Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
• • 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

Definitions

• the present invention relates to a galvannealed steel sheet and a member.
• This application claims priority based on Japanese Patent Application No. 2024-062779, filed on April 9, 2024, the contents of which are incorporated herein by reference.
• Patent Document 1 discloses a galvannealed steel sheet characterized in that the surface of the cold-rolled steel sheet has an Fe—Zn alloy coating consisting of a ⁇ 1 phase alone or a ⁇ 1 phase and a ⁇ 1 phase having a thickness of 1 ⁇ m or less, and the surface layer crystal has an average aspect ratio of 3 or less.
• Patent Document 1 is said to be capable of exhibiting excellent surface treatability and powdering resistance in addition to the sliding properties required for press forming.
• the technology of Patent Document 1 does not anticipate producing a high-strength galvannealed steel sheet having a tensile strength of 1180 MPa or more. Therefore, it can be said that the technology of Patent Document 1 has room for improvement in terms of achieving both excellent formability (coating adhesion and sliding properties) and high strength. It is also conceivable to adjust the components or manufacturing process to increase strength, but depending on the conditions, this can result in poor appearance, such as the appearance of metallic luster spots on the plated surface.
• the present invention therefore aims to provide a galvannealed steel sheet that is high in strength, has excellent formability, and is less susceptible to appearance defects, as well as a member that includes the galvannealed steel sheet.
• a first aspect of the present invention is a steel sheet having a chemical composition, in mass%, of C: 0.10 to 0.35%, Si: 0.01 to 2.00%, Mn: 2.8 to 4.0%, P: 0 to 0.100%, S: 0 to 0.100%, N: 0 to 0.020%, Al: 0.001 to 1.500%, O: 0 to 0.010%, Cr: 0 to 0.80%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ti: 0 to 0.1000%, Nb: 0 to 0.4000%, V: 0 to 0.50%, Ni: 0 to 1.0000%, Cu: 0 to 1.00%, REM: 0 to 0.0100%, As: 0 to 0.200%, Sb: 0 to 0.200 %, Sn: 0-0.20%, W: 0-0.100%, Co: 0-2.0%, Zn: 0-0.100%, Zr: 0-0.0
• the steel sheet has a Zn-containing plating layer on the surface of the steel material, wherein the aspect ratio of crystal grains on the surface of the plating layer is 4.0 or less, the thickness of the ⁇ phase in the plating layer is 1.0 ⁇ m or less, and the tensile strength is 1180 MPa or more.
• the above-described aspects of the present invention make it possible to provide a galvannealed steel sheet that is high in strength, has excellent workability, and is less susceptible to appearance defects, as well as a member that includes the galvannealed steel sheet.
• FIG. 2 is a schematic diagram showing the surface of a plating layer on which spot-shaped metallic luster portions appear.
• FIG. 2 is a schematic diagram showing a cross section of a galvannealed steel sheet in the vicinity of a metallic luster portion that appears in a plating layer.
• FIG. 1 is a cross-sectional view of a galvannealed steel sheet according to an embodiment of the present invention.
• FIG. 1 is a first schematic diagram for explaining a method for evaluating the adhesion of coating of a galvannealed steel sheet (after bending and unbending) in an example.
• FIG. 2 is a second schematic diagram for explaining a method for evaluating the adhesion of coating of a galvannealed steel sheet (after bending and unbending) in Examples.
• FIG. 1 is a first schematic diagram for explaining a method for evaluating the adhesion of coating of a galvannealed steel sheet (after bending and unbending) in Examples.
• FIG. 1 is a first schematic diagram
• FIG. 10 is a third schematic diagram for explaining a method for evaluating the adhesion of coating of a galvannealed steel sheet (after bending and unbending) in Examples.
• FIG. 1 is a first schematic diagram for explaining a method for evaluating the adhesion of the coating (after ironing), i.e., the sliding properties, of a galvannealed steel sheet in an example.
• FIG. 2 is a second schematic diagram for explaining a method for evaluating the adhesion of the coating (after ironing), i.e., the sliding properties, of the galvannealed steel sheet in the examples.
• 1 is a graph showing the relationship between the ⁇ phase thickness and the adhesion (after bending and unbending) for Examples.
• 1 is a graph showing the relationship between the aspect ratio of surface crystals and adhesion (after ironing) for Examples.
• the present inventors have conducted extensive research into a galvannealed steel sheet (and a member including the galvannealed steel sheet) that can suppress the occurrence of defects in appearance while achieving both strength and workability.
• a galvannealed steel sheet and a member including the galvannealed steel sheet
• the metal structure constituting the steel material is a structure mainly composed of a hard martensite phase. Therefore, after selecting an appropriate composition, the plating process and alloying process are controlled so that the austenite phase becomes the main phase, and the martensite phase is obtained by rapid cooling (quenching) after alloying heating.
• Excellent sliding properties can be obtained by suppressing the formation of a phase called the ⁇ phase, which is prone to adhesion to soft dies, on the surface of the plating layer.
• alloying heating should be performed under conditions that result in high and rapid temperature rise.
• Excellent plating adhesion can be achieved by suppressing the formation of a hard and brittle Fe-alloyed phase called the ⁇ phase in the plating layer and reducing the thickness of the ⁇ phase. If the ⁇ phase is thicker than a certain amount, the plating will peel off during press forming.
• FIG 1 is a schematic diagram showing the surface of a plating layer on which spot-like metallic luster areas a have appeared. As shown in Figure 1, each spot-like metallic luster area a has a circle-equivalent diameter of approximately 2 to 5 mm based on area measurement, and appears in rows. The inventors analyzed the poor appearance associated with numerous localized metallic luster areas a that occurs during rapid alloying heating.
• FIG. 2 is a schematic diagram showing a cross section of a galvannealed steel sheet 1001 in the vicinity of a metallic luster portion a that appears in the plating layer.
• the alloying reaction had progressed sufficiently outside the metallic luster part a, and an Fe-Zn alloy layer b (healthy part) had been formed, whereas the alloying reaction had not progressed locally within the metallic luster part a, and an initial alloy layer a1 consisting of Fe and Al remained, with an extremely thin Fe-Zn alloy layer a2 (defective part).
• the present inventors speculate that the reason for the above phenomenon is as follows.
• the steel material 1011 used as the base material for plating contains microscopic variations in structure, composition, etc.
• a C-deficient layer (decarburized layer) or a layer depleted of alloying elements such as Si and Mn is present in the surface layer of the steel sheet, the crystal grains in these depleted layers grow significantly when the steel sheet is heated to a high temperature during an annealing process, etc.
• the alloying reaction is less likely to occur during the temperature rise process, and the alloying reaction progresses rapidly at a higher temperature.
• the inventors have confirmed that when the heating rate is slowed, no significant defects in appearance occur. This is thought to be because, when the heating rate is slow, the alloying reaction proceeds slowly even during heating, and even if there are areas where the alloying reaction proceeds slowly due to some microscopic variation in reactivity, alloying begins before the alloying reaction in surrounding areas where the alloying reaction proceeds quickly has progressed sufficiently.
• improving the alloying rate is an effective heating method that does not result in poor appearance, even when rapid heating is used. They have also found that it is not necessary to selectively improve the alloying rate only in areas with poor reactivity that are several millimeters in diameter and result in poor appearance; they have found that improving the average alloying rate across the entire plating layer is sufficient to achieve the desired effect.
• the galvannealed steel sheet 1 according to this embodiment is a steel sheet used as a member.
• FIG. 3 is a cross-sectional view of a galvannealed steel sheet 1 according to this embodiment.
• the galvannealed steel sheet 1 has a Zn-containing plating layer 13 on the surface of a steel material 11 having a predetermined chemical composition. The chemical composition of the steel material will be explained below.
• C 0.10-0.35% C is an essential element for obtaining the desired tensile strength. If the C content is less than 0.10%, the desired tensile strength cannot be obtained, so the C content is 0.10% or more, preferably 0.13% or more, 0.15% or more, or 0.20% or more. On the other hand, if the C content exceeds 0.35%, the hydrogen embrittlement resistance and weldability of the steel material will decrease, so the C content is 0.35% or less, preferably 0.33% or less, and more preferably 0.30% or less.
• the Si content is 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more.
• the Si content is 2.00% or less, preferably 1.90% or less, 1.50% or less, and more preferably 1.00% or less.
• Mn is an austenite stabilizing element and is effective in improving the hardenability of steel materials. If the Mn content is less than 2.8%, hardening will be insufficient and the desired tensile strength will tend to be difficult to obtain. Therefore, the Mn content is 2.8% or more, preferably 2.9% or more, 3.0% or more, and more preferably 3.2% or more. If the Mn content exceeds 4.0%, oxides containing Mn that interfere with the alloying reaction are formed on the steel sheet surface, and the alloying rate is likely to vary due to the distribution of Mn oxides, resulting in a deterioration in appearance quality. Therefore, the Mn content is 4.0% or less, preferably 3.9% or less, 3.8% or less, or 3.5% or less.
• P is a solid solution strengthening element and is effective in increasing the strength of steel materials. However, if the P content exceeds 0.100%, the weldability and toughness of the steel material decrease, so the P content is 0.100% or less, preferably 0.050% or less, 0.040% or less, or 0.030% or less.
• the P content may be 0%, 0.0001% or more, or 0.001% or more.
• S is an impurity element, and the lower the content, the better. However, if the S content exceeds 0.100%, MnS is formed in the steel material, deteriorating toughness and hole expandability. Therefore, the S content is 0.100% or less, preferably 0.090% or less, 0.080% or less, 0.055% or less, 0.030% or less, or 0.020% or less.
• the S content may be 0%, 0.0001% or more, or 0.001% or more.
• N is an impurity element, and the lower the content, the better. If the N content exceeds 0.020%, coarse nitrides are formed in the steel material, reducing the hole expandability. Therefore, the N content is 0.020% or less, preferably 0.015% or less, 0.012% or less, or 0.010% or less.
• the N content may be 0%, 0.0001% or more, or 0.001% or more.
• Al is an element added for deoxidation.
• the Al content is set to 0.001% or more.
• the Al content is preferably 0.003% or more, 0.005% or more, 0.008% or more, or 0.010% or more.
• the Al content is set to 1.500% or less, preferably 1.000% or less, 0.500% or less, 0.300% or less, and more preferably 0.200% or less.
• the O content exceeds 0.010%, various oxides are formed, which may adversely affect the mechanical properties of the steel sheet, so the upper limit of the O content is 0.010%, preferably 0.008% or less, 0.006% or less, or 0.005% or less.
• the O content may be 0%, 0.0001% or more, or 0.001% or more.
• Cr, Mo, B, Ti, Nb, V, Ni, Cu, and REM are optional elements, and the lower limit for each is 0%.
• the preferred lower and upper limits for each element when included are explained below.
• the Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more.
• the upper limit of the Cr content is preferably 0.80%, and may also be 0.70% or less, 0.60% or less, or 0.50% or less.
• Mo 0.001-1.00%
• various properties such as strength, hole expandability, elongation, etc. of the steel material can be improved.
• the Mo content may be 0.01% or more, 0.05% or more, or 0.10% or more.
• the upper limit of the Mo content is preferably 1.00%, and may also be 0.80% or less, 0.50% or less, 0.20% or less, or 0.10% or less.
• B 0.0001-0.0100%
• various properties such as strength, hole expandability, and elongation of the steel material can be improved.
• the B content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more. Since a sufficient effect can be obtained even if the B content is 0.0100% or less, the upper limit of the B content is preferably 0.0100%, and may be 0.0080% or less, 0.0050% or less, or 0.0030% or less.
• Ti 0.0001-0.1000%
• various properties such as strength, hole expandability, elongation, etc. of the steel material can be improved.
• the Ti content may be 0.0003% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.
• the upper limit is preferably 0.1000%, and may be 0.0800% or less, 0.0600% or less, or 0.0500% or less.
• the Nb content is 0.0001% or more, various properties such as strength, hole expandability, elongation, etc. of the steel material can be improved.
• the Nb content may be 0.001% or more, or 0.005% or more.
• the upper limit is preferably 0.4000%, and may be 0.2000% or less, 0.1000% or less, or 0.0500% or less.
• V 0.0001-0.50%
• various properties such as strength, hole expandability, and elongation of the steel material can be improved.
• the V content may be 0.001% or more, 0.005% or more, or 0.010% or more.
• the upper limit is preferably 0.50%, and may be 0.30% or less, 0.20% or less, or 0.10% or less.
• Ni 0.0001-1.0000%
• various properties such as strength, hole expandability, elongation, etc. of the steel material can be improved.
• the Ni content may be 0.001% or more, 0.005% or more, 0.010% or more, 0.050% or more, or 0.100% or more. Since a sufficient effect can be obtained even if the Ni content is 1.0000% or less, the upper limit of the Ni content is preferably 1.0000%, and may be 0.8000% or less, 0.5000% or less, or 0.3000% or less.
• the Cu content is 0.001% or more, various properties such as strength, hole expandability, elongation, etc. of the steel material can be improved.
• the Cu content may be 0.005% or more, 0.010% or more, 0.050% or more, or 0.10% or more.
• the upper limit is preferably 1.00%, and may be 0.80% or less, 0.50% or less, or 0.30% or less.
• REM 0.0003-0.0100%
• the REM content may be 0.0005% or more, or 0.0010% or more. Since a sufficient effect can be obtained even with a REM content of 0.0100% or less, the upper limit of the REM content is preferably 0.0100%, and may be 0.0090% or less, 0.0080% or less, 0.0050% or less, or 0.0030% or less.
• REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the REM content refers to the total content of these elements.
• the upper limit is preferably 0.200%. However, since excessive removal increases the number of steps and man-hours, the content may be set to 0.001% or more.
• Sb is an element that is mixed in when scrap is used as a raw material. It may strongly segregate at grain boundaries, leading to embrittlement of the grain boundaries, a decrease in ductility, and a decrease in cold formability. Therefore, the upper limit is preferably 0.200%. However, since excessive removal increases the number of steps and man-hours, the content may be set to 0.001% or more.
• Sn is an element that is mixed in when scrap is used as a raw material. It may strongly segregate at grain boundaries, leading to embrittlement of the grain boundaries, a decrease in ductility, and a decrease in cold formability. Therefore, the upper limit is preferably 0.20%. However, since excessive removal increases the number of steps and man-hours, the content may be set to 0.001% or more.
• the W content is 0.001% or more, the strength of the steel material can be improved. If the W content exceeds 0.100%, it leads to a decrease in ductility and deteriorates the cold workability of the steel sheet, so the upper limit is preferably 0.100%.
• Co 0.01-2.0%
• the strength of the steel material can be improved. If the Co content exceeds 2.0%, the ductility is reduced and the cold workability of the steel sheet is deteriorated, so the upper limit is preferably 2.0%.
• the lower limit of the Zn content may be substantially 0%, or may be 0.0005%.
• Zr 0.0010-0.0500%
• the upper limit is preferably 0.0500%.
• Mg 0.001-0.050%
• the strength and bendability can be improved by adjusting the shape of the inclusions. Since a sufficient effect can be obtained even if the Mg content is 0.050% or less, the upper limit of the Mg content is preferably 0.050%.
• the Ca content is 0.001% or more, the strength and bendability can be improved by adjusting the shape of the inclusions. Since a sufficient effect can be obtained even if the Ca content is 0.050% or less, the upper limit of the Ca content is preferably 0.050%.
• Ta 0.001-0.100%
• the strength and bendability can be improved by adjusting the shape of the inclusions. Since a sufficient effect can be obtained even if the Ta content is 0.100% or less, the upper limit of the Ta content is preferably 0.100%.
• the Bi content is 0.001% or more, the strength and bendability can be improved by adjusting the shape of the inclusions. Since a sufficient effect can be obtained even if the Bi content is 0.050% or less, the upper limit of the Bi content is preferably 0.050%.
• Te 0.001-0.050%
• the strength and bendability can be improved by adjusting the shape of the inclusions. Since a sufficient effect can be obtained even if the Te content is 0.050% or less, the upper limit of the Te content is preferably 0.050%.
• the balance is composed of Fe and impurities.
• the impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to the embodiment of the present invention.
• the chemical composition of the steel material may be measured by a general analytical method.
• the chemical composition of the steel plate may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES) on chips in accordance with JIS G 1201:2014.
• ICP-AES inductively coupled plasma-atomic emission spectrometry
• a 35 mm square test piece is obtained from near the 1/4 position of the plate thickness of the steel material, and measured under conditions based on a previously prepared calibration curve using an ICPS-8100 or the like (measuring device) manufactured by Shimadzu Corporation. It can be identified by this.
• C and S which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-non-dispersive infrared absorption method.
• the plating layer on the surface of the steel material can be removed by mechanical grinding or the like before analyzing the chemical composition.
• the thickness of the steel material is not particularly limited, and should be between 0.6 mm and 5.0 mm, preferably between 1.0 mm and 3.0 mm, or between 1.2 mm and 2.4 mm.
• the plating layer contains, for example, 7.0 to 15.0% Fe, 0.1 to 1.0% Al, and the balance being Zn and impurities.
• impurities refers to components contained in raw materials or components mixed in during the manufacturing process, but not intentionally added. For example, trace amounts of components other than Fe may be mixed into the coating layer as impurities due to mutual atomic diffusion between the base steel and the coating bath.
• the above-mentioned content in the coating layer refers to the content of the coating layer as a whole, and not the content of localized portions, such as portions corresponding to specific phases in the coating layer.
• the plating layer is a galvannealed layer.
• the remainder of the plating layer, other than Fe, Al, and impurities, is Zn. It is preferable that the impurity content of the plating layer be 3.0% or less.
• the Fe content of the plating layer is 7.0% or more, the formation of a soft ⁇ phase can be suppressed, and adhesion between the die and the plating layer during press forming can be suppressed.
• the Fe content in the plating layer is preferably 8.0% or more, and more preferably 9.0% or more.
• the Fe content of the plating layer is 15.0% or less, the growth of the ⁇ phase can be suppressed and the adhesion of the plating can be ensured.
• the Fe content in the plating layer is preferably 13.0% or less, and more preferably 12.0% or less.
• the Al content in the plating layer is preferably 0.15% or more, and more preferably 0.2% or more.
• the Al content of the plating layer is 1.0% or less, the temperature required for alloying heating can be reduced, and the appearance quality after plating can be improved.
• the Al content in the plating layer is preferably 0.6% or less, and more preferably 0.5% or less.
• the chemical composition of the plating layer can be measured by a common analytical method.
• the plating layer of a sample is dissolved in dilute hydrochloric acid containing a commercially available inhibitor (e.g., Asahi Chemical's "Ivit 710K," concentration 0.04%), and the amount of Fe contained in the dilute hydrochloric acid after dissolving the plating layer is measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).
• the plating coating weight (g/m 2 ) is calculated from the mass difference between the sample before and after dissolving the plating layer, and the amount of Fe in the plating is calculated from the ratio of the Fe amount to the plating coating weight. Elements in the plating layer other than Fe can also be measured using a similar procedure.
• the ⁇ phase is an Fe-alloyed phase that forms in the coating layer near the interface with the steel substrate. Because the ⁇ phase is hard and brittle, if it exceeds a certain thickness, the coating may peel off during press forming. If the thickness of the ⁇ phase is 1.00 ⁇ m or less, peeling of the plating during press forming is suppressed and excellent plating adhesion is exhibited.
• the thickness of the ⁇ phase is preferably 0.90 ⁇ m or less, and more preferably 0.80 ⁇ m or less.
• the thickness of the ⁇ phase can be measured by an electrochemical method.
• a sample of a galvannealed steel sheet, in which the surface of the coating layer is masked with tape except for a measurement area of 20 mm diameter is immersed in a 150 g/L aqueous solution of NH 4 Cl, and a potential of ⁇ 0.940 V is applied to the sample.
• the sample is removed from the NH 4 Cl aqueous solution, and the surface of the measurement portion of the sample is rubbed vigorously with absorbent cotton to remove any residue.
• the sample is again immersed in the NH 4 Cl aqueous solution, and a voltage is applied to the sample so as to give a current density of 4 mA/cm 2.
• the ⁇ phase is a soft Fe-Zn alloyed phase that forms in the plating layer. Because the ⁇ phase on the surface of the plating layer is soft, it can adhere to the mold during press forming, causing the plating to peel off. In some cases, the sliding ability between the mold and the plating layer surface during press forming is insufficient, causing the steel material to crack along with the mold. By suppressing the formation of the ⁇ phase on the surface of the plating layer, sliding ability during press forming can be improved.
• the presence of the ⁇ phase on the surface of the coating layer is determined by the shape of the crystal grains.
• the ⁇ phase is a columnar crystal with a large aspect ratio. If the aspect ratio of the crystal grains on the surface of the coating layer is 4.0 or less, it can be said that the formation of the ⁇ phase is sufficiently suppressed. By controlling the aspect ratio of the crystal grains on the surface of the coating layer to 4.0 or less, the sliding properties of the galvannealed steel sheet can be improved.
• the aspect ratio of the crystal grains on the surface of the plating layer is preferably 3.5 or less, 2.0 or less, 1.5 or less, and more preferably 1.0 or less.
• the aspect ratio of the crystal grains on the surface of the plating layer can be measured as follows.
• the surface of the coating layer of the galvannealed steel sheet is ultrasonically cleaned with acetone, and then photographed at a magnification of 1000 times using an SEM (JEOL JSM-7001F, accelerating voltage 20 kV) to obtain an SEM image.
• SEM JEOL JSM-7001F, accelerating voltage 20 kV
• 20 or more crystal grains that are visually judged to have larger aspect ratios are selected, and the aspect ratio of each selected crystal grain is measured using the aspect ratio measurement function of the image analysis software "Image J 1.54f.” If 20 or more crystal grains cannot be observed in the 1000x SEM image, change the imaging range and take another SEM image under the same conditions for observation. If necessary, take additional SEM images until 20 or more crystal grains can be observed.
• the average value of the top 20 aspect ratios in descending order of the measured aspect ratios is calculated, and this value is defined as the "aspect ratio of the crystal grains on the surface of
• the galvannealed steel sheet according to this embodiment has a tensile strength of 1180 MPa (HV: 370) or more.
• the tensile strength is 1300 MPa (HV: 410) or more, and more preferably, the tensile strength is 1450 MPa (HV: 460) or more.
• the upper limit of the tensile strength of the galvannealed steel sheet according to this embodiment is not particularly limited, but the substantial upper limit is 2000 MPa (HV: 650).
• the Vickers hardness (HV) was measured at a position 1 ⁇ 4 of the sheet thickness from the surface of the steel sheet on the sheet thickness cross section of the test specimen, under a load of 0.490 N.
• the steel material contain hard martensite as the main phase.
• the steel material may contain trace amounts of bainite, ferrite, or austenite, but since this can cause a decrease in strength, it is preferable that at least 90% by volume, and preferably 95% by volume or more, be martensite.
• the martensite fraction (volume %) in the steel material is a value estimated from the tensile strength of the galvannealed steel sheet. If the tensile strength of the galvannealed steel sheet is 1180 MPa or more, it can be estimated that the martensite fraction in the steel material is 90% or more.
• the galvannealed steel sheet according to this embodiment has excellent appearance because the occurrence of metallic luster portions on the surface, which is likely to occur when the heating rate during alloying is high, is suppressed.
• the metallic luster portion is caused by coarsening of crystal grains in the surface layer portion of the steel material.
• the average crystal grain size in the surface layer portion of the steel material is preferably 15 ⁇ m or less, more preferably 12 ⁇ m or less, 10 ⁇ m or less, or 9 ⁇ m or less in terms of circle equivalent diameter.
• the average crystal grain size in the surface layer portion of the steel material may be, for example, 1 ⁇ m or more, or 2 ⁇ m or more in terms of circle equivalent diameter.
• the average crystal grain size in the surface layer portion of the steel material can be measured as follows: The plating layer of the sample is removed with dilute hydrochloric acid containing a commercially available inhibitor (for example, "Ivit 710K” manufactured by Asahi Chemical Industry, concentration 0.04%), and the sample is analyzed as is using EBSD (Electron Backscatter Diffraction) without being polished, to analyze the structure of the outermost surface of the steel material. Using an FE-SEM (JEOL JSM-7001F, accelerating voltage 20 kV) equipped with an EBSD detector (TSL OIM), the crystal orientation is analyzed at a magnification of 80x over a 1000 ⁇ m ⁇ 250 ⁇ m area with a measurement pitch of 1 ⁇ m.
• a commercially available inhibitor for example, "Ivit 710K” manufactured by Asahi Chemical Industry, concentration 0.04%
• EBSD Electro Backscatter Diffraction
• the boundary of an area with a crystal orientation difference of 5° or more is considered to be a grain boundary, and the area surrounded by the grain boundary is considered to be a crystal grain, and the circle equivalent diameter is measured based on the area of each crystal grain.
• the particle size distribution of the measured circle-equivalent diameters is evaluated in terms of area fraction. Measurement and analysis, including measurement of the circle-equivalent diameters and analysis of the particle size distribution, can be performed using the software "OIM ANALYSIS" attached to the EBSD detector (OIM manufactured by TSL).
• An example of the method for producing a galvannealed steel sheet according to this embodiment will be described below.
• An example of the manufacturing method includes (A) a hot rolling step, (B) a pickling and cold rolling step, (C) an annealing step, (D) a plating step, (E) an alloying step, and (F) a quenching and tempering step. Each step will be described below.
• a slab having the above-described chemical composition of the steel material is heated.
• the slab is not particularly limited as long as it has the above-described chemical composition.
• the slab may be one cast by a continuous casting method.
• the heating temperature of the slab is preferably 1100°C or higher.
• the upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower in view of the capacity of the heating equipment and productivity.
• the heated slab is subjected to rough rolling and finish rolling as appropriate, and then cooled to obtain a hot-rolled steel sheet of a predetermined thickness.
• the finish rolling completion temperature is preferably in the range of 860 to 960°C.
• the slab is cooled to a temperature range of 450 to 700°C, and the hot-rolled steel sheet is coiled.
• the thickness after finish rolling is preferably in the range of 2.0 to 4.0 mm.
• the parameter f(t, [Si]) consisting of the residence time t (s) of the hot-rolled steel sheet at 500°C or higher after coiling and the amount of Si in the steel [Si] (%) is 1.30 or more and 4.80 or less
• the solid solution concentration of the easily oxidizable elements decreases.
• the solid solution concentration of Si which is an easily oxidizable element, decreases in the surface layer of the steel sheet, an Si-depleted layer is formed in the surface layer of the steel sheet.
• the Si-depleted layer thus formed is thick, it is difficult for internal oxides containing Si to form in the surface layer of the steel sheet in the subsequent annealing process, and the crystal grains in the surface layer of the steel sheet are likely to become coarse.
• the Si-depleted layer in the surface layer of the steel sheet is generated by holding the hot-rolled steel sheet at a relatively high temperature (500°C or higher) for a long period of time after coiling. Furthermore, if the Si concentration in the steel sheet is low, it is difficult for Si-containing internal oxides to be formed in the annealing step (C) described below.
• f(t, [Si]) Log 10 (t+10)/[Si] 0.095 ...(2)
• the parameter f(t, [Si]) is preferably 4.70 or less, and more preferably 4.60 or less. It is more preferably 4.50 or less, 4.40 or less, 4.30 or less, or 4.10 or less, and even more preferably 3.50 or less.
• f(t, [Si]) is smaller, but making it too small would require strong cooling, such as immersion in water, after coiling. This would require large-scale equipment, which is not practical, and could also deteriorate the shape of the steel sheet and reduce subsequent threadability.
• the coiling temperature is set to 500°C or less, the residence time t at temperatures above 500°C can be reduced, but the coiling temperature may be limited by the amount of Si in the steel.
• the lower limit of f(t, [Si]) is preferably 1.30.
• the residence time t at temperatures above 500°C can be adjusted by controlling the coiling temperature during hot rolling, controlling the coil unit weight, or a combination of these. If necessary, the residence time t at temperatures above 500°C can also be adjusted by cooling the coil by spraying water mist onto it, or by keeping the coil warm by covering it with a cover made of insulating material.
• (C) Annealing Step the cold-rolled steel sheet is annealed under the following annealing conditions.
• the lower limit of the annealing temperature is 800°C or higher, preferably 820°C or higher. If the annealing temperature is lower than 800°C, the ferrite phase remains at the annealing temperature, and an austenite fraction of 90% or higher cannot be obtained. Therefore, it becomes difficult to obtain a martensite-based structure in the steel material, and the desired tensile strength cannot be achieved. If the annealing temperature is 880°C or higher, excessive energy is consumed and the furnace body may be damaged.
• Annealing is carried out in a nitrogen atmosphere containing 2% to 30% hydrogen. If the hydrogen concentration in the annealing atmosphere is low, the oxide film on the surface of the cold-rolled steel sheet will not be reduced, making it difficult for the plating to adhere in the plating process described below. On the other hand, there is no need to increase the hydrogen concentration in the annealing atmosphere more than necessary, and doing so would increase costs, so the upper limit is set at 30%.
• the dew point of the annealing atmosphere is set to be ⁇ 30° C. or higher and 15° C. or lower.
• the dew point of the annealing atmosphere is preferably ⁇ 25° C. or higher, and more preferably 0° C. or lower.
• a dew point of ⁇ 30° C. or lower internal oxides are not formed in the cold-rolled steel sheet, and the crystal grains in the surface layer of the cold-rolled steel sheet become coarse.
• the oxide film on the surface of the cold-rolled steel sheet is not sufficiently reduced, or an excessively thick decarburized layer is formed, resulting in a decrease in the strength of the steel material.
• the cooling conditions after annealing are not particularly limited.
• the cold-rolled steel sheet that has undergone the annealing step is plated with zinc to obtain a hot-dip galvanized steel sheet.
• the method for plating is not particularly limited, but the cold-rolled steel sheet can be plated by immersing it in a plating bath.
• the plating bath is a Zn bath containing 0.12% to 0.15% Al.
• the Al concentration in the plating bath is preferably 0.135% or more and 0.145% or less. If the Al concentration in the plating bath is low, the amount of bottom dross formed in the plating bath increases, which may cause poor appearance.
• the plating bath may contain additional elements such as Fe, Mg, Si, Ti, Sb, Sn, Pb, and Ca, as well as other impurities.
• the temperature of the plating bath is 440°C or higher and 480°C or lower.
• the temperature of the plating bath is preferably 450°C or higher and 470°C or lower. If the temperature of the plating bath is lower than 440°C, the low-temperature portion near the bath surface will solidify, which is likely to cause poor appearance. On the other hand, if the temperature of the plating bath is higher than 480°C, zinc will easily evaporate, resulting in loss of plating raw materials.
• the sheet temperature when the cold-rolled steel sheet is dipped into the coating bath is 400°C or higher and 490°C or lower. The sheet temperature is preferably 440°C or higher and 470°C or lower.
• the sheet temperature when the cold-rolled steel sheet is dipped into the coating bath is 400°C or lower, the amount of Fe eluted from the cold-rolled steel sheet decreases, and a coating defect called "bad plating" in which some coating is not formed may occur. If the sheet temperature is 490°C or higher, a very dense Fe-Zn alloy is formed in the coating bath, which may inhibit the reaction between Fe and molten Zn, thereby reducing the alloying processability in the alloying step described below.
• the coating weight is 30 to 80 g/ m2 , preferably 40 to 60 g/ m2 . If the coating weight is 30 g/m2 or less , it is difficult to obtain sufficient corrosion resistance. On the other hand, if the coating weight is 80 g/ m2 or more, the plating thickness becomes excessive, resulting in high costs.
• the hot-dip galvanized steel sheet obtained in the plating step is subjected to an alloying treatment to obtain an alloyed hot-dip galvanized steel sheet.
• the temperature rise rate when heating to the alloying temperature is set to 50° C./s or more and 400° C./s or less. If the heating rate is less than 50°C/s, it is difficult to suppress the formation of both the ⁇ phase and the ⁇ phase. Achieving a heating rate of more than 400°C/s requires an increase in the sheet running speed and a miniaturized, high-power heating device, which is industrially difficult.
• the alloying temperature is 500° C. or higher and 630° C. or lower.
• the alloying temperature is preferably 530° C. or higher and 600° C. or lower. If the alloying temperature is 500°C or lower, the ⁇ phase crystallizes, making it difficult to control the aspect ratio of the crystal grains on the surface of the plating layer to 4.0 or lower. If the alloying temperature is 630°C or higher, the alloying reaction proceeds rapidly in advance in part of the plating layer, which is likely to cause poor appearance and reduce plating adhesion during press forming. Any method such as electrical heating or induction heating can be used to heat the hot-dip galvanized steel sheet to the alloying temperature.
• the galvannealed steel sheet may be quenched for quenching, and then heated for tempering, if necessary.
• the cooling end temperature of quenching can be set arbitrarily, but may be set to, for example, 80°C or lower.
• the tempering temperature can also be set arbitrarily, but may be, for example, 200°C or higher and 350°C or lower.
• temper rolling may be performed to adjust the surface roughness and strength of the galvannealed steel sheet.
• the elongation of temper rolling can be set, for example, in the range of 0.1 to 5.0%.
• Each galvannealed steel sheet was manufactured under the following conditions:
• the cold-rolled steel sheet was annealed in a nitrogen atmosphere with a hydrogen concentration of 5%, a dew point of -5°C, and an annealing temperature of 850°C, and then hot-dip galvanized by immersing the cold-rolled steel sheet in a hot-dip galvanizing bath having an Al concentration of 0.135% and a bath temperature of 460°C at a sheet temperature of 460°C to obtain a hot-dip galvanized steel sheet.
• the coating weight was 55 g/ m2 .
• the hot-dip galvanized steel sheet obtained as described above was subjected to an alloying treatment under the manufacturing conditions shown in Table 3 to obtain a galvannealed steel sheet.
• the thickness of the obtained galvannealed steel sheet was 1.6 mm.
• the tensile strength (MPa) and the crystal grain size ( ⁇ m) in the surface layer of the steel material were measured for the obtained galvannealed steel sheet using the methods described above.
• Example Nos. 3, 4, 10, 11, 12, and 42 were 480, 450, 480, 480, 480, and 450, respectively, which were lower than 500°C, and therefore the residence time t at 500°C or higher after coiling was 0 (zero) seconds.
• the coiling temperature was 500°C or higher, and the residence time t (s) at 500°C or higher after coiling was as shown in Table 3.
• the appearance, adhesion, and sliding properties were evaluated as follows.
• FIGS. 4A to 4C The method for evaluating the adhesion of the coating of each galvannealed steel sheet (after bending and unbending) will be described with reference to FIGS. 4A to 4C.
• a disk-shaped sample 100a having a diameter of 70 mm obtained by punching from a galvannealed steel sheet is bent at 90°, and then the sample 100a is bent back to obtain a sample 100b.
• FIG. 4A a disk-shaped sample 100a having a diameter of 70 mm obtained by punching from a galvannealed steel sheet is bent at 90°, and then the sample 100a is bent back to obtain a sample 100b.
• a transparent cellophane tape (“CT405AP-24” manufactured by Nichiban Co., Ltd.) is applied to the inside of the bend (inside of the bend, valley fold side) of sample 100b and then peeled off, thereby obtaining a measurement cellophane tape 200 from sample 100b with plating 201 peeled off in a line along the inside of the bend.
• the cellophane tape for measurement 200 is attached to the whiteboard 300, and two locations with the largest width W (width in the direction perpendicular to the line) of the plating 201 attached in a line are selected, and the peel tape reflectance (%) is measured using a reflectometer 400 ("TC-6MC-D" manufactured by Tokyo Denshoku Co., Ltd.). Of the measured values thus obtained, the larger value was taken as the release tape reflectance (%) of the sample. The more the plating peeled off, the smaller the release tape reflectance (%). In other words, the less the plating peeled off, the larger the release tape reflectance (%), which is preferable. As an index of adhesion (after bending and unbending), a release tape reflectance of 40% or more was considered to be acceptable.
• Figs. 5A and 5B The method for evaluating the coating adhesion (after ironing) of each galvannealed steel sheet will be described with reference to Figs. 5A and 5B.
• a galvannealed steel sheet both sides of which were held down by beads formed by meshing a convex portion 510U of an upper die 500U with a concave portion 510L of a lower die 500L, was pressed with a punch 500C to perform ironing, thereby obtaining an ironed sample 600.
• the pressing load was set to 1200 kg
• the stroke amount of the punch 500C was set to 65 mm
• the curvature radius of both corners of the bottom surface of the convex portion 510U was set to 1 mm.
• a transparent cellophane tape (“CT405AP-24" manufactured by Nichiban Co., Ltd.) having a width of 24 mm and a length of 100 mm was attached to the lower end of the side wall portion 610 of the ironed sample 600 and then peeled off, thereby obtaining a cellophane tape 700 for visual evaluation having the plating 601 peeled off in a line from the ironed sample 600.
• This cellophane tape 700 for visual evaluation was attached to a whiteboard, and the adhesion (after ironing) was visually evaluated on a three-point scale of A to C based on the amount of plating adhered to the cellophane tape 700 for visual evaluation.
• a rating of A or B was considered a pass.
• C A large amount of plating adhered to the cellophane tape for visual evaluation (plating adhesion area 80% or more)
• Fig. 6 is a graph showing the relationship between the ⁇ phase thickness and the adhesion (after bending and unbending), and Fig. 7 is a graph showing the relationship between the aspect ratio of the surface crystals and the sliding properties. From the Examples, which are examples of the invention, it was confirmed that, by satisfying the ranges specified in the present application, a galvannealed steel sheet having high strength and excellent formability, and in which the occurrence of defective appearance is suppressed, can be obtained. On the other hand, it was confirmed that the comparative examples that did not satisfy the ranges specified in the present application did not provide galvannealed steel sheets that had high strength, excellent formability, and suppressed occurrence of appearance defects.
• the present disclosure provides a galvannealed steel sheet that is high in strength and excellent in formability, and that suppresses the occurrence of appearance defects, as well as a component that includes the galvannealed steel sheet.
• Such components can be used as automotive components.
• automotive components include structural components (framework components) for automobiles.

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