HOT-DIP GALVANIZED HIGH-STRENGTH STEEL SHEET AND MANUFACTURING METHOD THEREOF

Abstract

One objective is to provide a high-strength, hot-dip galvanized steel sheet suitable as a high-strength steel plate for an automotive energy-absorbing member, possessing a tensile strength (TS) of 980 MPa or greater and excellent fracture resistance characteristics in a crash, and to provide a manufacturing method for the same. The high-strength, hot-dip galvanized steel sheet, which includes a hot-dip galvanized coating layer on the steel sheet surface, has a component composition containing, in % by mass, C: 0.07% to 0.20%, Si: 0.1% to 2.0%, Mn: 2.0% to 3.5%, P: 0.05% or less, S: 0.05% or less, and Al: 0.005% to 0.1%, with the remainder being Fe and incidental impurities; and has a steel microstructure containing, by area fraction, 60% or less ferrite, 40% or more tempered martensite, and 10% or less fresh martensite and having a number density of gaps of 1,500 / mm2 or less in a bent portion in the VDA bend test.

Description

HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET AND MANUFACTURING METHOD THEREOF FIELD OF INVENTION The present invention relates to a high-strength hot-dip galvanized steel sheet that is suitable for uses such as an automotive steel sheet and is excellent in fracture resistance characteristics in a crash, and to a method of manufacturing the same. BACKGROUND OF THE INVENTION To reduce CO2 emissions for global environmental protection, reducing the weight of a car body while maintaining strength, thereby improving fuel efficiency, has always been a major challenge in the automotive industry. To reduce the weight of a car body while maintaining strength, it is effective to thin a sheet of steel used as a raw material for an automotive part by increasing its strength. Meanwhile, ensuring occupant safety in a crash is an essential requirement for an automotive part made from sheet steel. Therefore, excellent crash characteristics, in addition to desirable strength, are required for a high-strength sheet of steel used as a raw material for an automotive part. In recent years, high-strength steel sheets with a tensile strength exceeding 980 MPa (TS) have been increasingly used in automotive body construction. Based on crash characteristics, automotive parts are generally classified as energy-absorbing members and non-deformable members, such as pillars and bumpers. These respective members are required to possess the necessary crash characteristics to ensure passenger safety in the event of a collision. The strength of non-deformable members has already been increased to allow for the practical application of high-strength steel sheets exceeding 980 MPa.Meanwhile, for energy-absorbing members, a problem exists where high-strength steel sheets exceeding 980 MPa tend to fracture in impacts because a portion that has undergone primary processing during forming acts as a fracture starting point and consequently cannot exhibit stable impact energy absorption capacity. For this reason, high-strength steel sheets of 980 MPa or higher have not yet been applied to energy-absorbing members, thus offering an opportunity to contribute to environmental protection through weight reduction. Therefore, there is a need to apply high-strength steel sheets exceeding 980 MPa, with excellent fracture resistance characteristics, to energy-absorbing members. Regarding this need, Patent Document 1, for example, discloses a technique related to an ultra-high-strength steel sheet of grade 1,180 MPa in TS that is excellent in formability and impact resistance. Furthermore, Patent Document 2 discloses a technique related to a high-strength steel sheet having a maximum tensile strength of 780 MPa or more and applicable to impact-absorbing members in a crash. CQnRnn / Lznz / e / YiAi List of Appointments Patent Documents PTL 1: Publication of Unexamined Japanese Patent Application No. 2012-31462 PTL 2: Publication of Unexamined Japanese Patent Application No. 2015-175061 BRIEF DESCRIPTION OF THE INVENTION Technical Problem Although shock characteristics have been examined as impact resistance under the premise that a member does not fracture in a shock, Patent Document 1 has not examined shock characteristics in terms of a member's fracture resistance. Meanwhile, Patent Document 2 has observed a crack in each omega-profile member in a dynamic axial crush test using a drop weight to evaluate fracture resistance characteristics for a grade exceeding 780 MPa. However, it is impossible to evaluate, from a crack observed after crushing, the process from crack initiation that leads to fracture during crushing. The reasons are as follows: When a crack forms in the initial phase of the crushing process, even a minor crack that does not penetrate the sheet thickness is likely to reduce energy absorption.Furthermore, when a crack forms in the final stage of the crushing process, even a larger crack penetrating the sheet thickness is unlikely to have much effect on energy absorption. Therefore, a single crack observed after crushing is considered insufficient for evaluating fracture strength characteristics. The present invention was made in view of the foregoing, and one objective is to provide a high-strength hot-dip galvanized steel sheet that is suitable as a high-strength steel sheet for an automotive energy-absorbing member and that has a tensile strength (TS) of 980 MPa or more and excellent fracture resistance characteristics in a crash and to provide a method of manufacturing the same. Solution to the Problem To solve the problems mentioned above from the point of view of the composition of components and the microstructure of a steel sheet as well as a method of manufacturing the same, the inventors of the present continued with intensive studies and found the following as a result. It was found that it is possible to obtain a high-strength hot-dip galvanized steel sheet having a TS of 980 MPa or more and excellent fracture resistance characteristics in a shock by satisfying, in addition to a particular component composition, 60% or less ferrite, 40% or more tempered martensite, and 10% or less fresh martensite in area fraction; and a number density of gaps of 1,500 / mm2 or less in a bent portion in the VDA bend test. The present invention was made on the basis of these results and is summarized as follows. [1] A high-strength hot-dip galvanized steel sheet, which includes a hot-dip galvanized coating layer on a surface of the steel sheet, the steel sheet having a steel composition containing, in % by mass, C: 0.07% to 0.20%, Si: 0.1% to 2.0%, Mn: 2.0% to 3.5%, P: 0.05% or less, S: 0.05% or less, and Al: CQnRnn / Lznz / e / YiAi 0.005% to 0.1%, the remainder being Fe and incidental impurities; and a steel microstructure containing, by area fraction, 60% or less ferrite, 40% or more tempered martensite, and 10% or less fresh martensite and having a number density of gaps of 1,500 / mm2 or less in a bent portion in the VDA bend test. [2] High-strength hot-dip galvanized steel sheet according to [1], wherein the steel microstructure additionally contains, as an area fraction, 3% to 10% retained austenite. [3] High-strength hot-dip galvanized steel sheet in accordance with [1] or [2], wherein the steel composition additionally contains, in % by mass, one or two or more elements selected from Cr: 0.005% to 1.0%, Mo: 0.005% to 0.5%, and V: 0.005% to 0.5%. [4] High-strength hot-dip galvanized steel sheet conforming to any of [1] to [3], wherein the steel composition additionally contains, in % by mass, one or two or more elements selected from Ti: 0.005% to 0.5%, Nb: 0.005% to 0.5%, B: 0.0003% to 0.005%, Ni: 0.005% to 1.0%, and Cu: 0.005% to 1.0%. [5] High-strength hot-dip galvanized steel sheet conforming to any of [1] to [4], wherein the steel composition additionally contains, in % by mass, one or two elements selected from Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%. [6] High-strength hot-dip galvanized steel sheet conforming to any of [1] to [5], wherein the hot-dip zinc coating layer on the surface of the steel sheet is a galvano-annealed coating layer. [7] A method of manufacturing a high-strength hot-dip galvanized steel sheet, comprising: a hot rolling step for subjecting a block of steel having the steel composition conforming to any of [1] and [3] to [5] to hot rolling at a finish rolling temperature of 850°C to 950°C and coiling at a coiling temperature of 600°C or lower; a cold rolling step for cold rolling the hot-rolled steel sheet to a reduction that is greater than 20%; an annealing step for heating the cold-rolled steel sheet to an annealing temperature of 750°C or higher and holding for 30 seconds or more;subjecting the annealed steel sheet to a quenching and tempering stage which includes cooling at an average cooling rate of 20°C / s greater in a temperature range from the annealing temperature to a martensite start temperature (Ms), cooling at an average cooling rate of 2°C / s to 10°C / s to a stop-cooling temperature of (Ms - 200°C) to (Ms - 100°C), and then holding at 300°C to 500°C for 20 seconds or more; and a hot-dip galvanizing stage to subject the quenched and tempered steel sheet to hot-dip galvanizing. [8] The method of manufacturing a high-strength hot-dip galvanized steel sheet in accordance with [7], wherein the hot-dip galvanizing stage includes an alloying stage for subjecting the galvanized steel sheet to an alloying treatment after subjecting it to hot-dip galvanizing. Advantageous Effects of the Invention In accordance with the present invention, it is possible to obtain a steel sheet High-strength hot-dip galvanized CQnRnn / Lznz / e / YiAi is suitable as a high-strength steel sheet for an automotive energy absorption member and has a tensile strength (TS) of 980 MPa or more and excellent fracture resistance characteristics in a crash. DETAILED DESCRIPTION OF THE INVENTION Description of Modalities The details of the present invention will be described below. Herein, the “%” sign representing the content of each component element refers to “% by mass” unless otherwise stated. 1) Steel Composition C: 0.07% to 0.20% Carbon facilitates the formation of phases other than ferrite and forms an alloy compound with Nib, Ti, and similar elements. For these reasons, carbon is a necessary element for increasing strength. When the carbon content is less than 0.07%, it is impossible to guarantee the desired strength even with optimized manufacturing conditions. Meanwhile, when the carbon content exceeds 0.20%, it causes an increase in martensite, making it impossible in some cases to obtain the steel microstructure of the present invention, even with optimized manufacturing conditions. Preferably, the carbon content is set at 0.10% or more and 1.8% or less. Yes: 0.1% to 2.0% Silicon (Si) is a ferrite-forming element as well as a solid solution hardening agent. For this reason, Si contributes to improving the balance between strength and ductility. To achieve this effect, the Si content should be set at 0.1% or higher. Meanwhile, a Si content exceeding 2.0% leads to decreased adhesion of a galvanized coating and, in some cases, a deterioration in surface quality. Preferably, the Si content is set at 0.2% or higher and 1.5% or lower. Mn: 2.0% to 3.5% Manganese (Mn) is a martensite-forming element as well as a solid solution hardening element. Mn also contributes to stabilizing retained austenite. To achieve these effects, the Mn content should be set at 2.0% or higher. However, when the Mn content exceeds 3.5%, it causes an increase in the martensite fraction in the secondary phase, and formability deteriorates in some cases. Preferably, the Mn content is set at 2.1% or higher and 3.0% or lower. P: 0.05% or less Phosphorus (P) is an effective hardening agent for steel. However, a P content above 0.05% significantly slows the alloying rate. Furthermore, excessive content beyond 0.05% causes embrittlement due to segregation at grain boundaries, thereby impairing fracture resistance characteristics under shock in some cases. Preferably, the P content is set at 0.01% or less. The lower limit, although not specifically defined, is 0.0005% or more for economic reasons related to refining efficiency. S: 0.05% or less Sulfur forms an inclusion, such as MnS, to cause deterioration in impact resistance. CQnRnn / Lznz / e / YiAi as cracking along a metal flow in a weld. Therefore, the S content is preferably as small as possible but is set at 0.05% or less in view of manufacturing costs. Preferably, the S content is set at 0.01% or less. The lower limit is not particularly specified but is 0.0001% or more in view of economic efficiency in refining. In the Sun: 0.005% to 0.1% Aluminum acts as a deoxidizer and is also a solid solution hardening agent. When the solid solution aluminum content is less than 0.005%, these effects cannot be achieved. Meanwhile, a solid solution aluminum content exceeding 0.1% impairs block quality in steelmaking. Preferably, the solid solution aluminum content is set between 0.005% and 0.04%. The above are the basic components. The high-strength hot-dip galvanized steel sheet of the present invention has a component composition containing the above basic components, with the remainder being Fe (iron) and incidental impurities. Herein, the high-strength hot-dip galvanized steel sheet of the present invention preferably has a component composition containing the above basic components, with the remainder being Fe and incidental impurities. Meanwhile, it is acceptable to contain N as an incidental impurity within a range of 0.0060% or less. The high-strength hot-dip galvanized steel sheet of the present invention may have the component composition described above, which optionally contains additionally one or two or more elements selected from Cr, Mo, and V described below. Cr: 0.005% to 1.0%, Mo: 0.005% to 0.5%, V: 0.005% to 0.5% Cr, Mo, and V are effective elements for increasing the hardenability and toughening steel. The effect is achieved when the content of each element is 0.005% or more. However, when Cr, Mo, and V are added excessively beyond 1.0%, 0.5%, and 0.5%, respectively, the effect stabilizes, while raw material costs increase. Furthermore, the fraction of the secondary phase becomes excessive, potentially compromising fracture resistance characteristics under shock in some cases. The high-strength hot-dip galvanized steel sheet of the present invention may have the component composition described above, additionally containing one or two or more elements selected from Ti, Nb, B, Ni, and Cu described below. Ti: 0.005% to 0.5%, Nb: 0.005% to 0.5% Titanium (Ti) and Nibromyalgia (Nb) are effective for precipitation hardening of steel, and the effect is obtained when the content of each element is 0.005% or more. Within the specified ranges of the present invention, these elements can be used to harden steel. However, when the content of each element exceeds 0.5%, the fracture resistance characteristics under shock deteriorate in some cases. B: 0.0003% to 0.005% Boron contributes to increased hardenability by suppressing the formation and / or growth of ferrite from austenite grain boundaries. For this reason, boron can be added as needed. CQnAnn / Lznz / e / YiAi is required. The effect is obtained with a content of 0.0003% or more. Meanwhile, when the B content is greater than 0.005%, the fracture resistance characteristics under shock deteriorate in some cases. Ni: 0.005% to 1.0%, Cu: 0.005% to 1.0% Ni and Cu are effective elements for hardening steel and can be used within the ranges specified in the present invention for this purpose. To achieve the desired effect, each element is preferably contained at 0.005% or more. However, when the Ni and Cu content each exceeds 1.0%, the fracture resistance characteristics under impact deteriorate in some cases. The high-strength hot-dip galvanized steel sheet of the present invention may have the component composition described above, which optionally contains additionally one or two elements selected from Ca and REM described below. Ca: 0.001% to 0.005%, REM: 0.001% to 0.005% Both calcium (Ca) and mineral spirits (MS) are effective elements for improving workability by controlling the form of a sulfide. To achieve this effect, the content of each is preferably set at 0.001% or higher. However, when the content of either Ca or MS exceeds 0.005%, there is a risk of negatively impacting the purity of the steel and thus impairing its characteristics. 2) Steel Microstructure Ferrite Area Fraction: 60% or Less When the ferrite area fraction exceeds 60%, it is difficult to achieve both a tensile strength (TS) of 980 MPa or higher and excellent fracture resistance characteristics under shock. Therefore, the ferrite area fraction is set at 60% or less, and preferably at 40% or less. The lower limit for the area fraction is not specifically defined, but is preferably 10% or more. Area Fraction of Tempered Martensite: 40% or More Tempered martensite is effective in improving fracture resistance characteristics under shock conditions. When the area fraction of tempered martensite is less than 40%, this effect cannot be satisfactorily achieved. Preferably, the area fraction is set at 50% to 80%. Fresh Martensite Area Fraction: 10% or Less Fresh martensite is effective in increasing strength. However, gaps readily form at grain boundaries between fresh martensite and a soft phase. When the area fraction of fresh martensite exceeds 10%, fracture resistance characteristics under shock deteriorate in some cases. Preferably, the area fraction is set at 5% or less. The lower limit of the area fraction is not specifically defined but is preferably 1% or more. Numerical Density of Gaps in Bent Portion in VDA Bend Test: 1,500 / mm2 or Less In the high-strength hot-dip galvanized steel sheet of the present invention, excellent shock resistance characteristics are achieved by meeting a density CQnAnn / Lznz / e / YiAi numerical gaps of 1,500 / mm2 or less in a bent portion in the VDA bend test. The mechanism of this phenomenon is unclear but is assumed to be as follows. A fracture in a shock, which causes deterioration in shock resistance characteristics, originates from the initiation and propagation of a crack. Crack initiation is considered to be facilitated by the decrease in strain hardenability and by the formation and bonding of gaps observed within the microstructure of the steel sheet in a region of large hardness difference. Furthermore, in a shock to an actual member, the member deforms in such a way that a portion that has undergone primary processing bends back in a direction orthogonal to the primary processing.In this scenario, if a gap forms in a region of significant hardness difference due to primary processing, stress concentrates around the gap, promoting crack initiation and propagation, which ultimately leads to fracture. Conversely, a region of significant hardness difference can be reduced by using tempered martensite, and stress concentration during deformation in a primary processed portion can be suppressed by using retained austenite, if necessary. Consequently, it is possible to suppress crack initiation and propagation in the primary processed portion, as well as the resulting fracture of the member, thereby achieving excellent fracture resistance characteristics.To obtain these effects, the numerical density of gaps in a bent portion in the VDA bend test is set at 1,500 / mm2 or less and preferably at 1,000 / mm2 or less. Regarding the number of gaps in a bent portion in the VDA bend test, a desirable number of gaps can be achieved by controlling the cooling rate after annealing, as described below. In a high-temperature range, ferrite transformation during cooling is suppressed without decreasing Ms by using a rapid cooling rate. In a temperature range of Ms or lower, martensite is also tempered during cooling by reducing the cooling rate. Through further tempering by subsequent reheating, the martensite is fully tempered, further contributing to reducing the hardness difference. Consequently, gap formation during primary processing is suppressed. In this invention, the numerical density of gaps in a bent portion in the VDA bend test is the number of gaps observed within a steel sheet microstructure by metallographic observation of the bent portion after primary processing (after primary bending) in the orthogonal bend test (VDA bend test) according to the VDA standard (VDA 238-100) specified by the German Association of the Automotive Industry. As a measurement method of the present invention for the numerical density of gaps in a bent portion in the VDA bend test, the numerical density of gaps in a bent portion is measured according to the VDA standard by performing metallographic observation of a test specimen that has been subjected to primary bending using a 90-degree V-block under the following conditions. [Primary Bending Conditions] Punch tip radius: 5 mm Training load: 15 tons Stroke speed: 30 mm / min CQnRnn / Lznz / e / YiAi Hold time: 5 seconds Bending direction: direction parallel to rolling [Orthogonal Bending Conditions] Test method: roller support, punching Roller diameter 030 mm Punch tip radius: 0.4 mm Distance between rollers: (sheet thickness * 2) + 0.5 mm Stroke speed: 20 mm / min Test specimen size: 60 mm × 60 mm Bending direction: direction orthogonal to rolling Regarding the number density of gaps, a portion that had undergone primary processing was cut orthogonally to the rolling direction. The resulting cross-section in the sheet thickness direction was polished. Images of three fields of view were obtained under a scanning electron microscope (SEM) at 1,500x magnification for a surface layer in the sheet thickness direction on the inner side after bending during primary processing. Each gap number density was obtained from the resulting image data using Image-Pro from Media Cybernetics, Inc., and an average of the number densities for the three fields of view was considered the gap number density. The gaps are a darker black than the ferrite and are clearly distinguishable from each microstructure. Fraction of Retained Austenite Area: 3% to 10% (Preferred Condition) Retained austenite is effective in delaying crack initiation under shock and improving fracture toughness. When the retained austenite area fraction is less than 3%, this effect cannot be achieved. Conversely, when the retained austenite area fraction exceeds 10%, fracture toughness under shock deteriorates in some cases due to the fresh martensite formed through stress-induced transformation. Ideally, the area fraction should be set between 5% and 10%. Bainite, cementite, and / or pearlite may be present in 5% or less total in some cases as microstructures excluding ferrite, tempered martensite, fresh martensite, and retained austenite. Provided the conditions described above for the steel microstructure are met, the objective of the present invention is achieved. In this document, each ferrite, fresh martensite, and tempered martensite area fraction indicates the area ratio of each phase relative to an observed area. For the area fraction of each microstructure, a steel sheet was cut orthogonally to the rolling direction; the resulting cross-section in the sheet thickness direction was polished and then chemically etched with 3 wt% nital; three fields of view were imaged at the 1 / 4 position in the sheet thickness direction under a scanning electron microscope (SEM) at 1,500x magnification; the area fraction of each microstructure was obtained from the resulting image data using Image-Pro from Media Cybernetics, Inc.; and an average of the area fractions for the three fields of view was considered the area fraction of each microstructure.In the image data, each microstructure is distinguishable as black for ferrite, light gray for those containing fine carbides. CQnRnn / Lznz / e / YiAi randomly aligned for tempered martensite, and blank for retained austenite and fresh martensite. Furthermore, the volume fraction of retained austenite is a ratio of, in the 1 / 4 plane in the sheet thickness direction, integrated X-ray diffraction intensities for the (200), (220), and (311) planes of fcc iron to integrated X-ray diffraction intensities for the (200), (211), and (220) planes of bcc iron. Since fresh martensite and retained austenite are difficult to distinguish in a SEM image, the area fraction of fresh martensite is obtained by subtracting the area fraction of retained austenite from the total area fraction of fresh martensite and retained austenite. Furthermore, the hot-dip zinc coating layer on the surface of the steel sheet of the present invention is preferably an annealed galvanized coating layer. The term “surface” specified in the present invention refers to the interface between the coating layer and the steel sheet. 3) Manufacturing Conditions A method of manufacturing a high-strength steel sheet of the present invention is characterized by including: a hot rolling step for subjecting a block of steel having the steel composition described above to hot rolling at a finish rolling temperature of 850°C to 950°C and coiling at a coiling temperature of 600°C or lower; a cold rolling step for cold rolling to a reduction that is greater than 20%; an annealing step for heating to an annealing temperature of 750°C or higher and holding for 30 seconds or more;a quenching and tempering stage for cooling at an average cooling rate of 20°C / s or greater over a temperature range from the annealing temperature to a martensite start temperature (Ms), cooling at an average cooling rate of 2°C / s to 10°C / s to a stop-cooling temperature of (Ms - 200°C) to (Ms - 100°C), and then holding at 300°C to 500°C for 20 seconds or more; and a hot-dip galvanizing stage for undergoing hot-dip galvanizing. In addition, the hot-dip galvanizing stage may include an alloying stage for undergoing alloying treatment after hot-dip galvanizing. First, the respective conditions in the hot rolling stage will be described. Lamination Finishing Temperature: 850°C to 950°C When the rolling finish temperature is below 850°C, ferrite transformation occurs during rolling, locally reducing strength. Consequently, it is impossible to achieve the microstructure and characteristics of the present invention. Meanwhile, when the rolling finish temperature is above 950°C, the crystal grains become oily. Consequently, it is impossible to obtain the steel microstructure of the present invention. Therefore, the rolling finish temperature is set between 850°C and 950°C. Winding Temperature: 600°C or Lower When the rolling temperature exceeds 600°C, the carbides in a hot-rolled sheet become oily. Since these oily carbides do not completely dissolve during the holding temperature in annealing, the required strength cannot be achieved in some cases. CQnRnn / Lznz / e / YiAi A hot-rolled sheet obtained from the hot rolling stage is subjected to a preliminary treatment, such as pickling or degreasing, by a commonly known method, and subsequently to cold rolling as required. The conditions for the cold rolling stage, when subjected to cold rolling, will be described. Reduction in Cold Rolling: More than 20% When the reduction in cold rolling is 20% or less, ferrite recrystallization is not promoted, leaving unrecrystallized ferrite, thereby damaging workability in some cases. The conditions for the annealing stage for annealing a cold-rolled sheet obtained in the cold rolling stage will be described below. Annealing Temperature: 750°C or Higher, Holding Time: 30 Seconds or More When the annealing temperature is below 750°C, the steel microstructure of the present invention cannot be obtained because insufficient austenite forms while excessive ferrite forms simultaneously. Preferably, the annealing temperature is set between 750°C and 900°C. Furthermore, when the holding time is less than 30 seconds, the steel microstructure of the present invention cannot be obtained because insufficient austenite forms while excessive ferrite forms simultaneously. Preferably, the holding time is set between 30 seconds or more and 600 seconds or less. After the annealing stage, the steel sheet is subjected to quenching and tempering. The conditions for the quenching and tempering stage will be described. Average Cooling Rate in Temperature Range from Annealing Temperature to Martensite Start Temperature (Ms): 20°C / s Greater When the average cooling rate for the steel sheet after annealing at the aforementioned annealing temperature is less than 20°C / s over a temperature range from the annealing temperature to the martensite onset temperature (Ms), the fracture strength characteristics of the present invention cannot be achieved. The reason is unclear but is assumed to be the following: When the cooling rate is less than 20°C / s, ferrite and / or bainite form excessively during cooling, thereby lowering Ms. Consequently, the amount of martensite transformation decreases when cooling is stopped. Furthermore, since the martensite transformation occurs at a lower temperature, the martensite is under-tempered during cooling compared to a case with a higher Ms.As a result, it is considered that the effect of tempered martensite in reducing a hardness difference diminishes, and therefore gaps easily form during primary processing. Consequently, the average cooling rate is set at 20°C / s higher. Here, Ms can be obtained using the following formula. Ms (°C) = 539 - 423 χ {[C%] χ 100 / (100 - [% of area of ​​a])} - 30 χ [Mn%] - 12 χ [Cr%] - 18 χ [N¡%] - 8 χ [Mo%] where each element symbol represents the content (% by mass) of each element, and any element, if not contained, is set to zero. Furthermore, [% of area of ​​a] represents the ferrite area fraction during annealing. The ferrite area fraction CQnRnn / Lznz / e / YiAi during annealing is obtained in advance using a thermal expansion gauge by simulating a temperature rise rate, an annealing temperature, and a holding time during annealing. After annealing, rapid cooling at the fastest possible rate is preferable. Therefore, the average cooling rate in the temperature range from the annealing temperature to the martensite start temperature (Ms) is preferably 22°C / s higher. The average cooling rate in the temperature range from the annealing temperature to the martensite start temperature (Ms) is more preferably 50°C / s higher. However, considering the economic efficiency of the cooling system, the average cooling rate is preferably set at 100°C / s lower. Average Cooling Rate to Cooling Stop Temperature from (Ms 200°C) to (Ms - 100°C): 2°C / s 10°C / s When the average cooling rate is less than 2°C / s, excessive amounts of bainite-containing carbides form during cooling. Consequently, the steel microstructure of the present invention cannot be obtained. Meanwhile, when cooling is carried out at an average cooling rate greater than 10°C / s, the fracture toughness characteristics of the present invention cannot be achieved. The reason is unclear, but it is assumed to be the following: Setting the cooling rate to 10°C / s or less increases the time required to reach a cooling stop temperature from the Ms. As a result, it is considered that the martensite also tempers during cooling, thereby more effectively reducing the hardness difference due to the tempered martensite.Meanwhile, it is considered that such effects cannot be achieved when the cooling rate exceeds 10°C / s, so gaps easily form during primary processing. Consequently, the average cooling rate is set at 2°C / s or 10°C / s. Cooling Stop Temperature: (Ms - 200°C) to (Ms - 100°C) When the quenching stop temperature is above (Ms - 100°C), the steel microstructure of the present invention cannot be obtained because insufficient tempered martensite forms. Conversely, when the quenching stop temperature is below (Ms - 200°C), insufficient retained austenite forms in some cases due to excessive tempered martensite. Preferably, the quenching stop temperature is set between (Ms - 200°C) and (Ms - 150°C). Tempering Temperature: 300°C to 500°C, Holding Time: 20 Seconds or More When the tempering temperature is below 300°C, the fracture strength and steel microstructure characteristics of the present invention cannot be achieved because the martensite is sufficiently tempered. Meanwhile, when the tempering temperature is above 500°C, the steel microstructure of the present invention cannot be obtained because the ferrite forms excessively. Preferably, the tempering temperature is set at 350°C or higher and 450°C or lower. Furthermore, when the holding time is less than 20 seconds, the fracture strength and steel microstructure characteristics of the present invention cannot be achieved because the martensite is sufficiently tempered. Preferably, the holding time is set at 30 seconds or more and 500 seconds or less. CQnAnn / Lznz / e / YiAi The conditions for the hot-dip galvanizing stage will be described below. Hot-dip galvanizing is preferably carried out by immersing a steel sheet obtained as described above in a zinc bath at 440°C or higher and 500°C or lower, and then adjusting the coating weight by gas purging or similar means. The hot-dip galvanizing stage may be followed by an alloying stage for alloying treatment. A steel sheet that has undergone hot-dip galvanizing or post-galvanizing annealing can be subjected to fit rolling for shape correction, surface roughness adjustment, or similar purposes. However, when the reduction in fit rolling exceeds 0.5%, the bending capacity deteriorates in some cases due to hardening of the surface layer. Therefore, the reduction is preferably set at 0.5% or less, and more preferably at 0.3% or less. In addition, the steel sheet can also be treated with various coatings, such as resin and grease coatings. Other conditions for the manufacturing method are not particularly limited, but the following conditions are preferable. To avoid macrosegregation, a block is preferably manufactured using a continuous casting method and can also be manufactured using an ingot casting method or a thin-block casting method. For hot rolling of a block, it can be cooled once to room temperature, then reheated, and subjected to hot rolling. Alternatively, a block can also be charged into a heating furnace without cooling to room temperature and subjected to hot rolling. A more energy-efficient hot rolling process can also be employed immediately after a short heating and holding time. When heating a block, it is preferable to heat it to 1,100°C or higher to avoid an increase in the rolling load and to prevent the dissolution of carbides.Meanwhile, to avoid an increase in hull loss, a block heating temperature is preferably set at 1300°C or lower. When a block is hot-rolled, a rough-rolled block can be preheated to avoid problems during rolling associated with a low block heating temperature. Additionally, the so-called continuous rolling process can be used to join blocks and subject them continuously to finishing rolling. Furthermore, for reduced rolling loads and uniform shape and / or quality, rolling with lubrication at a coefficient of friction of 0.10 to 0.25 is preferably carried out in all or part of the passes during finishing rolling. Scale can be removed from a steel sheet after coiling by pickling or a similar process. After pickling, the steel sheet is subjected to cold rolling, annealing, and hot-dip galvanizing under the conditions described above. EXAMPLES Each steel having the component composition shown in Table 1 was refined in a vacuum melting furnace and rolled into a steel block. In Table 1, N is an incidental impurity. CQnRnn / Lznz / e / YiAi a ω o yi o N Ci Ü1 > B h C hcc (1 cct Table 1 Sidewalk Composition of components (% by mass) Note C Si Mn PS Al N Cr Mo V Ti Nb B Ni Cu Others A 0.12 0.7 2.7 0.008 0.0014 0.031 0.003 0.11 0.012 0.018 0.0015 Example B 0.13 1.5 2.4 0.006 0.0012 0.029 0.001 0.59 0.007 Example C 0.13 0.8 2.2 0.004 0.0016 0.031 0.002 0.06 0.015 0.0003 Example D 0.17 0.1 2.1 0.009 0.0013 0.029 0.002 0.42 0.021 0.0011 Ca:0.002;REM:0.002 Example E 0.19 0.2 1.7 0.008 0.0016 0.025 0.004 0.13 0.011 0.011 0.0015 0.2 Example F 0.11 1.7 2.6 0.009 0.0013 0.023 0.003 0.56 0.022 0.0012 0.49 REM:0.003 Example G 0.15 0.5 2.6 0.005 0.0016 0.026 0.001 0.11 0.016 0.0015 Example H 02 0.1 2.5 0.009 0.0015 0.022 0.001 0.14 0.013 0.0018 Comparative Example 1 06 0.3 2.8 0.007 0.0012 0.031 0.002 0.49 0.016 0.022 0.0011 Comparative Example J 0.13 0 2.2 0.008 0.0015 0.031 0.001 0.51 0.019 0.0012 Ca:0.002.REM:C.003 Comparative Example K 0.11 0.5 υ 0.006 0.0012 0.033 0.002 0.23 0.0019 Ca:0.002 Comparative Example L 0.14 0.4 3.3 0.009 0.0012 0.021 0.004 142 0.015 0.0009 REM:0.003 Comparative Example M 0.16 0.5 2.2 0.011 0.0013 0.022 0.003 071 0.014 Comparative Example N 0.17 0.2 2.5 0.007 0.0014 0.028 0.002 0.811 0.41 Comparative Example 0 0.09 1.5 2.1 0.006 0.0018 0.026 0.002 0.63 0.653 0.0013 0.3 Comparative Example P 0.15 0.4 2.7 0.008 0.0012 0.024 0.002 0.61 0.016 0.702 0.0015 Comparative Example Q 0.19 0.2 1.9 0.011 0.0008 0.031 0.003 0.032 0.018 0.6985 Comparative Example. The underlines indicate what is outside the scope of the invention Each of these steel blocks was heated and then subjected to rough rolling, finish rolling, and coiling to produce a hot-rolled sheet. Subsequently, the hot-rolled sheet was cold-rolled into a cold-rolled sheet, and the resulting cold-rolled sheet was annealed. The hot-rolling, cold-rolling, and annealing conditions are shown in Table 2. Each steel sheet prepared under the conditions shown in Table 2 was immersed in a coating bath to form a hot-dip galvanized (Gl) coating layer with a coating weight of 20 to 80 g / m². In addition, some steel sheets underwent alloy treatment after forming a hot-dip galvanized layer to produce galvano-annealed (GA) steel sheets. CQnRnn / Lznz / e / YiAi Β h C h A W W N N -1-4 OÜiOÜiOOiOUi C c Ü Tabla 2 No. of steel sheet too Hot rolling conditions Cold rolling conditions Annealing conditions Tempering and annealing conditions Coating conditions Note Finishing temperature of rolling Temperature of winding Reduction M Annealing temperature (5°C) Annealing holding time (5°C) Average cooling rate from annealing temperature to Ms w Ms (°C) Cooling stop temperature (ΐ) Average cooling rate from Ms to cooling stop temperature m Cooling temperature (ΐ) Tempering holding time Coating treatment 1 880 520 50 800 100 25 360 220 3 400 30 GA Epmpb 2 870 510 50 830 100 28 370 240 4 360 50 GA Ejempb 3 880 500 50 770 90 22 350 220 6 480 70 Gl Exampleb 4 860 550 55 820 150 31 340 200 4 410 100 Gl Exampleb 5 890 530 55 800 40 27 340 150 8 380 150 GA Exampleb 6 880 480 55 880 80 33 350 210 9 400 200 GA Exampleb 7 870 500 60 800 90 23 360 190 4 440 40 GA Exampleb 8 910 510 65 820 100 23 360 230 15370 50 Gl Comparative Example 5 ffl 500 50 760 100 í 340 220 5 400 30 GA Comparative Example 10 B 860 500 50 800 90 41 310 160 7 420 90 Gl Example b 11 870 560 65 890 80 23 380 190 5 390 40 GA Example b 12 900 ®0 65 860 80 21 380 200 18 450 60 Gl Comparative Example 13 870 530 15 820 100 26 350 190 14 400 50 GA Comparative Example 14 C 940 510 60 840 90 23 360 240 8 410 90 GA Example 15 910 M 55 780 100 21 220 180 6 470 250 GA Comparative Example 16 860 560 50 830 1» 37 220 160 8 400 30 GA Comparative Example 17 D 880 570 50 820 100 25 330 180 5 400 50 GA Example 18 890 520 50 800 80 27 340 270 9 380 40 GA Comparative Example 19 E 880 520 50 890 40 31 390 200 6 400 30 GA Example b 20 870 510 50 810 80 22 370 150 8 320 30 Gl Comparative Example 21 880 500 50 720 200 23 290 160 9 280 60 GA Comparative Example 22 F 890 520 55 840 150 32 350 150 4 410 40 GA Example b 23 860 550 55 820 40 49 360 190 5 400 Gl Example b 24 G 880 480 55 860 300 34 350 170 6 360 60 GA Exampleb 25 870 500 60 890 150 24 370 240 8 320 15GA Comparative Example 26 H 870 510 50 890 200 29 350 150 7 450 too GA Comparative Example 27 1 880 580 50 840 120 21 360 240 5 450 60 Gl Comparative Example 28 J 860 550 55 760 150 O 290 220 9 410 30 GA Comparative Example 29 K 890 530 55 850 100 38 430 250 21 380 190 GA Comparative Example 30 L 880 480 55 880 80 33 350 200 8 480 120 Gl Comparative Example 31 M 870 580 60 760 120 12 310 200 7 460 30 GA Comparative Example 32 N 910 M 55 850 100 26 370 200 7 510 30 GA Comparative Example 33 0 860 560 50 880 100 36 390 200 12 400 80 GA Comparative Example 34 P 880 570 50 880 100 26 350 160 14 400 70 GA Comparative Example 35 Q 890 520 50 900 100 37 380 170 18 380 50 GA Comparative Example bssüteyateinditaúquee^ The resulting hot-dip galvanized steel sheet or the resulting galvano-annealed steel sheet was subjected to fit rolling to a reduction of 0.3%. Subsequently, each area fraction of ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and retained austenite (RA) was obtained according to the method described above. In addition, the tensile characteristics and fracture resistance characteristics were obtained according to the following test methods. < Tensile Test > A JIS No. 5 tensile test specimen (JIS Z 2201) was taken in a direction orthogonal to the rolling direction and subjected to a tensile test in accordance with the instructions in JIS Z 2241 at a strain rate of 10⁻³ / s to obtain a tensile strength (TS). In the present case, a TS of 980 MPa or more was considered satisfactory. < Orthogonal Bending Test > The bending test was carried out under the following measurement conditions in accordance with the VDA standard (VDA 238-100) specified by the German Association of the Automotive Industry. A test specimen was previously subjected to primary bending using a 90-degree V-block under the following conditions. The fracture resistance characteristics were evaluated by assessing the cracking during the deformation process in the portion that had undergone primary bending. [Primary Bending Conditions] Punch tip radius: 5 mm Training load: 15 tons Stroke speed: 30 mm / min Hold time: 5 seconds Bending direction: direction parallel to rolling [Orthogonal Bending Conditions] Test method: roller support, punching Roller diameter: 030 mm Punch tip radius: 0.4 mm Distance between rollers: (sheet thickness x 2) + 0.5 mm Stroke speed: 20 mm / min Test specimen size: 60 mm × 60 mm Bending direction: direction orthogonal to rolling The stroke length from the moment a specimen flattens until the maximum load is reached was obtained from a stroke-load curve during the orthogonal bend test, and an average for the orthogonal bend test performed three times was indicated by AS. The point at which the specimen flattens is considered the point at which the load begins to increase again after becoming nearly constant on the stroke-load curve. An AS of 8 mm or more was considered satisfactory fracture strength characteristics. The results are shown in Table 3. CQnRnn / Lznz / e / YiAi & ω ω ιό □ σι ο σι ΙΌ ο σι Table 3 Steel Sheet No. Steel Microstructure Tensile Strength Fracture Strength Characteristics Note V(F)(%) V(TM1(%) V(FM](%) V(RA)(%) Numerical Density of Gaps ( / mm2) TS(MPa) áS(mm) 1 42 45 6 7 833 1098 12 Example 2 34 50 8 8 972 1156 11 Example 3 48 39 8 5 1389 1021 9 Example 4 49 41 4 6 972 1196 11 Example 5 53 42 3 2 1389 1113 8 Example 6 48 40 8 4 1250 1242 8 Example 7 43 48 6 3 694 1048 12 Example 8 37 49 6 8 2778 1124 7 Comparative Example 9 51 36 7 6 833 964 10 Comparative Example 10 50 43 3 4 1250 1101 10 Example 11 22 70 6 2 1389 1186 10 Example 12 20 63 5 6 1944 1182 7 Comparative Example 13 47 44 6 3 2222 1090 7 Comparative Example u 45 41 8 6 1250 1222 10 Example 15 77 7 12 4 1111 974 11 Comparative Example 16 77 10 11 2 2917 1003 6 Comparative Example 17 46 44 6 4 1528 999 10 Example 18 42 31 18 8 3194 1043 5 Comparative Example 19 22 68 7 3 1111 1492 8 Example 20 34 43 17 6 ​​3333 1336 6 Comparative Example 21 59 31 8 2 3056 1203 7 Comparative Example 2232 60 5 3 1250 1192 10 Example 23 20 68 7 5 1111 1342 10 Example 24 32 59 6 3 1380 1326 9 Example 25 13 67 18 2 3194 1397 6 Comparative Example 26 16 65 14 5 3056 1244 7 Comparative Example 27 11 20 6 1 972 972 10 Comparative Example 28 64 18 13 5 3611 1107 5 Comparative Example 29 24 64 11 1 3194 861 6 Comparative Example 30 10 73 11 6 3056 1428 6 Example Comparative Example 31 58 29 12 1 3194 1251 6 Comparative Example 32 15 71 11 3 2778 1005 7 Comparative Example 33 36 56 5 3 1944 963 11 Comparative Example 34 29 62 8 1 2917 1272 7 Comparative Example 35 20 72 5 1 3194 1233 6 Comparative Example The underlines indicate what is outside the scope of the invention V(F): ferrite V(TM): tempered martensite, V(FM): Iresca martensite, V(RA): austerlia reten da It is confirmed that all Examples have a TS of 980 MPa or more and excellent fracture resistance characteristics in a shock. As in the foregoing, in accordance with the present invention, it is possible to obtain a high-strength, hot-dip galvanized steel sheet with a tensile strength (TS) of 980 MPa or more and excellent fracture resistance characteristics in a crash. The present invention has excellent effects in contributing to the reduction of automobile weight and greatly contributes to the superior performance of automobile bodies. Industrial Application According to the present invention, it is possible to obtain a high-strength, hot-dip galvanized steel sheet 10 that has a tensile strength (TS) of 980 MPa or more and excellent fracture resistance characteristics in a crash. When used for automotive parts, the high-strength, hot-dip galvanized steel sheet of the present invention can contribute to reducing the weight of automobiles and greatly contribute to the superior performance of car bodies.

Claims

1. A high-strength hot-dip galvanized steel sheet having a hot-dip galvanized coating layer on a surface of the steel sheet, the steel sheet being characterized in that it comprises: a steel composition containing, by mass %, C: 0.07% to 0.20%, Si: 0.1% to 2.0%, Mn: 2.0% to 3.5%, P: 0.05% or less, S: 0.05% or less, and Al: 0.005% to 0.1%, the remainder being Fe and incidental impurities; and a steel microstructure containing, by area fraction, 60% or less ferrite, 40% or more tempered martensite, and 10% or less fresh martensite and having a gap number density of 1,500 / mm2 or less in a bent portion in the VDA bend test.

2. The high-strength hot-dip galvanized steel sheet according to claim 1, further characterized in that the steel microstructure additionally contains, as an area fraction, from 3% to 10% retained austenite.

3. The high-strength hot-dip galvanized steel sheet according to claim 1 or 2, further characterized in that the steel composition additionally contains, by mass %, one or two or more elements selected from Cr: 0.005% to 1.0%, Mo: 0.005% to 0.5%, and V: 0.005% to 0.5%.

4. The high-strength hot-dip galvanized steel sheet according to any of claims 1 to 3, further characterized in that the steel composition additionally contains, in % by mass, one or two or more elements selected from Ti: 0.005% to 0.5%, Nb: 0.005% to 0.5%, B: 0.0003% to 0.005%, Ni: 0.005% to 1.0%, and Cu: 0.005% to 1.0%.

5. The high-strength hot-dip galvanized steel sheet according to any of claims 1 to 4, further characterized in that the steel composition additionally contains, in % by mass, one or two elements selected from Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.

6. The high-strength hot-dip galvanized steel sheet according to any of claims 1 to 5, further characterized in that the hot-dip galvanized coating layer on the surface of the steel sheet is a galvano-annealed coating layer. CQnRnn / Lznz / e / YiAi 7. A method for manufacturing a high-strength hot-dip galvanized steel sheet, characterized in that it comprises: a hot rolling step for subjecting a steel block having the steel composition according to any of claims 1 and 3 to 5 to hot rolling at a finish rolling temperature of 850°C to 950°C and coiling at a coiling temperature of 600°C or lower; a cold rolling step for cold rolling the hot-rolled steel sheet to a reduction greater than 20%; an annealing step for heating the cold-rolled steel sheet to an annealing temperature of 750°C or higher and holding it for 30 seconds or more;subjecting the annealed steel sheet to a quenching and tempering stage which includes cooling at an average cooling rate of 20°C / s greater in a temperature range from the annealing temperature to a martensite start temperature (Ms), cooling at an average cooling rate of 2°C / s to 10°C / s to a stop-cooling temperature of (Ms - 200°C) to (Ms - 100°C), and then holding at 300°C to 500°C for 20 seconds or more; and a hot-dip galvanizing stage to subject the quenched and tempered steel sheet to hot-dip galvanizing.

8. The method of manufacturing a high-strength hot-dip galvanized steel sheet according to claim 7, further characterized in that the hot-dip galvanizing step includes an alloying step for subjecting the galvanized steel sheet to an alloying treatment after subjecting it to hot-dip galvanizing.