WO2016194272A1 - High-strength cold-rolled steel sheet, high-strength plated steel sheet, and method for producing same - Google Patents
High-strength cold-rolled steel sheet, high-strength plated steel sheet, and method for producing same Download PDFInfo
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- WO2016194272A1 WO2016194272A1 PCT/JP2016/001437 JP2016001437W WO2016194272A1 WO 2016194272 A1 WO2016194272 A1 WO 2016194272A1 JP 2016001437 W JP2016001437 W JP 2016001437W WO 2016194272 A1 WO2016194272 A1 WO 2016194272A1
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- 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
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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
- C22C38/38—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
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- 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
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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
- C22C38/22—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
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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
- C22C38/26—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
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- 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
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- 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
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- 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
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- 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”.
Abstract
Description
0.05[%C]-2.6[%Si]+1.2[%Mn]+2.6[%Mo]+[%Cr]≧3.15 (1)
(1)式における、[%C]、[%Si]、[%Mn]、[%Mo]および[%Cr]はそれぞれ質量%でのC、Si、Mn、MoおよびCr含有量を表す。
L1≧0.2×L2 (2)
(2)式における、L1は連結するマルテンサイト粒間の粒界の長さを表し、L2は連結するマルテンサイト粒のうち粒径の大きいマルテンサイト粒の周長を表す。 [1] By mass%, C: 0.07% or more and 0.17% or less, Si: less than 0.3%, Mn: 2.2% or more and 3.0% or less, 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 The following composition (1) is satisfied, the balance is composed of Fe and inevitable impurities, the ferrite phase area ratio is 30% to 70%, and the martensite phase area ratio is 30% to 70%. The average grain size of ferrite grains is 3.5 μm or less, the standard deviation of the grain diameter of ferrite grains is 1.5 μm 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 μm or less, the average aspect ratio of martensite grains is 2.5 or less, A steel structure in which the total area ratio of martensite grains satisfying the following formula (2) is 10% or less of the area ratio of the martensite phase: A high-strength cold-rolled steel sheet having a strength of 980 MPa.
0.05 [% C] -2.6 [% Si] +1.2 [% Mn] +2.6 [% Mo] + [% Cr] ≧ 3.15 (1)
In the formula (1), [% C], [% Si], [% Mn], [% Mo] and [% Cr] represent the contents of C, Si, Mn, Mo and Cr in mass%, respectively.
L1 ≧ 0.2 × L2 (2)
In the formula (2), L1 represents the length of the grain boundary between the martensite grains to be connected, and L2 represents the circumference of the martensite grains having a large particle diameter among the connected martensite grains.
成分組成
本発明の高強度冷延鋼板は、質量%で、C:0.07%以上0.17%以下、Si:0.3%未満、Mn:2.2%以上3.0%以下、P:0.03%以下、S:0.005%以下、Al:0.08%以下、N:0.0060%以下、Mo:0.07%以上0.50%以下、Cr:0.001%以上0.4%以下を含有する。以下の各成分の説明において「%」は「質量%」を意味する。 <High strength cold-rolled steel sheet>
Component composition 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. In the following description of each component, “%” means “% by mass”.
Cはマルテンサイトを硬化させ、実質的に鋼板の高強度化に寄与する元素である。本発明で求める引張強さ:980MPa以上を得るには、C含有量は0.07%以上にする必要がある。一方で、C含有量が0.17%を超えると、マルテンサイト相の面積率が過度に増大してしまい、フェライト相の加工硬化を活用することができなくなり、延性および曲げ性が低下する。したがって、C含有量は0.07%以上0.17%以下とする。下限について望ましいC含有量は0.08%以上であり、上限について望ましいC含有量は0.15%以下である。 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. In order to obtain the tensile strength required in the present invention: 980 MPa or more, the C content needs to be 0.07% or more. On the other hand, when the C content exceeds 0.17%, the area ratio of the martensite phase increases excessively, and the work hardening of the ferrite phase cannot be utilized, and ductility and bendability are reduced. Therefore, 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はフェライト相を生成させやすくする元素である。Si含有量が過剰になると、焼鈍中に粗大なフェライト粒が残存し、微細かつ整粒なフェライト相が得られなくなり、曲げ性が低下する。そのため、本発明ではSi含有量を0.3%未満とする必要がある。望ましいSi含有量は0.25%以下である。下限は特に定めないが、0.01%のSiは不可避的に鋼中に混入する場合がある。 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は、焼鈍中の鋼組織に含まれる粗大なフェライト粒を除去し、焼入性を高めて滞留過程で微細フェライト粒を生成させるために有効な元素である。一方で、過度に含有させると冷却および保持過程でフェライト相生成が阻害され、延性および曲げ性が低下する。以上の観点から、Mn含有量は2.2%以上3.0%以下とした。下限について好ましいMn含有量は2.3%以上であり、上限について好ましいMn含有量は2.8%以下である。 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は、粒界に偏析することで曲げ加工時に粒界割れの要因となるため、P含有量は極力低減することが好ましい。本発明ではP含有量が0.03%以下まで許容できる。好ましいP含有量は0.02%以下である。P含有量は極力低減する方が望ましいが、製造上、0.001%は不可避的に混入する場合がある。 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. In the present invention, 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は、鋼中でMnSなどの介在物として存在する。この介在物は、圧延により楔状に伸展した介在物となり、曲げ性に著しい悪影響をもたらす。したがって、本発明では、S含有量を極力低減することが好ましく、0.005%以下とする。好ましいS含有量は0.003%以下である。S含有量は極力低減する方が望ましいが、製造上、0.0001%は不可避的に混入する場合がある。 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.
Alを製鋼の段階で脱酸剤として添加する場合、Al含有量が0.02%以上であることが好ましい。一方で、Al含有量が0.08%を超えるとアルミナなどの介在物の影響により曲げ性への悪影響が顕在化する。したがって、Al含有量は0.08%以下とする。好ましくは0.07%以下である。 Al: 0.08% or less When Al is added as a deoxidizer at the stage of steelmaking, the Al content is preferably 0.02% or more. On the other hand, when the Al content exceeds 0.08%, an adverse effect on bendability becomes obvious due to the influence of inclusions such as alumina. Therefore, the Al content is 0.08% or less. Preferably it is 0.07% or less.
耐時効性に対し、Nは悪影響をもたらす。N含有量が0.0060%を上回ると、延性および曲げ性への経時劣化の影響が無視できなくなる。このため、N含有量上限は0.0060%とした。好ましいN含有量は0.0050%以下である。 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はMnと同様、焼入性を高め微細かつ整粒なフェライト相を生成させるために有効な元素である。この効果を得るには、Mo含有量が少なくとも0.07%である必要がある。一方で、Mo含有量が0.50%を上回るとマルテンサイト相の面積率が本発明で求める範囲から逸脱するため、延性および曲げ性が低下する。以上から、Mo含有量は0.07%以上0.50%以下とした。下限について好ましいMo含有量は0.07%以上であり、上限について好ましいMo含有量は0.30%以下である。 Mo: 0.07% or more and 0.50% or less Mo, like Mn, is an effective element for increasing the hardenability and generating a fine and sized ferrite phase. In order to obtain this effect, the Mo content needs to be at least 0.07%. On the other hand, if 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もMnおよびMoと同様に焼入性を高める効果がある元素である。この効果を得るには、Cr含有量が少なくとも0.001%である必要がある。一方で、Cr含有量が0.4%を超えると鋼板の表面特性に悪影響をもたらし、化成処理性やめっき品質が低下する。そこで、Cr含有量は0.001%以上0.4%以下とした。下限について好ましいCr含有量は0.005%以上であり、上限について好ましいCr含有量は0.3%以下である。 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.
0.05[%C]-2.6[%Si]+1.2[%Mn]+2.6[%Mo]+[%Cr]≧3.15 (1)
(1)式における、[%C]、[%Si]、[%Mn]、[%Mo]および[%Cr]はそれぞれ質量%でのC、Si、Mn、MoおよびCr含有量を表す。 Moreover, as for the component composition of this invention, C content, Si content, Mn content, Mo content, and Cr content satisfy | fill following (1) Formula.
0.05 [% C] -2.6 [% Si] +1.2 [% Mn] +2.6 [% Mo] + [% Cr] ≧ 3.15 (1)
In the formula (1), [% C], [% Si], [% Mn], [% Mo] and [% Cr] represent the contents of C, Si, Mn, Mo and Cr in mass%, respectively.
上記の元素は主に析出物として微細に分散するため、鋼板の高強度化に寄与する元素である。一方で、これらの元素を過度に含有させた場合は、スラブ加熱時に溶解せず粗大な炭化物として残存する。粗大な炭化物は曲げ性に悪影響をもたらす。以上の観点から、V含有量は0.001%以上0.3%以下、Ti含有量は0.001%以上0.1%以下、Nb含有量は0.001%以上0.08%以下とした。 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 Is an element that contributes to increasing the strength of the steel sheet. On the other hand, when these elements are contained excessively, they do not dissolve during slab heating and remain as coarse carbides. Coarse carbides adversely affect bendability. From the above viewpoint, the V content is 0.001% to 0.3%, the Ti content is 0.001% to 0.1%, and the Nb content is 0.001% to 0.08%. did.
本発明の高強度冷延鋼板の鋼組織は、フェライト相の面積率が30%以上70%以下、マルテンサイト相の面積率が30%以上70%以下であり、フェライト粒の平均粒径(フェライト平均粒径という場合がある。)が3.5μm以下、フェライト粒の粒径の標準偏差が1.5μm以下、フェライト粒の平均アスペクト比が1.8以下、マルテンサイト粒の平均粒径(マルテンサイト平均粒径)が3.0μm以下、マルテンサイト粒の平均アスペクト比が2.5以下であり、連結するマルテンサイト粒間の粒界の長さが(2)式を満たすマルテンサイト粒の面積率の合計がマルテンサイト相の面積率(全マルテンサイト粒の合計面積率)の10%以下である。 Steel structure 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%. (Sometimes referred to as ferrite average particle size) is 3.5 μm or less, the standard deviation of ferrite particle size is 1.5 μm or less, the average aspect ratio of ferrite particles is 1.8 or less, the average particle size of martensite particles (Martensite average particle diameter) is 3.0 μm or less, the average aspect ratio of martensite grains is 2.5 or less, and 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).
本発明では、フェライト相とマルテンサイト相との複合組織によってフェライト相の加工硬化上昇を実現している。この複合組織においては、フェライト相のフェライト粒の周囲にマルテンサイト粒が存在する傾向にある。この傾向にある鋼組織となることで、フェライト相を加工硬化させることができる。フェライト相およびマルテンサイト相のいずれかの組織の面積率が過度に大きくなると目的の組織が得られなくなる。この観点から、フェライト相の面積率が30%以上70%以下、マルテンサイト相の面積率が30%以上70%以下とする。下限について、好ましいフェライト相の面積率が35%以上であり、上限について好ましいフェライト相の面積率は65%以下である。また、下限について好ましいマルテンサイト相の面積率が35%以上であり、上限について好ましいマルテンサイト相の面積率は65%以下である。 Ferrite phase: 30% or more and 70% or less, martensite phase: 30% or more and 70% or less In the present invention, the work hardening of the ferrite phase is increased by the composite structure of the ferrite phase and the martensite phase. In this composite structure, martensite grains tend to exist around the ferrite grains of the ferrite phase. By forming a steel structure having this tendency, 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%. Regarding the lower limit, 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. Moreover, 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.
フェライト粒が粗大もしくは混粒した状態であると、全てのフェライト粒が均一に加工硬化されなくなる。さらに、フェライト粒が伸展した形状であっても加工硬化挙動に悪影響をもたらすため、加工硬化指数が高い鋼組織を得るには、微細で粒径にばらつきがない等軸のフェライト粒とする必要がある。そのため、フェライト粒の平均粒径が3.5μm以下、かつフェライト粒径の標準偏差およびフェライト粒の平均アスペクト比がそれぞれ1.5μm以下および1.8以下とした。好ましくは、フェライト粒の平均粒径が3.0μm以下、かつフェライト粒径の標準偏差およびフェライト粒の平均アスペクト比がそれぞれ1.0μm以下および1.5以下である。本発明で定義する混粒とは、平均アスペクト比が1.8より大きくフェライト粒径にばらつきがある(フェライト粒径の標準偏差が1.5μm超え)結晶粒群を指す。また、整粒は上記平均アスペクト比が1.8以下、標準偏差が1.5μm以下であることを指す。 The ferrite average particle size is 3.5 μm or less, the standard deviation of the ferrite particle size is 1.5 μm 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. Therefore, the average particle diameter of the ferrite grains is 3.5 μm or less, and the standard deviation of the ferrite grain diameter and the average aspect ratio of the ferrite grains are 1.5 μm or less and 1.8 or less, respectively. Preferably, the average particle diameter of the ferrite grains is 3.0 μm or less, and the standard deviation of the ferrite grain diameter and the average aspect ratio of the ferrite grains are 1.0 μm 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 μm). In addition, the sizing means that the average aspect ratio is 1.8 or less and the standard deviation is 1.5 μm or less.
上記の通り、多くのフェライト粒がマルテンサイト粒と接触した状態にすることでフェライト相の高い加工硬化を得ることができる。そのため、存在するマルテンサイト粒が粗大であった場合は、マルテンサイト粒の分布が局在化するため、所望の組織が得られない。また、微細なマルテンサイト粒が分散したマルテンサイト相としてもマルテンサイト粒同士が連結した状態であると加工硬化が低下する。そのため、微細なマルテンサイトであるためにマルテンサイト粒の平均粒径を3.0μm以下に限定する。また、マルテンサイト粒同士が連結した状態にほとんどならないことを特定するために、連結するマルテンサイト粒間の粒界の長さが下記(2)式を満たすマルテンサイト粒の合計面積率のマルテンサイト相の面積率に対する割合を10%以下、マルテンサイト粒の平均アスペクト比を2.5以下とする。好ましくは、上記平均粒径が2.5μm以下、上記割合が5%以下、上記平均アスペクト比が2.0以下である。
L1≧0.2×L2 (2)
(2)式における、L1は連結するマルテンサイト粒間の粒界の長さを表し、L2は連結するマルテンサイト粒のうち粒径の大きいマルテンサイト粒の周長を表す。 Martensite average particle size is 3.0 μm or less, martensite particles have an average aspect ratio of 2.5 or less, and 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 As described above, a high work hardening of the ferrite phase can be obtained by bringing many ferrite grains into contact with the martensite grains. Therefore, when the existing martensite grains are coarse, the distribution of the martensite grains is localized, so that a desired structure cannot be obtained. Moreover, when the martensite phase in which fine martensite grains are dispersed is in a state where the martensite grains are connected to each other, work hardening is reduced. Therefore, since it is a fine martensite, the average particle diameter of a martensite grain is limited to 3.0 micrometers or less. In addition, in order to specify that the martensite grains are hardly connected to each other, 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. Preferably, the average particle size is 2.5 μm or less, the ratio is 5% or less, and the average aspect ratio is 2.0 or less.
L1 ≧ 0.2 × L2 (2)
In the formula (2), L1 represents the length of the grain boundary between the martensite grains to be connected, and L2 represents the circumference of the martensite grains having a large particle diameter among the connected martensite grains.
次に、本発明の鋼板の製造方法について説明する。 <Method for producing high-strength cold-rolled steel sheet>
Next, the manufacturing method of the steel plate of this invention is demonstrated.
熱間圧延工程とは、上記成分組成を有する鋼素材を、1050℃以上1300℃以下に加熱し、800℃以上の仕上げ圧延温度で仕上げ圧延終了後、500℃以上700℃以下で巻き取る工程である。 Hot rolling process The 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.
冷間圧延工程とは、上記熱間圧延工程後に熱延板を冷間圧延する工程である。冷間圧延の条件は特に限定されず、圧下率が30~80%であることが好ましい。 Cold rolling step 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%.
本発明の製造方法においては、冷間圧延工程後に行われる第一焼鈍工程、該第一焼鈍工程後に行われる第二焼鈍工程の2回の焼鈍を行う。先ず、本発明において2回の焼鈍が必須である理由を説明する。 Annealing process In the manufacturing method of the present invention, the first annealing process performed after the cold rolling process and the second annealing process performed after the first annealing process are performed twice. First, the reason why two annealings are essential in the present invention will be described.
本発明の高強度めっき鋼板は、上記高強度冷延鋼板上にめっき層が形成されたものである。めっき層は一般的なものであればよく、その成分組成は、質量%で、Fe:20.0%以下、Al:0.001%以上1.0%以下を含有し、さらに、Pb、Sb、Si、Sn、Mg、Mn、Ni、Cr、Co、Ca、Cu、Li、Ti、Be、Bi及びREMから選択する1種または2種以上を合計で0%以上3.5%以下含有し、残部がZn及び不可避的不純物からなる成分組成である。このめっき層は合金化処理されたものであってもよい。なお、めっき層が溶融めっき層の場合にはFe含有量が5.0%未満であり、合金化溶融めっき層の場合にはFe含有量が5.0%以上20.0%以下である。 <High-strength plated steel sheet and method for producing the same>
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. In addition, when the plating layer is a hot dipping layer, the Fe content is less than 5.0%, and when it is an alloyed hot dipping layer, the Fe content is 5.0% or more and 20.0% or less.
各相の面積率は以下の手法により評価した。およそ1000mの冷延鋼板、めっき鋼板から、コイル長手方向に対し50m間隔で20個のサンプルを採取した。圧延方向に平行な断面が観察面となるよう切り出し、板厚中心部を1%ナイタールで腐食現出し、走査型電子顕微鏡で2000倍に拡大して板厚1/4t部(鋼板表面から板厚方向に1/4tの位置(tは板厚))を10視野分撮影した。フェライト相は粒内に腐食痕やセメンタイトが観察されない形態を有する組織であり、マルテンサイト相は粒内に炭化物が認められず、フェライト相よりも白いコントラストで観察される組織である。観察視野面積に対し、求める相が占める面積を各相の面積率とし、画像解析により求めた。また、この画像解析で、フェライト粒およびマルテンサイト粒の円相当径から求めた平均粒径、フェライト粒の標準偏差、フェライト粒およびマルテンサイト粒の平均アスペクト比も求めた。アスペクト比は圧延方向の結晶粒の長さを板厚方向の結晶粒の長さで割った商とした。また、この画像解析で、連結するマルテンサイト粒を確認し、該マルテンサイト粒を対象に、それぞれのマルテンサイト粒の周長を求め、長い方の周長をL2とし、粒界の長さL1が周長に対し20%以上であった連結マルテンサイト粒の面積率の合計を求め、視野に占める全マルテンサイトの面積率から割り付けた。表3では、この面積率を“連結マルテンサイト面積率”とした。 (I) Structure observation image The area ratio of each phase was evaluated by the following method. Twenty samples were collected from a cold rolled steel sheet and a plated steel sheet of about 1000 m at intervals of 50 m in the coil longitudinal direction. Cut out so that the cross-section parallel to the rolling direction becomes the observation surface, the center of the plate thickness appears to be corroded with 1% nital, and is magnified 2000 times with a scanning electron microscope to obtain a plate thickness of 1/4 t (from the steel plate surface to the plate thickness). 10 fields of view were taken at a position of 1/4 t in the direction (t is the plate thickness). The ferrite phase is a structure having a form in which corrosion marks and cementite are not observed in the grains, and 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. Further, in this image analysis, 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”.
得られた冷延鋼板、めっき鋼板から圧延方向に対して垂直方向にJIS5号引張試験片を作製し、JIS Z 2241(2011)の規定に準拠した引張試験を5回行い、引張強さ、降伏強さ、伸び、均一伸び、加工硬化指数(n値)を求めた。引張試験のクロスヘッドスピードは10mm/minとした。加工硬化指数はJIS Z 2253(1996)で定める方法に従って求められる値であり、真ひずみ域が0.02から0.05から求めた。 (Ii) Tensile test A JIS No. 5 tensile test piece was produced in the direction perpendicular to the rolling direction from the obtained cold-rolled steel sheet and plated steel sheet, and a tensile test based on the provisions of JIS Z 2241 (2011) was performed five times. Tensile strength, yield strength, elongation, uniform elongation, and work hardening index (n value) were determined. The crosshead speed in the tensile test was 10 mm / min. The work hardening index is a value determined according to the method defined in JIS Z 2253 (1996), and the true strain range was determined from 0.02 to 0.05.
冷延鋼板、めっき鋼板の長手方向に対し100m間隔で鋼板片を切り出し、圧延方向に対して垂直方向が試験片の長手方向となるようにJIS Z2248に記載の3号試験片を採取し、Vブロック法で曲げ試験を行った。曲げ稜線に割れが認められたときの押しつけ金具先端の半径(R(mm))を板厚(t(mm))で割ることにより限界曲げ半径(R/t)を求めた。全てのサンプルに対し、R/tが3.0以下であれば本発明で求める範囲として、表3では“○”と評価した。 (Iii) Bending test No. 3 test piece described in JIS Z2248 so that steel sheet pieces are cut out at intervals of 100 m with respect to the longitudinal direction of cold-rolled steel sheet and plated steel sheet, and the vertical direction to the rolling direction is the longitudinal direction of the test piece. Were collected and subjected to a bending test by the V-block method. The critical bending radius (R / t) was determined by dividing the radius (R (mm)) of the pressing metal tip when a crack was observed in the bending ridge line by the plate thickness (t (mm)). For all the samples, if R / t was 3.0 or less, it was evaluated as “◯” in Table 3 as a range required by the present invention.
It can be seen that all of the inventive examples have good bendability at a tensile strength of 980 MPa or more. And the comparative example which remove | deviates from the scope of the present invention cannot achieve both good bendability and high strength. In particular, only the first annealing step is comparative example No. in the scope of the present invention. 8 etc., and only the second annealing step is Comparative Example No. As is apparent from 4 etc., the two annealings are in a predetermined condition, so that the n value is as high as 0.14 or more, and the strength ductility balance is 18420 MPa ·%, which is favorable.
Claims (7)
- 質量%で、C:0.07%以上0.17%以下、Si:0.3%未満、Mn:2.2%以上3.0%以下、P:0.03%以下、S:0.005%以下、Al:0.08%以下、N:0.0060%以下、Mo:0.07%以上0.50%以下、Cr:0.001%以上0.4%以下を含有し、下記(1)式を満たし、残部がFeおよび不可避的不純物からなる成分組成と、
フェライト相の面積率が30%以上70%以下、マルテンサイト相の面積率が30%以上70%以下であり、フェライト粒の平均粒径が3.5μm以下、フェライト粒の粒径の標準偏差が1.5μm以下、フェライト粒の平均アスペクト比が1.8以下、マルテンサイト粒の平均粒径が3.0μm以下、マルテンサイト粒の平均アスペクト比が2.5以下であり、連結するマルテンサイト粒間の粒界の長さが下記(2)式を満たすマルテンサイト粒の面積率の合計がマルテンサイト相の面積率の10%以下である鋼組織と、を有し、
引張強さが980MPaである高強度冷延鋼板。
0.05[%C]-2.6[%Si]+1.2[%Mn]+2.6[%Mo]+[%Cr]≧3.15 (1)
(1)式における、[%C]、[%Si]、[%Mn]、[%Mo]および[%Cr]はそれぞれ質量%でのC、Si、Mn、MoおよびCr含有量を表す。
L1≧0.2×L2 (2)
(2)式における、L1は連結するマルテンサイト粒間の粒界の長さを表し、L2は連結するマルテンサイト粒のうち粒径の大きいマルテンサイト粒の周長を表す。 In mass%, C: 0.07% or more and 0.17% or less, Si: less than 0.3%, Mn: 2.2% or more and 3.0% or less, P: 0.03% or less, S: 0.0. 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, (1) satisfying the formula, with the balance being composed of Fe and inevitable impurities,
The area ratio of the ferrite phase is 30% or more and 70% or less, the area ratio of the martensite phase is 30% or more and 70% or less, the average grain diameter of the ferrite grains is 3.5 μm or less, and the standard deviation of the grain diameter of the ferrite grains is 1.5 μm or less, the average aspect ratio of ferrite grains is 1.8 or less, the average particle diameter of martensite grains is 3.0 μm or less, the average aspect ratio of martensite grains is 2.5 or less, and the martensite grains to be connected A steel structure in which the total area ratio of martensite grains satisfying the following formula (2) is 10% or less of the area ratio of the martensite phase:
A high-strength cold-rolled steel sheet having a tensile strength of 980 MPa.
0.05 [% C] -2.6 [% Si] +1.2 [% Mn] +2.6 [% Mo] + [% Cr] ≧ 3.15 (1)
In the formula (1), [% C], [% Si], [% Mn], [% Mo] and [% Cr] represent the contents of C, Si, Mn, Mo and Cr in mass%, respectively.
L1 ≧ 0.2 × L2 (2)
In the formula (2), L1 represents the length of the grain boundary between the martensite grains to be connected, and L2 represents the circumference of the martensite grains having a large particle diameter among the connected martensite grains. - 前記成分組成は、さらに、質量%で、V:0.001%以上0.3%以下、Ti:0.001%以上0.1%以下、Nb:0.001%以上0.08%以下の1種または2種以上を含有する請求項1に記載の高強度冷延鋼板。 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%. The high-strength cold-rolled steel sheet according to claim 1, containing one or more kinds.
- 請求項1または2に記載の高強度冷延鋼板と、
前記高強度冷延鋼板上に、質量%で、Fe:20.0%以下、Al:0.001%以上1.0%以下を含有し、さらに、Pb、Sb、Si、Sn、Mg、Mn、Ni、Cr、Co、Ca、Cu、Li、Ti、Be、Bi及びREMから選択する1種または2種以上を合計で0%以上3.5%以下含有し、残部がZn及び不可避的不純物からなる成分組成のめっき層と、有する高強度めっき鋼板。 The high-strength cold-rolled steel sheet according to claim 1 or 2,
On the high-strength cold-rolled steel sheet, Fe: 20.0% or less, Al: 0.001% or more and 1.0% or less in mass%, and further Pb, Sb, Si, Sn, Mg, Mn , Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, containing a total of 0% to 3.5%, with the balance being Zn and inevitable impurities And a high-strength plated steel sheet having a component composition. - 請求項3に記載の高強度めっき鋼板の前記めっき層が、溶融めっき層又は合金化溶融めっき層である高強度めっき鋼板。 A high-strength plated steel sheet, wherein the plated layer of the high-strength plated steel sheet according to claim 3 is a hot-dip plated layer or an alloyed hot-dip plated layer.
- 請求項1又は2に記載の成分組成を有する鋼素材を、1050℃以上1300℃以下に加熱し、800℃以上の仕上げ圧延温度で仕上げ圧延終了後、500℃以上700℃以下で巻き取る熱間圧延工程と、
前記熱間圧延工程後に熱延板を冷間圧延する冷間圧延工程と、
前記冷間圧延工程後の冷延板を100℃から825℃以上の最高到達温度までの平均加熱速度が1.5℃/s以上の条件で加熱し、最高到達温度まで加熱された冷延板を560℃までの平均冷却速度が12℃/s以上の条件で冷却し、200℃以上520℃以下の温度域に滞留される時間を30秒以上とする第一焼鈍工程と、
前記第一焼鈍工程後の焼鈍板を720℃以上820℃以下の最高到達温度まで加熱し、最高到達温度まで加熱された焼鈍板を560℃までの平均冷却速度が12℃/s以上の条件で冷却し、200℃以上500℃以下の温度域に滞留される時間を75秒以下とする第二焼鈍工程と、を有する高強度冷延鋼板の製造方法。 The steel material having the component composition according to claim 1 or 2 is heated to 1050 ° C. or more and 1300 ° C. or less, and after the finish rolling is finished at a finish rolling temperature of 800 ° C. or more, it is hot rolled up at 500 ° C. or more and 700 ° C. or less. Rolling process;
A cold rolling step of cold rolling the hot-rolled sheet after the hot rolling step;
The cold-rolled sheet after the cold rolling step is heated to a maximum temperature of 100 ° C. to a maximum temperature of 825 ° C. or higher at an average heating rate of 1.5 ° C./s or higher. The first annealing step of cooling at an average cooling rate of up to 560 ° C. under the condition of 12 ° C./s or more, and setting the residence time in the temperature range of 200 ° C. or more and 520 ° C. or less as 30 seconds or more,
The annealing plate after the first annealing step is heated to a maximum temperature of 720 ° C. or more and 820 ° C. or less, and the annealing plate heated to the maximum temperature is 560 ° C. under an average cooling rate of 12 ° C./s or more. A second annealing step of cooling and keeping the time in a temperature range of 200 ° C. or more and 500 ° C. or less for 75 seconds or less, and a method for producing a high-strength cold-rolled steel sheet. - 請求項5に記載の製造方法で製造された高強度冷延鋼板上に、質量%で、Fe:20.0%以下、Al:0.001%以上1.0%以下を含有し、さらにPb、Sb、Si、Sn、Mg、Mn、Ni、Cr、Co、Ca、Cu、Li、Ti、Be、Bi及びREMから選択する1種または2種以上を合計で0%以上3.5%以下を含有し、残部がZn及び不可避不純物からなるめっき層を形成するめっき工程を有する高強度めっき鋼板の製造方法。 It contains Fe: 20.0% or less, Al: 0.001% or more and 1.0% or less in mass% on the high-strength cold-rolled steel plate manufactured by the manufacturing method according to claim 5, and further Pb , Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM, or a total of 0% to 3.5% The manufacturing method of the high intensity | strength plated steel plate which has a plating process which forms a plating layer which contains Zn and the remainder consists of Zn and an inevitable impurity.
- 前記めっき層が溶融めっき層又は合金化溶融めっき層である請求項6に記載の高強度めっき鋼板の製造方法。 The method for producing a high-strength plated steel sheet according to claim 6, wherein the plated layer is a hot-dip plated layer or an alloyed hot-dip plated layer.
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CN107614731B (en) | 2019-07-23 |
KR102004077B1 (en) | 2019-07-25 |
CN107614731A (en) | 2018-01-19 |
JP6324512B2 (en) | 2018-05-16 |
JPWO2016194272A1 (en) | 2017-06-15 |
KR20170137899A (en) | 2017-12-13 |
MX2017015333A (en) | 2018-03-28 |
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