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
  • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
• • 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
  • C21D1/62—Quenching devices
  • C21D1/667—Quenching devices for spray quenching
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • 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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/34—Methods of heating
  • C21D1/42—Induction heating
• • 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
  • C21D2211/003—Cementite
• • 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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/25—Process efficiency

Definitions

• the present invention relates to a high carbon steel sheet, in particular a mild high carbon steel sheet containing 0.20 to 0.50 percent by mass of C, and a method for manufacturing the same.
• high carbon steel sheets are formed into various shapes and, thereafter, are subjected to a heat treatment for enhancing hardness, so as to be used for machine structural parts and the like.
• a high carbon steel sheet having a C content of 0.2 to 0.5 percent by mass can have enhanced mildness by being subjected to spheroidizing of cementite and is suitable for a raw material of sheet metal forming.
• Patent Literature 1 discloses a method for enhancing the mildness of a medium or high carbon steel sheet, wherein a hot rolled steel sheet of hypo-eutectoid steel containing C: 0.1 to 0.8 percent by mass is subjected to light reduction cold rolling at reduction ratio of more than 15% and 30% or less and, subsequently, is subjected to three-step annealing.
• Patent Literature 2 proposes a mild high carbon steel strip having a small heat treatment strain characterized by having a steel composition satisfying C: 0.10% to 0.80%, Si: 0.005% to 0.30%, Mn: 0.20% to 1.60%, sol. Al: 0.005% to 0.100%, N: 0.0010% to 0.0100%, Ti: 0.001% to 0.050%, on a percent by mass basis, and predetermined conditional expressions, wherein an average ferrite grain size and a shape in the steel satisfy predetermined conditional expressions.
• Patent Literature 3 proposes a very mild high carbon hot rolled steel sheet characterized by containing C: 0.2% to 0.7%, Si: 0.01% to 1.0%, Mn: 0.1% to 1.0%, P: 0.03% or less, S: 0.035% or less, Al: 0.08% or less, N: 0.01% or less, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, and having a microstructure in which the ferrite average grain size is 20 ⁇ m or more, the volume ratio of ferrite grains having grain sizes of 10 ⁇ m or more is 80% or more, and the average grain size of carbides (cementite) is 0.10 ⁇ m or more and less than 2.0 ⁇ m.
• Patent Literatures 1 to 3 have problems as described below.
• the method for enhancing the mildness of a medium or high carbon steel sheet described in Patent Literature 1 is to enhance the mildness by subjecting a hot rolled steel sheet to light reduction cold rolling at a reduction ratio of more than 15% and 30% or less and, then, annealing, so as to facilitate recrystallization during annealing because of forming strain.
• a relatively large amount of strain is introduced into the surface layer portion of the steel sheet, which is a material to be rolled, by the light reduction cold rolling at a small reduction ratio, only a small amount of strain is introduced into the sheet thickness central portion, so that the distribution of the introduced rolling strain becomes nonuniform in the sheet thickness direction.
• the steel sheet after annealing also has nonuniform microstructure and characteristic in the sheet thickness direction easily.
• the annealing after the cold rolling is composed of three steps including the combination of heating in specific temperature ranges just below and just above the Ac 1 transformation point, so that the temperature control during annealing is complicated and, from this point of view as well, nonuniformity occurs in the steel sheet characteristics easily.
• Patent Literature 2 the technology described in Patent Literature 2 is to obtain a steel sheet in which the average ferrite grain size is more than 35 ⁇ m and less than 100 ⁇ m and ferrite grains have predetermined expanded shapes by hot-rolling and coiling steel having a predetermined composition under a predetermined condition, and performing box annealing under the condition of the hydrogen concentration in the atmosphere of 90% or more, where cold rolling at a reduction ratio of 5% to 30% may be further combined. Therefore, so-called hydrogen annealing is necessary and practical annealing equipment is limited.
• the distribution of the introduced rolling strain becomes nonuniform in the sheet thickness direction and, thereby, the steel sheet after annealing has nonuniform microstructure and characteristic in the sheet thickness direction easily.
• Patent Literature 3 The technology described in Patent Literature 3 is to obtain a steel sheet having a microstructure including ferrite and carbides having predetermined grain sizes by roughly rolling the steel having a predetermined microstructure, performing finish rolling in which the reduction ratio of final pass is specified to be 10% or more and the finishing temperature is specified to be higher than or equal to (Ar 3 ⁇ 20° C.), performing cooling and coiling under a predetermined condition, and performing box annealing after pickling.
• this technology intends to enhance the mildness of the steel sheet by limiting the final pass condition of the hot rolling without employing light reduction cold rolling and, thereby, enhancing a grain growth driving force during spheroidizing.
• cooling to a temperature of 600° C.
• the present invention provides a mild high carbon steel sheet containing 0.20 to 0.50 percent by mass of C, being homogeneous in the sheet thickness direction, and having excellent formability and a method for manufacturing the same.
• the present inventors performed intensive research on the above-described high carbon steel sheet and, as a result, found the following.
• cooling is performed with two-step cooling pattern, in which gradual cooling is performed in a high temperature zone and then strong cooling is performed for a short time and, subsequently, annealing is performed after application of the cold rolling at a small reduction ratio.
• the present invention has been made on the basis of the above-described findings and provides a mild high carbon steel sheet characterized by having a chemical composition containing C: 0.20% to 0.50%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.02% or less, sol.Al: 0.08% or less, N: 0.02% or less, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, and a microstructure composed of ferrite and cementite, wherein each of the average grain size ds of the ferrite in the region from the surface of the steel sheet to the position at one-quarter of the sheet thickness and the average grain size dc of the ferrite in the region from the position at one-quarter of the sheet thickness of the steel sheet to the sheet thickness center is 20 to 40 ⁇ m 0.80 ⁇ ds/dc ⁇ 1.20 is satisfied, the average grain size of the above-described cementite is 1.0 ⁇ m or more, the spheroid
• the high carbon steel sheet according to the present invention can further contain at least one selected from the group consisting of Cr: 0.1% to 1.5%, Mo: 0.1% to 0.5%, Ni: 0.1% to 1.0%, Ti: 0.01% to 0.03%, Nb: 0.01% to 0.03%, V: 0.01% to 0.03%, and B: 0.0005% to 0.0050%, on a percent by mass basis, in addition to the above-described chemical composition.
• the above-described Si content is 0.1% or more and 0.5% or less on a percent by mass basis.
• the above-described Mn content is 0.2% or more and 1.0% or less on a percent by mass basis.
• the above-described cementite has an average grain size of 1.0 ⁇ m or more and 3.0 ⁇ m or less.
• the spheroidizing ratio of the above-described cementite is a ratio of the number of cementite grains having a cross-sectional aspect ratio, which is major axis/minor axis, of 3 or less to the number of all cementite grains.
• the mild high carbon steel sheet has a Vickers hardness of 150 or less.
• the high carbon steel sheet according to the present invention can be produced by hot-rolling a steel slab having the above-described chemical composition at a finishing temperature higher than or equal to the Ar 3 transformation point, performing primary cooling to a primary cooling stop temperature of 550° C. to 650° C. at an average cooling rate of 25° C./sec to 50° C./sec, then performing secondary cooling to a secondary cooling stop temperature of 500° C. to 600° C. at an average cooling rate of 120° C./sec or more for a cooling time of 1 s or less, performing coiling, removing scale on the steel sheet surface layer, then performing cold rolling at a reduction ratio of 20% to 30%, and performing annealing by holding for 20 hours or more at an annealing temperature of 680° C. or higher and lower than the Ac 1 transformation point.
• the average cooling rate of the above-described secondary cooling is 120° C./sec or more and 300° C./sec or less.
• the holding time of the above-described annealing temperature is 30 hours or more and 50 hours or less.
• the unit % of the content of component element refers to percent by mass.
• Carbon is an element indispensable for enhancing the strength after hardening. If the amount of C is less than 0.20%, the strength required of a machine structural part is not obtained. On the other hand, if the amount of C is more than 0.50%, the steel sheet has excessively high strength after the annealing, the formability is degraded and, in addition, embrittlement and a dimensional error of a part after hardening are caused. Therefore, the content of C is limited to 0.20% to 0.50%, and preferably 0.25% to 0.45%.
• Silicon has a function of deoxidizing steel and a function of enhancing the resistance to temper softening after hardening.
• the Si content is preferably 0.1% or more.
• the strength of the steel sheet is excessively enhanced and the surface quality of the steel sheet is degraded, so that the Si content is limited to 1.0% or less, preferably 0.5% or less, and further preferably 0.3% or less.
• Manganese has a function of enhancing hardenability and a function of enhancing the resistance to temper softening after hardening.
• the content is specified to be preferably 0.2% or more, and the content is specified to be further preferably 0.3% or more.
• the content of Mn is limited to 2.0% or less, preferably 1.0% or less, and further preferably 0.8% or less.
• Phosphorus degrades the formability of the steel sheet and the toughness after the heat treatment. Therefore, the content of P is limited to 0.03% or less, and preferably 0.02% or less.
• the content of S is limited to 0.02% or less, and preferably 0.01% or less.
• Aluminum is an element added for deoxidizing steel and can be contained as necessary.
• the steel sheet is held at a high temperature, AlN is formed in the steel, growth of austenite crystal grains during austenitization is suppressed, and the hardenability may be degraded.
• the above-described influence becomes remarkable easily by N which enters the steel from the atmosphere.
• the amount of sol.Al be 0.08% or less.
• the amount of sol.Al is preferably less than 0.04%, and further preferably the amount of sol.Al is specified to be less than 0.01%.
• the content of N is limited to 0.02% or less, and preferably 0.01% or less.
• the remainder is specified to be Fe and incidental impurities.
• at least one selected from the group consisting of Cr: 0.1% to 1.5%, Mo: 0.1% to 0.5%, Ni: 0.1% to 1.0%, Ti: 0.01% to 0.03%, Nb: 0.01% to 0.03%, V: 0.01% to 0.03%, and B: 0.0005% to 0.0050% can be further contained.
• the contents are less than the lower limits of the individual elements, the effects thereof are not obtained sufficiently. Meanwhile, if the contents are more than the upper limits, an increase in the production cost is caused and, in addition, the formability and/or the toughness may be degraded.
• the high carbon steel sheet according to an embodiment of the present invention has a microstructure composed of ferrite and spheroidized cementite in order to improve the formability because of enhanced mildness.
• Coarsening of ferrite grains contributes to enhance mildness significantly.
• the average grain size of the ferrite is less than 20 ⁇ m, enhancement of mildness is insufficient.
• the average grain size of the ferrite is more than 40 ⁇ m, a defect in surface quality, which is so-called orange peel, occurs easily during forming. Therefore, each of the average grain size ds of the ferrite in the region from the surface of the steel sheet to the position at one-quarter of the sheet thickness and the average grain size dc of the ferrite in the region from the position at one-quarter of the sheet thickness of the steel sheet to the sheet thickness center is specified to be 20 to 40 ⁇ m.
• Average grain size distribution of ferrite in sheet thickness direction the average grain size ds of the ferrite in the region from the surface of the steel sheet to the position at one-quarter of the sheet thickness and the average grain size dc of the ferrite in the region from the position at one-quarter of the sheet thickness of the steel sheet to the sheet thickness center satisfies 0.80 ⁇ ds/dc ⁇ 1.20
• Average Grain Size of Cementite 1.0 ⁇ m or More
• the average grain size of the cementite is less than 1.0 ⁇ m, enhancement of mildness becomes insufficient. However, if cementite becomes too coarse, the degree of concentration of stress around coarse cementite grains increases during forming, degradation in formability may be caused and, in addition, re-formation of solid solution of C along with decomposition of cementite during austenitization is suppressed. Therefore, the average grain size of the cementite is desirably 3.0 ⁇ m or less.
• the grain size of cementite refers to a geometric mean value of the major axis and the minor axis of cementite in an observation visual field of a predetermined steel sheet cross-section
• the average grain size of cementite refers to an arithmetic mean value of the individual cementite grain sizes.
• the spheroidizing ratio of the cementite is specified to be 90% or more.
• the spheroidizing ratio of the cementite refers to the ratio of the number of cementite grains having a cross-sectional aspect ratio (major axis/minor axis) of 3 or less to the number of all cementite grains in an observation visual field of a predetermined steel sheet cross-section.
• Ratio of the Number of Grains of Cementite Present Inside Ferrite Grains 90% or More
• the cementite in the steel sheet according to an embodiment of the present invention is spheroidized sufficiently, and the average grain size is a large 1.0 ⁇ m or more. If a large amount of such cementite is present at grain boundaries of ferrite, the cementite serves as a starting point of breakage easily when being subjected to forming, so that the formability is degraded. Therefore, it is necessary that 90% or more of cementite, on a ratio of the number of grains to the total number of cementite grains basis, be present inside ferrite grains.
• the high carbon steel sheet according to the present invention can be obtained by hot-rolling a steel slab having the above-described chemical composition at a finishing temperature higher than or equal to the Ar a transformation point, performing primary cooling to a primary cooling stop temperature of 550° C. to 650° C. at an average cooling rate of 25° C./sec to 50° C./sec, then performing secondary cooling to a secondary cooling stop temperature of 500° C. to 600° C. at an average cooling rate of 120° C./sec or more for a cooling time of 1 s or less, performing coiling, removing scale on the steel sheet surface layer, then performing cold rolling at a reduction ratio of 20% to 30%, and performing annealing by holding for 20 hours or more at an annealing temperature of 680° C. or higher and lower than the Ac 1 transformation point.
• the hot rolling step also plays a role in adjusting the microstructure prior to the following mildness enhancing treatment, in which cold rolling and annealing are combined, and is a step necessary and important for facilitating homogenization in the sheet thickness direction after the mildness enhancing treatment.
• the mildness enhancing treatment composed of light reduction cold rolling and annealing is a treatment to facilitate coarsening of ferrite grains by utilizing strain-induced grain growth due to a strain introduced on the basis of cold rolling and is a step responsible for enhancing mildness of the steel sheet. Although relatively much strain is introduced into the surface layer portion of the steel sheet, which is a material to be rolled, by the light reduction cold rolling at a small reduction ratio, strain is not introduced into the central portion of the sheet thickness easily. Consequently, a difference in the effect of facilitating ferrite grain growth occurs between the surface layer portion and the central portion, and the grain size distribution of ferrite and characteristics of the steel sheet become nonuniform.
• a specific microstructure distribution is preferably formed in advance in the steel sheet after the hot rolling.
• the cementite in the surface layer portion is made finer than the cementite in the sheet thickness central portion in the hot rolled steel sheet, ferrite grain growth in the surface layer portion is suppressed as compared with that in the sheet thickness central portion during annealing after the cold rolling, and the nonuniformity of the ferrite grain size distribution resulting from nonuniform introduction of rolling strain is eliminated.
• the hot rolling finishing temperature is lower than the Ar 3 transformation point, a microstructure in which a rolling texture remains is formed after the hot rolling, and the homogeneity in the sheet thickness direction after the annealing is degraded. Therefore, the finishing temperature is specified to be higher than or equal to the Ar 3 transformation point.
• the Ar 3 transformation point can be determined from, for example, an inflection point on the basis of a measurement of a heat shrinkage curve during the cooling process from an austenite temperature zone.
• Cooling to Cooling Stop Temperature 550° C. to 650° C. at Average Cooling Rate of 25° C./sec to 50° C./sec
• the steel sheet after the hot rolling is immediately primarily cooled to a cooling stop temperature of 550° C. to 650° C. at an average cooling rate of 25° C./sec to 50° C./sec. This is because if the average cooling rate is less than 25° C./sec or more than 50° C./sec, it becomes difficult to form a predetermined microstructure distribution, described later, after the hot rolling, the ferrite grain size distribution in the sheet thickness direction after the annealing becomes nonuniform, and the homogeneity in the sheet thickness direction is not attained.
• the cooling stop temperature is higher than 650° C.
• the microstructure after the hot rolling is coarsened easily, and a predetermined microstructure distribution after the hot rolling is not obtained easily.
• the temperature is lower than 550° C.
• hard phases e.g., bainite and martensite, are generated, the steel sheet becomes too strong, so that a coil shape during coiling and operability may be degraded, and the steel sheet shape becomes worse so that cooling variations may be caused.
• secondary cooling is performed on an as-is basis. It is desirable that the time interval after the primary cooling up to start of the secondary cooling be 3 sec or less to suppress excessive recuperation, and 1 sec or less is more desirable.
• Cooling to Cooling Stop Temperature 500° C. to 600° C. at Average Cooling Rate of 120° C./sec or More for Cooling Time of 1 sec or Less
• the steel sheet after the primary cooling is cooled to a cooling stop temperature of 500° C. to 600° C. at an average cooling rate of 120° C./sec or more within 1 sec or less and is coiled.
• the temperature zone of 500° C. to 600° C. is a zone in which transition from film boiling to nucleate boiling starts, so that cooling variations in the steel sheet occur easily.
• cooling variations in the steel sheet do not occur easily when accelerated water-cooling is performed under a nucleate boiling-dominated condition in such a way that the average cooling rate becomes 120° C./sec or more.
• the accelerated water-cooling at an average cooling rate of 240° C./sec or more is more preferable.
• the ferrite grain size distribution in the sheet thickness direction becomes uniform, and the homogeneity in the sheet thickness direction is attained. If the cooling time is more than 1 sec, the temperature distribution in the sheet thickness direction after the cooling becomes uniform easily, and predetermined microstructure distribution is not obtained easily.
• the cooling time is preferably 0.5 sec or less.
• the cooling stop temperature is more than 600° C., coarse pearlite is easily generated after the cooling, and it becomes difficult to form the predetermined microstructure distribution after the annealing.
• the cooling stop temperature is lower than 500° C., large amounts of hard phases, e.g., bainite and martensite, are generated, and the steel sheet becomes too strong, so that a coil shape during coiling and operability are degraded.
• the coiled steel sheet is subjected to pickling or the like to remove scale on the steel sheet surface layer and, thereafter, is cold-rolled in order to realize the enhancement of mildness of the steel sheet due to strain-induced grain growth during the annealing described next.
• the reduction ratio is less than 20%, a sufficient effect of accelerating grain growth is not obtained, and if the reduction ratio is more than 30%, ferrite grains become fine. Therefore, the cold rolling reduction ratio is limited to 20% to 30%.
• Annealing Holding for 20 Hours or More at Annealing Temperature of 680° C. or Higher and Lower Than Ac 1 Transformation Point
• the steel sheet after the cold rolling is annealed to facilitate spheroidization of cementite and coarsening of ferrite grains.
• the annealing temperature is lower than 680° C., the spheroidization of cementite and the coarsening of ferrite grains do not proceed smoothly, and if the annealing temperature is higher than or equal to the Ac 1 transformation point, austenite is partly generated during annealing, and pearlite, that is, cementite not spheroidized, becomes present together after the annealing, so that the formability and the hardenability are degraded. Therefore, the annealing temperature is limited to within the range of 680° C. or higher and lower than the Ac 1 transformation point, and preferably 690° C. or higher and (Ac 1 transformation point ⁇ 5° C.) or lower.
• the holding time at the annealing temperature be 20 hours or more in order to achieve the spheroidization of cementite and the coarsening of ferrite grains, and 30 to 50 hours is desirable.
• the Ac 1 transformation point can be determined from, for example, an inflection point on the basis of a measurement of a thermal expansion curve during the heating process from ambient temperature. Alternatively, it is also possible to roughly calculate from the contents of chemical components.
• the steel sheet after the annealing can be subjected to temper rolling, as necessary, to correct the shape or adjust the surface quality.
• Either a converter or an electric furnace can be employed for melting the high carbon steel used in the present invention.
• the melted steel is made into a steel slab (billet) by continuous casting or blooming after ingot making.
• the steel slab can undergo improvements, e.g., scarfing, as necessary.
• the steel slab before the hot rolling may be heated to a temperature at which a predetermined finishing temperature can be ensured in accordance with the capacity of the production equipment.
• the continuously cast steel slab may be hot-rolled directly or after a short time of heating without being cooled to ambient temperature.
• a former steel slab is heated by an induction heating apparatus, e.g., a bar heater or an edge heater, midway through the hot rolling.
• the Ar 3 transformation point and the Ac 1 transformation point shown in Table 1 were determined by rough calculation from the chemical compositions of the steel on the basis of the following formulae.
• [C], [Si], [Mn], [Cr], [Mo], and [Ni] represent contents (percent by mass) of C, Si, Mn, Cr, Mo, and Ni, respectively.
• a sample was taken from each of the resulting steel sheets, and the average grain size ds of the ferrite in the region from the steel sheet surface to the position at one-quarter of the sheet thickness, the average grain size dc of the ferrite in the region from the position at one-quarter of the sheet thickness to the sheet thickness center, the average grain size of the cementite, the spheroidizing ratio of the cementite, and the ratio of the number of cementite grains present inside ferrite grains were measured.
• the measurements were performed by using a microstructure photograph of each of the positions of the surface layer portion, the position at one-eighth of the sheet thickness, the position at one-quarter of the sheet thickness, the position at three-eighths of the sheet thickness, and the sheet thickness central portion, where a photograph was taken with a scanning electron microscope at a magnification of 500 to 3,000 times after a sheet thickness cross-section parallel to the steel sheet rolling direction of the sample was mirror-polished and was corroded with nital or picral. At this time, the nital corrosion photograph taken at each of the above-described positions was used, the grain size was determined in conformity with the method specified in Japanese Industrial Standards JIS G 0552, and the average grain size of the ferrite was calculated from the grain size number.
• the average grain size ds of the ferrite in the region from the steel sheet surface to the position at one-quarter of the sheet thickness was an average of the average grain sizes calculated at the surface layer portion, the position at one-eighth of the sheet thickness, and the position at one-quarter of the sheet thickness, and the average grain size dc of the ferrite in the region from the position at one-quarter of the sheet thickness to the sheet thickness center was an average of the average grain sizes calculated at the position at one-quarter of the sheet thickness, the position at three-eighths of the sheet thickness, and the sheet thickness central portion.
• each of the positions of the surface layer portion, the position at one-eighth of the sheet thickness, the position at one-quarter of the sheet thickness, and the position at three-eighths of the sheet thickness was observed with reference to both the surface and the back of the steel sheet. Meanwhile, the average grain size of the cementite, the spheroidizing ratio of the cementite, and the ratio of the number of cementite grains present inside ferrite grains were measured at the position at one-quarter of the sheet thickness, while the picral corrosion photograph was used in combination.
• the degree of the mildness and the homogeneity in the sheet thickness direction of the steel sheet were evaluated by mirror-polishing a sheet thickness cross-section parallel to the steel sheet rolling direction of the sample and measuring the Vickers hardness at the position at one-eighth of the sheet thickness and the position at three-eighths of the sheet thickness in conformity with the method specified in Japanese Industrial Standards JIS Z 2244 with a test forth of 9.8 N (1 kgf).
• at least five points of Vickers hardnesses were measured at each position and an average value thereof was specified to be HV.
• the HV at the position at one-eighth of the sheet thickness was taken as HVs
• the HV at the position at three-eighths of the sheet thickness was taken as HVc.
• the case where the HVs and the HVc were 150 or less was specified to be mild, and the case where the difference ⁇ HV between the HVs and the HVc was 5 or less was specified to be excellent in the homogeneity in the sheet thickness direction.
• the limit deformability was evaluated by performing a hole expanding test in conformity with Japanese Industrial Standards JIS Z 2256. At this time, in the case where the hole expanding ratio was 30% or more, the formability was evaluated as sufficient.
• the steel sheet according to an embodiment of the present invention is a mild high carbon steel sheet having a small ⁇ HV exhibiting homogeneity in the sheet thickness direction and having excellent formability.
• the steel sheet according to the comparative example has a large ⁇ HV exhibiting heterogeneity in the sheet thickness direction, insufficient mildness or poor formability.

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