Classifications

• • 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
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  • 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
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  • 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
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  • C21—METALLURGY OF IRON
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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
• • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • 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
  • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—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
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/22—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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • 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
  • C22C38/60—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
  • 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
  • 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

Definitions

• the present invention relates to steel sheet and a method of production of the same.
• Steel sheet containing, by mass%, carbon in an amount of 0.1 to 0.7% is being used as a material for production of gears, clutches, and other drive system parts of automobiles by being used press-formed, enlarged in holes, bent, drawn, thickened, and thinned and cold forged by combinations of the same from a blank.
• the strength of such parts is secured by quenching and tempering, so a high hardenability is demanded from steel sheet.
• PLT 1 discloses, as steel for machine structural use improving toughness by suppressing coarsening of crystal grains in carburization heat treatment, steel for machine structural use containing, by mass%, C: 0.10 to 0.30%, Si: 0.05 to 2.0%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.030% or less, Cr: 1.80 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.02 to 0.10%, and N: 0.0300% or less and having a balance of Fe and unavoidable impurities, having a structure before cold working comprised of ferrite and pearlite structures, and having an average value of ferrite grain size of 15 ⁇ m or more.
• PLT 2 discloses, as steel excellent in cold workability and carburizing and quenching ability, steel containing C: 0.15 to 0.40%, Si: 1.00% or less, Mn: 0.40% or less, sol. Al: 0.02% or less, N: 0.006% or less, and B: 0.005 to 0.050%, having a balance of Fe and unavoidable impurities, and having a structure mainly comprised of ferrite phases and graphite phases.
• PLT 3 discloses a steel material for carburized bevel gear use excellent in impact strength, a high toughness carburized bevel gear, and a method of production of the same.
• PLT 4 discloses, for a part produced by spheroidal annealing, then a cold forging and a carburizing, quenching, and tempering process, steel for carburized part use having excellent workability while suppressing coarsening of crystal grains even with subsequent carburization and having an excellent impact resistance characteristic and impact fatigue resistance characteristic.
• PLT 5 discloses as cold tool steel for plasma carburization use a steel containing C: 0.40 to 0.80%, Si: 0.05 to 1.50%, Mn: 0.05 to 1.50%, and V: 1.8 to 6.0%, further containing one or more of Ni: 0.10 to 2.50%, Cr: 0.1 to 2.0%, and Mo: 3.0% or less, and having a balance of Fe and unavoidable impurities.
• PLT 6 proposes prescribing the carbide grain size and spheroidization rate in steel containing C: 0.25 to 0.75% and improving the in-plane anisotropy by the cold rolling rate and box annealing conditions, the coiling temperature in hot rolling, and provisions on the texture so as to limit the "r" value and ⁇ r.
• PLTs 7 and 8 propose to prescribe the heating and annealing conditions of a hot rolled material between stands of a finish rolling machine so as to reduce the ⁇ r value and improve the in-plane anisotropy.
• PLT 8 proposes steel sheet reduced in in-plane anisotropy by prescribing hot rolling during which performing finish rolling at a temperature of the Ar3 point or more and coiling at 500 to 630°C.
• the present invention was made in consideration of the above situation in the prior art and has as its object the provision of steel sheet improved in hardenability and material formability, in particular, optimal for obtaining a gear or other part by thickening or other cold forging, 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, it becomes possible to increase the grain size so as to raise the material formability.
• Carbides by being made to suitably disperse in the metal structure, can maintain the material formability while imparting an excellent wear resistance and rolling fatigue characteristic, so are structures essential for drive system parts. Further, the carbides in the steel sheet are strong particles obstructing slip. By forming carbides at the ferrite grain boundaries, it is possible to prevent propagation of slip exceeding the crystal grain boundaries and suppress the formation of shear zones. The cold forgeability is improved and, simultaneously, the formability of steel sheet is also improved.
• cementite is a hard, brittle structure. If a laminar structure with ferrite present, that is, in the state of pearlite, the steel becomes hard and brittle, so it has to be present in a spheroidal form. If considering the cold forgeability and the occurrence of fractures at the time of forging, its grain size has to be a suitable range.
• the steel sheet is coiled at a relatively low temperature (400°C to 550°C). By coiling at a relatively low temperature, the cementite dispersed in the ferrite also easily becomes spheroidal.
• the cementite is partially made spheroidal by annealing at a temperature just under the Ac1 point as first stage annealing.
• part of the ferrite grains is left while part is transformed to austenite by annealing at a temperature between the Ac1 point and Ac3 point (so-called dual phase region of ferrite and austenite).
• dual phase region of ferrite and austenite By then making the remaining ferrite grains grow while slowly cooling the steel while using these as nuclei to transform the austenite to ferrite, it is possible to obtain large ferrite phases and make cementite precipitate at the grain boundaries to realize the above structure.
• the inventors found that it is difficult to realize a method of production of steel sheet satisfying both hardenability and formability even if adjusting the heat rolling conditions, annealing conditions, etc. separately and that it is possible to realize this by optimization by a so-called integrated process of hot rolling, annealing, etc.
• the present invention was made based on these findings and has as its gist the following:
• C is an element forming carbides and effective for strengthening the steel and refining the ferrite grains.
• suppression of coarsening of the ferrite grain size is necessary.
• C is less than 0.10%, the carbides become insufficient in volume fraction and coarsening of the carbides during annealing can no longer be suppressed, so C is made 0.10% or more. Preferably it is 0.14% or more.
• the content of C increases, the carbides increase in volume fraction, cracks are formed acting as starting points of breakage at the time of an instantaneous load, and there is the fear that the formability and impact resistance characteristic will fall. If making this drop as small as possible, C is made 0.40% or less. Preferably it is 0.38% or less.
• C is made over 0.40%.
• C is 0.44% or more.
• C is made 0.70% or less.
• it is 0.66% or less.
• Si is an element which acts as a deoxidizing agent and further has an effect on the form of the carbides and contributes to the improvement of the material formability.
• Si is made 0.01% or more. Preferably it is 0.07% or more.
• Si is made 0.30% or less. Preferably it is 0.28% or less.
• Mn is an element controlling the form of carbides in two-stage annealing. If less than 0.30%, in the gradual cooling after second stage annealing, it becomes difficult to form carbides at the ferrite grain boundaries, so Mn is made 0.30% or more. Preferably it is 0.40% or more.
• Mn is over 1.00%, after carburization, quenching, and tempering, the toughness falls, but on the other hand, the strength rises.
• Mn is made 1.00% or less. Preferably it is 0.96% or less.
• Mn When trying to raise the strength, Mn is made over 1.00%. Preferably it is 1.10% or more. If Mn is over 3.00%, after carburization, quenching, and tempering, the toughness remarkably falls, so Mn is made 3.00% or less. Preferably it is 2.70% or less.
• Al is an element which acts as a deoxidizing agent and stabilizes ferrite. If less than 0.001%, the effect of 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 0.09% or less.
• Cr is an element effective for stabilization of carbides at the time of heat treatment. If less than 0.010%, it becomes difficult to cause carbides to remain at the time of carburization, coarsening of the austenite grain size at the surface layer is invited, and the strength drops, so Cr is made 0.010% or more. Preferably it is 0.050% or more.
• Cr is made 0.50% or less.
• it is 0.40% or less.
• Mo like Mn and Cr, is an element effective for control of the form of carbides. If less than 0.001%, the effect of addition is not obtained, so Mo is made 0.001% or more. Preferably it is 0.005% or more.
• Mo concentrates at the carbides, stable carbides increase even in the austenite phases, carbides remain inside the ferrite grains after gradual cooling inviting an increase in the hardness and a decrease in the number of carbides at the ferrite grain boundaries, and the material formability falls, so Mo is made 0.50% or less. Preferably it is 0.40% or less.
• B is an element raising the hardenability and further raising the toughness.
• a predetermined hardenability is required, so 0.0004 to 0.01% is added. If less than 0.0004%, the effect of addition is not obtained, so B is made 0.0004% or more. Preferably it is 0.0010% or more.
• B is made 0.01% or less. Preferably it is 0.007% or less.
• Ti is an element forming nitrides and contributing to refinement of the crystal grains and works to effectively bring out the effect of addition of B. If less than 0.001%, the effect of addition is not obtained, so Ti is made 0.001% or more. Preferably it is 0.010% or more.
• Ti is made 0.10% or less. Preferably it is 0.07% or less.
• the following elements are impurities and have to be controlled to certain amounts or less.
• P is an element segregating at the ferrite grain boundaries and working to suppress the formation of carbides at the ferrite grain boundaries. For this reason, the smaller the amount of P, the better.
• the content of P 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 0.0013%.
• P is over 0.02%, formation of carbides at the ferrite grain boundaries is suppressed, the number of carbides decreases, and the material formability falls, so P is made 0.02% or less. Preferably it is 0.01% or less.
• S is an impurity element forming MnS and other nonmetallic inclusions.
• the nonmetallic inclusions form starting points of fracture at the time of cold forging, so the smaller the S, the better.
• the content of S may also be 0, but to lower S to less than 0.0001%, the refining costs greatly increase, so the substantive lower limit is 0.0001 to 0.0012%.
• S is over 0.01%, nonmetallic inclusions are formed and the material formability falls, so S is made 0.01% or less. Preferably it is 0.009% or less.
• N is an element which, if present in a large amount, causes embrittlement of the ferrite. For this reason, the smaller the amount of N, the better.
• the content of N may also be 0, but to lower N to less than 0.0001%, the refining costs greatly increase, so the substantive lower limit is 0.0001 to 0.0006%.
• N is over 0.02%, the ferrite becomes brittle and the material formability falls, so N is made 0.02% or less. Preferably it is 0.017% or less.
• the steel sheet of the present invention contains C: 0.10 to 0.40% and Mn: 0.30 to 1.00%, embrittlement of the ferrite is suppressed, so N is made 0.01% or less. Preferably it is 0.007% or less.
• O is an element which, if present in a large amount, promotes the formation of coarse oxides. For this reason, the smaller the amount of O, the better, but to lower O to less than 0.0001%, the refining costs greatly increase, so the amount is made 0.0001% or more. Preferably it is 0.0011% or more.
• Sn is an element which unavoidably enters from the steel starting materials. For this reason, the smaller the amount of Sn, the better.
• the content of S may also be 0, but to lower S to less than 0.001%, the refining costs greatly increase, so the substantive lower limit is 0.001 to 0.002%.
• Sn is made 0.05% or less.
• Sn is 0.04% or less.
• Sb like Sn, is an element which unavoidably enters from the steel starting materials, segregates at the ferrite grain boundaries, and reduces the number of carbides at the ferrite grain boundaries. For this reason, the smaller the amount of Sb, the better.
• the content of Sb may also be 0, but to lower Sb to less than 0.001%, the refining costs greatly increase, so the substantive lower limit is 0.001 to 0.002%.
• Sb segregates at the ferrite grain boundaries, the number of carbides at the ferrite grain boundaries decreases, and the material formability falls, so Sb is made 0.050% or less. Preferably it is 0.04% or less.
• As is an element which unavoidably enters from the steel starting materials and segregates at the ferrite grain boundaries. For this reason, the smaller the amount of As, the better.
• the content of As may also be 0, but to lower As to less than 0.001%, the refining costs greatly increase, so the substantive lower limit is 0.001 to 0.002%.
• the steel sheet of the present invention has the above elements as basic elements, but may further contain the following elements for the purpose of improving the cold forgeability of the steel sheet.
• the following elements are not essential for obtaining the effects of the present invention, so the contents may also be 0.
• Nb is an element effective for control of the form of the carbides. Further, it is an element refining the structure and contributing to improvement of the toughness. To obtain this effect of addition, Nb preferably is made 0.001% or more. More preferably it is 0.002% or more.
• Nb is made 0.10% or less. Preferably it is 0.09% or less.
• V 0.10% or less
• V is an element effective for control of the form of the carbides. Further, it is an element refining the structure and contributing to improvement of the toughness. To obtain this effect of addition, V preferably is made 0.01% or more. More preferably it is 0.004% or more.
• V is made 0.10% or less.
• it is 0.09% or less.
• Cu is an element segregating at the ferrite grain boundaries. Further, it is an element forming fine precipitates and contributing to the improvement of strength. To obtain the effect of improvement of strength, Cu preferably is made 0.001% or more. More preferably it is 0.008% or more.
• Cu is made 0.10% or less. Preferably it is 0.09% or less.
• W 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.
• Ta 0.001 to 0.10%
• Ta is an element effective for control of the form of carbides.
• Ta preferably is made 0.001% or more. More preferably it is 0.007% or more.
• T is made 0.100% or less. Preferably it is 0.09% or less.
• Ni is an element effective for improvement of the impact resistance characteristic of the formed part. To obtain this effect of addition, Ni preferably is made 0.001% or more. More preferably it is 0.002% or more.
• Ni is made 0.10% or less. Preferably it is 0.09% or less.
• Mg is an element which can control the form of sulfides by addition in a trace amount. To obtain this effect of addition, Mg preferably is made 0.0001% or more. More preferably it is 0.0008% or more.
• Mg is made 0.05% or less.
• Mg is 0.04% or less.
• Ca is an element which can control the form of sulfides by addition in a trace amount.
• Ca preferably is made 0.001% or more. More preferably it is 0.003% or more.
• Ca is made 0.05% or less.
• it is 0.04% or less.
• Y like Mg and Ca, is an element which can control the form of sulfides by addition in a trace amount. To obtain this effect of addition, Y preferably is made 0.001% or more. More preferably it is 0.003% or more.
• Y is made 0.05% or less.
• Zr like Mg, Ca, and Y, is an element which can control the form of sulfides by addition in a trace amount.
• Zr preferably is made 0.001% or more. More preferably it is 0.004% or more.
• Zr is made 0.05% or less.
• it is 0.04% or less.
• La is an element able to control the form of the sulfides by addition in a trace amount, but is an element which segregates at the grain boundaries and reduces the number of carbides at the ferrite grain boundaries.
• La preferably is made 0.001% or more. More preferably it is 0.003% or more.
• La is made 0.05% or less.
• it is 0.04% or less.
• Ce is an element able to control the form of the sulfides by addition in a trace amount, but is an element which segregates at the grain boundaries and reduces the number of carbides at the ferrite grain boundaries.
• Ce preferably is made 0.001% or more. More preferably it is 0.003% or more.
• Ce segregates at the ferrite grain boundaries, the number of carbides at the ferrite grain boundaries decreases, and the material formability falls, so Ce is made 0.050% or less. Preferably it is 0.04% or less.
• the balance of the chemical composition is Fe and unavoidable impurities.
• the structure of the steel sheet of the present invention is a structure substantially comprised of ferrite and carbides.
• Carbides are compounds of iron and carbon of cementite (Fe 3 C) plus compounds of cementite in which Fe atoms are substituted by Mn, Cr, and other alloy elements and alloy carbides (M 23 C 6 , M 6 C, MC, etc. [M: Fe and other metal elements added as alloys]).
• a shear zone is formed at the macrostructure of the steel sheet and slip deformation occurs concentrated near the shear zone.
• slip deformation along with proliferation of dislocations, a region of a high dislocation density is formed near the shear zone.
• slip deformation is promoted and the dislocation density increases.
• a shear zone can be understood as the phenomenon of slip occurring at a certain one grain crossing the crystal grain boundary and being continuously propagated to the adjoining grain. Accordingly, to suppress the formation of a shear zone, it is necessary to prevent propagation of slip crossing crystal grain boundaries.
• the carbides in steel sheet are strong particles inhibiting slip.
• carbides at the ferrite grain boundaries it becomes possible to prevent the propagation of slip crossing crystal grain boundaries and suppress the formation of a shear zone and improve the cold forgeability.
• the steel sheet is also improved in formability.
• the formability of steel sheet is largely due to the accumulation of strain inside the crystal grains (accumulation of dislocations). If propagation of strain to the adjoining crystal grains is blocked at the crystal grain boundaries, the amount of strain inside the crystal grains increases. As a result, the work hardening rate increases and the formability is improved.
• the average grain size of carbides is made 0.4 ⁇ m to 2.0 ⁇ m. If the average grain size of the carbides is less than 0.4 ⁇ m, the steel sheet remarkably increases in hardness and falls in cold forgeability. More preferably it is 0.6 ⁇ m or more.
• the average particle size of the carbides exceeds 2.0 ⁇ m, at the time of cold forming, the carbides form starting points of cracks. More preferably, it is 1.95 ⁇ m or less.
• cementite a carbide of iron
• cementite is a hard and brittle structure. If present in the form of pearlite, which is a layered structure with ferrite, the steel becomes hard and brittle. Therefore, pearlite has to be reduced as much as possible.
• the area ratio is made 6% or less.
• Pearlite has a unique lamellar structure, so can be discerned by observation by an SEM or optical microscope. By calculating the region of the lamellar structure at any cross-section, the area ratio of the pearlite can be found.
• cold forgeability is considered to be strongly affected by the rate of coverage of the ferrite grain boundaries by carbides.
• High precision measurement is sought, but measurement of the rate of coverage of ferrite grain boundaries by carbides in a three-dimensional space requires serial sectioning SEM observation using an FIB to repeatedly cut a sample for observation in a scanning electron microscope or 3D EBSP observation. A massive measurement time is required and technical knowhow has to be built up.
• Buckling, folding, and twisting of the steel sheet occurring at the time of cold forging occur due to localization of strain accompanying the formation of a shear zone, so by forming carbides at the ferrite grain boundaries, the formation of a shear zone and localization of strain are reduced and buckling, folding, and twisting are suppressed.
• 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. The magnification of the observation was made 3000X and images of eight fields of 30 ⁇ m ⁇ 40 ⁇ m at a sheet thickness 1/4 layer were captured at random.
• the obtained structural images were analyzed by image analyzing software (Win ROOF made by Mitani Shoji) to measure in detail the areas of the carbides contained in the analyzed regions.
• the number of carbides present at the ferrite grain boundaries are counted, the number of carbides at the grain boundaries are subtracted from the total number of carbides, and the number of carbides in the ferrite grains are found. Based on the measured and calculated number of carbides, the ratio B/A of the number B of carbides at the ferrite grain boundaries with respect to the number A of carbides inside the ferrite grains is calculated.
• the ferrite grain size is preferably 3 ⁇ m to 50 ⁇ m from the viewpoint of improvement of the cold forgeability. If the ferrite grain size is less than 3 ⁇ m, the hardness increases and fractures and cracks easily form at the time of cold forging, so the ferrite grain size is preferably 3 ⁇ m or more. More preferably it is 5 ⁇ m or more.
• the ferrite grain size is over 50.0 ⁇ m, the number of carbides on the crystal grain boundaries suppressing the propagation of slip is decreased and the cold forgeability falls, so the ferrite grain size is preferably 50 ⁇ m or less. More preferably it is 40 ⁇ m or less.
• the ferrite grain size is measured by using the above-mentioned procedure to polish the observed surface of the sample surface to a mirror finish, then etching it by a 3% nitric acid-alcohol solution and observing the structure by an optical microscope or scanning electron microscope and applying the line segment method to the captured image.
• the texture is evaluated by X-ray diffraction at a plane parallel to the sheet surface at a 1/2 sheet thickness part of the hot rolled steel sheet.
• X-rays from a Mo tube are used.
• the diffraction intensities at the diffraction orientations ⁇ 110 ⁇ , ⁇ 220 ⁇ , ⁇ 211 ⁇ , and ⁇ 310 ⁇ due to reflection are obtained and based on these an ODF is prepared.
• the diffraction intensity data of random orientations of iron is used. From this, the X-ray diffraction intensity of ⁇ 211 ⁇ 011> is found as I1 and the X-ray diffraction intensity of ⁇ 100 ⁇ 011> is found as I0. If this I1/I0 is less than 1, it means that the recrystallization necessary for a random texture appears at the time of hot rolling. If the random texture can be obtained, the plastic anisotropy is reduced and the formability is improved.
• the Vickers hardness of the steel sheet 100 HV to 150 HV (when C: 0.10 to 0.40% and Mn: 0.01 to 0.30%) or by making it 100 HV to 170 HV, it is possible to improve the formability at the time of cold forging. If the Vickers hardness is less than 100 HV, buckling easily occurs during the forming at the time of cold forging and the shaped part falls in precision, so the Vickers hardness is made 100 HV or more. Preferably, it is 110 HV or more.
• the Vickers hardness is made 170 HV or less.
• the Vickers hardness is preferably made 150 HV or less. More preferably, it is 140HV 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 and integrally manage the hot rolling conditions and annealing conditions to control the structure of the steel sheet.
• a steel slab obtained by continuously casting molten steel of the required chemical composition is used for hot rolling.
• the continuously cast slab may be directly used for hot rolling or may be used for hot rolling after cooling once, then heating.
• the heating temperature is preferably 1000°C to 1250°C and the heating time is preferably 0.5 hour to 3 hours. If directly using the continuously cast steel slab for hot rolling, the temperature of the steel slab used for the hot rolling is preferably made 1000°C to 1250°C.
• the temperature of the steel slab or the heating temperature of the steel slab is over 1250°C or the heating time of the steel slab is over 3 hours, decarburization from the surface layer of the steel slab becomes remarkable, at the time of heating before carburization and quenching, the austenite grains at the surface layer of the steel sheet abnormally grow, and the impact resistance falls.
• the temperature of the steel slab or the heating temperature of the steel slab is preferably 1250°C or less and the heating time is preferably 3 hours or less. More preferably, they are 1200°C or less and 2.5 hours or less.
• the temperature of the steel slab or the heating temperature of the steel slab is less than 1000°C or the heating time is less than 0.5 hour, the microsegregation and macrosegregation occurring in casting cannot be eliminated, regions remain inside the steel slab where Si, Mn, and other alloy elements locally concentrate, and the impact resistance falls.
• the temperature of the steel slab or the heating temperature of the steel slab is preferably 1000°C or more and the heating time is preferably 0.5 hour or more. More preferably they are 1050°C or more and 1 hour or more.
• the finish rolling in the hot rolling is completed at 820°C or more, preferably at 900°C to 950°C in temperature region. If the finish rolling temperature is less than 820°C, the steel sheet increases in deformation resistance, the rolling load remarkably rises, and, further, the amount of roll wear increases and the productivity falls. Along with this, the recrystallization required for improving the plastic anisotropy does not sufficiently proceed, so the finish rolling temperature is made 820°C or more. From the viewpoint of promoting recrystallization, it is preferably 900°C or more.
• the finish rolling temperature is made 950°C or less. Preferably it is 920°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 during the cooling. It is not possible to suppress the formation of flaws due to this and the impact resistance falls, so the cooling rate is preferably 10°C/sec or more. More preferably it is 15°C/sec or more.
• the outermost layer part is excessively heated and bainite, martensite, and other low temperature transformed structures are formed.
• microcracks form in the low temperature transformed structures. The microcracks are difficult to remove by pickling and cold rolling.
• 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 a 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 sheet 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 sheet 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 and in particular is a condition not generally used when the content of C is high.
• the structure of the steel sheet can be made a bainite structure comprised of fine ferrite in which carbides are dispersed.
• the austenite which was not transformed before coiling, transforms to hard martensite.
• the time of paying out the hot rolled steel sheet coil cracks form at the surface layer of the hot rolled steel sheet and the impact resistance falls.
• the coiling temperature is made 400°C or more. Preferably it is 430°C or more.
• the coiling temperature is made 550°C or less. Preferably it is 520°C or less.
• the hot rolled steel sheet coil is paid out and pickled, then is held in two temperature regions for two-stage step type of annealing (two-stage annealing).
• two-stage annealing By treating the hot rolled steel sheet by two-stage annealing, the stability of the carbides is controlled to promote the formation of carbides at the ferrite grain boundaries.
• the ferrite grains are refined, so the steel sheet becomes harder to soften. For this reason, in the present invention, it is not preferable to cold roll the steel before annealing. It is preferable to perform the annealing treatment without cold rolling after the pickling.
• the first stage of annealing is performed at 650 to 720°C, preferably the A c1 point or less in temperature region. Due to this annealing, the carbides are coarsened and partially spheroidized and the alloy elements are made to concentrate at the carbides to thereby 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 made 30°C/hour to 150°C/hour. If the first stage heating rate is less than 30°C/hour, raising the temperature takes time and the productivity falls, 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, the temperature difference between the outer circumferential part and the inside part of the hot rolled steel sheet coil increases, scratches and seizing occur due to the difference in heat expansion, and relief shapes are formed at the steel sheet surface.
• 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 of annealing (below, referred to as "the first stage annealing temperature”) is made 650°C to 720°C. If the first stage annealing temperature is less than 650°C, the carbides become insufficient in stability and it becomes difficult to form carbides remaining in the austenite in the second stage of annealing. Therefore, the first stage 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 of annealing (below, referred to as the "first stage annealing time") is made 3 hours to 60 hours. If the first stage annealing time is less than 3 hours, the carbides become insufficient in stability and it becomes difficult to form carbides remaining in the second stage of annealing. Therefore, the first stage annealing time is made 3 hours or more. Preferably it is 5 hours or more.
• the first stage 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 A c1 point to the A 3 point in temperature region, to form austenite in the structure.
• the carbides in the fine ferrite grains dissolve in the austenite, but the carbides coarsened due to the first stage of annealing remain in the austenite.
• the ferrite grain size does not become larger and the ideal structure cannot be obtained.
• the heating rate up to the annealing temperature in the second stage of annealing (below, referred to as the "second stage heating rate") is made 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 it becomes possible to suppress the formation of nuclei of austenite raise the rate of coverage of the grain boundaries by carbides in the structure formed by gradual cooling after annealing.
• the second stage heating rate preferably is slow, but if less than 1°C/hour, raising the temperature takes time and the productivity falls, 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, the temperature difference between the outer circumferential part and the inside part of the hot rolled steel sheet coil increases, scratches and seizing occur due to the large difference in heat expansion due to the transformation, and relief shapes are formed at the steel sheet surface.
• the second stage heating rate is made 80°C/hour or less. Preferably, it is 70°C/hour or less.
• the annealing temperature in the second stage of annealing (below, referred to as "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 austenite formed becomes smaller, the number of carbides at the ferrite grain boundaries decreases after cooling after the second stage of annealing, and, further, the ferrite grain size becomes smaller. Therefore, 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 in the austenite and it becomes difficult to control the changes in structure, 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 of annealing (“second stage annealing time”) is made 3 hours to 10 hours. If the second stage annealing time is less than 3 hours, the amount of formation of austenite becomes small, the carbides inside the ferrite grains do not sufficiently dissolve, it becomes difficult to make the number of carbides at the ferrite grain boundaries increase, and, further, the ferrite grain size becomes small. Therefore, the second stage annealing time is made 3 hours or more. Preferably it is 5 hours or more.
• the second stage annealing time exceeds 10 hours, it becomes difficult to make carbides remain in the austenite. Further, the manufacturing costs also increase. Therefore, the second stage annealing time is made less than 10 hours. Preferably it is 8 hours or less.
• the steel sheet After the two-stage annealing, the steel sheet is cooled by a 1°C/ hour to 100°C/hour cooling rate down to 650°C.
• the cooling rate is preferably slow, but if less than 1°C/hour, the time required for cooling increases and the productivity falls, so the cooling rate is made 1°C/hour or more. Preferably it is 10°C/hour or more.
• the cooling rate is made 100°C/hour or less. Preferably it is 80°C/hour or less.
• the steel sheet is cooled down to room temperature.
• the cooling rate at this time is not limited.
• the atmosphere in the two-stage annealing is not particularly limited.
• it may be any of a 95% or more nitrogen atmosphere, 95% or more hydrogen atmosphere, or air atmosphere.
• a continuously cast slab (steel slab) of each of the chemical compositions shown in Table 1 and Table 2 (continuation of Table 1) was heated at 1240°C for 1.8 hours, then hot rolled, cooled down to 530°C by a 45°C/sec cooling rate on the ROT after finish hot rolling at 920°C, and coiled at 520°C to produce sheet thickness 5.2 mm hot rolled steel sheet coil.
• the hot rolled steel sheet coil was paid out and pickled, then was loaded into a box type annealing furnace.
• the annealing atmosphere was controlled to 95% hydrogen -5% nitrogen, then the coil was heated from room temperature to 705°C by a 100°C/hour heating rate and was held at 710°C for 24 hours to obtain a uniform temperature distribution inside the hot rolled steel sheet coil.
• the coil was heated by a 5°C/hour heating rate to 740°C, was further held at 740°C for 5 hours, then was cooled down to 650°C by a 10°C/hour cooling rate, then was furnace cooled down to room temperature to prepare a sample for evaluation of performance.
• the structure of the sample was observed by the method explained above and the ferrite grain size and number of carbides were measured.
• Table 3 shows the ferrite grain size ( ⁇ m), average carbide grain size ( ⁇ m), pearlite area ratio (%), Vickers hardness (HV), number of grain boundary carbides/number of grain carbides, X-ray intensity ratio I1/I0, "r” value anisotropy index
• the critical cooling rate was found by preparing a CCT graph. If cooling hot rolled steel sheet by a cooling rate slower than the found critical cooling rate, the hardenability at the time of hardening after forming a part becomes poorer and pearlite structures are formed so sufficient strength cannot be obtained. For this reason, the critical cooling rate must be small in order to obtain a high hardening strength. If the critical cooling rate is 280°C/sec, it can be judged that the hardenability is improved.
• the average carbide grain size is 0.4 to 2.0 ⁇ m
• the pearlite area ratio is 6% or less
• the number of grain boundary carbides/number of grain carbides is over 1
• the I1/I0 is less than 1
• the Vickers hardness is 100HV to 170HV in range and
• the Vickers hardness is over 150, while the number of grain boundary carbides/number of grain carbides becomes less than 1.
• the critical cooling rate is over 280°C/sec and the hardenability falls.
• Table 4 shows the manufacturing conditions, while Table 5 shows the ferrite grain size ( ⁇ m), Vickers hardness (HV), number of grain boundary carbides/number of grain carbides, X-ray intensity ratio I1/I0, "r" value anisotropy index
• Table 4 No Hot rolling conditions Annealing conditions Finish rolling temp. (°C) Coiling temp.
• the present invention it is possible to provide steel sheet excellent in hardenability and formability as a material and a method of production of the same.
• the steel sheet of the present invention is suitable for forming a part by cold forging such as thickening to obtain a gear or other part. Accordingly, the present invention has high applicability in the manufacture of steel sheet and industries utilizing it.

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