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
  • C22C38/04—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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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
  • C21D1/26—Methods of annealing
  • C21D1/32—Soft annealing, e.g. spheroidising
• • 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
  • C21D1/84—Controlled slow cooling
• • 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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0257—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
• • 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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties 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 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
• • 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
  • C22C38/02—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
  • C22C38/06—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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/08—Iron or steel
• • 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
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite
• • 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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties 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
  • 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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the present invention relates to steel plate and a method of production of the same.
• Gears, clutches, and other auto parts are produced through stamping, forging, press-forming, and other working processes.
• improvement of the workability of the starting material carbon steel plate has been sought. Further, these parts are quenched and tempered, then used at a high strength, so excellent hardenability is demanded.
• PLT 1 discloses high carbon steel plate excellent in workability, hardenability, and toughness after heat treatment containing, by mass%, C: 0.20 to 0.45%, Mn: 0.40 to 1.50%, P: 0.03% or less, S: 0.02% or less, P+S: 0.010% or more, Cr: 0.01 to 0.80%, Ti: 0.005 to 0.050%, and B: 0.0003 to 0.0050% and having a balance of Fe and unavoidable impurities, further, containing Sn: 0.05% or less and Te: 0.05% or less and containing a total of Sn+Te of 0.005% or more, and comprised of a mixed structure of ferrite and pearlite or a mixed structure of ferrite and cementite.
• PLT 2 discloses a method of production of high hardenability high carbon hot rolled steel plate comprising hot rolling steel containing, by mass%, C: 0.2 to 0.7%, Si: 2% or less, Mn: 2% or less, P: 0.03% or less, S: 0.03% or less, sol.
• Al 0.08% or less
• N 0.01% or less and having a balance of iron and unavoidable impurities until the finishing temperature (Ar3 transformation point-20°C) or more, cooling it by a cooling rate of over 120°C/sec down to a cooling end temperature of 620°C or less, coiling it by a coiling temperature of 600°C or less to control it to a structure having over volume fraction 20% bainite phases, pickling it, and annealing it by an annealing temperature of 640°C to the Ac1 transformation point to obtain spheroidized structures.
• the high carbon steel plate described in PLT 1 also uses the high hardness pearlite in the starting material structure and is not necessarily excellent in workability.
• PLT 2 does not describe a specific form of structure excellent in workability.
• the present invention in consideration of the current state of the prior art, has as its object the provision of steel plate improved in formability and wear resistance, in particular suitable for obtaining gears, clutches, and other parts by forming thick gauge plate, and a method of production of the same.
• Ferrite phases are low in hardness and high in ductility. Therefore, in a structure mainly comprised of ferrite, by making the grain size larger, it becomes possible to raise the formability of the material.
• Carbides by being made to suitably disperse in the metal structure, can maintain the formability of the material while imparting excellent wear resistance and rolling fatigue characteristics, so are structures essential for drive system parts. Further, carbides in steel plate are strong grains inhibiting slip. By making carbides present at the ferrite grain boundaries, it is possible to prevent the propagation of slip crossing crystal grain boundaries and suppress the formation of a shear zone, improve the cold forgeability, and simultaneously improve the formability of the steel plate.
• cementite is a hard and brittle structure. If present in the state of a layered structure with ferrite, that is, pearlite, the steel becomes hard and brittle, so it must be made present in a spheroidal shape. If considering the cold forgeability and formation of cracks at the time of forging, the grain size has to be a suitable range.
• the plate should be coiled up at a relatively low temperature (400°C to 550°C). By coiling at a relatively low temperature, the cementite dispersed in the ferrite also becomes easy to spheroidize.
• the cementite should be partially spheroidized by annealing at a temperature of right below the Ac1 point.
• part of the ferrite grains should be left while causing part to transform to austenite by annealing at a temperature between the Ac1 point and the Ac3 point (so-called dual phase region of ferrite and austenite).
• the plate should be slowly cooled to cause the remaining ferrite grains to grow while using these as nuclei for transformation of austenite to ferrite to thereby obtain large ferrite phases while causing cementite to precipitate at the grain boundaries and realize the above structure.
• the present invention was made based on the above findings and has as its gist the following:
• C is an element which forms carbides in the steel and is effective for strengthening of steel and refinement of ferrite grains.
• C is made 0.10% or more. Preferably it is 0.12% or more.
• C is made 0.40% or less.
• it is 0.38% or less.
• Si is an element which acts as a deoxidizing agent and has an effect on the form of the carbides.
• Si is made 0.01% or more. Preferably, it is 0.05% or more.
• Si is 0.30% or less.
• it is 0.28% or less.
• Mn is an element which raises the hardenability and contributes to the improvement of the strength. If less than 1.00%, securing the strength after hardening and the residual carbides after hardening becomes difficult, so Mn is made 1.00% or more. Preferably, it is 1.09% or more.
• Mn is made 2.00% or less.
• it is 1.91% or less.
• Al is an element acting as a deoxidizing agent of the steel and stabilizing ferrite. With less than 0.001%, the effect due to addition is not sufficiently obtained, so Al is made 0.001% or more. Preferably it is 0.004% or more.
• Al is made 0.10% or less. Preferably it is 0.08% or less.
• the following elements are impurities and have to be controlled to certain amounts or less.
• P is an element which segregates at the ferrite grain boundaries and suppresses the formation of grain boundary carbides.
• P is made 0.020% or less. Preferably, it is 0.018% or less.
• S is an impurity element forming MnS and other nonmetallic inclusions.
• Nonmetallic inclusions become starting points of fracture at the time of cold working, so S is preferably as small as possible, but if reducing it to less than 0.0001%, the refining costs greatly increase, so S is made 0.0001% or more. Preferably it is 0.0012% or more.
• S is made 0.010% or less. Preferably, it is 0.007% or less.
• N is an element causing embrittlement of ferrite if contained in a large amount and is preferably as small as possible.
• the content of N may be made 0 as well, but if reducing it to less than 0.0001%, the refining costs greatly increase, so the substantive lower limit is 0.0001 to 0.0006%.
• N is made 0.010% or less. Preferably, it is 0.007% or less.
• O is an element forming coarse oxides in the steel if contained in a large amount and is preferably as small as possible.
• the content of O may also be 0%, but if reducing it to less than 0.0001%, the refining costs greatly increase, so the substantive lower limit is 0.0001 to 0.0011%.
• O is made 0.02% or less. Preferably it is 0.017% or less.
• Sn is an element which enters from the steel starting materials (scrap). It segregates at the grain boundaries and invites a drop in the number ratio of the grain boundary carbides, so is preferably as small as possible.
• the content of Sn may also be 0, but if reducing it to less than 0.001%, the refining costs greatly increase, so the substantive lower limit is 0.001 to 0.002%.
• Sn is made 0.050% or less. Preferably, it is 0.040% or less.
• Sb like Sn, is an element which enters from the steel starting materials (scrap). It segregates at the grain boundaries and invites a drop in the number ratio of the grain boundary carbides, so is preferably as small as possible.
• the content of Sb may also be 0, but if reducing it to less than 0.001%, the refining costs greatly increase, so the substantive lower limit is 0.001 to 0.002%.
• Sb is made 0.050% or less. Preferably, it is 0.040% or less.
• the content of As may also be 0, but if reducing it to less than 0.001%, the refining costs greatly increase, so the substantive lower limit is 0.001 to 0.002%.
• As is made 0.050% or less. Preferably, it is 0.040% or less.
• the steel plate of the present invention has the above elements as basic components, but may further contain the following elements for the purpose of improving the cold forgeability of the steel plate.
• the following elements are not essential for obtaining the effects of the present invention, so their contents may be 0 as well.
• Cr is an element which raises the hardenability and contributes to the improvement of the strength and, further, is an element which concentrates at the carbides and forms stable carbides even in the austenite phases.
• Cr preferably is made 0.001% or more. More preferably, it is 0.007% or more.
• Cr is made 0.50% or less. Preferably, it is 0.45% or less.
• Mo is an element effective for control of the form of carbides.
• Mo preferably is made 0.001% or more. More preferably, it is 0.010% or more.
• Mo is made 0.10% or less. Preferably, it is 0.08% or less.
• Nb is an element effective for control of the form of carbides. Further, it is an element refining the structure and contributing to improvement of the toughness. To obtain the effect of addition, Nb preferably is made 0.001% or more. More preferably, it is 0.002% or more. On the other hand, if over 0.10%, a large number of fine Nb carbides precipitate, the strength excessively rises, further, the number ratio of the grain boundary carbides decreases and the cold workability deteriorates, so Nb is made 0.10% or less. Preferably, it is 0.08% or less.
• V 0.10% or less
• V also, like Nb, is an element effective for control of the form of carbides. Further, it is an element refining the structure and contributing to improvement of the toughness. To obtain the effect of addition, V preferably is made 0.001% or more. More preferably, it is 0.004% or more. On the other hand, if over 0.10%, a large number of fine V carbides precipitate, the strength excessively rises, further, the number ratio of the grain boundary carbides decreases and the cold workability deteriorates, so V is made 0.10% or less. Preferably, it is 0.08% or less.
• Cu is an element which segregates at the crystal grain boundaries of the ferrite and forms fine precipitates to thereby contribute to the improvement of the strength.
• Cu preferably is made 0.001% or more. More preferably, it is 0.005% or more.
• Cu is made 0.10% or less. Preferably, it is 0.08% or less.
• W also, like Nb and V, is an element effective for control of the form of carbides.
• W preferably is made 0.001% or more. More preferably, it is 0.003% or more.
• W is made 0.10% or less. Preferably, it is 0.08% or less.
• Ta 0.10% or less
• Ta also, like Nb, V, and W, is an element effective for control of the form of carbides.
• Ta preferably is made 0.001% or more. More preferably, it is 0.005% or more.
• Ta is 0.10% or less. Preferably, it is 0.08% or less.
• Ni is an element effective for improving the toughness of a part.
• Ni preferably is made 0.001% or more. More preferably, it is 0.004% or more.
• Ni is made 0.10% or less. Preferably, it is 0.08% or less.
• Mg is an element able to control the form of sulfides by addition of a trace amount.
• Mg preferably is made 0.0001% or more. More preferably, it is 0.0008% or more.
• Mg is made 0.050% or less. Preferably, it is 0.040% or less.
• Ca is an element able to control the form of sulfides by addition of a trace amount.
• Ca preferably is made 0.001% or more. More preferably, it is 0.003% or more.
• Ca is made 0.050% or less. Preferably, it is 0.040% or less.
• Y like Mg and Ca, is an element able to control the form of sulfides by addition of a trace amount.
• Y preferably is made 0.001% or more. More preferably, it is 0.003% or more.
• Y is made 0.050% or less. Preferably, it is 0.035% or less.
• Zr like Mg, Ca, and Y, is an element able to control the form of sulfides by addition of a trace amount.
• Zr preferably is made 0.001% or more. More preferably, it is 0.004% or more.
• Zr is made 0.050% or less. Preferably, it is 0.045% or less.
• La is an element effective for control of the form of sulfides by addition of a trace amount, but is also an element which segregates at the grain boundaries and invites a drop in the number ratio of the grain boundary carbides.
• La preferably is made 0.001% or more. More preferably, it is 0.004% or more.
• La is made 0.050% or less. Preferably, it is 0.045% or less.
• Ce is an element able to control the form of sulfides by addition of a trace amount, but is also an element which segregates at the grain boundaries and invites a drop in the number ratio of the grain boundary carbides.
• Ce preferably is made 0.001% or more. More preferably, it is 0.004% or more.
• Ce is made 0.050% or less. Preferably, it is 0.046% or less.
• the balance of the chemical composition of the steel plate of the present invention is comprised of Fe and unavoidable impurities.
• part of the Fe may be replaced by one or both of Ti and B.
• Ti is an element effective for control of the form of carbides. Further, it is also an element refining the structure and contributing to improvement of the toughness. To obtain the effect of addition, Ti preferably is made 0.001% or more. More preferably, it is 0.005% or more. On the other hand, if over 0.10%, coarse Ti oxides are formed and become starting points of fracture at the time of cold working, so Ti is 0.10% or less. Preferably, it is 0.08% or less.
• B is an element which raises the hardenability at the time of heat treatment of a part and makes the structure uniform and which contributes to improvement of the toughness.
• B preferably is made 0.0001% or more. More preferably, it is 0.0006% or more.
• B is made 0.010% or less. Preferably, it is 0.009% or less.
• the structure of the steel plate of the present invention is substantially a structure comprised of ferrite and carbides.
• Carbides include not only the cementite (Fe 3 C) of the compound of iron and carbon but also compounds where the Fe atoms in cementite are replaced by Mn, Cr, and other alloy elements and alloy carbides (M 23 C 6 , M 6 C, MC, etc. [where M: Fe, and other metal elements added as alloys]).
• a shear zone is formed in the macrostructure of the steel plate and slip deformation occurs concentratedly near the shear zone. Slip deformation is accompanied with proliferation of dislocations. Near the shear zone, a region of high dislocation density is formed. Along with the increase of the amount of strain imparted to the steel plate, slip deformation is promoted and the dislocation density increases.
• a shear zone is understood as the phenomenon of slip occurring at a certain single crystal grain crossing crystal grain boundaries and continuously propagating to the adjoining crystal grains. Accordingly, to suppress formation of a shear zone, it is necessary to prevent propagation of slip crossing crystal grain boundaries.
• Carbides in steel plate are strong grains inhibiting slip. By forming carbides at the ferrite grain boundaries, propagation of slip crossing crystal grain boundaries can be prevented and formation of a shear zone can be suppressed so the cold formability can be improved. Simultaneously, the formability of the steel plate is improved.
• the formability of steel plate is greatly dependent on the accumulation of strain inside the crystal grains (accumulation of dislocations). If propagation of strain to the adjoining crystal grains is inhibited at the crystal grain boundaries, the amount of strain inside the crystal grains will increase. As a result, the work hardening rate will increase and the formability will be improved.
• the spheroidization rate of the carbides at the crystal grain boundaries is less than 80%, strain locally concentrates at the rod-shaped or plate-shaped carbides and voids and/or cracks easily are formed, so the spheroidization rate of the carbides at the crystal grain boundaries is preferably 80% or more, more preferably it is 90% or more.
• the average grain size of the carbides is less than 0.1 ⁇ m, the hardness of the steel plate remarkably increases and the workability deteriorates, so the average grain size of the carbides is preferably 0.1 ⁇ m or more. More preferably, it is 0.17 ⁇ m or more.
• the average grain size of carbides is preferably 2.0 ⁇ m or less. More preferably, it is 1.95 ⁇ m or less.
• the carbides are observed by a scanning electron microscope. Before observation, the sample for observation of the structure is polished by wet polishing by Emery paper and a diamond abrasive having an average particle size of 1 ⁇ m, the observed surface is polished to a mirror finish, then a 3% nitric acid-alcohol solution is used to etch the structure. For the magnification of the observation, magnification enabling judgment of the structure of ferrite and carbides in 3000X is selected. Eight images of fields of 30 ⁇ m ⁇ 40 ⁇ m at a plate thickness 1/4 layer are captured at random by the selected magnification.
• carbides with an area of 0.01 ⁇ m 2 or less are excluded from coverage by the evaluation.
• the number of carbides present at the ferrite grain boundaries is counted and the number of carbides at the ferrite grain boundaries is subtracted from the total number of carbides to calculate the number of carbides inside the ferrite grains. Based on the measured numbers, the number ratio of carbides at the ferrite grain boundaries to carbides inside the ferrite grains was found.
• the ferrite grain size 5.0 ⁇ m or more. If the ferrite grain size is less than 5 ⁇ m, the hardness increases and, at the time of cold working, fractures and cracks easily form, so the ferrite grain size is made 5 ⁇ m or more. Preferably, it is 7 ⁇ m or more.
• the ferrite grain size is 50 ⁇ m or less. Preferably, it is 37 ⁇ m or less.
• the ferrite grain size is measured by polishing the observed surface of the sample to a mirror finish by the above-mentioned polishing method, then etching the surface by a 3% nitric acid-alcohol solution and observing the structure of the observed surface by an optical microscope or scanning electron microscope and applying the line segment method to the captured images.
• the carbide of iron that is, cementite
• the carbide of iron is a hard and brittle structure. If present as a layered structure with ferrite, that is, in the state of pearlite, the steel becomes hard and brittle. Therefore, pearlite has to be reduced as much as possible.
• the steel plate of the present invention it is made an area ratio of 6% or less.
• Pearlite has a distinctive lamellar structure, so can be discerned by observation by an SEM or optical microscope. By calculating the regions of lamellar structures in any cross-section, it is possible to find the area ratio of pearlite.
• the Vickers hardness of the steel plate 100HV to 170HV, it is possible to improve the cold workability. If the Vickers hardness is less than 100HV, buckling easily occurs during cold working, so the Vickers hardness is made 100HV or more. Preferably, it is 110HV or more.
• the Vickers hardness is made 170HV or less.
• it is 168HV or less.
• the method of production of the present invention has as its basic idea to use a steel slab of the above-mentioned chemical composition, integrally manage the hot rolling conditions and annealing conditions, and control the structure of the steel plate.
• a steel slab obtained by continuously casting molten steel of each of the required chemical compositions was prepared for hot rolling.
• the continuously cast steel slab may be directly used for hot rolling or may be used for hot rolling after being cooled once, then heated.
• the heating temperature is preferably 1000°C to 1250°C and the heating time is preferably 0.5 hour to 3 hours.
• the temperature of the steel slab used for the hot rolling is preferably 1000°C to 1250°C.
• the steel slab temperature or steel slab heating temperature is preferably 1250°C or less and the heating time is preferably 3 hours or less. More preferably, it is 1200°C or less or 2.5 hours or less.
• the steel slab temperature or steel slab heating temperature is less than 1000°C or the heating time is less than 0.5 hour, the microsegregation or macrosegregation formed by the casting is not eliminated, regions remain inside the steel slab where Si, Mn, and other alloy elements locally concentrate, and the impact resistance deteriorates.
• the steel slab temperature or steel slab heating temperature is preferably 1000°C or more and the heating time is preferably 0.5 hour or more. More preferably, it is 1050°C or more or 1 hour or more.
• the finish rolling in the hot rolling is completed in the 750°C to 850°C temperature region. If the finish rolling temperature is less than 750°C, the deformation resistance of the steel plate increases and the rolling load remarkably rises. Further, the amount of roll wear increases and the productivity deteriorates. Along with this, the recrystallization required for improving the plasticity anisotropy does not sufficiently proceed. Therefore, the finish rolling temperature is made 750°C or more. In the point of promoting recrystallization, preferably it is 770°C or more.
• the finish rolling temperature is made 850°C or less. Preferably, it is 830°C or less.
• the cooling rate is preferably 10°C/sec to 100°C/sec. If the cooling rate is less than 10°C/sec, bulky scale is formed in the middle of cooling, formation of flaws due to the same cannot be suppressed, and the impact resistance deteriorates, so the cooling rate is preferably 10°C/sec or more. More preferably, it is 20°C/sec or more.
• the cooling rate is preferably 100°C/sec or less. More preferably, it is 90°C/sec or less.
• the cooling rate indicates the cooling ability received from the cooling facilities in each water spray section at the time when being cooled on the ROT down to the target temperature of coiling from the time when the hot rolled steel plate after finish rolling is water cooled at a water spray section after passing through a non-water spray section. It does not show the average cooling rate from the starting point of water spray to the temperature at which the steel plate is coiled up by the coiler.
• the coiling temperature is made 400°C to 550°C. This is a temperature lower than the general coiling temperature. In particular, it is a condition not usually applied when the content of C is high.
• the structure of the steel plate can be made a bainite structure comprised of fine ferrite in which carbides are dispersed.
• the austenite which had not yet been transformed before coiling, transforms to hard martensite.
• the time of payout of the hot rolled steel plate coil cracks form at the surface layer of the hot rolled steel plate and the impact resistance deteriorates.
• the coiling temperature is made 400°C or more. Preferably, it is 430°C or more.
• the coiling temperature is over 550°C, large lamellar spacing pearlite is formed and high heat stability, bulky needle-shaped carbides are formed. These needle-shaped carbides remain even after two-stage annealing. At the time of cold forging and other forming of steel plate, the needle-shaped carbides form starting points for cracks.
• the coiling temperature is made 550°C or less. Preferably, it is 520°C or less.
• the hot rolled steel plate coil is paid out and pickled, then is treated by two-stage step type annealing (two-stage annealing) holding it at two temperature regions.
• two-stage annealing By treating the hot rolled steel plate by two-stage annealing, the stability of the carbides is controlled and the formation of carbides at the ferrite grain boundaries is promoted.
• the first stage annealing is performed at 650 to 720°C, preferably the Ac1 point or less temperature region. Due to this annealing, the carbides are made to coarsen and are made to partially spheroidize and the alloy elements are made to concentrate at the carbides to raise the thermal stability of the carbides.
• the heating rate up to the annealing temperature (below, referred to as the "first stage heating rate") is 30°C/hour to 150°C/hour. If the first stage heating rate is less than 30°C/hour, time is raised for raising the temperature and the productivity deteriorates, so the first stage heating rate is made 3°C/hour or more. Preferably, it is 10°C/hour or more.
• the first stage heating rate is over 150°C/hour, at the hot rolled steel plate coil, the temperature difference between the peripheral parts and the inside increases whereby scratches and seizing occur due to the difference in heat expansion and relief shapes are formed at the surface of the steel plate.
• the relief shapes form starting points for cracks, the cold forgeability deteriorates, and the formability and the impact resistance after carburizing, quenching, and tempering deteriorates, so the first stage heating rate is made 150°C/hour or less. Preferably, it is 130°C/hour or less.
• the annealing temperature in the first stage annealing (below, called the "first annealing temperature”) is 650°C to 720°C. If the first annealing temperature is less than 650°C, the carbides are not sufficiently stabilized and at the time of the second stage annealing, it becomes difficult to make carbides remain in the austenite. For this reason, the first annealing temperature is made 650°C or more. Preferably, it is 670°C or more.
• the first stage annealing temperature is made 720°C or less.
• it is 700°C or less.
• the annealing time in the first stage annealing (below, called the "first annealing time") is 3 hours to 60 hours. If the first annealing time is less than 3 hours, the carbides are not sufficiently stabilized and at the time of the second stage annealing, it becomes difficult to make carbides remain in the austenite. For this reason, the first annealing time is made 3 hours or more. Preferably, it is 5 hours or more.
• the first annealing time is over 60 hours, much greater stabilization of the carbides cannot be expected. Furthermore, the productivity deteriorates, so the first annealing time is made 60 hours or less. Preferably, it is 55 hours or less.
• the temperature is raised to 725 to 790°C, preferably, the Ac1 point to the A 3 point in temperature range, and austenite is made to form in the structure.
• austenite is made to form in the structure.
• the carbides in the fine ferrite grains dissolve in the austenite, but the carbides coarsened by the first stage annealing remain in the austenite.
• the heating rate up to the annealing temperature of the second stage annealing (below, referred to as the "second stage heating rate") is 1°C/hour to 80°C/hour.
• austenite is formed and grows from the ferrite grain boundaries.
• by slowing the heating rate up to the annealing temperature formation of nuclei of austenite is suppressed and, in the structure formed by slow cooling after annealing, the rate of coverage of grain boundaries by carbides can be raised.
• the second stage heating rate is preferably slower, but if less than 1°C/hour, time is required to raise the temperature and the productivity deteriorates, so the second stage heating rate is made 1°C/hour or more. Preferably, it is 10°C/hour or more.
• the second stage heating rate is over 80°C/hour, at the hot rolled steel plate coil, the temperature difference between the peripheral parts and the inside increases, and scratches and seizing occur due to the large difference in heat expansion, and as a result, deformation and relief shapes are formed at the surface of the steel plate.
• the relief shapes form starting points for cracks, the cold forgeability and formability deteriorates, and the impact resistance after carburizing, quenching, and tempering also deteriorates, so the second stage heating rate is made 80°C/hour or less. Preferably, it is 70°C/hour or less.
• the annealing temperature at the second stage annealing (below, called the "second stage annealing temperature”) is made 725°C to 790°C. If the second stage annealing temperature is less than 725°C, the amount of formation of austenite becomes smaller and, after the cooling after the second stage annealing, the number of carbides at the ferrite grain boundaries decreases and the ferrite grain size becomes smaller. For this reason, the second stage annealing temperature is made 725°C or more. Preferably, it is 735°C or more.
• the second stage annealing temperature exceeds 790°C, it becomes difficult to make carbides remain at the austenite and control of changes in structure becomes difficult, so the second stage annealing temperature is made 790°C or less. Preferably, it is 770°C or less.
• the annealing time in the second stage annealing is made 3 hours to 50 hours. If the second stage annealing time is less than 3 hours, the amount of production of austenite becomes smaller and the carbides in the ferrite grains do not sufficiently dissolve so it becomes difficult to increase the number of carbides at the ferrite grain boundaries and, further, the ferrite grain size becomes smaller. For this reason, the second stage annealing time is made 3 hours or more. Preferably, it is 5 hours or more.
• the second stage annealing time is over 50 hours, it becomes difficult to make carbides remain in the austenite. Further, the manufacturing costs increase, so the second stage annealing time is made less than 50 hours. Preferably, it is 40 hours or less.
• the steel plate After the two-stage annealing, the steel plate is cooled by a 1°C/hour to 30°C/hour cooling rate down to 650°C.
• the austenite is transformed to ferrite, carbon atoms are adsorbed at the carbides remaining in the austenite, the carbides and austenite cover the ferrite grain boundaries, and, finally, it is possible to obtain a structure in which a large number of carbides are present at the ferrite grain boundaries.
• the cooling rate is preferably slow, but if less than 1°C/hour, the time required for cooling increases and the productivity deteriorates, so the cooling rate is made 1°C/hour or more. Preferably, it is 10°C/hour or more.
• the cooling rate is made 30°C/hour or less.
• it is 20°C/hour or less.
• the steel plate cooled down to 650°C is cooled down to room temperature.
• the cooling rate at this time is not limited.
• the atmosphere in the two-stage annealing is not limited to any specific atmosphere.
• it may also be any atmosphere of a 95% or more nitrogen atmosphere, 95% or more hydrogen atmosphere, and air atmosphere.
• the cold workability was evaluated by taking a JIS No. 5 tensile test piece from the plate thickness 3 mm material as annealed and conducting a tensile test.
• the total elongations in the direction of 0° from the rolling direction and the direction of 90° from the rolling direction were evaluated. In the case where, in both directions, they were 35% or more and the difference of the total elongations
• the hardenability was evaluated by grinding a plate thickness 3 mm material as annealed to a plate thickness 1.5 mm, holding it in a vacuum atmosphere at 880°C ⁇ 10 minutes, hardening it by a 30°C/sec cooling rate, and judging the hardenability was excellent if the fraction of martensite was 60% or more.
• the steel plates were cooled down to 650°C by a 10°C/hour cooling rate, then were furnace cooled down to room temperature to prepare samples for evaluation of characteristics. Note that, the structures of the samples were measured by the above-mentioned method.
• Table 2 shows the results of measurement or evaluation of the Vickers hardness of the produced samples, the ratio of the number of carbides at the ferrite grain boundaries to the number of carbides inside the ferrite grains, the pearlite area ratio, the cold workability, and the hardenability.
• Table 2 Hot rolling conditions Carbide size [ ⁇ m] Ferrite grain size [ ⁇ m] Pearlite area ratio [%] Vickers hardness [HV] No. of grain boundary carbides/ No. of grain carbides Total elongation [%] Elongation anisotropy Martensite fraction [%] Remarks Finish hot rolling temp. [°C] Coiling temp.
• Invention Steels B-1, E-1, F-1, H-1, J-1, K-1, L-1, M-1, N-1, P-1, R-1, T-1, W-1, X-1, Y-1, Z-1, AB-1, and AC-1 all have a ratio of the number of carbides at the ferrite grain boundaries with respect to the number of carbides inside the ferrite grains of over 1, a Vickers hardness of 170HV or less, and excellent cold workability and hardenability.
• Comparative Steel G-1 is high in amount of C and deteriorates in cold workability.
• Comparative Steel O-1 is high in amount of Mo and amount of Cr and is high in stability of carbides, so the carbides do not dissolve at the time of hardening, the amount of formation of austenite is small, and the hardenability is inferior.
• Comparative Steels Q-1 and AD-1 are high in amounts of Si and Al and high in A3 point, so the amount of formation of austenite at the time of hardening is small and the hardenability is inferior.
• Comparative Example U-1 is high in amount of S, has coarse MnS formed in the steel, and is low in cold workability.
• Comparative Example AA-1 is low in amount of Mn and inferior in hardenability.
• Comparative Example I-1 is low in finishing temperature of hot rolling and deteriorates in productivity.
• Comparative Example D-1 is high in finishing temperature of hot rolling and has scale flaws formed at the steel plate surface.
• Comparative Examples C-1 and S-1 are low in coiling temperature of hot rolling, are increased in number of bainite, martensite, and other low temperature transformed structures, become brittle resulting in frequent fracture at the time of pay out of the hot rolled coil, and deteriorates in productivity.
• Comparative Examples A-1 and V -1 are high in coiling temperature of hot rolling and have hot rolled structures formed with large lamellar spacing bulky pearlite and high heat stability needle-shaped coarse carbides. The carbides remain in the steel plate even after two-stage step type annealing and the cold workability deteriorates.
• steel slabs of the chemical compositions shown in Table 1 were heated at 1240°C for 1.8 hours, then used for hot rolling.
• the finish hot rolling was ended at 820°C, then the plates were cooled on the ROT by a 45°C/sec cooling rate down to 520°C and coiled at 510°C to produce plate thickness 3.0 mm hot rolled coils.
• These were annealed by two-stage step type box annealing under the annealing conditions shown in Table 3 to prepare plate thickness 3.0 mm samples.
• Table 3 shows the results of measurement or evaluation of the carbide size, ferrite grain size, Vickers hardness, ratio of the number of carbides at the ferrite grain boundaries to the number of carbides in the ferrite grains, pearlite area ratio, cold workability, and hardenability of the produced samples.
• Table 3 1st stage annealing 2nd stage Carbide size [ ⁇ m] Ferrite grain size [ ⁇ m] Pearlite area ratio [%] Vickers hardness [HV] No. of grain boundary carbides/ No. of grain carbides Total elongation [%] Elongation anisotropy Martensite fraction [%] Remarks Holding temp. [°C] Holding time [hr] Holding temp.
• the Invention Steels B-2, C-2, E-2, F-2, H-2, 1-2, J-2, K-2, M-2, N-2, R-2, S-2, V-2, Z-2, and AC-2 all have a ratio of the number of carbides at the ferrite grain boundaries with respect to the number of carbides inside the ferrite grains of over 1 and a Vickers hardness of 170HV or less and are excellent in cold workability and hardenability.
• Comparative Steel G-1 is high in amount of C and deteriorates in cold workability.
• Comparative Steel O-1 is high in amount of Mo and amount of Cr and deteriorates in cold workability.
• the carbides are high in stability, so at the time of hardening, the carbides will not dissolve, the amount of production of austenite is small, and the hardenability is inferior.
• Comparative Steel Q-1 is high in amount of Si and high in hardness of ferrite, so deteriorates in workability. Further, it is high in the A3 point, so the amount of production of austenite at the time of hardening is small and the hardenability is inferior. Comparative Steel AD-1 is high in amount of Al and high in A3 point, so the amount of production of austenite at the time of hardening is small and the hardenability is inferior. Comparative Steel U-1 is high in amount of S and is formed with coarse MnS in the steel, so deteriorates in cold workability. Comparative Steel AA-1 is low in amount of Mn and inferior in hardenability.
• Comparative Steel T-2 is low in holding temperature at the time of the first stage annealing of the two-stage step type box annealing, is insufficient in coarsening treatment of carbides at the Ac1 temperature or less, and is insufficient in thermal stability of the carbides, so is reduced in carbides remaining at the time of second stage annealing, cannot be suppressed in pearlite transformation in the structure after gradual cooling, and deteriorates in cold workability.
• Comparative Steel A-2 has a high holding temperature at the time of the first stage annealing of the two-stage step type box annealing, is formed with austenite during the annealing, cannot be raised in stability of carbides, is decreased in carbides remaining at the time of second stage annealing, cannot be suppressed in pearlite transformation in the structure after gradual cooling, and deteriorates in cold workability.
• Comparative Steel L-2 is short in holding time at the time of the first stage annealing of the two-stage step type annealing, is insufficient in the coarsening treatment of the carbides at the Ac1 temperature or less, and is insufficient in the thermal stability of the carbides, so is decreased in the carbides remaining at the time of the second stage annealing, cannot suppress pearlite transformation in the structure after gradual cooling, and deteriorates in cold workability.
• Comparative Steel W-2 is long in holding time at the time of the first stage annealing of the two-stage step type annealing and deteriorates in productivity.
• Comparative Steel X-2 is low in holding temperature at the time of the second stage annealing at the time of two-stage step annealing, is small in amount of production of austenite, cannot be increased in number ratio of carbides at the grain boundaries, and deteriorates in cold workability.
• Comparative Steel AB-2 is high in holding temperature at the time of second stage annealing in the two-stage step type box annealing and is promoted in dissolution of the carbides, so decreases the residual carbides, cannot suppress pearlite transformation in the structure after gradual cooling, and deteriorates in cold forgeability.
• Comparative Steel P-2 is low in holding temperature at the time of second stage annealing of the two-stage step type box annealing, has little formation of austenite, cannot be increased in the number ratio of carbides at the ferrite grain boundaries, and deteriorates in cold workability
• Comparative Steel Y-2 is long in holding time at the time of second stage annealing of the two-stage step type box annealing and is promoted in dissolution of carbides, so is decreased in remaining carbides, cannot suppress pearlite transformation in the structure after gradual cooling, and deteriorates in cold forgeability.
• Comparative Steel D-2 is large in cooling rate from the end of the second stage annealing of the two-stage step type box annealing down to 650°C, experiences pearlite transformation at the time of cooling, and deteriorates in cold workability.
• the steel plate of the present invention is steel plate suitable as a material for auto parts, edged tools, and other machine parts produced through stamping, bending, press-forming, and other working processes, so the present invention is high in industrial applicability.

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