Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—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

Definitions

• the disclosure relates to a cold-rolled steel sheet with a sheet thickness of 0.4 mm or more and 3.0 mm or less suitable for use in vehicles, electric appliances, etc., and particularly relates to a cold-rolled steel sheet having excellent spot weldability with a tensile strength of 980 MPa or more and a manufacturing method therefor.
• cold-rolled steel sheets typically the steel sheets that have been formed are joined by welding and made into a desired finished shape.
• the base material of the cold-rolled steel sheets typically the area including the weld metal and the heat-affected zone is required to have excellent mechanical property.
• a conventional measure to ensure excellent weld property as cold-rolled steel sheets for vehicles typically limits the addition amounts of alloying elements for enhancing quench hardenability such as C and Mn and the addition amounts of impurity elements for facilitating the microsegregation of welds such as P and S.
• the steel sheets are heated to the melting point and then quenched.
• the weld metal becomes a solidified martensite single-phase structure in coarse columnar form.
• the heat-affected zone heated to a temperature range of Ac 3 point or more also becomes a relatively coarse martensite structure.
• the weld metal and the heat-affected zone of Ac 3 point or more are therefore higher in hardness than the base material, and susceptible to embrittlement.
• the heat-affected zone heated only to a temperature range less than Ac 3 point (hereafter also referred to as "heat-affected zone less than Ac 3 point”) is likely to decrease in strength due to tempering effect, and tends to have a higher softening degree with respect to the base material when the base material has higher strength.
• the weld typically has a discontinuous shape unlike the base material, so that stress tends to concentrate and residual stress due to welding heat hysteresis is unavoidable.
• the discontinuity of strength in the area from the weld metal through the heat-affected zone to the base material is significant, and the fracture strength of the spot weld is likely to be lower than that of the base material.
• High strength steel sheets proposed in JP 2012-167338 A (PTL 1), JP 4530606 B2 (PTL 2), JP 4883216 B2 (PTL 3), JP 5142068 B2 (PTL 4), JP 5323552 B2 (PTL 5), and the like fail to achieve both high strength of 980 MPa or more in tensile strength and sufficiently improved spot weldability while ensuring sufficient economic efficiency and productivity.
• excellent spot weldability means that the cross tensile strength is 10 kN/spot or more and the failure mode is plug failure in a cross tensile test according to JIS Z 3137 (1999), and the difference ⁇ HV between the maximum and minimum values of Vickers hardness in the area from the weld metal portion to the base material portion is less than 120 in a spot weld section test according to JIS Z 3139 (2009).
• the use of the cold-rolled steel sheet according to the disclosure improves manufacturing efficiency when producing steel structures such as vehicles and safety for vehicle occupants, and also improves fuel efficiency and thus significantly contributes to lower environmental burden.
• C is the most important element in strengthening the steel, and has high solid solution strengthening ability. To achieve such effect, the C content needs to be 0.05% or more. If the C content is more than 0.13%, martensite phase in the base material increases and significantly hardens the material, causing degradation in hole expansion formability. The C content is therefore limited to the range of 0.05% to 0.13%. The C content is preferably in the range of 0.06% to 0.12%.
• Si is an element necessary in steelmaking, acting as a deoxidizing material. Si also has an effect of dissolving in the steel to strengthen the steel sheet by solid solution strengthening. To achieve such effects, the Si content needs to be 0.05% or more. If the Si content is more than 2.0%, the toughness of the weld metal and heat-affected zone degrades significantly, causing lower fracture strength of the weld. The Si content is therefore limited to the range of 0.05% to 2.0%. The Si content is preferably in the range of 0.10% to 1.60%.
• Mn has an effect of increasing the quench hardenability of the steel at relatively low cost.
• the Mn content needs to be 1.5% or more. If the Mn content is more than 4.0%, the fracture strength of the weld decreases, and the microsegregation of the base material increases, promoting a delayed fracture originating from the base material segregation area.
• the Mn content is therefore limited to the range of 1.5% to 4.0%.
• the Mn content is preferably in the range of 1.7% to 3.8%.
• P is an element having high solid solution strengthening ability, but promotes microsegregation as with Mn. Accordingly, if the P content is more than 0.05%, not only the base material embrittles but also the grain boundary segregation area tends to become a delayed fracture origin. Hence, the P content is desirably minimized with the upper limit being 0.05%. Excessively reducing P, however, involves high refining cost and is economically disadvantageous. Therefore, the lower limit of the P content is desirably about 0.005%.
• the S content is desirably minimized with the upper limit being 0.005%.
• Al acts as a deoxidizer, and is the most generally used element in the molten steel deoxidizing process for steel sheets. Al also has an effect of fixing solute N in the steel to form AlN, thus suppressing embrittlement caused by solute N. To achieve such effects, the Al content needs to be 0.01% or more. If the Al content is more than 0.10%, surface cracking during slab manufacture is promoted. The Al content is therefore limited to the range of 0.01% to 0.10%. The Al content is preferably in the range of 0.02% to 0.07%.
• the Cr has an effect of increasing the quench hardenability of the steel at relatively low cost, and is an element that delays the bainite transformation of intermediate hardness phase in the annealing process and generates martensite of high hardness phase to contribute to improved strength of the steel.
• the Cr content needs to be 0.05% or more. If the Cr content is more than 1.0%, not only an excessive strength increase promotes embrittlement, but also an economic disadvantage is entailed.
• the Cr content is therefore limited to the range of 0.05% to 1.0%.
• the Cr content is preferably in the range of 0.07% to 0.8%.
• Nb is an important element that, in annealing heating after cold rolling, exists as solute Nb to produce a solute drag effect and delay the recrystallization of the deformed microstructure generated in cold rolling, thus strengthening the steel sheet after annealing.
• NbC generated in the hot rolling process and annealing process refines the microstructure in the base material and heat-affected zone, and improves toughness.
• the Nb content needs to be 0.010% or more. If the Nb content is more than 0.070%, coarse carbonitride precipitates, which promotes surface cracking during slag manufacture and may also become a fracture origin.
• the Nb content is therefore limited to the range of 0.010% to 0.070%.
• the Nb content is preferably in the range of 0.015% to 0.060%.
• Ti is an important alloying element in the disclosure.
• solute N By fixing solute N to form TiN, Ti has an effect of suppressing the coarsening of crystal grains in the weld metal and heat-affected zone and an effect of suppressing embrittlement by reducing solute N.
• TiN By forming TiN, Ti suppresses the generation of Nb nitride to secure a predetermined amount of solute Nb in the hot rolling and annealing steps, thus effectively contributing to higher strength of the steel sheet after annealing.
• the Ti content needs to be 0.005% or more. If the Ti content is more than 0.040%, very hard and brittle TiC precipitates, which promotes embrittlement. The Ti content is therefore limited to the range of 0.005% to 0.040%. The Ti content is preferably in the range of 0.010% to 0.035%.
• N is contained in the steel as incidental impurity.
• N forms TiN, and thus has an effect of suppressing the coarsening of crystal grains in the weld metal and heat-affected zone during welding.
• the N content needs to be 0.0005% or more. If the N content is more than 0.0065%, an increase of solute N causes a significant decrease in anti-aging property. The N content is therefore limited to the range of 0.0005% to 0.0065%.
• the N content is preferably in the range of 0.0010% to 0.0060%.
• Ti/N 2.5 or more and 7.5 or less
• Ti/N is less than 2.5, solute N in the steel sheet increases, which promotes embrittlement. If Ti/N is more than 7.5, very hard and brittle TiC is generated in the steel sheet, causing lower ductility and significant embrittlement. Ti/N is therefore limited to the range of 2.5 to 7.5. Ti/N is preferably in the range of 3.0 to 7.0.
• Mo is an element that contributes to improved strength of the steel. To achieve such effect, the Mo content needs to be 0.01% or more. If the Mo content is more than 1.0%, not only an excessive strength increase promotes embrittlement, but also an economic disadvantage is entailed. Accordingly, in the case of adding Mo, the Mo content is in the range of 0.01% to 1.0%. The Mo content is preferably in the range of 0.03% to 0.8%.
• Cu is an element that contributes to improved strength of the steel. If the Cu content is more than 1.0%, however, hot shortness occurs and the surface characteristics of the steel sheet degrade. Accordingly, in the case of adding Cu, the Cu content is 1.0% or less.
• Ni is an element that contributes to improved strength of the steel. If the Ni content is more than 1.0%, however, the effect saturates, which is economically disadvantageous. Accordingly, in the case of adding Ni, the Ni content is 1.0% or less.
• V is an element that contributes to improved strength of the steel. If the V content is more than 0.1%, however, the ductility of the base material degrades. Accordingly, in the case of adding V, the V content is 0.1% or less.
• the balance other than the aforementioned components is Fe and incidental impurities.
• Proportion of Ti existing as precipitate in steel 70 mass% or more
• Ti precipitate refines the structure, thus improving the hole expansion formability of the eventually obtained cold-rolled steel sheet.
• Ti exists as a precipitate in the cold-rolled steel sheet after annealing, the coarsening of crystal grains in the heat-affected zone due to welding heat hysteresis during welding is suppressed, so that the fracture strength of the weld is improved.
• 70 mass% or more of Ti in the steel need to exist as a precipitate.
• the proportion of Ti existing as a precipitate in the steel is preferably 75 mass% or more.
• the upper limit of the proportion of Ti existing as a precipitate in the steel is not particularly limited. If the proportion is 100 mass%, however, toughness degrades significantly due to remaining solute N. Accordingly, the proportion of Ti existing as a precipitate in the steel is preferably less than 100 mass%, and more preferably less than 98 mass%.
• the form of the precipitate is mainly a single precipitate of TiN or a composite precipitate of TiN and another precipitate. Even when Ti oxide or Ti carbide is mixed, its effect is negligible as long as Ti oxide or Ti carbide is less than 10% of the total number of Ti-based precipitates. The existence form of Ti in the steel other than a precipitate is solute Ti.
• Nb existing as a solute has an effect of suppressing recrystallization during heating in the annealing process to effectively contribute to higher strength of the steel and also has an effect of suppressing the softening of the heat-affected zone less than Ac 3 point.
• Nb in the steel need to exist as solute Nb.
• the proportion ofNb existing as solute Nb in the steel is preferably 20 mass% or more.
• the upper limit of the proportion of Nb existing as solute Nb in the steel is not particularly limited. If the amount of solute Nb in the steel is excessively high, however, the aforementioned effects saturate, and the manufacturing cost increases. Accordingly, the proportion of Nb existing as solute Nb in the steel is preferably 70 mass% or less.
• Nb precipitate The existence form of Nb in the steel other than solute Nb is Nb precipitate.
• Nb precipitate examples include Nb carbide and Nb carbonitride such as NbC.
• the temperature of the steel sheet in the manufacturing conditions is the surface temperature of the steel sheet.
• Molten steel having the aforementioned chemical composition is obtained by steelmaking using a known method such as a converter or an electric heating furnace, and made into a steel material such as a slab having predetermined dimensions using a known method such as continuous casting or ingot casting and blooming.
• the molten steel may also be subjected to treatment such as refining with a ladle or vacuum degassing.
• the obtained steel material is immediately or temporarily cooled, heated to a temperature range of (Ts - 50) °C or more and (Ts + 200) °C or less, and hot rolled with a finisher delivery temperature of 850 °C or more.
• the steel material is then coiled at 650 °C or less, to form a hot-rolled steel sheet.
• Heating temperature (Ts - 50) °C or more and (Ts + 200) °C or less
• Carbonitride containing coarse Nb which has crystallized during the steelmaking of the steel material does not contribute to higher strength of the steel sheet. It is therefore important to temporarily dissolve such coarse Nb-based crystallized product in the steel in the heating stage before hot rolling, and precipitate it again as fine Nb carbide or carbonitride in the subsequent processes such as rolling, cooling, and annealing.
• the heating temperature is therefore (Ts - 50) °C or more and (Ts + 200) °C or less.
• the heating temperature is preferably (Ts - 20) °C or more and (Ts + 170) °C or less.
• Finisher delivery temperature 850 °C or more
• finisher delivery temperature is less than 850 °C, not only rolling efficiency drops, but also the rolling load increases, causing a greater load on the mill.
• the finisher delivery temperature is therefore 850 °C or more.
• Coiling temperature 650 °C or less
• the coiling temperature for the hot-rolled steel sheet is more than 650 °C, NbC which precipitates during coiling coarsens excessively, which facilitates embrittlement and is likely to provide a fracture origin.
• the coiling temperature for the hot-rolled steel sheet therefore needs to be 650 °C or less.
• the coiling temperature for the hot-rolled steel sheet is preferably 620 °C or less.
• the lower limit of the coiling temperature for the hot-rolled steel sheet need not be particularly limited. Given that an excessive temperature decrease causes lower manufacturing efficiency, however, the lower limit is preferably about 400 °C.
• the obtained hot-rolled steel sheet is then cold rolled into a cold-rolled steel sheet.
• the cold rolling conditions need not be particularly limited. To ensure desired strength after annealing, however, the total rolling reduction is preferably 30% or more. Moreover, to avoid an excessive load on the mill, the total rolling reduction is preferably 80% or less.
• the cold-rolled steel sheet obtained in this way is then continuously annealed under the following conditions.
• Heating temperature in continuous annealing 700 °C or more and 900 °C or less
• the heating temperature in continuous annealing is less than 700 °C, the reverse transformation of austenite is insufficient, and the amount of hard martensite or bainite generated in the subsequent cooling is insufficient, making it impossible to obtain desired strength. If the heating temperature in continuous annealing is more than 900 °C, austenite grains coarsen considerably, causing degradation in hole expansion formability of the base material and toughness of the heat-affected zone.
• the heating temperature in continuous annealing is therefore 700 °C or more and 900 °C or less.
• the heating temperature in continuous annealing is preferably 720 °C or more and 880 °C or less.
• the holding time after heating need not be particularly limited. To ensure a uniform temperature distribution and a stable microstructure, however, the holding time is preferably 15 s or more. Meanwhile, a long holding time causes not only lower manufacturing efficiency but also coarser austenite grains, and so the holding time is preferably 600 s or less.
• Average cooling rate 12 °C/s or more and 100 °C/s or less
• the average cooling rate in the cooling process after heating in continuous annealing is less than 12 °C/s, soft ferrite phase is generated excessively during cooling, making it difficult to ensure desired strength. Besides, Nb reprecipitates excessively in the middle of cooling, making it difficult to secure a desired amount of solute Nb. Further, coarse ferrite phase or pearlite phase is generated in the middle of cooling, leading to a decrease in strength. If the average cooling rate after annealing is more than 100 °C/s, it is difficult to secure the shape of the steel sheet.
• the average cooling rate after annealing treatment is therefore 12 °C/s or more and 100 °C/s or less.
• the average cooling rate is preferably 14 °C/s or more and 70 °C/s or less.
• Cooling stop temperature 200 °C or more and 450 °C or less
• the cooling stop temperature is less than 200 °C, the conveyance speed for the steel sheet is to be lowered extremely, which is not preferable in terms of manufacturing efficiency. If the cooling stop temperature is more than 450 °C, relatively soft bainite phase is generated excessively after the cooling stop, making it difficult to ensure desired strength. Besides, Nb reprecipitates excessively after the cooling stop, making it difficult to secure a desired amount of solute Nb. Further, a soft structure such as ferrite is generated excessively, leading to insufficient strength.
• the cooling stop temperature is therefore 200 °C or more and 450 °C or less.
• the cooling stop temperature is preferably 230 °C or more and 420 °C or less.
• the holding time in the cooling stop temperature range is less than 30 s, the uniformity of the temperature and material in the steel sheet decreases. If the holding time in the cooling stop temperature range is more than 600 s, manufacturing efficiency decreases. The holding time in the cooling stop temperature range is therefore 30 s or more and 600 s or less.
• An electroextraction test piece was collected from each cold-rolled steel sheet obtained as mentioned above, and subjected to electrolytic treatment using a AA electrolytic solution (ethanol solution of acetylacetone tetramethylammonium chloride), to extract a residue by filtration.
• AA electrolytic solution ethanol solution of acetylacetone tetramethylammonium chloride
• the extracted residue was set to a constant volume of 100 ml using pure water, and the amount of Ti was measured by high-frequency inductively coupled plasma (ICP) emission spectrometry as the amount of Ti existing as a precipitate. Likewise, the amount of Nb in the extracted residue was measured, and the measured amount of Nb was subtracted from the total amount of Nb in the test piece to calculate the amount of solute Nb.
• ICP inductively coupled plasma
• Each cold-rolled steel sheet obtained as mentioned above was used to form a cross tensile test piece according to JIS Z 3137 (1999). Spot welding in the formation of the cross tensile test piece was performed under the welding conditions of a nugget diameter of 6.0 mm according to the Japan Welding Engineering Society Standard: WES7301.
• the formed cross tensile test piece was then subjected to a cross tensile test according to JIS Z 3137 (1999). Each sample with a cross tensile strength of 10 kN/spot or more and a failure mode of plug failure was determined as excellent in spot weldability.
• Comparative Examples had insufficient performance in at least one of the tensile strength and total elongation of the base material and the cross tensile strength, the failure mode, and the difference ( ⁇ HV) between the maximum and minimum values of Vickers hardness in the spot weld test.

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