JP4627069B2 - Manufacturing method of high nitrogen steel - Google Patents
Manufacturing method of high nitrogen steel Download PDFInfo
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- JP4627069B2 JP4627069B2 JP2007123319A JP2007123319A JP4627069B2 JP 4627069 B2 JP4627069 B2 JP 4627069B2 JP 2007123319 A JP2007123319 A JP 2007123319A JP 2007123319 A JP2007123319 A JP 2007123319A JP 4627069 B2 JP4627069 B2 JP 4627069B2
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- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 title claims description 511
- 229910052757 nitrogen Inorganic materials 0.000 title claims description 263
- 229910000831 Steel Inorganic materials 0.000 title claims description 179
- 239000010959 steel Substances 0.000 title claims description 179
- 238000004519 manufacturing process Methods 0.000 title claims description 26
- 239000007789 gas Substances 0.000 claims description 66
- 238000007711 solidification Methods 0.000 claims description 47
- 230000008023 solidification Effects 0.000 claims description 47
- 239000007790 solid phase Substances 0.000 claims description 45
- 230000007547 defect Effects 0.000 claims description 40
- 239000007791 liquid phase Substances 0.000 claims description 39
- 229910000851 Alloy steel Inorganic materials 0.000 claims description 28
- 238000006243 chemical reaction Methods 0.000 claims description 19
- 229910045601 alloy Inorganic materials 0.000 claims description 16
- 239000000956 alloy Substances 0.000 claims description 16
- 239000000203 mixture Substances 0.000 claims description 15
- 239000007787 solid Substances 0.000 claims description 12
- 229910001566 austenite Inorganic materials 0.000 claims description 7
- 150000004767 nitrides Chemical class 0.000 claims description 6
- 239000013078 crystal Substances 0.000 claims description 5
- 229910000859 α-Fe Inorganic materials 0.000 claims description 5
- 230000015572 biosynthetic process Effects 0.000 claims description 4
- 210000000540 fraction c Anatomy 0.000 claims description 2
- 238000002844 melting Methods 0.000 description 25
- 230000008018 melting Effects 0.000 description 25
- 230000000052 comparative effect Effects 0.000 description 24
- 238000012360 testing method Methods 0.000 description 13
- 238000000034 method Methods 0.000 description 12
- 230000003068 static effect Effects 0.000 description 9
- 230000008859 change Effects 0.000 description 8
- 238000009792 diffusion process Methods 0.000 description 8
- 239000007788 liquid Substances 0.000 description 8
- 239000012071 phase Substances 0.000 description 8
- 230000015271 coagulation Effects 0.000 description 7
- 238000005345 coagulation Methods 0.000 description 7
- 238000005204 segregation Methods 0.000 description 7
- 230000006698 induction Effects 0.000 description 6
- 238000004364 calculation method Methods 0.000 description 5
- 238000010586 diagram Methods 0.000 description 5
- 239000000463 material Substances 0.000 description 5
- 230000007246 mechanism Effects 0.000 description 5
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 4
- 238000005266 casting Methods 0.000 description 4
- 229910052804 chromium Inorganic materials 0.000 description 4
- 230000003993 interaction Effects 0.000 description 4
- 239000002994 raw material Substances 0.000 description 4
- 239000000126 substance Substances 0.000 description 4
- 229910052799 carbon Inorganic materials 0.000 description 3
- 210000001787 dendrite Anatomy 0.000 description 3
- 229910001873 dinitrogen Inorganic materials 0.000 description 3
- 238000009826 distribution Methods 0.000 description 3
- 238000010276 construction Methods 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 239000012535 impurity Substances 0.000 description 2
- 229910052748 manganese Inorganic materials 0.000 description 2
- 229910052750 molybdenum Inorganic materials 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 229910052758 niobium Inorganic materials 0.000 description 2
- 229910052698 phosphorus Inorganic materials 0.000 description 2
- 239000002436 steel type Substances 0.000 description 2
- 229910052717 sulfur Inorganic materials 0.000 description 2
- 229910052721 tungsten Inorganic materials 0.000 description 2
- 229910052720 vanadium Inorganic materials 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 229910000846 In alloy Inorganic materials 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 238000012790 confirmation Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000005242 forging Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- QJGQUHMNIGDVPM-UHFFFAOYSA-N nitrogen group Chemical group [N] QJGQUHMNIGDVPM-UHFFFAOYSA-N 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 238000005192 partition Methods 0.000 description 1
- 230000000704 physical effect Effects 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 238000005096 rolling process Methods 0.000 description 1
- 238000004904 shortening Methods 0.000 description 1
- 239000002893 slag Substances 0.000 description 1
- 239000010935 stainless steel Substances 0.000 description 1
- 229910001220 stainless steel Inorganic materials 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
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- Treatment Of Steel In Its Molten State (AREA)
Description
本発明は、高窒素鋼の製造方法に関するものであり、さらに詳しくは、窒素を多量に含有する合金鋼において、ガス欠陥のない健全な鋼塊を得るための製造方法に関するものである。 The present invention relates to a method for producing high nitrogen steel, and more particularly to a production method for obtaining a healthy steel ingot free from gas defects in an alloy steel containing a large amount of nitrogen.
ステンレス鋼や耐熱鋼などの合金鋼において、窒素含有量を高めて耐食性や耐酸化性を向上させ、且つ室温強度や高温強度も向上させる試みがなされており、特許文献１や特許文献２のように高窒素鋼とその製造方法が提案されている。
一方で、窒素を多量に含有する合金鋼を製造する場合、常圧で鋳造した鋼塊はブローホールやポロシティと呼ばれる多くのガス欠陥を含むことがあり、圧延や鍛造時に割れの原因になったり、製品に残存して機械的特性の劣化を招いたりする。
そのため、このようなガス欠陥の生成を防止するため、従来から特許文献３や特許文献４、５のような、加圧雰囲気下で溶解し鋳造する方法が提案されている。
In alloy steels such as stainless steel and heat-resistant steel, attempts have been made to increase the nitrogen content to improve corrosion resistance and oxidation resistance, and to improve room temperature strength and high temperature strength. High nitrogen steel and its manufacturing method have been proposed.
On the other hand, when producing alloy steel containing a large amount of nitrogen, the steel ingot cast at normal pressure may contain many gas defects called blowholes and porosity, which may cause cracking during rolling and forging. Or remain in the product and cause deterioration of mechanical properties.
Therefore, in order to prevent the generation of such gas defects, conventionally, a method of melting and casting in a pressurized atmosphere as in Patent Document 3 and Patent Documents 4 and 5 has been proposed.
一般に、窒素ガスを含む雰囲気においては、合金鋼の溶湯の平衡窒素溶解度は、各合金化学成分の質量パーセントと日本学術振興会が推奨する相互作用助係数を用いて下記の式３により求めることができる（非特許文献１参照）。
Ｌｏｇ［％Ｎ］ｅ＝−５１８／Ｔ−１．０６３−０．００７＊［％Ｎｉ］−（−１４８／Ｔ＋０．０３３＋（１．５６／Ｔ−０．０００５３）＊［％Ｃｒ］）＊［％Ｃｒ］−（−３３．２／Ｔ＋０．００６４）＊［％Ｍｏ］−（−１４２０／Ｔ＋０．６３５）＊［％Ｖ］−（２８０／Ｔ＋０．０８１６）＊［％Ｎｂ］＋０．００２＊［％Ｗ］−０．０１２＊［％Ｃｏ］−０．０１＊［％Ａｌ］−０．１３＊［％Ｃ］−０．０４８＊［％Ｓｉ］＋０．０２＊［％Ｍｎ］−０．００７＊［％Ｓ］−０．０５９＊［％Ｐ］ ・・・（式３）
ここで、［％Ｎ］ｅ：１気圧における平衡窒素溶解度（質量パーセント）、Ｔ：溶鋼の温度である。
In general, in an atmosphere containing nitrogen gas, the equilibrium nitrogen solubility of the molten alloy steel can be obtained by the following Equation 3 using the mass percentage of each alloy chemical component and the interaction aid recommended by the Japan Society for the Promotion of Science. Yes (see Non-Patent Document 1).
Log [% N] e = −518 / T−1.063−0.007 * [% Ni] − (− 148 / T + 0.033 + (1.56 / T−0.00053) * [% Cr]) * [% Cr]-(-33.2 / T + 0.0064) * [% Mo]-(-1420 / T + 0.635) * [% V]-(280 / T + 0.0816) * [% Nb] +0.002 * [% W]-0.012 * [% Co]-0.01 * [% Al]-0.13 * [% C]-0.048 * [% Si] + 0.02 * [% Mn]- 0.007 * [% S] −0.059 * [% P] (Formula 3)
Here, [% N] e : equilibrium nitrogen solubility (mass percent) at 1 atm, T: temperature of molten steel.
また、常圧以下の雰囲気においては、溶鋼の平衡窒素溶解度はＳｉｅｖｅｒｔｚの法則に従うことが知られており、上記式によって求められた１気圧における平衡窒素溶解度と窒素分圧の１／２乗を用いて、下記の式４のように求めることができる。
［％Ｎ］Ｎ２＝［％Ｎ］ｅ＊ＰＮ２ １／２ ・・・（式４）
ここで、［％Ｎ］Ｎ２：窒素分圧ＰＮ２気圧における平衡窒素溶解度（質量パーセント）、ＰＮ２：窒素分圧（ａｔｍ）である。
Further, in an atmosphere of atmospheric pressure or lower, it is known that the equilibrium nitrogen solubility of molten steel follows Sievertz's law, using the equilibrium nitrogen solubility at 1 atm obtained from the above formula and the 1/2 power of the nitrogen partial pressure. Thus, it can be obtained as shown in Equation 4 below.
[% N] N2 = [% N] e * P N2 1/2 (Formula 4)
Here, [% N] N2 : nitrogen partial pressure PN2 equilibrium nitrogen solubility (mass percent) at PN2 atmospheric pressure, PN2 : nitrogen partial pressure (atm).
窒素分圧が常圧以上１０気圧以下の加圧雰囲気下においては、これらの式に窒素自身の相互作用助係数を加えた下記式５が成り立つことを発明者らは見出しており、合金鋼に添加する窒素量から必要となる窒素分圧を決定することができることから、溶解時に窒素分圧を調節することによって所定の窒素量を歩留り良く溶鋼に添加することが可能である。
Ｌｏｇ［％Ｎ］Ｎ２＝−５１８／Ｔ−１．０６３＋１／２＊Ｌｏｇ（ＰＮ２）−０．００７＊［％Ｎｉ］−（−１４８／Ｔ＋０．０３３＋（１．５６／Ｔ−０．０００５３）＊［％Ｃｒ］）＊［％Ｃｒ］−（−３３．２／Ｔ＋０．００６４）＊［％Ｍｏ］−（−１４２０／Ｔ＋０．６３５）＊［％Ｖ］−（２８０／Ｔ＋０．０８１６）＊［％Ｎｂ］＋０．００２＊［％Ｗ］−０．０１２＊［％Ｃｏ］−０．０１＊［％Ａｌ］−０．１３＊［％Ｃ］−０．０４８＊［％Ｓｉ］＋０．０２＊［％Ｍｎ］−０．００７＊［％Ｓ］−０．０５９＊［％Ｐ］−（０．０５１−０．００３４＊［％Ｎ］）＊［％Ｎ］ ・・・（式５）
ここで、［％Ｎ］Ｎ２：窒素分圧ＰＮ２気圧における平衡窒素溶解度（質量パーセント）、ＰＮ２：窒素分圧（ａｔｍ）である。
The inventors have found that in a pressurized atmosphere where the nitrogen partial pressure is greater than or equal to normal pressure and less than or equal to 10 atm, the following formula 5 is established by adding the interaction coefficient of nitrogen itself to these formulas. Since the necessary nitrogen partial pressure can be determined from the amount of nitrogen to be added, it is possible to add a predetermined amount of nitrogen to the molten steel with good yield by adjusting the nitrogen partial pressure during melting.
Log [% N] N2 = −518 / T−1.063 + 1/2 * Log (P N2 ) −0.007 * [% Ni] − (− 148 / T + 0.033 + (1.56 / T−0.00053) ) * [% Cr]) * [% Cr] − (− 33.2 / T + 0.0064) * [% Mo] − (− 1420 / T + 0.635) * [% V] − (280 / T + 0.0816) * [% Nb] +0.002 * [% W] −0.012 * [% Co] −0.01 * [% Al] −0.13 * [% C] −0.048 * [% Si] +0 .02 * [% Mn] −0.007 * [% S] −0.059 * [% P] − (0.051−0.0034 * [% N]) * [% N] (formula 5)
Here, [% N] N2 : nitrogen partial pressure PN2 equilibrium nitrogen solubility (mass percent) at PN2 atmospheric pressure, PN2 : nitrogen partial pressure (atm).
しかし、溶解時に平衡窒素溶解度まで窒素を添加した場合、凝固時には液相と晶出した固相との間で窒素を含めた合金化学成分の固液分配が生じ、晶出した固相は液相よりも窒素を含有することができないことから余分な窒素を液相へ排出する。その結果、加圧雰囲気下であっても合金組成によっては液相側の窒素濃度が平衡窒素溶解度を上回る現象が生じる。そのため、過剰となった分の窒素が凝固前面でガス気泡化して固相に捕捉され、凝固後の鋼塊中にガス欠陥が生成することがある。 However, when nitrogen is added to the equilibrium nitrogen solubility during dissolution, solid-liquid distribution of the alloy chemical components including nitrogen occurs between the liquid phase and the crystallized solid phase during solidification, and the crystallized solid phase is in the liquid phase. Excess nitrogen is discharged to the liquid phase because it cannot contain nitrogen. As a result, even in a pressurized atmosphere, depending on the alloy composition, a phenomenon occurs in which the nitrogen concentration on the liquid phase side exceeds the equilibrium nitrogen solubility. Therefore, the excess nitrogen is gas-bubbled on the solidification front surface and trapped in the solid phase, and gas defects may be generated in the steel ingot after solidification.
ガス気泡が生成する機構は、図１３に示す概念図により理解することができる。凝固前面には固相から固液分配により排出された窒素の濃化した濃化溶鋼が形成される。凝固時の偏析によって形成される濃化溶鋼中の合金元素および窒素の濃度は、既存の偏析の解析解を用いることによって推定することができる（例えば、非特許文献２）。特に炭素や窒素の偏析濃度は、固相内拡散を考慮した非特許文献３に示されている下記の式６を用いて推定することが望ましい。
ＣＬ／Ｃ０＝［１−｛１−βｋ／（１＋β）｝ｆｓ］（ｋ−１）／｛１−（βｋ／１＋β）｝・・・（式６）
ただし、
β＝４Ｄｓｔｆ／Ｌ２
ＣＬ：液相濃度（質量パーセント）
Ｃ０：初期濃度（質量パーセント）
ｋ：分配係数
ｆｓ：固相率
Ｄｓ：拡散係数（ｍ２／ｓ）
ｔｆ：部分凝固時間（ｓ）
Ｌ：デンドライトアーム間隔の１／２値（ｍ）である。
なお、発明者らの調査では、ｔｆとＬは連動して変化するが、３００＜ｔｆ＜４０００、５×１０−４＜Ｌ＜５×１０−３の範囲では、液相濃度はｔｆとＬが変化してもほとんど影響を受けないことが確認されている。
The mechanism by which gas bubbles are generated can be understood from the conceptual diagram shown in FIG. Concentrated molten steel enriched with nitrogen discharged from the solid phase by solid-liquid distribution is formed on the solidification front. The concentration of alloy elements and nitrogen in the concentrated molten steel formed by segregation during solidification can be estimated by using an existing analytical solution for segregation (for example, Non-Patent Document 2). In particular, it is desirable to estimate the segregation concentration of carbon or nitrogen using the following formula 6 shown in Non-Patent Document 3 in consideration of diffusion in the solid phase.
C L / C 0 = [1− {1−βk / (1 + β)} fs] (k−1) / {1− (βk / 1 + β)} (Expression 6)
However,
β = 4D s t f / L 2
C L : Liquid phase concentration (mass percent)
C 0 : Initial concentration (mass percent)
k: partition coefficient fs: solid phase ratio D s : diffusion coefficient (m 2 / s)
t f : Partial solidification time (s)
L: 1/2 value (m) of the dendrite arm interval.
In the investigation by the inventors, t f and L change in conjunction with each other. However, in the range of 300 <t f <4000, 5 × 10 −4 <L <5 × 10 −3 , the liquid phase concentration is t It has been confirmed that even if f and L change, they are hardly affected.
さらに、濃化溶鋼からガス気泡が核生成し成長するためには、非特許文献４に記載されているように、ガス気泡の内圧は下記の式７を満たさなければならない。
Ｐｂｕｂｂｌｅ≧Ｐ＋ρｈ／１０１３．２５＋１．９７４＊１０−６σ／ｒ ・・・（式７）
ここで、Ｐｂｕｂｂｌｅ：ガス気泡の内圧（ａｔｍ）、Ｐ：雰囲気圧力（ａｔｍ）、ρ：溶鋼密度（ｇ／ｃｍ３）、ガス気泡の浴高さ（ｃｍ）、σ：溶鋼表面張力（ｄｙｎ／ｃｍ）、ｒ：ガス気泡の半径（ｃｍ）である。
上記式右辺の第二項は溶鋼静圧の項であり第三項は表面張力の項であるが、一般的に第三項の寄与は小さいためガス気泡の内圧は雰囲気圧力と溶鋼静圧によって決定されるとみなすことができる。
Furthermore, in order for gas bubbles to nucleate and grow from the concentrated molten steel, as described in Non-Patent Document 4, the internal pressure of the gas bubbles must satisfy the following equation (7).
P bubble ≧ P + ρh / 1013.25 + 1.974 * 10 −6 σ / r (Expression 7)
Where P bubble : gas bubble internal pressure (atm), P: atmospheric pressure (atm), ρ: molten steel density (g / cm 3 ), gas bubble bath height (cm), σ: molten steel surface tension (dyn) / Cm), r: radius of gas bubbles (cm).
The second term on the right side of the above formula is the term of molten steel static pressure and the third term is the term of surface tension. Generally, the contribution of the third term is small, so the internal pressure of gas bubbles depends on the atmospheric pressure and the molten steel static pressure. It can be regarded as determined.
加えて、発明者らは母溶鋼と凝固前面（固相率ｆｓ＝０．３）に形成された濃化溶鋼の密度差の絶対値が０．０１（ｇ／ｃｍ３）以上の場合、固液共存領域における対流が発生してガス欠陥が生成することを発見した。母溶鋼および濃化溶鋼の密度は、各成分の密度と濃度から加成性が成り立つとして、下記の式８で計算することができ、下記の式９で溶鋼密度差を計算することができる。
ρ＝Σρｉ×Ｃｉ ・・・（式８）
ここで、ρ：溶鋼密度（ｇ／ｃｍ３）、ρｉ：各成分の密度（ｇ／ｃｍ３）、Ｃｉ：各成分の濃度（ｗｔ％）である。
Δρ＝ρ０−ρＬ ・・・（式９）
ここで、Δρ：溶鋼密度差（ｇ／ｃｍ３）、ρ０：母溶鋼の密度（ｇ／ｃｍ３）、ρＬ：濃化溶鋼の密度（ｇ／ｃｍ３）である。
この場合のガス欠陥が生成する機構は、図１４に示す概略図により理解することができる。密度差に起因する対流がデンドライト樹枝の間で発生すると母溶鋼と濃化溶鋼は不連続に接触することになり、窒素を多く含有する濃化溶鋼から窒素溶解度の小さい母溶鋼に多量の窒素が供給されるため、ガス気泡が生成することになる。
ρ = Σρ i × C i (Equation 8)
Here, ρ: molten steel density (g / cm 3 ), ρ i : density of each component (g / cm 3 ), C i : concentration of each component (wt%).
Δρ = ρ 0 −ρ L (Equation 9)
Here, Δρ is a molten steel density difference (g / cm 3 ), ρ 0 is a mother molten steel density (g / cm 3 ), and ρL is a concentrated molten steel density (g / cm 3 ).
The mechanism by which gas defects are generated in this case can be understood from the schematic diagram shown in FIG. When convection due to density difference occurs between dendritic dendrites, the mother molten steel and the concentrated molten steel will come into discontinuous contact, and a large amount of nitrogen will flow from the concentrated molten steel containing a large amount of nitrogen to the molten mother steel with a low nitrogen solubility. Since it is supplied, gas bubbles are generated.
ガス欠陥の生成を防止して健全な鋼塊を製造するために、従来の技術では前記のように高圧雰囲気を保って鋳造する方法が提案されている。特許文献２では鋳造時あるいは凝固時に全圧を窒素分圧の３倍よりも大きくするとしており、特許文献１では固相における窒素の平均溶解度に相当する窒素分圧の１．５倍以上の全圧を加えて鋳造するとしている。また、特許文献４では凝固時の鋼中に溶存し得る濃度を超えない窒素量を溶鋼に添加して窒素分圧を高く保ったまま鋳造するとしている。 In order to prevent the generation of gas defects and produce a healthy steel ingot, the conventional technique has proposed a method of casting while maintaining a high-pressure atmosphere as described above. In Patent Document 2, the total pressure is set to be larger than three times the nitrogen partial pressure during casting or solidification. In Patent Document 1, the total pressure is 1.5 times or more of the nitrogen partial pressure corresponding to the average solubility of nitrogen in the solid phase. It is supposed to cast with pressure. In Patent Document 4, the amount of nitrogen not exceeding the concentration that can be dissolved in the steel during solidification is added to the molten steel, and casting is performed while keeping the nitrogen partial pressure high.
しかし、いずれの先行技術も凝固偏析に基づくガス欠陥の生成現象を正確に反映しておらず、理論的に窒素分圧と全圧の関係を比例関係で整理することは困難であることから、ガス分圧の調節については根拠に乏しい。また、窒素固溶度の大きいオーステナイト系鋼では、窒素ガスのみの加圧雰囲気でも健全な鋼塊を鋳造することは可能であるが、オーステナイト系鋼以外の鋼種では、溶鋼の窒素含有量が平衡窒素溶解度に到達する前に鋳造しなければ健全な鋼塊を得られないので、窒素含有量が変わるたびに操業条件を変える必要が生じる。 However, none of the prior art accurately reflects the phenomenon of gas defect generation based on solidification segregation, and it is theoretically difficult to organize the relationship between nitrogen partial pressure and total pressure in a proportional relationship. There is little evidence for adjusting the gas partial pressure. In addition, with austenitic steels with high nitrogen solid solubility, it is possible to cast a healthy steel ingot even in a pressurized atmosphere of only nitrogen gas, but with steel types other than austenitic steels, the nitrogen content of the molten steel is balanced. Since a healthy steel ingot cannot be obtained unless it is cast before reaching nitrogen solubility, it is necessary to change operating conditions each time the nitrogen content changes.
加えて、いずれの先行技術においても大型鋼塊を製造する場合には上述の溶鋼静圧が無視できなくなるにもかかわらず考慮されていない。さらに、限定された製造設備で試行錯誤した上での開発技術であることから、必要となる圧力を過大に見積もる傾向があり、製造設備に依存し適用できる鋼種に制限があるなど汎用性に欠けている。
このように、従来の方法では不必要に高圧雰囲気に保つことから、製造設備に相応の耐圧構造が必要となるため製造コストの増大を招くといった問題が生じる。さらに、この問題を回避するために比較的低圧で鋳造することができる雰囲気圧力を探そうとすると、合金組成や鋼塊サイズ及び窒素添加量が変わるたびに試行錯誤しなければならなくなるという問題を有している。
In addition, in any of the prior arts, when manufacturing a large steel ingot, the above-mentioned molten steel static pressure is not taken into consideration even though it cannot be ignored. Furthermore, because it is a development technology after trial and error in a limited production facility, there is a tendency to overestimate the required pressure, and there is a lack of versatility such as restrictions on the types of steel that can be applied depending on the production facility. ing.
Thus, since the conventional method keeps the high-pressure atmosphere unnecessarily, a pressure-resistant structure corresponding to the manufacturing equipment is required, which causes an increase in manufacturing cost. Furthermore, when trying to find an atmospheric pressure that can be cast at a relatively low pressure in order to avoid this problem, there is a problem that trial and error must be performed each time the alloy composition, the steel ingot size, and the nitrogen addition amount change. Have.
本発明は、上記事情を背景としてなされたものであり、ガス欠陥の生成現象を正確に反映してガス欠陥の生成を効果的に防止する窒素分圧と全圧との調整を容易に行うことを可能にする高窒素鋼の製造方法を提供することを目的とする。 The present invention has been made against the background of the above circumstances, and easily adjusts the partial pressure of nitrogen and the total pressure to accurately prevent the generation of gas defects by accurately reflecting the phenomenon of generation of gas defects. It aims at providing the manufacturing method of the high nitrogen steel which makes it possible.
すなわち、本発明の高窒素鋼の製造方法のうち請求項１記載の発明は、窒素を多量に含有させると常圧雰囲気下では鋼塊中にガス欠陥が生成する合金鋼の溶鋼を、加圧雰囲気下で製造するにあたり、所定の窒素含有量を達成する窒素分圧を有する混合ガス雰囲気中で溶解し、平衡状態になった溶鋼から鋼塊を造塊する時、凝固前面における濃化溶鋼と母溶鋼との密度差の絶対値が０．０１（ｇ／ｃｍ ３ ）未満の場合は、下記式１に示す過剰窒素指数ＩＮＤＥＸ（１）が０よりも小さい値を持つように前記窒素分圧と前記全圧を調節して溶鋼を凝固させることを特徴とする高窒素鋼の製造方法。
ＩＮＤＥＸ（１）＝Ｃ Ｌ Ｎ （Ｐ Ｎ２ ，ｆｓ．ｐ）−Ｃ ｅ Ｎ （Ｐ ＴＯＴＡＬ ，ｆｓ．ｐ）・・・（式１）
ここで、
ＩＮＤＥＸ（１）：過剰窒素指数（ｗｔｐｐｍ）
ｆｓ．ｐ：包晶反応固相率
Ｃ Ｌ Ｎ （Ｐ Ｎ２ ，ｆｓ．ｐ）：窒素分圧Ｐ Ｎ２ （ａｔｍ）条件での包晶反応固相率における液相中の窒素濃度（ｗｔｐｐｍ）
Ｃ ｅ Ｎ （Ｐ ＴＯＴＡＬ ，ｆｓ．ｐ）：全圧Ｐ ＴＯＴＡＬ （ａｔｍ）条件での包晶反応固相率における液相の許容窒素溶解度（ｗｔｐｐｍ）
That is, the invention according to claim 1 of the method for producing high nitrogen steel of the present invention is to pressurize molten steel of alloy steel in which a gas defect is generated in a steel ingot under a normal pressure atmosphere when a large amount of nitrogen is contained. When producing steel ingots from molten steel that has been melted in a mixed gas atmosphere having a nitrogen partial pressure that achieves a predetermined nitrogen content and in equilibrium, when manufacturing in an atmosphere, When the absolute value of the density difference from the mother steel is less than 0.01 (g / cm 3 ), the nitrogen partial pressure is set so that the excess nitrogen index INDEX (1) shown in the following formula 1 has a value smaller than 0. And a method for producing high nitrogen steel, wherein the molten steel is solidified by adjusting the total pressure.
INDEX (1) = C L N (P N2 , fs.p) −C e N (P TOTAL , fs.p) (Formula 1)
here,
INDEX (1): excess nitrogen index (wtppm)
fs. p: Peritectic reaction solid fraction
C L N (P N2 , fs.p): Nitrogen concentration in the liquid phase (wtppm) in the peritectic reaction solid phase ratio under the condition of nitrogen partial pressure P N2 (atm)
C e N (P TOTAL , fs.p): Permissible nitrogen solubility (wtppm) of the liquid phase in the peritectic reaction solid phase ratio under the total pressure P TOTAL (atm) condition
なお、本発明では、溶鋼に加えられる雰囲気圧力と溶鋼静圧を加算した圧力を全圧と表記することとする。また、本発明では、包晶反応固相率は、凝固時にフェライト凝固からオーステナイト凝固に切り替わる固相率であり、固相率全率においてオーステナイト凝固する場合は０とする。 In the present invention, the pressure obtained by adding the atmospheric pressure applied to the molten steel and the molten steel static pressure is referred to as total pressure. In the present invention, the peritectic reaction solid phase ratio is a solid phase ratio that switches from ferrite solidification to austenite solidification at the time of solidification, and is 0 when the austenite solidification occurs at the total solid phase ratio.
請求項２記載の高窒素鋼の製造方法の発明は、窒素を多量に含有させると常圧雰囲気下では鋼塊中にガス欠陥が生成する合金鋼の溶鋼を、加圧雰囲気下で製造するにあたり、所定の窒素含有量を達成する窒素分圧を有する混合ガス雰囲気中で溶解し、平衡状態になった溶鋼から鋼塊を造塊する時、凝固前面における濃化溶鋼と母溶鋼との密度差の絶対値が０．０１（ｇ／ｃｍ３）以上の場合は、下記式２に示す過剰窒素指数ＩＮＤＥＸ（２）が０よりも小さい値を持つように前記窒素分圧と前記全圧を調節して溶鋼を凝固させることを特徴とする高窒素鋼の製造方法。
ＩＮＤＥＸ（２）＝ＣＬ Ｎ（ＰＮ２，ｆｓ＝０．７５）−Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ＝０）・・・（式２）
ここで、
ＩＮＤＥＸ（２）：過剰窒素指数（ｗｔｐｐｍ）
ＣＬ Ｎ（ＰＮ２，ｆｓ＝０．７５）：窒素分圧ＰＮ２（ａｔｍ）条件での固相率０．７５における液相中の窒素濃度（ｗｔｐｐｍ）
Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ＝０）：全圧ＰＴＯＴＡＬ（ａｔｍ）条件での固相率０における液相の許容窒素溶解度（ｗｔｐｐｍ）
The invention of the method for producing high nitrogen steel according to claim 2 is for producing a molten steel of alloy steel in which a gas defect is generated in a steel ingot under a normal pressure atmosphere when a large amount of nitrogen is contained. When a steel ingot is made from molten steel that has been melted in a mixed gas atmosphere having a nitrogen partial pressure to achieve a predetermined nitrogen content and is in an equilibrium state, the density difference between the concentrated molten steel and the mother molten steel at the solidification front When the absolute value of is not less than 0.01 (g / cm 3 ), the nitrogen partial pressure and the total pressure are adjusted so that the excess nitrogen index INDEX (2) shown in the following formula 2 has a value smaller than 0. And then solidifying the molten steel, a method for producing high nitrogen steel.
INDEX (2) = C L N (P N2 , fs = 0.75) −C e N (P TOTAL , fs = 0) (Equation 2)
here,
INDEX (2): excess nitrogen index (wtppm)
C L N (P N2 , fs = 0.75): nitrogen concentration (wtppm) in the liquid phase at a solid phase ratio of 0.75 under the condition of nitrogen partial pressure P N2 (atm)
C e N (P TOTAL , fs = 0): Permissible nitrogen solubility (wtppm) of the liquid phase at a solid phase ratio of 0 under the total pressure P TOTAL (atm) condition
請求項３記載の高窒素鋼の製造方法の発明は、請求項１または２に記載の発明において、前記合金鋼は、前記造塊時に液相線温度において晶出する初晶が窒化物以外となる窒素含有量および合金組成を有することを特徴とする。 The invention of the method for producing high nitrogen steel according to claim 3 is the invention according to claim 1 or 2 , wherein the alloy steel has a primary crystal that crystallizes at a liquidus temperature at the time of ingot formation other than nitride. It has the following nitrogen content and alloy composition.
前記したように濃化溶鋼に濃化する合金成分から、所定の窒素分圧における平衡窒素溶解度が既存の偏析の解析解と前記した相互作用助係数などを用いた算出方法（式５）などによって求められ、さらに、固相内拡散を考慮した濃化溶鋼中の窒素濃度が式６などを用いて算出することができる。
前記した相互作用助係数などを用いた算出方法（式５）の窒素分圧の項に、雰囲気圧力と溶鋼静圧の和を導入することによって液相の許容できる窒素溶解度を求めることができる。
そして、凝固前面に形成される濃化溶鋼中の窒素濃度がこの許容窒素溶解度を下回ればガス気泡は核生成せず、凝固後の鋼塊にはガス欠陥は生成しないものと考えられる。
As described above, from the alloy components concentrated in the concentrated molten steel, the equilibrium nitrogen solubility at a predetermined nitrogen partial pressure is calculated by the existing analytical solution for segregation and the calculation method (formula 5) using the above-mentioned interaction coefficient, etc. Further, the nitrogen concentration in the concentrated molten steel in consideration of diffusion in the solid phase can be calculated using Equation 6 or the like.
The allowable nitrogen solubility of the liquid phase can be obtained by introducing the sum of the atmospheric pressure and the molten steel static pressure into the term of the nitrogen partial pressure in the calculation method (formula 5) using the interaction aid coefficient described above.
And if the nitrogen concentration in the concentrated molten steel formed on the solidification front surface is lower than the allowable nitrogen solubility, gas bubbles are not nucleated, and it is considered that no gas defects are generated in the solidified steel ingot.
なお、加圧雰囲気下における溶解では、所定の窒素含有量を達成するのに必要十分な窒素分圧に調節する必要がある。その際、過分に窒素分圧を高めると余分な窒素が溶鋼に溶存してしまい、鋼塊を造塊する際にガス欠陥が生成してしまうため、窒素以外のガスも含んだ混合ガス雰囲気にして全圧を調節することが望ましい。窒素以外のガスには溶鋼と反応する空気や酸素が主成分となることは好ましくなく、溶鋼と反応しない希ガス（Ｈｅ、Ｎｅ、Ａｒ等）のような不活性ガスが主成分となることが望ましい。
また、加圧雰囲気下における造塊では、溶鋼に添加した窒素が雰囲気に放出されて所定の窒素含有量を下回らないように、凝固中も平衡窒素溶解度に合わせて窒素分圧を調節することが望ましい。
Note that, in melting under a pressurized atmosphere, it is necessary to adjust to a nitrogen partial pressure that is necessary and sufficient to achieve a predetermined nitrogen content. At that time, excessively increasing the nitrogen partial pressure causes excess nitrogen to be dissolved in the molten steel, and gas defects are generated when the steel ingot is formed, so a mixed gas atmosphere containing gas other than nitrogen is created. It is desirable to adjust the total pressure. It is not preferable that the gas other than nitrogen is mainly composed of air or oxygen that reacts with the molten steel, and an inert gas such as a rare gas (He, Ne, Ar, etc.) that does not react with the molten steel. desirable.
In addition, in ingot formation under a pressurized atmosphere, the nitrogen partial pressure can be adjusted according to the equilibrium nitrogen solubility during solidification so that the nitrogen added to the molten steel is not released into the atmosphere and falls below the predetermined nitrogen content. desirable.
本発明で定義した過剰窒素指数ＩＮＤＥＸ（１）は、凝固時の偏析過程で濃化していく濃化溶鋼中の窒素濃度と濃化溶鋼の許容窒素溶解度の差で表される過剰窒素濃度の最大値であり、図１に示すように、過剰窒素濃度は固相率を横軸にとった図面上ではピークを持った曲線となる。過剰窒素濃度が０以上となる領域は、凝固中に過剰となる窒素の総量を示すことになり、この過剰窒素がガス気泡化することによって鋼塊中にガス欠陥が生成する。 The excess nitrogen index INDEX (1) defined in the present invention is the maximum of the excess nitrogen concentration represented by the difference between the nitrogen concentration in the concentrated molten steel that is concentrated in the segregation process during solidification and the allowable nitrogen solubility of the concentrated molten steel. As shown in FIG. 1, the excess nitrogen concentration is a curve having a peak on the drawing in which the solid fraction is taken on the horizontal axis. The region where the excess nitrogen concentration becomes 0 or more indicates the total amount of nitrogen that becomes excessive during solidification, and gas defects are generated in the steel ingot due to the formation of gas bubbles in the excess nitrogen.
発明者らは、凝固前面における濃化溶鋼と母溶鋼との密度差の絶対値が０．０１（ｇ／ｃｍ３）未満の場合、凝固中に晶出する固相がフェライト相からオーステナイト相に切り替わる包晶反応凝固が開始される固相率において過剰窒素濃度が最大値を示すことを発見し、これを過剰窒素指数ＩＮＤＥＸ（１）と定義している。過剰窒素指数ＩＮＤＥＸ（１）が０となる全圧はガス欠陥が生成する臨界の圧力を意味する。 When the absolute value of the density difference between the concentrated molten steel and the mother molten steel at the solidification front is less than 0.01 (g / cm 3 ), the inventors changed the solid phase that crystallizes during solidification from the ferrite phase to the austenite phase. It has been found that the excess nitrogen concentration shows the maximum value at the solid phase ratio at which peritectic reaction coagulation starts, and this is defined as the excess nitrogen index INDEX (1). The total pressure at which the excess nitrogen index INDEX (1) becomes 0 means the critical pressure at which gas defects are generated.
また、本発明で定義した過剰窒素指数ＩＮＤＥＸ（２）は、固相率０．７５における濃化溶鋼の窒素濃度と固相率０に相当する母溶鋼の許容窒素溶解度の差で表される過剰窒素濃度であり、母溶鋼と濃化溶鋼の密度差が大きいと、固液共存領域において対流が発生し、流動限界固相率近傍における高窒素濃度の濃化溶鋼が固相率０である母溶鋼と不連続に接触する。この時の過剰窒素濃度が０以上となると、過剰窒素がガス気泡化し鋼塊中にガス欠陥が生成する。 The excess nitrogen index INDEX (2) defined in the present invention is an excess represented by the difference between the nitrogen concentration of the concentrated molten steel at a solid phase ratio of 0.75 and the allowable nitrogen solubility of the mother molten steel corresponding to a solid phase ratio of 0. If the density difference between the molten steel and the concentrated molten steel is large, convection occurs in the solid-liquid coexistence region, and the concentrated molten steel with a high nitrogen concentration near the flow limit solid phase ratio has a solid phase ratio of 0. Contact discontinuously with molten steel. If the excess nitrogen concentration at this time becomes 0 or more, excess nitrogen gas bubbles and gas defects are generated in the steel ingot.
発明者らは、凝固前面における濃化溶鋼と母溶鋼との密度差の絶対値が０．０１（ｇ／ｃｍ３）以上の場合に、固液共存領域における対流が発生してガス欠陥が生成することを発見し、固相率０．７５における濃化溶鋼の窒素濃度と固相率０に相当する母溶鋼の許容窒素溶解度の差で表される過剰窒素濃度を過剰窒素指数ＩＮＤＥＸ（２）と定義している。過剰窒素指数ＩＮＤＥＸ（２）が０となる全圧はガス欠陥が生成する臨界の圧力を意味する。
本発明では、凝固前面における濃化溶鋼と母溶鋼の密度差の絶対値によって、いずれかの過剰窒素指数を選択し、その過剰窒素指数が０よりも小さい値になるように全圧を調節することによって凝固中の過剰窒素を解消することから、先行技術より低圧条件であっても安定的に健全な鋼塊を製造することが可能となる。
When the absolute value of the density difference between the concentrated molten steel and the mother molten steel at the solidification front is 0.01 (g / cm 3 ) or more, the inventors generate convection in the solid-liquid coexistence region and generate gas defects. The excess nitrogen concentration represented by the difference between the nitrogen concentration of the concentrated molten steel at a solid phase ratio of 0.75 and the allowable nitrogen solubility of the mother molten steel corresponding to a solid phase ratio of 0 is expressed as the excess nitrogen index INDEX (2). It is defined as The total pressure at which the excess nitrogen index INDEX (2) becomes 0 means the critical pressure at which gas defects are generated.
In the present invention, any excess nitrogen index is selected according to the absolute value of the density difference between the concentrated molten steel and the mother molten steel on the solidification front surface, and the total pressure is adjusted so that the excess nitrogen index becomes a value smaller than 0. By eliminating excess nitrogen during solidification, it is possible to produce a stable steel ingot stably even under lower pressure conditions than in the prior art.
本発明の式は、液相線温度において晶出する初晶がフェライト相であってもオーステナイト相であっても適用することが可能であり、包晶反応凝固が生じる合金鋼であっても適用できる。
本発明では、窒素を添加する合金鋼に特に制限を設けず、窒素含有量も特に限定されるものではない。しかし、液相線温度において晶出する初晶が窒化物となる合金鋼では、窒化物が粗大に晶出し、ガス欠陥の無い鋼塊であっても熱間加工性などの機械的特性を劣化させることから、本発明を適用することは好ましくない。
また、本発明は、溶鋼に窒素を添加する方法は気相−液相間における窒素吸収反応に限定するものではなく、加圧雰囲気の窒素分圧を必要十分な窒素分圧に調節するかぎりにおいては、窒化合金を溶鋼に添加するような固相−液相間反応やスラグに添加した窒化物から溶鋼に窒素を吸収させるようなスラグ−液相間反応による窒素添加方法であっても構わない。
The formula of the present invention can be applied regardless of whether the primary crystal that crystallizes at the liquidus temperature is a ferrite phase or an austenite phase, and even an alloy steel that causes peritectic reaction solidification. it can.
In the present invention, no particular limitation is imposed on the alloy steel to which nitrogen is added, and the nitrogen content is not particularly limited. However, in alloy steels in which the primary crystals that crystallize at the liquidus temperature are nitrides, the nitrides crystallize coarsely, and even mechanical ingots with no gas defects deteriorate mechanical properties such as hot workability. Therefore, it is not preferable to apply the present invention.
In the present invention, the method for adding nitrogen to molten steel is not limited to the nitrogen absorption reaction between the gas phase and the liquid phase, as long as the nitrogen partial pressure in the pressurized atmosphere is adjusted to a necessary and sufficient nitrogen partial pressure. May be a method of adding nitrogen by a solid phase-liquid phase reaction in which a nitride alloy is added to molten steel or a slag-liquid phase reaction in which nitrogen is absorbed into molten steel from a nitride added to slag. .
さらに、本発明では、加圧雰囲気を維持できる設備であれば精錬・溶解方法に関する制限は特に無い。ただし、凝固収縮に起因するサクションなどによって凝固中に減圧環境が生じるとガス欠陥は生成しやすくなることから、指向性凝固が得やすく凝固方向を調節でき、ザクや引け巣などが生成する最終凝固部を押湯部に制御できるような造塊方法が望ましく、加圧雰囲気を維持できるエレクトロスラグ再溶解法やアーク再溶解法などを用いると確実に健全な鋼塊を製造することができる。 Furthermore, in the present invention, there is no particular limitation on the refining / dissolving method as long as the facility can maintain a pressurized atmosphere. However, if a reduced pressure environment occurs during solidification due to suction due to solidification shrinkage, etc., gas defects are likely to be generated. An ingot forming method in which the part can be controlled to a feeder part is desirable, and a sound steel ingot can be reliably produced by using an electroslag remelting method or an arc remelting method that can maintain a pressurized atmosphere.
すなわち、本発明の高窒素鋼の製造方法は、窒素含有合金鋼を加圧雰囲気下で製造するにあたり、所定の窒素含有量を達成する窒素分圧を有する混合ガス雰囲気中で前記合金鋼を溶解し、平衡状態になった溶鋼から鋼塊を造塊する際に、凝固時の固相率および窒素分圧に基づく液相中の窒素濃度が、凝固時の固相率および全圧に基づく液相中の許容窒素溶解度よりも小さくなるように、前記窒素分圧と前記全圧を調節して前記溶鋼を凝固させるので、物性値の情報によって確認試験を実施する必要も無く操業条件が決定できる。
前記窒素分圧と前記全圧の調節は、下式に示す過剰窒素指数ＩＮＤＥＸ（１）あるいはＩＮＤＥＸ（２）が０よりも小さい値を持つようにして行うことができる。
That is, in the method for producing high nitrogen steel of the present invention, when producing a nitrogen-containing alloy steel in a pressurized atmosphere, the alloy steel is dissolved in a mixed gas atmosphere having a nitrogen partial pressure that achieves a predetermined nitrogen content. However, when ingots are made from molten steel in an equilibrium state, the nitrogen concentration in the liquid phase based on the solid fraction and the nitrogen partial pressure during solidification is the same as the liquid concentration based on the solid fraction and total pressure during solidification. Since the molten steel is solidified by adjusting the nitrogen partial pressure and the total pressure so as to be smaller than the allowable nitrogen solubility in the phase, it is possible to determine the operating conditions without the need to carry out a confirmation test based on the information on the physical property values. .
The nitrogen partial pressure and the total pressure can be adjusted so that the excess nitrogen index INDEX (1) or INDEX (2) shown in the following equation has a value smaller than zero.
凝固前面における濃化溶鋼と母溶鋼との密度差の絶対値が０．０１（ｇ／ｃｍ３）未満の場合
ＩＮＤＥＸ（１）＝ＣＬ Ｎ（ＰＮ２，ｆｓ．ｐ）−Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ．ｐ）・・・（式１）
ここで、
ＩＮＤＥＸ（１）：過剰窒素指数（ｗｔｐｐｍ）
ｆｓ．ｐ：包晶反応固相率
ＣＬ Ｎ（ＰＮ２，ｆｓ．ｐ）：窒素分圧ＰＮ２（ａｔｍ）条件での包晶反応固相率における液相中の窒素濃度（ｗｔｐｐｍ）
Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ．ｐ）：全圧ＰＴＯＴＡＬ（ａｔｍ）条件での包晶反応固相率における液相の許容窒素溶解度（ｗｔｐｐｍ）
When the absolute value of the density difference between the concentrated molten steel and the mother molten steel at the solidification front is less than 0.01 (g / cm 3 ) INDEX (1) = C L N (P N2 , fs.p) −C e N ( P TOTAL , fs.p) (Formula 1)
here,
INDEX (1): excess nitrogen index (wtppm)
fs. p: peritectic reaction solid phase ratio C L N (P N2 , fs.p): nitrogen concentration in liquid phase (wtppm) in peritectic reaction solid phase ratio under nitrogen partial pressure P N2 (atm) condition
C e N (P TOTAL , fs.p): Permissible nitrogen solubility (wtppm) of the liquid phase in the peritectic reaction solid phase ratio under the total pressure P TOTAL (atm) condition
凝固前面における濃化溶鋼と母溶鋼との密度差の絶対値が０．０１（ｇ／ｃｍ３）以上の場合
ＩＮＤＥＸ（２）＝ＣＬ Ｎ（ＰＮ２，ｆｓ＝０．７５）−Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ＝０）・・・（式２）
ここで、
ＩＮＤＥＸ（２）：過剰窒素指数（ｗｔｐｐｍ）
ＣＬ Ｎ（ＰＮ２，ｆｓ＝０．７５）：窒素分圧ＰＮ２（ａｔｍ）条件での固相率０．７５における液相中の窒素濃度（ｗｔｐｐｍ）
Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ＝０）：全圧ＰＴＯＴＡＬ（ａｔｍ）条件での固相率０における液相の許容窒素溶解度（ｗｔｐｐｍ）
When the absolute value of the density difference between the concentrated molten steel and the mother molten steel at the solidification front is 0.01 (g / cm 3 ) or more INDEX (2) = C L N (P N2 , fs = 0.75) −C e N (P TOTAL , fs = 0) (Expression 2)
here,
INDEX (2): excess nitrogen index (wtppm)
C L N (P N2 , fs = 0.75): nitrogen concentration (wtppm) in the liquid phase at a solid phase ratio of 0.75 under the condition of nitrogen partial pressure P N2 (atm)
C e N (P TOTAL , fs = 0): Permissible nitrogen solubility (wtppm) of the liquid phase at a solid phase ratio of 0 under the total pressure P TOTAL (atm) condition
本発明の製造方法により、常圧雰囲気ではガス欠陥が生成して製品化が困難であった高窒素鋼を、過分な耐圧設備の建設や製造コストの増大を招くことなく、且つ鋼種が変わるたびに試行錯誤を繰り返すことなく、必要最小限の耐圧設備で確実に健全な鋼塊を製造することができる。また、先行技術よりも低圧条件で製造ができることから原材料費の低減や工期短縮の点で有利であり製造コストが低く抑えられる。
この結果、本発明の高窒素鋼の製造方法を採用して製造された高窒素鋼は、健全な内部品質を有しつつ安価且つ鋼塊質量の増大を図ることが可能となり、特に高窒素鋼の大型構造材への供給に対する貢献は多分に大きい。
By the production method of the present invention, high nitrogen steel, which has been difficult to produce due to the generation of gas defects in an atmospheric pressure atmosphere, without excessive construction of pressure-resistant equipment and an increase in production cost, and whenever the steel type changes. Thus, a sound steel ingot can be reliably produced with the minimum necessary pressure resistance equipment without repeating trial and error. Further, since the production can be performed under a lower pressure condition than the prior art, it is advantageous in terms of reducing raw material costs and shortening the construction period, and the production cost can be kept low.
As a result, the high nitrogen steel manufactured by adopting the manufacturing method of the high nitrogen steel of the present invention can be inexpensive and increase the mass of the steel ingot while having a sound internal quality. The contribution to the supply of large structural materials is much greater.
以下に、本発明の一実施形態について図表を交えて説明する。
例えば、表１の組成になるように窒素以外の原料を配合し、真空誘導溶解炉で溶解し合金鋼を作製する。次いで、得られた合金鋼から溶解母材を分割し、耐圧容器内に雰囲気加熱炉を設置した加圧溶解炉にて再溶解する。その際、加圧溶解炉内の雰囲気は窒素−Ａｒ混合ガス雰囲気とする。なお、本発明としては、窒素以外のガス種が限定されるものではなく、前記したように他の希ガスなどを用いることができる。溶解時の窒素分圧は、合金鋼で目標とする窒素含有量が得られるように設定する。設定方法は、常法により行うことができ、例えば前記式５を用いて目的の窒素含有量となる窒素分圧を求める方法により定めることができる。
なお、使用した加圧溶解炉は１１気圧まで加圧が可能であり、母材を溶解するるつぼが下降することにより上方に向かって一方向凝固ができる昇降機構を有していることから、最終凝固部を鋼塊上部に制御して鋼塊を製造することが可能である。
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
For example, raw materials other than nitrogen are blended so as to have the composition shown in Table 1, and melted in a vacuum induction melting furnace to produce alloy steel. Next, the molten base material is divided from the obtained alloy steel and re-melted in a pressure melting furnace in which an atmospheric heating furnace is installed in the pressure vessel. At that time, the atmosphere in the pressure melting furnace is a nitrogen-Ar mixed gas atmosphere. In the present invention, gas species other than nitrogen are not limited, and other rare gases can be used as described above. The partial pressure of nitrogen at the time of melting is set so that the target nitrogen content can be obtained with the alloy steel. The setting method can be performed by an ordinary method, and can be determined by, for example, a method for obtaining a partial pressure of nitrogen at which the target nitrogen content is obtained using the formula 5.
The pressure melting furnace used can be pressurized up to 11 atm. Since the crucible for melting the base material descends, it has a lifting mechanism that can solidify in one direction upward. It is possible to manufacture a steel ingot by controlling the solidification part at the top of the steel ingot.
上記組成の合金鋼では、前記式５を用いることによって成分に基づく加圧雰囲気下での液相における平衡窒素溶解度を算出することができる。この算出結果を用いて所定の窒素分圧下での液相における窒素濃度が算出される。
さらに、所定の窒素分圧下における窒素濃度を基にして前記式６により固相内拡散を考慮した窒素濃度ＣＬ Ｎ（ＰＮ２，ｆｓ．ｐ）を算出する。なお、凝固前面における濃化溶鋼と母溶鋼との密度差は、前記式８、９を用いると合金鋼の組成に従って０．００２〜０．００７（ｇ／ｃｍ３）となり、該算出における包晶反応固相率ｆｓ．ｐは、合金鋼の組成に従って０．６となる。
In the alloy steel having the above composition, the equilibrium nitrogen solubility in the liquid phase under a pressurized atmosphere based on the components can be calculated by using the above formula 5. Using this calculation result, the nitrogen concentration in the liquid phase under a predetermined nitrogen partial pressure is calculated.
Further, based on the nitrogen concentration under a predetermined nitrogen partial pressure, the nitrogen concentration C L N (P N2 , fs.p) in consideration of the diffusion in the solid phase is calculated by the above equation 6. The density difference between the concentrated molten steel and the mother molten steel on the solidified front surface is 0.002 to 0.007 (g / cm 3 ) according to the composition of the alloy steel when using the above formulas 8 and 9, and the peritectic in the calculation Reaction solid fraction fs. p becomes 0.6 according to the composition of the alloy steel.
式６におけるＣ０は、上記式５で算出された窒素濃度である。また、分配係数ｋは初晶の窒素含有量とＣ０の比により得ることができ、拡散係数Ｄｓ（ｍ２／ｓ）は既存のフェライト相あるいはオーステナイト相中の窒素の拡散係数により得ることができる。
さらに、部分凝固時間ｔｆ（ｓ）は、凝固時の液相線温度から固相線温度まで温度降下するのに要する時間を計測することにより得ることができ、デンドライトアーム間隔の１／２値Ｌ（ｍ）は、実際に製造した鋼塊の断面を観察することにより得ることができる。
加えて、発明者らは式６におけるｔｆ／Ｌ２の値を２．２１〜４．２７×１０９（ｓ／ｍ２）のいずれかに固定しても算出結果に変動が少ないことを見いだしており、計測や観察を行わずにこれらの数値を式６に導入する方法もある。
C 0 in Equation 6 is the nitrogen concentration calculated in Equation 5 above. The distribution coefficient k can be obtained from the ratio of the nitrogen content of primary crystal to C 0 , and the diffusion coefficient D s (m 2 / s) can be obtained from the diffusion coefficient of nitrogen in the existing ferrite phase or austenite phase. Can do.
Furthermore, the partial solidification time t f (s) can be obtained by measuring the time required for the temperature to drop from the liquidus temperature to the solidus temperature during solidification, and is half the dendrite arm interval. L (m) can be obtained by observing the cross section of the actually produced steel ingot.
In addition, the inventors found that even if the value of t f / L 2 in Equation 6 is fixed at any of 2.21 to 4.27 × 10 9 (s / m 2 ), there is little variation in the calculation result. There is also a method of introducing these numerical values into Equation 6 without performing measurement or observation.
一方、液相の許容窒素溶解度Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ．ｐ）は、既存の偏析の解析解と前記式５の窒素分圧の項に雰囲気圧力と溶鋼静圧の和を導入することによって液相の許容できる窒素溶解度を求めることができる。溶鋼静圧は、式７の右辺第二項を用いて算出することができる。この際にｈは、湯面と凝固前面との間の距離により求めることができる。
上記溶解において平衡状態に至った後は、溶鋼から鋼塊を造塊する際に、上記で求められたＣＬ Ｎ（ＰＮ２，ｆｓ．ｐ）とＣｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ．ｐ）の差（式１）で表される過剰窒素指数ＩＮＤＥＸ（１）が０未満となるようにして窒素分圧と全圧とを調整する。その結果、得られた高窒素鋼はガス欠陥が無く優れた品質を有している。
また、例えば、表２の組成（質量％、残部Ｆｅおよび不可避不純物）になるように窒素以外の原料を配合し、真空誘導溶解炉で溶解し合金鋼を作製する。次いで、得られた合金鋼から溶解母材を分割し、前記加圧溶解炉にて再溶解する。
On the other hand, the permissible nitrogen solubility C e N (P TOTAL , fs.p) in the liquid phase is obtained by introducing the sum of the atmospheric pressure and the molten steel static pressure into the existing analytical solution of segregation and the nitrogen partial pressure term of the above formula 5. Can determine the acceptable nitrogen solubility of the liquid phase. The molten steel static pressure can be calculated using the second term on the right side of Equation 7. In this case, h can be obtained from the distance between the molten metal surface and the solidification front surface.
After reaching the equilibrium state in the melting, when the steel ingot is formed from the molten steel, C L N (P N2 , fs.p) and C e N (P TOTAL , fs.p) obtained above are obtained. The partial pressure of nitrogen and the total pressure are adjusted so that the excess nitrogen index INDEX (1) represented by the difference (Equation 1) is less than 0. As a result, the obtained high nitrogen steel has excellent quality with no gas defects.
Further, for example, raw materials other than nitrogen are blended so as to have the composition shown in Table 2 (mass%, remaining Fe and inevitable impurities), and melted in a vacuum induction melting furnace to produce alloy steel. Next, the molten base material is divided from the obtained alloy steel and remelted in the pressure melting furnace.
凝固前面における濃化溶鋼と母溶鋼との密度差は、前記式８、９を用いると合金鋼の組成に従って０．０１５〜０．０１６（ｇ／ｃｍ３）となる。そのため、所定の窒素分圧下における窒素濃度を基にして前記式６により固相内拡散を考慮した窒素濃度ＣＬ Ｎ（ＰＮ２，ｆｓ＝０．７５）を算出する。
さらに、液相の許容窒素溶解度Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ＝０）は、母溶鋼の化学成分と前記式５の窒素分圧の項に雰囲気圧力と溶鋼静圧の和を導入することによって液相の許容できる窒素溶解度を求めることができる。
上記溶解において平衡状態に至った後は、溶鋼から鋼塊を造塊する際に、上記で求められたＣＬ Ｎ（ＰＮ２，ｆｓ＝０．７５）とＣｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ＝０）の差（式２）で表される過剰窒素指数ＩＮＤＥＸ（２）が０未満となるようにして窒素分圧と全圧とを調整する。その結果、得られた高窒素鋼はガス欠陥が無く優れた品質を有している。
また、例えば、表３の組成（質量％、残部Ｆｅおよび不可避不純物）になるように窒素以外の原料を配合し、真空誘導溶解炉で溶解し合金鋼を作製する。次いで、得られた合金鋼から溶解母材を分割し、前記加圧溶解炉にて再溶解する。
When the above formulas 8 and 9 are used, the density difference between the concentrated molten steel and the mother molten steel at the solidification front is 0.015 to 0.016 (g / cm 3 ) according to the composition of the alloy steel. Therefore, the nitrogen concentration C L N (P N2 , fs = 0.75) considering the diffusion in the solid phase is calculated by the above equation 6 based on the nitrogen concentration under a predetermined nitrogen partial pressure.
Further, the permissible nitrogen solubility C e N (P TOTAL , fs = 0) of the liquid phase is obtained by introducing the sum of the atmospheric pressure and the molten steel static pressure into the chemical component of the mother molten steel and the nitrogen partial pressure term of the formula 5 above. An acceptable nitrogen solubility of the liquid phase can be determined.
After reaching the equilibrium state in the melting, when the steel ingot is formed from the molten steel, C L N (P N2 , fs = 0.75) and C e N (P TOTAL , fs = The partial pressure of nitrogen and the total pressure are adjusted so that the excess nitrogen index INDEX (2) represented by the difference (Equation 2) of 0) is less than 0. As a result, the obtained high nitrogen steel has excellent quality with no gas defects.
Further, for example, raw materials other than nitrogen are blended so as to have the composition shown in Table 3 (mass%, remaining Fe and inevitable impurities), and melted in a vacuum induction melting furnace to produce alloy steel. Next, the molten base material is divided from the obtained alloy steel and remelted in the pressure melting furnace.
凝固前面における濃化溶鋼と母溶鋼との密度差は、前記式８、９を用いると合金鋼の組成に従って−０．０１６〜−０．０２０（ｇ／ｃｍ３）となる。そのため、所定の窒素分圧下における窒素濃度を基にして前記式６により固相内拡散を考慮した窒素濃度ＣＬ Ｎ（ＰＮ２，ｆｓ＝０．７５）を算出する。
さらに、液相の許容窒素溶解度Ｃｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ＝０）は、母溶鋼の化学成分と前記式５の窒素分圧の項に雰囲気圧力と溶鋼静圧の和を導入することによって液相の許容できる窒素溶解度を求めることができる。
上記溶解において平衡状態に至った後は、溶鋼から鋼塊を造塊する際に、上記で求められたＣＬ Ｎ（ＰＮ２，ｆｓ＝０．７５）とＣｅ Ｎ（ＰＴＯＴＡＬ，ｆｓ＝０）の差（式２）で表される過剰窒素指数ＩＮＤＥＸ（２）が０未満となるようにして窒素分圧と全圧とを調整する。その結果、得られた高窒素鋼はガス欠陥が無く優れた品質を有している。
以上、本発明について上記実施形態に基づいて説明をしたが、本発明は上記実施形態の説明内容に限定されるものではなく、本発明の範囲内において適宜の変更が可能である。
The density difference between the concentrated molten steel and the mother molten steel on the solidification front is −0.016 to −0.020 (g / cm 3 ) according to the composition of the alloy steel when the above formulas 8 and 9 are used. Therefore, the nitrogen concentration C L N (P N2 , fs = 0.75) considering the diffusion in the solid phase is calculated by the above equation 6 based on the nitrogen concentration under a predetermined nitrogen partial pressure.
Further, the permissible nitrogen solubility C e N (P TOTAL , fs = 0) of the liquid phase is obtained by introducing the sum of the atmospheric pressure and the molten steel static pressure into the chemical component of the mother molten steel and the nitrogen partial pressure term of the formula 5 above. An acceptable nitrogen solubility of the liquid phase can be determined.
After reaching the equilibrium state in the melting, when the steel ingot is formed from the molten steel, C L N (P N2 , fs = 0.75) and C e N (P TOTAL , fs = The partial pressure of nitrogen and the total pressure are adjusted so that the excess nitrogen index INDEX (2) represented by the difference (Equation 2) of 0) is less than 0. As a result, the obtained high nitrogen steel has excellent quality with no gas defects.
Although the present invention has been described based on the above embodiment, the present invention is not limited to the description of the above embodiment, and appropriate modifications can be made within the scope of the present invention.
以下に、本発明の実施例を説明する。
上記実施形態で示される表１の組成の合金鋼および真空誘導溶解炉および加圧溶解炉を用いて造塊を行った。
その際に、実施例１として窒素分圧を３ａｔｍ、Ａｒ分圧を４ａｔｍとした全圧７ａｔｍの加圧雰囲気下で、加圧溶解炉を用いて凝固試験を行った。続いて、比較例１として窒素分圧を３ａｔｍ、Ａｒ分圧を２．５ａｔｍとした全圧５．５ａｔｍの加圧雰囲気下で実施例と同様に凝固試験を行った。
Examples of the present invention will be described below.
Ingot forming was performed using an alloy steel having the composition shown in Table 1 shown in the above embodiment, a vacuum induction melting furnace, and a pressure melting furnace.
At that time, as Example 1, a solidification test was performed using a pressure melting furnace in a pressurized atmosphere with a nitrogen partial pressure of 3 atm and an Ar partial pressure of 4 atm and a total pressure of 7 atm. Subsequently, as Comparative Example 1, a coagulation test was performed in the same manner as in the example under a pressurized atmosphere with a nitrogen partial pressure of 3 atm and an Ar partial pressure of 2.5 atm and a total pressure of 5.5 atm.
図２に実施例１の液相窒素濃度と許容窒素溶解度を、図３に比較例１の液相窒素濃度と許容窒素溶解度を示す。図２に示すように、実施例１では全固相率にわたって液相の窒素濃度は許容窒素溶解度を下回ることが想定された。一方、図３に示したように、比較例１では固相率０．５２〜０．７７にかけて、許容窒素溶解度を超える窒素濃化が想定された。 FIG. 2 shows the liquid phase nitrogen concentration and allowable nitrogen solubility of Example 1, and FIG. 3 shows the liquid nitrogen concentration and allowable nitrogen solubility of Comparative Example 1. As shown in FIG. 2, in Example 1, it was assumed that the nitrogen concentration in the liquid phase was below the allowable nitrogen solubility over the entire solid phase ratio. On the other hand, as shown in FIG. 3, in Comparative Example 1, nitrogen concentration exceeding the allowable nitrogen solubility was assumed over the solid phase ratio of 0.52 to 0.77.
図４に実施例１および比較例１の固相率−過剰窒素濃度図を示す。両者の過剰窒素濃度の最大値は包晶反応固相率である固相率０．６０で現れ、本発明の式より、実施例１では過剰窒素指数が０を下回るためガス欠陥が生成せず、比較例１では過剰窒素指数が０を上回るためガス欠陥が生成すると判断された。
図５に実施例１の鋼塊の縦断面を、図６に比較例１の鋼塊の縦断面を図面代用写真（倍率０．５倍）によって示す。図５および図６に示したように、本発明の式が判断したように、比較例１にはガス欠陥が生成し実施例１の鋼塊にはガス欠陥は存在しなかった。次に、窒素分圧、全圧の条件を変えて同様の凝固試験を行った。窒素分圧の変更によって目標窒素含有量、包晶反応固相率も変動する。
表４に本発明の実施例として行った表１の合金鋼の試験結果をまとめて示す。表４に示したように、本発明を用いることによって簡便にガス欠陥生成臨界圧力が推定され、種々の窒素含有量を有する合金鋼をガス欠陥の無い健全な鋼塊として製造できることが証明された。
FIG. 4 shows solid phase ratio-excess nitrogen concentration diagrams of Example 1 and Comparative Example 1. The maximum value of the excess nitrogen concentration of both appears at a solid phase ratio of 0.60, which is the peritectic reaction solid phase ratio. From the formula of the present invention, in Example 1, the excess nitrogen index is less than 0, so no gas defects are generated. In Comparative Example 1, it was determined that a gas defect was generated because the excess nitrogen index exceeded 0.
FIG. 5 shows a longitudinal section of the steel ingot of Example 1, and FIG. 6 shows a longitudinal section of the steel ingot of Comparative Example 1 by a drawing substitute photograph (0.5 times magnification). As shown in FIG. 5 and FIG. 6, as judged by the formula of the present invention, gas defects were generated in Comparative Example 1 and there were no gas defects in the steel ingot of Example 1. Next, the same coagulation test was performed by changing the conditions of the nitrogen partial pressure and the total pressure. By changing the nitrogen partial pressure, the target nitrogen content and the peritectic reaction solid fraction also vary.
Table 4 summarizes the test results of the alloy steels of Table 1 performed as examples of the present invention. As shown in Table 4, the critical pressure for gas defect generation was easily estimated by using the present invention, and it was proved that alloy steels having various nitrogen contents can be produced as healthy steel ingots without gas defects. .
次いで、上記実施形態で示される表２の組成の合金鋼および真空誘導溶解炉および加圧溶解炉を用いて造塊を行った。
その際に、実施例４として窒素分圧を６ａｔｍ、Ａｒ分圧を５ａｔｍとした全圧１１ａｔｍの加圧雰囲気下で、加圧溶解炉を用いて凝固試験を行った。続いて、比較例４として窒素分圧を６ａｔｍ、Ａｒ分圧を４ａｔｍとした全圧１０ａｔｍの加圧雰囲気下で実施例と同様に凝固試験を行った。
Next, ingot forming was performed using an alloy steel having a composition shown in Table 2 shown in the above embodiment, a vacuum induction melting furnace, and a pressure melting furnace.
At that time, as Example 4, a solidification test was performed using a pressure melting furnace in a pressurized atmosphere of a total pressure of 11 atm with a nitrogen partial pressure of 6 atm and an Ar partial pressure of 5 atm. Subsequently, as Comparative Example 4, a coagulation test was performed in the same manner as in the example in a pressurized atmosphere with a nitrogen partial pressure of 6 atm and an Ar partial pressure of 4 atm and a total pressure of 10 atm.
図７に実施例４と比較例４の液相窒素濃度と許容窒素溶解度を示す。図７に示すように、実施例４では固相率０．７５の濃化溶鋼の窒素濃度は母溶鋼の許容窒素溶解度を下回ることが想定された。一方、比較例４では母溶鋼の許容窒素溶解度を上回ることが想定された。
図８に実施例４の鋼塊の縦断面を、図９に比較例４の鋼塊の縦断面を図面代用写真（倍率０．５倍）によって示す。図８および図９に示したように、本発明の式が判断したように、比較例４にはガス欠陥が生成し実施例４の鋼塊にはガス欠陥は存在しなかった。次に、窒素分圧、全圧の条件を変えて同様の凝固試験を行った。
表５に本発明の実施例として行った表２の合金鋼の試験結果をまとめて示す。表５に示したように、本発明を用いることによって簡便にガス欠陥生成臨界圧力が推定され、種々の窒素含有量を有する合金鋼をガス欠陥の無い健全な鋼塊として製造できることが証明された。
FIG. 7 shows the liquid phase nitrogen concentration and allowable nitrogen solubility in Example 4 and Comparative Example 4. As shown in FIG. 7, in Example 4, it was assumed that the nitrogen concentration of the concentrated molten steel having a solid phase ratio of 0.75 was lower than the allowable nitrogen solubility of the mother molten steel. On the other hand, in Comparative Example 4, it was assumed that the allowable nitrogen solubility of the mother molten steel was exceeded.
FIG. 8 shows a longitudinal section of the steel ingot of Example 4, and FIG. 9 shows a longitudinal section of the steel ingot of Comparative Example 4 by a drawing substitute photograph (0.5 times magnification). As shown in FIGS. 8 and 9, as judged by the equation of the present invention, gas defects were generated in Comparative Example 4, and there were no gas defects in the steel ingot of Example 4. Next, the same coagulation test was performed by changing the conditions of the nitrogen partial pressure and the total pressure.
Table 5 summarizes the test results of the alloy steels of Table 2 that were used as examples of the present invention. As shown in Table 5, by using the present invention, gas critical critical pressure was easily estimated, and it was proved that alloy steels having various nitrogen contents can be produced as healthy steel ingots without gas defects. .
次いで、上記実施形態で示される表３の組成の合金鋼および真空誘導溶解炉および加圧溶解炉を用いて造塊を行った。
その際に、実施例６として窒素分圧を１．５ａｔｍ、Ａｒ分圧を７．５ａｔｍとした全圧９ａｔｍの加圧雰囲気下で、加圧溶解炉を用いて凝固試験を行った。続いて、比較例６として窒素分圧を１．５ａｔｍ、Ａｒ分圧を５．５ａｔｍとした全圧７ａｔｍの加圧雰囲気下で実施例と同様に凝固試験を行った。
Next, ingot forming was performed using an alloy steel having the composition of Table 3 shown in the above embodiment, a vacuum induction melting furnace, and a pressure melting furnace.
At that time, as Example 6, a solidification test was performed using a pressure melting furnace in a pressurized atmosphere with a nitrogen partial pressure of 1.5 atm and an Ar partial pressure of 7.5 atm and a total pressure of 9 atm. Subsequently, as Comparative Example 6, a coagulation test was performed in the same manner as in the example in a pressurized atmosphere with a nitrogen partial pressure of 1.5 atm and an Ar partial pressure of 5.5 atm and a total pressure of 7 atm.
図１０に実施例６と比較例６の液相窒素濃度と許容窒素溶解度を示す。図１０に示すように、実施例６では固相率０．７５の濃化溶鋼の窒素濃度は母溶鋼の許容窒素溶解度を下回ることが想定された。一方、比較例６では母溶鋼の許容窒素溶解度を上回ることが想定された。
図１１に実施例６の鋼塊の縦断面を、図１２に比較例６の鋼塊の縦断面を図面代用写真（倍率０．５倍）によって示す。図１１および図１２に示したように、本発明の式が判断したように、比較例６にはガス欠陥が生成し実施例６の鋼塊にはガス欠陥は存在しなかった。次に、窒素分圧、全圧の条件を変えて同様の凝固試験を行った。
表６に本発明の実施例として行った表３の合金鋼の試験結果をまとめて示す。表６に示したように、本発明を用いることによって簡便にガス欠陥生成臨界圧力が推定され、種々の窒素含有量を有する合金鋼をガス欠陥の無い健全な鋼塊として製造できることが証明された。
FIG. 10 shows the liquid phase nitrogen concentration and allowable nitrogen solubility in Example 6 and Comparative Example 6. As shown in FIG. 10, in Example 6, the nitrogen concentration of the concentrated molten steel having a solid phase ratio of 0.75 was assumed to be lower than the allowable nitrogen solubility of the mother molten steel. On the other hand, in Comparative Example 6, it was assumed that the allowable nitrogen solubility of the mother molten steel was exceeded.
FIG. 11 shows a longitudinal section of the steel ingot of Example 6, and FIG. 12 shows a longitudinal section of the steel ingot of Comparative Example 6 with a drawing substitute photograph (0.5 magnification). As shown in FIG. 11 and FIG. 12, as judged by the formula of the present invention, gas defects were generated in Comparative Example 6, and no gas defects were present in the steel ingot of Example 6. Next, the same coagulation test was performed by changing the conditions of the nitrogen partial pressure and the total pressure.
Table 6 summarizes the test results of the alloy steels shown in Table 3 that were conducted as examples of the present invention. As shown in Table 6, by using the present invention, the critical pressure for gas defect generation was easily estimated, and it was proved that alloy steels having various nitrogen contents could be produced as healthy steel ingots without gas defects. .
Claims (3)
ＩＮＤＥＸ（１）＝ＣINDEX (1) = C ＬL ＮN （Ｐ(P Ｎ２N2 ，ｆｓ．ｐ）−Ｃ, Fs. p) -C ｅe ＮN （Ｐ(P ＴＯＴＡＬTOTAL ，ｆｓ．ｐ）・・・（式１）, Fs. p) (Formula 1)
ここで、here,
ＩＮＤＥＸ（１）：過剰窒素指数（ｗｔｐｐｍ）INDEX (1): excess nitrogen index (wtppm)
ｆｓ．ｐ：包晶反応固相率fs. p: Peritectic reaction solid fraction
ＣC ＬL ＮN （Ｐ(P Ｎ２N2 ，ｆｓ．ｐ）：窒素分圧Ｐ, Fs. p): Nitrogen partial pressure P Ｎ２N2 （ａｔｍ）条件での包晶反応固相率における液相中の窒素濃度（ｗｔｐｐｍ）Nitrogen concentration in liquid phase (wtppm) in peritectic reaction solid fraction under (atm) conditions
ＣC ｅe ＮN （Ｐ(P ＴＯＴＡＬTOTAL ，ｆｓ．ｐ）：全圧Ｐ, Fs. p): Total pressure P ＴＯＴＡＬTOTAL （ａｔｍ）条件での包晶反応固相率における液相の許容窒素溶解度（ｗｔｐｐｍ）Permissible nitrogen solubility (wtppm) of liquid phase at peritectic reaction solid fraction under (atm) conditions
ただし、包晶反応固相率は、凝固時にフェライト凝固からオーステナイト凝固に切り替わる固相率であり、固相率全率においてオーステナイト凝固する場合は０とする。However, the peritectic reaction solid phase ratio is a solid phase ratio that switches from ferrite solidification to austenite solidification at the time of solidification, and is 0 when the austenite solidification occurs at the total solid fraction.
ＩＮＤＥＸ（２）＝ＣINDEX (2) = C ＬL ＮN （Ｐ(P Ｎ２N2 ，ｆｓ＝０．７５）−Ｃ, Fs = 0.75) -C ｅe ＮN （Ｐ(P ＴＯＴＡＬTOTAL ，ｆｓ＝０）・・・（式２）, Fs = 0) (Expression 2)
ここで、here,
ＩＮＤＥＸ（２）：過剰窒素指数（ｗｔｐｐｍ）INDEX (2): excess nitrogen index (wtppm)
ＣC ＬL ＮN （Ｐ(P Ｎ２N2 ，ｆｓ＝０．７５）：窒素分圧Ｐ, Fs = 0.75): nitrogen partial pressure P Ｎ２N2 （ａｔｍ）条件での固相率０．７５における液相中の窒素濃度（ｗｔｐｐｍ）Nitrogen concentration in liquid phase (wtppm) at a solid phase ratio of 0.75 under (atm) conditions
ＣC ｅe ＮN （Ｐ(P ＴＯＴＡＬTOTAL ，ｆｓ＝０）：全圧Ｐ, Fs = 0): total pressure P ＴＯＴＡＬTOTAL （ａｔｍ）条件での固相率０における液相の許容窒素溶解度（ｗｔｐｐｍ）Allowable nitrogen solubility in liquid phase (wtppm) at a solid phase ratio of 0 under (atm) conditions
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| JP2000212631A (en) * | 1999-01-22 | 2000-08-02 | Daido Steel Co Ltd | Production of high nitrogen steel |
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