Classifications

• • 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
  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
• • 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
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • 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
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/02—Hardening by precipitation
• • 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
  • 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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0236—Cold rolling
• • 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
  • C21D8/0247—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 characterised by the heat treatment
• • 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
  • C21D8/0247—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 characterised by the heat treatment
  • C21D8/0263—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 characterised by the heat treatment following hot rolling
• • 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
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—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
  • C22C38/46—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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present disclosure relates to cold-rolled steel sheets, especially to cold-rolled steel sheets with excellent press-blanking workability.
• the present disclosure further relates to a method of producing the cold-rolled steel sheets.
• Press blanking is a widely used method for processing cold-rolled steel sheets into part shapes.
• cold-rolled steel sheets are worked into part shapes by press blanking, then subjected to working such as cutting, drawing, and polishing, and heat treatment such as quenching and tempering before the final textile machinery parts are produced.
• burrs occur on edge surfaces when punching material.
• the occurrence of burrs can cause problems when parts with burrs are used in textile machinery such as knitting machines. Therefore, grinding or polishing is used to remove burrs after press blanking, but removing burrs sufficiently is difficult depending on the dimensions and the complexity of shape of a part.
• JP 2019-039056 A proposes a medium- to high-carbon cold-rolled steel sheet in which a controlled microstructure suppresses the generation of waviness and rollover on the punched end surface due to blanking.
• JP H05-171288 A (PTL 2) proposes a method for producing high-carbon steel sheets that are soft and have excellent formability by optimizing the chemical composition and production conditions.
• WO 2019/163828 A proposes a high-carbon cold-rolled steel sheet with improved fine blanking workability by optimizing the grain size of cementite and ferrite, etc.
• the technology proposed in PTL 2 reduces variations in material properties within a coil to suppress the reduction in workability caused by the variations and does not improve the intrinsic blanking workability of the steel sheet.
• the present disclosure has been developed to provide a cold-rolled steel sheet with excellent blanking workability.
• the present inventors have studied methods of further improving the blanking workability of cold-rolled steel sheets and have found the following.
• the present disclosure can provide a cold-rolled steel sheet with excellent blanking workability.
• the cold-rolled steel sheet according to the present disclosure is extremely suitable for use as a material for press blanking, especially for textile machinery parts such as knitting needles, because burr generation is suppressed when press blanking is performed, and residual stress is smaller.
• the cold-rolled steel sheet of the present disclosure has the chemical composition described above. The reason for this limitation is described below. As used herein, “%” as a unit of content refers to “mass percent” unless otherwise specified.
• C is an element that has an effect of increasing hardness through quenching and plays an important role in blanking workability.
• C forms cementite with Fe, and as a result a boundary occurs between the generated cementite and ferrite. This boundary then becomes the initiation point for a void during blanking.
• shearing occurs with a void as an initiation point, plastic deformation of ferrite is suppressed and the burr height lowers.
• the C content is less than 0.60 %, carbon is consumed in cementite formation and carbides are not formed in grains, resulting in plastic deformation of ferrite grains growing. As a result, burrs increase in height and residual stress increases, and shape and dimensional accuracy decrease.
• the C content is therefore 0.60 % or more, preferably 0.65 % or more, and more preferably 0.70 % or more.
• the C content is therefore 1.25 % or less, preferably 1.20 % or less, and more preferably 1.15 % or less.
• Si is an element that has an effect of increasing the strength of a ferrite microstructure through solid solution strengthening, and the addition of Si can improve blanking workability.
• a Si content is 0.1 % or more, preferably 0.12 % or more, and more preferably 0.14 % or more.
• excessive Si content promotes ferrite formation and grain growth, and ferrite strength decreases.
• the promoted ferrite formation also promotes the precipitation of coarse cementite to the grain boundary, and the frequency of void generation decreases. As a result, plastic deformation increases and blanking workability decreases.
• the Si content is therefore 0.55 % or less, preferably 0.52 % or less, and more preferably 0.50 % or less.
• Mn is an element that mixes with cementite and inhibits cementite growth.
• the refinement of cementite formed at ferrite grain boundaries can suppress the plastic deformation of ferrite and improve blanking workability.
• a Mn content is 0.5 % or more, preferably 0.52 % or more, and more preferably 0.54 % or more.
• the Mn content exceeds 2.0 %, the segregation of Mn sulfides generates an extensive band-like structure in the rolling direction, resulting in an abnormal microstructure formation.
• the Mn content is 2.0 % or less, preferably 1.95 % or less, more preferably 1.90 % or less, and even more preferably 1.85 % or less.
• P is an element that strengthens ferrite. Therefore, the addition of a trace amount of P can suppress the plastic deformation of ferrite and improve the blanking workability.
• a P content is therefore 0.0005 % or more, and preferably 0.0010 % or more.
• the P content exceeds 0.05 %, the grain boundary segregation of P suppresses cementite formation at the grain boundary, increases the plastic deformation of ferrite, resulting in blanking workability decreasing.
• the P content is therefore 0.05 % or less, and preferably 0.04 % or less.
• S forms sulfides with Mn contained in the steel.
• MnS metal-oxide-semiconductor
• a S content is therefore 0.0001 % or more, and preferably 0.0005 % or more.
• S content is therefore 0.01 % or less, and preferably 0.008 % or less.
• Al is dispersed in the steel as oxide and forms a solid solution to strengthen ferrite, thereby suppressing plastic deformation of ferrite and improving blanking workability.
• An Al content is therefore 0.001 % or more, and preferably 0.002 % or more.
• the Al content exceeds 0.10 %, ferrite grain growth is promoted and plastic deformation increases, resulting in blanking workability decreasing.
• the Al content is therefore 0.10 % or less, preferably 0.08 % or less, and more preferably 0.06 % or less.
• N combines with Al in steel to form AlN.
• a N content is less than 0.001 %, ferrite crystal grains coarsen and blanking workability decreases. Therefore, the N content is 0.001 % or more.
• the N content exceeds 0.009 %, AlN precipitates at ferrite grain boundaries of the hot-rolled steel sheet, which is an intermediate product, and the ferrite grains expand and coarsen, resulting in blanking workability decreasing.
• the N content is therefore 0.009 % or less, and preferably 0.006 % or less.
• Cr is an element that increases the hardenability of steel and improves its strength and affects blanking workability.
• the Cr content is 0.05 % or more, preferably 0.08 % or more, more preferably 0.10 % or more, and even more preferably 0.15 % or more.
• excessive Cr content leads to the formation of coarse Cr carbides and Cr nitrides, which precede the voids that occur at the interface between cementite and ferrite.
• the formation of coarse Cr carbides suppresses carbide formation within the grains and reduces the strength of the ferrite. This localizes deformation and blanking workability decreases. Therefore, the Cr content is 0.65 % or less, and preferably 0.60 % or less.
• the above chemical composition contains at least one selected from the group consisting of Ti: 0.001 % to 0.30 %, Nb: 0.01 % to 0.1 %, and V: 0.005 % to 0.5%.
• Ti forms fine TiC within the ferrite grains, which strengthens the ferrite grains and suppresses the amount of plastic deformation. Therefore, the addition of Ti can improve the blanking workability.
• Ti content is less than 0.001 %, Ti is consumed by the precipitation of TiN before TiC, resulting in a blanking workability improvement effect being unobtainable. Therefore, when adding Ti, a Ti content is 0.001 % or more and preferably 0.005 % or more.
• the Ti content exceeds 0.30 %, coarse TiC is formed, and void formation and growth occur locally around the coarse TiC. As a result, plastic deformation is localized and blanking workability decreases.
• the Ti content is therefore 0.30 % or less, preferably 0.28 % or less, and more preferably 0.26 % or less.
• Nb forms fine NbC in the ferrite grains, which strengthens the ferrite grains and suppresses plastic deformation. Therefore, the addition of Nb can improve the blanking workability.
• the Nb content is 0.01 % or more and preferably 0.015 % or more.
• the Nb content exceeds 0.1 %, coarse Nb(CN) is formed and voids localize around the coarse Nb(CN), and deformations are localized, resulting in blanking workability decreasing. Therefore, the Nb content is 0.1 % or less, and preferably 0.09 % or less.
• V 0.005 % to 0.5 %
• V forms fine VC in the ferrite grains, which strengthens the ferrite grains and suppresses plastic deformation. Therefore, the addition of V can improve the blanking workability.
• a V content is less than 0.005 %, the amount of VC precipitation is small, resulting in the blanking workability improvement effect being unobtainable. Therefore, when adding V, the V content is 0.005 % or more and preferably 0.010 % or more.
• the V content exceeds 0.5 %, coarse V(CN) is formed and voids localize around the coarse V(CN), resulting in uneven deformation and blanking workability decreases.
• the V content is therefore 0.5 % or less, preferably 0.45 % or less, and more preferably 0.40 % or less.
• the cold-rolled steel sheet according to one of the embodiments of the present disclosure comprises a chemical composition containing the above components, with the balance being Fe and inevitable impurities.
• the above composition may optionally further contain at least one selected from the group consisting of Sb: 0.1 % or less, Hf: 0.5 % or less, REM: 0.1 % or less, Cu: 0.5 % or less, Ni: 3.0 % or less, Sn: 0.5 % or less, Mo: 1 % or less, and Zr: 0.5 % or less.
• the Sb is an effective element for improving corrosion resistance, but when added in excess, a rich Sb layer forms under scale generated during hot rolling, which causes surface scabs (scratches) on the steel sheet after hot rolling. Therefore, the Sb content is 0.1 % or less. Although the lower limit of the Sb content is not particularly limited, in terms of increasing the effect of adding Sb, the Sb content is preferably 0.0003 % or more.
• Hf is an effective element for improving corrosion resistance, but when added in excess, a rich Hf layer is formed under the scale generated during hot rolling, which causes surface scabs (scratches) on the steel sheet after hot rolling. Therefore, the Hf content is 0.5 % or less. Although the lower limit of the Hf content is not particularly limited, in terms of increasing the effect of adding Hf, the Hf content is preferably 0.001 % or more.
• An REM (rare earth metal) is an element that increases the strength of steel.
• excessive addition of REM may retard carbide refinement, which may promote inhomogeneous deformation during cold working and degrade surface characteristics. Therefore, the REM content is 0.1 % or less.
• the lower limit of the REM content is not particularly limited, in terms of increasing the effect of adding REM, the REM content is preferably 0.005 % or more.
• the Cu is an effective element for improving corrosion resistance, but when added in excess, a rich Cu layer is formed under the scale generated during hot rolling, which causes surface scabs (scratches) on the steel sheet after hot rolling. Therefore, the Cu content is 0.5 % or less. Although the lower limit of the Cu content is not particularly limited, in terms of increasing the effect of adding Cu, the Cu content is preferably 0.01 % or more.
• Ni is an element that increases the strength of steel. However, excessive addition may retard carbide refinement, which may promote inhomogeneous deformation during cold working and degrade surface characteristics. Therefore, the Ni content is 3.0 % or less. Although the lower limit of the Ni content is not particularly limited, in terms of increasing the effect of adding Ni, the Ni content is preferably 0.01 % or more.
• the Sn content is 0.5 % or less.
• the lower limit of the Sn content is not particularly limited, in terms of increasing the effect of adding Sn, the Sn content is preferably 0.0001 % or more.
• Mo is an element that increases the strength of steel. However, excessive addition may retard carbide refinement, which may promote inhomogeneous deformation during cold working and degrade surface characteristics. Therefore, the Mo content is 1 % or less. Although the lower limit of the Mo content is not particularly limited, in terms of increasing the effect of adding Mo, the Mo content is preferably 0.001 % or more.
• the Zr is an effective element for improving corrosion resistance, but when added in excess, a rich Zr layer is formed under the scale generated during hot rolling, which causes surface scabs (scratches) on the steel sheet after hot rolling. Therefore, the Zr content is 0.5 % or less. Although the lower limit of the Zr content is not particularly limited, in terms of increasing the effect of adding Zr, the Zr content is preferably 0.01 % or more.
• Average grain size of ferrite 10 ⁇ m or less
• the average grain size of ferrite is 10 ⁇ m or less.
• the finer the ferrite the more desirable, and therefore the lower limit of the average grain size is not limited.
• the average particle size may be 0.5 ⁇ m or more. The average particle size of ferrite can be measured by the method described in the EXAMPLES section below.
• Average grain size of cementite at ferrite grain boundaries 5 ⁇ m or less
• cementite is present both within the ferrite grains and at the ferrite grain boundaries, and the cementite at the ferrite grain boundaries is relatively coarser than the cementite within the ferrite grains.
• the present inventors found that the blanking workability can be improved by controlling the average grain size of the cementite present at ferrite grain boundaries.
• the average grain size of cementite at ferrite grain boundaries is 5 ⁇ m or less.
• cementite at the grain boundary tends to grow because annealing is repeatedly performed in the later-described production method. Therefore, realistically, the average particle size is 0.5 ⁇ m or more.
• the average grain size of cementite at ferrite grain boundaries can be measured by the method described in the EXAMPLES section below.
• the spheronization ratio of grain boundary cementite is not particularly limited but is preferably 2.5 or less.
• La and Lb are determined as the average values of the major axis lengths and minor axis lengths, respectively, of all grain boundary cementite in an image obtained by photographing a cross-section of a cold-rolled steel sheet cut in the sheet thickness direction using a scanning electron microscope (SEM) at a magnification of 1000x for 3 observation fields.
• SEM scanning electron microscope
• the major axis lengths and minor axis lengths are the values when cementite is assumed to be an ellipsoid or sphere.
• Average grain size of NaCl-type carbides in ferrite grains 0.5 ⁇ m or less
• the cold-rolled steel sheet according to the present disclosure further contains at least one of Ti, Nb, and V. These elements form NaCl-type carbides and then precipitate in the ferrite grain and at ferrite grain boundaries.
• the NaCl-type carbides are finely dispersed within the ferrite grains to harden the ferrite and may reduce the amount of plastic deformation of the ferrite grains. As a result, the burr height during press blanking can be reduced.
• the average grain size of NaCl-type carbides containing at least one of Nb, Ti, and V in the ferrite grains is 0.5 ⁇ m or less.
• the smaller the average grain size the more effective the average grain size is in strengthening ferrite, and therefore the lower limit of the average grain size is not particularly limited.
• precipitates tend to grow because annealing is repeatedly performed in the later-described production method. Therefore, realistically, the average particle size is 0.01 ⁇ m or more.
• the average particle size can be measured by the method described in the EXAMPLES section below.
• NaCl-type carbides containing at least one of Nb, Ti, and V in ferrite grains may by simply referred to as "NaCl-type carbides".
• Average spacing of NaCl-type carbides 710 nm or less
• the strengthening of ferrite by the above NaCl-type carbides is due to the finely dispersed NaCl-type carbides acting as an obstacle to dislocations, and such strengthening is referred to as strengthening by precipitation.
• strengthening by precipitation the smaller the distance between precipitates, the greater the strengthening.
• the average spacing of said NaCl-type carbides in the ferrite grains is 710 nm or less, preferably 250 nm or less.
• the lower limit of the average spacing is not particularly limited but is 30 nm or more in a realistic production environment.
• the average spacing of NaCl-type carbides in the ferrite grains can be measured by the method described in the EXAMPLES section below.
• the number density of NaCl-type carbides, which contain at least one of Nb, Ti, and V and are present in the ferrite grain, is not particularly limited but is preferably less than 100 grains/ ⁇ m 2 .
• the number density of grain boundary cementite with a grain size of 0.5 ⁇ m or more is not particularly limited but is preferably not less than 5 grains/100 ⁇ m 2 .
• the upper limit of the number density of grain boundary cementite with a grain size of 0.5 ⁇ m or more is also not particularly limited but is preferably not more than 50 grains/100 ⁇ m 2 .
• the cold-rolled steel sheet according to the present disclosure has a ferrite-containing microstructure.
• the ferrite area ratio is not particularly limited, the cold-rolled steel sheet preferably has a ferrite-dominated microstructure. "Ferrite-dominated" is defined as having a ferrite area ratio of 50 % or more. The ferrite area ratio is preferably 68 % or more.
• the microstructure can also contain any microstructure other than ferrite.
• the cementite area ratio is preferably less than 30 %.
• a cold-rolled steel sheet according to one of the embodiments of the present disclosure may, for example, have a microstructure containing, in area ratio, 68 % or more ferrite, less than 30 % cementite, with the balance being precipitates other than cementite.
• the "precipitates other than cementite” may include, for example, carbides excluding cementite (Fe 3 C), nitrides, carbonitrides, sulfides, and carbon sulfides. More specific examples include carbides, nitrides, and carbonitrides of at least one of Ti, V, and Nb, as well as Mn-based sulfides and Ti-based complex carbosulfides.
• the sheet thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness. Considering the press blanking and a use as material for textile machinery parts, the sheet thickness is preferably 0.1 mm or more. The sheet thickness is preferably 1.6 mm or less. In particular, considering a use as a material for knitting needles, the sheet thickness is preferably 0.2 mm or more. The sheet thickness is preferably 0.8 mm or less.
• the following describes a method of producing a cold-rolled steel sheet according to one of the embodiments of the present disclosure.
• Cold-rolled steel sheets can be produced by subjecting steel slabs having the above chemical composition to the following processes in order.
• a steel slab with the above chemical composition is heated.
• the method of producing the steel slab is not particularly limited and any method may be used.
• adjustments to the chemical composition of the steel slab may be performed by blast furnace converter steelmaking process or electric furnace steelmaking process.
• Casting from molten steel into slabs may be performed by continuous casting or by blooming.
• a heating temperature of the steel slab is not particularly limited but may be adjusted such that a temperature of the steel slab, at the stage when the next hot rolling starts, is in the austenite region as described later.
• the heated steel slab subjected to hot rolling to obtain a hot-rolled steel sheet.
• rough rolling and finish rolling may be performed according to conventional methods.
• Hot rolling start temperature Ac3 point or more
• the hot rolling start temperature is the Ac3 point or more.
• the Ac3 point (°C) is obtained by the following formula (1).
• Ac 3 ° C 910 ⁇ 203 ⁇ C 1 / 2 + 44.7 ⁇ Si ⁇ 30 ⁇ Mn ⁇ 11 ⁇ Cr + 400 ⁇ Ti + 460 ⁇ Al + 700 ⁇ P + 104 ⁇ V + 38
• the element symbols denote the contents (mass%) of the respective elements, and the content of any element not contained is 0 mass%.
• Finisher delivery temperature 800 °C or more
• the finisher delivery temperature is 800 °C or more.
• Time to cooling start 5.0 seconds or less
• the hot-rolled steel sheet is cooled.
• the time from hot-rolling finish to cooling start (hereinafter simply referred to as "time to cooling start") is 5.0 seconds or less, preferably 4.5 seconds or less, and more preferably 4.0 seconds or less.
• the lower limit of the time to cooling start is not particularly limited, in terms of compatibility with general production lines, the time to start cooling is preferably 0.2 seconds or more and more preferably 0.5 seconds or more.
• Average cooling rate 25 °C/s or more
• the average cooling rate in the above cooling is less than 25 °C/s, elongated grains generate in the cold-rolled steel sheet, which is the final product, and as a result blanking workability decreases. Therefore, the average cooling rate is 25 °C /s or more.
• the upper limit of the average cooling rate is not particularly limited, in terms of compatibility with general production lines, the average cooling rate is preferably 80 °C/s or less, more preferably 60 °C/s or less, and even more preferably 50 °C/s or less.
• Cooling stop temperature 620 °C to 740 °C
• the cooling stop temperature is 740 °C or less.
• the cooling stop temperature is 620 °C or more, and preferably 630 °C or more.
• the cooled hot-rolled steel sheet is wound into a coil.
• the winding temperature is not particularly limited, the winding temperature is preferably 600 °C or more. The winding temperature is preferably 730 °C or less.
• the hot-rolled steel sheet is also preferably pickled after the winding and prior to the next first annealing.
• the hot-rolled steel sheet after winding has a pearlitic microstructure. Therefore, the cementite contained in the pearlite is decomposed by the first annealing of the hot-rolled steel sheet after winding. By decomposing the cementite, the cementite becomes fine in the subsequent second annealing and cold rolling. Therefore, as a result, the ferrite is refined, and the plastic deformation of ferrite grains can be suppressed.
• Annealing temperature 730 °C or less
• the annealing temperature in the first annealing process is higher than 730 °C, phase transformation preferentially proceeds in one area, resulting in local coarsening of ferrite grains, and as a result an increase in plastic deformation. A locally coarse microstructure also results in inhomogeneous working and poor part shape accuracy. Therefore, the annealing temperature is 730 °C or less.
• the lower limit of said annealing temperature is not particularly limited, in terms of cementite reforming a solid solution in pearlite to promote cementite decomposition, the annealing temperature is preferably 450 °C or more, more preferably 500 °C or more, and even more preferably 520 °C or more.
• Annealing time 5 hours or more
• the annealing time in the first annealing is less than 5 hours, the decomposition of cementite does not progress.
• the annealing time is 5 hours or more.
• the upper limit of the annealing time is not particularly limited.
• the microstructural change saturates after cementite decomposition begins, and therefore in terms of manufacturing efficiency, the annealing temperature is preferably 50 hours or less, and more preferably 40 hours or less.
• the hot-rolled steel sheet is also preferably pickled after the first annealing and prior to the next bending and reverse bending.
• the hot-rolled steel sheet after the first annealing is subjected to bending and reverse bending.
• This bending and reverse bending is extremely important for achieving a desired microstructure in the finally-obtained cold-rolled steel sheet.
• the hot-rolled steel sheet is subjected to bending and reverse bending after the cementite is decomposed by the first annealing.
• the bending and reverse bending provides processing strain which introduces strain energy.
• a later-described second annealing promotes cementite refinement. Without bending and reverse bending, the coarsened cementite localizes, and the amount of plastic deformation increases locally, resulting in blanking workability decreasing.
• processing strain by bending and reverse bending can be done by any method without any particular limitation.
• bending and reverse bending may be applied using a leveler used in shape adjustment, a skin pass mill, or a slitter for shearing steel sheets, and bending and reverse bending may be applied during unwinding from a coil and rewinding into a coil.
• the diameter of the rolls is preferably 300 mm or more, and more preferably 450 mm or more.
• the rolls may be bridle rolls. When bridle rolls are used, strain is introduced by passing the sheet through the bridle rolls.
• the hot-rolled steel sheet is subjected to second annealing.
• the second annealing promotes cementite refinement.
• Annealing temperature 600 °C or more
• the annealing temperature in the second annealing is less than 600 °C, cementite refinement does not progress and the formation of NaCl-type carbides containing at least one of Nb, Ti and V is suppressed.
• the annealing temperature in the second annealing is 600 °C or more.
• the upper limit of the annealing temperature is not particularly limited, when the upper limit is too high, the structure becomes coarse and the burrs increase in height, and therefore the annealing temperature is preferably 790 °C or less, and more preferably 770 °C or less.
• the hot-rolled steel sheet after the second annealing is subjected to two or more repetitions of cold rolling and third annealing.
• the cold rolling adjusts the sheet thickness of the final cold-rolled steel sheet.
• the third annealing removes strain caused by the cold rolling. Performing the cold rolling and third annealing two or more times improves the uniformity of the microstructure and strengthens the ferrite by refining the ferrite microstructure, resulting in improved blanking workability.
• the rolling ratio in the cold rolling is 15 % or more
• the annealing temperature in the third annealing is 600 °C or more.
• the upper limit of the rolling ratio is not particularly limited, when the rolling ratio is excessively high, the microstructure becomes locally coarse, and the burrs increase in height. Therefore, the rolling ratio is preferably 52 % or less, and more preferably 50 % or less.
• the upper limit of the annealing temperature in a third annealing is not particularly limited, but an excessively high annealing temperature coarsens the microstructure and burrs increase in height. Therefore, the annealing temperature is preferably 750 °C or less, and more preferably 720 °C or less.
• the rolling ratio in the final cold rolling is not particularly limited but is preferably 20 % or more.
• the upper limit of the rolling reduction in the final cold rolling is also not particularly limited but is preferably 50 % or less.
• the finally-obtained cold-rolled steel sheets may also be subjected to further optional surface treatment.
• cold-rolled steel sheets were produced by the following procedure, and the blanking workability of each of the resulting cold-rolled sheets was evaluated.
• microstructures of resulting cold-rolled steel sheets were evaluated by the following procedure.
• a test piece for microstructure observation was taken from each of the resulting cold-rolled steel sheets. After polishing a rolling direction cross-section (L-section) of the test piece for microstructure observation, the polished surface was corroded with a 3 vol% nital solution to reveal the microstructure. The surface of the test piece for microstructure observation was then imaged using the scanning electron microscope (SEM) at a magnification of 3,000x to obtain a microstructure image. The ferrite grain sizes were measured from the obtained microstructure image by the cutting method according to Japan Industrial Standard (JIS) G0551:2020. The average ferrite grain size measured in 5 observation fields was calculated and used as the average grain size.
• JIS Japan Industrial Standard
• a test piece for microstructure observation was taken from each of the resulting cold-rolled steel sheets. After polishing a rolling direction cross-section (L-section) of the test piece for microstructure observation, the polished surface was corroded with a 3 vol% nital solution to reveal the microstructure. The surface of the test piece for microstructure observation was then imaged using the SEM at a magnification of 3000x to obtain a microstructure image. The grain size of only the grain boundary cementite was measured from the obtained microstructure image, and the measurement was made by the cut-off method. The average grain size of the grain boundary cementite measured in 3 observation fields was calculated and used as the average grain size of the grain boundary cementite. The number density of grain boundary cementite with a grain size of 0.5 ⁇ m or more was determined from the microstructure image.
• TEM transmission electron microscope
• the individual grain diameters of NaCl-type carbides, which contain at least one of Nb, Ti, and V and are present the ferrite grains, in the obtained microstructure image were determined and their average value was calculated.
• the average spacing of NaCl-type carbides which contain at least one of Nb, Ti, and V and are present in the ferrite grain, was determined by measuring the spacing of all NaCl-type carbides that could be seen in an observation field at 80,000x and calculating the average value for five observation fields.
• the measurement results are as listed in Tables 4 and 5.
• the NaCl-type carbides in Tables 4 and 5 refer to NaCl-type carbides which contain at least one of Nb, Ti, and V and are present in ferrite grains.
• test piece 20 mm wide, 150 mm long, and 0.4 mm thick was taken from each cold-rolled steel sheet.
• a ⁇ 10 SKD (die steel) or cemented carbide punch was then used to perform blanking on the test piece.
• the clearance in the blanking was 100 ⁇ m.
• the blanking was performed 10 times for each test piece. In this case, the distance from the edge of the test piece to the blanking hole was 5 mm or more for the first blanking. The distance between adjacent blanking holes was 5 mm or more for the second and subsequent blankings.
• the height of burrs occurring in the circumferential direction was then observed using a microscope.
• the height of the burrs was measured at five locations evenly in the circumferential direction for each hole, and the average of the burr heights at the five locations was calculated.
• the same measurements were then performed on 10 holes, and the average value of the burr heights calculated for each hole was adopted as the burr height.
• Table 2 No. Steel s ample ID Ac3 Hot rolling Cooling First annealing Bending and revers e bending Second annealing Cold rolling Third annealing Repetition frequency Remarks Hot rolling Start temp. Finisher delivery temp. Time to cooling start Average cooling rate Cooling stop temp. Annealing temp. Annealing time Roll diameter Annealing temp. Rolling ratio Annealing temp. °C °C °C seconds °C/s °C °C h mm °C % °C No.

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