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
  • B22D11/11—Treating the molten metal
  • B22D11/114—Treating the molten metal by using agitating or vibrating means
  • B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/0006—Adding metallic additives
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/06—Deoxidising, e.g. killing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/068—Decarburising
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/10—Handling in a vacuum
• • 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
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper

Definitions

• the present invention relates to extremely low carbon steel plate excellent in surface characteristics, workability, and formability and a method of producing extremely low carbon cast slab.
• Molten steel refined in a converter or vacuum treatment vessel contains a large amount of dissolved oxygen. This excess oxygen is generally removed by Al which is a strong deoxidizing element with a strong affinity with oxygen. However, Al forms alumina-based inclusions by deoxidation. These aggregate and form coarse alumina clusters.
• the alumina clusters become the cause of the formation of surface defects during steel plate production and greatly deteriorate the quality of the thin-gauge steel plate.
• the amount of the alumina clusters is extremely large and the rate of formation of surface defects is extremely high, so measures for reducing the alumina-based inclusions have become major issues.
• Japanese Patent Publication (A) No. 5-302112 discloses the method of producing molten steel for thin-gauge steel plate not being deoxidized much at all by Al by deoxidizing the molten steel by Mg.
• the vapor pressure of the Mg is high and the yield to molten steel is extremely low, so deoxidizing molten steel with a high concentration of dissolved oxygen such as with extremely low carbon steel by just Mg requires a large amount of Mg. Considering the production costs, the process cannot be said to be practical.
• the present invention has as its object to provide extremely low carbon steel plate reliably preventing surface defects and superior in workability and formability by finely dispersing oxides at the time of solidification without forming almost any inclusions in the molten steel and a method of production of the same.
• the present invention has the following as its gist:
• a method of producing an extremely low carbon steel cast slab characterized by casting molten steel obtained by reducing the carbon concentration of the molten steel to 0.005 mass % or less, then adding Cu, Nb, and B to the molten steel, furthermore, and adjusting the concentration of dissolved oxygen in the molten steel to 0.01 mass % to 0.06 mass %.
• a method of producing an extremely low carbon steel cast slab characterized by casting molten steel obtained by reducing the carbon concentration of the molten steel to 0.005 mass % or less, then adding Cu, Nb, and B to the molten steel to make the molten steel include Cu by 0.01 to 3.0 mass % and Nb and B by respectively
• a method of producing an extremely low carbon steel cast slab characterized by casting molten steel obtained by reducing the carbon concentration of the molten steel to 0.005 mass % or less, adding Cu, Ni, Nb, and B to the molten steel to make the molten steel include Cu by 0.01 to 3.0 mass %, Ni by 0.5 ⁇ Cu concentration or less, and Nb and B by respectively
• a method of producing an extremely low carbon steel cast slab according to (5) characterized by casting said molten steel by casting it while electromagnetically stirring it to make the molten steel at the meniscus position swirl by an average flow rate of 40 cm/s to 100 cm/s.
• Extremely low carbon steel plate having excellent surface characteristics, workability, and formability comprised of steel containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass % or less, and further Cu, Nb, and B, characterized in that the steel has fine oxides of a diameter of 0.5 ⁇ m to 30 ⁇ m dispersed in it in an amount of 1000 particles/cm 2 to 1,000,000 particles/cm 2 .
• Extremely low carbon steel plate having excellent surface characteristics, workability, and formability comprised of steel containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass % or less, Cu: 0.01 to 3.0 mass %, and further Nb and B of respectively
• the steel has fine oxides of a diameter of 0.5 ⁇ m to 30 ⁇ m dispersed in it in an amount of 1000 particles/cm 2 to 1,000,000 particles/cm 2 .
• the steel has fine oxides of a diameter of 0.5 ⁇ m to 30 ⁇ m dispersed in it in an amount of 1000 particles/cm 2 to 1,000,000 particles/cm 2 .
• At least 40% of the number of oxides present in the steel include at least Si, Mn, and Fe.
• Extremely low carbon steel plate having excellent surface characteristics, workability, and formability comprised of steel containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass % or less, Cu: 0.01 to 3.0 mass %, Ni: 0.5 ⁇ Cu mass % or less, and further Nb and B of respectively
• the steel characterized in that at least 40% of the number of. oxides present in the steel include at least Si, Mn, and
• An extremely low carbon steel plate having excellent surface characteristics, workability, and formability according to any one of (10) to (12) characterized in that the contents of the at least Si oxides, Mn oxides, and Fe oxides which at least 40% of the number of oxides present in the steel include is 20 mass % or more.
• oxides can be made to finely precipitate in the molten steel at the time of solidification without causing formation of almost any inclusions, so it is possible to reliably prevent surface defects and fix the C and N in the steel plate and possible to control the texture of the hot rolled steel plate, so it is possible to produce a thin-gauge steel plate excellent in workability and formability.
• the method of production of the present invention adds Cu, Nb, and B to molten steel refined in a converter, electric furnace, or other steelmaking furnace or treated by vacuum degassing or the like to reduce the carbon concentration in the molten steel to 0.005 mass % or less and adjusts the concentration of dissolved oxygen to 0.01 to 0.06 mass %.
• This melting method is to reduce the carbon concentration to an extent where it does not react with the oxygen during casting to generate CO gas and leave behind a large amount of dissolved oxygen without adding almost any Al so as to prevent the formation of almost all inclusions in the molten steel and to add Cu, Nb, and B extremely weak in deoxidizing power to fix the C and N and control the texture control and thereby secure quality as steel plate for sheet use.
• the molten steel decarburized in the converter or vacuum treatment vessel contains a large amount of dissolved oxygen. This dissolved oxygen forms a large amount of alumina-based inclusions since almost all of it is removed by addition of Al (reaction of Formula 1):
• the alumina-based inclusions aggregate immediately after deoxidation to become coarse alumina-based inclusions which become the cause of formation of surface defects at the time of steel plate production.
• a large amount of dissolved oxygen is contained in the molten steel, but almost no inclusions are formed and molten steel of an extremely high cleanliness can be obtained.
• the present invention has focused attention on not adding Al at all or not adding much at all and leaving dissolved oxygen, but instead greatly lowering the C concentration so as to suppress formation of CO gas during solidification.
• the C concentration is made 0.005 mass % or less, the rate of formation of CO gas during solidification drops by an extremely large margin.
• Nb and B function to increase the workability of the steel plate by fixing mainly C and mainly N respectively as precipitates.
• the Lankford value (referred to as the “r value”) becomes a somewhat low value compared with Al-deoxidized Ti-added extremely low carbon steel.
• the concentration of dissolved oxygen in the molten steel has to be 0.01 mass % to 0.06 mass %.
• the concentration of dissolved oxygen in the molten steel can be analyzed by an oxygen sensor using a solid electrolyte, while the concentration of C can be analyzed by the molten steel sampling method.
• Nb and B increase the workability of the steel plate by fixing mainly C and mainly N respectively as precipitates.
• the preferred range of addition of Nb and B to the molten steel can be suitably expressed if using the middle part of the following formulas described using the chemical equivalents of the elements as indicators.
• the middle part of [Formula 2] means the amount of free Nb not bonding with C and forming a carbide
• the middle part of [Formula 3] means the amount of free N not bonding with N and forming a nitride.
• the oxygen concentration balanced with Nb and B is 0.01 mass % or more. Even if adding Nb and B, dissolved oxygen of 0.01 mass % or more can be secured.
• Cu has the effect of promoting the formation of a texture of the ⁇ 111 ⁇ orientation where a high r value is easily obtained in the steel plate. At the minimum, if adding 0.01 mass % or more, this effect does not easily appear, so the amount of addition is preferably made 0.01 mass % or more.
• the upper limit is preferably made 3.0 mass %.
• Ni has the effect of easing the deterioration of the hot rolled surface characteristics due to Cu. On a mass base, it is general to add the equivalent of more than half of the Cu as a rule. It was discovered in steel plate with a high oxygen concentration of the present invention, when the concentration of dissolved oxygen in the molten steel is 0.01 mass % or more, Cu-embrittlement is inhibited by smoothing the scale and ferrite boundaries of the hot rolled plate and improving the scale peelability.
• Ni is added in a state where Ni is not added, the surface characteristics of the hot rolled plate become good and the features of the present invention can be extracted to the maximum, but when it is necessary, it is sufficient to add Ni in an amount of less than half of the Cu.
• the upper limit of Ni is preferably made less than 1 ⁇ 2 of the. Cu concentration.
• the Si concentration in the molten steel is preferably 0.005 mass % to 0.03 mass %. If Si concentration is less than 0.005 mass %, the strength of the steel plate easily becomes insufficient, while if the Si concentration is over 0.03 mass %, the workability of the steel plate decreases.
• the Si concentration is 0.03 mass % or less, the equilibrium oxygen concentration also becomes more than 0.02 mass %.
• the Si concentration it is possible to secure a concentration of dissolved oxygen of over 0.02 mass % to 0.06 mass %.
• a concentration of dissolved oxygen in the molten steel of 0.01 mass % to 0.06 mass % can be secured.
• the Mn concentration in the molten steel is less than 0.08 mass %, scab flaws are easily formed at the time of hot rolling of the slab. Further, if the Mn concentration is over 0.3 mass %, the workability of the steel plate decreases. Because of this, the Mn concentration in the molten steel is preferably 0.08 mass % to 0.3 mass %.
• Mn has an extremely weak deoxidizing power compared to Si, because the Mn concentration is 0.3 mass %, the equilibrium oxygen concentration is in excess of 0.1 mass %, furthermore, by adding elements having deoxidizing power, a concentration of dissolved oxygen in the molten steel no less than 0.01 mass % and no greater than 0.06 mass % can be guaranteed.
• Mn has an extremely weak deoxidizing power
• Mn concentration is 0.3 mass % or less
• Mn oxides are formed under equilibrium conditions, but if adding Mn in high oxygen molten steel after converter blowing or after vacuum degassing, the Mn is adding in the form of large clumps of ferromanganese or manganese ore, so sometimes regions of high Mn concentration are locally formed in the molten steel. In such regions, while small in amount, Mn oxides end up being formed.
• the present invention it is more preferable not to form inclusions in the molten steel, so it is more preferable to adjust the Mn concentration under operating conditions with no addition of Mn after converter blowing or after vacuum degassing.
• molten iron contains Mn. Even without the addition of Mn, by the operating conditions, it is possible to obtain an Mn concentration of about 0.15 mass %. Consequently, if considering even the inclusions in addition to the quality, the more preferable range of Mn concentration is 0.08 to 0.15 mass % where production is possible without the addition of Mn after converter blowing or after vacuum degassing.
• Al is not added to the molten steel or almost not added at all.
• the acid soluble Al concentration of the steel plate exceeds 0.005 mass %, the alumina-based inclusions in the steel plate increase, so the upper limit was made 0.005 mass %. Since no addition of Al in the molten steel is preferable, of course the lower limit of Al concentration includes 0 mass %.
• the acid soluble Al is the amount of Al dissolved in acid. Usually, this corresponds to the dissolved Al concentration (concentration of Al not forming Al 2 O 3 ).
• the alumina-based inclusions inevitably entering from the refractories etc. do not pose a problem. This if because with a small amount of alumina-based inclusions, the dissolved oxygen in the molten steel is high, so the boundary energy of the molten steel and alumina-based inclusions decreases and almost no texture is formed.
• the Ti in the molten steel fixes the C and N as TiN or TiC, so is effective in improving the workability, but if the amount of addition of Ti also becomes greater, for example, if the Ti concentration becomes more than 0.01 mass %, the equilibrium oxygen concentration becomes less than 0.01 mass %, so a sufficient concentration of dissolved oxygen cannot be secured. Consequently, when adding Ti from the necessity of further raising the workability, it should be added in the range of 0.01 mass % or less.
• the present inventors discovered that if securing a molten steel flow rate at the meniscus in the casting mold at the time of electromagnetic stirring during solidification of 40 to 100 cm/s, casting is possible with almost no CO bubbles trapped in the cast slab even if making the concentration of dissolved oxygen about 0.06 mass %, so this is more preferable.
• molten steel reduced in C concentration to 0.05 mass % or so by converter blowing is further reduced in C concentration to 0.005 mass % by a vacuum degassing apparatus.
• concentration of dissolved oxygen in the molten steel is controlled to approach 0.01 to 0.06 mass % after the end of decarburization considering the amount of decarburization.
• Mn and Si are not added or not added as much as possible, but Cu, Nb, B, Ni, and the like are added. Further, when it is necessary to finely adjust the concentration of dissolved oxygen in the molten steel to the target value, simultaneously small amounts of Al and Ti are added to adjust the ingredients.
• the melted steel produced in this way is continuously cast to produce a cast slab using continuous casting or electromagnetic stirring.
• the steel plate of the present invention will be explained.
• the hot rolled steel plate obtained by hot rolling the cast slab produced by the above method, cold rolled steel plate obtained by cold rolling, or other steel plate obtained by working the cast slab is defined as the “steel plate” in the present invention.
• the steel plate of the present invention contains Cu, Nb, and B.
• other elements for example, it is possible to include Si, Mn, etc. from the viewpoint of securing the strength and a trace amount of Ti and acid soluble Al at 0.005 mass % or less from the viewpoint of securing workability.
• the dissolved oxygen precipitates during the casting as Fe oxide-based inclusions.
• the Fe oxide-based inclusions are not formed in the molten steel, but precipitate during solidification, so disperse finely in the cast slab without aggregating together.
• Fe oxide-based inclusions are not just pure Fe oxides and also contain oxides of Si oxides, Mn oxides, etc. combined.
• At least Si, Mn, and Fe are included as oxides.
• at least one type of oxide of Si, Mn, and Fe is included.
• various oxides such as oxides of Mg, Ca, and Al may also be included.
• fine oxides of a size of 0.5 ⁇ m to 30 ⁇ m are dispersed in the steel plate in an amount of 1000 particles/cm 2 to 1,000,000 particles/cm 2 .
• the size of the fine oxides is made from 0.5 ⁇ m to 30 ⁇ m because the size of the inclusions in the steel plate of the present invention falls in the range of about 0.5 ⁇ m to 30 ⁇ m. If the inclusions are of a size of 30 ⁇ m or so, surface defects can be sufficiently prevented.
• the state of dispersion of inclusions was made 1000 particles/cm 2 to 1,000,000 particles/cm 2 because if the inclusions of the steel plate in the present invention are in this range of particle density, surface defects are not formed.
• the state of dispersion of inclusions was evaluated by observing the polished surface of the steel plate by an optical microscope at 100 ⁇ and 1000 ⁇ power and assessing the distribution of particle size of the inclusions in a unit area.
• the particle size of the inclusions that is, the diameter, was obtained by measuring the major axis and minor axis and calculating (major axisxminor axis) 0.5 .
• the number of oxides present in the steel plate contain at least Si, Mn, and Fe, almost all inclusions will be formed during solidification and the time for them to aggregate will be short, so they can finely disperse and surface defects will be difficult to form. This is therefore preferable.
• “contain at least Si, Mn, and Fe” means at least one type of Si, Mn, and Fe. This is used in a similar sense later as well.
• the number of oxides present in the steel plate have a content of at least Si oxides, Mn oxides, and Fe oxides of 20 mass % or more, more preferably 50 mass % or more, almost all of the oxides will be formed at a timing close to the end of solidification and the time for them to aggregate will be extremely short, so the inclusions will finely disperse and surface defects will be difficult to form. This is therefore more preferable.
• a steel plate having this kind of dispersed state of oxides and composition is resistant to the formation of surface defects.
• the Fe oxide-based inclusions can be made to precipitate and finely disperse during solidification without allowing the formation of almost any inclusions in the molten steel, so the inclusions do not become the cause of formation of surface defects at the time of production of the steel plate. Further, the workability is greatly improved due to the Nb, B, and Cu in the steel plate, so the quality and material of the steel plate for sheet use can be greatly improved.
• Steel plate for sheet use is used for automobile external sheet and other applications where processing is harsh, so workability must be added.
• decreasing the C concentration as much as possible and further fixing the C and N in solid solution in the steel by the addition of other elements are important.
• the C concentration is made 0.01 mass % or less, preferably 0.005 mass % or less, from the viewpoint of workability.
• the condition for prevention of the formation of CO bubbles during solidification is a C concentration of 0.005 mass % or less, so in the present invention, the C concentration determined from the condition of workability is sufficiently satisfied.
• the lower limit of the C concentration is not particularly limited.
• 300 t of molten steel with a C concentration of 0.0019 mass % was produced by refining at a converter and treatment at a rotary flow type vacuum degassing apparatus.
• alloys of Cu, Nb, and B were added, without adding Al, to give 0.011 mass % Si, 0.16 mass % Mn, 0.014 mass % Nb, 0.003 mass % B, 0.07 mass % Cu, 0.0016 mass % N, 0.043 mass % dissolved oxygen, and 0.001 mass % or less acid soluble Al.
• This molten steel was cast into a slab of a thickness of 250 mm and a width of 1800 mm by continuous casting.
• the cast slab was cut into 8500 mm lengths for use as coil units.
• the thus obtained slab was hot rolled and cold rolled by ordinary methods to finally obtain a coil of cold rolled steel plate of 0.7 mm thickness and a width of 1800 mm.
• the quality was visually examined on an inspection line after cold rolling and the number of surface defects formed per coil was evaluated.
• the obtained cold rolled steel plate was evaluated for workability. It was high workability steel plate with a total elongation of 57% and an r value of 2.6.
• 300 t of molten steel with a C concentration of 0.003 mass % was produced by refining at a converter and treatment at a rotary flow type vacuum degassing apparatus.
• alloys of Cu, Nb, B, and Ni were added, without adding Al, to give 0.01 mass % Si, 0.15 mass % Mn, 0.035 mass % Nb, 0.005 mass % B, 1.8 mass % Cu, 0.5 mass % Ni, 0.0025 mass % N, 0.004 mass % Ti, 0.015 mass % dissolved oxygen, and 0.001 mass % acid soluble Al.
• This molten steel was cast to a slab of a thickness of 250 mm and a width of 1800 mm using a continuous casting machine with an in-mold electromagnetic stirring device while electromagnetically stirring the molten metal by an average flow rate of 50 cm/s at the meniscus.
• the cast slab was cut into 8500 mm lengths for use as coil units.
• the thus obtained slab was hot rolled and cold rolled by ordinary methods to finally obtain a coil of cold rolled steel plate of 0.7 mm thickness and a width of 1800 mm.
• the quality of the slab was visually examined on an inspection line after cold rolling and the number of surface defects formed per coil was evaluated.
• the obtained cold rolled steel plate was evaluated for workability. It was high workability steel plate with a total elongation of 56% and an r value of 2.7.
• Alloys of Ti and Cu were added to molten steel in a ladle reduced in carbon concentration to 0.0015 mass % by refining at a converter and treatment at a rotary flow type vacuum degassing apparatus and the steel was deoxidized by Al to give 0.01 mass % Si, 0.15 mass % Mn, 0.02 mass % Ti, 0.3 mass % Cu, 0.002 mass % N, 0.04 mass % Al, and 0.0002 mass % concentration of dissolved oxygen.
• This molten steel was cast into a slab of a thickness of 250 mm and a width of 1800 mm by continuous casting.
• the cast slab was cut into 8500 mm lengths for use as coil units.
• the thus obtained slab was hot rolled and cold rolled by ordinary methods to finally obtain a coil of cold rolled steel plate of 0.7 mm thickness and a width of 1800 mm.
• the quality of the slab was visually examined on an inspection line after cold rolling and the number of surface defects formed per coil was evaluated.

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• 2005
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