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
• • 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
  • 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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0426—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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing 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
  • C22C38/002—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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • 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
• • 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

Definitions

• the present invention is a high-strength steel sheet with excellent formability that can be applied to automobile parts and the like, and in particular, tensile strength TS is 600 to 700 MPa, elongation El is 25% or more (plate thickness 1.6 mm, JIS No. 5 specimen), high strength steel plate with a hole expansion ratio ⁇ of 80% or more, which is an index of stretch flangeability, and its manufacturing method (high strength, steel, sheet, and method for for producing same).
• Patent Document 1 As chemical components, C: 0.02 to 0.16% by mass, P ⁇ 0.010%, S ⁇ 0.003%, one or two of Si and Al in a total amount of 0.2 to 4%, One or more of Mn, Ni, Cr, Mo, Cu are included in a total amount of 0.5-4%, C / (Si + Al + P) is 0.1 or less, the remainder Fe and inevitable impurities A steel sheet having a cross-sectional microstructure of one or two of martensite and retained austenite in a total area ratio of less than 3%, ferrite and bainite.
• the total area ratio of one or two of (bainite) is 80% or more, the balance is pearlite, the maximum length of pearlite, martensite, and retained austenite is 10 microns or less.
• Strength-hole expansion ratio balance and shape fixability characterized by the inclusion of inclusions of 20 microns or more in the cross section within 0.3 per square mm An excellent high strength steel sheet for processing is disclosed.
• Patent Document 2 includes mass%, C: 0.05 to 0.15%, Mn: 0.8 to 1.2%, Si: 0.02 to 2.0%, sol.Al: 0.002% to less than 0.05%, N: 0.001% to 0.005
• the balance consists of Fe and impurities, and Ti, Nb and V in the impurities are all less than 0.005%, and the microstructure is ferrite with an average particle size of 1.1-5.0 ⁇ m as the main phase and pearlite as the second phase.
• a hot-rolled steel material containing either one or both of cementite and cementite and satisfying Mn ⁇ / Mn ⁇ ⁇ 1 is disclosed.
• Mn ⁇ is the amount of Mn in cementite containing cementite in pearlite
• Mn ⁇ is the amount of Mn in ferrite as the main phase.
• Punched when the tensile strength is 50 kgf / mm 2 or more characterized in that the structure ratio of cementite with an equivalent circle radius of 0.1 ⁇ m or more is 0.1% or less and / or the structure ratio of martensite is 5% or less.
• the structure ratio of cementite with an equivalent circle radius of 0.1 ⁇ m or more is 0.1% or less and / or the structure ratio of martensite is 5% or less.
• the present invention provides a high-strength steel sheet having excellent workability with TS of 600 to 700 MPa, El of 25% or more (in the case of a plate thickness of 1.6 mm, JIS No. 5 test piece) and ⁇ of 80% or more, and a method for producing the same.
• the purpose is to do.
• the inventors of the present invention have studied the above-mentioned high-strength steel sheet, and have ferrite and pearlite, the ferrite volume fraction is 70% to 97%, the pearlite volume fraction is 3% or more, and the ferrite It is effective to have a microstructure in which the volume fraction of cementite existing at grain boundaries is 2% or less, the volume fraction of other phases is less than 3% in total, and the average grain size of ferrite is 7 ⁇ m or less I found out.
• the present invention has been made based on such findings, in mass%, C: 0.10% or more and 0.18% or less, Si: more than 0.5% and 1.5% or less, Mn: 0.5% or more and 1.5% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.05% or less, having a composition composed of the remaining Fe and inevitable impurities, the microstructure has ferrite and pearlite, and the volume fraction of the ferrite is 70% or more 97% or less, the pearlite volume fraction is 3% or more, the volume fraction of cementite present in the ferrite grain boundary is 2% or less, and the volume fraction of phases other than the ferrite, pearlite, and cementite is 3 in total. And providing a high-strength steel sheet having an average grain size of 7 ⁇ m or less.
• the high-strength steel sheet of the present invention further includes at least one selected from mass%, Cr: 0.01% to 1.0%, Ti: 0.01% to 0.1%, V: 0.01% to 0.1%. It is preferable to do.
• the high-strength steel sheet of the present invention preferably has a tensile strength TS of 600 to 700 MPa.
• the high-strength steel sheet of the present invention preferably has a hole expansion ratio ⁇ of 80% or more.
• the high-strength steel sheet of the present invention preferably has a ferrite volume fraction of 80% to 95%.
• the high strength steel sheet of the present invention preferably has a pearlite volume fraction of 3% to 30%. The volume ratio of pearlite is more preferably 5% or more and 28% or less.
• the production method of the high strength steel sheet of the present invention is mass%, C: 0.10% or more and 0.18% or less, Si: more than 0.5% and 1.5% or less, Mn: 0.5% or more and 1.5% or less, P: 0.05% or less, S: 0.005% or less, Al: including 0.05% or less, a step of preparing a steel slab having a chemical composition consisting of the balance Fe and inevitable impurities, a step of subjecting the steel slab to hot rolling to form a hot rolled sheet, The hot-rolled sheet is heated to a two-phase temperature range between the Ac 1 transformation point and the Ac 3 transformation point, and then the average cooling rate is 5 ° C./s to 30 ° C./s in a temperature range of 450 ° C. to 600 ° C. Cooling and performing annealing for staying in the temperature range for 100 s or more.
• the steel slab further contains at least one selected from mass%, Cr: 0.01% to 1.0%, Ti: 0.01% to 0.1%, V: 0.01% to 0.1%. preferable.
• the annealing is performed by heating to a two-phase temperature range between the Ac 1 transformation point and the Ac 3 transformation point, and then to an average cooling rate of 10 ° C./s to 20 ° C./s to a temperature range of 450 ° C. to 600 ° C. It is preferable that it is cooled at a temperature of 100s to 300s.
• the present invention makes it possible to produce a high-strength steel sheet having excellent workability with TS of 600 to 700 MPa, El of 25% or more, and ⁇ of 80% or more.
• C 0.10% or more and 0.18% or less C forms a second phase such as pearlite, martensite, and cementite, and contributes to an increase in the strength of the steel sheet.
• a C amount of 0.10% or more is necessary.
• the second phase increases too much, so TS exceeds 700MPa and El and ⁇ decrease.
• the C content is 0.10% to 0.18%. Preferably it is 0.12% or more and 0.16% or less.
• Si more than 0.5% and less than 1.5% Si is an element contributing to solid solution strengthening. In order to obtain a TS of 600 MPa or more, a Si amount exceeding 0.5% is required. However, if it exceeds 1.5%, the surface properties of the steel sheet deteriorate due to the generation of scale. From the above, the Si content is 0.5% to 1.5%. Preferably they are 0.7% or more and 1.2% or less.
• Mn 0.5% or more and 1.5% or less Mn is an element contributing to solid solution strengthening. In order to obtain a TS of 600 MPa or more, an Mn amount of 0.5% or more is necessary. However, if it exceeds 1.5%, TS exceeds 700 MPa, or ⁇ decreases due to segregation. From the above, the Mn content is 0.5% or more and 1.5% or less. Preferably they are 1.1% or more and 1.5% or less.
• P 0.05% or less
• P is an element contributing to solid solution strengthening. However, if it exceeds 0.05%, El decreases due to segregation. Based on the above, the P content is 0.05% or less. Preferably it is 0.03% or less.
• S 0.005% or less
• S segregation occurs in the prior austenite grain boundaries, or MnS precipitates in the steel sheet, leading to a decrease in ⁇ . From the above, the amount of S is set to 0.005% or less, but the smaller the amount, the better.
• Al 0.05% or less Al is added as a deoxidizer for steel and is an effective element for improving the cleanliness of steel. However, if it exceeds 0.05%, a large amount of inclusions are generated, which causes surface defects of the steel sheet. From the above, the Al content is 0.05% or less. Preferably it is 0.03% or less.
• the balance is Fe and inevitable impurities, and further contains at least one selected from Cr: 0.01% to 1.0%, Ti: 0.01% to 0.1%, V: 0.01% to 0.1%. be able to.
• Cr, Ti, and V suppress the recrystallization and recovery of austenite in the hot rolling temperature range, promote ferrite refinement, form carbides, or form a solid solution of ferrite. This is because it works to strengthen Note that Nb is an element that has the same effect. However, the addition of these elements does not lower the ductility (El) as much as when the same amount of Nb is added.
• Cr 0.02% to 0.5%
• Ti 0.02% to 0.05%
• V 0.02% to 0.05%.
• O is 0.003% or less
• Cu, Ni, Sn, and Sb are each 0.05% or less.
• the volume fraction of ferrite is 70% or more and 97% or less. If the volume fraction of the entire ferrite structure is less than 70%, TS exceeds 700 MPa or ⁇ of 80% or more cannot be obtained. On the other hand, when the volume ratio exceeds 97%, the amount of pearlite decreases, so that a TS of 600 MPa or more cannot be obtained. From the above, the volume fraction of ferrite is 70% or more and 97% or less. It is preferably 95% or less, and more preferably 80% or more and 90% or less.
• Perlite volume ratio 3% or more ⁇ improves when the pearlite volume ratio is 3% or more. Preferably it is 5% or more. This is because pearlite is soft compared to cementite, martensite, and retained austenite, so compared to the number of voids generated at the interface between ferrite and martensite and the interface between ferrite and retained austenite after processing, This is probably because the number of voids generated at the interface is small.
• the volume fraction of cementite present at the ferrite grain boundaries 2% or less
• the steel sheet of the present invention may contain cementite, martensite, etc. in addition to ferrite and pearlite. If the volume fraction of cementite in the entire grain structure of cementite exceeds 2%, the number of voids generated at the interface between ferrite and cementite during hole expansion increases, leading to a decrease in ⁇ . Therefore, the volume fraction of cementite existing at the ferrite grain boundary is set to 2% or less. It may be 0%.
• volume fraction of phases other than cementite present in ferrite, pearlite, and ferrite grain boundaries less than 3% in total
• Other phases other than cementite present in ferrite, pearlite, and ferrite grain boundaries include martensite and retained austenite
• the volume fraction of phases other than cementite should be less than 3% in total. Preferably it is 2.5% or less, and may be 0%.
• Average ferrite particle diameter 7 ⁇ m or less Since the average particle diameter of ferrite exceeds 7 ⁇ m, the strength is reduced, so a TS of 600 MPa or more cannot be obtained. From the above, the average grain size of ferrite is 7 ⁇ m or less. Preferably, it is 5 ⁇ m or less.
• the volume fraction of the entire structure of ferrite, pearlite, cementite, martensite, and retained austenite is corroded with a nital solution after polishing the plate thickness section parallel to the rolling direction of the steel plate, and the magnification is 1000 times with an optical microscope. 3 fields of view were taken, and the type of tissue was selected by image processing. At the same time, the average particle diameter of the ferrite was calculated by a cutting method.
• an image taken with an optical microscope at a magnification of 1000 times is divided into 20 lines vertically and 20 lines horizontally.
• the segment was plotted, and the value obtained by dividing the total length of ferrite grains cut by one line segment by the number of ferrite to be cut was taken as the cut length, and the average intercept length L at each line segment was calculated. And the average particle diameter d was calculated
• required by following Formula. d 1.13 ⁇ L
• the volume ratio of the cementite existing in the ferrite grain boundary in the entire structure was obtained by taking three fields of view with a scanning electron microscope at a magnification of 3000 and extracting the cementite existing in the ferrite grain boundary by image processing.
• the steel slab to be used is preferably manufactured by a continuous casting method in order to prevent macrosegregation of the component molten steel melted in the above component composition by a known method such as a converter. However, it can also be produced by an ingot-making method.
• Hot rolling The steel slab thus produced is hot-rolled after being cooled to room temperature or reheated in a heating furnace without being cooled to room temperature, or kept at a high temperature without passing through the heating furnace.
• the hot rolling conditions need not be particularly limited, but after heating the steel slab to the range of 1100 ° C to 1300 ° C, the hot rolling (finish rolling) is finished at 850 ° C to 950 ° C, and the coil is wound at 720 ° C or lower. It is preferable to take. This is due to the following reason.
• the heating temperature is less than 1100 ° C.
• the deformation resistance of the steel is high, so that hot rolling may be difficult, and if it exceeds 1300 ° C., the crystal grain size becomes coarse and TS may decrease.
• the finish rolling finish temperature is less than 850 ° C, ferrite is formed during rolling, so that extended ferrite is formed, which may lead to a decrease in ⁇ , and if it exceeds 950 ° C, the crystal grain size becomes coarse , TS may decrease.
• the winding temperature exceeds 720 ° C., the formation of the internal oxide layer becomes remarkable, and the chemical conversion property and the corrosion resistance after coating may be deteriorated.
• the hot-rolled sheet after hot rolling is pickled to remove scale generated on the surface of the steel sheet.
• Annealing The hot-rolled sheet after pickling treatment is heated to a two-phase temperature range between the Ac 1 transformation point and Ac 3 transformation point, and then an average cooling rate of 5 ° C / s in the temperature range of 450 ° C to 600 ° C. It is cooled at 30 ° C./s or less and annealed to stay in the temperature range for 100 s or more.
• the reason for heating to the two-phase temperature range between the Ac 1 transformation point and the Ac 3 transformation point is to form a microstructure having ferrite and pearlite.
• cooling to a temperature range of 450 ° C to 600 ° C at an average cooling rate of 5 ° C / s to 30 ° C / s is because the volume fraction of cementite present at the ferrite grain boundaries exceeds 600 ° C.
• the average cooling rate is preferably 10 ° C./s or more and 20 ° C./s or less. The reason for staying in the temperature range of 450 ° C.
• the staying time be 150 seconds or longer.
• the effect is only saturated when staying for an excessively long time, it is preferably set to 300 s or less from the viewpoint of production efficiency.
• annealing can be performed by a continuous annealing facility or the like.
• the steel sheet thus obtained was examined for the microstructure by the above method, and a tensile test was performed according to JIS Z 2241 using a JIS No. 5 test piece to measure TS and El. Further, using a 100 mm square test piece, a hole expansion test was performed in accordance with JFST 1001-1996, and ⁇ was measured.
• the steel plates of the examples of the present invention are high-strength steel plates having excellent workability with TS of 600 to 700 MPa, El of 25% or more, and ⁇ of 80% or more.
• the target TS or ⁇ was not obtained in the steel plate of the comparative example.

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• Crystallography & Structural Chemistry (AREA)
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• 2011
  • 2011-12-19 JP JP2011276997A patent/JP5316634B2/ja active Active
• 2012
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