. use. As such mold powder, there is a flux called FFM1. The flux is composed of CaO: 27-33%, SiO ₂ : 22-33%, and B ₂ O ₃ : 5-7% by weight, in addition to Al ₂ O ₃ : 3-5%, Na ₂ O : 8-12%, K ₂ O: 3-4%, F: 3-4%, Li ₂ O: about 0.6% or less, MgO: about 1.4% or less.

CaO and SiO ₂ are the basicity adjusting components in the mold powder, and when the CaO is added at 33% or the SiO ₂ is added at less than 22%, the viscosity of slag decreases. _{When 2} is added to 33% or more, the viscosity of slag becomes high, which makes it difficult to introduce slag into molten steel, thereby reducing the lubricating ability and causing break-out. B ₂ O ₃ in the mold powder component is 5-7% for securing the weak solidification shell thickness and stable inflow of slag due to increased consumption. If it exceeds 7%, there is a lot of heat transfer, so the shrinkage of cast steel is severe.

(4) secondary cooling conditions

The castings, which are continuously cast out of the mold and reach the secondary cooling zone, are sprinkled with 0.40 to 0.55 (l / kg) of cooling water, which is a specific amount of the weak cooling pattern. The most important is the temperature in the zone where the cast piece is re-stretched in the continuous casting machine (hereafter referred to as 'straightener'). This is the area where most of the cracks are extended and advanced here, usually controlled by the amount of secondary coolant.

As a result of simultaneously promoting the correlation test with the secondary cooling in the present invention, it was found that the quality is ensured only when the slab temperature passing through this part is maintained at 850 ° C. or higher, and in order to maintain the temperature of the straightener portion above 850 ° C. It was confirmed that it is obtained when the specific amount of cooling stand is managed within the range of 0.40 to 0.55 (L / kg). At temperatures other than the quantity (non-aqueous quantity), when the casting is reduced, the degree of reduction in reduction rate is insignificant, so that the effect of reducing coagulation brittleness is small, so that a good surface and internal quality cannot be obtained.

(5) casting speed

Casting speed is 1.0 ~ 1.2m / min regardless of width considering break-out and cast cooling. The ordinary steel (low coal) passes through the well reaction (0.16% [C], 1493 ℃) section and passes through the gamma region as a single phase, while high carbon alloy steels (greater than 0.70%) solidify directly into the gamma or FCC phase. When the solid coexistence zone is about 4 times higher than steel, solidification is delayed. If sufficient solidification time is not given, break-out occurs and operation accidents occur.

Therefore, the casting speed should be low speed casting in the range of 1.0 ~ 1.2m / min, which is a low speed casting speed compared to the general steel in order to ensure good internal quality and prevent operation accidents. If the casting speed is less than 1.0m / min, casting is impossible due to the long casting time (about 73 minutes). If the casting speed is over 1.2m / min, the casting is broken due to the deterioration of the solidification shell. Becomes difficult.

Continuously cast high-carbon alloy steel has a work-type rolling mill (HCR) that is hot rolled immediately after casting is completed because the stress contained in the cast during cooling at room temperature reduces the stress during cooling. Doing. In the present invention, such a work-induction rolling operation is also performed.

Hereinafter, the present invention will be described in detail through examples.

Example 1

The molten steel in the 320-ton converter was first drilled into a ladle with 2.0 tons of carburizing agent, and then dispersed by about 0.5 tons in the LF process and 0.5 tons in the RH, resulting in a weak molten steel or molten steel. There was no anxiety or visible dust. In the RH, the molten steel was degassed to control the content of the [N] component below 50 ppm, and compared to the uncontrolled molten steel (A and B in Table 2) to confirm cast quality.

division Chemical composition (% by weight) C Si Mn P S S.Al Cr N (ppm) A 0.85 0.20 0.50 0.02 0.005 0.015 0.015 40 B 0.83 0.20 0.40 0.02 0.004 0.020 - 70

In addition, the steel casting manufacturing standards were tested for continuous casting conditions varying the degree of superheat, mold powder (Table 3), and secondary cooling conditions to produce casts having a width of 1100 mm. Sulfur Print, Surface and internal quality were examined by the Macro Test and the results are shown in Table 4.

division Mold Powder Properties basicity Melting Point (℃) Viscosity Solidification temperature SGP20-04 1.01 1040 1.20 1128 FF-M1 0.96 993 1.03 1026 ST-SP / Y5 0.96 975 0.67 1031

division Steel grade Casting speed (m / min) Specific quantity (ℓ / kg) Calibration point temperature (℃) Cast steel cooling Mold Powder Cast Surface Defect Index Central segregation index Comparative Example a A 1.20 0.69 800 HCR Rolled SGP20-04 0.32 2 Inventive Example 1 A 1.10 0.47 880 FF-M1 0.01 0 Comparative Example b B 1.10 0.77 750 SGP20-04 0.05 One Inventive Example 2 B 1.20 0.47 880 FF-M1 0.4 0 Comparative Example c A 1.10 0.70 750 ST-SP / Y5 0.3 2 Comparative example d B 1.20 0.60 800 FF-M1 0.2 2 Comparative example A 1.30 0.47 860 ST-SP / Y5 0.2 3

As shown in Table 4, in the case of the invention examples (1, 2) in which the casting speed and the secondary cooling were controlled so that the calibration point passing temperature was 850 ° C. or more, the surface defects were relatively decreased, and the internal quality was also good. On the other hand, in the case of Comparative Example (ae) which increased the casting speed and slightly cooled the secondary cooling, surface defects and internal quality were also inferior. ST-SP / Y5 is CaO: 28.7%, SiO ₂ : 30.4%, Al ₂ O ₃ : 5%, Na ₂ O: 10.8%, F: 9.5%, Li ₂ O: 1%, MgO: 7.9%, B ₂ O ₃ : 1% is to be included. SGP20-04 contains CaO: 31.8%, SiO ₂ : 31.5%, Al ₂ O ₃ : 4.39%, Na ₂ O: 13.2%, F: 7.3%, Li ₂ O: 0.55%, MgO: 1.4% Will be.

Through this experiment, it was confirmed that not only the quality but also mold powder was important for securing workability. In the case of using SGP20-04 and ST-SP / Y5 powders as shown in Table 4, surface defects occurred due to insufficient thermal conductivity and consumption, and the restraint break-out was detected by the instrument and required precautions. Therefore, in order to ensure stable casting of the present invention, in the case of high carbon alloy steel, since the absolute molten steel temperature is low, the mold powder composition is designed to cool the mold somewhat while securing sufficient powder consumption by using low viscosity and low melting point powder. It is preferable.

As described above, according to the present invention can solve the problem of unstable operation and quality of high-carbon steel, there is an effect that can be stably supply the hot-burned material having excellent surface quality as well as the inside.

1 is a graph showing the change in cross-sectional reduction rate according to temperature and nitrogen content

Claims (1)

In the method of continuously casting high carbon steel of 0.7% by weight or more of carbon, Injecting molten steel with a N content of 50 ppm or less into a tundish; Superheat of molten steel taken in tundish is injected into the mold while managing it at 15-20 ℃, and CaO: 27-33%, SiO ₂ : 22-33%, B ₂ O ₃ : 5-7%, Al ₂ O ₃ : 3-5%, Na ₂ O: 8-12%, K ₂ O: 3-4%, F: 3-4%, Li ₂ O: 0.6% or less, MgO: Injecting a mold powder including 1.4% or less; Casting at a rate of 1.0-1.2 m / min while secondary cooling in a non-amount of 0.4-0.5 L / kg to the cast from the mold; continuous casting method of a high carbon alloy tool steel comprising a.