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
  • C22C38/18—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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—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
  • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—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
  • C22C38/36—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.7% by weight of carbon
• • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/24—Nitriding
  • C23C8/26—Nitriding of ferrous surfaces
• • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/28—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in one step
  • C23C8/30—Carbo-nitriding
  • C23C8/32—Carbo-nitriding of ferrous surfaces
• • 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
  • C21D2211/002—Bainite
• • 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

Definitions

• This disclosure relates to a steel sheet for nitriding which is used by being subjected to nitriding treatment to improve durability and which is suitable as a material for machine parts, in particular, a steel sheet for nitriding having excellent formability and punchability before nitriding treatment, and a method of producing the same.
• steel as a material is formed into a desired part shape and then often subjected to a surface-hardening treatment before use.
• Typical examples of such a surface-hardening treatment are carburizing treatment and nitriding treatment.
• Carburizing treatment is the most common surface-hardening treatment.
• carbon is diffused and infiltrated (carburized) into the surface portion of steel at a temperature of the A 3 transformation point or more, and then the steel is subjected to quenching. Consequently, under the influence of distortion caused by quenching from the high temperature, a decrease in shape accuracy of parts is unavoidable.
• the toughness of the steel is markedly decreased. Accordingly, after quenching, it is necessary to perform tempering to recover toughness and perform correction of the shape of parts. Therefore, when a carburizing treatment is employed, the number of processes required to manufacture parts increases, resulting in an increase in manufacturing costs, which is disadvantageous.
• nitriding treatment usually, steel is heated to a temperature of about 500° C. to 600° C. that is lower than the A 1 transformation temperature so that nitrogen is diffused and infiltrated (nitrided) into the surface portion of steel, and the surface of steel is hardened without quenching, unlike the carburizing treatment. That is, in nitriding treatment, the treatment temperature is relatively low, and treatment is not accompanied by phase transformation of steel during cooling. Therefore, a decrease in shape accuracy of parts due to transformation strain does not occur, which is advantageous. Furthermore, the volume variation of the surface portion of steel due to nitriding is small, and it is easy to maintain good shape accuracy of parts, which is also advantageous.
• nitriding using ammonia gas since the time required for nitriding is significantly long, the nitriding treatment is not suitable for automotive parts and the like that are supposed to be mass-produced.
• a nitriding treatment referred to as nitrocarburizing in which a nitriding reaction is allowed to proceed rapidly by utilizing a carburizing atmosphere has become common.
• the significantly long treatment time which is the problem in the existing techniques, is in the process of being solved.
• nitrocarburizing treatment by holding an object to be treated in a treatment atmosphere at 550° C. to 600° C. for several hours, nitrogen is diffused and introduced into steel from the surface of steel by an iron carbide generating reaction.
• the surface hardness obtained after the treatment is lower than that in the existing nitriding treatment, the time required for nitriding can be markedly reduced.
• the number of cases in which nitrocarburizing treatment is employed as the surface-hardening treatment, as an alternative to carburizing treatment has been increasing.
• machine parts used for automotive transmissions and the like are generally manufactured by subjecting intermediate products obtained by casting or forging to machining
• thin steel sheets have been used as a material for machine parts
• machine parts have been manufactured by subjecting thin steel sheets to press working or the like to have a desired shape.
• the reason for this is that, by replacing parts manufactured by subjecting intermediate products obtained by casting or forging to machining in the existing techniques with sheet-metal working products of steel sheets, shortening of manufacturing processes and reduction in manufacturing costs can be achieved.
• Japanese Unexamined Patent Application Publication No. 9-25513 and Japanese Unexamined Patent Application Publication No. 9-25543 propose techniques for producing a steel sheet for nitriding in which a steel having a composition including, in weight ratio, 0.01% to less than 0.08% of C, 0.005% to 1.00% of Si, 0.010% to 3.00% of Mn, 0.001% to 0.150% of P, 0.0002% to 0.0100% of N, more than 0.15% to 5.00% of Cr, more than 0.060% to 2.00% of Al, and one or two of Ti and V is hot-rolled, followed by coiling at 500° C.
• Japanese Unexamined Patent Application Publication No. 2005-171331 proposes a technique on a steel sheet for nitrocarburizing in which the steel sheet has a composition including 0.01% to 0.10% by mass of C, 0.1% by mass or less of Si, 0.1% to 1.0% by mass of Mn, 0.05% by mass or less of P, 0.01% by mass or less of S, 0.01% to 0.06% by mass of Al, 0.05% to 0.50% by mass of Cr, 0.01% to 0.30% by mass of V, 0.01% by mass or less of N, and the balance being Fe and incidental impurities.
• the composition includes a large amount of Al as a nitriding accelerating element. Therefore, there is a concern that inner defects and surface defects resulting from Al-containing inclusions may occur, and generation of a large amount of Al-containing slag increases refining costs during smelting.
• a steel sheet for nitriding having a composition and microstructure; the composition including, in percent by mass, 0.02% to 0.08% of C, 0.1% or less of Si, 0.2% to 1.8% of Mn, 0.05% or less of P, 0.02% or less of S, 0.01% to 0.06% of Al, 0.5% to 1.5% of Cr, 0.01% or less of N, and the balance being Fe and incidental impurities, and the microstructure including ferrite as a main phase and pearlite and/or bainite as a secondary phase, wherein the ferrite has an area fraction of 70% or more in the entire microstructure and an average grain diameter of 5 to 25 ⁇ m, and cementite present in the secondary phase has an average length of the major axis of 3.0 ⁇ m or less in a cross section in the rolling direction of the steel sheet.
• composition further includes, in percent by mass, at least one selected from the group consisting of 0.005% to 0.075% of V, 0.005% to 0.025% of Nb, and 0.005% to 0.025% of Ti.
• a method of producing a steel sheet for nitriding including:
• a steel slab having a composition including, in percent by mass, 0.02% to 0.08% of C, 0.1% or less of Si, 0.2% to 1.8% of Mn, 0.05% or less of P, 0.02% or less of S, 0.01% to 0.06% of Al, 0.5% to 1.5% of Cr, 0.01% or less of N, and the balance being Fe and incidental impurities, to 1,050° C. to 1,250° C.;
• cooling the hot rolled steel sheet at a cooling rate of 40° C./s to 80° C./s in the temperature range from the finishing temperature to 750° C. and at a cooling rate of 15° C./s to 35° C./s in the temperature range from 750° C. to a cooling stop temperature of 500° C. to 650° C.; and coiling the cooled steel sheet at a coiling temperature of 500° C. to 650° C.
• composition of the steel slab further includes, in percent by mass, at least one selected from the group consisting of 0.005% to 0.075% of V, 0.005% to 0.025% of Nb, and 0.005% to 0.025% of Ti.
• the steel sheet is very suitable as a material for formed parts to be subjected to nitriding treatment such as automotive transmission parts, thus exhibiting industrially marked effects.
• the steel sheet is not limited to being used for gas nitrocarburizing treatment and salt bath nitrocarburizing treatment, but can also be suitably used as any of various steel sheets for nitriding such as plasma nitriding, gas nitriding, carbonitriding, and nitrosulphurizing.
• the steel sheet has a microstructure including ferrite (which may also be referred to as “polygonal ferrite”), which is a main phase, and a secondary phase.
• the secondary phase includes pearlite and/or bainite.
• the fraction of the ferrite in the entire microstructure is 70% or more, the average grain diameter of the ferrite is 5 to 25 ⁇ m, and the average length of the major axis of cementite present in the secondary phase in a cross section in the rolling direction of the steel sheet is 3.0 ⁇ m or less.
• the steel sheet by using soft ferrite as a main phase, formability of the steel sheet can be secured.
• a material other than ferrite is used as the main phase, it is not possible to impart good formability to the steel sheet.
• the steel sheet has a microstructure including ferrite as the main phase, and the secondary phase described below.
• the secondary phase which is the remainder other than ferrite, includes one or two selected from the group consisting of pearlite and bainite.
• the secondary phase in the steel sheet microstructure has a role in reinforcing the strength of the steel sheet having soft ferrite as the main phase.
• martensite is softened by an increase in temperature during nitriding treatment, resulting in an increased variation in the strength of the steel sheet. Therefore, to stably maintain the strength of the steel sheet even after being subjected to nitriding treatment in which the steel sheet is held at about 500° C. to 600° C., the secondary phase in the steel sheet microstructure is required to be composed of pearlite and/or bainite.
• the area fraction of the ferrite which is the main phase, to be 70% or more to impart good formability to the steel sheet.
• the area fraction of the ferrite is less than 70%, formability of the steel sheet is likely to be at an insufficient level. Furthermore, punchability of the steel sheet decreases. For example, during punching of the steel sheet, the sheared surface ratio of punched surfaces decreases.
• the area fraction of the ferrite is preferably 97% or less, and more preferably 95% or less.
• the average grain diameter of the ferrite When the average grain diameter of the ferrite is more than 25 ⁇ m, the surface properties of the steel sheet may be degraded during forming, and the smoothness of punched surfaces may be degraded, resulting in degradation of punchability of the steel sheet. Furthermore, when the grain diameter of the ferrite becomes coarse, the number of grain boundaries decreases. Consequently, grain boundary diffusion of N during nitriding treatment is suppressed, and there is a concern that the time required for the nitriding treatment may be elongated. On the other hand, when the average grain diameter of the ferrite is less than 5 ⁇ m, the steel sheet hardens, and formability is likely to be degraded. Therefore, the average grain diameter of the ferrite is 5 to 25 ⁇ m, and preferably 5 to 15 ⁇ m.
• the average length of the major axis of cementite present in the secondary phase in a cross section in the rolling direction of the steel sheet exceeds 3.0 ⁇ m, the stress concentration ratio increases at the interface between the cementite and the ferrite during punching of the steel sheet, which makes it easy to cause microcracks, and the fracture surface ratio at punched surfaces increases. Thus, punchability of the steel sheet is degraded. Therefore, the average length of the major axis is 3.0 ⁇ m or less. However, when the cementite becomes extremely small, microcracks are likely to occur in punched surfaces of the steel sheet. Therefore, the average length of the major axis is preferably 1.0 ⁇ m or more.
• % which is the unit of measure for the constituent element content, means “percent by mass” unless otherwise indicated.
• the C is an element having an effect of increasing the strength of steel through solid solution strengthening and formation of the secondary phase.
• the C content is less than 0.02%, it is not possible to secure a sufficient strength of the steel sheet as a material for parts.
• the C content is exceeded 0.08%, the strength of the steel sheet increases excessively, resulting in a decrease in formability.
• the fraction of the secondary phase increases, and it is difficult to obtain cementite having a desired form. Therefore, the C content is 0.02% to 0.08%, and preferably 0.04% to 0.06%.
• Si is an element effective in deoxidizing steel, and also has an effect of strengthening steel by solid solution strengthening.
• the Si content is preferably 0.01% or more to obtain these effects.
• the Si content is 0.1% or less, and preferably 0.05% or less.
• Mn is an element that strengthens steel by solid solution strengthening. Furthermore, Mn has an effect of fixing, as precipitates, S which is present as an impurity in the steel, thus reducing adverse effects caused by S.
• the Mn content is less than 0.2%, the effects cannot be obtained sufficiently, and it is not possible to secure the required strength of the steel sheet.
• the Mn content is more than 1.8%, the strength of the steel sheet increases excessively, and a band-like microstructure due to microsegregation is likely to be formed, resulting in degradation in formability and punchability of the steel sheet. Therefore, the Mn content is 0.2% to 1.8%, and preferably 0.2% to 1.2%.
• the P is an element present as an impurity in the steel and, when the P content is large, formability and toughness of the steel sheet degrade. Therefore, the P content is 0.05% or less, and preferably 0.03% or less.
• S is also an element present as an impurity in the steel and, when the S content is large, formability and toughness of the steel sheet degrade. Therefore, the S content is 0.02% or less, and preferably 0.01% or less.
• Al is an element added for the purpose of deoxidizing steel.
• the Al content in the steel is less than 0.01%, it is not possible to obtain a sufficient deoxidizing effect.
• the Al content in the steel is more than 0.06%, the deoxidizing effect is saturated, and there is a possibility that inner defects and surface defects will increase due to an increase in inclusions in the steel. Therefore, the Al content is 0.01% to 0.06%, and preferably 0.02% to 0.05%.
• Cr has an effect of increasing the hardness of the surface portion of the steel sheet by forming nitrides in the steel by nitriding treatment, and is an important alloy element. Cr also has an effect of refining cementite in the steel. In terms of sufficient exhibition of such effects, it is necessary to set the Cr content at 0.5% or more. However, when the Cr content exceeds 1.5%, the hardened portion of the outermost layer is significantly embrittled by nitriding treatment, and the depth of the hardened portion may be decreased in some cases. Therefore, the Cr content is 0.5% to 1.5%, and preferably 0.5% to 1.0%.
• N is an element present as an impurity in the steel. A large amount of N degrades formability of the steel sheet, and there is a possibility that N will combine with nitriding accelerating elements such as Cr, before nitriding treatment, thus degrading the hardening property due to nitriding. Therefore, the N content is 0.01% or less, and preferably 0.005% or less.
• the steel sheet may contain, in addition to the composition described above, one or two or more selected from the group consisting of 0.005% to 0.075% of V, 0.005% to 0.025% of Nb, and 0.005% to 0.025% of Ti.
• V 0.005% to 0.075%
• V is an element having an effect of increasing the hardness of the surface portion of the steel sheet by forming nitrides in the steel by nitriding treatment. Furthermore, since V is a carbide/nitride forming element, V also has an effect of increasing the strength of steel by particle dispersion strengthening (precipitation strengthening). Accordingly, V can be incorporated for the purpose of controlling the hardening property due to nitriding treatment and adjusting the level of strength of the steel sheet. In terms of sufficient exhibition of such effects, the V content is preferably 0.005% or more.
• the V content is preferably 0.005% to 0.075%, and more preferably 0.025% to 0.075%.
• Nb is a carbide/nitride forming element having an effect of increasing the strength of steel by particle dispersion strengthening (precipitation strengthening).
• precipitation strengthening particle dispersion strengthening
• the Nb content is preferably 0.005% to 0.025%, and more preferably 0.005% to 0.015%.
• Ti is also a carbide/nitride forming element having an effect of increasing the strength of steel by particle dispersion strengthening (precipitation strengthening).
• particle dispersion strengthening particle dispersion strengthening
• the Ti content is preferably 0.005% to 0.025%, and more preferably 0.005% to 0.015%.
• the balance other than the components described above includes Fe and incidental impurities.
• incidental impurities 0.03% or less of Cu, 0.03% or less of Ni, 0.03% or less of Mo, 0.003% or less of Sn, 0.003% or less of Sb, 0.005% or less of 0, and the like are permissible.
• a steel sheet can be obtained by heating, hot rolling, cooling, and coiling steel having the chemical composition described above.
• the steel can be refined by any of known refining processes such as a converter process or an electric furnace process.
• the refined steel is formed into a steel (slab) by continuous casting or ingot casting and bloom rolling or the like. As necessary, preliminary treatments, secondary smelting, cleaning of the steel surface can be performed.
• Heating Temperature of Steel 1,050° C. to 1,250° C.
• the heating temperature of the steel is 1,050° C. to 1,250° C., and preferably 1,100° C. to 1,200° C.
• the steel cooled to normal temperature may be reheated, or the steel being cooled after casting may be subjected to additional heating or heat-retained.
• rough rolling and finish rolling are performed.
• the rough rolling may be performed under ordinary conditions, and rough rolling conditions are not particularly limited.
• the finishing temperature in the hot rolling step is lower than the Ar 3 transformation temperature, an un-recrystallized ferrite microstructure which is elongated in the rolling direction and a pancake-shaped, coarse ferrite microstructure are formed, and it is not possible to obtain ferrite with a desired grain diameter. Furthermore, formability and punchability of the steel sheet are degraded. Moreover, the in-plane anisotropy of mechanical properties of the steel sheet is increased. On the other hand, when the finishing temperature exceeds (the Ar 3 transformation temperature+100° C.), surface properties of the steel sheet are likely to be degraded, the ferrite microstructure is likely to be coarsened, and it is difficult to obtain ferrite with a desired grain diameter.
• the finishing temperature is the Ar 3 transformation temperature to (the Ar 3 transformation temperature+100° C.), and preferably (the Ar 3 transformation temperature+20° C.) to (the Ar 3 transformation temperature+100° C.).
• the steel sheet under rolling may be subjected to additional heating using a heating device such as a sheet bar heater or an edge heater to secure a necessary finishing temperature.
• Cooling Rate from Finishing Temperature to 750° C. 40° C./s to 80° C./s
• the steel sheet which has been subjected to hot rolling is cooled (forced cooled) in the temperature range from the finishing temperature to 750° C. at a cooling rate of 40° C./s to 80° C./s, and preferably 45° C./s to 75° C./s.
• the cooling rate in the temperature range is less than 40° C./s, the microstructure of the hot-rolled steel sheet is likely to be coarsened, and it is not possible to obtain ferrite or cementite having a desired shape.
• Cooling Stop Temperature 500° C. to 650° C.
• cooling In the temperature range from 750° C. to a cooling stop temperature, cooling (forced cooling) is performed at a cooling rate of 15° C./s to 35° C./s, and preferably 15° C./s to 25° C./s.
• the cooling rate in the temperature range is less than 15° C./s, the microstructure of the hot-rolled steel sheet is likely to be coarsened, and it is difficult to obtain ferrite or cementite having a desired shape.
• the cooling rate in the temperature range is more than 35° C./s, progression of ferrite transformation is suppressed and it is not possible to obtain ferrite with a desired fraction.
• the cooling stop temperature is lower than 500° C.
• the steel sheet is hardened because martensite and an excessively large amount of bainite are generated.
• formability of the steel sheet is degraded and the strength of the steel sheet after nitriding treatment becomes unstable.
• the cooling stop temperature is higher than 650° C., pearlite is coarsened, and it is not possible to obtain cementite having a desired shape. Therefore, the cooling stop temperature is 500° C. to 650° C., and preferably 500° C. to 600° C.
• the steel sheet cooled to the cooling stop temperature may be directly coiled, or may be allowed to cool for a short period of time until it is coiled with a coiler.
• the term “being allowed to cool” refers to air cooling in the air in which forced cooling by pouring water is not performed. However, to remove cooling water remaining on the steel sheet, it is permissible to spray high-pressure water or compressed air for a very short period of time onto the steel sheet which is being allowed to cool because a decrease in the temperature of the steel sheet due to this is very small.
• the coiling temperature is lower than 500° C.
• the steel sheet is hardened because martensite and an excessively large amount of bainite are generated.
• formability of the steel sheet is degraded and the strength of the steel sheet after nitriding treatment becomes unstable.
• the coiling temperature is higher than 650° C., pearlite is coarsened, and it is not possible to obtain cementite having a desired shape. Therefore, the coiling temperature is 500° C. to 650° C., and preferably 500° C. to 600° C.
• the coiled steel sheet is used after scale is removed by pickling or shot peening. Furthermore, the steel sheet may be subjected to temper rolling for the purpose of shape straightening and adjusting surface roughness. Performing such descaling or temper rolling does not impair the advantages.
• a specimen of a cross section in the thickness direction parallel to the rolling direction at the 1/4 width position was taken from each of the steel sheets before the nitriding treatment, and the specimen was subjected to mirror polishing and etched with nital. Using an image obtained by photographing the 1 ⁇ 4 thickness position at an appropriate magnification of 500 to 5,000 times with an optical microscope or a scanning electron microscope, the microstructure was confirmed.
• the area ratio of ferrite was obtained by image analysis, which was defined as the fraction of ferrite.
• grain diameters were determined in accordance with the method stipulated in Japan Industrial Standard JIS G 0551-2005, and the average grain diameter was calculated from the grain size number.
• Formability of the steel sheet was evaluated on the basis of ductility determined by a tensile test.
• the tensile test was carried out in accordance with JIS Z 2241-2011 using a No. 5 test specimen stipulated in JIS Z 2241-2011 which was taken at the 1 ⁇ 4 width position of a steel sheet such that the testing direction corresponded to the rolling direction.
• Tensile strength (TS) and elongation after fracture (EL) were measured, and a strength-elongation balance (TS ⁇ EL) was calculated.
• the steel sheet having a strength-elongation balance value of 16 GPa ⁇ % or more was evaluated as having good formability.
• a disk-shaped test specimen with a diameter of 50 mm was punched out from a steel sheet before nitriding treatment (clearance: 5% of steel sheet thickness), the sheared surface ratio at the punched surface of the test specimen was measured, and presence or absence of microcracks in the fracture surface region was confirmed.
• the sheared surface ratio was 60% or more, and no cracks were observed in the fracture surface region, the steel sheet was evaluated as having good punchability.
• a hot-rolled steel sheet after temper rolling was subjected to gas nitrocarburizing treatment, and the cross section hardness of the steel sheet after the gas nitrocarburizing treatment (nitrided layer cross section hardness) was measured.
• Gas obtained by mixing ammonia (NH 3 ) and endothermic converted gas at the equal volume ratio was used as nitriding gas.
• the gas nitrocarburizing treatment temperature was 570° C., and the holding time at the gas nitrocarburizing treatment temperature was 150 minutes. After holding, oil cooling was performed.
• the cross section hardness of the steel sheet a specimen of a cross section in the thickness direction parallel to the rolling direction was taken from the steel sheet after the gas nitrocarburizing treatment, and the Vickers hardness (HV0.1) at a depth of 0.2 mm from the surface of the steel sheet was measured in accordance with JIS Z 2244-2009. When the measured Vickers hardness was 250 or more, the surface hardening property of the steel sheet by nitriding treatment was evaluated to be good.

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