WO2015029994A1 - Cu-Sn共存鋼およびその製造方法 - Google Patents
Cu-Sn共存鋼およびその製造方法 Download PDFInfo
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- WO2015029994A1 WO2015029994A1 PCT/JP2014/072287 JP2014072287W WO2015029994A1 WO 2015029994 A1 WO2015029994 A1 WO 2015029994A1 JP 2014072287 W JP2014072287 W JP 2014072287W WO 2015029994 A1 WO2015029994 A1 WO 2015029994A1
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
Definitions
- the present invention relates to a corrosion-resistant low alloy steel (Cu—Sn coexisting steel) containing Cu and Sn and a method for producing the same.
- the present invention relates to a steel that does not have surface cracks or surface defects and that does not generate surface cracks or surface defects even when rolled into a thick plate, and a method for producing the same.
- Cu and Sn are both trump elements in scrap iron, and are useful in that they are elements that improve the corrosion resistance of steel.
- Cu causes cracking during hot working of steel, causing so-called red heat embrittlement (hereinafter, red heat embrittlement caused by Cu is also referred to as “Cu embrittlement”)
- Sn causes Cu embrittlement. It is known to encourage. Therefore, when manufacturing a steel material containing both Cu and Sn, suppression of surface cracks and surface flaws is the biggest issue.
- Patent Document 1 discloses a steel material containing both Cu and Sn and having excellent beach weather resistance, and a structure using the steel material. However, this document does not pay attention to the prevention of surface embrittlement of the slab during hot casting and surface flaws during continuous casting.
- Patent Document 2 discloses a hot rolled steel containing both Cu and Sn for producing without surface flaws during hot working. Moreover, although the literature can prevent the crack of the steel surface resulting from Cu by adding Ni to Cu containing steel, in steel containing Sn other than Cu, the crack prevention effect of Ni may fall. Are listed. However, in the same document, Ni is considered to be low in resources and high in cost, and is intended to provide hot rolled steel having a good surface texture without adding Ni, Cu and Sn, There is no sufficient description of the effects when Ni coexists.
- Patent Document 3 discloses a technique for preventing the occurrence of surface flaws by continuously casting the value of the component ratio Cu / Sn and the value of (Cu + Ni) / Sn in the corrosion-resistant low alloy steel within a predetermined range. Is disclosed.
- Patent Document 2 and Patent Document 3 are low alloy steels in which the Sn content is at least twice the Cu content.
- the component ratio Cu / Sn in mass% (hereinafter referred to as “Cu / Sn ratio”) has an upper limit of 0.5, and when the Cu / Sn ratio is too high. Since surface cracks occur, it is difficult to increase the Cu / Sn ratio for the purpose of improving characteristics such as corrosion resistance.
- Non-Patent Document 1 includes the following items a and b as influences of Cu and Sn on hot work cracking due to surface red hot embrittlement (liquid embrittlement).
- the steel material heated to 1000 ° C. or more generates a scale on the surface by atmospheric oxidation.
- Fe which is the main component of the parent phase is selectively oxidized, and Cu is concentrated in the surface layer portion of the steel material.
- Cu having a melting point lower than that of Fe precipitates as a liquid phase on the surface layer portion of the steel material, and this penetrates into the crystal grain boundary and causes liquid film embrittlement.
- Cu, Sn, and Ni are all metal elements that are less likely to be oxidized than Fe, which is the main component of the parent phase, that is, nobler than Fe.
- steel containing Cu and Sn (Cu: 0.3% by mass, Sn: 0.04% by mass) is a steel containing only Cu among these elements (Cu: 0.3% by mass).
- the surface cracking of the steel material is remarkable compared to Moreover, the surface crack does not arise in the steel (Sn: 0.04 mass%) containing only Sn among these elements.
- Non-Patent Document 1 also examines the effect of Ni that suppresses embrittlement caused by Cu and Sn. In the same document, it is sufficient to add 0.15% by mass of Ni for suppressing the embrittlement of the steel containing only Cu, while suppressing the embrittlement of the steel containing Cu and Sn. It is said that it is necessary to add 0.3% by mass of Ni.
- Non-Patent Document 1 Sn and Ni affect the suppression of embrittlement of steel containing only Cu, and that embrittlement does not occur in steel containing only Sn. It is only described.
- the present invention has been made in view of these problems, that is, the occurrence of surface cracks and surface defects due to Cu embrittlement when producing steel containing Cu and Sn. It is an object of the present invention to provide a Cu—Sn coexisting steel capable of maintaining good surface quality and a method for producing the same.
- the present inventors selected a low alloy steel containing Cu and Sn and serving as a thick plate material having good corrosion resistance. Specifically, by mass, C: 0.04 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, Cu: 0.20 to 1.%. It is a Cu—Sn coexisting steel containing 50% and Sn: 0.06 to 0.50%. With this component composition, satisfactory corrosion resistance can be obtained while satisfying the mechanical properties as a material for the thick plate. In this steel, the Cu / Sn ratio (mass ratio) preferably satisfies 1.0 to 8.0 in order to improve the corrosion resistance. However, since Cu and Sn coexist in this steel, Cu embrittlement tends to occur remarkably.
- the present inventors examined the component composition which can suppress Cu embrittlement which arises with the selective oxidation of Fe in the said Cu-Sn coexisting steel.
- the internal oxide layer is a pre-oxidation layer formed by oxidizing an alloy element that is lower than Fe before the parent phase Fe is oxidized.
- a fine oxide of Si and Mn is formed in the Cu—Sn coexisting steel.
- This is a layer in which (SiO 2 , MnO, and SiMnO (manganese silicate) are main components) are dispersed.
- SiO 2 , MnO, and SiMnO mangaganese silicate
- the composition of the molten steel is adjusted so that the contents of Si, Mn, Cu, Sn, Al, and Ni satisfy predetermined conditions by adding Al and Ni to the molten steel of the Cu—Sn coexisting steel.
- the surface of the slab is oxidized during the cooling process of the slab to form an internal oxide layer, and Al 2 O 3 is contained in the composite oxide formed in the internal oxide layer, whereby Cu It was found that the occurrence of surface cracks accompanying embrittlement can be suppressed.
- Al and Ni are elements having an effect of increasing the solid solution ratio of Cu in steel. On the other hand, when Al or Ni was contained alone, a great effect was not obtained with respect to the suppression of the occurrence of surface cracks. The examination content of the addition conditions of Al and Ni will be described later.
- the present invention has been made on the basis of this finding, and the gist thereof lies in the following method for producing a Cu—Sn coexisting steel and the Cu—Sn coexisting steel produced by this production method.
- composition in mass% C: 0.04 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, P: 0.05% or less, S : 0.02% or less, Cu: 0.20 to 1.50%, and Sn: 0.06 to 0.50%, Al: 0.06 to 1.00% and Ni: 0.05
- the composition of the molten steel is expressed by the following formulas (1) to (3).
- the inner oxide layer is formed by oxidizing the slab surface during the cooling process of the slab, and the composite oxide formed in the inner oxide layer contains Al 2 O 3
- a method for producing a Cu—Sn coexisting steel characterized by comprising: [Al] / (3 [Si] + [Mn]) ⁇ 0.050 (1) [Ni] / ([Cu] +5 [Sn]) ⁇ 0.10 (2) [Al] / [Ni] ⁇ 0.20 (3)
- [Al], [Si], [Mn], [Ni], [Cu], and [Sn] are the contents (mass of Al, Si, Mn, Ni, Cu, and Sn in the molten steel, respectively. %).
- the content of Al 2 O 3 in the composite oxide produced in the internal oxide layer is preferably 15 to 40% by mass. Further, it is preferable to adjust the component composition of the molten steel so as to satisfy the condition represented by the following formula (4), that is, the Cu / Sn ratio is 1.0 to 8.0. 1.0 ⁇ [Cu] / [Sn] ⁇ 8.0 (4)
- “mass%” for the component composition of steel and composite oxide is also simply expressed as “%”.
- “steel material” includes a cast slab and a processed product obtained by subjecting the slab to processing such as rolling.
- Al in oxide means Al as one of constituent elements of the oxide. Therefore, in addition to Al in a single Al 2 O 3 , Al in a complex oxide such as Al in an oxide containing Al, Si, and Mn is also included.
- content of Al 2 O 3 in the composite oxide in the present invention, the content of Al 2 O 3 when the composite oxide is obtained by assuming Al 2 O 3, SiO 2, and shall consist of MnO
- An actual composite oxide contains an oxide having a complicated composition of a ternary system or more, and it is difficult to calculate the content of Al 2 O 3 in the composite oxide.
- the O content in the composite oxide depends on the stoichiometric ratio depending on the content of each metal element of Al, Si, and Mn and the valence of each metal element. Therefore, it was assumed that the composite oxide was composed of Al 2 O 3 , SiO 2, and MnO, and the content ratio of Al 2 O 3 in the composite oxide was calculated. A specific calculation method will be described later.
- the Cu—Sn coexisting steel of the present invention has no surface cracks or surface flaws, and surface cracks do not occur even during the subsequent hot rolling. Therefore, by using the Cu—Sn coexisting steel of the present invention as a raw material, a steel material having a good surface quality can be produced.
- FIG. 3 is a flowchart illustrating a method for producing a Cu—Sn coexisting steel according to an embodiment of the present invention. It is a flowchart explaining the other example of a form of the manufacturing method of the Cu-Sn coexisting steel which concerns on one Embodiment of this invention. It is a figure explaining Cu-Sn coexisting steel concerning one embodiment of the present invention.
- the notation “A to B” means “A to B”. In such a notation, when a unit is attached only to the numerical value B, the unit is also applied to the numerical value A.
- Cu embrittlement due to Cu in Cu-containing steel is caused by the penetration of the Cu liquid phase into the grain boundary of the austenite phase of Fe as the parent phase. It is thought to be caused by weakening. Precipitation of the Cu liquid phase is likely to occur at a temperature around 1100 ° C. (for example, a temperature range of about 1050 to 1150 ° C.).
- the Cu liquid phase has a lower melting point of Cu than Fe, when Fe, which is the main component of steel, is selectively oxidized, Cu noble than Fe is concentrated locally, and the Cu concentration is the parent phase. This is caused by exceeding the solid solubility limit of a certain Fe in the austenite phase. That is, the solid solubility limit of Cu in Fe at high temperature is one of the important factors that cause Cu embrittlement.
- Ni is an element more noble than Fe like Cu. Ni increases the solid solubility limit of Cu in Fe and raises the melting point of Cu, thereby suppressing Cu embrittlement. Therefore, in general, in Cu-containing steel, Ni is added to prevent the steel material from cracking.
- Sn coexisting with Cu in the steel in the present invention is an element nobler than Fe, like Cu. Sn reduces the solid solubility limit of Cu with respect to Fe and lowers the melting point of Cu, thus promoting Cu embrittlement. For this reason, when Cu and Sn coexist in the steel, the cracking susceptibility becomes extremely large, and it is difficult to completely prevent cracking even if Ni is simply added.
- a measure for preventing the formation of a low melting point liquid phase that is, a measure for limiting the content of Sn can be considered. Since the addition of Sn lowers the melting point of Cu and promotes Cu embrittlement, it is difficult to produce a slab without causing surface cracks while actively coexisting Cu and Sn in steel. It is.
- Al is an element that is less basic than Fe, unlike Cu and Ni. Al has the effect
- the phenomenon related to Cu embrittlement corresponds to the selective oxidation behavior of steel. That is, an alloy element that is lower than Fe is oxidized prior to the parent phase, and then Fe in the parent phase is oxidized, and an alloy element that is nobler than Fe is concentrated in the parent phase.
- Cu embrittlement behavior in Cu-Sn coexisting steel was investigated.
- C 0.04 to 0.20%
- Si 0.05 to 1.00%
- Mn 0.20 to 2.50%
- Cu 0.20 to 1.50%
- Sn A Cu—Sn coexisting steel having a composition suitable for a thick plate structure material containing 0.06 to 0.50% and the balance being Fe and impurities was used.
- This Cu—Sn coexisting steel is a material with extremely high cracking susceptibility due to the fact that Cu and Sn coexist and Cu embrittlement is remarkable due to the coexistence of Cu and Sn.
- the following knowledge was obtained.
- the shape of the interface between the scale and the parent phase of the steel surface layer becomes uneven. This unevenness of the interface has the effect of suppressing the accumulation of the liquid phase at the interface, and is therefore advantageous for rejecting the deposited Cu liquid phase into the scale and suppressing the occurrence of Cu embrittlement.
- the Cu liquid phase deposited on the steel surface layer and the FeNi alloy phase formed on the steel surface layer suppress the oxidation inside the portion where the alloy phase of the steel surface layer is formed and also suppress the growth of the internal oxide layer. To do. However, since the thickness of the alloy phase in the steel surface layer portion is non-uniform, the thickness of the internal oxide layer on the inner side becomes non-uniform.
- the formation of the internal oxide layer is promoted by selective oxidation.
- Si and Mn which are less basic than Fe, which is the main component
- oxides of Si and Mn are first formed in the steel surface layer, and in the internal oxide layer
- Composite oxide particles enriched with Si and Mn are dispersed, and an oxide (scale) of Fe is formed.
- Al like Mn and Si, is more easily oxidized than Fe. Therefore, in steel materials containing Al, formation of an internal oxide layer accompanying selective oxidation is promoted.
- an oxide enriched with Si and Mn and an oxide enriched with Al are generated independently, and the internal oxide layer is a dispersion of more oxide particles than a normal steel material not containing Al. It becomes.
- the oxide particles in the internal oxide layer are very fine because O in the steel material increases as the oxidation of the surface layer increases, exceeds the solubility limit, and precipitates from the solid phase.
- particles having a fine diameter of 0.1 ⁇ m or less may exist, but usually oxide particles having a particle diameter of 0.2 ⁇ m or more can be easily observed with an optical microscope or an electron microscope.
- oxide particles having a particle size in the range of about 0.2 to 1.0 ⁇ m are dispersed in the internal oxide layer having a thickness of 20 to 200 ⁇ m.
- the dispersion number density of observable oxide particles of 0.2 ⁇ m or more is about 100,000 to 1,200,000 particles / mm 2 .
- the effects i and j can be obtained in addition to these effects.
- the depth of unevenness at the interface between the scale and the parent phase of the steel surface layer portion is, for example, about 20 to 100 ⁇ m
- the interval between the concave and convex portions is, for example, about 20 to 50 ⁇ m.
- the Cu—Sn coexisting steel of the present invention was made on the basis of the knowledge obtained as a result of the above study, and C: 0.04 to 0 20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, P: 0.05% or less, S: 0.02% or less, Cu: 0.20 to 1. 50%, Sn: 0.06 to 0.50%, Al: 0.06 to 1.00%, and Ni: 0.05 to 1.00%, with the balance being composed of Fe and impurities is there.
- impurities include H, N, O, Mg, Ca, Sr, As, Se, Sb, and Te.
- a part of Fe may be replaced with other alloy components.
- other alloy components in the present invention include B, Ti, Zr, V, Nb, Cr, Mo, and W.
- C 0.04 to 0.20%
- C is an element having an effect of increasing the strength of the material. In order to obtain this effect, the C content is 0.04% or more. On the other hand, if the C content exceeds 0.20%, the toughness is lowered and the weld crack sensitivity is increased. Therefore, the C content is 0.04 to 0.20%.
- Si 0.05 to 1.00% Si is an element effective for deoxidation. In order to obtain this effect, the Si content is set to 0.05% or more. On the other hand, if the Si content exceeds 1.00%, the toughness may decrease. Therefore, the Si content is 0.05 to 1.00%.
- Mn 0.20 to 2.50%
- Mn is an element having an effect of increasing the strength of the material. In order to obtain this effect, the Mn content is 0.20% or more. On the other hand, if the Mn content exceeds 2.50%, the toughness may decrease. Therefore, the Mn content is 0.20 to 2.50%.
- P 0.05% or less
- P is an impurity element inevitably contained in the steel material, and it is better that the content is smaller.
- the upper limit of P is preferably 0.03%.
- S 0.02% or less S is an impurity element inevitably contained in the steel material, and it is better that the content is smaller.
- S content exceeds 0.02, the hot cracking sensitivity is increased.
- the amount of MnS inclusions that become the starting point of corrosion of the steel material increases, and the corrosion resistance is impaired. Therefore, the S content is 0.02% or less, and the smaller the content, the better.
- the upper limit of S is preferably 0.010%.
- Cu 0.20 to 1.50%
- Cu is an element having an effect of improving the corrosion resistance of steel. In order to obtain this effect, the Cu content is 0.20% or more.
- Cu content shall be 1.50% or less.
- Sn 0.06-0.50%
- Sn is an element having an effect of improving the corrosion resistance of steel. In order to obtain this effect, the Sn content is set to 0.06% or more. On the other hand, when the Sn content exceeds 0.50%, the corrosion resistance is saturated. Further, when Sn is contained in steel containing Cu, corrosion resistance is improved, but red heat embrittlement is promoted, and surface flaws are easily generated in the manufacturing process. For this reason, Sn content shall be 0.50% or less.
- Al is an element originally used for deoxidation of steel, and is contained in the present invention for suppressing Cu embrittlement.
- the Al content is less than 0.06%, a sufficient effect of suppressing embrittlement cannot be obtained.
- the Al content is set to 0.06 to 1.00%. This Al content is the content of acid-soluble Al.
- Ni 0.05-1.00%
- Ni is an element that expands the solid solubility limit of Cu in Fe, makes the interface between the scale and the parent phase of the steel material surface layer uneven, and promotes the rejection of the deposited Cu liquid phase to the scale side.
- it is an element which forms the FeNi alloy phase in the surface layer part of the steel material and suppresses the progress of the oxidation of the parent phase.
- the Ni content is less than 0.05%, a sufficient effect of suppressing embrittlement cannot be obtained.
- the Ni content exceeds 1.00%, it is not only economically undesirable, but also the growth of the internal oxide layer in the alloy phase is suppressed because the FeNi alloy phase is easily formed during the selective oxidation of the steel surface layer portion. This will promote the progress of grain boundary oxidation.
- the Ni content is set to 0.05 to 1.00%.
- the component composition of the molten steel is further adjusted so as to satisfy the relationships of the following formulas (1) to (3).
- K1 [Al] / (3 [Si] + [Mn]) ⁇ 0.050
- K2 [Ni] / ([Cu] +5 [Sn]) ⁇ 0.10
- K3 [Al] / [Ni] ⁇ 0.20
- [Al], [Si], [Mn], [Ni], [Cu], and [Sn] are the contents (mass of Al, Si, Mn, Ni, Cu, and Sn in the molten steel, respectively. %).
- the component composition of molten steel may satisfy
- K1 [Al] / (3 [Si] + [Mn]) ⁇ 0.050
- K1 is a value represented by the contents of Al, Si, and Mn, and is a value that affects the formation of the internal oxide layer.
- Al, Si, and Mn are all elements that are lower than Fe, and are oxidized prior to Fe during the progress of oxidation of the steel material, and many fine oxide particles are generated in the surface layer portion of the steel material.
- An internal oxide layer is formed by oxide particles of these elements.
- the oxide generated in the internal oxide layer is a composite oxide composed of Al, Si, Mn, and O.
- the composition of each composite oxide is Si—Mn system containing SiO 2 and MnO as main components and containing less than 10% Al 2 O 3 , and SiO 2 and Al 2 O 3 as main components and containing less than 20% MnO. They are roughly classified into Si—Al system containing Al—MnO, Al—Mn system containing Al 2 O 3 and MnO as main components and containing less than 10% SiO 2 .
- the content of Al 2 O 3 is preferably 15% or more and 40% or less as a total amount in the composite oxide in the internal oxide layer.
- the oxide in the internal oxide layer is mainly SiMn oxide. Further, inside the partially precipitated Cu liquid phase, oxygen is not sufficiently diffused and does not react with Si and Mn, so that the internal oxide layer does not grow and the thickness of the internal oxide layer becomes non-uniform. As a result, the oxidation (grain boundary oxidation) at the grain boundaries of the steel material proceeds remarkably, the infiltration of the precipitated Cu liquid phase into the grain boundaries is facilitated, and Cu embrittlement is caused.
- the value of K1 is preferably 2.0 or less.
- the value of K1 is larger than 2.0, Al 2 O 3 is excessively formed inside the internal oxide layer.
- the oxide of each element constituting the steel material grows along the crystal grain boundary of the steel material.
- the oxidation of the steel material is promoted, the infiltration of the precipitated Cu liquid phase into the grain boundary becomes easy, and Cu embrittlement occurs. Is triggered.
- K2 [Ni] / ([Cu] +5 [Sn]) ⁇ 0.10 K2 is a value represented by the contents of Ni, Cu, and Sn, and is a value that affects the selective oxidation behavior of Fe when the oxidation of the steel material proceeds.
- K2 is preferably 1.2 or less. This is because if the value of K2 is too large, the effect is saturated, which is economically undesirable.
- K3 [Al] / [Ni] ⁇ 0.20 K3 is the ratio of the content ratio of Al and Ni, and is a value that affects the uniformity of the thickness of the formed internal oxide layer.
- the thickness of the internal oxide layer becomes non-uniform.
- the thickness of the internal oxide layer is not uniform, the growth of oxides of each element constituting the steel material along the crystal grain boundary of the steel material is promoted, and the infiltration of the precipitated Cu liquid phase into the grain boundary Therefore, Cu embrittlement is caused.
- the value of K3 is preferably 2.0 or less. If the value of K3 is larger than 2.0, Al 2 O 3 is excessively formed inside the internal oxide layer. In particular, an oxide of each element constituting the steel material grows along the crystal grain boundary of the steel material, which promotes the oxidation of the steel material and facilitates infiltration of the precipitated Cu liquid phase into the grain boundary. Is triggered.
- Al 2 O 3 content in the composite oxide formed in the internal oxide layer 15 to 40%
- the content of Al 2 O 3 in the composite oxide formed in the internal oxide layer is preferably 15 to 40%.
- the thickness of the internal oxide layer is nonuniform. This is because internal oxidation proceeds with surface layer oxidation (scale growth), but Ni is partially concentrated to form an FeNi alloy phase. In this FeNi alloy phase, internal oxidation hardly proceeds, and as a result, the thickness of the internal oxide layer becomes non-uniform. In the region where the internal oxidation does not proceed, only the grain boundary oxidation proceeds remarkably while the oxidation within the crystal grains is suppressed, and this becomes the starting point of cracking.
- the Al content in the composite oxide is increased.
- the content of Al 2 O 3 causes defects occur during hot working to be a rigid when it comes to 40% by mass or more. Therefore, the Al 2 O 3 content in the composite oxide formed in the internal oxide layer is preferably 40% or less.
- the content of Al 2 O 3 in the composite oxide formed in the internal oxide layer can be obtained, for example, by sequentially performing the following 1) to 7).
- 1) An observation sample is collected from a steel material, and a surface layer portion (longitudinal section) is observed using a scanning electron microscope (SEM).
- SEM scanning electron microscope
- 2) The composition difference is observed in the reflected electron image, and the oxide is selected.
- the oxide forming elements O, Al, Si, and Mn are all lighter than Fe, the oxide is observed with a lower luminance than the Fe matrix and can be distinguished.
- the oxide composition is evaluated by an energy dispersive X-ray detector (EDS).
- the composition of the oxide region is evaluated by the atomic ratio (atomic concentration). 4) Obtain the atomic concentration ratio of the metal element excluding the light elements C and O and the main component Fe of the parent phase from the constituent elements of the composition based on the atomic number concentration (Al, Si, Obtain the ratio of Mn). 5) Considering the valence of oxide formation, it is converted to a constituent oxide. The molecular weight of each of Al 2 O 3 , SiO 2 , and MnO (Al 2 O 3 (AlO 1.5 ): 50.98, SiO 2 : 60.10, MnO: 70.94) is converted into a weight concentration. 6) The Al 2 O 3 content in the composite oxide is calculated. 7) The above is performed on at least 10 composite inclusions, and the average value is obtained.
- Cu] / [Sn] is the ratio of Cu and Sn content, that is, the above-mentioned Cu / Sn ratio.
- the Cu / Sn ratio is set to 1.0 to 8.0.
- the method for producing a Cu—Sn coexisting steel of the present invention is the same as the above (1) when the cast steel is continuously cast using the molten steel having the above composition.
- the internal oxide layer is formed by oxidizing the slab surface during the cooling process of the slab and adjusting the condition expressed by the formula (3).
- Al 2 O 3 is produced.
- the manufacturing method S1 includes a molten steel component composition adjusting step S11 (hereinafter, sometimes abbreviated as “S11”) and an internal oxide layer forming step S12 (hereinafter abbreviated as “S12”).
- S11 molten steel component composition adjusting step S11
- S12 internal oxide layer forming step S12
- the molten steel component composition adjusting step S11 when the cast slab is continuously cast using the molten steel having the above component composition, the component composition of the molten steel is adjusted so as to satisfy the conditions expressed by the above formulas (1) to (3). It is a process to do.
- the adjustment of the composition of the molten steel in S11 is performed by adding an alloy at the refining stage.
- the internal oxide layer forming step S12 is a step of forming an internal oxide layer by oxidizing the surface of the slab in the cooling process of the slab obtained by cooling the molten steel whose component composition is adjusted in S11. .
- Al 2 O 3 is included in the composite oxide generated in the internal oxide layer formed in S12.
- the content of Al 2 O 3 in the composite oxide formed in the internal oxide layer formed in S12 is preferably 15 to 40% by mass.
- the molten steel component composition adjusting step satisfies the conditions represented by the above formulas (1) to (3) and the condition represented by the above formula (4). It is preferable that it is the process of adjusting the component composition of molten steel.
- FIG. 2 shows a flow chart for explaining the production method S2 of Cu—Sn coexisting steel according to this embodiment (hereinafter sometimes referred to as “production method S2”). As shown in FIG. 2, the manufacturing method S2 includes a molten steel component composition adjusting step S21 (hereinafter sometimes abbreviated as “S21”) and an internal oxide layer forming step S22 (hereinafter abbreviated as “S22”). In the above order.
- the molten steel component composition adjustment step S21 adjusts the component composition of the molten steel to satisfy the conditions expressed by the above formulas (1) to (4) when continuously casting a slab using molten steel having the above component composition. It is a process to do. Adjustment of the composition of the molten steel in S21 is performed by adding an alloy at the refining stage.
- the internal oxide layer forming step S22 is a step of forming an internal oxide layer by oxidizing the surface of the slab in the cooling process of the slab obtained by cooling the molten steel whose component composition has been adjusted in S21. .
- Al 2 O 3 is included in the composite oxide generated in the internal oxide layer formed in S22.
- the content of Al 2 O 3 in the composite oxide formed in the internal oxide layer formed in S22 is preferably 15 to 40% by mass.
- the slab manufactured by this method does not have surface cracks or surface defects, and surface cracks do not occur during hot rolling in the subsequent process. Therefore, by using the Cu—Sn coexisting steel of the present invention as a raw material, a steel material having a good surface quality can be produced.
- An ingot produced by pouring molten steel satisfying the above component composition and the above formulas (1) to (3) or the above formulas (1) to (4) into a bottomed mold is also divided and subjected to hot rolling.
- the surface of the slab is oxidized to form an internal oxide layer, and Al 2 O 3 is generated in the composite oxide formed in the internal oxide layer. Cu embrittlement during cooling can be suppressed.
- FIG. 3 is a diagram illustrating a Cu—Sn coexisting steel 10 according to one embodiment of the present invention.
- the Cu—Sn coexisting steel 10 shown in FIG. 3 is a slab produced by the production method S1. According to the production method S1, it is possible to produce a high quality slab (Cu—Sn coexisting steel 10) in which generation of surface cracks and surface flaws associated with Cu embrittlement is suppressed. Therefore, the Cu—Sn coexisting steel 10 is a steel material with good quality in which generation of surface cracks and surface flaws accompanying Cu embrittlement is suppressed.
- the slab manufactured by the manufacturing method S1 shows the slab manufactured by the manufacturing method S1
- the Cu—Sn coexisting steel of the present invention can also be manufactured by the manufacturing method S2. Also with the production method S2, it is possible to produce a slab of good quality in which the occurrence of surface cracks and surface defects accompanying Cu embrittlement is suppressed.
- Preliminary test 1-1 Test method No. 1 shown in Table 1.
- a Cu—Sn coexisting steel having a component composition of 1 to 22 was melted in a vacuum melting furnace to obtain a 50 kg ingot.
- the Al content indicates the content of acid-soluble Al.
- the obtained ingot was forged, and the forged product was heated and rolled to obtain a test steel material.
- Each test steel was held in an electric furnace at 1100 ° C. in an air atmosphere for 15 minutes to oxidize the surface to form a scale, and then allowed to cool to room temperature.
- No. Reference numerals 1 to 12 are reference examples in which the component composition and the values of K1 to K3 satisfy the provisions of the present invention.
- 17 is a comparative example in which the component composition does not satisfy the definition of the present invention.
- No. 18 and no. No. 20 is a comparative example in which the component composition does not satisfy the provisions of the present invention.
- No. 19 is a comparative example in which the component composition and the value of K2 do not satisfy the provisions of the present invention.
- No. 21 is a comparative example in which the value of K1 does not satisfy the provisions of the present invention.
- 22 is a comparative example in which the value of K3 does not satisfy the definition of the present invention.
- the evaluation items were the following a to d. a. Precipitation state of Cu liquid phase in surface layer part of test steel material. This is because if the Cu liquid phase is precipitated, the steel material tends to become brittle.
- the liquid phase has a property of being accumulated in a film form once it is precipitated, and it can be determined whether it is liquid phase precipitation or solid phase precipitation from the form of precipitation. When the precipitation form is granular and is as fine as less than 1 ⁇ m, it can be determined that the Cu solid phase does not precipitate, that is, the liquid film does not become brittle. Therefore, it was judged that the cracking susceptibility was high if it was deposited in the form of a film.
- the precipitated Cu liquid phase does not accumulate at the interface and is taken into the scale, so it does not become brittle. Therefore, if the undulation at the boundary between the scale and the parent phase was less than 50 ⁇ m, it was judged that the cracking sensitivity was high.
- Table 2 shows the ratio of Al 2 O 3 in the composite oxide formed in the internal oxide layer, in addition to the evaluation of crack sensitivity as each of items a to d and overall evaluation.
- Table 2 when there was an undulation of 50 ⁇ m or more at the boundary between the scale and the parent phase of the surface layer of the test steel material, it was judged as “uneven”, and when there was no undulation, it was judged as “smooth”.
- the “thickness of the internal oxide layer” was determined to be “uniform” when the difference between the maximum and minimum thickness was less than 30 ⁇ m, and “non-uniform” when the difference was 30 ⁇ m or more.
- the “ratio of Al 2 O 3 in the oxide contained in the internal oxide layer” was evaluated as ⁇ when the ratio was 15% or more and 40% or less, and x when the ratio was less than 15% or more than 40%.
- each complex oxide both of which are contained within the oxide layer is more than 5%, and containing Al 2 O 3 of less than 90%, the total amount of Al 2 O 3 composite oxide contains ( The content ratio of Al 2 O 3 in the composite oxide was 15% or more and 40% or less.
- No. which is a comparative example.
- precipitation of the Cu liquid phase occurs at the entire surface or part of the interface between the scale and the parent phase of the surface layer of the test steel material, and the precipitated Cu liquid phase enters the old ⁇ grain boundary of the surface layer of the test steel material.
- the cracking sensitivity was high.
- Some old ⁇ grain boundaries were accompanied by open cracks.
- the ratio of Al 2 O 3 in the oxide contained in the internal oxide layer was less than 15% by mass or more than 40% by mass.
- No. 20 which is a comparative example.
- the steel surface layer part became an FeNi alloy, and the growth of the internal oxide layer was partially suppressed, the thickness of the internal oxide layer became nonuniform, and the grain boundary oxidation was promoted.
- Precipitation of the Cu liquid phase partially occurred at the interface between the scale and the parent phase of the surface layer of the test steel material, and the precipitated Cu liquid phase invaded the old ⁇ grain boundary of the surface layer of the test steel material.
- all of the Cu liquid phase precipitates at the interface between the scale and the parent phase of the surface layer of the test steel material, and the precipitated Cu liquid phase penetrates into the old ⁇ grain boundary of the surface layer of the test steel material. Therefore, cracking sensitivity was high.
- the ratio of Al 2 O 3 in the oxide contained in the internal oxide layer was less than 15% by mass or more than 40% by mass.
- the thickness of the internal oxide layer became non-uniform, and the oxide in the internal oxide layer was mainly SiMn oxide. Grain boundary oxidation progressed remarkably, and the precipitated Cu liquid phase infiltrated deeply at the grain boundary and became brittle.
- the steel surface layer part was made into a FeNi alloy, and the oxidation inside thereof was partially suppressed. As a result, the thickness of the internal oxide layer became non-uniform and grain boundary oxidation was promoted. Precipitation of the Cu liquid phase occurred at a part of the interface between the scale and the parent phase of the surface layer of the test steel material, and the precipitated Cu liquid phase entered the old ⁇ grain boundary of the surface layer of the test steel material.
- Test method No. shown in Table 3 Cu—Sn coexisting steels having component compositions of 23 and 24 were melted in a melting furnace.
- No. 23 is an example of the present invention in which the component composition and the values of K1 to K3 satisfy the definition of the present invention.
- No. 24 is a comparative example in which the values of K1 and K3 do not satisfy the definition of the present invention.
- the molten 2.5t molten steel is poured into a tundish through a ladle, and supplied into an oscillating internal water-cooled copper plate mold through an immersion nozzle with a superheat of 50-70 ° C.
- Continuous casting was performed with a vertical continuous casting machine at a casting speed of 0.8 m / min.
- the mold flux disposed on the molten steel in the mold has a physical property value of 0.04 Pa ⁇ s at a solidification temperature of 1235 ° C. and 1300 ° C., and a basicity (CaO content (mass%) as SiO 2 content ( The value obtained by dividing by (mass%) was 1.8.
- a part of the cooled slab slab was cut, and a test piece for investigating the presence or absence of a slab surface crack and a steel material for a hot rolling test were collected.
- the hot rolling test was performed by heating the collected steel material to 1100 ° C. in the air and then rolling it at a reduction rate of 75%.
- the evaluation items were the presence or absence of surface cracks in the slab slab and the presence or absence of surface cracks in the steel material after rolling (hereinafter referred to as “rolled steel material”). Each surface crack was examined for the presence of grain boundary cracks by die check (dye penetrant inspection).
- the Cu—Sn coexisting steel of the present invention has no surface cracks or surface flaws, and surface cracks do not occur even during the subsequent hot rolling. Therefore, by using the Cu—Sn coexisting steel of the present invention as a raw material, a steel material having a good surface quality can be produced.
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Abstract
Description
[Al]/(3[Si]+[Mn])≧0.050 …(1)
[Ni]/([Cu]+5[Sn])≧0.10 …(2)
[Al]/[Ni]≧0.20 …(3)
ここで、[Al]、[Si]、[Mn]、[Ni]、[Cu]、および[Sn]は、それぞれ溶鋼中におけるAl、Si、Mn、Ni、Cu、およびSnの含有率(質量%)である。
1.0≦[Cu]/[Sn]≦8.0 …(4)
1-1.添加元素の検討
元来、Cu含有鋼における、Cuに起因する赤熱脆化(Cu脆化)は、Cu液相が、母相であるFeのオーステナイト相の結晶粒界に浸透し、結晶粒界を脆弱化させることにより生じると考えられている。Cu液相の析出は、1100℃付近の温度(例えば1050~1150℃程度の温度領域)で生じやすい。
Niは、Cuと同様に、Feより貴な元素である。Niは、Fe中のCuの固溶限を拡大してCuの融点を上昇させるため、Cu脆化を抑制する。そのため、一般に、Cu含有鋼ではNiを添加して、鋼材の割れの発生を防止している。
上記Cu-Sn共存鋼において、Ni含有率が0.1~0.5%となるようにNiを添加し、Al含有率を0.05%以下とした場合、鋼材の表層部が酸化してスケールが形成される。このスケールについて、以下のa~dの効果が生じる。
上記Cu-Sn共存鋼において、Al含有率が0.1~0.5%となるようにAlを添加し、Ni含有率を0.05%未満とした場合も、鋼材の表層部が酸化してスケールが形成される。Alによって以下のeおよびfの効果が生じる。
以上のように、Cu-Sn共存鋼において、NiやAlを添加すると、母相のFeの選択酸化挙動に影響があるものの、NiまたはAlの一方が不足すると、Cu液相を伴ったCu脆化の抑制には効果が少ないことをつきとめた。
h.Fe中のCuの固溶限を拡大する。
i.鋼材表層部におけるCu液相およびFeNi合金相の内側で、厚さが均一な内部酸化層が形成される。
j.内部酸化層の酸化物粒子が鋼材表層の合金相の内側にも生成しやすく、Cu液相がスケール中に排斥されやすい。
本発明のCu-Sn共存鋼は、以上の検討の結果得られた知見に基づいてなされたものであり、C:0.04~0.20%、Si:0.05~1.00%、Mn:0.20~2.50%、P:0.05%以下、S:0.02%以下、Cu:0.20~1.50%、Sn:0.06~0.50%、Al:0.06~1.00%、およびNi:0.05~1.00%を含有し、残部がFeおよび不純物からなる成分組成である。本発明において不純物としては、H、N、O、Mg、Ca、Sr、As、Se、Sb、Teが例示される。また、Feの一部をその他の合金成分に代えてもよい。本発明においてその他の合金成分としては、B、Ti、Zr、V、Nb、Cr、Mo、Wが例示される。
Cは材料の強度を高める効果を有する元素である。この効果を得るためにC含有量は0.04%以上とする。一方、C含有量が0.20%を超えると靱性が低下するとともに溶接割れ感受性が高まる。したがって、C含有量は0.04~0.20%とする。
Siは脱酸に有効な元素である。この効果を得るためにSi含有量は0.05%以上とする。一方、Si含有量が1.00%を超えると靱性が低下する恐れがある。したがって、Si含有量は0.05~1.00%とする。
Mnは材料の強度を高める効果を有する元素である。この効果を得るためにMn含有量は0.20%以上とする。一方、Mn含有量が2.50%を超えると靱性が低下する恐れがある。したがって、Mn含有量は0.20~2.50%とする。
Pは鋼材中に不可避に含まれる不純物元素であり、少ない方がよい。P含有量が0.05%を超えると、熱間での割れ感受性が高くなる。したがって、P含有量は0.05%以下とし、少ないほど好ましい。Pの上限は0.03%であることが好ましい。
Sは鋼材中に不可避に含まれる不純物元素であり、少ない方がよい。S含有量が0.02を超えると、熱間での割れ感受性が高くなる。また、鋼材の腐食起点となるMnS介在物の量が多くなって、耐食性が損なわれる。したがって、S含有量は0.02%以下とし、少ないほど好ましい。Sの上限は0.010%であることが好ましい。
Cuは鋼の耐食性を向上させる効果を有する元素である。この効果を得るために、Cu含有量は0.20%以上とする。一方、鋼材中のCuが過剰に存在すると、鋼の製造工程において、高温酸化を伴う熱間工程、たとえば、連続鋳造工程および熱間圧延工程において、赤熱脆化が生じ、鋼材の表面に割れまたは疵が発生する。このためCu含有量は1.50%以下とする。
Snは鋼の耐食性を向上させる効果を有する元素である。この効果を得るために、Sn含有量は0.06%以上とする。一方、Sn含有量が0.50%を超えると、耐食性は飽和する。また、Cuを含有する鋼にSnを含有させると耐食性が向上するが、赤熱脆化は助長され、製造工程で表面疵が発生しやすくなる。このためSn含有量は0.50%以下とする。
Al:0.06~1.00%
Alは、元来、鋼の脱酸に用いられる元素であり、本発明ではCu脆化の抑制のため含有させる。しかし、Al含有率が0.06%未満では十分な脆化抑制の効果が得られない。一方、1.00%を超えると、鋳片の冷却過程で形成される内部酸化層内に生じるAl2O3含有率が過剰となり、脆化抑制の効果が損なわれる。これらのことから、本発明では、Al含有率を0.06~1.00%とする。このAl含有率は酸可溶Alの含有率である。
Niは、Fe中のCuの固溶限を拡大するとともに、スケールと鋼材表層部の母相との界面を凹凸化し、析出したCu液相のスケール側への排斥を促進する元素である。また、鋼材の表層部でFeNi合金相を形成させ、母相の酸化の進行を抑制する元素である。しかし、Ni含有率が0.05%未満では十分な脆化抑制の効果が得られない。また、Ni含有率が1.00%を超えると経済的に好ましくないだけでなく、鋼材表層部の選択酸化時にFeNi合金相を容易に形成させるため合金相内の内部酸化層の成長が抑制され、粒界酸化の進行を助長することになる。これらのことから、本発明では、Ni含有率を0.05~1.00%とする。
本発明では、さらに、溶鋼の成分組成が下記(1)~(3)式の関係を満足するように調整する。
K1=[Al]/(3[Si]+[Mn])≧0.050 …(1)
K2=[Ni]/([Cu]+5[Sn])≧0.10 …(2)
K3=[Al]/[Ni]≧0.20 …(3)
ここで、[Al]、[Si]、[Mn]、[Ni]、[Cu]、および[Sn]は、それぞれ溶鋼中におけるAl、Si、Mn、Ni、Cu、およびSnの含有率(質量%)である。
1.0≦[Cu]/[Sn]≦8.0 …(4)
K1は、Al、Si、およびMnの含有率で表される値であり、内部酸化層の形成に影響を及ぼす値である。Al、Si、およびMnはいずれもFeより卑な元素であり、鋼材の酸化進行時には、Feに先行して酸化し、鋼材の表層部で微細な酸化物粒子を多数生成する。これらの元素の酸化物粒子によって形成されるのが内部酸化層である。
K2は、Ni、Cu、およびSnの含有率とで表される値であり、鋼材の酸化が進行する際のFeの選択酸化挙動に影響を及ぼす値である。
K3は、AlとNiの含有率の比であり、形成される内部酸化層の厚さの均一さに影響を及ぼす値である。
内部酸化層中に生成する複合酸化物中のAl2O3の含有率は、15~40%とすることが好ましい。複合酸化物中のAl2O3の含有率が15%未満になると、内部酸化層の厚さに不均一が生じる。これは、表層部の酸化(スケールの成長)とともに内部酸化が進行するが、部分的にNiが濃化してFeNi合金相を形成する。このFeNi合金相では内部酸化がほとんど進行せず、その結果、内部酸化層の厚さが不均一となる。内部酸化の進行しない領域では、結晶粒内での酸化が抑制されたまま粒界酸化のみが著しく進行し、これが割れの起点となる。また、著しい粒界酸化が進行した粒界には、Cu液相の浸潤も容易となって、Cu脆化を引き起こす。他方、FeNi合金相でもAl2O3を生成させれば、内部酸化層の厚さが均一となる。その結果、Cu脆化が抑制される。このときの複合酸化物中のAl2O3の含有率は15%以上である。
1)鋼材から観察試料を採取し、走査型電子顕微鏡(SEM)を用いて表層部(縦断面)を観察する。
2)反射電子像で組成差を観察し、酸化物を選択する。ここで、反射電子像は、重い元素ほど輝度が高い。酸化物の形成元素O、Al、Si、およびMnは、いずれもFeよりも軽いので、酸化物はFe母相よりも低輝度で観察され、判別できる。
3)エネルギー分散型X線検出器(EDS)にて酸化物の組成を評価する。その際に、酸化物領域について原子数比(原子数濃度)で組成を評価する。
4)原子数濃度による組成の構成元素から、軽元素C、O、および母相の主成分Feを除いた金属元素の原子数濃度比率を求める(複合酸化物の主要構成元素としてAl、Si、Mnの比率を求める)。
5)酸化物形成の価数を考慮し、構成酸化物に換算する。Al2O3、SiO2、およびMnOそれぞれの分子量(Al2O3(AlO1.5):50.98、SiO2:60.10、MnO:70.94)から、重量濃度に換算する。
6)複合酸化物中のAl2O3含有量を算出する。
7)以上を少なくとも10個以上の複合介在物について行い、平均値を求める。
[Cu]/[Sn]は、CuとSnの含有率の比、すなわち上述のCu/Sn比である。Cu/Sn比を1.0~8.0とすることにより、塩化物環境、酸化性環境等、過酷な環境下においても十分な耐食性を得ることができる。
本発明のCu-Sn共存鋼の製造方法は、上記成分組成の溶鋼を用いて鋳片を連続鋳造する際に、溶鋼の成分組成を上記(1)~(3)式で表される条件を満足するように調整し、鋳片の冷却過程で鋳片表面を酸化させて内部酸化層を形成し、この内部酸化層内に生成する複合酸化物中にAl2O3を生じさせる方法である。これにより、Cu脆化に伴う表面割れや表面疵の発生が抑制された、品質の良好な鋳片を製造することができる。
図1は、本発明の一の実施形態に係るCu-Sn共存鋼の製造方法S1(以下において、「製造方法S1」と称することがある。)を説明するフローチャートである。図1に示すように、製造方法S1は、溶鋼成分組成調整工程S11(以下において、「S11」と略記することがある。)と、内部酸化層形成工程S12(以下において、「S12」と略記することがある。)とを、上記順に有する。溶鋼成分組成調整工程S11は、上記成分組成の溶鋼を用いて鋳片を連続鋳造する際に、溶鋼の成分組成を上記(1)~(3)式で表される条件を満足するように調整する工程である。S11における溶綱の成分組成の調整は、精錬段階で合金を添加することにより行う。内部酸化層形成工程S12は、S11で成分組成が調整された溶鋼を冷却することにより得られた、鋳片の冷却過程で鋳片表面を酸化させることにより、内部酸化層を形成する工程である。製造方法S1では、S12で形成した内部酸化層内に生成する複合酸化物中に、Al2O3を含有させる。製造方法S1において、S12で形成した内部酸化層内に生成する複合酸化物中のAl2O3の含有率は、質量%で、15~40%であることが好ましい。
また、本発明のCu-Sn共存鋼の製造方法において、溶鋼成分組成調整工程は、上記(1)~(3)式で表わされる条件、および、上記(4)式で表される条件を満足するように、溶鋼の成分組成を調整する工程であることが好ましい。この形態に係るCu-Sn共存鋼の製造方法S2(以下において、「製造方法S2」と称することがある。)を説明するフローチャートを図2に示す。図2に示すように、製造方法S2は、溶鋼成分組成調整工程S21(以下において、「S21」と略記することがある。)と、内部酸化層形成工程S22(以下において、「S22」と略記することがある。)とを、上記順に有する。溶鋼成分組成調整工程S21は、上記成分組成の溶鋼を用いて鋳片を連続鋳造する際に、溶鋼の成分組成を上記(1)~(4)式で表される条件を満足するように調整する工程である。S21における溶綱の成分組成の調整は、精錬段階で合金を添加することにより行う。内部酸化層形成工程S22は、S21で成分組成が調整された溶鋼を冷却することにより得られた、鋳片の冷却過程で鋳片表面を酸化させることにより、内部酸化層を形成する工程である。製造方法S2では、S22で形成した内部酸化層内に生成する複合酸化物中に、Al2O3を含有させる。製造方法S2において、S22で形成した内部酸化層内に生成する複合酸化物中のAl2O3の含有率は、質量%で、15~40%であることが好ましい。
図3は、本発明の一の実施形態に係るCu-Sn共存鋼10を説明する図である。図3に示したCu-Sn共存鋼10は、上記製造方法S1で製造された鋳片である。製造方法S1によれば、Cu脆化に伴う表面割れや表面疵の発生が抑制された、品質の良好な鋳片(Cu-Sn共存鋼10)を製造することができる。したがって、Cu-Sn共存鋼10は、Cu脆化に伴う表面割れや表面疵の発生が抑制された、品質が良好な鋼材である。なお、図3には、製造方法S1で製造された鋳片を示したが、本発明のCu-Sn共存鋼は、製造方法S2で製造することも可能である。製造方法S2によっても、Cu脆化に伴う表面割れや表面疵の発生が抑制された、品質の良好な鋳片を製造することができる。
1-1.試験方法
表1に示すNo.1~22の成分組成を有するCu-Sn共存鋼を真空溶解炉で溶製して50kgのインゴットを得た。同表において、Alの含有率は、酸可溶Alの含有率を示した。得られたインゴットを鍛造し、その鍛造品に対して加熱および圧延を行い、試験鋼材を得た。各試験鋼材は、1100℃の大気雰囲気の電気炉内で15分間保持して表面を酸化させてスケールを生成させた後、室温まで放冷した。
各試験鋼材は、割れ感受性によって評価した。割れ感受性の評価は、放冷後の試験鋼材の表層部の断面についての光学顕微鏡を用いた組織観察、ならびにSEM/EDSを用いた組織観察および元素分析によって行った。
a.試験鋼材の表層部におけるCu液相の析出状態。Cu液相の析出があると鋼材が脆化しやすいからである。液相は一旦析出すると膜状に集積する性質があり、析出形態から液相析出か固相析出かを判定できる。析出形態が粒状でかつ1μm未満と微細なときはCu固相の析出すなわち液膜脆化しないものと判断できる。そこで、膜状に析出していれば割れ感受性が大きいと判断した。
以上のa~dについての評価を総合して、割れ感受性を評価した。表2には、a~dの各項目および総合評価としての割れ感受性の評価に加え、内部酸化層内に生成した複合酸化物中のAl2O3の割合についても示した。表2において、スケールと試験鋼材表層部の母相との境界に50μm以上の起伏がある場合に「凹凸あり」とし、当該起伏がない場合に「平滑」と判断した。また、「内部酸化層の厚さ」は、厚さの最大と最小の差が30μm未満の場合に「均一」とし、差が30μm以上の場合に「不均一」と判断した。また、「内部酸化層に含まれる酸化物中のAl2O3の割合」は、15%以上40%以下の場合を○、15%未満または40%を超える場合を×とした。なお、表1に示したNo.8の鋼材では、内部酸化層に含まれる複合酸化物中のAl2O3含有量が、任意の複合酸化物10個を選びEDSで組成分析した平均値で29.3%であった。
続いて、予備試験の結果を踏まえ、連続鋳造機を用いた本試験を行った。
表3に示すNo.23および24の成分組成を有するCu-Sn共存鋼を溶解炉にて溶製した。No.23は、成分組成およびK1~K3の値が本発明の規定を満足する本発明例である。No.24は、K1およびK3の値が本発明の規定を満足しない比較例である。
評価項目は、スラブ鋳片の表面割れの有無と、圧延後の鋼材(以下「圧延鋼材」という。)の表面割れの有無とした。いずれの表面割れともダイチェック(染色浸透探傷検査)によって粒界割れの有無の調査を行った。
本発明例であるNo.23のスラブ鋳片および圧延鋼材のいずれも表面割れは皆無であり、Cu脆化は抑制されていた。
S11、S21…溶鋼成分組成調整工程
S12、S22…内部酸化層形成工程
10…Cu-Sn共存鋼
Claims (4)
- 化学組成が、質量%で、C:0.04~0.20%、Si:0.05~1.00%、Mn:0.20~2.50%、P:0.05%以下、S:0.02%以下、Cu:0.20~1.50%、およびSn:0.06~0.50%を含有し、さらに、Al:0.06~1.00%およびNi:0.05~1.00%を含有し、残部がFeおよび不純物からなる溶鋼を連続鋳造し、Cu-Sn共存鋼を製造する際、
前記溶鋼の成分組成を下記(1)~(3)式で表される条件を満足するように調整し、鋳片の冷却過程で鋳片表面を酸化させることにより内部酸化層を形成し、当該内部酸化層内に生成する複合酸化物中にAl2O3を含有させることを特徴とするCu-Sn共存鋼の製造方法。
[Al]/(3[Si]+[Mn])≧0.050 …(1)
[Ni]/([Cu]+5[Sn])≧0.10 …(2)
[Al]/[Ni]≧0.20 …(3)
ここで、[Al]、[Si]、[Mn]、[Ni]、[Cu]、および[Sn]は、それぞれ溶鋼中におけるAl、Si、Mn、Ni、Cu、およびSnの含有率(質量%)である。 - 前記内部酸化層内に生成する複合酸化物中のAl2O3の含有率が、質量%で、15~40%であることを特徴とする請求項1に記載のCu-Sn共存鋼の製造方法。
- 前記溶鋼の成分組成を下記(4)式で表される条件を満足するように調整することを特徴とする請求項1または2に記載のCu-Sn共存鋼の製造方法。
1.0≦[Cu]/[Sn]≦8.0 …(4) - 請求項1~3のいずれかに記載のCu-Sn共存鋼の製造方法で製造されていることを特徴とするCu-Sn共存鋼。
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