Classifications

• • 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
• • 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
  • B22D11/10—Supplying or treating molten metal
  • B22D11/108—Feeding additives, powders, or the like
• • 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
  • B22D11/12—Accessories for subsequent treating or working cast stock in situ
• • 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
  • B22D11/16—Controlling or regulating processes or operations
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/001—Heat treatment of ferrous alloys containing Ni
• • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
• • 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
• • 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
• • 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
• • 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
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

• the present invention relates to a steel slab containing Ni (nickel), a continuous casting method, and a method for producing a steel slab.
• Ni-containing steel Steel containing around 9% by mass of Ni (hereinafter also referred to as Ni-containing steel) is also referred to as 9% Ni steel.
• 9% Ni steel can withstand use at temperatures below -160°C, and is therefore widely used as a welded structural steel for low-temperature applications such as LNG tanks, for example.
• Ni-containing steel is prone to scratches on its surface.
• a slab after being cast has many cracks (hereinafter also referred to as surface cracks) on and near the surface.
• S (sulfur), P (phosphorus), etc. contained in the steel slab become concentrated in a specific region.
• An increase in the concentration of S (sulfur) and P (phosphorus) causes grain boundary embrittlement of the steel slab. Therefore, in this region, surface cracks occur because the embrittled grain boundaries are destroyed by tensile stress.
• Patent Document 1 discloses controlling the cooling rate and the surface temperature of the slab in a secondary cooling zone when continuously casting molten steel containing 5 to 10% by mass of Ni. ing.
• Patent Document 2 describes that when continuously casting Ni-containing steel containing 8 to 10% by mass of Ni, the reduction of area at the time of casting is estimated, and the secondary Controlling the cooling intensity is disclosed.
• Ni-containing steels having a Ni content of less than 7.5% by mass is increasing significantly.
• the present invention has been made in view of the above problems, and provides a steel slab containing 2.0% by mass or more and less than 7.5% by mass of Ni and less surface cracking, a continuous casting method, and a continuous casting method for producing a steel slab.
• the purpose is to provide a manufacturing method.
• the present invention has the following features.
• a continuous casting method for casting the steel slab according to [1] or [2] A continuous casting method that includes the step of vibrating a mold at a frequency of 80 cycles or more per minute.
• a continuous casting method for casting the steel slab according to [1] or [2] A step of adding mold powder having a viscosity of 0.5 Pa ⁇ s (5 poise) or more at 1300° C. into the mold, A continuous casting method comprising the step of vibrating a mold at a frequency of 50 cycles or more per minute.
• the size of the solidification cells can be made smaller than the conventional size. Thereby, segregation of S (sulfur) and P (phosphorus) at the interface of the solidification cell can be reduced more than before. As a result, embrittlement at the interface of the solidified cells can be suppressed. Moreover, the stress acting on the interface of the coagulation cell can also be dispersed. Thereby, it is possible to suppress the occurrence of cracks at the interface of the solidification cells, and it is possible to reduce the occurrence of cracks on the surface of the steel slab. As a result, it is possible to reduce the processing time for the treatment for removing cracks on the surface of the steel slab, and it is possible to improve productivity and reduce manufacturing costs.
• the Ni-containing steel slab of the present invention (hereinafter also simply referred to as steel slab) contains 2.0% by mass or more and less than 7.5% by mass of Ni.
• the steel slab can be used, for example, as a low-temperature steel used in a temperature range lower than room temperature.
• the Ni-containing steel slab of the present invention has, in mass %, C: 0.03% or more and 0.10% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.10% or more and 1 .00% or less, P: 0.001% or more and 0.010% or less, S: 0.0001% or more and 0.0050% or less, Ni: 2.0% or more and less than 7.5%, Al: 0.010% N: 0.0010% to 0.0050%, O: 0.0005% to 0.0040%, and the remainder consists of Fe and inevitable impurities.
• the steel slab By containing C (carbon) as a composition, the steel slab can ensure the strength of the base material. In particular, by setting the C content in the steel slab to 0.03% by mass (hereinafter simply referred to as "%") or more, the strength of the base material can be improved. When the content of C contained in the steel slab becomes excessive, cementite and island martensite, which become the starting point of brittle fracture, increase, and there is a possibility that suitable toughness cannot be obtained. By controlling the content of C contained in the steel slab to 0.10% or less, appropriate toughness of the steel slab can be obtained.
• the steel slab By containing Si (silicon) as a composition, the steel slab can enhance the deoxidizing effect of removing oxygen contained in the steel slab. Moreover, the strength of the base material can be ensured by containing Si as a composition of the steel slab. When the amount of Si added increases, island-like martensite is generated in the structure of the weld heat-affected zone (HAZ), and good toughness of the HAZ tends to not be obtained.
• Si silicon
• HAZ weld heat-affected zone
• the steel slab By containing Mn as a composition, the steel slab can ensure the strength of the base material.
• the amount of Mn added increases, there is a tendency that good HAZ toughness cannot be obtained.
• the strength of the base material can be improved.
• suitable toughness of the HAZ can be ensured.
• the steel slab When steel slabs contain P (phosphorus) as a composition, they tend to cause grain boundary embrittlement. For this reason, the steel slab preferably contains P (phosphorus) in the lowest possible amount.
• P (phosphorus) content of the steel slab is 0.010% or less, it is possible to suppress the promotion of surface cracking due to grain boundary embrittlement. Thereby, the toughness of the base material and the HAZ can be improved.
• the P (phosphorus) content of the steel slab is set to 0.001% or more, it is possible to suppress an increase in the load of dephosphorization refining in the steelmaking process, and to suppress an increase in manufacturing costs. Can be done.
• the steel slab When steel slabs contain S (sulfur) as a composition, they tend to cause grain boundary embrittlement. For this reason, the steel slab preferably contains as low a content of S (sulfur) as possible.
• S (sulfur) content of the steel slab is 0.0050% or less, it is possible to suppress the occurrence of grain boundary embrittlement and the promotion of surface cracking. Thereby, the toughness of the base material and the HAZ can be improved.
• S (sulfur) reduces the toughness of steel slabs as inclusions such as MnS. For this reason, it is desirable that the S (sulfur) content of the steel slab be low.
• the S (sulfur) content of the steel slab is set to 0.0001% or more, it is possible to suppress an increase in the load of dephosphorization refining in the steelmaking process, and to suppress an increase in manufacturing costs. Can be done.
• the steel slab contains Ni (nickel) in a composition of 2.0% or more and less than 7.5%.
• Ni nickel
• the steel slab can realize physical properties equivalent to 9% Ni steel with a lower Ni content than 9% Ni steel.
• the Ni content of the steel slab is preferably 2.0% or more and less than 7.5%, and preferably 6.5% or more and less than 7.5%. Note that if the Ni content is less than 2.0%, there is a tendency that the toughness at low temperatures due to Ni cannot be obtained.
• the steel slab By containing Al (aluminum) as a composition, the steel slab can enhance the deoxidizing effect of removing oxygen contained in the steel slab. Moreover, the strength of the base material can be ensured by containing Al (aluminum) as a composition of the steel slab. When the amount of Al (aluminum) added increases, the base material and HAZ toughness tend to decrease due to coarse AlN.
• the steel slab When a steel slab contains N (nitrogen) as a composition, coarse metal nitrides such as AlN are generated, which tends to reduce the toughness of the base material and HAZ. For this reason, the steel slab preferably contains as low a content of N (nitrogen) as possible.
• N (nitrogen) content of the steel slab By setting the N (nitrogen) content of the steel slab to 0.0050% or less, suitable toughness of the base material and HAZ can be ensured.
• N (nitrogen) content of the steel slab By setting the N (nitrogen) content of the steel slab to 0.0010% or more, it is possible to suppress the increase in the burden of denitrification treatment and nitrogen absorption prevention treatment in the steel manufacturing process, which leads to an increase in manufacturing costs. This can be suppressed.
• the steel slab When a steel slab contains O (oxygen) as a composition, inclusions tend to form and the base metal and HAZ toughness tend to decrease. For this reason, the steel slab preferably contains as low a content of O (oxygen) as possible. When the O (oxygen) content of the steel slab is 0.0040% or less, suitable toughness of the base material and HAZ can be ensured.
• the O (oxygen) content of the steel slab is set to 0.0005% or more, it is possible to suppress an increase in the load of inclusion removal treatment in the steel manufacturing process, and to suppress an increase in manufacturing costs. be able to.
• the steel slab is selected from Cu, Cr, Mo, Nb, V, Ti, B, Ca, and Mg in order to improve the strength and toughness of the base metal and joint. It is preferable to contain one or more kinds.
• the steel slab preferably contains Cu (copper) as a composition.
• Cu (copper) as a composition, the steel slab can ensure the strength of the base material.
• the amount of Cu (copper) added increases, there is a tendency that good HAZ toughness cannot be obtained.
• the steel slab preferably contains Cr (chromium) as a composition.
• Cr chromium
• the steel slab can ensure the strength of the base material.
• the amount of Cr (chromium) added increases, there is a tendency that good HAZ toughness cannot be obtained.
• the steel slab preferably contains Mo (molybdenum) as a composition.
• Mo (molybdenum) as a composition, the steel slab can ensure the strength of the base material.
• the amount of Mo (molybdenum) added increases, there is a tendency that good HAZ toughness cannot be obtained.
• the steel slab preferably contains Nb (niobium) as a composition.
• Nb (niobium) As a composition, the steel slab can ensure the strength of the base material and can achieve finer crystal grains. When the amount of Nb (niobium) added increases, there is a tendency that good HAZ toughness cannot be obtained.
• Nb (niobium) in the steel slab by setting the content of Nb (niobium) in the steel slab to 0.003% or more, it is possible to obtain good strength of the base metal and to make the crystals in the slab finer. . Further, by controlling the content of Nb (niobium) in the steel slab to 0.100% or less, suitable toughness of the HAZ can be ensured.
• the steel slab preferably contains V (vanadium) as a composition.
• V (vanadium) as a composition, the steel slab can ensure the strength of the base material and can achieve finer crystal grains.
• the amount of V (vanadium) added increases, there is a tendency that good HAZ toughness cannot be obtained.
• V (vanadium) in the steel slab by setting the content of V (vanadium) in the steel slab to 0.003% or more, it is possible to obtain good base material strength and to make the crystals in the slab finer. . Further, by controlling the content of V (vanadium) in the steel slab to 0.100% or less, suitable toughness of the HAZ can be ensured.
• the steel slab preferably contains Ti (titanium) as a composition.
• Ti (titanium) as a composition, the steel slab can ensure the strength of the base material and can achieve finer grains of crystals in the slab.
• HAZ toughness tends to decrease due to coarse TiN.
• the steel slab preferably contains B (boron) as a composition.
• B (boron) as a composition, the steel slab can improve hardenability even in a very small amount. As a result, when performing controlled cooling and quenching heat treatment, a significant increase in strength can be achieved.
• HAZ toughness tends to decrease due to the precipitation of coarse boron nitrides and carbides.
• good strength can be obtained by setting the B (boron) content of the steel slab to 0.0002% or more. Further, by controlling the B (boron) content of the steel slab to 0.0025% or less, suitable HAZ toughness can be ensured.
• the steel slab preferably contains Ca (calcium) as a composition.
• Ca (calcium) combines with S to become CaS.
• CaS suppresses ductility-degrading cracking at grain boundaries and reduces surface cracking.
• HAZ toughness decreases due to the formation of coarse Ca-containing inclusions. There is a tendency to
• the steel slab preferably contains Mg (magnesium) as a composition.
• Mg manganesium
• the steel slab can control the form of inclusions and improve toughness.
• Mg (magnesium) combines with S to become MgS.
• MgS can suppress ductility reduction cracking at grain boundaries.
• MgS has a large effect of making the austenite grain size fine, and can reduce surface cracking during continuous casting or rolling.
• the amount of Mg (magnesium) added increases, the HAZ toughness tends to decrease due to the formation of coarse Mg-containing inclusions.
• Mg (magnesium) content of the steel slab to 0.0005% or more, good strength can be obtained. Further, by controlling the content of Mg (magnesium) in the steel slab to 0.0030% or less, suitable toughness of the HAZ can be ensured.
• the density of solidification nuclei on the surface of the steel slab is 0.35 pieces/mm 2 or more.
• the density of coagulation nuclei on the surface is preferably 0.35 pieces/ mm2 or more and less than 5.00 pieces/ mm2 , and preferably 0.50 pieces/mm2 or more and less than 5.00 pieces/ mm2. More preferred.
• the density of solidified nuclei is 5.00 pieces/mm 2 or more, it is not preferable because it is necessary to use mold powder that is cooled more intensely and to set the mold vibration frequency to a very high value. That is, if the cooling of the mold becomes too strong, there is a tendency for vertical cracks to occur more prominently due to non-uniform cooling of the steel slab within the mold. In addition, operational problems such as breakouts due to insufficient inflow of mold powder are likely to occur. From the above, it cannot be said that it is effective to extremely increase the density of coagulation nuclei (5.00 pieces/mm 2 or more).
• the density of solidification nuclei on the surface of a steel slab can be measured by the following method. For example, on the surface of a steel slab, a lump (solidification cell or dendrite cell) in which dendrite branches are oriented in approximately the same direction can be considered to have grown from one solidification nucleus. That is, by calculating the number of the lumps per predetermined area, the density of the solidified core can be calculated.
• the number of solidification nuclei be as large as possible in order to suppress surface cracking. Specifically, if the density of solidification nuclei on the surface of the steel slab is 0.35 pieces/mm 2 or more, surface cracking can be effectively suppressed.
• the continuous casting method for casting steel slabs explained above will be explained.
• One way to increase the density of solidification nuclei on the surface of a continuously cast steel slab is to strengthen the cooling during the initial solidification of continuous casting, that is, to strengthen the cooling within the mold. .
• continuous casting of steel slabs containing Ni includes a step of adding mold powder from above the surface of molten steel in a mold.
• the mold powder functions as an antioxidant, a heat insulator, and a lubricant between the mold and the solidified shell.
• the thickness of the inflow layer can be made thin, and the heat removal ability of the mold can be increased, thereby increasing the density of the solidification cores.
• the mold powder is composed of CaO, SiO2, Na2O , CaF2 , Al2O3 , etc.
• the thermal conductivity of the mold powder is significantly lower than that of the metals molten steel and copper that constitutes the continuous casting mold.
• Heat removal from molten steel to the mold depends on the thickness of the inflow layer of mold powder.
• the thickness of the inflow layer of mold powder can be estimated from the consumption amount of mold powder.
• the thickness of the molding powder inflow layer is usually about 0.1 to 0.3 mm.
• the steel slab is preferably continuously cast by adding mold powder having a viscosity of 0.5 Pa ⁇ s (5 poise) or more at 1300° C. into the mold.
• the viscosity of the mold powder at 1300°C is preferably 0.5 Pa ⁇ s (5 poise) or more and 5.0 Pa ⁇ s (50 poise) or less, and 1.0 Pa ⁇ s (10 poise) or more and 5.0 Pa ⁇ s (50 poise) or less is more preferable.
• the viscosity of the molding powder By setting the viscosity of the molding powder at 1300° C. to 0.5 Pa ⁇ s (5 poise) or more, it is possible to make it difficult for the molding powder to flow into the gap between the solidified shell and the mold. Therefore, the thickness of the inflow layer of mold powder can be reduced, and the heat removal from the molten steel to the mold can be increased. Thereby, the density of solidification nuclei can be increased, that is, 0.35 pieces/mm 2 or more, and the occurrence of surface cracks can be suppressed.
• the density of solidified nuclei can also be controlled by vibrating the mold at a predetermined frequency (oscillation cycle). For example, when the mold vibrates, some of the dendrite dendrites in the middle of solidification dissociate and adhere to the surface of the mold powder inflow layer. When dendrite dendrites adhere to the surface of the mold powder inflow layer, solidification nuclei are generated from that location. Thereby, the density of coagulation nuclei can be increased.
• Continuous casting preferably includes, for example, a step of vibrating the mold at a frequency of 80 cycles or more per minute.
• the frequency at which the mold is vibrated is preferably 80 to 400 cycles, more preferably 100 to 400 cycles.
• the frequency of vibration of the mold is less than 80 cycles, there is a possibility that sufficient solidification core density cannot be ensured. Furthermore, if the frequency of vibration of the mold exceeds 400 cycles, the mold tends to resonate and casting becomes unstable.
• the viscosity of the mold powder can be changed depending on the frequency (oscillation cycle) of the mold. For example, when the frequency (oscillation cycle) of the mold is 50 cycles per minute, it is preferable to use a molding powder with a viscosity of 0.5 Pa ⁇ s (5 poise) or more at 1300°C. Even in this case, the density of solidification nuclei on the surface of the Ni-containing steel slab can be set to 0.35 pieces/mm 2 or more.
• the vibration frequency (oscillation cycle) of the mold is 80 cycles per minute, it is preferable to use a molding powder having a viscosity of 0.15 Pa ⁇ s (1.5 poise) or more at 1300°C. Even in this case, the density of solidification nuclei on the surface of the Ni-containing steel slab can be set to 0.35 pieces/mm 2 or more.
• a molding powder with a mold frequency (oscillation cycle) of 80 cycles or more per minute and a viscosity of 0.5 Pa ⁇ s (5 poise) or more at 1300°C. Under such conditions, surface cracking of the steel slab can be significantly reduced.
• the flat surface is thought to have been formed by a type of solidification cracking. More specifically, when molten steel solidifies, C, S, P, etc. are concentrated in the final solidified part, and the melting point is lowered.
• the flat surface is formed by shrinking the surrounding area that has already been solidified while a low melting point liquid phase is present in the final solidified part.
• surface cracking occurs when solute elements such as C, S, and P are concentrated at the boundary between two solidified cells that make up the solidified shell when the solidified shell grows within the mold.
• solute elements such as C, S, and P
• a liquid phase with a low melting point is generated, resulting in solidification cracking.
• the crack progresses further due to thermal stress in the secondary cooling zone, stress due to bending straightening, etc.
• Solidification cracking is less likely to occur as the concentration of solute elements in the final solidification zone decreases, and as the thermal stress acting on the final solidification zone decreases. For example, when the size of the solidification cell becomes smaller, the cooling rate naturally increases and concentration of solute elements is suppressed. Moreover, when the size of the solidification cells is small, thermal stress is dispersed, and the thermal stress acting on the interface of each solidification cell becomes small. Reducing the size of the solidification cells is effective in preventing solidification cracking.
• the size of the solidification cell can be reduced by increasing the density of solidification nuclei in the portion where the molten steel contacts the mold.
• the Ni-containing steel slab of the present invention has a high density of solidification nuclei, the size of the solidification cells can be reduced. As a result, a Ni-containing steel slab with less surface cracking can be provided.
• initial solidification on the surface of a steel slab can be controlled.
• a large number of solidification nuclei are generated, and the concentration of impurity elements such as P (phosphorus) and S (sulfur) and C (carbon) on the interface of the solidification cell is reduced. Therefore, solidification cracking at the interface of the solidification cells can be suppressed. Therefore, so-called surface cracks occurring on the surface of the steel slab can be suppressed.
• the Ni-containing steel slab according to the present invention has a density of solidification nuclei on the slab surface of 0.35 pieces/mm 2 or more. Therefore, the size of the coagulation cell can be reduced. This reduces the concentration of S and P at the interface of the solidification cells, so that embrittlement at the interface of the solidification cells can be suppressed. Moreover, the stress acting on the interface of the solidification cell is also dispersed, and solidification cracking at the interface of the solidification cell can be suppressed. As a result, the occurrence of cracks on the surface of the steel slab can be reduced.
• the method for manufacturing steel slabs containing Ni includes a care process in which a slab manufactured by a continuous casting method is treated, and after the care process, the slab is heated at a heating temperature of 1100°C or less in a heating furnace. and a heating step.
• steel slabs are manufactured using slabs produced by continuous casting of molten steel.
• the generated slab is heated at 1000 to 1200° C. (first heat treatment).
• the slab subjected to the first heating is subjected to preliminary rolling (light blooming) to a thickness of about 60 to 90%.
• the pre-rolled slab is ground until there are no flaws, and a care process is performed to remove the flaws.
• the treated slab is heated at 1000 to 1200°C (second heat treatment). Rolling (main rolling) is performed on the slab that has been subjected to the second heat treatment.
• Fe 2 SiO 4 scale is produced in steel containing 0.05% or more of silicon (Si) as Fe 2 SiO 4 is produced. Note that the eutectic temperature of Fe 2 SiO 4 with wustite (FeO) is 1170°C. Fe 2 SiO 4 is a liquid phase oxide above the eutectic temperature.
• Fe 2 SiO 4 scale When Fe 2 SiO 4 scale is generated, grain boundaries become brittle. Since Fe 2 SiO 4 scale is in a liquid phase at high temperatures, it easily diffuses into grain boundaries and deep into the matrix.
• the manufacturing process be performed at a temperature equal to or lower than the eutectic temperature of Fe 2 SiO 4 .
• the heating temperature of the heating furnace be equal to or lower than the eutectic temperature of Fe 2 SiO 4 .
• Fe 2 SiO 4 scale at grain boundaries is also related to the segregation of elements such as P and S in the regions. For this reason, even if the manufacturing process is performed at a temperature below the eutectic temperature of about 1100° C., for example, liquid phase Fe 2 SiO 4 scale may be generated in a part of the region.
• the heating step is performed at a temperature of 1100° C. or lower, and it is more preferable that the heating step is performed at a temperature of 1050° C. or lower. . Further, it is preferable that the rolling step is performed after the slab is heated in the heating step under such conditions.
• a slab when a slab is produced by controlling the density of solidified cores by the continuous casting method of the present invention, it has superior toughness than conventional slabs, so it can be used to produce steel slabs without pre-rolling such as light blooming. can be manufactured.
• a steel slab when a slab is produced by controlling the density of solidification nuclei by the continuous casting method of the present invention, a steel slab can be produced by performing the following (1) to (3).
• a care process is performed to remove the flaws by grinding approximately 3 to 6 mm from the surface until the generated slab is free of flaws (care process).
• Heating the treated slab at 1100° C. or lower heating step.
• the slab subjected to the heating process is rolled (main rolling).
• Ni steel with a Ni content of 3.5% by mass was created by melting steel.
• a converter and an RH vacuum degassing device were used to create the molten steel.
• test Nos. 1 to 20 A test was conducted to cast this molten steel using a vertical bending type continuous slab casting machine.
• the vertical bending continuous slab casting machine had a thickness of 250 mm and a width of 2100 mm.
• the test was conducted in a total of 20 heats (Test Nos. 1 to 20).
• Table 1 shows the chemical components of Test Nos. 1 to 20.
• Table 2 shows the casting conditions in the continuous casting machine for test Nos. 1 to 20.
• the casting speed was 0.8 m/min.
• the amplitude of the mold oscillation was 8 mm.
• the frequency of the oscillation was 60 cycles per minute.
• the molding powder used had a viscosity of 0.20 Pa ⁇ s at 1300°C.
• the casting speed was 0.8 m/min.
• the amplitude of the mold oscillation was 8 mm.
• the frequency of the oscillation was 60 cycles per minute.
• the molding powder used had a viscosity of 0.06 to 2.00 Pa ⁇ s at 1300°C.
• the casting speed was 0.8 m/min.
• the amplitude of the mold oscillation was 8 mm.
• the frequency of the oscillation was 80 cycles per minute.
• the molding powder used had a viscosity of 0.20 Pa ⁇ s at 1300°C.
• the casting speed was 0.8 m/min.
• the amplitude of the mold oscillation was 8 mm.
• the frequency of the oscillation was 80 cycles per minute.
• the molding powder used had a viscosity of 0.5 Pa ⁇ s at 1300°C.
• the cast steel slab was cut into a length of 300 mm.
• the cut samples were subjected to the following treatments and then evaluated for surface cracks.
• the surface of each sample was subjected to shot blasting to remove the oxide film on the surface. Thereafter, surface cracks were determined by penetrant testing. Regarding the determined surface cracks, the length and number of cracks were measured.
• the density of solidified nuclei on the surface of the slab was measured by the following method. A sample was taken from the surface of a steel slab, and the oxide film on the surface was removed by shot blasting. The surface of the steel slab from which the oxide film had been removed was mirror-ground and corroded with picric acid to reveal the solidified structure.
• the coagulated tissue that appeared was photographed.
• a mass coagulation cell or dendrite cell
• dendrite branches were oriented in almost the same direction was considered to have grown from one coagulation nucleus.
• the density of coagulation nuclei was calculated by calculating the number of the lumps per predetermined area.
• the number of coagulated cells was counted as the mass shown in the photograph of the coagulated tissue, and the density was determined by dividing by the area occupied by the coagulated cells. Furthermore, the size of the coagulation cell tends to be small near the oscillation mark and to become large away from the oscillation mark. For this reason, the range in which coagulation cells were counted was from one oscillation mark adjacent to the other oscillation marks, and the average value was determined.
• Table 3 shows test no. The results of the investigation of the density of solidified nuclei from 1 to 20 and the total crack length (crack length x number of cracks) are shown.
• Test Nos. 1, 6, 11, and 16 the density of coagulation nuclei was less than 0.35 pieces/mm 2 . In Test Nos. 1, 5, 9, and 13, many surface cracks occurred. In addition, in Test Nos. 1, 6, 11, and 16, cracks occurred at a distance of 3 mm or 6 mm from the surface.
• test No. 2 in which the density of coagulation nuclei exceeds 1.50 pieces/ mm2 .
• samples No. 5, 10, 15, and 20 a significant reduction in surface cracking was confirmed.
• tests within the scope of the present invention are indicated as "invention examples”, and other tests are indicated as “comparative examples”.
• Method A A care process is performed to remove the flaws by grinding approximately 3 to 6 mm from the surface until the generated slab is free of flaws.
• the treated slab is heated at 1050°C or 1200°C (first heat treatment).
• the slab subjected to the first heating is pre-rolled (light blooming) to a thickness of 250 mm to 190 mm.
• the pre-rolled slab is cleaned by grinding approximately 3 to 6 mm from the surface to remove any flaws until there are no flaws.
• the treated slab is heated at 1050°C or 1200°C (second heat treatment).
• the slab subjected to the second heat treatment was subjected to main rolling, and its thickness was changed from 190 mm to 25 mm. Thereafter, flaws were inspected on surfaces at distances of 3 mm, 6 mm, and 9 mm from the surface (hereinafter also referred to as product inspection).
• Method B A care process is performed to remove the flaws by grinding approximately 3 to 6 mm from the surface until the generated slab is free of flaws (care process).
• the treated slab is heated at 1050°C or 1200°C (heating step).
• the slab subjected to the heating process was subjected to main rolling to a thickness of 25 mm from 250 mm. Thereafter, product inspection was performed on surfaces at distances of 3 mm, 6 mm, and 9 mm from the surface.
• Table 4 shows the occurrence of surface flaws after rolling each product.
• Test No. Slabs 2 to 5, 7 to 10, 12 to 15, and 17 to 20 heated at a heating temperature of 1050°C were evaluated as " ⁇ : No product defects" even if light blooming was not performed. It was done. Therefore, for these tests, process costs can be significantly reduced and product quality can be stably improved.

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• 2023
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