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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • 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
  • C22C38/18—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
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips
• • 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
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a high-strength cold-rolled steel sheet having high tensile strength (TS): 980 MPa or more and excellent bendability, a high-strength plated steel sheet, and a method for producing them, which are useful for the use of a framework member for automobiles. .
• TS tensile strength
• Patent Document 1 in mass%, C: 0.05 to 0.30%, Si: 3.0% or less, Mn: 0.1 to 5.0%, P: 0.1% or less, S : Ferrite containing 0.02% or less, Al: 0.01 to 1.0%, N: 0.01% or less, with the balance being composed of iron and inevitable impurities, and being a soft first phase
• a technique for improving the bendability by relaxing the tensile / compressive stress sometimes applied to the surface layer portion is disclosed.
• Patent Document 2 in mass%, C: 0.050% to 0.40%, Si: 0.50% to 3.0%, Mn: 3.0% to 8.0%, P : 0.05% or less, S: 0.01% or less, sol. Al: 0.001% or more and 3.0% or less, and N: 0.01% or less, and by containing austenite with an area ratio of 10% or more and 40% or less, high strength, ductility and impact properties are obtained.
• An improved steel material is disclosed.
• Patent Document 3 in mass%, C: 0.075 to 0.300%, Si: 0.30 to 2.50%, Mn: 1.30 to 3.50%, P: 0.001 to 0 0.050%, S: 0.0001 to 0.0100%, Al: 0.005 to 1.500%, N: 0.0001 to 0.0100%, O: 0.0001 to 0.0100%
• a high-strength galvanized steel sheet having improved bendability by controlling the hardness distribution in the thickness direction of the high-strength galvanized steel sheet is disclosed.
• JP 2013-249502 A JP 2014-25091 A International Publication Number WO2013 / 018739
• Patent Document 2 utilizes the austenite phase, and stably austenite even to the steel sheet surface layer, which is important in bending, due to the effects of decarburization from the surface layer and changes in thermal history in the thickness direction. It is extremely difficult to generate a phase. Therefore, it is difficult to improve the bendability by the technique described in Patent Document 2.
• Patent Document 3 may cause a crack when strain is locally concentrated during bending.
• the present invention has been made in order to solve the above-mentioned problems, and the purpose thereof is tensile strength (TS): a high strength cold-rolled steel sheet having a high strength of 980 MPa or more and an excellent bendability, and a high strength plated steel sheet. And providing a manufacturing method thereof.
• TS tensile strength
• the average grain size of ferrite grains is 3.5 ⁇ m or less, the standard deviation of the grain diameter of ferrite grains is 1.5 ⁇ m or less, the average aspect ratio of ferrite grains is 1.8 or less, and the average grain diameter of martensite grains is 3. 0 ⁇ m or less, the average aspect ratio of martensite grains is 2.5 or less,
• the component composition further includes, in mass%, V: 0.001% to 0.3%, Ti: 0.001% to 0.1%, Nb: 0.001% to 0.08 % High-strength cold-rolled steel sheet according to [1].
• Fe 20.0% or less
• Al 0.001% or more.
• a steel material having the composition described in [1] or [2] is heated to 1050 ° C. or higher and 1300 ° C. or lower, and after finish rolling at a finish rolling temperature of 800 ° C. or higher, 500 ° C. or higher and 700 ° C. or lower.
• the first annealing step in which the time of staying in the temperature range of °C to 520 °C is 30 seconds or more, and the annealed plate after the first annealing step is heated to the highest reached temperature of 720 to 820 °C, Up to 560 ° C for annealed plates heated to the ultimate temperature
• a high-strength cold-rolled steel sheet or a high-strength plated steel sheet that is suitable for use as a structural member of an automobile and has good ductility and bendability can be obtained.
• the present invention improves the weight reduction and reliability of automobile parts.
• the high-strength cold-rolled steel sheet of the present invention is, in mass%, C: 0.07% to 0.17%, Si: less than 0.3%, Mn: 2.2% to 3.0%, P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.0060% or less, Mo: 0.07% or more and 0.50% or less, Cr: 0.001 % Or more and 0.4% or less.
• “%” means “% by mass”.
• C 0.07% or more and 0.17% or less C is an element that hardens martensite and substantially contributes to increasing the strength of the steel sheet.
• the C content needs to be 0.07% or more.
• the C content is set to 0.07% or more and 0.17% or less.
• the desirable C content for the lower limit is 0.08% or more, and the desirable C content for the upper limit is 0.15% or less.
• Si Less than 0.3% Si is an element that facilitates the formation of a ferrite phase. When the Si content is excessive, coarse ferrite grains remain during annealing, and a fine and sized ferrite phase cannot be obtained, resulting in a decrease in bendability. Therefore, in the present invention, the Si content needs to be less than 0.3%. A desirable Si content is 0.25% or less. Although the lower limit is not particularly defined, 0.01% Si may inevitably be mixed into the steel.
• Mn 2.2% or more and 3.0% or less Mn is effective for removing coarse ferrite grains contained in the steel structure during annealing, enhancing hardenability and generating fine ferrite grains during the staying process. It is an element. On the other hand, if it is excessively contained, ferrite phase formation is inhibited during the cooling and holding processes, and ductility and bendability are reduced. From the above viewpoint, the Mn content is set to 2.2% or more and 3.0% or less. The preferable Mn content for the lower limit is 2.3% or more, and the preferable Mn content for the upper limit is 2.8% or less.
• P 0.03% or less P is segregated at the grain boundary to cause grain boundary cracking during bending, and therefore, the P content is preferably reduced as much as possible.
• the P content is acceptable up to 0.03% or less.
• a preferable P content is 0.02% or less. Although it is desirable to reduce the P content as much as possible, 0.001% may be inevitably mixed in production.
• S 0.005% or less S is present as an inclusion such as MnS in steel. This inclusion becomes an inclusion extended in a wedge shape by rolling, and has a significant adverse effect on bendability. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and set it to 0.005% or less. A preferable S content is 0.003% or less. Although it is desirable to reduce the S content as much as possible, 0.0001% may be inevitably mixed in production.
• the Al content is preferably 0.02% or more.
• the Al content is 0.08% or less.
• the Al content is 0.08% or less.
• it is 0.07% or less.
• N 0.0060% or less N has an adverse effect on aging resistance. If the N content exceeds 0.0060%, the influence of deterioration over time on ductility and bendability cannot be ignored. For this reason, the upper limit of the N content is set to 0.0060%. A preferable N content is 0.0050% or less.
• Mo 0.07% or more and 0.50% or less
• Mo is an effective element for increasing the hardenability and generating a fine and sized ferrite phase.
• the Mo content needs to be at least 0.07%.
• the Mo content exceeds 0.50%, the area ratio of the martensite phase deviates from the range required by the present invention, so that ductility and bendability are reduced. From the above, the Mo content is set to 0.07% or more and 0.50% or less.
• the preferable Mo content for the lower limit is 0.07% or more, and the preferable Mo content for the upper limit is 0.30% or less.
• Cr 0.001% or more and 0.4% or less Cr, like Mn and Mo, is an element having an effect of improving hardenability. In order to obtain this effect, the Cr content needs to be at least 0.001%. On the other hand, when the Cr content exceeds 0.4%, the surface properties of the steel sheet are adversely affected, and the chemical conversion property and plating quality are deteriorated. Therefore, the Cr content is set to be 0.001% or more and 0.4% or less. A preferable Cr content for the lower limit is 0.005% or more, and a preferable Cr content for the upper limit is 0.3% or less.
• C content, Si content, Mn content, Mo content, and Cr content satisfy
• [% C], [% Si], [% Mn], [% Mo] and [% Cr] represent the contents of C, Si, Mn, Mo and Cr in mass%, respectively.
• the coefficient of each element of the formula (1) was obtained from the result of investigating the effect of each element.
• the left side of equation (1) is less than 3.15, coarse ferrite grains remain, or fine and sized ferrite phases cannot be obtained due to ferrite grain growth at high temperatures due to insufficient hardenability.
• the left side of equation (1) exceeds 4.30, it may be difficult to obtain a ferrite phase. Therefore, the left side of equation (1) is preferably 4.30 or less.
• the above is an essential component of the component composition of the high-strength cold-rolled steel sheet of the present invention.
• the component composition may include at least one selected from V, Ti, and Nb as an optional component.
• V 0.001% or more and 0.3% or less
• Ti 0.001% or more and 0.1% or less
• Nb 0.001% or more and 0.08% or less
• V 0.001% or more and 0.3% or less
• Ti 0.001% or more and 0.1% or less
• Nb 0.001% or more and 0.08% or less
• Components other than the above essential components and optional components are Fe and inevitable impurities.
• Inevitable impurities include components added in a range that does not impair the effects of the present invention in order to impart desired characteristics in addition to components inevitably mixed during production.
• content of the said arbitrary component is less than the said lower limit, the said arbitrary component shall be contained as an unavoidable impurity.
• the steel structure of the high-strength cold-rolled steel sheet of the present invention has a ferrite phase area ratio of 30% to 70% and a martensite phase area ratio of 30% to 70%.
• ferrite average particle size is 3.5 ⁇ m or less
• the standard deviation of ferrite particle size is 1.5 ⁇ m or less
• the average aspect ratio of ferrite particles is 1.8 or less
• the average particle size of martensite particles is 3.0 ⁇ m or less
• the average aspect ratio of martensite grains is 2.5 or less
• the length of the grain boundary between connected martensite grains satisfies the formula (2).
• the total area ratio is 10% or less of the area ratio of the martensite phase (total area ratio of all martensite grains).
• the work hardening of the ferrite phase is increased by the composite structure of the ferrite phase and the martensite phase.
• martensite grains tend to exist around the ferrite grains of the ferrite phase.
• the ferrite phase can be work-hardened. If the area ratio of either the ferrite phase or the martensite phase is excessively large, the desired structure cannot be obtained. From this viewpoint, the area ratio of the ferrite phase is set to 30% to 70% and the area ratio of the martensite phase is set to 30% to 70%.
• the area ratio of the preferable ferrite phase is 35% or more, and the area ratio of the ferrite phase preferable for the upper limit is 65% or less.
• the area ratio of the martensite phase preferable about the lower limit is 35% or more, and the area ratio of the martensite phase preferable about the upper limit is 65% or less.
• the ferrite average particle size is 3.5 ⁇ m or less, the standard deviation of the ferrite particle size is 1.5 ⁇ m or less, the average aspect ratio of the ferrite particles is 1.8 or less, and all ferrite particles are coarse or mixed. Is not uniformly cured by work. Furthermore, even if the shape of the ferrite grains is extended, the work hardening behavior is adversely affected. Therefore, in order to obtain a steel structure with a high work hardening index, it is necessary to use fine, equiaxed ferrite grains with no variation in grain size. is there.
• the average particle diameter of the ferrite grains is 3.5 ⁇ m or less, and the standard deviation of the ferrite grain diameter and the average aspect ratio of the ferrite grains are 1.5 ⁇ m or less and 1.8 or less, respectively.
• the average particle diameter of the ferrite grains is 3.0 ⁇ m or less, and the standard deviation of the ferrite grain diameter and the average aspect ratio of the ferrite grains are 1.0 ⁇ m or less and 1.5 or less, respectively.
• the mixed grain defined in the present invention refers to a group of crystal grains having an average aspect ratio larger than 1.8 and a variation in ferrite grain size (standard deviation of ferrite grain size exceeds 1.5 ⁇ m).
• the sizing means that the average aspect ratio is 1.8 or less and the standard deviation is 1.5 ⁇ m or less.
• Martensite average particle size is 3.0 ⁇ m or less
• martensite particles have an average aspect ratio of 2.5 or less
• the total area of martensite particles satisfying the following formula (2) is the length of grain boundaries between connected martensite particles
• the ratio of the ratio to the area ratio of the martensite phase 10% or less
• the average particle diameter of a martensite grain is limited to 3.0 micrometers or less.
• the length of the grain boundary between the connected martensite grains satisfies the following formula (2), and the martensite has a total area ratio of martensite.
• the ratio with respect to the area ratio of a phase shall be 10% or less, and the average aspect-ratio of a martensite grain shall be 2.5 or less.
• the average particle size is 2.5 ⁇ m or less, the ratio is 5% or less, and the average aspect ratio is 2.0 or less.
• L1 represents the length of the grain boundary between the martensite grains to be connected
• L2 represents the circumference of the martensite grains having a large particle diameter among the connected martensite grains.
• the area ratio, average particle diameter, standard deviation, average aspect ratio of each phase, and the ratio of the total area ratio of martensite grains satisfying the above formula (2) to the area ratio of the martensite phase are derived by the following method. .
• the average grain diameter of these ferrite grains (the diameter when the ferrite grains are approximated to a circle), and the average value of the aspect ratio of the ferrite grains (Average aspect ratio) and standard deviation of ferrite grain size are derived.
• the average value in one sample (10 fields of view) is calculated, and the average value in the whole (20 pieces) is calculated in the same manner for the other 19 samples.
• the average particle diameter and average aspect ratio of the martensite grains are calculated by the same method as the average grain diameter of ferrite grains and the average aspect ratio of ferrite grains.
• fills said (2) Formula is calculated
• the martensite to be connected refers to those in which martensite grains are connected with a grain boundary interposed therebetween. That is, in the martensite grains to be connected, the grain boundary is a part of the circumference of one martensite grain and a part of the circumference of the other martensite grain. The length of this grain boundary corresponds to “the length of the grain boundary between connected martensite grains”.
• the longer one is defined as L2.
• the total area ratio of the martensite grains satisfying the formula (2) is calculated, and the martensite of the total area ratio of the martensite grains satisfying the formula (2) is calculated from the area ratio and the area ratio of the martensite phase. The ratio to the area ratio of the phase is calculated.
• the average value is calculated in the same manner in all fields of view, and this average value is the ratio of the total area ratio of martensite grains satisfying the above formula (2) to the area ratio of the martensite phase.
• the sum total of the area ratio of the two martensite grain to connect will be an area ratio of the martensite grain which satisfy
• the right martensite grain is connected.
• the martensite grains in the center are martensite grains in the martensite phase that satisfy the formula (2).
• the manufacturing method of the high-strength cold-rolled steel sheet of the present invention includes a hot rolling process, a cold rolling process, a first annealing process, and a second annealing process.
• a hot rolling process a hot rolling process
• a cold rolling process a cold rolling process
• a first annealing process a first annealing process
• a second annealing process a second annealing process
• Hot rolling process is a process in which a steel material having the above component composition is heated to 1050 ° C. or higher and 1300 ° C. or lower, and finish rolling is finished at a finish rolling temperature of 800 ° C. or higher. This is a winding process.
• Steel melting method for obtaining a steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Then, it is preferable to use a slab (steel material) by a continuous casting method from the viewpoint of productivity and quality.
• the slab may be formed by a known casting method such as ingot-bundling rolling or continuous slab casting.
• the heating temperature of the steel material is 1050 ° C. or higher and 1300 ° C. or lower.
• the steel material obtained as described above is subjected to rough rolling and finish rolling. In the present invention, it is necessary to heat the steel material prior to rough rolling to obtain a substantially homogeneous austenite phase. If the heating temperature is lower than 1050 ° C, the hot rolling cannot be completed at a finish rolling temperature of 800 ° C or higher. On the other hand, when the heating temperature exceeds 1300 ° C., the scale is bitten, the surface properties of the hot-rolled steel sheet are deteriorated, and the yield is reduced due to the scale loss. Therefore, the heating temperature of the steel material is set to 1050 ° C. or higher and 1300 ° C. or lower.
• the steel material is 1100 degreeC or more and 1270 degrees C or less.
• the steel material after casting is in a temperature range of 1050 ° C. or more and 1300 ° C. or less, or if the carbide of the steel material is dissolved, the steel material is heated. Direct rolling may be performed without any problem.
• the rough rolling conditions are not particularly limited.
• the finish rolling temperature is 800 ° C or higher.
• the finish rolling temperature is 800 ° C. or higher.
• it is 820 degreeC or more.
• the upper limit of finish rolling temperature is not specifically limited, In this invention, it is 1000 degrees C or less normally.
• the winding temperature is 500 ° C or higher and 700 ° C or lower.
• the coiling temperature is below 500 ° C.
• the martensite phase is excessively generated, the deformation resistance during cold rolling is increased, and the thickness accuracy is adversely affected.
• the plate thickness accuracy is lowered, strain is localized during bending, which causes cracks.
• the coiling temperature range is set to 500 ° C. or more and 700 ° C. or less.
• a preferable winding temperature is 520 ° C. or more and 670 ° C. or less.
• the cold rolling step is a step of cold rolling the hot-rolled sheet after the hot rolling step.
• the conditions for cold rolling are not particularly limited, and the rolling reduction is preferably 30 to 80%.
• the first annealing process performed after the cold rolling process and the second annealing process performed after the first annealing process are performed twice.
• the reason why two annealings are essential in the present invention will be described.
• first annealing step it is necessary to completely recrystallize from the structure extended by cold rolling by heating, to eliminate coarse ferrite grains, and to generate a bainite single phase by staying after heating.
• Bainite is a structure containing cementite, which becomes a nucleation site for ferrite formation during the second annealing, so that the ferrite grains after the second annealing become fine and sized by increasing the number density of cementite.
• the first annealing step and the second annealing step will be described.
• the cold-rolled sheet after the cold rolling step is heated from 100 ° C. to a maximum attainable temperature of 825 ° C. or more under the condition that the average heating rate is 1.5 ° C./s or more and heated to the maximum attainable temperature.
• This is a step in which the cold-rolled sheet is cooled under the condition that the average cooling rate up to 560 ° C. is 12 ° C./s or more, and the residence time in the temperature range of 200 ° C. or more and 520 ° C. or less is 30 seconds or more.
• the heating condition of the cold-rolled sheet after the cold rolling step was set to an average heating rate from 100 ° C. to the highest temperature reached 1.5 ° C./s or higher, and the highest temperature reached 825 ° C.
• the heating conditions of the cold-rolled sheet after the cold rolling step are an average heating rate from 100 ° C. to the highest temperature reached 2.1 ° C./s or higher, and the highest temperature reached 830 ° C. or higher.
• the average heating rate becomes extremely high, the reverse transformation is caused without recrystallization, and the microstructure is likely to be non-uniform, so that the temperature is preferably 100 ° C./s or less.
• the upper limit of the maximum temperature reached is not particularly limited, but is usually 870 ° C. or lower in the present invention.
• the cold-rolled sheet heated to the highest temperature is cooled under the condition that the average cooling rate up to 560 ° C. is 12 ° C./s or more, and the time for staying in the temperature range of 200 ° C. or more and 520 ° C. or less is 30 seconds or more.
• the average cooling rate from the highest attained temperature to 560 ° C. needs to be 12 ° C./s or more. Preferably, it is 15 ° C./s or more.
• the time of staying in the temperature range of 300 ° C. or more and 500 ° C. or less is 30 seconds or more.
• the upper limit of the average cooling rate is not particularly limited, but it is preferably 50 ° C./s or less for the reason of suppressing the fluttering of the steel sheet in the annealing line and performing stable operation.
• the upper limit of the residence time is not particularly limited, but is preferably 300 seconds or less because productivity is lowered.
• the second annealing step means that the annealed plate after the first annealing step is heated to a maximum temperature of 720 ° C. or more and 820 ° C. or less, and an average cooling rate up to 560 ° C. is applied to the cold-rolled plate heated to the maximum temperature. This is a step of cooling under a condition of 12 ° C./s or more and setting the time for staying in a temperature range of 200 ° C. or more and 500 ° C. or less to 75 seconds or less.
• the second annealing process When heating in the second annealing process (second annealing process), it is necessary to have a structure containing fine ferrite and austenite. When the maximum temperature reached is below 720 ° C., austenite is not generated. When heated to a temperature exceeding 820 ° C., austenite becomes coarse and a desired structure cannot be obtained. From the above, the highest temperature reached in the heating of the annealed plate after the first annealing step was set to 720 ° C. or more and 820 ° C. or less. A preferable maximum temperature is 720 ° C. or higher and 810 ° C.
• the heated cold-rolled sheet up to the highest temperature is cooled under the condition that the average cooling rate up to 560 ° C. is 12 ° C./s or more, and the residence time in the temperature range of 200 ° C. to 500 ° C. is 75 seconds.
• the average cooling rate from the highest attained temperature to 560 ° C. is set to 12 ° C./s or more.
• a preferable average cooling rate is 15 ° C./s or more. Since the austenite phase is martensitic transformed after cooling, it is necessary to suppress the bainite transformation after cooling.
• the residence time in the temperature range from 200 ° C. to 500 ° C. was set to 75 seconds or less.
• the upper limit of the average cooling rate is not particularly limited, but it is preferably 50 ° C./s or less for the reason of suppressing the fluttering of the steel sheet in the annealing line and performing stable operation.
• the upper limit of the residence time is not particularly limited, but is preferably 300 seconds or less because productivity is lowered.
• the high-strength plated steel sheet of the present invention is obtained by forming a plating layer on the high-strength cold-rolled steel sheet.
• the plating layer may be a general one, and its component composition is, by mass%, Fe: 20.0% or less, Al: 0.001% or more and 1.0% or less, and further Pb, Sb , Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in total containing 0% to 3.5%
• the balance is a component composition consisting of Zn and inevitable impurities.
• This plating layer may be alloyed.
• the Fe content is less than 5.0%
• when it is an alloyed hot dipping layer the Fe content is 5.0% or more and 20.0% or less.
• the manufacturing method of the high-strength plated steel sheet can be performed by a general method. Moreover, what is necessary is just to perform the alloying process implemented as needed after the plating process which forms a plating layer by a general method.
• a steel material having a component composition shown in Table 1 and a wall thickness of 250 mm was made into a hot-rolled steel sheet under the hot-rolling conditions shown in Table 2, and cold-rolled at a cold rolling rate of 30% to 80%.
• an annealing process (first annealing process, second annealing process) is performed in a continuous annealing line under the conditions shown in Table 2, and a cold-rolled steel sheet (CR material) or melted in a continuous annealing line or continuous annealing galvanizing line.
• Galvanized steel sheets (“GI material” and “GA material”) were produced.
• the temperature was based on the steel sheet surface temperature measured with a multiple reflection thermometer.
• the temperature of the plating bath immersed in the continuous annealing hot dip galvanizing line (plating composition: Zn—0.13 mass% Al) is 460 ° C., and the amount of plating adhesion is 45 to 65 g / side for both GI and GA materials. and m 2, Fe content of the zinc plating layer is 6-14 wt%, Al content was 0.001 to 1.0 mass%.
• Specimens were collected from the cold-rolled steel sheet or plated steel sheet obtained as described above, and the structure was observed by the following method to evaluate the performance.
• the ferrite phase is a structure having a form in which corrosion marks and cementite are not observed in the grains
• the martensite phase is a structure in which no carbide is observed in the grains and observed with a white contrast compared to the ferrite phase.
• the area occupied by the desired phase with respect to the observation visual field area was defined as the area ratio of each phase, and was determined by image analysis. In this image analysis, the average particle diameter obtained from the equivalent circle diameters of ferrite grains and martensite grains, the standard deviation of ferrite grains, and the average aspect ratio of ferrite grains and martensite grains were also obtained.
• the aspect ratio was a quotient obtained by dividing the length of crystal grains in the rolling direction by the length of crystal grains in the plate thickness direction.
• the martensite grains to be connected are confirmed, the circumference of each martensite grain is obtained for the martensite grains, the longer circumference is L2, and the grain boundary length L1
• the total area ratio of the connected martensite grains which was 20% or more of the circumference, was determined and assigned from the area ratio of all martensite in the field of view. In Table 3, this area ratio was defined as “connected martensite area ratio”.

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