US20150252456A1 - Cold-rolled steel sheet with excellent shape fixability and method of manufacturing the same - Google Patents

Cold-rolled steel sheet with excellent shape fixability and method of manufacturing the same Download PDF

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US20150252456A1
US20150252456A1 US14/433,869 US201214433869A US2015252456A1 US 20150252456 A1 US20150252456 A1 US 20150252456A1 US 201214433869 A US201214433869 A US 201214433869A US 2015252456 A1 US2015252456 A1 US 2015252456A1
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cold
steel sheet
equal
rolled steel
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Taro Kizu
Koichiro Fujita
Hideharu Koga
Masahide Morikawa
Kenji Tahara
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JFE Steel Corp
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JFE Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C21D8/0221—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
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation 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
    • 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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • 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
    • 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
    • C22C38/26—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
    • 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

Definitions

  • This disclosure relates to a cold-rolled steel sheet suitable for members of parts requiring strict dimensional accuracy in the electrical, automotive, building material, and other fields and which has excellent shape fixability and also relates to a method of manufacturing the same.
  • the disclosure particularly relates to the enhancement of shape fixability.
  • WO 00/06791 discloses a ferritic steel sheet with excellent shape fixability.
  • steel having a composition containing 0.0001% to 0.05% C, 0.01% to 1.0% Si, 0.01% to 2.0% Mn, 0.15% or less P, 0.03% or less S, 0.01% or less Al, 0.01% or less N, and 0.007% or less O on a mass basis is hot-rolled such that the sum of rolling reductions at a temperature of not lower than the Ar 3 transformation temperature to 950° C. is 25% or more and the coefficient of friction during hot rolling at 950° C.
  • Japanese Unexamined Patent Application Publication No. 2002-66637 discloses a method of press-forming a formed product with excellent dimensional accuracy.
  • forming is performed using a steel sheet in which the ratio of the ⁇ 100 ⁇ plane to ⁇ 111 ⁇ plane parallel to a sheet surface is 1.0 or more such that a tensile stress equal to 40% to 100% of the tensile strength of material is applied to a vertical wall portion of a hat-shaped member. According to that technique, a member having significantly increased hat bendability, small springback, and excellent shape fixability can be provided.
  • the technique described in WO '791 has problems such as: the degree of improvement in shape fixability is small in performing press forming other than bending, and springback may be large due to the influence of grain boundary sliding or the like even when performing bending. Furthermore, the technique described in JP '637 has a problem that the effect of improving the dimensional accuracy of a formed product is not obtained in performing press forming other than hat forming and a problem that the blank holding pressure needs to be large to apply stress to a vertical wall in performing hat forming and therefore the power of a press needs to be increased, leading to an increase in cost.
  • the strain of a flat portion of a formed member is significantly affected by the proportional limit of a steel sheet used.
  • the strain of a flat portion of a formed member is significantly increased particularly when the proportional limit is more than 100 MPa.
  • an ultra-low carbon based chemical composition essentially containing Ti and B needs to be adjusted such that the ratio, B/C, of the content of B to the content of C satisfies 0.5 or more such that the proportional limit is 100 MPa or less.
  • a cold-rolled steel sheet having a significantly reduced proportional limit and excellent shape fixability after forming can be readily manufactured at low cost. This is industrially particularly advantageous. Furthermore, there is an effect that the reduction in gauge of a member can be accelerated.
  • FIG. 1 is a schematic view showing a test specimen for punch stretch forming and a flange-suppressing region (hatched portion) during a forming test.
  • FIG. 2 is a schematic view showing a method of measuring the maximum strain height after a punch stretch forming test.
  • FIG. 3 is a graph showing the relationship between the proportional limit and the maximum strain height.
  • FIG. 4 is a graph showing the relationship between B/C and the proportional limit.
  • C is an element which forms a solid solution to promote formation of coarse B precipitates and which contributes to a reduction in proportional limit. Such an effect is remarkable when the content thereof is 0.0010% or more. However, when the content thereof is high, more than 0.0030%, the reduction of ductility is caused because the amount of solute C and/or carbides is large and the strength is excessively high. Therefore, C is limited to 0.0010% to 0.0030%. It is preferably 0.0020% or less.
  • Si is 0.05% or less.
  • Mn combines with S, where S significantly reduces hot ductility and is harmful, in steel to form MnS, contributes to rendering S harmless, and has the effect of hardening steel.
  • the content thereof needs to be 0.1% or more to achieve such effects.
  • Mn is 0.1% to 0.5%. It is preferably 0.3% or less and more preferably 0.2% or less.
  • P segregates at grain boundaries and has the function of reducing ductility. Therefore, P is preferably minimized and up to 0.05% is acceptable. Hence, P is 0.05% or less. It is preferably 0.03% or less and more preferably 0.02% or less.
  • S is an impurity element and is preferably minimized.
  • S significantly reduces hot ductility, causes hot cracking, significantly deteriorates surface properties, and has adverse influences. Furthermore, S hardly contributes to strength and forms coarse MnS to reduce ductility. This becomes significant when S is more than 0.02%. Therefore, S is 0.02% or less. It is preferably 0.01% or less.
  • Al is an element acting as a deoxidizer. 0.02% or more is preferably contained to achieve such an effect.
  • Al has the function of increasing the ⁇ -to- ⁇ transformation temperature of steel. Therefore, when the content is high, more than 0.10%, it is difficult to complete rolling in a ⁇ -region during hot rolling. Therefore, Al is 0.10% or less.
  • N is an element which combines with a nitride-forming element to form a nitride and has the function of hardening steel by precipitation hardening.
  • N is 0.0050% or less. It is preferably 0.0030% or less and more preferably 0.0020% or less.
  • Ti is an element which fixes N in the form of a nitride and has the function of suppressing hardening and aging deterioration due to solute N. 0.021% or more needs to be contained to achieve such effects. However, when the content is high, more than 0.060%, the precipitation of carbides is promoted and the amount of solute C is reduced. Hence, production of coarse B precipitates containing C and Fe is suppressed. Therefore, a desired reduction in proportional limit cannot be achieved. Thus, Ti is 0.021% to 0.060%. It is preferably 0.050% or less.
  • B is an important element and forms coarse B precipitates to contribute to a reduction in proportional limit. 0.0005% or more needs to be contained to achieve such an effect. However, when the content is high, more than 0.0050%, slab cracking is caused. Therefore, B is 0.0005% to 0.0050%. It is preferably 0.0010% or more, more preferably 0.0020% or more, and further more preferably 0.0030% or more.
  • C and B are contained in the above ranges and the contents of C and B are adjusted such that the ratio, B/C, of the content of B to the content of C satisfies 0.5 or more.
  • B/C is less than 0.5, it is difficult to form coarse B precipitates. Therefore, B/C is limited to 0.5 or more. Incidentally, it is preferably 1.0 or more, more preferably 1.5 or more, and further more preferably 2.0 or more.
  • the above components are fundamental components. 0.009% or less Nb and/or 0.06% or less Cr may be contained as a selective element in addition to the fundamental components as required.
  • Nb as well as Ti, is an element which combines with N to form a nitride, which fixes N, which suppresses hardening and aging deterioration due to solute N, and contributes to enhancement of shape fixability and may be contained as required. 0.001% or more is preferably contained to achieve such effects. However, the content is high, more than 0.009%, grains become fine. Therefore, when Nb is contained, Nb is preferably 0.009% or less.
  • Cr is an element which destabilizes C in a solid solution to promote production of coarse B precipitates containing C and may be contained as required. 0.001% or more is preferably contained to achieve such an effect. However, when the content of Cr is high, more than 0.06%, the production of the coarse B precipitates containing C is inhibited instead. Therefore, when Cr is contained, Cr is preferably 0.06% or less. The remainder other than the above components are Fe and incidental impurities.
  • the cold-rolled steel sheet has a microstructure dominated by ferrite with an average grain size of 10 ⁇ m to 30 ⁇ m.
  • the microstructure dominated by ferrite allows the steel sheet to be soft and therefore allows workability thereof to be enhanced.
  • the term “microstructure dominated by ferrite” as used herein refers to a microstructure in which ferrite (polygonal ferrite) accounts for 95% or more, and more preferably 100%, in terms of area fraction.
  • a secondary phase other than ferrite is preferably cementite or bainite. If the average grain size of ferrite is 10 ⁇ m or more, the concentration of strain at grain boundaries can be suppressed, strain can be concentrated around precipitates, and the proportional limit can be reduced.
  • the average grain size of ferrite is 10 ⁇ m to 30 ⁇ m. It is preferably 15 ⁇ m to 25 ⁇ m.
  • a steel material (slab) with the above composition is used as a starting material.
  • a method of manufacturing the steel material is not particularly limited.
  • Molten steel with the above composition is preferably produced in a regular converter, an electric furnace, or the like and is then solidified into a slab (steel material) by a continuous casting process or an ingot casting-blooming process.
  • the slab is preferably directly hot-rolled without cooling the slab to room temperature when having heat sufficient for hot rolling.
  • the slab is preferably hot-rolled after the slab is temporally charged into a furnace and is heat-retained or the slab is cooled to room temperature and is then reheated to a temperature of 1,100° C. to 1,250° C. by charging the slab into a furnace.
  • the heated steel material is subjected to a hot rolling step.
  • hot rolling step hot rolling including rough rolling and finish rolling is performed and coiling is then performed.
  • Finish rolling is performed at a finishing delivery temperature of 870° C. to 950° C.
  • the finishing delivery temperature When the finishing delivery temperature is low, lower than 870° C., the microstructure is transformed from austenite into ferrite in the course of rolling and therefore it is difficult to control the load of a rolling machine. Hence, the risk of causing fracture or the like during processing increases. Incidentally, if rolling is performed from the finishing entry side in a ferrite region, the fracture or the like during processing can be avoided. However, there is a problem in that the microstructure of the hot-rolled sheet is transformed into unrecrystallized ferrite because of the decrease of the rolling temperature and therefore the load for cold rolling is increased. On the other hand, when the finishing delivery temperature is high, higher than 950° C., the hot-rolled sheet has a large ferrite grain size.
  • the finishing delivery temperature is 870° C. to 950° C.
  • the hot-rolled sheet is coiled. Cooling until coiling after finish rolling is not particularly limited and it is sufficient that the rate of cooling is higher than that of air cooling. There is no particular problem even if quenching is performed at 100° C./s or more as required.
  • the coiling temperature after the completion of finish rolling is 450° C. to 630° C.
  • the coiling temperature is lower than 450° C.
  • acicular ferrite is produced and a steel sheet is hardened.
  • the load for subsequent cold rolling is increased and, also, leads to the difficulty in operating hot rolling.
  • the coiling temperature is high, higher than 630° C.
  • the precipitation of carbides is promoted, the amount of solute C is reduced and, therefore, a desired amount of solute C cannot be ensured during hot rolling process.
  • the coiling temperature is 450° C. to 630° C.
  • the coiled hot-rolled sheet is subjected to an ordinary pickling step and then subjected to a cold-rolling step, whereby a cold-rolled sheet is obtained.
  • the cold-rolled sheet is obtained by performing cold rolling at a cold-rolling reduction of 90% or less.
  • the cold-rolling reduction is limited to 90% or less. It is preferably 80% or less.
  • the lower limit of the cold-rolling reduction is not particularly limited. However, when the cold rolling reduction is low, the thickness of the hot-rolled sheet needs to be reduced with respect to the predetermined thickness of products and, therefore, productivity of hot rolling and pickling is reduced. Hence, the cold-rolling reduction is preferably 50% or more.
  • the cold-rolled sheet is subjected to an annealing step, whereby a cold-rolled annealed sheet is obtained.
  • the annealing step is a step in which heating is performed up to a holding temperature of 700° C. to 850° C. at an average heating rate of 1° C./s to 30° C./s in a temperature region not lower than 600° C., retention is performed at the holding temperature for 30 s to 200 s, and cooling is then performed at a cooling rate of 3° C./s or more down to 600° C. or lower.
  • cold-rolled worked ferrite is recrystallized to have a desired average grain size and coarse B precipitates containing C and Fe are distributed at grain boundaries and in grains. Heating rate: 1° C./s to 30° C./s
  • the average heating rate in a temperature region ranging from 600° C. to the holding temperature is less than 1° C./s, ferrite grains grow significantly and therefore ferrite with a desired average grain size cannot be obtained.
  • the heating rate is high, more than 30° C./s, TiC is precipitated during heating instead of the production of B precipitates and therefore it is difficult to form desired coarse B precipitates.
  • the average heating rate in a temperature region not lower than 600° C. is limited to 1° C./s to 30° C./s. It is preferably 5° C./s or more and more preferably 10° C./s or more.
  • the holding temperature is 700° C. or higher because the recrystallization of cold-worked ferrite needs to be completed.
  • the holding temperature is high, higher than 850° C., ferrite grains become coarse and therefore ferrite with a desired average grain size cannot be obtained.
  • the holding temperature is 700° C. to 850° C.
  • the holding time is 30 s or more to complete the recrystallization of cold-worked ferrite.
  • the holding time is short, the recrystallization thereof is not completed or ferrite grains remain fine.
  • the holding time is long, more than 200 s, ferrite grains grow excessively.
  • the holding time is of 30 s to 200 s.
  • Cooling Rate 3° C./s or More
  • the average cooling rate in a temperature region ranging from the holding temperature to 600° C. is 3° C./s or more.
  • the upper limit of the cooling rate need not be particularly limited and is determined depending on the capacity of a cooling facility. In ordinary cooling facilities, the upper limit of the cooling rate is about 30° C./s.
  • Coarsening of the microstructure due to growth of ferrite grains can be suppressed by cooling to 600° C., whereby a microstructure dominated by ferrite with a desired average grain size can be obtained.
  • Conditions for cooling to 600° C. or less need not be particularly limited and arbitrary cooling is not particularly problematic.
  • galvanizing may be performed at about 480° C. as required.
  • galvannealing may be performed by reheating to 500° C. or higher.
  • Thermal history including retention during cooling may be performed.
  • temper rolling may be performed at about 0.5% to 2% as required.
  • electrogalvanizing may be performed for the purpose of enhancing corrosion resistance.
  • a coating may be provided on the cold-rolled steel sheet or a plated steel sheet using chemical conversion or the like.
  • Steel materials having a composition containing 0.0010% to 0.035% C, 0.01% to 0.03% Si, 0.10% to 0.45% Mn, 0.03% to 0.08% Al, 0.022% to 0.060% Ti, 0.0003% to 0.0048% B, and 0.0015% to 0.0040% N on a mass basis were subjected to hot rolling and cold rolling and further subjected to annealing under various heating, holding, and cooling conditions, whereby cold-rolled annealed sheets were obtained.
  • a JIS #5 test specimen was taken from each obtained cold-rolled annealed sheet such that a tensile direction coincided with a rolling direction, followed by determining the proportional limit thereof.
  • a 5 mm strain gauge was attached to a parallel portion of the tensile test specimen and tensile testing was performed at a cross head speed of 1 mm/min. The stress at which the slope of the stress-strain curve thereof began to decrease was defined as the proportional limit thereof.
  • a test specimen (a size of 120 mm ⁇ 120 mm) was taken from each obtained cold-rolled annealed sheet and then punch stretch formed.
  • Punch stretch forming was performed by press forming such that a central portion of the test specimen was stretched by 8 mm using a spherical punch with a diameter of 20 mm.
  • a region (hatched portion) with a diameter of 28 mm to 54 mm was pressed with a load of 100 kN and formed as shown in FIG. 1 .
  • the formed test specimen was placed on a platen and a flange portion thereof was measured for maximum strain height. Observation of the obtained cold-rolled annealed sheets showed that all the cold-rolled annealed sheets had a microstructure dominated by ferrite.
  • FIGS. 3 and 4 show the relationship between the proportional limit and maximum strain height of each flange portion.
  • FIG. 4 shows the relationship between B/C and the proportional limit.
  • steel materials having a chemical composition shown in Table 1 were used as starting materials.
  • the slabs were heated to 1,200° C.
  • the slabs were subjected to a hot-rolling step, a pickling step, a cold-rolling step, and an annealing step in that order, whereby cold-rolled annealed sheets were obtained.
  • each steel material was roughly rolled into a sheet bar and the sheet bar was finish-rolled at a finishing delivery temperature equal to a temperature (FT) shown in Table 2 and was then coiled at a coiling temperature (CT) shown in Table 2, whereby a hot-rolled sheet with a thickness shown in Table 2.
  • CT coiling temperature
  • the cold-rolled sheet was subjected to the annealing step, whereby a cold-rolled annealed sheet was obtained.
  • annealing was performed at a heating rate, a holding temperature, a holding time, and a cooling rate as shown in Table 2. Cooling from 600° C. or lower to room temperature was performed at a similar cooling rate. After the annealing step was performed, temper rolling was performed at a rolling reduction of 1.0%.
  • the obtained cold-rolled annealed sheets (cold-rolled steel sheets) were subjected to microstructure observation, a tensile test, and a punch stretch forming test. Testing methods were as described below.
  • a test specimen for microstructure observation was taken from each obtained cold-rolled annealed sheet; a cross section (L-cross section) in a rolling direction was polished and etched; the microstructure thereof was observed and photographed using an optical microscope (a magnification of 100 times) and a scanning electron microscope (a magnification of 1,000 times); and the average grain size of ferrite, the fraction of ferrite, and the type and fraction of a secondary phase were determined by image analysis.
  • the average intercept length of ferrite grains in a 300 ⁇ m ⁇ 300 ⁇ m region was determined in the rolling and thickness directions and the value of 2/(1/A+1/B) was defined as the average grain size, where A is the average intercept length of the ferrite grains in the rolling direction and B is the average intercept length of the ferrite grains in the thickness direction.
  • the fraction of ferrite was measured in a 300 ⁇ m ⁇ 300 ⁇ m region.
  • a JIS #5 test specimen was taken from each obtained cold-rolled annealed sheet such that a tensile direction coincided with the rolling direction, followed by determining the proportional limit thereof.
  • a strain gauge was attached to a parallel portion of the tensile test specimen and tensile testing was performed at a cross head speed of 1 mm/min, whereby tensile properties (proportional limit, tensile strength, and elongation) were determined.
  • the proportional limit was defined as the stress at which the slope of the stress-strain curve thereof began to decrease.
  • test specimen (a size of 120 mm ⁇ 120 mm) was taken from each obtained cold-rolled annealed sheet and was then punch stretch formed.
  • Punch stretch forming was performed by press forming such that a central portion of the test specimen was stretched by 8 mm using a spherical punch with a diameter of 20 mm.
  • punch stretch forming a region (hatched portion) with a diameter of 28 mm to 54 mm was depressed with a load of 100 kN and formed as shown in FIG. 1 .
  • FIG. 2 After forming, as shown in FIG. 2 , the test specimen was placed on a platen and a flange portion thereof was measured for maximum strain height. Obtained results are shown in Table 3.
  • cold-rolled steel sheets have excellent shape fixability with a low proportional limit of 100 MPa or less and flat portions of punch stretch formed members having a maximum strain height of 0.8 mm or less.
  • the proportional limit is more than 100 MPa or the maximum strain height is large, more than 0.8 mm, and shape fixability is low.

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US11421294B2 (en) * 2016-08-12 2022-08-23 Posco High strength steel sheet having excellent formability and manufacturing method thereof
CN115135792A (zh) * 2019-12-19 2022-09-30 Posco公司 硬度和加工性优良的结构部用冷轧钢板及其制造方法
US20230287543A1 (en) * 2020-07-08 2023-09-14 Jfe Steel Corporation Method for producing ultra-low carbon steel product

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US11421294B2 (en) * 2016-08-12 2022-08-23 Posco High strength steel sheet having excellent formability and manufacturing method thereof
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CN104870678A (zh) 2015-08-26
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KR20150060957A (ko) 2015-06-03

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