Galvanized steel sheet, member, and method for producing same

Abstract

Provided is a technique for a galvanized steel sheet having a high yield ratio, excellent bendability and stretch flangeability, and excellent hydrogen embrittlement resistance, and having a TS of 780 MPa or more and less than 1180 MPa. Provided is a galvanized steel sheet. A base steel sheet having less than 65% ferrite, a total of 25% or more martensite and bainite, and 3-10% inclusive of retained austenite in area ratio, the base steel sheet having a thickness of 1 / 8-8 70% or more of the total martensite in the steel structure at 3 / 8 is a steel structure having tempered martensite having carbides having an average particle diameter of 50-200 nm inclusive, and a zinc plating layer formed on a base steel sheet, the galvanized steel sheet having a TS of 780 MPa or more and less than 1180 MPa, a yield ratio of 0.60 or more, and a diffusible hydrogen content of 0.45 ppm by mass or less.

Description

Technical Field

[0001] This invention relates to high-strength galvanized steel sheets with excellent workability, components made from such high-strength galvanized steel sheets, and methods for manufacturing them. The galvanized steel sheets of this invention are primarily suitable for use as automotive steel sheets. Background Technology

[0002] In recent years, from the perspective of protecting the Earth's environment, there has been a strong demand to improve fuel efficiency in order to reduce CO2 emissions from automobiles. Along with this, the trend towards lightweighting of vehicle bodies through thinner wall thicknesses has become active, increasing the demand for high-strength steel sheets used in body components. Particularly from the perspective of ensuring vehicle crash safety, steel sheets used around the passenger compartment require not only high tensile strength but also high yield strength. Furthermore, to avoid compromising the freedom of shape design due to high strength, stamping formability is also necessary. In addition, if corrosion develops during use as a vehicle component, the material may not be able to maintain its original strength during a collision; therefore, the application of hot-dip galvanized steel sheets with excellent corrosion resistance is expanding.

[0003] On the other hand, the increase in strength of steel sheets is accompanied by concerns about hydrogen embrittlement. Hydrogen embrittlement in high-strength steel sheets is caused not only by hydrogen intruding from the outside due to corrosion during use, but sometimes also by hydrogen intruding during the manufacturing process. This is particularly true in the case of hot-dip galvanized steel sheets, which are typically manufactured using a series of processes from annealing to hot-dip galvanizing in a hydrogen-containing atmosphere, with the aim of reducing and inhibiting oxidation. Therefore, the zinc coating is formed while residual hydrogen has intruded from the atmosphere into the steel. Hydrogen diffusion in the zinc coating is very slow, hindering the external release of diffusible hydrogen from the steel. This residual diffusible hydrogen can sometimes contribute to cracking due to hydrogen embrittlement during processing, and there is a particular risk of localized deterioration in ductility, such as reduced bending and flange extension. Therefore, it is important to establish hot-dip galvanized steel sheets with excellent resistance to hydrogen embrittlement and their manufacturing methods that sufficiently reduce the amount of hydrogen in the steel.

[0004] Regarding issues related to the workability and hydrogen embrittlement resistance of high-strength hot-dip galvanized steel sheets, for example, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet with excellent bending properties and a method for manufacturing the same. Furthermore, Patent Document 2 discloses a high-strength steel sheet with excellent ductility and resistance to delayed fracture (TS) of 900 MPa or higher, a method for manufacturing a high-strength cold-rolled steel sheet, and a method for manufacturing a high-strength galvanized steel sheet.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: International Publication No. 2016 / 113788

[0008] Patent Document 2: Japanese Patent Application Publication No. 2011-111671 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, while Patent Document 1 describes improving flexibility by refining martensite grains, it does not disclose a method for improving hydrogen embrittlement resistance. Furthermore, while Patent Document 2 describes a balance between ductility and hydrogen embrittlement resistance, it addresses post-processing hydrogen embrittlement resistance and does not describe improving hydrogen embrittlement resistance caused by hydrogen intrusion into the steel during manufacturing.

[0011] Based on the manufacturing methods disclosed in these patent documents, it is believed that the presence of a large amount of residual hydrogen in the steel poses a risk of hydrogen embrittlement during processing, such as deterioration of bending and extension flange properties, and cracking in the weld nugget of spot welds.

[0012] The present invention was made in view of the above circumstances, and provides galvanized steel sheets, components, and methods thereof suitable for automotive applications, having a high yield ratio (YR), excellent bending and elongation flange properties, excellent resistance to hydrogen embrittlement, and a tensile strength (TS) of 780 MPa or more and less than 1180 MPa.

[0013] It should be noted that, here, "high yield ratio" refers to the condition where a JIS 5 tensile test piece (JIS Z2201) is cut along a direction perpendicular to the rolling direction, and the strain rate is set to 10. -3 The tensile test specified in JIS Z2241 (2011) is performed with a YR of 0.60 or higher.

[0014] Tensile strength (TS) refers to the tensile strength measured by cutting a JIS5 tensile test piece (JISZ2201) perpendicular to the rolling direction and testing it at a strain rate of 10. -3 The tensile strength is obtained by tensile testing as specified in JIS Z2241 (2011).

[0015] Excellent bending performance refers to the following: a 35mm × 100mm strip test piece is cut from a galvanized steel sheet with the direction parallel to the rolling direction as the bending test axis. Under the conditions of a stroke speed of 50mm / s, an indentation load of 10 tons, and a pressing and holding time of 5 seconds, a 90-degree V-bending test is performed with various bending radii. The edge of the bending apex of the test piece is observed using a 10x magnifying glass. The value R / t obtained by dividing the minimum bending radius R (mm) without observed cracks of more than 0.5mm in length by the plate thickness (mm) is obtained, which satisfies either (A) or (B) below.

[0016] (A) TS is above 780MPa and below 980MPa and R / t is below 4.5.

[0017] (B) TS is above 980MPa and below 1180MPa and R / t is below 5.0.

[0018] Excellent extension flange properties refer to the following: a 100mm×100mm test piece is cut from a galvanized steel sheet, and a 10mm diameter hole is formed in the center of the test piece with a 12.5% ​​gap through punching. Then, a blanking force of 9 tons (88.26kN) is applied around the hole using a die with an inner diameter of 75mm, and a conical punch with a 60° apex angle is pressed into the hole to enlarge the hole. The diameter of the hole when cracks occur is measured, and the limiting enlargement rate λ (%) calculated by the following formula (1) is 35% or more.

[0019] λ(%)={(D f -D0) / D0}×100 …Equation (1)

[0020] D f Diameter of the pore (mm) at which the crack forms.

[0021] D0: Diameter of the hole before reaming (mm)

[0022] Excellent resistance to hydrogen embrittlement means that when spot welding is performed using the following methods, the cracks in the weld nugget are less than 100 μm.

[0023] (1) A 30mm×100mm test piece cut from a galvanized steel plate is clamped at both ends with a 2mm thick spacer, and the spacers are joined together by spot welding to make a welded test piece.

[0024] (2) In spot welding, an inverter DC resistance spot welding machine is used, and the electrodes are made of chrome copper with a dome-shaped tip diameter of 6mm. The applied pressure is set to 380kgf, the energizing time is set to 16 cycles / 50Hz, and the holding time is set to 5 cycles / 50Hz.

[0025] (3) The welding current value is adjusted in a way that forms a weld nugget diameter corresponding to the plate thickness.

[0026] The diameter of the melting nucleus satisfies the following equation (2).

[0027] 3.0×t 1 / 2 <Molten core diameter < 3.5 × t 1 / 2 …Formula (2)

[0028] In equation (2), t is the plate thickness (mm).

[0029] (4) After 24 hours from the spot welding, cut off the spacer part and observe the cross-section of the weld nugget.

[0030] Methods for solving problems

[0031] To solve the above problems, the inventors conducted repeated and in-depth research. The results showed that by optimizing the steel sheet structure and appropriately controlling the precipitation morphology of carbides in the steel to effectively serve as hydrogen capture points, diffusible hydrogen in the steel can be reduced. This results in high-strength galvanized steel sheets with high yield ratios, excellent bending properties, and a low risk of crack formation in the weld nugget of the spot weld.

[0032] This invention was made based on the following insight, the gist of which is as follows.

[0033] [1] A galvanized steel sheet having a base steel sheet and a zinc coating formed on the base steel sheet, wherein,

[0034] The aforementioned base steel plate has a steel microstructure consisting of less than 65% ferrite by area ratio, more than 25% martensite and bainite combined, and more than 3% and less than 10% retained austenite.

[0035] In terms of area ratio, more than 70% of the martensite in the microstructure of the aforementioned base steel plate at 1 / 8 to 3 / 8 of its thickness is tempered martensite with carbides having an average grain size of 50 nm or more and 200 nm or less.

[0036] The galvanized steel sheet has a tensile strength of 780 MPa or higher and less than 1180 MPa, and a yield ratio of 0.60 or higher.

[0037] The cumulative amount of hydrogen released when the base steel plate is heated from room temperature to 200°C is less than 0.45 ppm by mass.

[0038] [2] According to the galvanized steel sheet described in [1] above, the steel composition of the base steel sheet contains, by mass %: C: 0.080% or more and 0.300% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.00% or more and 4.00% or less, P: 0.10% or less, S: 0.0200% or less, Al: 0.003% or more and 0.100% or less, N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities.

[0039] [3] According to the galvanized steel sheet described in [2] above, the steel composition further comprises, by mass%, B: less than 0.0100%, Ti: less than 0.200%, Nb: less than 0.200%, Sb: less than 0.200%, Sn: less than 0.200%, V: less than 0.100%, Cu: less than 2.00%, Cr: less than 2.00%, Ni: less than 2.00%, Mo: less than 1.00%, Ta: less than 0.100%, W: less than 0.500%, Zr: less than 0.020%, and Ca: less than 0.0200%. The following are at least one of the following: Mg: less than 0.0200%, Zn: less than 0.020%, Co: less than 0.020%, Ce: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0200%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM (excluding Ce): less than 0.0200%.

[0040] [4] According to any one of [1] to [3] above, wherein, in the base steel plate, when the area less than 200 μm in the thickness direction from the surface of the base steel plate is used as the surface layer, the surface layer has a soft surface layer with a Vickers hardness of 85% or less at a position where the Vickers hardness is 1 / 4 of the plate thickness.

[0041] [5] According to the galvanized steel sheet described in [4] above, when measuring the nanohardness at 300 points or more in a 50μm × 50μm region on the surface of the base steel sheet at positions 1 / 4 and 1 / 2 of the thickness direction depth of the surface soft layer, respectively.

[0042] The proportion of samples with a nanohardness of 7.0 GPa or higher located at 1 / 4 of the thickness of the aforementioned surface soft layer from the surface of the base steel plate was less than 0.10 of the total samples.

[0043] Furthermore, the standard deviation σ of the nanohardness of the plate surface at a position one-quarter of the thickness of the aforementioned surface soft layer from the surface of the base steel plate is less than 1.8 GPa.

[0044] Furthermore, the standard deviation σ of the nanohardness of the plate surface at a position half the thickness of the aforementioned surface soft layer from the surface of the base steel plate is less than 2.2 GPa.

[0045] [6] The galvanized steel sheet according to any one of [1] to [5] above, wherein a metal coating is formed between the base steel sheet and the zinc coating on one or both sides of the galvanized steel sheet.

[0046] [7] The galvanized steel sheet according to any one of [1] to [6] above, wherein the zinc coating is an alloyed zinc coating.

[0047] [8] The galvanized steel sheet according to any one of [1] to [7] above, wherein the cumulative amount of hydrogen released in the temperature range of 350 to 600°C when the base steel sheet is heated from room temperature to 600°C is 0.05 ppm by mass or more.

[0048] [9] A component formed using any one of the galvanized steel sheets described in [1] to [8] above.

[0049]

[10] A method for manufacturing galvanized steel sheet, comprising:

[0050] The hot rolling process, wherein a steel billet having the steel composition described in [2] or [3] above is heated to a temperature range of 1100 to 1350°C, hot rolling is performed at a finishing rolling end temperature of 800 to 950°C, and coiling is performed at a coiling temperature of 650°C or below.

[0051] The reduction process involves holding the steel plate after the hot rolling process at a temperature of 700°C or higher in a reducing atmosphere with a hydrogen concentration of 8% by volume or higher and 30% by volume or lower for at least 20 seconds.

[0052] The homogenization process involves holding the steel plate after the reduction process at a temperature of 750°C or higher in a homogenization atmosphere with a hydrogen concentration of 0.2% by volume or higher and 8% by volume or lower for 20 seconds or more and 300 seconds or less.

[0053] The first cooling process involves cooling the steel plate after the above-mentioned heat soaking process.

[0054] The galvanizing process, wherein a zinc coating is formed on the surface of the steel plate after the first cooling process described above;

[0055] In the second cooling process, for the steel sheet after the galvanizing process, the average cooling rate from the temperature range of 350-450°C to (Ms point - 100°C) is set to less than 20°C / s, and the sheet is held at a temperature range of 100-350°C for more than 5 seconds, cooling to a cooling stop temperature of 100-300°C; and

[0056] In the reheating process, the steel plate after the second cooling process is kept at a temperature range of 5 to 600 s above the cooling stop temperature and below 450 ℃.

[0057]

[11] According to the manufacturing method of galvanized steel sheet described in

[10] above, after the galvanizing process, the zinc coating is formed on the surface of the steel sheet and then further alloying treatment is performed.

[0058]

[12] The method for manufacturing galvanized steel sheet according to

[10] or

[11] above includes a cold rolling process in which cold rolling is performed at a reduction rate of 20% or more after the hot rolling process and before the reduction process.

[0059]

[13] The method for manufacturing galvanized steel sheet according to any one of

[10] to

[12] above, wherein, in the first cooling step above, the steel sheet after the soaking process above is cooled from 600 to 900°C to 150 to 500°C in an atmosphere with a hydrogen concentration of 0.5% to 30% by volume and a dew point of 0°C or less at an average cooling rate of 20°C / s or less.

[0060]

[14] In the method for manufacturing galvanized steel sheet according to any one of

[10] to

[13] above, in the second cooling process, the average cooling rate from the temperature range of 350 to 450°C to the point (Ms point - 100°C) is set to 10°C / s or less, and the temperature is held for more than 10s in the temperature range of 100 to 350°C to cool to a cooling stop temperature of 100 to 300°C.

[0061]

[15] The method for manufacturing galvanized steel sheet according to any one of

[10] to

[14] above, wherein the heat-soaking atmosphere in the heat-soaking process is an atmosphere with a dew point of -30°C or higher.

[0062]

[16] The method for manufacturing galvanized steel sheet according to any one of

[10] to

[15] above, wherein a pre-plating step is performed on one or both sides of the steel sheet after the hot rolling step and before the reduction step to form a metal coating.

[0063]

[17] The method for manufacturing galvanized steel sheet according to any one of

[10] to

[16] above includes an oxidation process in which the steel sheet after the hot rolling process and before the reduction process is heated to 600°C or higher in an oxidizing atmosphere with an oxygen concentration of 1,000 ppm or more and 30,000 ppm or less.

[0064]

[18] A method for manufacturing a component, comprising a step of forming a component by performing at least one of forming or joining processes on a galvanized steel sheet as described in any one of [1] to [8].

[0065] Invention Effects

[0066] The galvanized steel sheet provided by the present invention has a tensile strength of 780 MPa or more and less than 1180 MPa, a yield ratio of 0.60 or more, and excellent bending properties, elongation flange properties and hydrogen embrittlement resistance.

[0067] Furthermore, the component provided by the present invention has a tensile strength of 780 MPa or more and less than 1180 MPa, a yield ratio of 0.60 or more, and excellent bending properties, elongation flange properties, and resistance to hydrogen embrittlement. Attached Figure Description

[0068] Figure 1 This is a diagram illustrating (a) a primary bending process and (b) a secondary bending process in the U-bending + tight bending test and the V-bending + orthogonal VDA bending test of the embodiments.

[0069] Figure 2 The diagram shows (a) a front view, (b) a perspective view, and (c) a schematic diagram illustrating the axial crush test, of a test member manufactured by spot welding a cap-shaped member to a steel plate for conducting the axial crush test of the embodiment. Detailed Implementation

[0070] The following is a detailed description of the present invention.

[0071] <Galvanized steel sheet>

[0072] The galvanized steel sheet of the present invention is a galvanized steel sheet having a base steel sheet and a zinc coating formed on the base steel sheet. The base steel sheet is characterized by having a steel microstructure with ferrite less than 65% by area ratio, martensite and bainite totaling 25% or more, and retained austenite of 3% or more and 10% or less. In terms of area ratio, at least 70% of the martensite in the steel microstructure at 1 / 8 to 3 / 8 of the plate thickness of the base steel sheet is tempered martensite with carbides having an average grain size of 50 nm or more and 200 nm or less. The tensile strength is 780 MPa or more and less than 1180 MPa, the yield ratio is 0.60 or more, and the cumulative amount of hydrogen released when the base steel sheet after the zinc coating is peeled off is heated from room temperature to 200°C is 0.45 ppm by mass or less.

[0073] steel structure

[0074] First, the area ratio of each microstructure in the base steel plate of the present invention will be explained. The area ratio of each microstructure refers to the area ratio of each constituent phase in the total observed area. The area ratio of each microstructure can be calculated as follows: after grinding the cross section of the steel plate parallel to the rolling direction, etching is performed with nitric acid ethanol solution, and the image is taken using SEM (scanning electron microscope) at a magnification of 1500x. The area ratio of each microstructure is calculated from the obtained image data. The measurement area is set as a region of 1 / 8 of the plate thickness from the surface and a region of 1 / 8 to 3 / 8 of the plate thickness.

[0075] In the image data, ferrite can be distinguished as black, bainite as black containing island-like retained austenite or gray containing uniformly oriented carbides, tempered martensite as light gray containing fine and randomly oriented carbides, and retained austenite as white. Here, both quenched martensite and retained austenite appear white, making them sometimes difficult to distinguish from SEM images. Therefore, the area ratio of retained austenite is calculated separately by measuring the X-ray diffraction intensity, and subtracted from the area ratio of the white portion to obtain the area ratio of quenched martensite. The area ratio of retained austenite is calculated based on the ratio of the X-ray diffraction integral intensity of the (200), (220), and (311) planes of fcc iron in the quarter-thickness plane to the X-ray diffraction integral intensity of the (200), (211), and (220) planes of bcc iron. The retained austenite was calculated as a volume fraction based on the above measurements, but the retained austenite was considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite was used as the area fraction of the retained austenite.

[0076] It should be noted that the martensite specified in this invention includes both quenched martensite and tempered martensite, and can be composed of quenched martensite and tempered martensite.

[0077] Ferrite area fraction: less than 65%

[0078] When the ferrite area fraction is 65% or more, a tensile strength (TS) of 780 MPa or more is sometimes not obtained. Therefore, the ferrite area fraction is less than 65%, preferably less than 60%. There is no particular limitation on the lower limit, and it can be 0%, but from the viewpoint of balancing tensile strength and ductility, the ferrite area fraction is preferably 5% or more, and more preferably 10% or more.

[0079] The combined area ratio of martensite (including tempered martensite) and bainite is 25% or more.

[0080] Martensite is a hard microstructure formed from austenite at low temperatures below the martensitic transformation initiation point (Ms point). The martensite in this invention includes not only quenched martensite (quenched martensite) but also tempered martensite obtained by tempering the generated martensite at a specified temperature.

[0081] Bainite is a hard microstructure formed from austenite at a relatively low temperature above the Ms point and containing fine carbides dispersed in acicular or plate-like ferrite. Martensite and bainite are the microstructures responsible for the strength of the steel sheet of the present invention; when their combined area fraction is less than 25%, a strength tolerance (TS) of 780 MPa or higher is sometimes not obtained. Therefore, the combined area fraction of martensite and bainite is 25% or more, preferably 30% or more. It should be noted that only either martensite or bainite may fall within the above range. There is no particular upper limit, but from the viewpoint of more appropriately controlling the balance between TS and ductility, the combined area fraction of martensite (including tempered martensite) and bainite is preferably 85% or less, more preferably 80% or less.

[0082] The area ratio of retained austenite is 3% or more but less than 10%.

[0083] When there is an excessive amount of retained austenite, during bending or punching-based hole-making processes, the process may induce a phase transformation into hard martensite, thereby promoting cracking and deteriorating bending and hole-opening properties. This effect becomes significant when the retained austenite content exceeds 10%. Therefore, the area ratio of retained austenite is 10% or less. The area ratio of retained austenite is preferably 9% or less, more preferably 8% or less. On the other hand, when the retained austenite content is less than 3%, sufficient ductility cannot be obtained. Therefore, the retained austenite content is 3% or more. The area ratio of retained austenite is preferably 4% or more, more preferably 5% or more.

[0084] If ferrite, martensite, bainite, and retained austenite meet the above-mentioned ranges, other microstructures may be included in the microstructure allowance at an area ratio of less than 5%. Examples of other microstructures include pearlite.

[0085] In terms of area ratio, more than 70% of the total martensite in the microstructure of the base steel plate at 1 / 8 to 3 / 8 of the plate thickness is tempered martensite with carbides having an average grain size of 50 nm or more and 200 nm or less.

[0086] In this invention, tempered martensite with a specified size comprising at least 70% of the martensite in the microstructure at 1 / 8 to 3 / 8 of the thickness of the base steel plate is obtained by precipitation strengthening, thereby achieving a high yield ratio.

[0087] Simultaneously, by effectively utilizing fine carbides as hydrogen trapping points in the steel, diffusible hydrogen is reduced, thereby achieving excellent resistance to hydrogen embrittlement. To fully obtain these effects, the area fraction of tempered martensite with carbides of a specified size in all martensite contained in the microstructure at 1 / 8 to 3 / 8 of the thickness of the base steel plate is 70% or more, preferably 80% or more, and can also be 100%. This effect is achieved by ensuring that the carbide size is in the range of an average grain size of 50 nm or more and 200 nm or less. Generally, given a constant volume fraction of precipitates, smaller precipitate grain size is more advantageous for precipitation strengthening. Furthermore, from the viewpoint of hydrogen trapping, a larger surface area of ​​carbides is more advantageous; therefore, if the volume fraction is constant, smaller carbide grain size is more preferred. In addition, if the carbides become coarse, they become a cause of deterioration in bending performance. Therefore, the average grain size of the aforementioned carbides is 200 nm or less. On the other hand, when the average particle size of the carbide is less than 50 nm, the volume fraction of the carbide sometimes decreases, and the precipitation enhancement and hydrogen capture effects required by the present invention cannot be fully obtained.

[0088] Therefore, in this invention, tempered martensite with an average grain size of 50 nm or more and 200 nm or less carbides at 1 / 8 to 3 / 8 of the thickness of the base steel plate is set to be 70% or more of all martensite.

[0089] Here, the average particle size is the average of the major axis length and the minor axis length when approximating an ellipse. The average particle size can be determined using the method shown in the examples.

[0090] The cumulative amount of hydrogen released when the base steel plate after the coating is removed is heated from room temperature to 200°C: less than 0.45 ppm by mass.

[0091] The galvanized steel sheet of the present invention effectively captures hydrogen that infiltrates into the steel during the manufacturing process by controlling the carbides in the martensite to the range described above, thereby reducing diffusible hydrogen and exhibiting excellent flexibility, elongated flange properties, and resistance to hydrogen embrittlement. In the present invention, the cumulative amount of hydrogen released from the base steel sheet after the coating (which is a zinc coating, and also includes a metal coating if a metal coating described later is formed) is defined as the amount of diffusible hydrogen in the steel when heated from room temperature (15–35°C) to 200°C at a rate of 200°C / hour.

[0092] When the diffusible hydrogen content exceeds 0.45 ppm by mass, good flexibility, elongation flange properties, and resistance to hydrogen embrittlement are sometimes not obtained. Therefore, the diffusible hydrogen content is 0.45 ppm by mass or less, preferably 0.40 ppm by mass or less.

[0093] There is no specific lower limit; the lower the diffusible hydrogen content, the better, so it can be 0 ppm by mass.

[0094] When the base steel plate after the coating is removed is heated from room temperature to 600°C, the cumulative amount of hydrogen released within the temperature range of 350–600°C is 0.05 ppm by mass or more (preferred condition).

[0095] The galvanized steel sheet of the present invention effectively captures hydrogen that infiltrates into the steel during the manufacturing process by controlling the carbides in the martensite to the range described above, thereby reducing diffusible hydrogen. Compared with diffusible hydrogen, hydrogen captured in the carbides is less likely to detach from the steel, is not released when heated to 200°C, and is released at 350°C or higher. To fully obtain the effects of the present invention, the cumulative value of hydrogen released at 350-600°C when the steel sheet after stripping the coating (which is a zinc coating, and also includes a metal coating if a metal coating described later is formed) is preferably 0.05 ppm by mass or more when heated from room temperature (15-35°C) to 600°C at a rate of 200°C / hour. More preferably, it is 0.06 ppm by mass or more, and even more preferably, it is 0.07 ppm by mass or more.

[0096] There is no specific upper limit, and the cumulative amount of hydrogen released at 350–600°C can be below 1.00 ppm by mass.

[0097] As a quantitative method for hydrogen in the aforementioned steel, a hydrogen analysis test piece of approximately 5 mm × 30 mm was cut from a galvanized steel sheet. The zinc coating (including any metallic coating if present) was removed using a precision grinder. The piece was then rapidly placed into a quartz tube purged with Ar gas and heated to 600 °C at a rate of 200 °C / hour. The amount of hydrogen released during the heating process was determined by gas chromatography. Specifically, the cumulative amount of hydrogen released within the temperature range from room temperature to 200 °C was calculated as the "diffuse hydrogen amount," and the cumulative amount of hydrogen released within the temperature range above 350 °C and below 600 °C was calculated as the "captured hydrogen amount."

[0098] Here, the hydrogen content is preferably measured after the steel plate has been manufactured. That is, the diffusible hydrogen content is preferably measured after the steel plate has been manufactured. In addition, the captured hydrogen content is preferably measured after the steel plate has been manufactured.

[0099] Furthermore, the determination of diffusible hydrogen and the determination of captured hydrogen are preferably carried out within 72 hours after the steel plate initially reaches room temperature (below 40°C) following the reheating process specified in this invention.

[0100] Zinc coating

[0101] The galvanized steel sheet of the present invention has a zinc coating on the base steel sheet. The zinc coating can be an alloyed zinc coating, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer.

[0102] The galvanized steel sheet of the present invention may have a zinc coating (a second coating if a metal coating (first coating) is formed) formed on a base steel sheet (on the surface of the base steel sheet or on the surface of the metal coating if a metal coating is formed) as the outermost layer. The zinc coating may be provided on only one surface of the base steel sheet or on both sides.

[0103] That is, the galvanized steel sheet of the present invention may have a base steel sheet and a second coating (zinc coating, aluminum coating, etc.) formed on the base steel sheet. Alternatively, it may have a base steel sheet and a metal coating (a first coating (a second coating excluding the zinc coating)) and a second coating (zinc coating, aluminum coating, etc.) may be formed sequentially on the base steel sheet.

[0104] It should be noted that the zinc coating mentioned here refers to a coating with Zn as the main component (Zn content of 50.0% or more), such as hot-dip galvanized coatings and alloyed hot-dip galvanized coatings.

[0105] Here, the hot-dip galvanized layer (zinc coating) is preferably composed, for example, of Zn, 20.0% or less of Fe, and 0.001% or more but less than 1.0% by mass of Al. Additionally, the hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, totaling 0.0% or more but less than 3.5% by mass. Furthermore, the Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. It should be noted that the balance other than the aforementioned elements represents unavoidable impurities.

[0106] Furthermore, the alloyed hot-dip galvanized layer (alloyed zinc coating) is preferably composed, for example, of 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al. Additionally, the alloyed hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, totaling 0.0% by mass or more and 3.5% by mass or less. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. Furthermore, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0% by mass or less, and even more preferably 13.0% by mass or less. It should be noted that the balance other than the aforementioned elements represents unavoidable impurities.

[0107] Furthermore, there is no particular limitation on the amount of zinc coating applied per single side, but it is preferably set to 20 g / m². 2 That's all. Additionally, the zinc coating thickness on each side is preferably set at 80 g / m².2 the following.

[0108] It should be noted that the coating adhesion of the zinc coating was measured as follows.

[0109] Specifically, a treatment solution was prepared by adding 0.6 g of a corrosion inhibitor for Fe (manufactured by Asahi Chemical Industry Co., Ltd., "IBIT 700BK" (registered trademark)) to 1 L of a 10% hydrochloric acid aqueous solution. Next, a steel plate used as the test material was immersed in this treatment solution to dissolve the zinc coating. Then, the mass reduction of the test material before and after dissolution was measured, and this value was divided by the surface area of ​​the base steel plate (the surface area of ​​the portion covered by the coating) to calculate the coating adhesion amount (g / m²). 2 ).

[0110] Composition

[0111] Next, the preferred composition of the steel sheet (base steel sheet (hereinafter sometimes simply referred to as steel sheet)) constituting the galvanized steel sheet will be described. It should be noted that, unless otherwise specified, the "%" indicating the content of the constituent elements refers to "mass %".

[0112] C: Above 0.080% and below 0.300%

[0113] Carbon (C) improves hardenability, facilitating the formation of martensite and bainite. Furthermore, by precipitating as fine carbides within the microstructure, it traps hydrogen in the steel, reducing diffusible hydrogen and improving resistance to hydrogen embrittlement. When the C content is less than 0.080%, the required strength and flexural properties cannot be simultaneously achieved. Therefore, the C content is preferably 0.080% or more, and more preferably 0.100% or more.

[0114] On the other hand, when the C content exceeds 0.300%, the hydrogen capture effect brought about by the fine carbides sometimes becomes saturated, and the strength of the martensite becomes too high, thereby impairing ductility. Therefore, the C content is preferably 0.300% or less, and more preferably 0.250% or less.

[0115] Si: 0.20% or more and 2.00% or less

[0116] Si is an effective strengthening element for ferrite, which has excellent ductility. Furthermore, when annealed steel sheets are kept within a relatively low temperature range, it promotes the formation of retained austenite by suppressing the formation of coarse carbides, thus contributing to a better balance between strength and ductility. This effect cannot be fully achieved when the Si content is less than 0.20%. Therefore, the Si content is 0.20% or more, preferably 0.30% or more.

[0117] On the other hand, when the Si content exceeds 2.00%, the formation of carbides sometimes becomes excessive, making it impossible to obtain the fine carbides in the martensite required by the present invention. This leads to an increase in diffusible hydrogen in the steel, resulting in the inability to achieve the desired yield ratio and hydrogen embrittlement resistance of the present invention. Therefore, the Si content is preferably 2.00% or less, more preferably 1.50% or less.

[0118] Mn: Above 1.00% and below 4.00%

[0119] Mn improves the hardenability of steel, thus facilitating the formation of martensite and bainite. When the Mn content is less than 1.00%, the area fraction of ferrite sometimes becomes excessive, failing to achieve the desired strength and yield ratio. Therefore, the Mn content is preferably 1.00% or more, and more preferably 1.50% or more.

[0120] On the other hand, when the Mn content exceeds 4.00%, the strength of the martensite sometimes becomes too high, and it promotes the formation of coarse MnS, making it impossible to obtain excellent bending properties. Therefore, the Mn content is preferably 4.00% or less, and more preferably 3.50% or less.

[0121] P: below 0.10%

[0122] When phosphorus (P) is present in excess, it segregates at the original austenite grain boundaries, causing grain boundary embrittlement and deteriorating bending performance. Therefore, the P content is preferably 0.10% or less, more preferably 0.05% or less. The P content is preferably 0.03% or less, more preferably 0.02% or less.

[0123] There is no specific lower limit, but phosphorus (P) is also an effective element for increasing the strength of steel plates through solid solution strengthening. Therefore, to achieve such an effect, the P content is preferably set to 0.001% or higher. The P content can be 0.002% or higher, or 0.005% or higher.

[0124] S: Below 0.0200%

[0125] S forms inclusions such as MnS, which contribute to the deterioration of bending performance. Therefore, the S content is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0126] There is no specific lower limit, but due to limitations in production technology, the sulfur content can be above 0.0001%. The sulfur content can be above 0.0002% or above, or even above 0.0004%.

[0127] Al: Above 0.003% and below 0.100%

[0128] Al is used as a deoxidizer and also contributes to the solid solution strengthening of steel. These effects are sometimes not achieved when the Al content is less than 0.003%. Therefore, the Al content is preferably 0.003% or more. The Al content is preferably 0.005% or more, and more preferably 0.007% or more.

[0129] On the other hand, when the Al content exceeds 0.100%, it leads to a deterioration in the quality of the steel billet during steelmaking. Therefore, the Al content is preferably 0.100% or less, and more preferably 0.050% or less.

[0130] N: below 0.0100%

[0131] Nitrogen (N) can sometimes form large nitrides, which can become the starting point for void formation and reduce flexibility. By keeping the N content below 0.0100%, the formation of large nitrides can be prevented. Therefore, the N content is preferably below 0.0100%, and more preferably below 0.0060%.

[0132] There is no specific lower limit, but due to limitations in production technology, the nitrogen content can be above 0.0005%.

[0133] The balance other than those mentioned above consists of Fe and unavoidable impurities. The steel plate of the present invention preferably has a composition containing the above-mentioned components, with the balance consisting of Fe and unavoidable impurities.

[0134] The composition may also optionally contain a specified amount of at least one element selected from the group consisting of the following elements. When the content of any of the following optional elements is less than a preferred lower limit, the optional elements may be included as unavoidable impurities.

[0135] B: Below 0.0100%

[0136] Boron (B) is an element effective in improving the hardenability of steel. To improve hardenability, the B content is preferably set to 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0003% or more. The B content is preferably set to 0.0005% or more, and more preferably 0.0007% or more.

[0137] To obtain better formability, when B is present, the B content is preferably set to 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less.

[0138] Ti: below 0.200%

[0139] Ti causes the precipitation of fine carbides, which helps to increase strength and improve hydrogen capture. There is no particular limit to the lower limit of Ti, but in order to obtain these effects, it is preferred to set it to 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0140] On the other hand, when Ti is present in excessive amounts, the carbides may become coarse, which can lead to a deterioration in flexibility. Therefore, when Ti is present, the Ti content is preferably set to 0.200% or less, more preferably 0.100% or less, and even more preferably 0.060% or less.

[0141] Nb: below 0.200%

[0142] Nb causes the precipitation of fine carbides, which helps to increase strength and improve hydrogen capture. There is no particular limit to the lower limit of Nb, but in order to obtain these effects, it is preferred to set it to 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0143] On the other hand, when Nb is present in excessive amounts, the carbides may become coarse, which can lead to a deterioration in flexibility. Therefore, when Nb is present, the Nb content is preferably set to 0.200% or less, more preferably 0.100% or less, and even more preferably 0.060% or less.

[0144] Sb: below 0.200%

[0145] Sb is an effective element for suppressing excessive decarburization on the surface of steel plates and preventing a reduction in the amount of martensite formation. To achieve this effect, the Sb content is preferably set to 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0146] On the other hand, in order to obtain better toughness, when Sb is present, the Sb content is preferably set to 0.200% or less. More preferably, the Sb content is set to 0.060% or less, and even more preferably, it is set to 0.020% or less.

[0147] Sn: below 0.200%

[0148] Sn is an effective element for inhibiting decarburization, denitrification, and other processes that reduce the strength of steel. To achieve this effect, the Sn content is preferably set to 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0149] On the other hand, in order to obtain better impact resistance, when Sn is present, the Sn content is preferably set to 0.200% or less. The Sn content is more preferably set to 0.060% or less, and even more preferably set to 0.020% or less.

[0150] V: Below 0.100%

[0151] V causes the precipitation of fine carbides, which helps to increase strength and improve hydrogen capture. There is no particular limitation on the lower limit of V, but to achieve these effects, it is preferably set to 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. The V content is preferably set to 0.007% or more, and more preferably 0.009% or more.

[0152] On the other hand, when there is an excessive amount of V, the carbides may become coarse, which can lead to a deterioration in flexibility. Therefore, when V is present, the V content is preferably set to 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less.

[0153] Cu: below 2.00%

[0154] Cu is an element that increases hardenability and is effective in increasing the area ratio of the hard phase within a more preferred range. To achieve this effect, the Cu content is preferably set to 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more. The Cu content is preferably set to 0.040% or more, and more preferably 0.060% or more.

[0155] Furthermore, in the case of Cu, from the viewpoint of preventing cost increases, the Cu content is preferably set to 2.00% or less, more preferably 1.00% or less, and even more preferably 0.50% or less.

[0156] Cr: less than 2.00%

[0157] Adding Cr can improve hardenability and enhance the balance between strength and ductility. To achieve this effect, the Cr content is preferably set to 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. The Cr content is preferably set to 0.007% or more, and more preferably 0.009% or more.

[0158] Furthermore, in the case of Cr, from the viewpoint of preventing cost increases, the Cr content is preferably set to 2.00% or less, more preferably 1.00% or less, and even more preferably 0.80% or less.

[0159] Ni: below 2.00%

[0160] Adding Ni improves hardenability and enhances the balance between strength and ductility. To achieve this effect, the Ni content is preferably set to 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more. The Ni content is preferably set to 0.030% or more, and more preferably 0.040% or more.

[0161] Furthermore, in the case of containing Ni, from the viewpoint of preventing cost increases, the Ni content is preferably set to 2.00% or less, more preferably 1.00% or less, and even more preferably 0.80% or less. The Ni content is preferably set to 0.60% or less, and more preferably 0.40% or less.

[0162] Mo: 1.00% or less

[0163] By adding Mo, an intensity adjustment effect can be achieved. To achieve this effect, the Mo content is preferably set to 0.005% or more, more preferably 0.01% or more, and even more preferably 0.02% or more. The Mo content is preferably set to 0.03% or more, and more preferably 0.04% or more.

[0164] Furthermore, in the case of containing Mo, from the viewpoint of preventing cost increases, the Mo content is preferably set to 1.00% or less, more preferably 0.80% or less, and even more preferably 0.60% or less. The Mo content is preferably set to 0.50% or less, and more preferably 0.40% or less.

[0165] Ta: Below 0.100%

[0166] By adding Ta, the strength can be improved. To achieve this effect, the Ta content is preferably set to 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

[0167] Furthermore, when Ta is present, from the viewpoint of preventing cost increases, the Ta content is preferably set to 0.100% or less. More preferably, the Ta content is set to 0.050% or less, and even more preferably, 0.020% or less. The Ta content is preferably set to 0.010% or less, and more preferably 0.008% or less.

[0168] W: Below 0.500%

[0169] By adding W, an increase in strength can be achieved. To achieve this effect, the W content is preferably set to 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. The W content is preferably set to 0.005% or more, and more preferably 0.007% or more.

[0170] Furthermore, when W is present, from the viewpoint of preventing cost increases, the W content is preferably set to 0.500% or less, more preferably 0.450% or less, and even more preferably 0.400% or less. The W content is preferably set to 0.350% or less, and more preferably 0.300% or less.

[0171] Zr: below 0.020%

[0172] Adding Zr can improve the ultimate deformation capacity and elongation flange properties of steel plates. To achieve this effect, the Zr content is preferably set to 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0173] Furthermore, when Zr is present, from the viewpoint of preventing cost increases, the Zr content is preferably set to 0.020% or less. More preferably, the Zr content is set to 0.010% or less, and even more preferably, to 0.0050% or less.

[0174] Ca: below 0.0200%

[0175] By including Ca, the morphology of sulfides can be controlled, further improving ductility and toughness. To obtain better ductility, when Ca is included, the Ca content is preferably set to 0.0200% or less. More preferably, the Ca content is set to 0.0100% or less, and even more preferably, 0.0050% or less. The Ca content is preferably set to 0.0040% or less, and most preferably 0.0030% or less.

[0176] Furthermore, there is no particular limitation on the lower limit of the Ca content, which can be 0.0000%, but due to limitations in production technology, the Ca content is preferably set to 0.0001% or more. In addition, from the viewpoint of controlling the morphology of the aforementioned sulfides and further improving ductility and toughness, the Ca content is more preferably set to 0.0005% or more.

[0177] Mg: below 0.0200%

[0178] By containing 0.0001% or more of Mg, the morphology of sulfides can be controlled, thereby improving ductility and toughness. More preferably, it is 0.0005% or more, and even more preferably, it is 0.001% or more.

[0179] Furthermore, to obtain better ductility, when Mg is present, the Mg content is preferably set to 0.0200% or less. More preferably, the Mg content is set to 0.0100% or less, and even more preferably, it is set to 0.0050% or less.

[0180] Zn: below 0.020%

[0181] Adding Zn can improve the ultimate deformation capacity and elongation flange properties of steel plates. To achieve this effect, the Zn content is preferably set to 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

[0182] Furthermore, when Zn is present, from the viewpoint of preventing cost increases, the Zn content is preferably set to 0.020% or less. More preferably, the Zn content is set to 0.010% or less, and even more preferably, to 0.008% or less.

[0183] Co: below 0.020%

[0184] Adding Co can improve the ultimate deformation capacity and elongation flange properties of steel plates. To achieve this effect, the Co content is preferably set to 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

[0185] Furthermore, in the case of containing Co, from the viewpoint of preventing cost increases, the Co content is preferably set to 0.020% or less. More preferably, the Co content is set to 0.010% or less, and even more preferably, it is set to 0.008% or less.

[0186] The content of each of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM (excluding Ce) is below 0.0200%.

[0187] By adding these elements, the ultimate deformation capacity and elongation flange properties of the steel sheet can be improved. To achieve this effect, it is preferable to contain at least one of these elements in an amount of 0.0001% or more.

[0188] On the other hand, from the viewpoint of preventing cost increases, when containing at least one of these elements, the content of each is preferably set to 0.0200% or less.

[0189] Ce is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Furthermore, Ce is more preferably 0.0150% or less, and even more preferably 0.0100% or less. Ce is preferably set to 0.0080% or less, and more preferably 0.0060% or less.

[0190] Se is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Se is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. Se is more preferably 0.0150% or less, and even more preferably 0.0100% or less. Se is preferably set to 0.0080% or less, and even more preferably 0.0060% or less.

[0191] Te is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Te is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. Te is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0192] Ge is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Ge is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. Ge is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0193] The content of As is more preferably 0.0002% or more, and even more preferably 0.0005% or more. The content of As is preferably 0.0007% or more, and even more preferably 0.0009% or more. The content of As is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0194] Sr is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Sr is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. Sr is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0195] Cs is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Cs is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. Cs is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0196] Hf is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Hf is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. Hf is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0197] Pb is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Pb is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. Pb is more preferably 0.0150% or less, and even more preferably 0.0100% or less. Pb is preferably set to 0.0080% or less, and even more preferably 0.0060% or less.

[0198] Bi is more preferably 0.0002% or more, and even more preferably 0.0005% or more. Bi is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. Bi is more preferably 0.0150% or less, and even more preferably 0.0100% or less. Bi is preferably set to 0.0080% or less, and even more preferably 0.0060% or less.

[0199] REM is more preferably 0.0002% or more, and even more preferably 0.0005% or more. REM is preferably set to 0.0007% or more, and even more preferably 0.0009% or more. REM is more preferably 0.0150% or less, and even more preferably 0.0100% or less. It should be noted that REM as defined in this invention does not include Ce as described above.

[0200] It should be noted that, in this invention, REM refers to the lanthanide elements, namely scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in this invention refers to the total content of one or more elements selected from the aforementioned REM elements.

[0201] As REM, there are no special restrictions, but Sc, Y, and La are preferred.

[0202] Surface soft layer

[0203] According to one embodiment of the present invention, the base steel sheet of the galvanized steel sheet preferably has a surface soft layer. During stamping and vehicle body collisions, this surface soft layer helps to suppress the propagation of bending cracks, thus further improving the bending fracture resistance. It should be noted that the surface soft layer refers to a decarburized layer, which is a surface region where the Vickers hardness of a section (a plane parallel to the steel sheet surface) at 1 / 4 of the sheet thickness is 85% or less.

[0204] Here, the surface soft layer is formed with a thickness of 200 μm or less in the thickness direction from the surface of the base steel plate. The area where the surface soft layer is formed is preferably 150 μm or less in the thickness direction from the surface of the base steel plate, more preferably 120 μm or less. It should be noted that the lower limit of the thickness of the surface soft layer is not particularly specified, but it is preferably 7 μm or more, more preferably 14 μm or more, and even more preferably 17 μm or more. Furthermore, the surface soft layer is preferably 30 μm or more, more preferably 40 μm or more.

[0205] In addition, the base steel plate at 1 / 4 of its thickness where the Vickers hardness was measured was a non-surface soft layer (a layer that does not meet the hardness requirements for the surface soft layer specified in this invention).

[0206] Vickers hardness was measured based on JIS Z 2244-1 (2020) with a load of 10 gf.

[0207] Nanoscale hardness of the surface soft layer

[0208] Preferably, in the galvanized steel sheet of the present invention, when measuring the nanohardness at 300 or more points in a 50μm×50μm region on the sheet surface at positions 1 / 4 and 1 / 2 of the thickness direction depth of the surface soft layer from the base steel sheet, the proportion of measurements where the nanohardness of the sheet surface at positions 1 / 4 of the thickness direction depth of the surface soft layer from the base steel sheet is 7.0 GPa or higher is 0.10 or less relative to the total number of measurements; and the standard deviation σ of the nanohardness of the sheet surface at positions 1 / 4 of the thickness direction depth of the surface soft layer from the base steel sheet is 1.8 GPa or less, and the standard deviation σ of the nanohardness of the sheet surface at positions 1 / 2 of the thickness direction depth of the surface soft layer from the base steel sheet is 2.2 GPa or less.

[0209] When the proportion of the number of nanoscale hardnesses above 7.0 GPa is less than 0.10 relative to the total number of measurements, it means that the proportion of hard structures (such as martensite) and inclusions is small. This can further suppress the generation, connection, and cracking of voids during stamping and collision of the hard structures (such as martensite) and inclusions, resulting in excellent R / t and SFmax.

[0210] In addition, in order to obtain excellent bending properties during stamping and excellent bending fracture properties during impact, it is preferable that the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the thickness direction depth of the surface soft layer from the surface of the base steel plate is 1.8 GPa or less, and the standard deviation σ of the nanohardness of the plate surface at a position 1 / 2 of the thickness direction depth of the surface soft layer from the surface of the base steel plate is 2.2 GPa or less.

[0211] When the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the thickness of the surface soft layer from the base steel plate surface is less than 1.8 GPa, and the standard deviation σ of the nanohardness of the plate surface at a position 1 / 2 of the thickness of the surface soft layer from the base steel plate surface is less than 2.2 GPa, it means that the difference in microstructure hardness in the micro-region is small. Because the difference in microstructure hardness in the micro-region is small, it can further suppress the generation, connection, and cracking of voids during stamping and collision, and can obtain excellent R / t and SFmax.

[0212] Furthermore, the preferred range for the standard deviation σ of the nanohardness of the plate surface at a location representing one-quarter of the thickness depth of the surface soft layer from the base steel plate surface is 1.7 GPa or less. More preferably, the standard deviation σ of the nanohardness of the plate surface at a location representing one-quarter of the thickness depth of the surface soft layer from the base steel plate surface is 1.3 GPa or less. While the lower limit is not particularly limited, the standard deviation σ of the nanohardness of the plate surface at a location representing one-quarter of the thickness depth of the surface soft layer from the base steel plate surface can be 0.5 GPa or more.

[0213] The preferred range for the standard deviation σ of the nanohardness of the plate surface at a location half the thickness depth of the surface soft layer from the base steel plate surface is 2.1 GPa or less. More preferably, the standard deviation σ of the nanohardness of the plate surface at a location half the thickness depth of the surface soft layer from the base steel plate surface is 1.7 GPa or less. While the lower limit is not particularly limited, the standard deviation σ of the nanohardness of the plate surface at a location half the thickness depth of the surface soft layer from the base steel plate surface can be 0.6 GPa or more.

[0214] Here, the nanohardness of the plate surface at 1 / 4 and 1 / 2 of the thickness direction refers to the hardness measured by the following method.

[0215] First, after the zinc coating is stripped, for steel sheets with further metal coatings such as electroplated metal layers, mechanical grinding is performed after stripping the metal coating until one-quarter of the thickness of the surface soft layer is reached from the surface of the base steel sheet. Polishing with diamond and alumina is then performed, followed by colloidal silica polishing. Nanoscale hardness is measured using a Hysitron Tribo-950 with a Beaufort-shaped diamond indenter under conditions of a load of 500 μN, a measurement area of ​​50 μm × 50 μm, and a dot spacing of 2 μm.

[0216] In addition, mechanical grinding was performed until half the thickness of the surface soft layer, followed by polishing with diamond and alumina, and then colloidal silica grinding. Then, using a Hysitron Tribo-950 with a Bose-shaped diamond indenter, the nanohardness was measured under conditions of a load of 500 μN, a measurement area of ​​50 μm × 50 μm, and a dot spacing of 2 μm.

[0217] Nanoscale hardness was measured at 1 / 4 of the thickness depth of the plate, with more than 300 points. Additionally, nanoscale hardness was also measured at 1 / 2 of the thickness depth of the plate.

[0218] For example, when the thickness of the surface soft layer is 100 μm, the 1 / 4 position is located 25 μm from the surface of the surface soft layer, and the 1 / 2 position is located 50 μm from the surface of the surface soft layer. At the 25 μm position, more than 300 points of nanohardness were measured, and at the 50 μm position, more than 300 points of nanohardness were also measured.

[0219] Metal plating

[0220] Furthermore, according to one embodiment of the present invention, the galvanized steel sheet preferably has a metal coating (first coating, pre-coating) formed between the base steel sheet and the zinc coating on one or both sides of the base steel sheet (it should be noted that the metal coating does not include hot-dip galvanized layers or alloyed hot-dip galvanized layers). The metal coating is preferably a metal electroplated layer, and the following description will use a metal electroplated layer as an example.

[0221] By forming a metal electroplating layer on the base steel plate, the outermost metal electroplating layer helps to suppress the generation of bending cracks during stamping and body collision, thus further improving the bending fracture resistance.

[0222] In this invention, by setting the dew point in the homogenization process to above -5°C, the thickness of the soft layer can be further increased, resulting in excellent axial crush characteristics. Furthermore, in this invention, by having a metal coating, axial crush characteristics equivalent to those obtained with a large soft layer thickness can be achieved even when the dew point is below -5°C and the soft layer thickness is small.

[0223] The metal used for electroplating can be any one of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi, with Fe being more preferred. The following explanation uses an Fe-based electroplating layer as an example, but the same conditions for Fe can be applied to other metal types.

[0224] The adhesion amount of the Fe-based electroplated layer is set to be greater than 0 g / m. 2 The preferred setting is 2.0 g / m 2 The above describes the upper limit of the amount of Fe-based electroplated coating applied to the base steel plate per side. While there is no specific limit, from a cost perspective, it is preferable to set the amount of Fe-based electroplated coating applied to each side at 60 g / m². 2 The following is a preferred adhesion amount for Fe-based electroplated layers: 50 g / m². 2 The following is more preferably 40g / m 2 The following is a further preferred value: 30g / m 2 the following.

[0225] The adhesion amount of Fe-based electroplated coatings was determined as follows. A 10×15mm sample was cut from an Fe-based electroplated steel sheet and embedded in resin to create a cross-section. Using a scanning electron microscope (SEM) at an accelerating voltage of 15kV, three arbitrary locations on this cross-section were observed at magnifications ranging from 2000 to 10000 times, depending on the thickness of the Fe-based electroplated coating. The average thickness of the three fields of view was multiplied by the density of iron to calculate the adhesion amount of the Fe-based electroplated coating per single side.

[0226] As an Fe-based electroplating layer, in addition to pure Fe, alloy plating layers such as Fe-B alloys, Fe-C alloys, Fe-P alloys, Fe-N alloys, Fe-O alloys, Fe-Ni alloys, Fe-Mn alloys, Fe-Mo alloys, and Fe-W alloys can also be used. The composition of the Fe-based electroplating layer is not particularly limited, but it is preferably set to contain one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the balance consisting of Fe and unavoidable impurities, totaling 10% or less by mass. By setting the amount of elements other than Fe to a total of 10% or less by mass, a decrease in electrolysis efficiency can be prevented, and an Fe-based electroplating layer can be formed at low cost. In the case of Fe-C alloys, the C content is preferably set to 0.08% or less by mass.

[0227] <Manufacturing Method of Galvanized Steel Sheet>

[0228] Next, a method for manufacturing a galvanized steel sheet according to one embodiment of the present invention will be described. It should be noted that, unless otherwise specified, the temperatures used to heat or cool the steel billet (steel raw material), steel sheet, etc., as shown below refer to the surface temperature of the steel billet (steel raw material), steel sheet, etc.

[0229] The method for manufacturing galvanized steel sheet of the present invention includes: a hot rolling process, wherein a steel billet having the above-mentioned composition is heated to a temperature range of 1100 to 1350°C, hot-rolled at a finishing rolling temperature of 800 to 950°C, and coiled at a coiling temperature of 650°C or lower; a reduction process, wherein the steel sheet after the hot rolling process is held at a temperature of 700°C or higher in a reducing atmosphere with a hydrogen concentration of 8% by volume or higher and 30% by volume or lower for at least 20 seconds; and a homogenizing process, wherein the steel sheet after the reduction process is held at a temperature of 750°C or higher in a homogenizing atmosphere with a hydrogen concentration of 0.2% by volume or higher and 8% by volume or lower for at least 20 seconds and at least 300 seconds. The process includes a first cooling step, in which the steel sheet after the soaking process is cooled; a galvanizing step, in which a zinc coating is formed on the surface of the steel sheet after the first cooling step; a second cooling step, in which the average cooling rate of the steel sheet after the galvanizing step is set to less than 20°C / s from a temperature range of 350 to 450°C until it reaches (Ms point - 100°C), and the sheet is held at a temperature range of 100 to 350°C for more than 5 seconds to cool to a cooling stop temperature of 100 to 300°C; and a reheating step, in which the steel sheet after the second cooling step is held at a temperature range of 5 to 600 seconds above the cooling stop temperature and below 450°C.

[0230] [Hot rolling process]

[0231] The above-mentioned steel billet is hot-rolled under the following conditions.

[0232] Billet heating temperature: 1100~1350℃

[0233] To dissolve carbides and reduce rolling load, the billet heating temperature is set to 1100°C or higher. Preferably, the billet heating temperature is 1150°C or higher. Furthermore, to prevent increased oxide scale loss, the billet heating temperature is 1350°C or lower, preferably 1300°C or lower.

[0234] Finishing rolling temperature: 800~950℃

[0235] When the finishing rolling temperature is below 800°C, ferrite phase transformation sometimes occurs during rolling, forming extended ferrite on the surface of the hot-rolled plate. This ferrite remains in the next process, leading to a deterioration in the final bending properties. Therefore, the finishing rolling temperature is 800°C or higher, preferably 850°C or higher. On the other hand, when the finishing rolling temperature exceeds 950°C, grain coarsening sometimes occurs, resulting in insufficient strength and deterioration in bending properties. Therefore, the finishing rolling temperature is 950°C or lower, preferably 930°C or lower.

[0236] Winding temperature: below 650℃

[0237] When the coiling temperature exceeds 650°C, the carbides in the hot-rolled steel sheet sometimes coarsen and do not completely dissolve before the homogenization process, leading to deterioration in bendability. Therefore, the coiling temperature is preferably below 650°C, and more preferably below 600°C. While there is no particular limitation on the lower limit of the coiling temperature, it is preferable to set it to 400°C or higher from the viewpoint of suppressing shape defects in the steel sheet and preventing excessive hardening of the steel sheet.

[0238] [Cold rolling process (preferred conditions)]

[0239] Before supplying the base steel sheet obtained after the hot rolling process to the reduction process, cold rolling can be performed as needed. In the case of oxidation treatment in the oxidation process described later, cold rolling is performed before the oxidation process; in the case of metal plating in the pre-plating process, cold rolling is performed before the pre-plating process. When performing cold rolling, it is preferable to pre-treat the hot-rolled steel sheet obtained after the hot rolling process using known methods, such as pickling and degreasing, before performing cold rolling.

[0240] A reduction rate of over 20%

[0241] When cold rolling is performed, the reduction rate (cumulative reduction rate) is not particularly limited, but it is preferably set to 20% or more, more preferably 30% or more, in order to promote the recrystallization of ferrite. There is no particular upper limit to the reduction rate, but it is preferably set to 80% or less.

[0242] [Pre-plating (metal plating, metal electroplating, first plating) process (preferred conditions)]

[0243] In one embodiment of the present invention, the base steel plate can be pre-plated (metal plating) after the hot rolling process (or after the cold rolling process in the case of cold rolling) and before the reduction process to produce a pre-annealed metal-plated steel plate (pre-annealed metal electroplated steel plate) with a metal plating layer or the like formed on one or both sides of the base steel plate. It should be noted that the metal plating described herein does not include zinc plating (second plating).

[0244] There is no particular limitation on the metal plating process, but as mentioned above, as a metal plating layer formed on the base steel plate, it is preferable to set it as a metal electroplating layer, and therefore it is preferable to perform metal electroplating.

[0245] For example, when using an Fe-based electroplating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be used. Furthermore, the amount of metal plating layer adhering before annealing can be adjusted according to factors such as the energizing time. It should be noted that metal-plated steel sheet before annealing means that the metal plating layer has not undergone oxidation, reduction, or soaking processes; this does not exclude the possibility that the hot-rolled or cold-rolled steel sheet was pre-annealed before the metal plating treatment.

[0246] Here, the metal used as the electroplating layer can be any one of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi, with Fe being more preferred. The following explanation uses an Fe-based electroplating layer as an example, but the same conditions for Fe-based electroplating layers can be applied to other metal-based electroplating layers as well.

[0247] The Fe ion content in the Fe-based electroplating bath before energization is determined by Fe. 2+ The optimal concentration is set at 0.5 mol / L or higher. The Fe ion content in the Fe-based electroplating bath is determined by Fe... 2+ Sufficient Fe adhesion can be obtained when the Fe ion concentration is 0.5 mol / L or higher. Furthermore, to obtain sufficient Fe adhesion, the Fe ion content in the Fe-based electroplating bath before energizing is preferably set to 2.0 mol / L or lower.

[0248] Furthermore, the Fe-based electroplating bath may contain Fe ions and at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the Fe-based electroplating bath is preferably set such that the total content of these elements in the Fe-based electroplated layer before annealing is 10% by mass or less. It should be noted that metallic elements can be contained in the form of metal ions, and non-metallic elements can be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. Additionally, the ferric sulfate plating bath may also contain conductivity aids, chelating agents, pH buffers, etc., such as sodium sulfate and potassium sulfate.

[0249] There are no particular limitations on other conditions for the Fe-based electroplating bath. Considering temperature stability, the temperature of the Fe-based electroplating bath is preferably set to 30°C or higher. Furthermore, the temperature of the Fe-based electroplating bath is preferably set to 85°C or lower. There are no particular requirements for the pH of the Fe-based electroplating bath, but from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, it is preferably set to 1.0 or higher. Additionally, considering the conductivity of the Fe-based electroplating bath, it is preferably 3.0 or lower. Regarding the current density, from a productivity viewpoint, it is preferably set to 10 A / dm³. 2 From the perspective of easily controlling the adhesion amount of Fe-based electroplated layers, a value of 150 A / dm is preferred. 2 Regarding the plate speed, from a productivity point of view, it is preferable to set it to 5 mpm or higher; from the point of view of stable control of the amount of adhesion, it is preferable to set it to 150 mpm or lower.

[0250] It should be noted that, as a pretreatment before Fe-based electroplating, degreasing and rinsing for cleaning the steel plate surface, and pickling and rinsing for activating the steel plate surface can be performed. Following these pretreatments, Fe-based electroplating is performed. The methods for degreasing and rinsing are not particularly limited, and conventional methods can be used. In the pickling process, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Sulfuric acid, hydrochloric acid, or mixtures thereof are preferred. The concentration of the acid is not particularly specified, but considering the ability to remove the oxide film and the prevention of surface roughness (surface defects) caused by over-pickling, it is preferably 1 to 20% by mass. Furthermore, the pickling solution may contain defoamers, pickling accelerators, pickling inhibitors, etc.

[0251] [Oxidation process (preferred conditions)]

[0252] For steel plates after the aforementioned hot rolling process, or for steel plates after the cold rolling process in the case of cold rolling, an oxidation treatment can be performed in the oxidation process before the reduction process described later. By performing the oxidation treatment in the oxidation process, an iron oxide layer can be generated on the surface of the steel plate, and a reduced iron layer can be generated in the subsequent annealing in a reducing atmosphere, thereby improving the wettability of the steel plate.

[0253] Oxygen concentration: ≥1000 ppm by volume and ≤30000 ppm by volume

[0254] When oxidation is performed in an oxidation process, an oxygen concentration in the atmosphere of less than 1000 ppm by volume may not promote the formation of an iron oxide layer, resulting in minimal improvement in wettability. Therefore, an oxygen concentration of 1000 ppm by volume or higher is preferred, and 1500 ppm by volume or higher is preferable.

[0255] On the other hand, when the oxygen concentration exceeds 30,000 ppm by volume, excessive iron oxide may form. If reduction is incomplete before galvanizing processes such as hot-dip galvanizing, the wettability of the coating deteriorates due to the residual iron oxide. Therefore, the oxygen concentration is preferably 30,000 ppm by volume or less, and more preferably 25,000 ppm by volume or less. The oxygen concentration is preferably 22,000 ppm by volume or less, and more preferably 20,000 ppm by volume or less.

[0256] Steel plate temperature (maximum temperature reached during oxidation treatment): above 600℃

[0257] When oxidation is performed in the oxidation process, even if the atmosphere described above is set, if the steel plate temperature (the highest temperature reached during oxidation) is below 600°C, the amount of iron oxide formed on the surface may become small, resulting in a diminished effect on improving the wettability of the coating. Therefore, the steel plate temperature (the highest temperature reached during oxidation) in the oxidation process is 600°C or higher, preferably 620°C or higher. While there is no particular upper limit to the steel plate temperature, to more appropriately prevent excessive oxidation of the steel plate and the resulting unreduced iron oxide residue in the subsequent reduction process, and to obtain more stable and excellent coating properties, it is preferably 900°C or lower, more preferably 880°C or lower. The steel plate temperature (the highest temperature reached during oxidation) in the oxidation process is preferably 860°C or lower, more preferably 840°C or lower.

[0258] [Restoration Process]

[0259] A reduced iron layer is formed by heating the naturally formed oxide film on the steel plate surface, or the iron oxide layer generated during the oxidation process, in an iron-reducing atmosphere. In the subsequent homogenization process, the atmosphere becomes lower in hydrogen concentration, thus slowing down the iron reduction reaction. Therefore, the reduction of iron oxide needs to be completed in the reduction process.

[0260] Steel plate temperature (maximum temperature reached during reduction treatment): above 700℃

[0261] When the steel plate temperature (the highest temperature reached during reduction treatment) is below 700°C, the reduction rate is sometimes slow, leaving unreduced iron oxide residue. Therefore, the steel plate temperature in the reduction process is 700°C or higher, preferably 750°C or higher. There is no particular upper limit to the steel plate temperature, but from the viewpoint of reducing furnace load, it is preferable to set it below 950°C. The steel plate temperature in the reduction process is preferably below 920°C, more preferably below 900°C.

[0262] Hold time (hold time during restoration): 20 seconds or more

[0263] When the holding time in the reduction process (the holding time during the reduction treatment) is less than 20 seconds, the reduction of iron oxide may not be completed. Therefore, the holding time in the reduction process should be 20 seconds or more, preferably 25 seconds or more.

[0264] There is no particular upper limit to the holding time, but from a productivity point of view, it is preferable to set it to 150 seconds or less. The holding time is preferably 120 seconds or less, and more preferably 100 seconds or less.

[0265] Hydrogen concentration (hydrogen concentration during reduction treatment): 8% by volume or more and 30% by volume or less

[0266] When the hydrogen concentration in the atmosphere during the reduction process (hydrogen concentration during reduction treatment) is less than 8% by volume, unreduced iron oxide may sometimes remain. Therefore, the hydrogen concentration in the reduction process should be 8% by volume or more, preferably 10% by volume or more.

[0267] On the other hand, when the hydrogen concentration exceeds 30% by volume, the reduction rate sometimes saturates, and the intrusion of hydrogen into the steel increases excessively, making it difficult to sufficiently reduce the hydrogen content in the steel during the subsequent soaking process. Therefore, the hydrogen concentration in the reduction process is 30% by volume or less, preferably 28% by volume or less. The hydrogen concentration is preferably 26% by volume or less, and more preferably 24% by volume or less.

[0268] The dew point of the atmosphere in the reduction process (dew point during reduction treatment) is not particularly limited. However, in order to properly prevent oxidation of the surface of the base steel plate or the Fe-based electroplated layer and to ensure good adhesion of the coating when the hot-dip galvanized layer is applied, it is preferably set to 30°C or below, and more preferably to 25°C or below. During reduction treatment, the dew point is preferably set to 22°C or below, and more preferably to 20°C or below.

[0269] Furthermore, there is no particular limitation on the lower limit of the dew point. From the perspective of ease of industrial control, it is preferable to set it to -50°C or higher, and more preferably to -45°C or higher. During reduction treatment, the dew point is preferably set to -40°C or higher, and more preferably to -35°C or higher.

[0270] [Heating Process]

[0271] In the homogenization process, annealing is performed to obtain the microstructure of the galvanized steel sheet of the present invention, and the reduction of hydrogen intrusion into the steel during the reduction process is achieved.

[0272] Steel plate temperature (annealing temperature): above 750℃

[0273] If the steel plate temperature (annealing temperature) during the soaking process is below 750°C, the steel plate surface may be re-oxidized. Furthermore, sometimes unrecrystallized ferrite remains, or the austenite formation during annealing becomes insufficient, thus failing to obtain the microstructure required for the steel plate of this invention. Therefore, the steel plate temperature is 750°C or higher, preferably 760°C or higher.

[0274] On the other hand, there is no particular upper limit to the temperature of the steel plate in the soaking process, but from the viewpoint of reducing the furnace load and preventing excessive coarsening of austenite grains, it is preferable to set it to 950°C or below. The annealing temperature is preferably 930°C or below, and more preferably 910°C or below.

[0275] Holding time (holding time during soaking heat treatment): 20 seconds or more but less than 300 seconds

[0276] When the holding time in the soaking process is less than 20 seconds, the hydrogen in the steel is sometimes not sufficiently reduced. Furthermore, austenite formation is sometimes insufficient, and the steel sheet microstructure of the present invention cannot be obtained. Therefore, the holding time is 20 seconds or more, preferably 50 seconds or more.

[0277] On the other hand, when the holding time exceeds 300 seconds, the austenite grains may coarsen and the surface may decarburize, resulting in the steel sheet structure of the present invention not being obtained. Therefore, the holding time is 300 seconds or less, preferably 200 seconds or less. The holding time is preferably 180 seconds or less, more preferably 160 seconds or less.

[0278] Hydrogen concentration (hydrogen concentration during homogenization): ≥0.2% by volume and ≤8% by volume

[0279] Since the reduction of iron oxide is completed in the reduction process, the atmosphere in the soaking process is set to a low hydrogen concentration within a range that prevents the reduced iron from oxidizing again, thereby reducing the amount of hydrogen that has penetrated into the steel during the reduction process. When the hydrogen concentration exceeds 8% by volume, the hydrogen in the steel cannot be sufficiently reduced, and the steel plate of the present invention cannot be obtained. Therefore, the hydrogen concentration is 8% by volume or less, preferably 5% by volume or less.

[0280] On the other hand, when the hydrogen concentration is less than 0.2% by volume, the reduced iron may be re-oxidized. Therefore, the hydrogen concentration in the homogenizing atmosphere is 0.2% by volume or more, preferably 0.5% by volume or more. The hydrogen concentration is preferably 0.8% by volume or more, and more preferably 1.0% by volume or more.

[0281] Dew point of the soaking atmosphere (annealing atmosphere) during the soaking process (dew point during soaking treatment): -30°C or higher (preferred condition).

[0282] In one embodiment of the invention, the dew point of the soaking atmosphere in the soaking process is preferably set to -30°C. By setting the dew point of the atmosphere in the soaking process to -30°C or higher for soaking treatment, the decarburization reaction is promoted, and a deeper surface soft layer can be formed. More preferably, the dew point of the atmosphere in the soaking process is -15°C or higher, and even more preferably -5°C or higher. There is no particular upper limit to the dew point of the atmosphere in the soaking process; however, to properly prevent oxidation of the substrate steel plate surface or the Fe-based electroplated layer surface and to ensure good adhesion of the coating when the hot-dip galvanized layer is applied, it is preferably set to 30°C or lower, more preferably 25°C or lower, and even more preferably 20°C or lower.

[0283] [First Cooling Process]

[0284] The steel sheet after the soaking process is cooled to a temperature suitable for hot-dip galvanizing. The cooling conditions are not necessarily limited, but the following conditions are preferred.

[0285] Hydrogen concentration (hydrogen concentration during the first cooling process): 0.5% by volume or more and 30% by volume or less; Dew point (dew point during the first cooling process): 0°C or less (preferred conditions).

[0286] By setting the hydrogen concentration in the cooling atmosphere during the first cooling process to 0.5% by volume or more and 30% by volume or less, and setting the dew point to 0°C or less, re-oxidation during cooling can be suppressed. More preferably, the hydrogen concentration is 1.0% by volume or more and the dew point is -20°C or less.

[0287] The hydrogen concentration is preferably 1.5% by volume or more, and more preferably 2.0% by volume or more.

[0288] The dew point is preferably set to below -25°C, and more preferably to below -30°C.

[0289] In addition, there is no particular limitation on the lower limit of the dew point, but the dew point is preferably set to -55°C or higher, and more preferably to -50°C or higher.

[0290] On the other hand, by keeping the hydrogen concentration at 30% by volume or less, the increase in hydrogen in the steel caused by hydrogen intrusion during the cooling process can be suppressed. More preferably, the hydrogen concentration is 20% by volume or less, and even more preferably 10% by volume or less.

[0291] The steel plate is cooled from a temperature of 600–900°C (the temperature of the steel plate during the first cooling treatment) to 150–500°C at an average cooling rate of less than 20°C / s (the average cooling rate during the first cooling treatment) (preferred condition).

[0292] Furthermore, to more appropriately prevent an increase in hydrogen content in the steel during cooling, it is preferable to cool the steel plate from a temperature of 600°C to 900°C with an average cooling rate of 20°C / s or less. Additionally, the average cooling rate is preferably set to 10°C / s or more.

[0293] Furthermore, the steel sheet temperature needs to be cooled to the same level as the galvanizing bath temperature in subsequent galvanizing processes. Therefore, the cooling stop temperature (cooling stop temperature during the first cooling process) is below 500°C. Additionally, if the cooling stop temperature is below 150°C, excessive martensitic transformation sometimes occurs before the galvanizing process, which is then tempered afterward, resulting in insufficient strength in the final microstructure. Therefore, the cooling stop temperature is above 150°C.

[0294] Here, the average cooling rate (°C / s) is obtained from (steel plate temperature (600~900°C) - cooling stop temperature (150~500°C)) / cooling time (s) from the start of cooling to the end of cooling.

[0295] [Zinc plating process]

[0296] A zinc coating is formed on the surface of the steel sheet after the first cooling process. A representative method is to immerse the steel sheet in a hot-dip galvanizing bath.

[0297] The following explanation uses immersion in a hot-dip galvanizing bath as an example.

[0298] The immersion conditions in the hot-dip galvanizing bath are not particularly limited and can be carried out using general methods. The hot-dip galvanizing bath consists of Al, Zn, and unavoidable impurities, and its composition is not particularly specified. In one example, the Al concentration in the bath can be 0.05% by mass or more, and in another example, it can be 0.190% by mass or less. When the Al concentration in the bath is 0.05% by mass or more, the formation of bottom slag can be more effectively prevented. In addition, when the Al concentration in the bath is 0.190% by mass or less, the formation of top slag can be more effectively prevented. From a cost perspective, it is also preferable to set the Al concentration in the bath to 0.190% by mass or less. The galvanizing bath temperature is not particularly specified and can be 440°C or more, or 500°C or less. It should be noted that the galvanizing used in this invention is not limited to the above-described hot-dip galvanizing, as long as the desired steel plate temperature is met in the preceding and following processes, it can also be electro-galvanizing or other methods such as electro-galvanizing.

[0299] There is no particular limit to the amount of coating applied per single side; in one example, it is set to 25 g / m². 2 The above is additionally set at 120g / m 2 The coating thickness per single side is 25g / m². 2 At the above levels, corrosion resistance is particularly good, and the coating adhesion is particularly easy to control. Furthermore, the coating adhesion per single side is 120 g / m². 2 The coating adhesion is particularly good under the following conditions. There are no particular limitations on the method for adjusting the coating adhesion. As an example of hot-dip galvanizing, gas wiping can be used, and the adjustment can be made by adjusting the gas pressure and the distance between the wiping nozzle and the steel plate.

[0300] After a zinc coating is formed during the galvanizing process, alloying treatment can be performed to produce alloyed galvanized steel sheets. By making the zinc coating an alloyed zinc coating, the removal of hydrogen from the steel during the subsequent reheating process can be further promoted. There are no particular limitations on the conditions for alloying treatment, as long as the desired degree of alloying is obtained. For example, in the alloying treatment during hot-dip galvanizing, it is preferable to hold the steel sheet at a specified temperature for a specified time. For example, the steel sheet temperature can be set to 440°C or higher. The steel sheet temperature can be set to 600°C or lower. Furthermore, the holding time can be set to 5 seconds or more. The holding time can be set to 60 seconds or lower.

[0301] When alloying is performed, it is preferable to set the Fe content (degree of alloying) in the coating to 7% by mass or more. Furthermore, it is preferable to set this Fe content (degree of alloying) to 15% by mass or less. By setting the degree of alloying to 7% by mass or more, no η-Zn phase remains in the alloyed zinc coating, allowing for a more appropriate reduction of hydrogen in the steel during the reheating process. Additionally, when the degree of alloying exceeds 15% by mass, the formation of the Γ phase at the interface between the alloyed zinc coating and the base steel sheet is sometimes promoted, reducing the coating adhesion; therefore, it is preferable to set it to 15% by mass or less.

[0302] [Second Cooling Process]

[0303] The steel sheet after the above galvanizing process is cooled to a cooling stop temperature of 100-300°C. It should be noted that there are no particular limitations on the cooling method, as long as the following cooling conditions are met, such as N2 gas cooling, water cooling, or a combination thereof.

[0304] Cooling stop temperature (cooling stop temperature during the second cooling process): 100℃ or higher and 300℃ or lower

[0305] In the second cooling process, martensite is generated by cooling the untransformed austenite to a temperature range lower than the Ms point. Compared to austenite with an FCC crystal structure, hydrogen diffusion is faster in martensite with a BCT crystal structure, thus promoting hydrogen reduction in subsequent reheating processes. As described above, the steel sheet of the present invention needs to be controlled so that at least 70% of the martensite in the microstructure consists of tempered martensite of a specified size by area, which cannot be achieved when the cooling stop temperature exceeds 300°C. Therefore, the cooling stop temperature is 300°C or lower, preferably 280°C or lower. Furthermore, when the cooling stop temperature is below 100°C, sometimes almost all the austenite undergoes martensitic transformation at that moment, leaving almost no untransformed austenite in the final microstructure, impairing ductility. Additionally, the desired strength is sometimes not obtained.

[0306] Therefore, the cooling stop temperature is set to 100°C or higher, preferably 120°C or higher.

[0307] Average cooling rate from the temperature range of 350–450°C until reaching (Ms point - 100°C) (average cooling rate during the second cooling process): below 20°C / s

[0308] In the second cooling step, cooling is preferably performed by holding the material in a temperature range below Ms and above the cooling stop temperature for a predetermined time. This allows the martensite formed slightly below Ms to undergo self-tempering during cooling, further promoting the formation of carbides in the martensite required by the present invention. When the average cooling rate exceeds 20°C / s, self-tempering sometimes becomes insufficient, and the aforementioned carbides cannot be adequately obtained.

[0309] Therefore, the average cooling rate is 20°C / s or less, preferably 10°C / s or less, and more preferably 5°C / s or less. Furthermore, while there is no particular limitation on the lower limit of the average cooling rate, from the viewpoint of more stably preventing the carbide size (the average particle size of the carbides contained in the tempered martensite) from exceeding 200 nm due to excessively slow cooling, it is preferably 1°C / s or more.

[0310] Here, the average cooling rate (°C / s) is obtained from "(cooling start temperature (350~450°C)) - (Ms point - 100°C)) / cooling time (s) from cooling start to (Ms point - 100°C)".

[0311] Stay in a temperature range of 100–350℃ for more than 5 seconds.

[0312] In addition, during the second cooling process, if the residence time in the temperature range of 100 to 350°C is less than 5 seconds, the self-tempering may become insufficient and the aforementioned carbides may not be fully generated.

[0313] Therefore, the aforementioned residence time is 5 seconds or more, preferably 10 seconds or more, and more preferably 15 seconds or more. Furthermore, there is no particular upper limit to the residence time, but from the viewpoint of more stably preventing the carbide size from exceeding 200 nm, it is preferably 60 seconds or less.

[0314] It should be noted that the Ms point is the temperature at which the martensitic phase transformation begins, and it can be calculated using the following formula.

[0315] Ms (°C) = 539 - 423 × {[C mass%] × 100 / (100 - [α area%])} - 30 × [Mn mass%] - 12 × [Cr mass%] - 18 × [Ni mass%] - 8 × [Mo mass%]

[0316] In the above formula, [M mass%] (M: element) represents the amount of each element contained in the steel plate. Additionally, [α area%] represents the area fraction (%) of ferrite in the microstructure of the annealed steel plate.

[0317] [Reheating process]

[0318] In this process, by reheating the steel plate from the second cooling process described above, a reduction in diffusible hydrogen is achieved while tempering the martensite required to obtain the steel plate of the present invention.

[0319] In the reheating process, since a zinc coating already exists on the surface of the steel plate, there is no need to consider the re-oxidation of iron as in the soaking process, and it can be carried out in an atmosphere with a low hydrogen concentration, such as in the atmosphere. On the other hand, if the hydrogen concentration is too high, the diffusible hydrogen remaining in the steel may not decrease, but may even increase again at excessively high hydrogen concentrations. Therefore, although there is no particular limitation, the hydrogen concentration in the reheating process is preferably set to 0.2% by volume or less.

[0320] Reheating temperature: Above the cooling stop temperature (the cooling stop temperature during the second cooling process) and below 450°C

[0321] When the reheating temperature is lower than the cooling stop temperature (the cooling stop temperature during the second cooling process), the tempering of martensite is insufficient, and the carbides and tempered martensite required by the present invention cannot be obtained. Furthermore, when the temperature is lower than the cooling stop temperature (the cooling stop temperature during the second cooling process), the reduction effect of diffusible hydrogen in the steel also becomes insufficient. Therefore, the reheating temperature is at or above the cooling stop temperature (the cooling stop temperature during the second cooling process), preferably 120°C or higher. On the other hand, when the reheating temperature exceeds 450°C, excessive tempering leads to a decrease in tensile strength and coarsening of carbides, and the steel sheet of the present invention cannot be obtained. Additionally, temperatures exceeding 450°C sometimes damage the appearance of the zinc coating. Therefore, the reheating temperature is 450°C or lower, preferably 430°C or lower.

[0322] Holding time (holding time during reheat treatment): 5–600 s

[0323] When the holding time is less than 5 seconds, the tempering of martensite is insufficient, and the carbides and tempered martensite required by the present invention cannot be obtained, nor can the diffusible hydrogen in the steel be sufficiently reduced. Therefore, the holding time is 5 seconds or more, preferably 10 seconds or more. The holding time is more preferably 15 seconds or more, and more preferably 20 seconds or more.

[0324] On the other hand, when the holding time exceeds 600 seconds, it sometimes not only reduces production efficiency but also leads to a decrease in tensile strength and coarsening of carbides due to excessive tempering, thus failing to obtain the steel sheet of the present invention. Therefore, the holding time is 600 seconds or less, preferably 500 seconds or less. The holding time is more preferably 400 seconds or less, and even more preferably 300 seconds or less.

[0325] It should be noted that manufacturing conditions other than those mentioned above can be handled using conventional methods.

[0326] The thickness of the galvanized steel sheet of the present invention is not particularly limited, but is preferably 0.4 mm or more, more preferably 0.6 mm or more. The sheet thickness is preferably greater than 0.8 mm. The sheet thickness is more preferably 0.9 mm or more. The sheet thickness is further preferably 1.0 mm or more. The sheet thickness is even more preferably 1.2 mm or more.

[0327] In addition, the plate thickness is preferably set to 3.2 mm or less, and more preferably to 3.0 mm or less.

[0328] <Components>

[0329] Next, a component according to one embodiment of the present invention will be described.

[0330] According to one embodiment of the present invention, the component is a component formed using the aforementioned galvanized steel sheet (as a raw material). For example, the component is made by performing at least one of forming or joining processes on the galvanized steel sheet as a raw material.

[0331] Here, the aforementioned galvanized steel sheet exhibits a high yield ratio (YR), excellent bending and elongation flange properties, excellent resistance to hydrogen embrittlement, and a tensile strength (TS) of 780 MPa or more and less than 1180 MPa. Therefore, the component according to one embodiment of the present invention has a high yield ratio (YR), excellent bending and elongation flange properties, excellent resistance to hydrogen embrittlement, and a tensile strength (TS) of 780 MPa or more and less than 1180 MPa. Therefore, the component according to one embodiment of the present invention is particularly suitable as a component for use in the automotive field.

[0332] <Methods for manufacturing components>

[0333] Next, a method for manufacturing a component according to one embodiment of the present invention will be described.

[0334] According to one embodiment of the present invention, a method for manufacturing a component includes a step of performing at least one of forming processing and joining processing on the galvanized steel sheet (e.g., a galvanized steel sheet manufactured by the above-described method for manufacturing galvanized steel sheet) to produce a component.

[0335] Here, there are no particular limitations on the forming process; for example, general processing methods such as stamping can be used. Similarly, there are no particular limitations on the joining process; for example, general welding methods such as spot welding, laser welding, and arc welding, as well as riveting and rivet joining, can be used. It should be noted that there are no particular limitations on the forming and joining conditions; conventional methods are acceptable.

[0336] Example

[0337] The present invention will be specifically described with reference to the embodiments. The scope of the present invention is not limited to the following embodiments.

[0338] <Example 1>

[0339] Using steel with the composition shown in Table 1, cold-rolled steel sheets with a thickness of 1.4 mm or hot-rolled steel sheets with a thickness of 1.8 mm were manufactured under the conditions shown in Table 2. Galvanized steel sheets (hot-dip galvanized steel sheets) were then manufactured and evaluated as follows.

[0340] For all steel plates, in the hot rolling process, the billet heating temperature is set at 1250℃, the finishing rolling end temperature is set at 900℃, and the coiling temperature is set at 520℃. The reheating process is carried out in the atmosphere.

[0341] The evaluation results are shown in Table 3.

[0342] It should be noted that the galvanizing and alloying processes in the galvanizing process are carried out under the following conditions. Furthermore, all evaluations below are conducted within 72 hours after the steel sheet initially reaches room temperature (below 40°C) following the reheating process specified in this invention.

[0343] • Plating bath composition: Zn bath containing 0.13% by mass Al

[0344] Plating bath temperature: 460℃

[0345] • Coating adhesion range: 30~60g / m -2

[0346] Alloying temperature: 450~560℃

[0347] • Alloying degree range: 8.0~14.0% by mass

[0348] [Table 1]

[0349]

[0350] [Table 2]

[0351]

[0352] <Tension Test>

[0353] JIS5 tensile test specimens (JISZ2201) were cut from the obtained galvanized steel sheet along a direction perpendicular to the rolling direction, and the strain rate was set to 10. -3 The yield strength (YS), tensile strength (TS), elongation (EL), and yield ratio (YR = YS / TS) are determined by tensile testing according to JIS Z2241 (2011). It should be noted that examples with TS of 780 MPa or higher and less than 1180 MPa and YR of 0.60 or higher are used as examples in this invention.

[0354] Cases where the product of EL and TS is 10000 or more are considered to have excellent TS-EL balance.

[0355] <Bore Enlargement Test>

[0356] As an evaluation of the extended flange performance, a hole enlargement test was conducted according to the method of JIS Z 2256. A 100mm × 100mm test piece was cut from the obtained galvanized steel sheet, and a 10mm diameter hole was formed in the center of the test piece by punching. The clearance during punching was set to 12.5%. Next, a blank holder force of 9 tons (88.26kN) was applied around the hole using a die with an inner diameter of 75mm, and a conical punch with a 60° apex angle was pressed into the hole to enlarge it. The diameter of the hole at which cracking occurred was measured. The limiting hole enlargement rate λ (%) was calculated according to the following formula. It should be noted that examples with λ of 35% or more are used as examples of this invention.

[0357] λ(%)={(D f -D0) / D0}×100

[0358] D f Diameter of the pore (mm) at which the crack forms.

[0359] D0: Diameter of the hole before reaming (mm)

[0360] <Organizational Observation>

[0361] Test pieces for microstructure observation were cut from the obtained galvanized steel sheet. After grinding the section of the plate thickness parallel to the rolling direction, the plate was etched with nitric acid ethanol solution. The microstructure of the area 1 / 8 of the plate thickness from the surface and the area 1 / 8 to 3 / 8 of the plate thickness were photographed by SEM at a magnification of 1500x in three fields each.

[0362] The area ratio of retained austenite was determined separately by measuring the X-ray diffraction intensity. The area ratio of the quenched martensite was obtained by subtracting the area ratio of the white portion (quenched martensite and retained austenite). The area ratio of retained austenite was calculated based on the ratio of the integrated X-ray diffraction intensity of the (200), (220), and (311) planes of fcc iron in the quarter-thickness plane to the integrated X-ray diffraction intensity of the (200), (211), and (220) planes of bcc iron. The retained austenite was calculated as a volume ratio based on the above measurements, but since the retained austenite was considered three-dimensionally homogeneous, the volume ratio of the retained austenite was used as the area ratio of the retained austenite.

[0363] The area ratio of each constituent phase is calculated from the obtained image using the method described above. The area ratio of the constituent phase in each region is the average value from all images taken separately.

[0364] In addition, the microstructure of a region from 1 / 8 to 3 / 8 of the thickness of the substrate steel plate was photographed at 5000x magnification within a single field of view using SEM. The number and total area of ​​carbides within the original austenite grains containing the aforementioned martensite were determined from the image. The area of ​​each carbide was then calculated, yielding the average grain size of the carbides. Here, the average grain size is the average of the major and minor axes when approximating an ellipse. It should be noted that for each carbide, a continuously formed, integral region is considered as a single measurement in the SEM image.

[0365] <Quantitative Analysis of Hydrogen in Steel>

[0366] A hydrogen analysis test piece of approximately 5 mm × 30 mm was cut from the obtained galvanized steel sheet. The surface coating was removed using a precision grinder. The piece was then placed in a quartz tube purged with Ar gas and heated to 600 °C at a heating rate of 200 °C / hour. The amount of hydrogen released during the heating process was determined by gas chromatography. Specifically, the cumulative amount of hydrogen released within the temperature range from room temperature to 200 °C was calculated as the "diffuse hydrogen content," and the cumulative amount of hydrogen released within the temperature range of 350 °C to 600 °C was calculated as the "captured hydrogen content." It should be noted that an example with a diffuse hydrogen content of 0.45 ppm or less is used as an example of this invention.

[0367] Here, the determination of diffusible hydrogen and the determination of captured hydrogen are carried out after the steel plate is manufactured.

[0368] <Bending Test>

[0369] Using the direction parallel to the rolling process as the bending test axis, a 35mm × 100mm strip test piece was cut from the obtained galvanized steel sheet. Under conditions of a stroke speed of 50mm / s, a pressing load of 10 tons, and a holding time of 5 seconds, 90-degree V-bending tests were conducted with various bending radii. It should be noted that for steel sheets with a thickness exceeding 1.4mm, the sheet was ground on one side to 1.4mm before the test. The ground surface was used as the inner side (valley side) of the bend, ensuring that the grinding process did not affect the bending test results. After the test, the edge of the bending apex of the test piece was observed using a 10x magnifying glass. The value R / t was calculated by dividing the minimum bending radius R (mm) without observed cracks longer than 0.5mm by the sheet thickness t (mm) (t = 1.4, 1.8mm). It should be noted that examples satisfying either (A) or (B) below are considered examples of this invention.

[0370] (A) TS is above 780MPa and below 980MPa and R / t is below 4.5.

[0371] (B) TS is above 980MPa and below 1180MPa and R / t is below 5.0.

[0372] <Evaluation of Hydrogen Embrittlement Resistance>

[0373] A 30mm × 100mm test piece was cut from the obtained galvanized steel sheet. Spacers with a thickness of 2mm were clamped at both ends, and the spacers were spot-welded together to form a welded test piece. For spot welding, an inverter DC resistance spot welding machine was used, with chrome-copper dome-shaped electrodes with a front diameter of 6mm. The applied pressure was set to 380kgf, the energizing time to 16 cycles / 50Hz, and the holding time to 5 cycles / 50Hz. The welding current was adjusted to form a weld nugget diameter corresponding to the plate thickness. To satisfy the following formula (2), the weld nugget diameter was set to 3.8mm for a plate thickness of 1.4mm and 4.4mm for a plate thickness of 1.8mm.

[0374] 3.0×t 1 / 2 <Molten core diameter < 3.5 × t 1 / 2 …Formula (2)

[0375] In equation (2), t is the plate thickness (mm).

[0376] After 24 hours from spot welding, the spacer section is cut off, and the cross-section of the weld nugget is observed. The results are evaluated according to the following criteria, with cases rated 1 or 2 considered as the preferred scope of the present invention.

[0377] Crack observation result level

[0378] No cracking occurred: 1 (Exceptionally excellent resistance to hydrogen embrittlement)

[0379] It produces only tiny cracks smaller than 100μm: 2 (Excellent resistance to hydrogen embrittlement)

[0380] There are cracks larger than 100μm: 3 (Poor resistance to hydrogen embrittlement)

[0381] [Table 3]

[0382]

[0383] <Example 2>

[0384] Using steel with the composition shown in Table 1, cold-rolled steel sheets with a thickness of 1.4 mm were manufactured under the conditions shown in Table 4. Hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets were manufactured and evaluated in the same way as in Example 1. Further evaluations were performed as described below.

[0385] For all steel plates, in the hot rolling process, the billet heating temperature is set at 1250℃, the finishing rolling end temperature is set at 900℃, and the coiling temperature is set at 520℃. The reheating process is carried out in the atmosphere.

[0386] In Example 2, for a portion of steel plates (steel plates with "Pre-plating process" in Table 4), compared to Example 1, the metal plating process is performed in the pre-plating process after the cold rolling process and before the reduction process (or before the oxidation process if the treatment in the oxidation process is performed).

[0387] The evaluation results are shown in Table 5.

[0388] The galvanizing and alloying processes in the galvanizing process are carried out in the same manner as in Example 1.

[0389]

[0390] The tensile test, hole expansion test, microstructure observation, quantitative determination of hydrogen in steel, bending test, and evaluation of hydrogen embrittlement resistance were performed in the same manner as in Example 1.

[0391] <U-bend + tight bend test>

[0392] The U-bending + tight bending test is conducted as follows.

[0393] Test pieces of 60mm × 30mm were cut from the obtained galvanized steel sheet by shearing and end face grinding. Here, the 60mm edge is parallel to the width (C) direction. A U-bending process (single bending) was performed in the width (C) direction with a curvature radius / sheet thickness of 4.2 and the rolling (L) direction as the axis to prepare the test pieces. In the U-bending process (single bending), as... Figure 1 As shown in (a), a test piece T1 is obtained by pressing a steel plate placed on roller A1 into punch B1. Then, as... Figure 1 As shown in (b), the test piece T1, placed on the lower mold A2, undergoes a close-fitting bending process (secondary bending) by flattening it with the upper mold B2. Figure 1 (a) and Figure 1 In (b), D1 represents the width (C) direction and D2 represents the rolling (L) direction.

[0394] The conditions for U-bending in the U-bending + tight bending test are as follows.

[0395] Test method: roller support, punch pressing

[0396] Punch tip radius (R): 5.0mm

[0397] Roller spacing: plate thickness × 2mm

[0398] Stroke speed: 10mm / min

[0399] Bending direction: Rolling right angle (C) direction

[0400] The conditions for the tight bending in the U-bending + tight bending test are as follows.

[0401] Spacer thickness: varies in 0.5mm intervals.

[0402] Test method: Die support, punch pressing

[0403] Molding load: 10 tons

[0404] Test speed: 10 mm / min

[0405] Duration: 5 seconds

[0406] Bending direction: Rolling right angle (C) direction

[0407] Three U-bending + tight bending tests were performed, and the limiting spacer thickness (ST) was evaluated when no cracks appeared in any of the three tests. Additionally, using a Leica stereomicroscope, cracks longer than 200 μm at 25x magnification were defined as cracks. It should be noted that ST is an indicator for evaluating fracture resistance under impact (fracture resistance of the longitudinal wall in axial crush tests). The results are recorded in Table 5.

[0408] The following (A) or (B) conditions are evaluated as having excellent fracture resistance characteristics during impact (fracture resistance characteristics of the longitudinal wall in the axial crush test).

[0409] (A) When 780MPa≤TS<980MPa, 2.0mm≥ST

[0410] (B) When 980MPa≤TS<1180MPa, 3.0mm≥ST

[0411] <V-bending + orthogonal VDA bending test>

[0412] The V-bending + orthogonal VDA bending test is conducted as follows.

[0413] Test pieces measuring 60mm × 65mm were cut from the obtained galvanized steel sheet through shearing and end-face grinding. Here, the 60mm edge is parallel to the rolling (L) direction. The test pieces were prepared by performing a 90° bend (single bend) in the rolling (L) direction with a curvature radius / sheet thickness of 4.2 and the width (C) direction as the axis. During the 90° bend (single bend), as... Figure 1 As shown in (a), a steel plate placed on a die A1 with a V-groove is pressed into a punch B1 to obtain test piece T1. Then, as... Figure 1As shown in (b), the test piece T1, placed on the support roller A2, is pressed into the punch B2 with the bending direction perpendicular to the rolling direction to perform orthogonal bending (secondary bending process). Figure 1 (a) and Figure 1 In (b), D1 represents the width (C) direction and D2 represents the rolling (L) direction.

[0414] The conditions for V-bending in the V-bending + orthogonal VDA bending test are as follows.

[0415] Test method: Die support, punch pressing

[0416] Molding load: 10 tons

[0417] Test speed: 30 mm / min

[0418] Duration: 5 seconds

[0419] Bending direction: Rolling (L) direction

[0420] The conditions for VDA bending in the V-bending + orthogonal VDA bending test are as follows.

[0421] Test method: roller support, punch pressing

[0422] Roller diameter: φ30mm

[0423] Punch tip radius (R): 0.4mm

[0424] Roller spacing: (plate thickness × 2) + 0.5mm

[0425] Stroke speed: 20mm / minute

[0426] Test piece size: 60mm × 60mm

[0427] Bending direction: Rolling right angle (C) direction

[0428] From the stroke-load curves obtained during the VDA bending test described above, the stroke at which the load is maximum is determined. The average stroke at which the load is maximum during three V-bending + orthogonal VDA bending tests is taken as SFmax (mm). It should be noted that SFmax is an index for evaluating the fracture resistance characteristics during impact (fracture resistance characteristics of the bending ridge portion in the axial crush test). The results are recorded in Table 5. Cases (A) or (B) below are evaluated as having excellent fracture resistance characteristics during impact (fracture resistance characteristics of the bending ridge portion in the axial crush test).

[0429] (A) When 780MPa≤TS<980MPa, 29.5mm≤SFmax

[0430] (B) When 980MPa≤TS<1180MPa, 28.5mm≤SFmax

[0431] <Axial Compression Test>

[0432] A 160mm × 200mm test piece was cut from the obtained hot-dip galvanized steel sheet by shearing. Here, the 160mm edge is parallel to the rolling (L) direction. A forming process (bending process) was performed using a die with a punch shoulder radius of 5.0mm and a die shoulder radius of 5.0mm to achieve a depth of 40mm to produce the test piece. Figure 2 (a) and Figure 2 (b) shows the hat-shaped component 10. Additionally, a steel plate used as the raw material for the hat-shaped component is separately cut into a size of 80mm × 200mm. Next, the cut steel plate 20 is spot-welded to the hat-shaped component 10 to create a shape as shown. Figure 2 (a) and Figure 2 (b) shows the test component 30. Figure 2 (a) is a front view of the test component 30 made by spot welding the cap-shaped component 10 to the steel plate 20. Figure 2 (b) is a perspective view of the test component 30. The location of the spot weld 40 is as follows: Figure 2 (b) shows the setting such that the distance between the end of the steel plate and the weld is 10 mm, and the distance between the welds is 20 mm. Next, as shown... Figure 2 As shown in (c), a test component 30 is joined to a base plate 50 using TIG welding to create a sample for axial crush testing. Next, an impactor 60 is brought into contact with the sample at a constant velocity of 10 mm / min, crushing the sample by 70 mm. Figure 2 As shown in (c), the crushing direction D3 is set to be parallel to the length direction of the test member 30. The results are recorded in Table 5.

[0433] Determination of axial crush fracture (visual cracks)

[0434] More preferably (◎): No visible cracks were observed in the samples after the axial crush test.

[0435] Preferred (○): No more than one visible crack is observed in the sample after the axial crush test.

[0436] In the U-bending + close-fitting bending test, V-bending + orthogonal VDA bending test, and axial crush test of galvanized steel sheets with a thickness greater than 1.2 mm, all tests were conducted using steel sheets with a thickness of 1.2 mm, taking into account the influence of sheet thickness. For steel sheets with a thickness greater than 1.2 mm, single-sided grinding was performed to reduce the thickness to 1.2 mm. Since grinding may affect the bendability of the steel sheet surface, in the U-bending + close-fitting bending test, the grinding surface was set to the inner side of the bend (valley side); in the V-bending + orthogonal VDA bending test, the grinding surface was set to the outer side of the bend (peak side) during the V-bending test, and the grinding surface was set to the inner side of the bend (valley side) during the subsequent VDA bending test. On the other hand, in the U-bending + close-fitting bending test, V-bending + orthogonal VDA bending test, and axial crush test of galvanized steel sheets with a thickness of less than 1.2 mm, the influence of sheet thickness was small, so the tests were conducted without grinding.

[0437] In a 50μm×50μm region on the plate surface at a position 1 / 4 of the thickness of the surface soft layer from the base steel plate, when the nanohardness of more than 300 points was measured, and the proportion of nanohardness of 7.0GPa or higher was less than 0.10, it was possible to further suppress the generation, connection, and cracking of voids during stamping and collision of hard microstructures (such as martensite) and inclusions, resulting in excellent R / t and SFmax.

[0438] After the coating was stripped, mechanical grinding was performed until 5 μm below the thickness of the soft outer layer on the substrate steel plate, which is 1 / 4 of the thickness of the soft outer layer. Diamond and alumina polishing was then performed until the same depth was reached, followed by colloidal silica polishing. Using a Hysitron Tribo-950 indenter with a Beaufort-shaped diamond indenter, the nanohardness of 512 points was measured under conditions of a 500 μN load, a 50 μm × 50 μm measurement area, and a 2 μm dot spacing.

[0439] When the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the thickness direction depth of the surface soft layer from the surface of the base steel plate is less than 1.8 GPa, and the standard deviation σ of the nanohardness of the plate surface at a position 1 / 2 of the thickness direction depth of the surface soft layer from the surface of the base steel plate is less than 2.2 GPa, it is possible to further suppress the generation, connection, and cracking progress of voids during stamping and collision, resulting in excellent R / t and SFmax.

[0440] After the zinc coating is stripped, for steel sheets with a further metal coating, mechanical grinding, polishing with diamond and alumina, and colloidal silica are performed until the thickness of the aforementioned surface soft layer is 1 / 4 of the distance from the base steel sheet surface. Using a Hysitron Tribo-950 with a Beaufort-shaped diamond indenter, the nanohardness of a total of 512 points was measured under the conditions of a load of 500 μN, a measurement area of ​​50 μm × 50 μm, and a dot spacing of 2 μm.

[0441] Next, mechanical grinding, polishing with diamond and alumina, and colloidal silica grinding were performed until half the thickness of the aforementioned surface soft layer was reached. Using a Hysitron Tribo-950 with a Bose-shaped diamond indenter, the nanohardness of a total of 512 points was measured under the conditions of a load of 500 μN, a measurement area of ​​50 μm × 50 μm, and a dot spacing of 2 μm.

[0442]

[0443] According to Examples 1 and 2, for the galvanized steel sheet of the invention examples, TS is 780 MPa or more and 1180 MPa or less, YR is 0.60 or more, and the diffusible hydrogen in the steel is 0.45 ppm by mass or less, exhibiting excellent elongation flange properties, bending properties, and hydrogen embrittlement resistance. For the steel sheet of the comparative examples, at least one of TS, YR, elongation flange properties, bending properties, and hydrogen embrittlement resistance is poor.

[0444] It should be noted that in Example 2, when the dew point is -15°C, although there are cases where the thickness of the surface soft layer is less than 17 μm and the fracture (visual crack) determination in the axial crush test is "○", even when the thickness of the surface soft layer is less than 17 μm, the fracture resistance is excellent when there is a metal coating, and the fracture (visual crack) determination in the axial crush test is also "◎".

[0445] Furthermore, for components obtained by forming or joining using the galvanized steel sheet of the invention, the TS is 780 MPa or more and 1180 MPa or less, the YR is 0.60 or more, the diffusible hydrogen in the steel is 0.45 ppm by mass or less, and the elongation flange properties, bending properties and hydrogen embrittlement resistance are excellent.

[0446] Symbol Explanation

[0447] 10. Hat-shaped components

[0448] 20 Galvanized steel sheet

[0449] 30 Test components

[0450] 40 spot welds

[0451] 50 base plate

[0452] 60 impactors

[0453] A1 Die

[0454] A2 Support Roller

[0455] B1 punch

[0456] B2 punch

[0457] D1 Width (C) Direction

[0458] D2 Rolling (L) direction

[0459] D3 Damage direction

[0460] T1 test piece

[0461] Industrial availability

[0462] According to the present invention, it is possible to obtain galvanized steel sheets that are mainly suitable for automotive parts, have a tensile strength TS of 780 MPa or more and less than 1180 MPa, a yield ratio of 0.60 or more, and excellent bending properties, elongation flange properties and resistance to hydrogen embrittlement.

Claims

1. A galvanized steel sheet which is a galvanized steel sheet having a base steel sheet and a zinc plated layer formed on the base steel sheet, wherein the base steel sheet has a steel structure in which ferrite is less than 65% by area ratio, the total of martensite and bainite is 25% or more, retained austenite is 3% or more and 10% or less by area ratio, 70% or more of all the martensite in the steel structure at a position of 1 / 8 to 3 / 8 of the sheet thickness of the base steel sheet is tempered martensite having carbides with an average particle diameter of 50 nm or more and 200 nm or less, the tensile strength of the galvanized steel sheet is 780 MPa or more and less than 1180 MPa, the yield ratio is 0.60 or more, and the cumulative value of the amount of hydrogen released when the base steel sheet is warmed from room temperature to 200°C is 0.45 mass ppm or less. The steel composition of the base steel sheet contains, in mass %, C: 0.080% or more and 0.300% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.00% or more and 4.00% or less, P: 0.10% or less, S: 0.0200% or less, Al: 0.003% or more and 0.100% or less, N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities. The steel composition further contains at least one selected from the group consisting of, in mass %, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, Sb: 0.200% or less, Sn: 0.200% or less, V: 0.100% or less, Cu: 2.00% or less, Cr: 2.00% or less, Ni: 2.00% or less, Mo: 1.00% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.020% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM (excluding Ce): 0.0200% or less. In the base steel sheet, when a region of 200 μm or less in the sheet thickness direction from the surface of the base steel sheet is taken as a surface layer, a surface layer soft layer having a Vickers hardness of 85% or less of the Vickers hardness at a position of 1 / 4 of the sheet thickness is present in the surface layer.

5. The galvanized steel sheet according to claim 4, wherein when the nano hardness is measured at 300 points or more in a 50 μm x 50 μm region of the sheet surface at each of a position of 1 / 4 of the sheet thickness direction depth and a position of 1 / 2 of the sheet thickness direction depth from the surface of the base steel sheet, the surface layer soft layer has a nano hardness of 2.0 GPa or more at the position of 1 / 4 of the sheet thickness direction depth and a nano hardness of 1.5 GPa or more at the position of 1 / 2 of the sheet thickness direction depth.

2. Zinc-coated steel sheet according to claim 1, wherein, ​ 3. Galvanised steel sheet according to claim 2 wherein, ​ 4. The galvannealed steel sheet according to any one of claims 1 to 3, wherein, ​ ​ ​ The proportion of the number of measurements of the nano-hardness of the plate surface at a position 1 / 4 of the depth of the surface layer soft layer from the surface of the base steel plate is 0.10 or less with respect to the total number of measurements, The standard deviation σ of the nano-hardness of the plate surface at a position 1 / 4 of the depth of the surface layer soft layer from the surface of the base steel plate is 1.8 GPa or less, The standard deviation σ of the nano-hardness of the plate surface at a position 1 / 2 of the depth of the surface layer soft layer from the surface of the base steel plate is 2.2 GPa or less.

6. The galvannealed steel sheet according to any one of claims 1 to 5, wherein, The zinc plated layer is an alloyed zinc plated layer.

7. Galvanised steel sheet according to anyone of claims 1 to 6 wherein, The zinc plated layer is an alloyed zinc plated layer.

8. Galvanised steel sheet according to anyone of claims 1 to 7 wherein, The cumulative value of the amount of hydrogen released in the temperature range of 350 to 600°C when the base steel plate is warmed from room temperature to 600°C is 0.05 mass ppm or more.

9. A member formed using the galvanized steel sheet described in any one of claims 1 to 8.

10. A method of manufacturing a galvanized steel sheet, comprising: a hot rolling step in which a steel billet having the steel composition described in claim 2 or 3 is heated to a temperature range of 1100 to 1350°C, hot-rolled at a finish rolling end temperature of 800 to 950°C, and coiled at a coiling temperature of 650°C or less; a reduction step in which the steel sheet after the hot rolling step is held in a reduction atmosphere having a hydrogen concentration of 8% by volume or more and 30% by volume or less at 700°C or more for 20 seconds or more; a soaking step in which the steel sheet after the reduction step is held in a soaking atmosphere having a hydrogen concentration of 0.2% by volume or more and 8% by volume or less at 750°C or more for 20 seconds or more and 300 seconds or less; a first cooling step in which the steel sheet after the soaking step is cooled; a zinc plating step in which a zinc plated layer is formed on the surface of the steel sheet after the first cooling step; a second cooling step in which, for the steel sheet after the zinc plating step, the average cooling rate from the temperature range of 350 to 450°C until (Ms point - 100°C) is reached is set to 20°C / s or less, and the steel sheet is held in the temperature range of 100 to 350°C for 5 seconds or more, and cooled to a cooling stop temperature of 100 to 300°C; and a reheating step in which the steel sheet after the second cooling step is held in the temperature range of the cooling stop temperature or more and 450°C or less for 5 to 600 seconds.

11. The method of producing a galvanized steel sheet according to claim 10, wherein The zinc plated layer is formed on the surface of the steel sheet after the zinc plating step, and an alloying treatment is further performed.

12. The method of producing a galvannealed steel sheet according to claim 10 or 11, wherein, A cold rolling step in which cold rolling is performed at a reduction ratio of 20% or more after the hot rolling step and before the reduction step is included.

13. The method of producing a galvannealed steel sheet according to any one of claims 10 to 12, wherein, In the first cooling step, the steel sheet after the soaking step is cooled from 600 to 900°C to 150 to 500°C at an average cooling rate of 20°C / s or less in an atmosphere having a hydrogen concentration of 0.5% by volume or more and 30% by volume or less and a dew point of 0°C or less.

14. The method of producing a galvannealed steel sheet according to any one of claims 10 to 13, wherein In the second cooling step, the average cooling rate from the temperature range of 350 to 450°C until reaching (Ms point - 100°C) is set to 10°C / s or less, and the temperature range of 100 to 350°C is stayed for 10 seconds or more, and cooled to the cooling stop temperature of 100 to 300°C.

15. The method of producing a galvanized steel sheet according to any one of claims 10 to 14, wherein The soaking atmosphere in the soaking step is an atmosphere having a dew point of -30°C or higher.

16. The method of producing a galvanized steel sheet according to any one of claims 10 to 15, wherein A pre-plating step of forming a metal plated layer by performing metal plating on one side or both sides of the steel sheet after the hot rolling step and before the reduction step.

17. The method of producing a galvanized steel sheet according to any one of claims 10 to 16, wherein An oxidizing step of heating the steel sheet after the hot rolling step and before the reduction step in an oxidizing atmosphere having an oxygen concentration of 1000 volume ppm or more and 30000 volume ppm or less to 600°C or higher.

18. A method for manufacturing a member, comprising a step of manufacturing a member by performing at least one of a forming process and a joining process on the galvanized steel sheet according to any one of claims 1 to 8.

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