WO2022004820A1 - 亜鉛めっき鋼板、部材及びそれらの製造方法 - Google Patents
亜鉛めっき鋼板、部材及びそれらの製造方法 Download PDFInfo
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- WO2022004820A1 WO2022004820A1 PCT/JP2021/024846 JP2021024846W WO2022004820A1 WO 2022004820 A1 WO2022004820 A1 WO 2022004820A1 JP 2021024846 W JP2021024846 W JP 2021024846W WO 2022004820 A1 WO2022004820 A1 WO 2022004820A1
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- Prior art keywords
- less
- steel sheet
- retained austenite
- rolling
- zinc
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- 239000010959 steel Substances 0.000 title claims abstract description 131
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- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 claims abstract description 7
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- 238000001816 cooling Methods 0.000 claims description 37
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- B23K2101/34—Coated articles, e.g. plated or painted; Surface treated articles
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2103/00—Materials to be soldered, welded or cut
- B23K2103/02—Iron or ferrous alloys
- B23K2103/04—Steel or steel alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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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
- C21D2211/002—Bainite
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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
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12785—Group IIB metal-base component
- Y10T428/12792—Zn-base component
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12861—Group VIII or IB metal-base component
- Y10T428/12951—Fe-base component
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12861—Group VIII or IB metal-base component
- Y10T428/12951—Fe-base component
- Y10T428/12958—Next to Fe-base component
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12861—Group VIII or IB metal-base component
- Y10T428/12951—Fe-base component
- Y10T428/12958—Next to Fe-base component
- Y10T428/12965—Both containing 0.01-1.7% carbon [i.e., steel]
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12861—Group VIII or IB metal-base component
- Y10T428/12951—Fe-base component
- Y10T428/12972—Containing 0.01-1.7% carbon [i.e., steel]
Definitions
- the present invention relates to galvanized steel sheets, members, and methods for manufacturing them, which have high strength and excellent collision characteristics.
- the galvanized steel sheet of the present invention can be suitably used mainly as a steel sheet for automobiles.
- the strength of the energy absorbing member has not been increased, and the strength level of the practical steel is up to about 590 MPa class for TS.
- the reason why the strength of energy absorbing members has not been increased is that high-strength galvanized steel sheets are prone to breakage of members starting from the part that has undergone primary processing by molding at the time of collision, and exhibit stable collision energy absorption capacity. There are things you can't do. Therefore, there is room to contribute to environmental conservation by reducing the weight while ensuring safety at the time of collision by suppressing member breakage at the time of collision and stably exerting high absorption energy. From the above, it is necessary to apply a high-strength galvanized steel sheet having a TS of 590 MPa or more, which has excellent collision characteristics, to the energy absorbing member.
- Patent Document 1 discloses a technique relating to an ultra-high-strength galvanized steel sheet having a TS of 1200 MPa class, which is excellent in formability and impact resistance.
- Patent Document 2 discloses a technique relating to a high-strength galvanized steel sheet having a maximum tensile strength of 780 MPa or more and applicable to a shock absorbing member at the time of a collision.
- Patent Document 1 Although the collision characteristics are examined in Patent Document 1, the impact resistance on the premise that the member does not break at the time of collision is examined, and the collision characteristics from the viewpoint of the member breakage are examined. Not.
- the hat material is subjected to a crack determination in a dynamic shaft crushing test by a drop weight, and the fracture resistance property of TS over 780 MPa is evaluated.
- the crack determination after crushing the process from cracking to rupture during crushing, which is important for collision characteristics, cannot be evaluated. The reason is that if cracks occur at an early stage in the process of crushing, even a slight crack that does not penetrate the plate thickness may reduce the absorbed energy. In addition, if cracks occur in the later stage of the crushing process, even large cracks that penetrate the plate thickness may have little effect on the absorbed energy. Therefore, it is considered that the evaluation of the fracture resistance is not sufficient only by the crack determination after crushing.
- the present invention has been made in view of such circumstances, and is suitable for an energy absorbing member of an automobile, has a tensile strength (TS) of 590 MPa or more, and has excellent collision characteristics.
- the purpose is to provide a manufacturing method.
- the present inventors have found the following as a result of repeated diligent research to solve the above problems.
- a zinc-plated steel sheet having a composition and an area ratio of a carbon equivalent Ceq of 0.35% or more and less than 0.60%, ferrite: 40 to 80%, tempered martensite and bainite total: 15 to 55%, residual.
- the ratio of the residual austenite grains having an aspect ratio of less than 2.0 is 50% among the total retained austenite grains in which the amount of solid-dissolved C in the retained austenite is 0.6% by mass or more.
- the void number density is 1000 pieces / mm 2 or less in the L cross section within the region of 0 to 50 ⁇ m from the surface of the steel plate on the compression side when the predetermined 90 ° bending process is performed. From these, it was found that a steel sheet having high strength and excellent collision characteristics can be obtained.
- the present invention has been made based on such findings, and the gist thereof is as follows.
- the proportion of residual austenite grains having an aspect ratio of less than 2.0 among all residual austenite grains is 50% or more.
- Radius of curvature / Plate thickness Number of voids in the L cross section within the region of 0 to 50 ⁇ m from the surface of the steel plate on the compression side when bending 90 ° in the rolling (L) direction with the width (C) direction as the axis at 4.2. The density is 1000 pieces / mm 2 or less, A galvanized steel sheet having a tensile strength of 590 MPa or more.
- the composition of the components is mass%. C: 0.03 to 0.20%, Si: 0.10 to 2.00%, Mn: 0.5-2.5%, P: 0.05% or less, S: 0.05% or less, Sol.
- the composition of the components is further increased by mass%. Cr: 1.0% or less, Mo: 0.5% or less, V: 0.5% or less, Ti: 0.5% or less, Nb: 0.5% or less, B: 0.005% or less, Ni: 1.0% or less, Cu: 1.0% or less, Sb: 1.0% or less, Sn: 1.0% or less,
- the hot rolling process of rolling and winding at a winding temperature of 600 ° C or less A cold rolling step of cold rolling a hot-rolled steel sheet after the hot rolling step at a reduction ratio of more than 20%, and a cold rolling step.
- a method for producing a galvanized steel sheet comprising: after the quenching and tempering steps, a cooling step of cooling from the tempering temperature to 50 ° C. at an average cooling rate of 20 ° C./s or more.
- a galvanized steel sheet having a tensile strength (TS) of 590 MPa or more and excellent collision characteristics can be obtained.
- the member obtained by performing molding or welding on the galvanized steel sheet of the present invention can be suitably used as an energy absorbing member used in the automobile field.
- the zinc-plated steel sheet of the present invention has a component composition having a carbon equivalent Ceq of 0.35% or more and less than 0.60%, an area ratio of ferrite: 40 to 80%, and a total of tempered martensite and bainite: 15 to 55. %, Retained austenite: 3 to 20%, Fresh martensite: 10% or less, Ferrite, Tempered martensite, Bainite, Retained austenite and Fresh martensite total: 90% or more Steel structure with steel structure and steel plate surface It is provided with a zinc-plated layer on top.
- Carbon equivalent Ceq 0.35% or more and less than 0.60% Carbon equivalent Ceq is the effect of elements other than C converted into C as an index of steel strength.
- the area ratio of each metal structure such as ferrite, which will be described later can be controlled within the range of the present invention.
- the strength of the present invention can be obtained by setting the carbon equivalent Ceq to 0.35% or more, preferably 0.40% or more. Further, when the carbon equivalent Ceq is 0.60% or more, the effect of improving the collision characteristics of the present invention cannot be obtained. Therefore, the carbon equivalent Ceq is less than 0.60%, preferably 0.55% or less.
- the carbon equivalent Ceq can be calculated by the following formula.
- Area ratio of ferrite 40-80% If the area ratio of ferrite is less than 40%, the tempered martensite fraction may become excessive and the collision characteristics may deteriorate. Therefore, the area ratio of ferrite is 40% or more, preferably 45% or more. On the other hand, if the area ratio of ferrite exceeds 80%, the ferrite fraction may become excessive and the TS may decrease. Therefore, the area ratio of ferrite is 80% or less, preferably 75% or less.
- Total area ratio of tempered martensite and bainite 15-55% Tempering martensite and bainite are effective in improving absorption energy and increasing strength while suppressing member breakage during collision deformation. If the total area ratio of tempered martensite and bainite is less than 15%, TS may decrease. Therefore, the total area ratio is 15% or more, preferably 20% or more. More preferably, it is 22% or more, and even more preferably, it is 24% or more. Further, if the total area ratio of tempered martensite and bainite exceeds 55%, the collision characteristics may deteriorate. Therefore, the total area ratio is 55% or less, preferably 50% or less. It is more preferably 48% or less, still more preferably 46% or less.
- Area ratio of retained austenite 3-20% Retained austenite is effective in delaying the occurrence of cracks at the time of collision and improving the collision characteristics.
- the mechanism is not clear, but it is thought to be as follows.
- the residual austenite is work-hardened during collision deformation, and the radius of curvature during bending deformation increases, so that the strain at the bent portion is dispersed. By dispersing the strain, the stress concentration on the void generation part due to the primary processing is relaxed, and as a result, the collision characteristics are improved. If the area ratio of retained austenite is less than 3%, such an effect cannot be obtained. Therefore, the area ratio of retained austenite is 3% or more, preferably 5% or more. More preferably, it is 7% or more.
- the area ratio of retained austenite exceeds 20%, the fresh martensite generated by the work-induced transformation may reduce the fracture resistance at the time of collision. Therefore, the area ratio of retained austenite is 20% or less, preferably 15% or less. More preferably, it is 10% or less.
- Fresh martensite 10% or less Fresh martensite is effective for increasing strength. However, voids are likely to occur at the grain boundaries with the soft phase, and if the area ratio of fresh martensite exceeds 10%, the collision characteristics may deteriorate. Therefore, the area ratio of fresh martensite is 10% or less, preferably 5% or less.
- the lower limit is not particularly limited, but is preferably 1% or more, more preferably 2% or more, from the viewpoint of ensuring strength.
- Total area ratio of ferrite, tempered martensite, bainite, retained austenite and fresh martensite 90% or more
- the area ratio of the phase becomes high, and it becomes difficult to achieve both strength and collision characteristics.
- the phase other than the above include pearlite and cementite, and when these phases increase, they may become a starting point of void generation at the time of collision deformation and deteriorate the collision characteristics.
- the strength may decrease. If the total area ratio is 90% or more, high strength and collision characteristics can be obtained regardless of the type and area ratio of the remaining phases.
- the total area ratio is preferably 93% or more.
- the remaining structures other than the above include pearlite and cementite, and the total area ratio of these remaining structures is 10% or less.
- the total area ratio of the remaining tissue is 7% or less.
- the area ratio of each tissue is the ratio of the area of each phase to the observed area.
- the area ratio of each tissue is measured as follows. After polishing the plate thickness cross section of the steel plate cut at right angles to the rolling direction, it is corroded with 3% by volume nital, and the plate thickness 1/4 position is photographed in 3 fields with a SEM (scanning electron microscope) at a magnification of 1500 times. From the obtained image data, the area ratio of each structure is obtained using Image-Pro manufactured by Media Cybernetics. The average value of the area ratio of the three visual fields is defined as the area ratio of each tissue of the present invention.
- ferrite is black
- bainite is black with island-like retained austenite or gray with aligned carbides
- tempered martensite is light gray with fine misaligned carbides
- retained austenite is white. Can be distinguished.
- fresh martensite also exhibits white color, and it is difficult to distinguish between fresh martensite and retained austenite in the SEM image. Therefore, the area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite obtained by the method described later from the total area ratio of fresh martensite and retained austenite.
- the X-ray diffraction intensity was measured to determine the volume fraction of retained austenite, and the volume fraction was regarded as the area fraction of retained austenite.
- the volume fraction of the retained austenite is the X-ray diffraction integral intensity of the bcc iron (200), (211), (220) planes on the plate thickness 1/4 plane, and the volume fraction of the fcc iron (200), (220), (311). It is obtained by the ratio of the X-ray diffraction integrated intensity of the surface.
- Amount of solid solution C in retained austenite 0.6% by mass or more
- the amount of solid solution C in retained austenite is less than 0.6% by mass, a large amount of retained austenite becomes martensite in the initial low strain region during the collision deformation process.
- the fracture resistance at the time of collision may be deteriorated by the fresh martensite generated by the work-induced transformation during the transformation process. Therefore, the amount of solid solution C in the retained austenite is 0.6% by mass or more, more preferably 0.7% by mass or more.
- the upper limit of the amount of solid solution C in the retained austenite is not particularly limited, but if C is excessively concentrated in the untransformed austenite, the untransformed austenite may be decomposed and the retained austenite may decrease. It shall be 5.5% by mass or less.
- the amount of solid-dissolved C in the retained austenite is the residual austenite grains at the plate thickness 1/4 position of the steel plate cross section cut at right angles to the rolling direction using FE-EPMA (field emission electron probe microanalyzer). It can be measured by analyzing the C amount of C and averaging the C amount of each retained austenite grain obtained from the analysis result.
- FE-EPMA field emission electron probe microanalyzer
- the characteristics may deteriorate.
- Residual austenite is work-hardened and disperses the strain of the bending deformation part to improve the collision characteristics, but the fresh martensite generated by the work-induced transformation during the deformation process tends to be the starting point of voids.
- the voids When voids are formed at the interface of work-induced martensite with a high aspect ratio of retained austenite, the voids rapidly coarsen along the interface and promote crack growth.
- the proportion of retained austenite grains having an aspect ratio of less than 2.0 is set to 50% or more among all retained austenite grains.
- the ratio is more preferably 60% or more. The higher the ratio, the better, so the upper limit is not particularly limited.
- Three visual fields are measured, and (the number of retained austenite grains having an aspect ratio of less than 2.0) / (the number of total retained austenite grains) are measured, respectively.
- the average of the measured values in the three fields of view is defined as the ratio of the retained austenite grains having an aspect ratio of less than 2.0 among all the retained austenite grains.
- Radius of curvature / Plate thickness When bending 90 ° in the rolling (L) direction with the width (C) direction as the axis at 4.2, the void number density in the L cross section within the region of 0 to 50 ⁇ m from the surface of the steel sheet on the compression side. : 1000 pieces / mm 2 or less
- high collision characteristics can be obtained by setting the void number density to 1000 pieces / mm 2 or less. This mechanism is not clear, but it is thought to be as follows. Fracture at the time of collision, which causes deterioration of collision characteristics, starts from the occurrence and propagation of cracks.
- the void number density is set to 1000 pieces / mm 2 or less. Further, since it is considered that the smaller the void number density is, the more the fracture at the time of shaft crushing is suppressed, the lower limit is not particularly limited.
- the desired void number density can be obtained by holding before quenching and controlling the cooling rate after annealing, which will be described later, and by performing a plating treatment before the quenching and tempering steps.
- Bainite produced by holding before quenching is tempered in the plating step and tempering step, and by softening, void formation at the interface with soft ferrite is suppressed.
- the bainite produced in the tempering process suppresses softening due to tempering during cooling by increasing the cooling rate after tempering, and further suppresses softening due to tempering during the plating process by performing a plating process before the tempering process. By doing so, void formation at the interface with hard fresh martensite is suppressed.
- the void number density (pieces / mm 2 ) in the present invention is the radius of curvature / plate thickness: 4.2 and is the compression side when rolled 90 ° in the rolling (L) direction with the width (C) direction as the axis. It is the number of voids per 1 mm 2 in the L cross section in the region of 0 to 50 ⁇ m from the surface of the steel sheet.
- the processing method is not limited as long as the primary bending processing conditions are satisfied.
- Examples of the primary bending method include bending by the V-block method and bending by draw forming.
- the method for measuring the void number density is as follows.
- the galvanized steel sheet is bent 90 ° in the rolling (L) direction with the width (C) direction as the axis at a radius of curvature / plate thickness of 4.2, and then the plate thickness cross section is polished to 0 from the surface of the steel plate on the compression side.
- the L cross section is photographed in three fields with a SEM (scanning electron microscope) at a magnification of 1500 times, and the number density of voids is determined from the obtained image data using Image-Pro manufactured by Media Cybernetics.
- the average value of the number densities of the three visual fields is defined as the void number density.
- the void is darker black than ferrite and can be clearly distinguished from each structure.
- the measurement position of the void in the rolling direction after bending is defined as a region including the corner portion X0 formed by bending and extending in the width (C) direction (see reference numeral D1 in FIG. 1). More specifically, in the region that is the lowest in the width direction and the direction perpendicular to the rolling direction (pressing direction of the pressing portion such as a punch) due to bending, the region is 0 to 50 ⁇ m in the plate thickness direction (reference numeral in FIG. 1). (See XA) to measure the number density of voids.
- performing 90 ° bending in the rolling (L) direction about the width (C) direction means that when the steel plate is viewed in the width (C) direction (see reference numeral D1 in FIG. 1) (width direction).
- One of the steel plate surfaces in the width direction and the direction perpendicular to the rolling direction see reference numerals D1 and D2 in FIG. 1) so that the distance between both ends is shortened in the steel plate view (horizontal vertical cross-sectional view in the width direction). It refers to bending by pressing from the side to make the angle of the lowermost part of the V-shape in the L cross section 90 °.
- the surface of the steel plate on the compression side refers to the surface of the steel plate on one side of the pressing (the surface of the steel plate in contact with the pressing portion such as a punch to be pressed).
- the L cross section after bending is a cross section formed by cutting parallel to the direction of deformation due to bending and perpendicular to the surface of the steel sheet, and is a cross section perpendicular to the width direction. Point to.
- the galvanized steel sheet of the present invention has a galvanized layer on the surface of the steel sheet.
- the galvanized layer is, for example, an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer.
- the tensile strength (TS) of the galvanized steel sheet of the present invention is 590 MPa or more.
- the high strength in the present invention means that the tensile strength (TS) is 590 MPa or more.
- the upper limit of the tensile strength (TS) is not particularly limited, but is preferably less than 980 MPa from the viewpoint of harmony with other characteristics.
- JIS Z2241 (2011) in which a JIS No. 5 tensile test piece (JIS Z2201) is collected from a steel plate in a direction perpendicular to the rolling direction and the strain rate is 10 -3 / s. ), A tensile test is performed to determine the tensile strength (TS).
- the thickness of the galvanized steel sheet of the present invention is preferably 0.2 mm or more and 3.2 mm or less from the viewpoint of effectively obtaining the effect of the present invention.
- the galvanized steel sheet of the present invention has excellent collision characteristics.
- excellent in collision characteristics means that the fracture resistance characteristics are good and the absorption energy is good.
- Good fracture resistance in the present invention means that the average value ⁇ S of the stroke at the maximum load when the bending-orthogonal bending test described below is performed is 30 mm or more.
- Good collision characteristics in the present invention means that the shaft crushing test described below is carried out, and the average value Ave of the area in the range of stroke 0 to 100 mm in the stroke-load graph at the time of crushing is 35,000 N or more. It means that it is.
- a test piece is prepared by subjecting a steel sheet to a 90 ° bending process (primary bending process) in the rolling (L) direction with the width (C) direction as the axis at a radius of curvature / plate thickness of 4.2.
- the punch B1 is pushed into the steel plate placed on the die A1 having the V groove to obtain the test piece T1.
- the punch B2 is pushed into the test piece T1 placed on the support roll A2 so that the bending direction is perpendicular to the rolling direction, and orthogonal bending (secondary bending) is performed.
- D1 indicates the width (C) direction
- D2 indicates the rolling (L) direction.
- FIG. 4 shows a test piece T1 obtained by bending a steel sheet by 90 ° (primary bending).
- FIG. 5 shows a test piece T2 that has been subjected to orthogonal bending (secondary bending process) to the test piece T1.
- the position shown by the broken line on the test piece T2 of FIG. 5 corresponds to the position shown by the broken line on the test piece T1 of FIG. 4 before performing the orthogonal bending.
- the conditions for orthogonal bending are as follows. [Orthogonal bending conditions] Test method: Roll support, punch pushing roll diameter: ⁇ 30 mm Punch tip R: 0.4 mm Distance between rolls: (plate thickness x 2) + 0.5 mm Stroke speed: 20 mm / min Test piece size: 60 mm x 60 mm Bending direction: Rolling orthogonal direction In the stroke-load curve obtained when the above orthogonal bending is performed, the stroke at the maximum load is obtained. Let ⁇ S be the average value of the strokes at the maximum load when the bending-orthogonal bending test is performed three times.
- the above shaft crush test is performed as follows. In the shaft crushing test, considering the influence of the plate thickness, all are carried out with a galvanized steel plate having a plate thickness of 1.2 mm.
- the galvanized steel sheet obtained in the above manufacturing process is cut out and molded (bending) to a depth of 40 mm using a die having a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm. ), And the hat-shaped member 10 shown in FIGS. 6 and 7 is manufactured. Further, the galvanized steel sheet used as the material of the hat-shaped member is separately cut into a size of 200 mm ⁇ 80 mm.
- FIG. 6 is a front view of a test member 30 manufactured by spot welding a hat-shaped member 10 and a galvanized steel sheet 20.
- FIG. 7 is a perspective view of the test member 30. As shown in FIG. 7, the positions of the spot welded portions 40 are such that the end portion of the galvanized steel sheet and the welded portion are 10 mm apart, and the welded portion is 45 mm apart.
- the test member 30 is joined to the main plate 50 by TIG welding to prepare a sample for a shaft crush test.
- the impactor 60 is collided with the prepared sample for the shaft crush test at a constant velocity at a collision speed of 10 m / s, and the sample for the shaft crush test is crushed by 100 mm.
- the crushing direction D3 is a direction parallel to the longitudinal direction of the test member 30.
- the area in the range of stroke 0 to 100 mm in the stroke-load graph at the time of crushing is obtained, and the average value of the area when the test is performed three times is defined as the absorbed energy ( Fave ).
- C 0.03 to 0.20% C is an element necessary for improving the strength because it facilitates the formation of a phase other than ferrite and forms an alloy compound with Nb, Ti, and the like. If the C content is less than 0.03%, the desired strength may not be secured even if the production conditions are optimized. Therefore, the C content is preferably 0.03% or more, more preferably 0.05% or more. On the other hand, if the C content exceeds 0.20%, the strength of martensite increases excessively, and the collision characteristics of the present invention may not be obtained even if the production conditions are optimized. Therefore, the C content is preferably 0.20% or less, more preferably 0.18% or less.
- Si 0.10 to 2.00% Since Si suppresses the formation of carbides, retained austenite can be obtained. It is also a solid solution strengthening element and contributes to improving the balance between strength and ductility. In order to obtain this effect, the Si content is preferably 0.10% or more, more preferably 0.50% or more. On the other hand, if the Si content exceeds 2.00%, zinc plating adhesion, deterioration of adhesion and deterioration of surface texture may occur. Therefore, the Si content is preferably 2.00% or less, more preferably 1.50% or less.
- Mn 0.5-2.5%
- Mn is a martensite-forming element and is also a solid solution-enhancing element. It also contributes to the stabilization of retained austenite.
- the Mn content is preferably 0.5% or more.
- the Mn content is more preferably 1.0% or more.
- the Mn content is preferably 2.5% or less, more preferably 2.0% or less.
- P 0.05% or less
- the P content is preferably 0.05% or less, more preferably 0.01% or less.
- the lower limit that is currently industrially feasible is 0.002%, preferably 0.002% or more.
- the S content should be as low as possible, but the S content is preferably 0.05% or less from the viewpoint of manufacturing cost.
- the S content is more preferably 0.01% or less.
- Sol. Al acts as a deoxidizing agent and is also a solid solution strengthening element. Sol. If the Al content is less than 0.005%, these effects may not be obtained, and even if the steel structure of the present invention is satisfied, the strength may decrease. Therefore, Sol. The Al content is preferably 0.005% or more. On the other hand, Sol. If the Al content exceeds 0.100%, the slab quality during steelmaking deteriorates. Therefore, Sol. The Al content is preferably 0.100% or less, more preferably 0.050% or less.
- N 0.010% or less Since N forms a coarse nitride, it becomes the starting point of void formation during collision deformation and may deteriorate the collision characteristics. Therefore, the N content should be as small as possible, but the N content is preferably 0.010% or less, more preferably 0.006% or less from the viewpoint of manufacturing cost.
- the lower limit of the N content is not particularly limited, but the lower limit that can be industrially implemented at present is 0.0003%, preferably 0.0003% or more.
- the component composition of the steel sheet according to the present invention contains the above-mentioned component elements as basic components, and the balance contains iron (Fe) and unavoidable impurities.
- the steel sheet of the present invention contains the above-mentioned basic components, and the balance has a component composition consisting of iron (Fe) and unavoidable impurities.
- the steel sheet according to the present invention can appropriately contain the components (arbitrary elements) described below according to desired characteristics.
- Cr 1.0% or less
- Mo 0.5% or less
- V 0.5% or less
- Ti 0.5% or less
- Nb 0.5% or less
- B 0.005% or less
- Ni Select from 1.0% or less
- Cu 1.0% or less
- Sb 1.0% or less
- Sn 1.0% or less
- Ca 1.0% or less
- REM 0.005% or less
- At least one kind of Cr, Mo, and V is an element that improves hardenability and is effective for strengthening steel.
- the second phase fraction may become excessive and the fracture resistance at the time of collision may be deteriorated.
- the Cr content is preferably 1.0% or less, the Mo content is preferably 0.5% or less, and the V content is preferably 0.5% or less. Is. More preferably, the Cr content is 0.8% or less, the Mo content is 0.4% or less, and the V content is 0.4% or less. Since the effect of the present invention can be obtained even if the content of Cr, Mo, and V is small, the lower limit of the content of each is not particularly limited. In order to obtain the effect of quenchability more effectively, the contents of Cr, Mo and V are preferably 0.005% or more, respectively. More preferably, the contents of Cr, Mo and V are 0.01% or more, respectively.
- Ti and Nb are elements that are effective in strengthening the precipitation of steel. However, if the Ti content and the Nb content each exceed 0.5%, the fracture resistance at the time of collision may deteriorate. Therefore, when any of Ti and Nb is contained, the Ti content and the Nb content are preferably 0.5% or less, respectively. More preferably, the Ti content and the Nb content are 0.4% or less, respectively. Since the effect of the present invention can be obtained even if the contents of Ti and Nb are small, the lower limit of the respective contents is not particularly limited. In order to more effectively obtain the effect of steel precipitation strengthening, the Ti content and the Nb content are preferably 0.005% or more, respectively. More preferably, the Ti content and the Nb content are 0.01% or more, respectively.
- the B contributes to the improvement of hardenability by suppressing the formation and growth of ferrite from the austenite grain boundaries, and can be added as needed. However, if the B content exceeds 0.005%, the fracture resistance at the time of collision may deteriorate. Therefore, when B is contained, the B content is preferably 0.005% or less. More preferably, the B content is 0.004% or less. Since the effect of the present invention can be obtained even if the B content is small, the lower limit of the B content is not particularly limited. In order to more effectively obtain the effect of improving hardenability, the B content is preferably 0.0003% or more. More preferably, the B content is 0.0005% or more.
- Ni and Cu are effective elements for strengthening steel. However, if Ni and Cu each exceed 1.0%, the fracture resistance at the time of collision may deteriorate. Therefore, when any of Ni and Cu is contained, the content of Ni and Cu is preferably 1.0% or less, respectively. More preferably, the Ni content and the Cu content are 0.9% or less, respectively. Since the effect of the present invention can be obtained even if the contents of Ni and Cu are small, the lower limit of the respective contents is not particularly limited. In order to obtain the effect of strengthening the steel more effectively, the Ni content and the Cu content are preferably 0.005% or more, respectively. More preferably, the Ni content and the Cu content are 0.01% or more, respectively.
- Sn and Sb can be added as needed from the viewpoint of suppressing nitriding and oxidation of the surface of the steel sheet and decarburization of the region near the surface of the steel sheet. Suppressing such nitriding and oxidation has the effect of preventing the amount of martensite produced on the surface of the steel sheet from decreasing and improving the collision characteristics. However, if Sb and Sn each exceed 1.0%, the collision characteristics may deteriorate due to grain boundary embrittlement. Therefore, when any of Sb and Sn is contained, the Sb content and the Sn content are preferably 1.0% or less, respectively. More preferably, the Sb content and the Sn content are 0.9% or less, respectively.
- the lower limit of the respective contents is not particularly limited.
- the Sb content and the Sn content are preferably 0.005% or more, respectively. More preferably, the Sb content and the Sn content are 0.01% or more, respectively.
- Both Ca and REM are effective elements for improving processability by controlling the morphology of sulfides.
- the content of Ca and REM is preferably 0.005% or less, respectively. More preferably, the Ca content and the REM content are 0.004% or less, respectively. Since the effect of the present invention can be obtained even if the contents of Ca and REM are small, the lower limit of the respective contents is not particularly limited.
- the contents of Ca and REM are preferably 0.001% or more, respectively. More preferably, the Ca content and the REM content are 0.002% or more, respectively.
- the element is considered to be contained as an unavoidable impurity.
- the temperature at which the steel slab (steel material), steel plate, etc. shown below is heated or cooled means the surface temperature of the steel slab (steel material), steel plate, etc., unless otherwise specified.
- the method for producing a zinc-plated steel sheet of the present invention comprises a hot rolling step in which a steel slab having the above composition is hot-rolled at a finish rolling temperature of 850 to 950 ° C. and wound at a winding temperature of 600 ° C. or lower.
- the cold-rolled steel sheet after the hot-rolled step is cold-rolled at a reduction rate of over 20%, and the cold-rolled steel sheet after the cold-rolled step is heated to a shrinking temperature of 720 to 860 ° C.
- the composition of the steel slab used in the method for producing a steel sheet of the present invention satisfies the carbon equivalent Ceq: 0.35% or more and less than 0.60%. Carbon equivalent Ceq: 0.35% or more and less than 0.60% is a range optimized for producing the steel sheet of the present invention under the production conditions of the present invention.
- Finish rolling temperature 850-950 ° C If the finish rolling temperature is less than 850 ° C., ferrite transformation occurs during rolling and the strength is locally reduced, so that the strength may not be obtained even if the structure of the present invention is satisfied. Therefore, the finish rolling temperature is 850 ° C. or higher, preferably 880 ° C. or higher. On the other hand, when the finish rolling temperature exceeds 950 ° C., the crystal grains become coarse and the strength may not be obtained even if the structure of the present invention is satisfied. Therefore, the finish rolling temperature is 950 ° C. or lower, preferably 930 ° C. or lower.
- Winding temperature 600 ° C or less
- the winding temperature exceeds 600 ° C, the carbides in the hot-rolled steel sheet become coarse, and such coarsened carbides cannot be completely melted during soaking during annealing, so the necessary collisions are required. It may not be possible to obtain the characteristics. Therefore, the winding temperature is 600 ° C. or lower, preferably 580 ° C. or lower.
- the lower limit of the take-up temperature is not particularly limited, but it is preferable that the take-up temperature is 400 ° C. or higher from the viewpoint of preventing the steel sheet from having a shape defect and preventing the steel sheet from becoming excessively hard.
- the hot-rolled steel sheet obtained by the hot-rolling process is subjected to pretreatment such as pickling and degreasing by a commonly known method, and then cold-rolled as necessary.
- pretreatment such as pickling and degreasing by a commonly known method
- cold-rolled as necessary.
- the conditions of the cold rolling process when cold rolling is performed will be described.
- Cold rolling reduction rate 20% or more When the cold rolling reduction rate is 20% or less, recrystallization of ferrite is not promoted, unrecrystallized ferrite remains, and the steel structure of the present invention may not be obtained. be. Therefore, the rolling reduction of cold rolling is more than 20%, preferably 30% or more.
- the annealing temperature is 720 to 860 ° C. and the holding time is 30 seconds or more and the annealing temperature is less than 720 ° C., the formation of austenite is insufficient and excess ferrite is formed, so that the steel structure of the present invention cannot be obtained. Therefore, the annealing temperature is 720 ° C. or higher, preferably 740 ° C. or higher. On the other hand, if the annealing temperature exceeds 860 ° C., the ferrite fraction of the present invention cannot be secured. Therefore, the annealing temperature is 860 ° C or lower, preferably 840 ° C or lower.
- the holding time is 30 seconds or more, preferably 60 seconds or more.
- the upper limit of the holding time is not particularly limited, but the holding time is preferably 600 seconds or less so as not to impair the productivity.
- the conditions of the plating process will be explained.
- the steel sheet is cooled to a temperature range of 300 to 600 ° C., held in the temperature range for 10 to 300 seconds, and then galvanized on the surface of the steel sheet.
- Retention time in the temperature range of 300 to 600 ° C 10 to 300 seconds
- cooling to the temperature range of 300 to 600 ° C and holding in the temperature range of 300 to 600 ° C for 10 to 300 seconds is to obtain bainite. It is valid.
- a large amount of retained austenite can be obtained by concentrating C in untransformed austenite by bainite formation. These effects cannot be obtained in less than 10 seconds.
- bainite may be excessively generated, C may be excessively concentrated in untransformed austenite, pearlite may be generated, and a desired residual austenite amount may not be obtained. Therefore, the holding time is 300 seconds or less, preferably 100 seconds or less.
- the zinc plating treatment is, for example, a treatment of applying electrogalvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing to the surface of a steel plate.
- hot-dip galvanizing is applied to the surface of a steel sheet, for example, it is preferable to immerse the steel sheet obtained above in a zinc plating bath having a temperature of 440 ° C. or higher and 500 ° C. or lower to form a hot-dip galvanized layer on the surface of the steel sheet.
- the steel sheet after the hot dip galvanizing treatment may be alloyed.
- the treatment conditions for the electrozinc plating treatment are not particularly limited, and a conventional method may be followed.
- Cooling shutdown temperature (Ms-250 ° C) to (Ms-50 ° C) If the cooling shutdown temperature exceeds (Ms-50 ° C.), the formation of tempered martensite is insufficient, and the steel structure of the present invention cannot be obtained. On the other hand, if the cooling shutdown temperature is less than (Ms-250 ° C.), tempered martensite may be excessive and the production of retained austenite may be insufficient. Therefore, the cooling shutdown temperature is (Ms ⁇ 250 ° C.) to (Ms ⁇ 50 ° C.). It is preferably (Ms-200 ° C.) or higher. Further, it is preferably (Ms-100 ° C.) or lower. If the cooling stop temperature satisfies the above range, the cooling rate up to the cooling stop temperature is not limited, and the steel structure of the present invention can be obtained.
- Ms is the martensitic transformation start temperature, which can be calculated by the following formula.
- [element symbol%] represents the content (mass%) of each element, and the element not contained is 0.
- [ ⁇ area%] is the ferrite area ratio (%) after annealing. The ferrite area ratio after annealing is determined in advance by simulating the heating rate, annealing temperature, and holding time during annealing with a thermal expansion measuring device.
- Tempering temperature 300 to 500 ° C, holding time: 20 to 500 seconds If the tempering temperature is less than 300 ° C, the tempering of martensite becomes insufficient, and voids are likely to occur at the interface between the tempered martensite and ferrite during primary processing, resulting in collision. It is thought that the characteristics will deteriorate. Therefore, the tempering temperature is 300 ° C. or higher, preferably 350 ° C. or higher. On the other hand, if the tempering temperature exceeds 500 ° C., the tempering of martensite and bainite becomes excessive, and it is considered that voids are likely to be generated at the interface between fresh martensite and tempered martensite and bainite during the primary processing, and the collision characteristics are deteriorated.
- the tempering temperature is 500 ° C. or lower, preferably 450 ° C. or lower.
- the holding time is 20 seconds or more, preferably 30 seconds or more.
- the holding time exceeds 500 seconds, the proportion of retained austenite having an aspect ratio of less than 2.0 may decrease. Therefore, the upper limit of the holding time is 500 seconds or less, preferably 450 seconds or less.
- Average cooling rate from the tempering temperature to 50 ° C . 20 ° C./s or more If the average cooling rate from the tempering temperature to 50 ° C. is less than 20 ° C./s, the collision characteristics of the present invention cannot be obtained. The reason for this is not clear, but it is thought to be as follows. In order to suppress the formation of voids in the primary processed part and improve the collision characteristics, it is necessary to reduce the hardness difference between the soft phase (ferrite) and the hard phase (fresh martensite) with the intermediate hardness phase (tempering martensite, bainite). There is.
- bainite generated before the plating treatment and martensite generated during quenching are softened in the tempering process to reduce the difference in hardness from the soft phase and suppress the formation of voids.
- the latter suppresses the formation of voids with bainite produced in the tempering process.
- bainite produced in the tempering process is softened, the difference in hardness from the hard phase becomes large.
- the average cooling rate to room temperature after the tempering step is less than 20 ° C./s, bainite is tempered during cooling and the hardness difference from the hard phase becomes large, so that voids are formed at the interface during the primary processing. Is likely to be generated, and it is considered that the collision characteristics are deteriorated.
- the average cooling rate is preferably 25 ° C./s or higher.
- the upper limit of the average cooling rate is not particularly limited, but is preferably 70 ° C./s or less from the viewpoint of energy saving of the cooling equipment.
- the galvanized steel sheet of the present invention can be tempered and rolled for the purpose of shape correction and adjustment of surface roughness.
- the pressure adjusting ratio exceeds 0.5%, the bendability may deteriorate due to surface hardening, so the pressure adjusting ratio is preferably 0.5% or less. More preferably, it is 0.3% or less.
- various coating treatments such as resin and oil coating can be applied.
- the conditions of other manufacturing methods are not particularly limited, but it is preferable to carry out under the following conditions.
- the slab is preferably manufactured by a continuous casting method in order to prevent macrosegregation, and can also be manufactured by an ingot forming method or a thin slab casting method.
- To hot-roll the slab the slab may be cooled to room temperature and then reheated for hot rolling. It is also possible to charge the slab into a heating furnace without cooling it to room temperature and perform hot rolling. In addition, an energy-saving process of hot rolling immediately after performing a small amount of heat retention can also be applied.
- heating the slab it is preferable to heat it to 1100 ° C. or higher because it prevents the rolling load from increasing and the carbides dissolve. Further, in order to prevent an increase in scale loss, the heating temperature of the slab is preferably 1300 ° C. or lower.
- the scale of the rolled steel sheet may be removed by pickling or the like. After pickling, cold rolling, annealing, and galvanization are performed under the above conditions.
- the member of the present invention is formed by subjecting the galvanized steel sheet of the present invention to at least one of molding and welding. Further, the method for manufacturing a member of the present invention includes a step of performing at least one of molding and welding on the galvanized steel sheet manufactured by the method for manufacturing a galvanized steel sheet of the present invention.
- the galvanized steel sheet of the present invention has high strength and excellent collision characteristics. Therefore, the member obtained by using the galvanized steel sheet of the present invention also has high strength, excellent collision characteristics, and is less likely to break the member at the time of collision deformation. Therefore, the member of the present invention can be suitably used as an energy absorbing member in an automobile part.
- general processing methods such as press processing can be used without limitation.
- welding general welding such as spot welding and arc welding can be used without limitation.
- Example 1 The steel having the composition shown in Table 1 was melted in a vacuum melting furnace and lump-rolled to obtain a steel slab. These steel slabs were heated and subjected to hot rolling, cold rolling, annealing, plating treatment, quenching and tempering, and cooling under the conditions shown in Table 2 to produce galvanized steel sheets. In the plating treatment, an electrogalvanized layer (EG), a hot-dip galvanized layer (GI) or an alloyed hot-dip galvanized layer (GA) was formed on the surface of the steel plate.
- EG electrogalvanized layer
- GI hot-dip galvanized layer
- GA alloyed hot-dip galvanized layer
- the steel sheet was immersed in a zinc solution and energized to form an electrozinc plating layer (EG) having a plating adhesion amount of 10 to 100 g / m 2. Further, in the hot-dip galvanizing treatment, the steel sheet was immersed in a plating bath to form a hot-dip galvanizing layer (GI) having a plating adhesion amount of 10 to 100 g / m 2. Further, in alloyed hot-dip galvanizing, an alloyed hot-dip galvanized layer (GA) was formed by forming a hot-dip galvanized layer on a steel sheet and then performing an alloying treatment. The final thickness of each galvanized steel sheet was 1.2 mm.
- the obtained galvanized steel sheet is subjected to temper rolling with a rolling reduction of 0.2%, and then ferrite (F), bainite (B), tempered martensite (TM), and fresh martensite (FM) are according to the following method. ) And the area ratio of retained austenite (RA) were determined respectively. Further, the galvanized steel sheet is bent 90 ° in the rolling (L) direction with the radius of curvature / plate thickness: 4.2 and the width (C) direction as the axis according to the following method, and then the surface of the steel sheet on the compression side. The number of voids per 1 mm 2 in the L cross section in the region from 0 to 50 ⁇ m was measured.
- the area ratio of each of the above tissues is measured as follows. After polishing the plate thickness cross section of the steel plate cut at right angles to the rolling direction, it is corroded with 3% by volume nital, and the plate thickness 1/4 position is photographed in 3 fields with a SEM (scanning electron microscope) at a magnification of 1500 times. From the obtained image data, the area ratio of each structure was determined using Image-Pro manufactured by Media Cybernetics. The average value of the area ratios of the three visual fields was taken as the area ratio of each tissue of the present invention.
- ferrite is black
- bainite is black with island-like retained austenite or gray with aligned carbides
- tempered martensite is light gray with fine misaligned carbides
- retained austenite is white.
- fresh martensite also exhibits white color, and it is difficult to distinguish between fresh martensite and retained austenite in the SEM image. Therefore, the area ratio of fresh martensite was obtained by subtracting the area ratio of retained austenite obtained by the method described later from the total area ratio of fresh martensite and retained austenite.
- the total area ratio of ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM) and retained austenite (RA) is 100. Determined by subtracting from%, these remnants were determined to be pearlite and / or cementite.
- the X-ray diffraction intensity was measured to obtain the volume fraction of retained austenite, and the volume fraction was regarded as the area ratio of retained austenite.
- the volume fraction of the retained austenite is the X-ray diffraction integral intensity of the bcc iron (200), (211), (220) planes on the plate thickness 1/4 plane, and the volume fraction of the fcc iron (200), (220), (311). It was determined by the ratio of the X-ray diffraction integrated intensity of the surface.
- the amount of solid solution C in the retained austenite was measured by analysis with FE-EPMA (field emission electron probe microanalyzer).
- the method for measuring the void number density is as follows.
- the galvanized steel sheet is bent 90 ° in the rolling (L) direction with the width (C) direction as the axis at a radius of curvature / plate thickness of 4.2, and then the plate thickness cross section is polished to 0 from the surface of the steel plate on the compression side.
- the L cross section in the ⁇ 50 ⁇ m region was observed.
- the L cross section was photographed in three fields with a SEM (scanning electron microscope) at a magnification of 1500 times, and the number density of voids was determined from the obtained image data using Image-Pro manufactured by Media Cybernetics. The average value of the number densities of the three visual fields was defined as the void number density.
- the measurement position of the void in the rolling direction after the bending process is a region including the corner portion X0 formed by the bending process and extending in the width (C) direction (see reference numeral D1 in FIG. 1). More specifically, in the region that is the lowest in the width direction and the direction perpendicular to the rolling direction (pressing direction of the pressing portion such as a punch) due to bending, the region is 0 to 50 ⁇ m in the plate thickness direction (reference numeral in FIG. 1). The number density of voids was measured by XA).
- TS tensile strength
- FIG. 4 shows a test piece T1 obtained by bending a steel sheet by 90 ° (primary bending).
- FIG. 5 shows a test piece T2 that has been subjected to orthogonal bending (secondary bending process) to the test piece T1.
- the position shown by the broken line on the test piece T2 of FIG. 5 corresponds to the position shown by the broken line on the test piece T1 of FIG. 4 before performing the orthogonal bending.
- the conditions for orthogonal bending are as follows. [Orthogonal bending conditions] Test method: Roll support, punch pushing roll diameter: ⁇ 30 mm Punch tip R: 0.4 mm Distance between rolls: (plate thickness x 2) + 0.5 mm Stroke speed: 20 mm / min Test piece size: 60 mm x 60 mm Bending direction: Rolling orthogonal direction In the stroke-load curve obtained when the above orthogonal bending is performed, the stroke at the maximum load is obtained. The average value of the strokes at the maximum load when the bending-orthogonal bending test was performed three times was defined as ⁇ S. It was evaluated that the fracture resistance was good when ⁇ S was 30 mm or more.
- FIG. 6 is a front view of a test member 30 manufactured by spot welding a hat-shaped member 10 and a galvanized steel sheet 20.
- FIG. 7 is a perspective view of the test member 30. As shown in FIG. 7, the positions of the spot welded portions 40 are such that the end portion of the galvanized steel sheet and the welded portion are 10 mm apart, and the welded portion is 45 mm apart.
- the test member 30 was joined to the main plate 50 by TIG welding to prepare a sample for a shaft crush test.
- the impactor 60 was collided with the prepared sample for the shaft crush test at a constant velocity at a collision speed of 10 m / s, and the sample for the shaft crush test was crushed by 100 mm.
- the crushing direction D3 is a direction parallel to the longitudinal direction of the test member 30.
- the area in the range of stroke 0 to 100 mm was obtained, and the average value of the area when the test was performed three times was taken as the absorbed energy ( Fave ).
- F ave was evaluated as good absorption energy at more than 35000N.
- the collision property was evaluated as good.
- the galvanized steel sheet of the invention example had a TS of 590 MPa or more and was excellent in collision characteristics.
- the galvanized steel sheet of the comparative example had a TS of less than 590 MPa or poor collision characteristics.
- Example 2 No. 1 in Table 3 of Example 1.
- the galvanized steel sheet of No. 1 (example of the present invention) was formed by press working to manufacture the member of the example of the present invention. Further, No. 1 in Table 3 of Example 1.
- the galvanized steel sheet of No. 1 and No. 1 of Table 3 of Example 1. 3 (Example of the present invention) was joined to the galvanized steel sheet by spot welding to manufacture the member of the example of the present invention.
- the member of the present invention example manufactured by using the steel plate of the present invention has excellent collision characteristics and high strength, and No. 1 in Table 3 of Example 1 is used.
- a galvanized steel sheet having a TS of 590 MPa or more and excellent collision characteristics can be obtained. If the member obtained from the galvanized steel sheet of the present invention is used as an automobile part, it can contribute to the weight reduction of the automobile and greatly contribute to the high performance of the automobile body.
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Abstract
Description
[1]炭素当量Ceqが0.35%以上0.60%未満を満たす成分組成と、
面積率で、フェライト:40~80%、焼戻しマルテンサイト及びベイナイトの合計:15~55%、残留オーステナイト:3~20%、フレッシュマルテンサイト:10%以下、フェライト、焼戻しマルテンサイト、ベイナイト、残留オーステナイト及びフレッシュマルテンサイトの合計:90%以上である鋼組織と、を有する鋼板と、
前記鋼板表面上に亜鉛めっき層と、を備え、
前記残留オーステナイト中の固溶C量が0.6質量%以上であり、
全残留オーステナイト粒のうち、アスペクト比が2.0未満の残留オーステナイト粒の割合が、50%以上であり、
曲率半径/板厚:4.2で幅(C)方向を軸に圧延(L)方向に90°曲げ加工した際に、圧縮側の鋼板表面から0~50μm領域内のL断面において、ボイド数密度が1000個/mm2以下であり、
引張強度が590MPa以上である亜鉛めっき鋼板。
[2]前記成分組成は、質量%で、
C:0.03~0.20%、
Si:0.10~2.00%、
Mn:0.5~2.5%、
P:0.05%以下、
S:0.05%以下、
Sol.Al:0.005~0.100%、及び
N:0.010%以下を含有し、残部がFe及び不可避的不純物からなる[1]に記載の亜鉛めっき鋼板。
[3]前記成分組成は、さらに、質量%で、
Cr:1.0%以下、
Mo:0.5%以下、
V:0.5%以下、
Ti:0.5%以下、
Nb:0.5%以下、
B:0.005%以下、
Ni:1.0%以下、
Cu:1.0%以下、
Sb:1.0%以下、
Sn:1.0%以下、
Ca:0.005%以下、及び
REM:0.005%以下のうちから選ばれる少なくとも1種を含有する[2]に記載の亜鉛めっき鋼板。
[4]前記亜鉛めっき層が、電気亜鉛めっき層、溶融亜鉛めっき層又は合金化溶融亜鉛めっき層である[1]から[3]までのいずれか一つに記載の亜鉛めっき鋼板。
[5][1]から[4]までのいずれか一つに記載の亜鉛めっき鋼板に対して、成形加工及び溶接の少なくとも一方を施してなる部材。
[6]炭素当量Ceqが0.35%以上0.60%未満を満たし、かつ[2]又は[3]に記載の成分組成を有する鋼スラブを、仕上げ圧延温度を850~950℃として熱間圧延を施し、600℃以下の巻取温度で巻取る熱間圧延工程と、
前記熱間圧延工程後の熱延鋼板を20%超えの圧下率で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鋼板を720~860℃の焼鈍温度まで加熱し、30秒以上保持する焼鈍工程と、
前記焼鈍工程後、300~600℃の温度域まで冷却し、当該温度域で10~300秒保持した後、鋼板表面に亜鉛めっき処理を施すめっき工程と、
前記めっき工程後、(Ms-250℃)~(Ms-50℃)の冷却停止温度まで冷却した後、300~500℃の焼戻し温度で20~500秒保持する焼入れ及び焼戻し工程と、
前記焼入れ及び焼戻し工程後に、前記焼戻し温度から50℃までを平均冷却速度20℃/s以上で冷却する冷却工程と、を有する亜鉛めっき鋼板の製造方法。
[7]前記亜鉛めっき処理は、鋼板表面に、電気亜鉛めっき、溶融亜鉛めっき、又は合金化溶融亜鉛めっきを施す処理である[6]に記載の亜鉛めっき鋼板の製造方法。
[8][6]又は[7]に記載の亜鉛めっき鋼板の製造方法によって製造された亜鉛めっき鋼板に対して、成形加工及び溶接の少なくとも一方を施す工程を有する部材の製造方法。
炭素当量Ceqは鋼の強度における指標としてC以外の元素の影響をC量に換算したものである。炭素当量Ceqを0.35%以上0.60%未満とすることで、後述するフェライト等の各金属組織の面積率を本発明の範囲内に制御することができる。炭素当量Ceqを0.35%以上、好ましくは0.40%以上とすることで、本発明の強度を得ることができる。また、炭素当量Ceqが0.60%以上の場合、本発明の衝突特性向上の効果が得られない。したがって、炭素当量Ceqは0.60%未満であり、好ましくは0.55%以下である。
ただし、上記式中の[元素記号%]は、各元素の含有量(質量%)を表し、含有しない元素は0とする。
フェライトの面積率が40%未満では、焼戻しマルテンサイト分率が過剰となり、衝突特性が低下する場合がある。したがって、フェライトの面積率は40%以上であり、好ましくは45%以上である。一方、フェライトの面積率が80%を超えるとフェライト分率が過剰となりTSが低下する場合がある。したがって、フェライトの面積率は80%以下であり、好ましくは75%以下である。
焼戻しマルテンサイトとベイナイトは、衝突変形時に部材破断を抑制しつつ吸収エネルギーを向上させ、高強度化させるのに有効である。焼戻しマルテンサイト及びベイナイトの合計面積率が15%未満では、TSが低下してしまう場合がある。したがって、合計面積率は15%以上であり、好ましくは20%以上である。より好ましくは、22%以上であり、さらに好ましくは、24%以上である。また、焼戻しマルテンサイト及びベイナイトの合計面積率が55%を超えると衝突特性が低下する場合がある。したがって、合計面積率は55%以下であり、好ましくは50%以下である。より好ましくは、48%以下であり、さらに好ましくは、46%以下である。
残留オーステナイトは衝突時の割れ発生を遅延させ、衝突特性を向上させるのに有効である。メカニズムは明らかではないが、次のように考えられる。残留オーステナイトは衝突変形時に加工硬化することで曲げ変形中の曲率半径が大きくなることで曲げ部のひずみが分散される。ひずみが分散されることによって一次加工によるボイド生成部への応力集中が緩和され、その結果衝突特性が向上する。残留オーステナイトの面積率が3%未満ではこうした効果を得られない。したがって、残留オーステナイトの面積率は3%以上であり、好ましくは5%以上である。より好ましくは、7%以上である。一方、残留オーステナイトの面積率が20%を超えると、加工誘起変態によって生成したフレッシュマルテンサイトによって衝突時の耐破断特性を低下させる場合がある。したがって、残留オーステナイトの面積率は20%以下であり、好ましくは15%以下である。より好ましくは、10%以下である。
フレッシュマルテンサイトは高強度化には有効である。しかしながら、軟質相との粒界でボイドを生じやすく、フレッシュマルテンサイトの面積率が10%を超えると衝突特性を低下させる場合がある。したがって、フレッシュマルテンサイトの面積率は10%以下であり、好ましくは5%以下である。また、下限は特に限定されないが、強度確保の点から、1%以上であることが好ましく、より好ましくは2%以上である。
フェライト、焼戻しマルテンサイト、ベイナイト、残留オーステナイト及びフレッシュマルテンサイトの合計面積率が90%未満になると、上記以外の相の面積率が高くなり、強度と衝突特性を両立することが困難となる。上記以外の相には、例えば、パーライト、セメンタイトが挙げられ、これらの相が増加すると、衝突変形時にボイド生成の起点となり衝突特性を低下させる場合がある。また、パーライトやセメンタイトが増加すると、強度が低下する場合がある。上記合計面積率が90%以上であれば残りの相の種類や面積率にかかわらず高い強度及び衝突特性が得られる。合計面積率は好ましくは93%以上とする。なお、上記以外の残部の組織としては、パーライト及びセメンタイトがあり、これら残部の組織の合計面積率は10%以下である。好ましくは、この残部の組織の合計面積率は7%以下である。
残留オーステナイト中の固溶C量が0.6質量%未満になると衝突変形過程における初期の低ひずみ域で多くの残留オーステナイトがマルテンサイトに変態し、その後の変形過程で加工誘起変態によって生成したフレッシュマルテンサイトによって衝突時の耐破断特性が低下する場合がある。したがって、残留オーステナイト中の固溶C量は0.6質量%以上であり、より好ましくは0.7質量%以上である。残留オーステナイト中の固溶C量の上限は特に限定はしないが、未変態オーステナイト中に過度にCを濃化させると未変態オーステナイトが分解し、残留オーステナイトが減少する場合があるため、好ましくは1.5質量%以下とする。
全残留オーステナイト粒のうち、アスペクト比が2.0未満の残留オーステナイト粒の割合が50%未満では衝突特性が低下する場合がある。この理由は明らかではないが次のように考えられる。残留オーステナイトは加工硬化し曲げ変形部のひずみを分散することで衝突特性を向上させるが、変形過程で加工誘起変態によって生成したフレッシュマルテンサイトはボイドの起点となりやすい。残留オーステナイトのアスペクト比の高い加工誘起マルテンサイトの界面でボイドが生成した場合、ボイドが界面に沿って急激に粗大化し、割れの進展を助長する。したがって、割れの進展を抑制しつつ残留オーステナイトのひずみ分散能を活用するために全残留オーステナイト粒のうち、アスペクト比が2.0未満の残留オーステナイト粒の割合は50%以上とする。当該割合は、より好ましくは60%以上とする。当該割合は高い方がよいため、上限は特に限定されない。
本発明の亜鉛めっき鋼板において、上記ボイド数密度を1000個/mm2以下とすることで高い衝突特性が得られる。このメカニズムは明らかではないが、次のように考えられる。衝突特性劣化の原因となる衝突時の破断は、割れの発生及び進展が起点となる。割れは加工硬化能の低下及び高硬度差領域でのボイドの生成及び連結によって発生しやすくなると考えられる。また、実部材の衝突では一次加工を受けた箇所で一次加工と直交方向に曲げ戻されるように変形する。このとき一次加工の高硬度差領域でボイドが発生するとボイドの周辺に応力が集中し、割れの発生・進展が助長され、その結果破断に至る。そこで、焼戻しマルテンサイト及びベイナイトによって高硬度差領域を減少させ、さらに必要に応じて残留オーステナイトを活用し変形中に一次加工部での応力集中を抑制することで、一次加工部におけるボイド発生、進展及びそれに伴う部材破断を抑制し、高い耐破断特性が得られる。したがって、これらの効果を得るために上記ボイド数密度を1000個/mm2以下とする。また、上記ボイド数密度が小さいほど軸圧壊時の破断が抑制されると考えられることから、下限は特に限定しない。
また、曲げ加工後のボイドの圧延方向における測定位置については、曲げ加工により形成され、幅(C)方向(図1の符号D1参照)に延びた角部X0を含む領域とする。より具体的には、曲げ加工により幅方向及び圧延方向に垂直な方向(パンチ等の押圧部の押圧方向)で最下部となる領域において、板厚方向に0~50μm領域内(図1の符号XA参照)でボイドの数密度を測定する。
また、圧縮側の鋼板表面とは、上記の押圧した一方の側の鋼板表面(押圧を施すパンチ等の押圧部と接触する方の鋼板表面)のことを指す。
また、曲げ加工後のL断面については、曲げ加工による変形の方向に対し平行に、且つ鋼板表面に対し垂直に切断することで形成される断面であって、幅方向に対し垂直な断面のことを指す。
まず、鋼板に対して、曲率半径/板厚:4.2で幅(C)方向を軸に圧延(L)方向に90°曲げ加工(一次曲げ加工)を施し、試験片を準備する。90°曲げ加工(一次曲げ加工)では、図2に示すように、V溝を有するダイA1の上に載せた鋼板に対して、パンチB1を押し込んで試験片T1を得る。次に、図3に示すように、支持ロールA2の上に載せた試験片T1に対して、曲げ方向が圧延直角方向となるようにして、パンチB2を押し込んで直交曲げ(二次曲げ加工)を施した。図2及び図3において、D1は幅(C)方向、D2は圧延(L)方向を示している。
[直交曲げ条件]
試験方法:ロール支持、パンチ押し込み
ロール径:φ30mm
パンチ先端R:0.4mm
ロール間距離:(板厚×2)+0.5mm
ストローク速度:20mm/min
試験片サイズ:60mm×60mm
曲げ方向:圧延直角方向
上記直交曲げを施した際に得られるストローク-荷重曲線において、荷重最大時のストロークを求める。上記曲げ-直交曲げ試験を3回実施した際の当該荷重最大時のストロークの平均値をΔSとする。
軸圧壊試験では板厚の影響を考慮し、全て板厚1.2mmの亜鉛めっき鋼板で実施する。上記製造工程で得られた亜鉛めっき鋼板を切り出し、パンチ肩半径が5.0mmであり、ダイ肩半径が5.0mmである金型を用いて、深さ40mmとなるように成形加工(曲げ加工)して、図6及び図7に示すハット型部材10を作製する。また、ハット型部材の素材として用いた亜鉛めっき鋼板を、200mm×80mmの大きさに別途切り出す。次に、その切り出した後の亜鉛めっき鋼板20と、ハット型部材10とをスポット溶接し、図6及び図7に示すような試験用部材30を作製した。図6は、ハット型部材10と亜鉛めっき鋼板20とをスポット溶接して作製した試験用部材30の正面図である。図7は、試験用部材30の斜視図である。スポット溶接部40の位置は、図7に示すように、亜鉛めっき鋼板の端部と溶接部が10mm、溶接部間が45mmの間隔となるようにする。次に、図8に示すように、試験用部材30を地板50とTIG溶接により接合して軸圧壊試験用サンプルを作製する。次に、作製した軸圧壊試験用サンプルにインパクター60を衝突速度10m/sで等速衝突させ、軸圧壊試験用のサンプルを100mm圧壊する。図8に示すように、圧壊方向D3は、試験用部材30の長手方向と平行な方向とする。圧壊時のストローク-荷重のグラフにおける、ストローク0~100mmの範囲における面積を求め、3回試験を行った際の当該面積の平均値を吸収エネルギー(Fave)とする。
Cはフェライト以外の相を生成しやすくし、また、NbやTiなどと合金化合物を形成するため、強度向上に必要な元素である。C含有量が0.03%未満では、製造条件の最適化を図っても、所望の強度を確保できない場合がある。したがって、C含有量は好ましくは0.03%以上であり、より好ましくは0.05%以上である。一方、C含有量が0.20%を超えるとマルテンサイトの強度が過剰に増加し、製造条件の最適化を図っても本発明の衝突特性が得られない場合がある。したがって、C含有量は好ましくは0.20%以下であり、より好ましくは0.18%以下である。
Siは炭化物生成を抑制するため、残留オーステナイトが得られる。また、固溶強化元素でもあり、強度と延性のバランスの向上に寄与する。この効果を得るために、Si含有量は好ましくは0.10%以上であり、より好ましくは0.50%以上である。一方、Si含有量が2.00%を超えると、亜鉛めっき付着、密着性の低下及び表面性状の劣化を引き起こす場合がある。したがって、Si含有量は好ましくは2.00%以下であり、より好ましくは1.50%以下である。
Mnはマルテンサイトの生成元素であり、また、固溶強化元素でもある。また、残留オーステナイト安定化に寄与する。これらの効果を得るために、Mn含有量は好ましくは0.5%以上である。Mn含有量は、より好ましくは1.0%以上である。一方、Mn含有量が2.5%を超えると残留オーステナイト分率が過剰に増加し、衝突特性が低下する場合がある。したがって、Mn含有量は好ましくは2.5%以下であり、より好ましくは2.0%以下である。
Pは、鋼の強化に有効な元素である。しかしながら、P含有量が0.05%を超えると合金化速度を大幅に遅延させる場合がある。また、Pを0.05%超えて過剰に含有させると、粒界偏析により脆化を引き起こし、本発明の鋼組織を満たしても衝突時の耐破断特性を劣化させる場合がある。したがって、P含有量は好ましくは0.05%以下であり、より好ましくは0.01%以下である。P含有量に特に下限は無いが、現在工業的に実施可能な下限は0.002%であり、0.002%以上であることが好ましい。
Sは、MnSなどの介在物となって、溶接部のメタルフローに沿った割れの原因となり、本発明の鋼組織を満たしても衝突特性が低下する場合がある。したがって、S量は極力低い方がよいが、製造コストの面からS含有量は好ましくは0.05%以下である。S含有量は、より好ましくは、0.01%以下である。S含有量に特に下限は無いが、現在工業的に実施可能な下限は0.0002%であり、0.0002%以上であることが好ましい。
Alは脱酸剤として作用し、また、固溶強化元素でもある。Sol.Al含有量が0.005%未満ではこれらの効果は得られない場合があり、本発明の鋼組織を満たしても強度が低下する場合がある。したがって、Sol.Al含有量は、好ましくは0.005%以上である。一方、Sol.Al含有量が0.100%を超えると製鋼時におけるスラブ品質を劣化させる。したがって、Sol.Al含有量は、好ましくは0.100%以下であり、より好ましくは0.050%以下である。
Nは粗大な窒化物を形成するため、衝突変形時にボイド生成の起点となり、衝突特性を低下させる場合がある。したがってN量は極力少ない方がよいが、製造コストの面からN含有量は好ましくは0.010%以下であり、より好ましくは0.006%以下である。なお、N含有量の下限は特に限定されるものではないが、現在工業的に実施可能な下限は0.0003%であり、0.0003%以上であることが好ましい。
Cr、Mo、Vは焼き入れ性を上げ、鋼の強化に有効な元素である。しかし、Cr:1.0%、Mo:0.5%、V:0.5%を超えて過剰に添加すると、上記の効果が飽和し、さらに原料コストが増加する。また、第2相分率が過大となり衝突時の耐破断特性を劣化させる場合がある。したがって、Cr、Mo、Vのいずれかを含有する場合、Cr含有量は好ましくは1.0%以下、Mo含有量は好ましくは0.5%以下、V含有量は好ましくは0.5%以下である。より好ましくは、Cr含有量は0.8%以下であり、Mo含有量は0.4%以下であり、V含有量は0.4%以下である。Cr、Mo、Vの含有量が少なくても本発明の効果は得られるので、それぞれの含有量の下限は特に限定されない。焼き入れ性の効果をより有効に得るためには、Cr、Mo、Vの含有量はそれぞれ0.005%以上であることが好ましい。より好ましくは、Cr、Mo、Vの含有量はそれぞれ0.01%以上である。
仕上げ圧延温度が850℃未満の場合、圧延時にフェライト変態が起こり、局所的に強度が低下するため、本発明の組織を満たしても強度が得られない場合がある。したがって、仕上げ圧延温度は850℃以上であり、好ましくは880℃以上である。一方、仕上げ圧延温度が950℃を超えると結晶粒が粗大化し、本発明の組織を満たしても強度が得られない場合がある。したがって、仕上げ圧延温度は950℃以下であり、好ましくは930℃以下である。
巻取温度が600℃を超えた場合、熱延鋼板中の炭化物が粗大化し、このような粗大化した炭化物は焼鈍時の均熱中に溶けきらないため、必要な衝突特性を得ることができない場合がある。したがって、巻取温度は、600℃以下であり、好ましくは580℃以下である。巻取温度の下限は特に限定されないが、鋼板の形状不良を発生しにくくし、かつ鋼板が過度に硬質化することを防ぐ観点から、巻取温度を400℃以上とすることが好ましい。
冷間圧延の圧下率が20%以下では、フェライトの再結晶が促進されず、未再結晶フェライトが残存し、本発明の鋼組織が得られない場合がある。したがって、冷間圧延の圧下率は20%超えであり、好ましくは30%以上である。
焼鈍温度が720℃未満では、オーステナイトの生成が不十分となり、過剰なフェライトが生成して本発明の鋼組織が得られない。したがって、焼鈍温度は720℃以上であり、好ましくは740℃以上である。一方、焼鈍温度が860℃を超えると本発明のフェライト分率を確保できなくなる。したがって、焼鈍温度は860℃以下であり、好ましくは840℃以下である。また、保持時間が30秒未満では、オーステナイトの生成が不十分となり、過剰なフェライトが生成して本発明の鋼組織が得られない。したがって、保持時間は30秒以上であり、好ましくは60秒以上である。保持時間の上限は特に限定されないが、生産性を損なわないようにするために、保持時間を600秒以下とすることが好ましい。
焼鈍工程後300~600℃の温度域まで冷却し、300~600℃の温度域で10~300秒保持することはベイナイトを得るために有効である。また、ベイナイト生成によって未変態オーステナイト中にCが濃化することで多量の残留オーステナイトが得られる。10秒未満ではこれらの効果が得られない。また、300秒を超えるとベイナイトが過剰に生成し、未変態オーステナイト中にCが過剰に濃化し、パーライトが生成し、所望の残留オーステナイト量が得られない場合がある。したがって、保持時間は300秒以下であり、好ましくは100秒以下である。
冷却停止温度が(Ms-50℃)超えでは焼戻しマルテンサイトの生成が不十分であり、本発明の鋼組織が得られない。一方、冷却停止温度が(Ms-250℃)未満では焼戻しマルテンサイトが過剰になり、残留オーステナイトの生成が不十分となる場合がある。したがって、冷却停止温度は(Ms-250℃)~(Ms-50℃)である。好ましくは(Ms-200℃)以上である。また、好ましくは、(Ms-100℃)以下である。なお、冷却停止温度が上記の範囲を満たしていれば冷却停止温度までの冷却速度は限定されず、本発明の鋼組織が得られる。
上記式において、[元素記号%]は、各元素の含有量(質量%)を表し、含有しない元素は0とする。また、[α面積%]は焼鈍後のフェライト面積率(%)である。焼鈍後のフェライト面積率は、熱膨張測定装置で昇温速度、焼鈍温度及び焼鈍時の保持時間を模擬することによって事前に求める。
焼戻し温度が300℃未満ではマルテンサイトの焼戻しが不十分となり、一次加工時に焼戻しマルテンサイトとフェライトの界面でボイドが発生しやすくなり、衝突特性が低下すると考えられる。したがって、焼戻し温度は300℃以上であり、好ましくは350℃以上である。一方、焼戻し温度が500℃を超えるとマルテンサイト及びベイナイトの焼戻しが過剰となり、一次加工時にフレッシュマルテンサイトと焼戻しマルテンサイト及びベイナイトの界面でボイドが生成しやすくなり、衝突特性が低下すると考えられる。したがって、焼戻し温度は500℃以下であり、好ましくは450℃以下である。また、保持時間が20秒未満ではマルテンサイトの焼戻しが不十分となり、衝突特性が低下すると考えられる。したがって、保持時間は20秒以上であり、好ましくは30秒以上である。また、保持時間が500秒を超えるとアスペクト比が2.0未満の残留オーステナイトの割合が減少する場合がある。したがって保持時間の上限は500秒以下であり、好ましくは450秒以下である。
上記焼戻し温度から50℃までの平均冷却速度が20℃/s未満では本発明の衝突特性が得られない。この理由は明らかではないが次のように考えられる。一次加工部のボイド生成を抑制し、衝突特性を向上させるためには軟質相(フェライト)と硬質相(フレッシュマルテンサイト)との硬度差を中間硬度相(焼戻しマルテンサイト、ベイナイト)で低減する必要がある。前者はめっき処理前に生成させたベイナイト及び焼入れ時に生成させたマルテンサイトを焼戻し工程で軟化さることで軟質相との硬度差を低減しボイドの生成を抑制している。後者は焼戻し工程で生成させたベイナイトでボイドの生成を抑制している。焼戻し工程で生成させたベイナイトは軟化すると硬質相との硬度差が大きくなるため、ベイナイトが生成する焼戻し工程前に高温にさらされるめっき処理を行い、さらに焼戻し工程後の冷却速度を速くすることで冷却中のベイナイトの焼戻しを抑制することで、軟質相との硬度差を低減しボイドの生成を抑制している。したがって、焼戻し工程後における室温までの平均冷却速度が20℃/s未満では、冷却中にベイナイトが焼戻され、硬質相との硬度差が大きくなってしまうことにより、一次加工時にその界面でボイドが生成しやすくなり、衝突特性が低下すると考えられる。平均冷却速度は、好ましくは25℃/s以上である。平均冷却速度の上限は特に限定されないが、冷却設備の省エネルギーの観点から、70℃/s以下が好ましい。
表1に示す成分組成の鋼を真空溶解炉により溶製し、分塊圧延して鋼スラブとした。これらの鋼スラブを加熱し、表2に示す条件で、熱間圧延、冷間圧延、焼鈍、めっき処理、焼入れ及び焼戻し、並びに冷却を行い、亜鉛めっき鋼板を製造した。めっき処理では、鋼板表面に電気亜鉛めっき層(EG)、溶融亜鉛めっき層(GI)又は合金化溶融亜鉛めっき層(GA)を形成させた。電気亜鉛めっき処理では、鋼板を亜鉛溶液に浸漬しつつ通電し、めっき付着量10~100g/m2の電気亜鉛めっき層(EG)を形成させた。また、溶融亜鉛めっき処理では、鋼板をめっき浴中に浸漬し、めっき付着量10~100g/m2の溶融亜鉛めっき層(GI)を形成させた。また、合金化溶融亜鉛めっきでは、鋼板に溶融亜鉛めっき層を形成した後に合金化処理を行い、合金化溶融亜鉛めっき層(GA)を形成させた。なお、最終的な各亜鉛めっき鋼板の板厚は、1.2mmであった。
また、曲げ加工後のボイドの圧延方向における測定位置については、曲げ加工により形成され、幅(C)方向(図1の符号D1参照)に延びた角部X0を含む領域とした。より具体的には、曲げ加工により幅方向及び圧延方向に垂直な方向(パンチ等の押圧部の押圧方向)で最下部となる領域において、板厚方向に0~50μm領域内(図1の符号XA参照)でボイドの数密度を測定した。
得られた各亜鉛めっき鋼板から圧延方向に対して直角方向にJIS5号引張試験片(JIS Z2201)を採取し、歪速度が10-3/sとするJIS Z2241(2011)の規定に準拠した引張試験を行い、引張強度(TS)を求めた。なお、TSが590MPa以上を合格とした。
得られた鋼板に対して、曲率半径/板厚:4.2で幅(C)方向を軸に圧延(L)方向に90°曲げ加工(一次曲げ加工)を施し、試験片を準備した。90°曲げ加工(一次曲げ加工)では、図2に示すように、V溝を有するダイA1の上に載せた鋼板に対して、パンチB1を押し込んで試験片T1を得た。次に、図3に示すように、支持ロールA2の上に載せた試験片T1に対して、曲げ方向が圧延直角方向となるようにして、パンチB2を押し込んで直交曲げ(二次曲げ加工)を施した。図2及び図3において、D1は幅(C)方向、D2は圧延(L)方向を示している。
[直交曲げ条件]
試験方法:ロール支持、パンチ押し込み
ロール径:φ30mm
パンチ先端R:0.4mm
ロール間距離:(板厚×2)+0.5mm
ストローク速度:20mm/min
試験片サイズ:60mm×60mm
曲げ方向:圧延直角方向
上記直交曲げを施した際に得られるストローク-荷重曲線において、荷重最大時のストロークを求めた。上記曲げ-直交曲げ試験を3回実施した際の当該荷重最大時のストロークの平均値をΔSとした。ΔSが30mm以上で耐破断特性が良好と評価した。
軸圧壊試験では板厚の影響を考慮し、全て板厚1.2mmの亜鉛めっき鋼板で実施した。上記製造工程で得られた亜鉛めっき鋼板を切り出し、パンチ肩半径が5.0mmであり、ダイ肩半径が5.0mmである金型を用いて、深さ40mmとなるように成形加工(曲げ加工)して、図6及び図7に示すハット型部材10を作製した。また、ハット型部材の素材として用いた亜鉛めっき鋼板を、200mm×80mmの大きさに別途切り出した。次に、その切り出した後の亜鉛めっき鋼板20と、ハット型部材10とをスポット溶接し、図6及び図7に示すような試験用部材30を作製した。図6は、ハット型部材10と亜鉛めっき鋼板20とをスポット溶接して作製した試験用部材30の正面図である。図7は、試験用部材30の斜視図である。スポット溶接部40の位置は、図7に示すように、亜鉛めっき鋼板の端部と溶接部が10mm、溶接部間が45mmの間隔となるようにした。次に、図8に示すように、試験用部材30を地板50とTIG溶接により接合して軸圧壊試験用サンプルを作製した。次に、作製した軸圧壊試験用サンプルにインパクター60を衝突速度10m/sで等速衝突させ、軸圧壊試験用のサンプルを100mm圧壊した。図8に示すように、圧壊方向D3は、試験用部材30の長手方向と平行な方向とした。圧壊時のストローク-荷重のグラフにおける、ストローク0~100mmの範囲における面積を求め、3回試験を行った際の当該面積の平均値を吸収エネルギー(Fave)とした。Faveが35000N以上で吸収エネルギーが良好と評価した。また、耐破断特性及び吸収エネルギーの両方が良好の場合、衝突特性が良好と評価した。
実施例1の表3のNo.1(本発明例)の亜鉛めっき鋼板を、プレス加工により成形加工して、本発明例の部材を製造した。さらに、実施例1の表3のNo.1の亜鉛めっき鋼板と、実施例1の表3のNo.3(本発明例)の亜鉛めっき鋼板とをスポット溶接により接合し、本発明例の部材を製造した。本発明の鋼板を用いて製造した本発明例の部材は、衝突特性に優れており、高強度であり、実施例1の表3のNo.1(本発明例)の鋼板の成形加工により製造した部材、および実施例1の表3のNo.1の鋼板と、実施例1の表3のNo.3(本発明例)の鋼板とをスポット溶接して製造した部材のすべてにおいて、自動車用骨格部品等に好適に用いることができることを確認できた。
20 亜鉛めっき鋼板
30 試験用部材
40 スポット溶接部
50 地板
60 インパクター
A1 ダイ
A2 支持ロール
B1 パンチ
B2 パンチ
D1 幅(C)方向
D2 圧延(L)方向
D3 圧壊方向
T1 試験片
T2 試験片
X0 角部
XA 曲げ加工後のボイドの測定位置(測定領域)
Claims (8)
- 炭素当量Ceqが0.35%以上0.60%未満を満たす成分組成と、
面積率で、フェライト:40~80%、焼戻しマルテンサイト及びベイナイトの合計:15~55%、残留オーステナイト:3~20%、フレッシュマルテンサイト:10%以下、フェライト、焼戻しマルテンサイト、ベイナイト、残留オーステナイト及びフレッシュマルテンサイトの合計:90%以上である鋼組織と、を有する鋼板と、
前記鋼板表面上に亜鉛めっき層と、を備え、
前記残留オーステナイト中の固溶C量が0.6質量%以上であり、
全残留オーステナイト粒のうち、アスペクト比が2.0未満の残留オーステナイト粒の割合が、50%以上であり、
曲率半径/板厚:4.2で幅(C)方向を軸に圧延(L)方向に90°曲げ加工した際に、圧縮側の鋼板表面から0~50μm領域内のL断面において、ボイド数密度が1000個/mm2以下であり、
引張強度が590MPa以上である亜鉛めっき鋼板。 - 前記成分組成は、質量%で、
C:0.03~0.20%、
Si:0.10~2.00%、
Mn:0.5~2.5%、
P:0.05%以下、
S:0.05%以下、
Sol.Al:0.005~0.100%、及び
N:0.010%以下を含有し、残部がFe及び不可避的不純物からなる請求項1に記載の亜鉛めっき鋼板。 - 前記成分組成は、さらに、質量%で、
Cr:1.0%以下、
Mo:0.5%以下、
V:0.5%以下、
Ti:0.5%以下、
Nb:0.5%以下、
B:0.005%以下、
Ni:1.0%以下、
Cu:1.0%以下、
Sb:1.0%以下、
Sn:1.0%以下、
Ca:0.005%以下、及び
REM:0.005%以下のうちから選ばれる少なくとも1種を含有する請求項2に記載の亜鉛めっき鋼板。 - 前記亜鉛めっき層が、電気亜鉛めっき層、溶融亜鉛めっき層又は合金化溶融亜鉛めっき層である請求項1から請求項3までのいずれか一項に記載の亜鉛めっき鋼板。
- 請求項1から請求項4までのいずれか一項に記載の亜鉛めっき鋼板に対して、成形加工及び溶接の少なくとも一方を施してなる部材。
- 炭素当量Ceqが0.35%以上0.60%未満を満たし、かつ請求項2又は請求項3に記載の成分組成を有する鋼スラブを、仕上げ圧延温度を850~950℃として熱間圧延を施し、600℃以下の巻取温度で巻取る熱間圧延工程と、
前記熱間圧延工程後の熱延鋼板を20%超えの圧下率で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鋼板を720~860℃の焼鈍温度まで加熱し、30秒以上保持する焼鈍工程と、
前記焼鈍工程後、300~600℃の温度域まで冷却し、当該温度域で10~300秒保持した後、鋼板表面に亜鉛めっき処理を施すめっき工程と、
前記めっき工程後、(Ms-250℃)~(Ms-50℃)の冷却停止温度まで冷却した後、300~500℃の焼戻し温度で20~500秒保持する焼入れ及び焼戻し工程と、
前記焼入れ及び焼戻し工程後に、前記焼戻し温度から50℃までを平均冷却速度20℃/s以上で冷却する冷却工程と、を有する亜鉛めっき鋼板の製造方法。 - 前記亜鉛めっき処理は、鋼板表面に、電気亜鉛めっき、溶融亜鉛めっき、又は合金化溶融亜鉛めっきを施す処理である請求項6に記載の亜鉛めっき鋼板の製造方法。
- 請求項6又は請求項7に記載の亜鉛めっき鋼板の製造方法によって製造された亜鉛めっき鋼板に対して、成形加工及び溶接の少なくとも一方を施す工程を有する部材の製造方法。
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EP4141132A4 (en) | 2023-10-11 |
CN115715332A (zh) | 2023-02-24 |
CN115715332B (zh) | 2024-04-12 |
US20230243014A1 (en) | 2023-08-03 |
JP7031801B1 (ja) | 2022-03-08 |
JPWO2022004820A1 (ja) | 2022-01-06 |
JP2022034016A (ja) | 2022-03-02 |
KR20230013273A (ko) | 2023-01-26 |
MX2022016016A (es) | 2023-02-02 |
EP4141132A1 (en) | 2023-03-01 |
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