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

Definitions

• the present invention relates to steel sheets, and more specifically to steel sheets with excellent appearance, primarily used for, for example, exterior panel components for automobiles.
• Patent Document 1 describes a steel sheet for hot-dip galvanizing, which contains, by mass%, C: 0.02-0.3%, Si: 0.1-2.0%, Mn: less than 1.0%, Cr: more than 1.0-3.0%, P: 0.02% or less, S: 0.02% or less, Al: 0.014% or less, and N: 0.001-0.008%, and satisfies 2.5 ⁇ 1.5Mn%+Cr%, 4.1-2.3Mn%-1.2Cr% ⁇ Si%, with the balance being Fe and unavoidable impurities.
• Patent Document 1 also teaches that by optimizing the amounts of Mn, Cr, and Si added, it is possible to achieve both the workability of a steel sheet for hot-dip galvanizing with a tensile strength of 390 MPa or more and an appearance after processing that allows it to be used as an automotive exterior panel. Furthermore, Patent Document 1 teaches that by setting the area ratio of the main phase, ferrite, to 70% or more and the area ratio of the hard second phase, including martensite, to 30% or less, it is possible to keep the strength, yield strength, yield ratio, and strength-ductility balance all within a good range.
• the present invention describes a cold-rolled steel sheet having a composition that contains Ti* in a range that satisfies 0 ⁇ Ti* ⁇ 0.02 and further satisfies (Sb%) ⁇ (Cu%)/5, with the balance being Fe and unavoidable impurities, and that the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the plate thickness surface layer portion up to 10 ⁇ m from each surface on both sides of the steel sheet is 9% or less of the total Ti content (mass%) in the steel sheet.
• Patent Document 2 also teaches that by setting the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the plate thickness surface layer portion up to 10 ⁇ m from each surface on both sides of the steel sheet to 9% or less of the total Ti content (mass%) in the steel sheet, it is possible to avoid the occurrence of appearance unevenness caused by such fine Ti-based precipitates, and to obtain a cold-rolled steel sheet with excellent surface properties, and further that the cold-rolled steel sheet can be suitably used for parts that require excellent surface quality after forming, mainly for the outer panels of automobiles.
• Patent Document 1 considers improving formability and appearance after forming, mainly from the perspective of chemical composition, but does not necessarily consider sufficiently from the perspective of making the metal structure appropriate. Therefore, in the steel sheets of the prior art, there is still room for improvement in terms of improving formability and appearance after forming.
• the steel plate according to the embodiment of the present invention has, in mass%, C: 0.030-0.100%, Mn: 1.00-2.80%, Si: 0.005-1.500%, Al: 1.000% or less, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0-0.200%, Nb: 0 to 0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%,
• the inventors have found that the elongation of the steel sheet can be significantly improved by increasing the ratio A of the region in the ferrite where the KAM value is less than 0.5° (in the direction of lowering the dislocation density) while decreasing the ratio B of the region in the ferrite where the KAM value is 1.0° or more (similarly in the direction of lowering the dislocation density), thereby controlling the value of the ratio A/B to be larger, more specifically, by controlling the ratio A/B to be 0.60 or more.
• Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel.
• the Al content may be 0%, but in order to fully obtain these effects, the Al content is preferably 0.001% or more.
• the Al content may be 0.005% or more, 0.010% or more, 0.025% or more, 0.050% or more, or 0.080% or more.
• the Al content is set to 1.000% or less.
• the Ac3 point can be reduced by reducing the Al content. Therefore, the Al content may be 0.800% or less, 0.700% or less, 0.600% or less, or 0.300% or less.
• N is an element that causes blowholes during welding. Therefore, the N content is set to 0.0150% or less.
• the N content may be 0.0120% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less.
• the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• Ni is an element effective in improving the strength of steel plate.
• the Ni content may be 0%, but in order to obtain the above effect, the Ni content is preferably 0.001% or more.
• the Ni content may be 0.01% or more or 0.05% or more.
• the Ni content is preferably 1.00% or less.
• the Ni content may be 0.80% or less, 0.40% or less, or 0.20% or less.
• Ta is an element effective in controlling the morphology of carbides and improving the strength of steel sheets.
• the Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more.
• the Ta content may be 0.01% or more or 0.03% or more.
• the Ta content is preferably 0.10% or less.
• the Ta content may be 0.08% or less, 0.06% or less, or 0.04% or less.
• As is an element that may be contained in a steel sheet when scrap is used as a raw material for the steel sheet.
• As is an element that strongly segregates at grain boundaries, and the lower the As content, the better.
• the As content is preferably 0.10% or less, and may be 0.05% or less, 0.04% or less, or 0.02% or less.
• the As content may be 0%, but reducing the As content to less than 0.001% leads to an excessive increase in refining costs. For this reason, the As content may be 0.001% or more, 0.005% or more, or 0.01% or more.
• the remainder excluding the above elements consists of Fe and impurities.
• 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 plate according to the embodiment of the present invention.
• the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method.
• the chemical composition of the steel plate may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for chips in accordance with JIS G 1201:2014.
• ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
• a 35 mm square test piece may be obtained from the vicinity of the 1/4 position of the plate thickness of the steel plate, and the composition may be identified by measuring under conditions based on a calibration curve created in advance using a Shimadzu ICPS-8100 or the like (measuring device).
• C and S which cannot be measured by ICP-AES, may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. If the steel sheet has a plating layer on its surface, the plating layer can be removed by mechanical grinding or other methods before analyzing the chemical composition.
• ferrite Since ferrite is a soft structure, it is easily deformed and contributes to improving elongation. By making the area ratio of ferrite 75% or more, a certain degree of formability can be ensured. From the viewpoint of improving formability, the higher the area ratio of ferrite, the more preferable it is, and it may be, for example, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is excessively contained, the steel sheet may not be able to achieve the desired strength. Therefore, the area ratio of ferrite is set to 95% or less. The area ratio of ferrite may be 93% or less, 90% or less, or 87% or less.
• Martensite is a structure with high dislocation density and hardness, and therefore contributes to improving tensile strength. If the area ratio of martensite is excessively low, the desired strength may not be achieved, and/or elongation may occur unevenly during press forming, resulting in the generation of a streak-like pattern called stretcher strain. By setting the area ratio of martensite to 5% or more, it is possible to ensure a tensile strength of, for example, 540 MPa or more without such problems. From the viewpoint of improving strength, the higher the area ratio of martensite, the more preferable it is, and it may be, for example, 7% or more, 8% or more, 10% or more, or 13% or more.
• martensite includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.
• Total of ferrite and martensite 90% or more
• the total area ratio of ferrite and martensite may be 92% or more, 93% or more, 94% or more, 95% or more, 96% or more, 97% or more, 98% or more, 99% or more, or may be 100%.
• the thickness of the steel plate to be measured is thin and a measurement area of 100 ⁇ m in the plate thickness direction cannot be secured, the length in the plate thickness direction is reduced and a measurement area of 10,000 ⁇ m 2 is secured.
• a measurement area of 20 ⁇ m in the plate thickness direction and 500 ⁇ m in the direction perpendicular to the plate thickness direction may be observed.
• the measurement length in the sheet thickness direction is set to 10 ⁇ m or more, preferably 50 ⁇ m or more. The same applies to the "100 ⁇ m ⁇ 100 ⁇ m region" in the following explanation.
• plate thickness x/y position (where x and y are natural numbers satisfying x ⁇ y) refers to a position that has moved in the plate thickness direction from the surface (plate surface) of the steel plate in the plate thickness direction toward the center of the steel plate by a distance (depth) of x/y of the plate thickness t. For example, if the plate thickness t of the steel plate is 2 mm, then “plate thickness 1/8 position” refers to a position that is 0.25 mm deep in the plate thickness direction from the surface of the steel plate.
• the area ratio of ferrite and the area ratio of martensite are determined by the following procedure. First, the observation surface of the sample is etched with a Nital reagent (3% nitric acid in ethanol), and then a 100 ⁇ m x 100 ⁇ m area within the range of 1/8 to 3/8 of the plate thickness position, centered at the 1/4 position, is observed with a FE-SEM (field emission scanning electron microscope). In Nital corrosion, martensite and retained austenite are not corroded, so the area ratio of the uncorroded area corresponds to the total area ratio of martensite and retained austenite. Specifically, the image analysis software Image J (Ver.
• the average grain spacing of martensite is preferably 2.4 ⁇ m or less, more preferably 2.2 ⁇ m or less, and most preferably 2.0 ⁇ m or less or 1.8 ⁇ m or less. Although there is no particular lower limit, for example, the average grain spacing of martensite may be 0.5 ⁇ m or more, 0.8 ⁇ m or more, or 1.0 ⁇ m or more.
• Observation may be performed in multiple fields of view.
• a plane parallel to the plane rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed using the above method.
• the average value of the length of the major axis of the multiple inclusions in each cross section obtained is calculated for each cross section, and the cross section with the largest average value of the length of the major axis of the inclusions is identified.
• the direction parallel to the major axis of the inclusions in that cross section is determined to be the rolling direction.
• the steel plate according to the embodiment of the present invention by controlling the area ratio of martensite, which is a hard structure, to 5 to 25%, and further controlling the chemical composition of the steel plate within a predetermined range, high strength, for example high strength with a tensile strength of 540 MPa or more, is ensured, and by controlling the area ratio of ferrite, which is a soft structure, to 75 to 95%, a certain level of formability is ensured, and further by controlling the ratio A/B, which is the ratio A of the region in ferrite where the KAM value is less than 0.5° and the ratio B of the region in ferrite where the KAM value is 1.0° or more, to 0.60 or more, it is possible to significantly improve the balance between strength and formability.
• the average grain size of ferrite in the metal structure is 3.0 to 25.0 ⁇ m.
• the average grain size of ferrite may be 5.0 ⁇ m or more, 7.0 ⁇ m or more, 8.0 ⁇ m or more, 9.0 ⁇ m or more, or 10.0 ⁇ m or more.
• the average grain size of ferrite may be 22.0 ⁇ m or less, 20.0 ⁇ m or less, 16.0 ⁇ m or less, 14.0 ⁇ m or less, or 12.0 ⁇ m or less.
• the average grain size of ferrite in steel plate is determined as follows. First, the circle equivalent diameter is calculated for all grains (ferrite grains) located in the region corresponding to the ferrite determined by GAIQ analysis in the EBSD measurement results. Here, the grains are defined as the region surrounded by grain boundaries, which are the boundaries of regions where the crystal orientation differs by 15° or more. Next, the value obtained by arithmetically averaging these is determined as the average grain size of ferrite.
• the average grain size of martensite in the metal structure is 1.0 to 5.0 ⁇ m.
• the average grain size of martensite may be 1.2 ⁇ m or more, 1.5 ⁇ m or more, 1.7 ⁇ m or more, or 2.0 ⁇ m or more.
• the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite.
• the area ratio of retained austenite is sufficiently low compared to the area ratio of martensite, so the white structure can be considered as martensite.
• calculate the circle equivalent diameter of all the identified martensite This operation is performed in the other two observation regions, and the circle equivalent diameters of all the martensite obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average crystal grain size of the martensite (strictly speaking, particles containing martensite and/or retained austenite).
• the average aspect ratio of martensite in the metal structure is 2.5 or more.
• the average aspect ratio of martensite may be 2.6 or more, 2.8 or more, or 3.0 or more.
• the upper limit is not particularly limited, but for example, the average aspect ratio of martensite may be 4.0 or less, 3.8 or less, or 3.6 or less.
• the average aspect ratio of martensite is determined as follows. First, the aspect ratios of all martensite grains are calculated using the image analysis software Image J (Ver. 1.54f) in the image data of one observation area obtained when measuring the average crystal grain size of martensite. The aspect ratio of the particles (crystal grains) on the image can be measured using a function built into the image analysis software Image J (Ver. 1.54f). Next, this operation is performed in the other two observation areas, and the aspect ratios of all martensite grains obtained in the three observation areas are arithmetically averaged, and the obtained value is determined as the average aspect ratio of martensite (strictly speaking, particles containing martensite and/or retained austenite).
• the steel plate according to the embodiment of the present invention has a plate thickness of, for example, 0.2 to 2.0 mm, but is not particularly limited thereto.
• a steel plate having such a plate thickness is suitable for use as a material for an exterior plate member such as a door or a hood.
• the plate thickness may be 0.3 mm or more, 0.4 mm or more, or 0.6 mm or more.
• the plate thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less.
• the plate thickness 0.2 mm or more it becomes easier to maintain the shape of the molded product flat, and an additional effect of improving the dimensional accuracy and shape accuracy can be obtained.
• the plate thickness 1.0 mm or less the weight reduction effect of the member becomes remarkable.
• the plate thickness of the steel plate is measured by a micrometer.
• the steel sheet according to the embodiment of the present invention is a cold-rolled steel sheet, but may further include a plating layer on the surface for the purpose of improving corrosion resistance or the like.
• the plating layer may be either a hot-dip plating layer or an electroplating layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplating layer on its surface.
• the hot-dip plating layer includes, for example, a hot-dip galvanized layer (GI), a hot-dip galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
• the electroplating layer includes, for example, an electrogalvanized layer (EG), an electrogalvanized Zn-Ni alloy plating layer, and the like.
• the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
• the coating weight of the plating layer is not particularly limited and may be a general coating weight.
• TS Tensile strength
• El total elongation
• TS tensile strength
• the tensile strength is preferably 570 MPa or more, more preferably 600 MPa or more.
• the upper limit is not particularly limited, but for example, the tensile strength may be 980 MPa or less, 900 MPa or less, 850 MPa or less, 830 MPa or less, 810 MPa or less, or 800 MPa or less.
• exterior plate members particularly automobile exterior plate members
• the characteristics of the metal structure do not change particularly before and after forming.
• the method for producing a steel sheet according to an embodiment of the present invention comprises: a hot rolling process comprising: heating a slab having the chemical composition described above in relation to the steel plate to a temperature of 1100-1300°C, finish rolling it and then coiling it at a temperature of 500-670°C, the end temperature of said finish rolling being 800-1250°C; a pickling step of pickling the obtained hot-rolled steel sheet; a cold rolling process in which the pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 20 to 90%;
• the cold-rolled steel sheet thus obtained is heated to a temperature of Ac3+10°C or higher, and then cooled at an average cooling rate CR of 30 to 200°C/s to a cooling stop temperature of 350°C or lower, and held at a temperature T1 of 600°C or lower, in which CR, T1, and an index X represented by the following formula 1 or 2 satisfy the following formula 3; and
• the cold-rolled steel sheet is heated at an average heating rate HR°C/s to
• the slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method.
• the slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to dissolve the alloying elements in the slab. If the heating temperature is less than 1100°C, the alloying elements may not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. For this reason, the heating temperature is preferably 1100°C or higher.
• the upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower from the viewpoint of the capacity and productivity of the heating equipment.
• the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
• the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
• the heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling. Since the slab used as described above contains a relatively large amount of alloy elements, it is necessary to increase the rolling load during hot rolling. For this reason, it is preferable to perform hot rolling at a high temperature.
• the end temperature of the finish rolling is important in terms of controlling the metal structure of the steel sheet. If the end temperature of the finish rolling is low, the metal structure may become non-uniform and the formability may decrease. For this reason, the end temperature of the finish rolling is set to 800°C or higher. On the other hand, in order to suppress the coarsening of austenite, the end temperature of the finish rolling is set to 1250°C or lower.
• the finish-rolled hot-rolled steel sheet is coiled at a coiling temperature of 500 to 670°C.
• a coiling temperature 500 to 670°C.
• the metal structure cannot be constituted by a structure mainly composed of bainite and/or martensite in the primary annealing process, and the desired dispersion state of martensite cannot be obtained even in the subsequent secondary annealing process. More specifically, even after the subsequent secondary annealing step, it is not possible to control the average grain spacing of martensite to 2.5 ⁇ m or less, and/or it is not possible to control the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less, i.e., it is not possible to obtain a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions. In this case, it is not possible to sufficiently suppress the occurrence of ghost lines, etc., and the appearance after forming is deteriorated.
• the heating temperature in the first annealing step is less than Ac3+10°C, austenitization will be insufficient, and the metal structure in the steel sheet will not be composed mainly of bainite and/or martensite even after subsequent cooling, meaning that the total area ratio of bainite and martensite will not be 90% or more.
• the heating temperature in the first annealing step be 1050°C or less.
• the holding time at the above heating temperature is preferably 10 to 500 seconds.
• the subsequent cooling and holding are also important. That is, the cold-rolled steel sheet is cooled at an average cooling rate CR of 30 to 200°C/s to a cooling stop temperature of 350°C or less, reheated as necessary, and then held at a temperature T1 of 600°C or less. It is also important to control CR, T1, and the index X expressed by the above formula 1 or 2 so as to satisfy the above formula 3.
• the cooling in the first stage ensures that the metal structure in the steel sheet is transformed into a structure mainly composed of bainite and/or martensite, and the holding in the second stage makes it possible to reduce the dislocation density in the structure by at least partially tempering these structures.
• the ratio A/B of the ratio A of the region in ferrite where the KAM value is less than 0.5° and the ratio B of the region in ferrite where the KAM value is 1.0° or more can be controlled to 0.60 or more to sufficiently reduce the dislocation density in ferrite, and the balance between strength and formability can be significantly improved.
• the Ac1 point (°C) is determined, as in the case of the Ac3 point, by cutting a small piece from a cold-rolled steel sheet and measuring the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C/s. First, the steel sheet after the primary cooling is heated to a maximum heating temperature T2 of Ac1 to (Ac3-10)°C, whereby carbides can be generated and dispersed on many interfaces contained within the bainite and/or martensite in the metal structure.
• the ratio A/B of the ratio A of the area in the ferrite where the KAM value is less than 0.5° to the ratio B of the area in the ferrite where the KAM value is 1.0° or more can be controlled to 0.60 or more, thereby sufficiently reducing the dislocation density in the metal structure, particularly the dislocation density in the ferrite, and making it possible to significantly improve the balance between strength and formability.
• Mo, Ti, and Nb have the effect of suppressing the recovery of dislocations and suppressing recrystallization, and these effects are strengthened by the combination with B. Therefore, in order to reduce the dislocation density in ferrite in the final structure, it is necessary to appropriately determine the average heating rate HR and the maximum heating temperature T2 during heating in the secondary annealing process while taking into account the effects of Mo, Ti, Nb, and B. Therefore, the present inventors have studied the effects of these elements, the average heating rate HR, and the maximum heating temperature T2 on the dislocation density in ferrite in the final structure.
• the present inventors have found that it is possible to promote recrystallization of the metal structure by using an index X calculated by the following formula 1 or 2 according to the B content, and selecting the average heating rate HR and the maximum heating temperature T2 in the secondary annealing process so that the value calculated by the left side of the following formula 4 is 3200 or more.
• the present inventors have found that it is possible to reduce the dislocation density in ferrite in the final structure to a level at which the ratio A/B of the KAM value is 0.60 or more.
• the dislocation density in ferrite in the final structure may not be reduced to a desired level.
• the ratio A/B between the ratio A of the region in ferrite where the KAM value is less than 0.5° and the ratio B of the region in ferrite where the KAM value is 1.0° or more cannot be controlled to 0.60 or more, and as a result, the balance between strength and formability cannot be sufficiently improved. From the viewpoint of further improving the balance between strength and formability, the higher the value of (1000/HR+T2)/index X, the more preferable, for example, 3300 or more.
• the upper limit is not particularly limited, but the value of (1000/HR+T2)/index X may be 6000 or less or 5500 or less, for example.
• the slower the average heating rate HR the more preferable.
• productivity will decrease, so it is preferable to select the HR appropriately, for example, within the range of 1.0 to 30.0°C/sec.
• a plating treatment may be applied to the surface of the obtained cold-rolled steel sheet as necessary.
• the plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating.
• the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or the alloying treatment may be performed after the hot-dip galvanizing treatment.
• the specific conditions of the plating treatment and the alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art.
• the alloying temperature may be 450 to 600°C.
• a slab having the chemical composition shown in Table 1 and a thickness of 200 to 300 mm was cast by continuous casting.
• the remainder of the components shown in Table 1 was Fe and impurities.
• the obtained slab was heated to 1230°C and then hot rolled. Hot rolling was performed by rough rolling and finish rolling, with the finishing temperature of the finish rolling being 910°C and the coiling temperature being 550°C.
• the obtained hot-rolled steel sheet was pickled and then cold-rolled at a reduction rate of 84% to obtain a cold-rolled steel sheet with a thickness of 0.4 mm.
• the obtained cold-rolled steel sheet was subjected to primary annealing, specifically, the cold-rolled steel sheet was heated to a temperature of (Ac3+10)°C and held for 100 seconds, then cooled to a cooling stop temperature of 250°C at an average cooling rate CR shown in Table 2, reheated as necessary, and held at a temperature T1 shown in Table 2 for 200 seconds.
• the cold-rolled steel sheet subjected to primary annealing was subjected to secondary annealing, specifically, the cold-rolled steel sheet was heated to a maximum heating temperature T2 also shown in Table 2 at an average heating rate HR shown in Table 2, then held at the heating temperature for 100 seconds, and further cooled to 500°C at an average cooling rate of 10°C.
• the surface of the obtained cold-rolled steel sheet was subjected to plating treatment as necessary to appropriately form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrolytic galvanized layer (EG).
• GI hot-dip galvanized layer
• the properties of the resulting steel plates were measured and evaluated using the following methods.

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