WO2020004501A1 - 鋼の製造方法 - Google Patents
鋼の製造方法 Download PDFInfo
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- WO2020004501A1 WO2020004501A1 PCT/JP2019/025471 JP2019025471W WO2020004501A1 WO 2020004501 A1 WO2020004501 A1 WO 2020004501A1 JP 2019025471 W JP2019025471 W JP 2019025471W WO 2020004501 A1 WO2020004501 A1 WO 2020004501A1
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- ladle
- steel
- molten steel
- auxiliary material
- tapping
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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
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
- C21C5/28—Manufacture of steel in the converter
- C21C5/42—Constructional features of converters
- C21C5/46—Details or accessories
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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
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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
- C21C7/076—Use of slags or fluxes as treating agents
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Definitions
- the present disclosure relates to a method for producing steel.
- the molten steel may be vigorously stirred by the bottom blow, and the nitrogen concentration in the molten steel at the end of the converter blowing is reduced to about 10 ppm.
- the molten steel is tapped from the converter to the ladle, but the tapping flow involves the atmosphere during tapping, and the nitrogen concentration in the molten steel increases. .
- molten steel whose nitrogen concentration has been reduced to about 10 ppm in a converter is tapped into a ladle without nitriding, and the vacuum degassing system removes molten steel.
- the state in which nitrogen is suppressed is maintained, and the process proceeds to the next step, continuous casting.
- Patent Literature 1 proposes a technique of tapping low-nitrogen molten steel that has been denitrified while sealing it with an inert gas.
- Patent Literature 2 in a steel receiving ladle having a lid, the fuel is burned by oxygen-enriched air to preheat the steel receiving ladle, and is replaced with combustion exhaust gas so that the atmosphere in the steel receiving ladle is reduced.
- ⁇ Circle around (3) ⁇ (3) is a method of tapping in undeoxidized or semi-deoxidized state during tapping as described in Patent Document 4, which is a general method found in many prior art documents.
- Patent Document 5 a tapping flow is received by a ladle along the inclined ladle wall, and an inert gas is supplied to a tapping outlet of a steelmaking furnace such as a converter to supply tapping flow. There has been proposed a technique of mixing an inert gas into a gas.
- Patent Document 5 is a method of reducing the size of the waterfall pot itself of tapping flow.
- the area of the reaction interface where nitrogen absorption occurs is also reduced, so that the effect of suppressing nitrogen absorption can be obtained.
- making the tapping flow along the wall of the ladle increases the risk of erosion of the refractory. For this reason, even if a waterfall pot is generated, a technique of a different cut is required that can reduce a boundary area where nitriding occurs in the waterfall pot.
- An object of the present disclosure is to provide a method of manufacturing steel that can effectively suppress nitrification at a waterfall pot formed by a tapping flow when tapping molten steel into a ladle.
- the gist of the present disclosure is as follows. ⁇ 1> a process of receiving molten steel discharged from a molten steel furnace into a ladle; Discharging the molten steel received in the ladle from the ladle and casting, When the molten steel discharged from the molten steel furnace is received in the ladle, a slag thickness T calculated by the following equation (1) is 0.02 m or more. Prior to the start of receiving the molten steel, it is placed in the bottom of the ladle or put into the ladle with the start of the steel receiving, and the molten steel discharged from the molten steel furnace is received by the ladle. Steel production method.
- the composition of the auxiliary material is CaO / Al 2 O 3 : 0.8 to 4.0 (2) 5% ⁇ SiO 2 ⁇ 10% (3) MgO ⁇ 10% (4) CaO + Al 2 O 3 + SiO 2 + MgO ⁇ 90% (5)
- ⁇ 3> The method for producing steel according to ⁇ 1> or ⁇ 2>, wherein the amount W of the auxiliary raw material is an amount that satisfies the slag thickness T calculated by the equation (1) of 0.1 m or less.
- ⁇ 4> The steel according to any one of ⁇ 1> to ⁇ 3>, wherein before the start of receiving the molten steel, the amount W of the auxiliary material is placed in the bottom of the ladle. Production method.
- ⁇ 5> The steel production according to ⁇ 4>, wherein the auxiliary material placed in the ladle is preheated, and the molten steel is received by the ladle in a state where the temperature of the auxiliary material is 800 ° C or higher.
- a method of manufacturing steel capable of effectively suppressing nitrogen absorption in a waterfall pot formed by a tapping flow when tapping molten steel into a ladle.
- the smelting furnace refers to a holding vessel for smelting molten steel, such as a converter, an AOD (Argon Oxygen Decarburization) furnace, or an electric furnace.
- Tapping refers to an operation of transferring molten metal (molten steel) held in a steelmaking furnace from a steelmaking furnace to a transport container such as a ladle.
- the term "steel receiving" means that the ladle receives molten steel from the molten steel furnace, and the tapping and the steel receiving are performed at the same timing.
- the auxiliary material refers to an additive other than iron necessary for refining molten steel.
- Ladle diameter D means the inner diameter of the ladle. Normally, the inside diameter of the bottom and top (opening) is the same inside the ladle, but if the inside diameters of the bottom and top are different, the average value of each diameter (inside diameter) at the bottom and top of the ladle is I do. When the cross section inside the ladle perpendicular to the height direction of the ladle is elliptical, the average value of the major axis and the minor axis is defined as the ladle diameter D.
- the inventor conducted a gas absorption experiment using a dissolved oxygen concentration meter and a water model device, and investigated in detail the bubble entrainment behavior and the gas absorption behavior in the waterfall pot.
- a dissolved oxygen concentration meter Usually, about 8 ppm of oxygen is dissolved in water and can be measured using a dissolved oxygen concentration meter.
- the dissolved oxygen amount was reduced to 0.8 ppm by blowing Ar beforehand.
- the amount of dissolved oxygen in the converter and the ladle of the water model device is continuously measured (see Non-Patent Document 2).
- the surface of the molten metal can be covered with molten oxide, and when tapping proceeds in this state, a situation in which the molten oxide is entrained in the waterfall pot portion is intentionally created and absorbed. Nitrogen can be suppressed. Further, it is desirable that the material (sub-raw material) entrained in the water basin at this time is in a molten state, but even if a solid phase remains, it still covers a part of the gas-liquid interface. The effect of suppressing nitrogen can be expected.
- the method for producing steel according to the present disclosure includes: Receiving the molten steel from the molten steel furnace into the ladle, Discharging the molten steel received in the ladle from the ladle and casting, When the molten steel discharged from the molten steel furnace is received in the ladle, a slag thickness T calculated by the following equation (1) is 0.02 m or more.
- T (W / ⁇ ) / (( ⁇ ⁇ D 2 ) / 4) (1)
- T Slag thickness (m)
- D Ladle diameter (m)
- W Amount of auxiliary material (kg)
- the slag composition after tapping is the same as that of the conventional method, but the timing of adding an auxiliary material in order to effectively suppress the nitrogen absorption at the beginning of tapping is earlier than before, and the slag composition is shorter than before.
- a certain amount of auxiliary material must be kept in the ladle before steel, or it must be put into the ladle together with tapping, and the amount of auxiliary material melted immediately after starting tapping. This is a big difference from the conventional method.
- ⁇ [N] under the condition where no synthetic flux was kept was 26 ppm.
- ⁇ [N] was 21 ppm, and a clear nitriding effect was recognized.
- the synthetic flux melted when it came into contact with the molten steel injected into the ladle, and the solid flux existing around the ladle was exposed.
- the composition of the synthetic flux to be stored was changed according to the composition shown in Table 1, and the effect of suppressing nitrogen absorption under the condition without preheating was investigated.
- the synthetic flux composition was CaO / Al 2 O 3 : 0.8 to 4.0 (formula (2)), 5% ⁇ SiO 2 ⁇ 10% (formula (3)), and MgO ⁇ 10% ((4 Under the conditions of the formula (1), a stable nitrogen absorption control effect was obtained.
- the synthetic flux composition when stable nitrogen absorption suppression effect was obtained is consistent with the condition that the ratio of the liquid phase near the molten steel temperature is high. It is considered that the coating effect is large.
- the synthetic flux placed in the ladle was preheated with a burner, and the synthetic flux temperature immediately before tapping was changed to investigate the effect of preventing nitrogen absorption.
- the temperature of the synthetic flux was investigated with a thermocouple installed in a ladle. As a result, as shown in FIG. 2, when the temperature of the synthetic flux was heated to 800 ° C. or higher, a remarkable effect of suppressing nitrogen absorption was obtained. On the other hand, when the preheating temperature of the synthetic flux exceeded 1150 ° C., the effect of suppressing nitrogen absorption was saturated. It is considered that the preheating shortens the time until the flux is melted and suppresses nitrogen absorption immediately after the start of tapping.
- a ladle is preheated by a burner, and then conveyed to a position immediately below a steel smelting furnace by a carrier trolley to receive the molten steel.
- auxiliary materials such as quick lime are often added to molten steel after tapping, but when applying the method for producing steel according to the present disclosure, a certain amount or more of auxiliary materials It is necessary to keep the ladle in the ladle, or to receive molten steel and to put a certain amount or more of auxiliary materials into the ladle.
- the auxiliary material is charged into the ladle before or during the preheating of the ladle.
- the auxiliary raw material is granular so as not to be dissipated by an ascending air current during preheating or during tapping, but when performing preheating, the preheating is usually performed with the ladle upper part covered with a lid. Powdered auxiliary materials can also be used.
- the slag thickness T represented by the formula (1) is stored in the ladle.
- the amount W of the auxiliary material determined so as to be 0.02 m or more (preferably 0.1 m or less, more preferably 0.05 m or less) is introduced. Further, it is necessary to melt the steel immediately after the start of tapping.
- the auxiliary raw material is put into the ladle at the same time as the start of the steel receiving, preferably, the molten steel is poured into the ladle within 10 seconds after starting to be poured, more preferably within 5 seconds, and further preferably, the molten steel is poured. At the same time, the introduction of auxiliary materials into the ladle starts.
- the slag thickness T is preferably set within 60 seconds, more preferably within 40 seconds, even more preferably within 20 seconds after the start of steel receiving.
- the feeding of the auxiliary material in an amount W of not less than .02 m is completed.
- the auxiliary raw material may be a combination of the storage of the auxiliary raw material in the ladle before the start of the steel receiving and the input of the auxiliary raw material into the ladle together with the start of the steel receiving.
- the amount W1 of the auxiliary material is put in the ladle, and further, the amount W2 of the auxiliary material is put into the ladle with the start of the steel receiving, so that the total amount of the auxiliary material (W1 + W2) is obtained.
- ) May be the amount W determined so that the slag thickness T represented by the expression (1) satisfies 0.02 m or more.
- an Al alloy or the like may be added several minutes after the start of steel receiving for the purpose of deoxidation or the like. It is not included in the amount of auxiliary raw materials in the amount W determined so that T satisfies 0.02 m or more.
- molten slag refers to a state in which auxiliary materials placed or charged in a ladle have been melted into a liquid phase or a liquid phase including a solid phase.
- a state in which the liquid phase ratio is 50% or more by calculation using general-purpose thermodynamic calculation software or the like is defined as a liquid phase slag.
- the basin refers to the part where bubbles are entrained and raised by entraining the gas phase around the injection flow when the injection flow enters the molten steel in the ladle. Occurs just below the part in contact with the molten steel. If the waterhole is covered with molten slag during tapping, the effect of reducing nitrogen according to the present disclosure can be obtained.
- the auxiliary raw material composed of the oxide before the disclosure of the steel reception or at the start of the steel reception is converted to a slag thickness T represented by the formula (1) of 0.02 m or more. (Preferably 0.1 m or less, more preferably 0.05 m or less) is placed or put into a ladle with an amount W determined to satisfy the condition, and the molten steel discharged from the molten steel furnace is received in the ladle. Thereby, molten slag can be made to exist in a waterfall pot part during steel receiving.
- the auxiliary material placed or put in the ladle is an auxiliary material composed of an oxide. Therefore, it does not include carbonates, fluorides, carbides, and the like.
- Patent Document 3 discloses an invention in which calcium carbonate is stored for the purpose of reducing the nitrogen concentration in the atmosphere in a ladle.
- calcium carbonate is not added because the purpose is to prevent the nitriding phenomenon in the waterfall pot by the molten slag on the molten steel surface. Calcium carbonate is not preferred from the viewpoint of lowering the temperature of molten steel because it involves an endothermic reaction during decomposition.
- fluorides such as fluorite does not hinder the recycling of the produced slag, so the fluorides are not added.
- a carbide such as calcium carbide is not added.
- the auxiliary material composed of oxide to be put or put in the ladle has a composition of CaO / Al 2 O 3 : 0.8 to 4.0 (formula (2)), 5% ⁇ SiO 2 ⁇ It is preferable to add after adjusting to 10% (formula (3)) and MgO ⁇ 10% (formula (4)). With such a composition range, the melting temperature of the auxiliary material can be preferably reduced. More preferably, the MgO content is 5% or more.
- the components contained in the auxiliary raw materials are acceptable even if they contain less than 5% of oxide components such as MnO and FeO in addition to the above-mentioned CaO, Al 2 O 3 , SiO 2 and MgO. It is also allowed that volatile components and impurities are contained. That is, it is preferable that the above formula (5) is satisfied.
- the auxiliary material placed in the ladle is pre-heated together with the ladle, and is preferably pre-heated to 800 ° C. or higher.
- the preheating temperature of the auxiliary material can be evaluated by measuring the surface temperature of the auxiliary material placed in the ladle with a radiation thermometer.
- auxiliary material Before tapping, before preheating the ladle, or after preheating the ladle, place a specified amount of auxiliary material at the bottom of the ladle, transport the ladle to immediately below the converter, and receive molten steel. Made steel. Alternatively, the auxiliary material was put into a ladle together with the molten steel. At the time of tapping, an alloy containing Al was charged in such a manner that it was involved in the tapping flow two minutes after the tapping was started. Further, 3 to 4 minutes after the start of tapping, additional raw materials (oxides) were additionally charged into the ladle to obtain the “final slag thickness t” shown in Table 2.
- ⁇ [N] when ⁇ [N] was 15 ppm or less, it was judged that there was a remarkable effect of suppressing nitrogen absorption, and was determined to be “A”. When ⁇ [N] is more than 20 ppm, it was set to “D” because no effect of suppressing nitrogen absorption was observed.
- Test No. No. 1 was a condition in which no auxiliary material was placed in the ladle
- No. No. 3 is a comparative example in which the auxiliary raw material is put in the ladle, but the slag thickness is out of the range of the present disclosure.
- Test no. 2 and test no. The ⁇ [N] of No. 3 was 23 to 24 ppm, and no effect of suppressing nitrogen absorption was observed.
- Test No. 4 to Test No. 17 are examples satisfying the requirements of the present disclosure, ⁇ [N] is 20 ppm or less, and the effect of suppressing nitrogen absorption is recognized.
- Test No. Test No. 10 to No. 10 The conditions up to 13 were conditions in which the composition of the auxiliary raw material to be placed in the ladle was adjusted to a suitable range, ⁇ [N] was 17 ppm or less, and it was determined that there was an excellent nitrogen absorption control effect.
- Test No. 3, 5, 9 and Test No. Test No. 14 to No. 14 The conditions up to 16 are conditions in which the preheating temperature of the stored auxiliary raw material is changed.
- Test No. 5 and test no. 7 shows that Test No.
- Test No. 7 in which the preheating temperature of the auxiliary material was high It can be seen that No. 5 has a larger effect of suppressing nitrogen absorption, and that by increasing the preheating temperature of the auxiliary material, an excellent effect of suppressing nitrogen absorption can be obtained.
- Test No. 11 and Test No. 14 is apparent from the comparison of Test No. In No. 14, in addition to controlling the composition of the auxiliary raw material to a preferable range according to the present disclosure, by setting the preheating temperature of the auxiliary raw material to 800 ° C. or higher, a remarkable nitrogen absorption suppressing effect is obtained.
- Test No. 15 and 16 are also the same.
- Test No. Reference numeral 18 is an embodiment in which the auxiliary material is put into the ladle together with the steel receiving material. ⁇ [N] was 20 ppm, which was lower than that of the comparative example, and the effect of suppressing nitrogen absorption was recognized.
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Abstract
Description
特許文献1では、脱窒された低窒素溶鋼を不活性ガスでシールしながら出鋼する技術が提案されている。
特許文献2では、蓋を有する受鋼用取鍋内において、酸素富化空気によって燃料を燃焼させ受鋼取鍋を予熱し、且つ燃焼排ガスで置換することにより受鋼用取鍋内の雰囲気中の窒素を低下せしめた後に、転炉出鋼時に受鋼用取鍋の蓋に設けられた溶鋼流を囲む円環状に配設されたノズルからアルゴンガスを溶鋼流に吹き付けることを特徴とする技術が提案されている。
特許文献3では、炭酸カルシウムを入れた取鍋内に溶鋼を出鋼し、出鋼時及び出鋼中の取鍋内の雰囲気をCO2ガス雰囲気として、溶鋼が空気と接触するのを抑制する方法が開示されている。
<1> 溶鋼炉から出鋼された溶鋼を取鍋に受鋼する工程と、
前記取鍋に受鋼した前記溶鋼を前記取鍋から排出して鋳造する工程と、を含み、
前記溶鋼炉から出鋼された前記溶鋼を前記取鍋に受鋼する際、下記(1)式によって算出されるスラグ厚みTが0.02m以上を満たす量Wの酸化物からなる副原料を、前記溶鋼の受鋼開始前に前記取鍋内の底部に入れ置きし又は受鋼開始と共に前記取鍋内に投入し、前記溶鋼炉から出鋼された前記溶鋼を前記取鍋に受鋼する、鋼の製造方法。
T=(W/ρ)/((π・D2)/4) (1)
T:スラグ厚み(m)
D:取鍋直径(m)
ρ:溶融酸化物密度(=3000kg/m3)
W:副原料の量(kg)
<2> 前記副原料の組成が、
CaO/Al2O3:0.8~4.0 (2)
5%≦SiO2≦10% (3)
MgO≦10% (4)
CaO+Al2O3+SiO2+MgO≧90% (5)
を満たしている、<1>に記載の鋼の製造方法。
ただし、(2)~(5)式中の分子記号は当該分子の含有量(質量%)を意味する。
<3> 前記副原料の量Wが、前記(1)式によって算出される前記スラグ厚みTが0.1m以下を満たす量である、<1>又は<2>に記載の鋼の製造方法。
<4> 前記溶鋼の受鋼開始前に、前記量Wの前記副原料を前記取鍋内の底部に入れ置きしておく、<1>~<3>のいずれか1つに記載の鋼の製造方法。
<5> 前記取鍋内に入れ置きした前記副原料を予熱し、前記副原料の温度が800℃以上の状態で前記溶鋼を前記取鍋に受鋼する、<4>に記載の鋼の製造方法。
溶鋼炉(製鋼炉)とは、転炉、AOD(Argon Oxygen Decarburization)炉、電気炉といった、溶鋼を溶製するための保持容器を指す。
出鋼とは、製鋼炉に保持された溶融金属(溶鋼)を製鋼炉から取鍋といった搬送用の容器に移し替える操作を指す。また、受鋼とは、溶鋼炉から出た溶鋼を取鍋が受けることを意味し、出鋼と受鋼は同じタイミングで行われることになる。
副原料とは、溶鋼を精錬するのに必要な鉄分以外の添加物を指す。本開示では、酸化物からなる副原料を対象とし、鉄以外の成分が含まれる酸化物からなるものを副原料とする。具体的には、生石灰、珪砂、カルシウムアルミネート系造滓剤、アルミナレンガ屑、焼成ドロマイト等が使用できる。
取鍋直径Dとは、取鍋の内径を意味する。通常、取鍋内は底部と上部(開口部)の内径が同じ作りになっているが、底部と上部の内径が異なる場合は、取鍋底部と上部での各直径(内径)の平均値とする。また、取鍋の高さ方向に垂直な取鍋内部の断面が楕円形である場合は、長径と短径との平均値を取鍋直径Dとする。
すなわち、本開示に係る鋼の製造方法は、
溶鋼炉から出鋼された溶鋼を取鍋に受鋼する工程と、
前記取鍋に受鋼した前記溶鋼を前記取鍋から排出して鋳造する工程と、を含み、
前記溶鋼炉から出鋼された前記溶鋼を前記取鍋に受鋼する際、下記(1)式によって算出されるスラグ厚みTが0.02m以上を満たす量Wの酸化物からなる副原料を、前記溶鋼の受鋼開始前に前記取鍋内の底部に入れ置きし又は受鋼開始と共に前記取鍋内に投入し、前記溶鋼炉から出鋼された前記溶鋼を前記取鍋に受鋼する、鋼の製造方法である。
T=(W/ρ)/((π・D2)/4) (1)
T:スラグ厚み(m)
D:取鍋直径(m)
ρ:溶融酸化物密度(=3000kg/m3)
W:副原料の量(kg)
一方、合成フラックスを取鍋内の底部から少し浮かせた壁面に吊り下げ、出鋼開始から15秒後に溶鋼面に添加される条件で出鋼したところ、Δ[N]は24ppmであり、明確な吸窒抑制効果は認められなかった。この場合、合成フラックスは出鋼末期になって溶融していることが確認されたが、最も吸窒量が多い出鋼前半から中盤にかけては添加した合成フラックスの溶融は見られていないことから、溶鋼と空気との反応界面積低減には至らなかったと推定される。
低窒素鋼を製造する場合、高炉あるいは電気炉から搬送された炭素濃度の高い溶銑を転炉などの溶鋼炉に装入し、酸素吹錬により鋼中の炭素をCOガスとして除去する。その際、溶鋼炉ではC+O=CO反応によって炉内の窒素分圧が低下することに加え、底吹きおよび上吹きによる撹拌作用とも相まって鋼中の窒素濃度は10ppm程度まで低下する。脱炭処理後の溶鋼は成分調整や脱ガスを行うため、溶鋼炉から取鍋に出鋼される。その後、成分や温度が調整された溶鋼は鋳造プロセスに供され、鋳造された後は加熱、圧延、熱処理、表面処理といった工程を経て製品として出荷される。
副原料の形態としては、予熱中もしくは出鋼時の上昇気流で散逸しないように粒状であることが好ましいが、予熱を行う際は通常取鍋上部を蓋で覆った状態で予熱を行う為、粉状の副原料も使用可能である。好ましくは取鍋が溶鋼炉直下まで搬送された時点で、遅くとも溶鋼炉からの溶鋼の出鋼開始(受鋼開始)と共に、取鍋内には、(1)式で示されたスラグ厚みTが0.02m以上(好ましくは0.1m以下、より好ましくは0.05m以下)となるように求めた量Wの副原料が投入されることが必要である。また、出鋼開始後は速やかに溶融させることが必要である。なお、受鋼開始と共に副原料を取鍋に投入する場合、好ましくは、溶鋼炉から取鍋に溶鋼が注入され始めてから10秒以内に、より好ましくは5秒以内に、更に好ましくは溶鋼の注入と同時に取鍋内への副原料の投入を開始する。また、受鋼開始と共に副原料を取鍋に投入する場合は、受鋼開始後、好ましくは60秒以内に、より好ましくは40秒以内に、更に好ましくは20秒以内に、スラグ厚みTが0.02m以上となる量Wの副原料の投入を完了する。
また、副原料は、受鋼開始前の取鍋内の副原料の入れ置きと受鋼開始と共に取鍋内への副原料の投入を組み合わせてもよい。すなわち、受鋼開始前に量W1の副原料を取鍋内に入れ置きしておき、さらに受鋼開始と共に量W2の副原料を取鍋内に投入することで、副原料の合計量(W1+W2)が、(1)式で示されたスラグ厚みTが0.02m以上を満たすように求めた量Wとなるようにしてもよい。
なお、受鋼開始から数分後、脱酸等の目的でAl合金等を添加する場合があるが、このような目的、タイミングで添加される成分は、(1)式で示されたスラグ厚みTが0.02m以上を満たすように求めた量Wの副原料に含まれない。
滝壷部とは、注入流が取鍋内の溶鋼に進入する際に注入流周りの気相を巻き込んで生じる気泡の巻込みおよび上昇が生じている部分を指し、通常は注入流が取鍋内の溶鋼と接する部分の直下に生じる。出鋼中に滝壺部が溶融スラグに覆われていれば本開示による低窒素化の効果が得られる。溶鋼炉から出鋼された溶鋼を取鍋に受鋼する際、受鋼開示前又は受鋼開始と共に酸化物からなる副原料を、(1)式で示されたスラグ厚みTが0.02m以上(好ましくは0.1m以下、より好ましくは0.05m以下)を満たすように求めた量Wで取鍋内に入れ置き又は投入し、溶鋼炉から出鋼された溶鋼を取鍋に受鋼することにより、受鋼中において滝壷部に溶融スラグを存在させることができる。
このような本開示に係る鋼の製造方法は、炭素鋼に非常に有効であるが、炭素鋼以外のステンレス鋼、合金鋼の製造にも有効である。
高炉から搬送された溶銑(炭素含有量4.5%相当)を転炉に装入し、酸素吹錬を行った。転炉吹錬後の成分は、[C]=0.06~0.14%、[Si]=0.01~0.05%、[Mn]=0.1~0.4%、[P]=0.01~0.03%、[N]=9~12ppm、残部がFeおよび不純物である。処理量は300ton規模、取鍋直径(内径)は3.9mであり、出鋼時間はおよそ5分である。出鋼前、取鍋を予熱する前段階、もしくは、取鍋予熱後に、取鍋底部に成分調整した所定量の副原料を入れ置きし、取鍋を転炉直下まで搬送した後、溶鋼を受鋼した。あるいは、溶鋼の受鋼と共に副原料を取鍋に投入した。出鋼の際、出鋼を開始してから2分後に出鋼流に巻き込ませる形でAlを含む合金を投入した。また、出鋼開始から3~4分後に取鍋内に副原料(酸化物)を追加投入することで、表2に示す「最終スラグ厚みt」とした。
試験No.10から試験No.13までは取鍋内に入れ置きする副原料の組成を好適な範囲に調整した条件であり、Δ[N]は17ppm以下となり、優れた吸窒抑制効果があったと判断した。
試験No.3,5,9および試験No.14から試験No.16までは、入れ置きした副原料の予熱温度を変化させた条件である。試験No.5と試験No.7を比較すると、副原料の予熱温度が高い試験No.5の方が吸窒抑制効果が大きく、副原料予熱温度を高くすることで、優れた吸窒抑制効果が得られることが分かる。このことは、試験No.11と試験No.14を比較しても明らかであり、試験No.14は副原料組成を本開示の好適な範囲に制御することに加え、副原料予熱温度を800℃以上とすることで、顕著な吸窒抑制効果が得られている。試験No.15、16も同様である。
試験No.18は、取鍋内に受鋼と共に副原料を投入した実施例である。Δ[N]は20ppmであり、比較例よりも低く、吸窒抑制効果が認められた。
Claims (5)
- 溶鋼炉から出鋼された溶鋼を取鍋に受鋼する工程と、
前記取鍋に受鋼した前記溶鋼を前記取鍋から排出して鋳造する工程と、を含み、
前記溶鋼炉から出鋼された前記溶鋼を前記取鍋に受鋼する際、下記(1)式によって算出されるスラグ厚みTが0.02m以上を満たす量Wの酸化物からなる副原料を、前記溶鋼の受鋼開始前に前記取鍋内の底部に入れ置きし又は受鋼開始と共に前記取鍋内に投入し、前記溶鋼炉から出鋼された前記溶鋼を前記取鍋に受鋼する、鋼の製造方法。
T=(W/ρ)/((π・D2)/4) (1)
T:スラグ厚み(m)
D:取鍋直径(m)
ρ:溶融酸化物密度(=3000kg/m3)
W:副原料の量(kg) - 前記副原料の組成が、
CaO/Al2O3:0.8~4.0 (2)
5%≦SiO2≦10% (3)
MgO≦10% (4)
CaO+Al2O3+SiO2+MgO≧90% (5)
を満たしている、請求項1に記載の鋼の製造方法。
ただし、(2)~(5)式中の分子記号は当該分子の含有量(質量%)を意味する。 - 前記副原料の量Wが、前記(1)式によって算出される前記スラグ厚みTが0.1m以下を満たす量である、請求項1又は請求項2に記載の鋼の製造方法。
- 前記溶鋼の受鋼開始前に、前記量Wの前記副原料を前記取鍋内の底部に入れ置きしておく、請求項1~請求項3のいずれか1項に記載の鋼の製造方法。
- 前記取鍋内に入れ置きした前記副原料を予熱し、前記副原料の温度が800℃以上の状態で前記溶鋼を前記取鍋に受鋼する、請求項4に記載の鋼の製造方法。
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