KR100301984B1 - Method for manufacturing billets for plate spring steel - Google Patents

Method for manufacturing billets for plate spring steel Download PDF

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KR100301984B1
KR100301984B1 KR1019960049026A KR19960049026A KR100301984B1 KR 100301984 B1 KR100301984 B1 KR 100301984B1 KR 1019960049026 A KR1019960049026 A KR 1019960049026A KR 19960049026 A KR19960049026 A KR 19960049026A KR 100301984 B1 KR100301984 B1 KR 100301984B1
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molten steel
mold
tundish
shielding material
steel
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KR1019960049026A
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KR19980029706A (en
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강석은
윤원규
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이구택
포항종합제철 주식회사
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/06—Deoxidising, e.g. killing

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Treatment Of Steel In Its Molten State (AREA)

Abstract

PURPOSE: A method is provided which not only maintains an equal level of fatigue life compared to a conventional method but also directly manufactures the molten steel into billets without passing through blooms by appropriately changing carburization and injection conditions of mole steel, thereby minimizing formation of scum in the billets. CONSTITUTION: In a billet manufacturing method by an open mold(2) comprising the processes of deoxidizing the molten steel(6) during tapping of molten steel for a pan spring from a ladle, and pouring the deoxidized molten steel into the mold in the atmosphere not using an immersion nozzle from a tundish(1), the method for manufacturing billets for plate spring steel is characterized in that the deoxidization is performed by injecting ferro silicon into the molten steel, and the pouring of the molten steel(6) is performed after maintaining a nitrogen atmosphere in a space between the tundish and the mold, wherein an amount of ferro silicon injected into the molten steel is selected in the range of 3.4 to 3.6 kg, and the nitrogen atmosphere in a space between the tundish and the mold(2) is made by supplying nitrogen into the inner space of the shielding material(4) through a nitrogen supply pipe(5) penetrating the heat resistant cloth(3) and shielding material after positioning a shielding material(4) with an inner space through which the molten steel passes between the lower part of the tundish(1) and the mold so that the molten steel discharged from the tundish is supplied into an injection hole of the mold, and covering the outer part of the shielding material with a heat resistant cloth(3).

Description

판스프링강용 강편의 제조방법Manufacturing method of steel strip for plate spring steel

본 발명은 자동차 등에 사용되는 겹판 스프링용 강편(billet)의 제조방법에 관한 것으로, 보다 상세하게는 오픈 몰드(open mold)를 이용하여 개재물을 최소화한 판스프링용 강편의 제조방법에 관한 것이다.BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a billet for a leaf spring used in automobiles and the like, and more particularly to a method for manufacturing a billet for a leaf spring in which inclusions are minimized by using an open mold.

일반적으로 자동차 하부의 충격을 완화할 목적으로 사용되는 겹판 스프링은, 연속적인 피로충격에 대한 특성이 수요가 요구에 만족되어야 하며, 무엇보다도 반제품으로 얻어지는 강편(billet)의 표면결함이 없어야 하고, 또한 강편 내부의 개재물을 최소화시킬 필요가 있다. 만일 판스프링용 강편에 개재물이 다수 존재하면, 연속적인 피로응력에 의해 개재물 주변에서부터 파괴 기점으로 작용되어 조기에 파괴를 일으키게 되므로 바람직하지 않다. 보통 판스프링에 요구되는 피로수명은 약 5만회 이상이면 되는데, 이러한 판스프링용 강편을 제조하기 위해 종래에는 우선 대강편(bloom)으로 제조한 다음 강편(billet)을 얻는 방법을 주로 이용하였다.In general, the leaf springs used for the purpose of mitigating the impact of the lower part of the automobile must satisfy the demand for the characteristics of continuous fatigue shock, and among other things, there should be no surface defects of the billet obtained by the semi-finished product, It is necessary to minimize inclusions inside the slab. If a large number of inclusions are present in the slab for a leaf spring, the continuous fatigue stress acts as a fracture origin from the periphery of the inclusions, causing premature fracture. Generally, the fatigue life required for a leaf spring is at least about 50,000 times. In order to manufacture such a leaf spring for a leaf spring, a method of obtaining a billet by first making a bloom has been used.

즉, 종래 스프링강의 제조공정을 살펴보면 아래와 같다.That is, a conventional spring steel manufacturing process will be described below.

먼저, 전로에 용강과 고철을 장입한 후 취련을 실시하는데 취련중 용강내에 용해되어 있는 산소를 제거하기 위해 용강톤당 2.0~2.5kg 정도의 알루미늄을 투입하였다. 그리고, 용강중의 종점탄소를 0.10∼0.20중량%로 한 후, 출강을 실시하고, 출강중에 목표성분을 얻기 위한 합금철 적절량 투입하고 계속하여 생석회를 용가톤당 3kg 투입하여 슬래그와 용강간의 반응을 일으켜 개재물을 제어하게 된다.First, 2.0 ~ 2.5kg of aluminum was added per ton of molten steel in order to remove the dissolved oxygen in the molten steel during the blowing after charging molten steel and scrap iron into the converter. Then, the end carbon of the molten steel is adjusted to 0.10 to 0.20 wt%, the steel is poured, and a proper amount of ferrous iron is added to obtain the target component during pouring. Then, 3 kg of the calcium oxide is charged per ton of the molten steel to cause a reaction between the slag and the molten steel The inclusions are controlled.

출강이 완료된 용강을 연속주조를 위한 설비로 이송하기 전에 승온설비로 옮겨 슬래그중의 산소를 제거할 목적으로 가탄제를 50kg 정도 투입하는데, 그 이유는 상기 알루미늄만으로는 슬래그중의 산소를 제거하기 힘들기 때문이다.Before transferring the molten steel to the equipment for continuous casting, the molten steel having been subjected to lubrication is transferred to a temperature elevating apparatus and charged with about 50 kg of a gadolinizer for removing oxygen in the slag. This is because the aluminum alone can not remove oxygen in the slag Because.

이렇게 가탄제를 투입한 후 승온을 실시하고 슬래그 정련을 위한 플럭스를 투입하는데, 플럭스로는 경소돌로마이트 약 200kg 과 규사 약 300kg 정도이다. 승온과 플럭스 투입이 완료되면 유량 5~10N㎥/hr 의 아르곤 가스를 이용하여 버브링(bubbling)을 31분 이상 실시하고, 상기 승온설비에서 처리가 끝난 용강을 다시 탈개스 설비로 이송시켜 탈수소와 탈질소의 목적으로 진공도 2Torr 이하의 고진공에서 약 15분간 처리를 실시하였다.After adding the gadolinizer, the temperature is raised and the flux for slag refining is injected. The flux is about 200 kg of light dolomite and about 300 kg of silica sand. When the temperature is raised and the flux is input, bubbling is performed for at least 31 minutes using an argon gas having a flow rate of 5 to 10 Nm 3 / hr. After the treated molten steel is transferred to the degassing equipment, For the purpose of denitrification, the treatment was carried out for about 15 minutes under a high vacuum of 2 Torr or less in vacuum degree.

그 다음, 탈개스 설비에서 처리가 끝난 용강을 연주로 이송하여 주조를 실시하는데, 기존의 방법은 단면의 크기가 250x330mm인 연주기를 이용하여 주조를 한 후 다시 강편공장으로 이송시켜 120x120mm의 크기로 강편압연한 후 수요가로 불출시킨다. 이때, 250x330mm인 연주기로 주조를 할 경우 연주기 특성상 몰드내 침적노즐이 취부되어 있는 상태로 주조를 실시하기 때문에 품질확보 측면에서는 효율적이지만, 이후 압연설비에 적합하도록 다시 강편공정을 거쳐 반제품강편(120x120mm)으로 제조해야 하는 등 불필요한 공정이 수반되는 단점이 있다.The casting is then carried out by transferring the treated steel from the gas-fired facility to the performance. In the conventional method, casting is carried out using a continuous casting machine having a cross-sectional size of 250x330 mm, After rolling, the demand is released to the customer. In this case, when casting with a 250x330mm casting machine, casting is carried out with the deposition nozzle in the mold attached due to the characteristics of the casting machine, which is efficient in terms of quality assurance. It is required to be manufactured by a conventional method.

이러한 단점을 해결하기 위해, 최근에는 판스프링용 반제품 제조시 대강편을 거치지 않고 직접 강편으로 제조하는 오픈몰드(open mold)에 의한 방법을 적용하고 있다.In order to solve these drawbacks, recently, an open mold method has been applied in which a semi-finished product for plate spring is directly manufactured into a slab without passing through a general slab.

오픈 몰드에 의한 연주방법은, 용강의 1차 응고가 발생되는 몰드와 이 몰드로 용강을 공급해주는 턴디쉬 사이에 침적노즐을 사용하지 않고 직접 용강이 몰드로 유입되도록 하는 방법으로서, 주로 연강용 저급 소재를 생산하는데 사용되는 연속주조 방식이다. 그러나, 판스프링과 같은 고급강을 주조하기 위해서는 특별한 방법이 요구된다. 즉, 종래 오픈 몰드에 의한 연주방법에서는, 턴디쉬와 몰드 사이에 형성된 공간이 대기에 노출되어, 용강이 직접 대기와 접촉하여 용강중 산화성 원소가 식(1)과 같이 재산화반응을 일으켰다.A method of performing an open mold is to introduce molten steel directly into a mold without using a dipping nozzle between a mold in which molten steel is firstly solidified and a tundish that supplies molten steel to the mold. It is the continuous casting method used to produce the material. However, special methods are required to cast high-grade steels such as leaf springs. That is, in the conventional performance method using the open mold, the space formed between the tundish and the mold is exposed to the atmosphere, and the molten steel directly comes into contact with the atmosphere, and the oxidizing element in the molten steel causes the re-oxidation reaction as shown in equation (1).

상기 식 1에 나타난 바와 같은 재산화반응에 의해 형성된 스컴(scum)이라는 비금속 개재물이 몰드 상부에 존재하여 이후 강편내에 혼입되어 버리면, 강편의 청정도가 저하되어 결국 내부품질을 열악하게 되며, 용강의 응고를 불균일하게 하여 이상 조업현상을 유발하는 것이다.If a nonmetallic inclusion called scum formed by the reification reaction as shown in the above-mentioned formula 1 is present in the upper part of the mold and then mixed in the steel billet, the cleanliness of the billet is lowered and the internal quality is lowered, Thereby causing an abnormal operation phenomenon.

이에, 본 발명은 용강의 탈탄처리 및 주입조건을 적절히 변경함으로써, 강편내에 스컴의 발생을 최소화하여 기존의 방법으로 제조된 것과 피로수명에 있어 동등판 수준을 유지하면서 대강편을 거치지 않고 직접 강편으로 제조할 수 있는 방법을 제공하고자 하는데 그 목적이 있다.Accordingly, it is an object of the present invention to minimize the generation of scum in the steel strip by appropriately changing the decarburization treatment and injection conditions of molten steel, and to provide a steel strip which is manufactured by a conventional method, And a method for manufacturing the same.

제1도는 본 발명에 따른 강편을 제조하기 위한 실링(sealing)장치의 일례도FIG. 1 is an example of a sealing device for manufacturing a steel piece according to the present invention.

제2도는 종래 및 본 발명방법을 적용했을 때 주조시간에 따른 몰드 상부에서의 대기중 산소농도 변화를 나타낸 그래프FIG. 2 is a graph showing changes in atmospheric oxygen concentration in the upper portion of the mold according to the casting time when the conventional and inventive methods are applied.

상기 목적 달성을 위한 본 발명은, 판스프링용 용강을 레들로부터 출강중에 탈산처리하고, 탈산된 용강을 턴디쉬로부터 침지노즐을 사용하지 않고 대기중에서 몰드에 주입하는 오픈몰드에 의한 강편(billet)을 제조하는 방법에 있어서,In order to accomplish the above object, the present invention provides a method of manufacturing a steel plate for a flat spring, comprising the steps of: deoxidizing a molten steel for a leaf spring from the reed during the casting of the molten steel and injecting the deoxidized molten steel from the tundish into the mold without using an immersion nozzle In the method of manufacturing,

상기 탈산처리는 훼로실리콘으로 행하고, 상기 용강의 주입은 턴디쉬와 몰드사이를 질소분위기로 한 후 실시하는 것을 특징으로 하는 판스프링강용 강편의 제조방법에 관한 것이다.Wherein the deoxidizing treatment is carried out with ferro silicon and the molten steel is injected after a nitrogen atmosphere is established between the tundish and the mold.

이하, 본 발명에 대하여 상세히 설명한다.Hereinafter, the present invention will be described in detail.

통상과 같이, 턴디쉬로부터 몰드에 용강을 주입할 때 침지노즐을 사용하지 않고, 직접 턴디쉬로부터 용강을 대기중에서 몰드에 주입시키는 오픈 몰드 방식을 적용하면, 대강편을 거치지 않고 바로 강편을 제조할 수 있다.When an open mold method in which molten steel is directly injected into a mold from a tundish from a tundish without using an immersion nozzle when injecting molten steel into a mold from a tundish as usual, .

본 발명에서는 이러한 오픈 몰드 방식을 적용하는데, 이 경우, 청정성이 우수한 강편을 얻기 위해서는, 먼저 레들에서 용강을 출강시 종래방법과는 다리 알루미늄 대신 훼로실리콘을 사용하여 탈산처리를 행하는 것이 바람직하다. 즉, 알루미늄으로 탈산처리를 하게 되면 상대적으로 산화성이 강한 알루미늄에 인해 주입시 재산화되어 오픈몰드에서 스컴이라는 개재물을 발생시킬 뿐만 아니라 매우 적은 주입구멍(약 15mmØ)을 폐쇄하기도 하는 반면, 훼로실리콘으로 탈산처리를 하게 되면 상기 (1)식에서와 같이 산화성이 적은 SiO2, Fe0 등의 스컴구성 개성물들이 형성되기는 하지만, 슬래그 배제후 주입시 이들이 재산화되는 경우가 드물게 된다.In the present invention, such an open mold method is applied. In this case, in order to obtain a steel piece having excellent cleanliness, it is preferable to perform deoxidation treatment using molybdenum silicon instead of the brass aluminum in the case of introducing molten steel first in the reel. That is, when deoxidized by aluminum, it is reoxidized during injection due to aluminum which is relatively oxidative to not only generate an inclusion of scum in the open mold but also to close very small injection holes (about 15 mm Ø ) The scum constituents such as SiO 2 and FeO having low oxidation are formed as in the above formula (1), but they are rarely reoxidized after the slag exclusion.

본 발명에 따른 탈산처리시 투입되는 훼로슬리콘의 양은, 용강톤당 3.4~3.6kg 정도로 설정하는 것이 바람직하다.The amount of the feroxylicone introduced in the deoxidation treatment according to the present invention is preferably set to about 3.4 to 3.6 kg per ton of molten steel.

한편, 본 발명에서는, 상기와 같이 탈산처리를 한 후, 종래 오픈 몰드 연주방법에서 문제시 된 바와 같이, 턴디쉬와 몰드 사이에 형성된 공간에서 용강이 대기와 접촉되어 몰드상부에서 개재물이 형성되는 것을 방지하기 위해, 오픈몰드 주입시 턴디쉬와 몰드 사이에 형성된 공간의 분위기를 질소분위기로 형성하는 것이 바람직하다. 예를 들면, 제1도와 같이, 턴디쉬의 배출구 주위를 따라 턴디쉬(1)와 몰드(2)사이에 차폐재(4)를 마련하고 상기 차폐재(4)의 측면에 연결된 질소공급관(5)을 통해 질소를 취입하는 방법을 이용하여, 턴디쉬로부터 주입되는 용강(6)이 대기와 접촉되지 않도록 하면 된다. 이때, 상기 차폐재(4)의 외부에는 내열천(3)을 이용하여 몰드(2)의 상부까지 닿도록 하면 보다 바람직하다.According to the present invention, molten steel is contacted with the atmosphere in a space formed between the tundish and the mold so that inclusions are formed at the upper portion of the mold, as dealt with in the conventional open mold playing method after deacidification as described above It is preferable to form the atmosphere of the space formed between the tundish and the mold in the nitrogen atmosphere when the open mold is injected. A shielding material 4 is provided between the tundish 1 and the mold 2 along the periphery of the outlet of the tundish and the nitrogen supply pipe 5 connected to the side of the shielding material 4 is provided The molten steel 6 injected from the tundish can be prevented from coming into contact with the atmosphere. At this time, it is more preferable that the outer surface of the shielding material 4 is brought into contact with the upper portion of the mold 2 by using the heat-resistant cloth 3.

이하, 본 발명을 실시예를 통하여 구체적으로 설명한다.Hereinafter, the present invention will be described in detail with reference to examples.

[실시예][Example]

중량%로, C:0.6%, Si:0.25%, Mn:0.85%, P:0.02% 이하, S:0.01% 이하, Al:0.03%, Cr:0.85% 및 잔부 Fe로 조성되도록 용강을 취련하고, 취련중에 99%의 Al 을 용강톤당 2.3kg 첨가하여 탈산처리를 하고, 이를 종래강으로 하였다. 또한, 상기 용강성분중 Al을 0.005% 이하로 하고, Al 대신 하기 표 1과 같은 조성을 갖는 훼로실리콘을 용강톤당 3.5kg 첨가하여 이를 발명강으로 하였다.The molten steel is made to be composed of 0.6% of C, 0.25% of Si, 0.85% of Mn, 0.02% or less of P, 0.01% or less of S, 0.03% of Al, 0.85% , 99% of Al was added to 2.3 kg of molten steel during the blowing process, followed by deoxidation, and this was used as a conventional steel. Further, 3.5 kg of ferro silicon was added to the inventive steel in place of Al and 0.005% or less of Al in the above molten steel components and having the composition shown in Table 1 below.

[표 1][Table 1]

상기와 같이 탈산처리된 종래의 용강은 정련한 다음, 250x330mm 연주기를 이용하여 대강편(bloom)으로 제조하였다. 반면, 본 발명의 용강은 정련한 다음, 도 1과 같이 실링장치를 이용하여 시간당 5~6N㎥ 의 질소를 취입하고, 상기 질소분위기하에서 오픈 몰드에 의해 주조하여 120x120mm의 강편으로 제조하였다.The conventional deoxidized molten steel was refined and then manufactured into a bloom using a 250x330 mm casting machine. On the other hand, after refining the molten steel of the present invention, nitrogen of 5 to 6 Nm 3 per hour was blown by using a sealing device as shown in FIG. 1, and cast by an open mold under the nitrogen atmosphere to prepare a 120 x 120 mm steel piece.

먼저, 종래강 및 본 발명강의 주조시, 주조시간에 따라 몰드상부에서의 대기중 산소농도를 측정하고, 그 결과를 도2에 나타내었다.First, the oxygen concentration in the atmosphere at the upper portion of the mold was measured according to the casting time at the time of casting the conventional steel and the present invention steel, and the results are shown in FIG.

도 2에 나타난 바와 같이, 실링장치를 하지 않은 종래방법의 경우에 비하여 실링장치를 취부한 본 발명의 경우 몰드상부에서의 산소농도가 0.2% 이하로 됨을 알 수 있는데, 이는 본 발명을 적용하는 경우 용강의 재산화가 이루어질 수 없음을 의미한다.As shown in FIG. 2, the oxygen concentration at the top of the mold is 0.2% or less in the case of the present invention, in which the sealing device is mounted, as compared with the conventional method without the sealing device. Which means that molten steel can not be re-oxidized.

또한, 각각의 제조된 반제품을 이용하여 통상의 방법으로 판스프링을 만들고, 만든 판스프링에서 시편을 채취하여 피로특성 및 인장특성을 시험하였다.In addition, plate springs were made by the usual method using each manufactured semi-finished product, and the specimens were taken from the plate springs, and fatigue characteristics and tensile properties were tested.

그 결과, 피로수명의 경우 종래 및 본 발명에 따른 반제품으로 제조된 경우 각각 69000회, 52108회 였고, 인장가도의 경우 각각 142kg/㎟, 150.6kg/㎟ 였다. 결국, 본 발명에 의한 강편으로 제조된 스프링의 경우 종래방법에 비하여 동등한 기계적 성질을 갖고 있는데, 종래방법의 경우 대강편(bloom)을 다시 강편(billet)로 강편 압연해야 한다는 점을 고려할 때 상당한 공정단축이 가능함을 알 수 있다.As a result, the fatigue life was 69,000 times and 52,108 times for the conventional and semi-manufactured products according to the present invention, respectively, and 142 kg / mm 2 and 150.6 kg / mm 2 for the tensile fatigue, respectively. As a result, the spring made of the steel billet according to the present invention has the same mechanical properties as those of the conventional method. Considering that the conventional method requires rolling a billet to a billet, It can be seen that shortening is possible.

한편, 훼로실리콘으로 탈산처리를 행하고 실링처리를 하지 않은 통상적인 오픈 몰드에 의한 방법과 본 발명을 비교하기 위해 각각 제조된 강편내의 청정도를 측정한 결과, 본 발명의 경우 0.05인 반면 종래의 경우 0.1이였다. 따라서, 본 발명에 따른 오픈 몰드 방식은 판스프링용 강편제조에 보다 적합함을 알 수 있었다.On the other hand, in order to compare the present invention with the conventional open mold method in which deoxidation treatment with ferrosilicon was not performed and the sealing treatment was not carried out, cleanliness in each of the manufactured steel strips was measured. As a result, it was 0.05 in the case of the present invention, . Therefore, it can be seen that the open mold method according to the present invention is more suitable for manufacturing the steel strip for plate spring.

상술한 바와 같이, 본 발명은 용강의 탈산처리 및 주입조건을 적절히 변경함으로써 청정성이 우수한 오픈 몰드방식에 의한 강편의 제조가 가능하며, 제조된 반제품 강편은 기존 대강편의 품질특성과 동등한 수준을 갖고 있어 판스프링용 강편의 공정을 크게 개선하는 효과가 있다.INDUSTRIAL APPLICABILITY As described above, the present invention enables the production of a steel strip by an open mold method with excellent cleanliness by suitably changing the deoxidation treatment and injection conditions of molten steel, and the produced semi-finished steel strip has a level equivalent to that of existing steel strip There is an effect of greatly improving the process of the slab for leaf spring.

Claims (3)

판스프링용 용강을 레들로부터 출강중에 탈산처리하고, 탈산된 용강을 턴디쉬로부터 침지노즐을 사용하지 않고 대기중에서 몰드에 주입하는 오픈몰드에 의한 강편(billet)을 제조하는 방법에 있어서,A method for producing a billet by an open mold in which a molten steel for a leaf spring is subjected to a deoxidation treatment while being discharged from a reed and a deoxidized molten steel is injected from a tundish into a mold in an atmosphere without using an immersion nozzle, 상기 탈산처리는 훼로실리콘으로 행하고, 상기 용강의 주입은 턴디쉬와 몰드사이를 질소분위기로 한 후 실시하는 것을 특징으로 하는 판스프링강용 강편의 제조방법.Wherein the deoxidation treatment is carried out with ferro silicon and the molten steel is injected after a nitrogen atmosphere is established between the tundish and the mold. 제1항에 있어서, 상기 훼로실리콘의 투입량은 3.4-3.6kg 의 범위로 선정함을 특징으로 하는 제조방법The method according to claim 1, wherein the amount of the ferro silicon is in the range of 3.4 to 3.6 kg 제1항에 있어서, 턴디쉬와 몰드사이의 질소분위기화는, 용강이 통과하도록 형성된 내부공간을 갖는 차폐재를 턴디쉬에서 배출되는 용강이 몰드의 주입구에 공급되도록 상기 턴디쉬의 하부와 몰드사이에 위치시킨 다음, 상기 차폐재의 외부를 내열천으로 감싼 후, 상기 내열천 및 차폐재를 관통하는 질소공급판을 통하여 상기 차폐재의 내부공간에 질소를 공급함으로써 이루어지는 것을 특징으로 하는 강편의 제조방법.The method according to claim 1, wherein the nitrogen atmosphere between the tundish and the mold is such that a shielding material having an internal space formed to allow molten steel to pass therethrough is disposed between the lower portion of the tundish and the mold so that molten steel discharged from the tundish is supplied to the injection port of the mold. Wherein the shielding material is wrapped with a heat-resistant cloth, and nitrogen is supplied to the inner space of the shielding material through the heat-resistant cloth and the nitrogen supply plate passing through the shielding material.
KR1019960049026A 1996-10-28 1996-10-28 Method for manufacturing billets for plate spring steel KR100301984B1 (en)

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