HOT-DIP GALVANIZED STEEL SHEET AND METHOD FOR PRODUCING THE SAME

Abstract

A hot-dip galvanized steel sheet is provided comprising a base steel sheet having a predetermined chemical composition and containing ferrite: 50% or less, retained austenite: 30% or less, quenched martensite: 5% or more, fresh martensite: 10% or less, and pearlite and cementite in total: 5% or less, the remaining structures consisting of bainite and a numerical ratio of quenched martensite with a Mn concentration profile that satisfies [Mn]b / [Mn]a>1.2 and [Mn]a / [Mn]<2.0 ([Mn] is the Mn content in the base steel sheet, [Mn]a is the average Mn concentration in the quenched martensite and [Mn]b is the Mn concentration at the interfaces of different phases of the quenched martensite and ferrite phase and the bainite phase) is 0.2 or more with respect to the total number of quenched martensite, and a method for producing the same.

Description

HOT-DIP GALVANIZED STEEL SHEET AND METHOD FOR PRODUCING IT L LCRnn / Lznz / E / YIL Field of invention [1] The present invention relates to a hot-dip galvanized steel sheet and a method for producing the same, mainly relating to a high-strength hot-dip galvanized steel sheet for working into various shapes by pressing, etc., such as a steel sheet for use in automobiles and a method for producing the same. Background of the invention [2] In recent years, efforts have focused on improving the energy efficiency of automobiles from the perspective of controlling greenhouse gas emissions, a key component of the campaign against global warming. The use of high-strength steel sheets to reduce the weight of vehicle bodies and ensure collision safety has become increasingly widespread. In particular, the demand for ultra-high-strength steel sheets with a tensile strength of 980 MPa or higher has grown significantly recently. Furthermore, high-strength, hot-dip galvanized steel sheets with a hot-dip galvanized surface are sought after for body parts where rust prevention is required. [3] Hot-dip galvanized steel sheet used for automotive parts requires not only strength but also press formability, weldability, and various other workability properties necessary for part forming. Specifically, from the standpoint of press formability, excellent elongation (total elongation in tensile testing: E) and stretch flanging capability (hole expansion rate: A) of the steel sheet are required. Furthermore, considering the application of high-strength steel sheet to automobiles used in cold regions, the high-strength steel sheet is required to not exhibit brittle fracture in a low-temperature environment; that is, it must have excellent low-temperature toughness. [4] In general, press formability deteriorates along with the increased strength of the steel sheet. As a means of achieving greater strength and press formability of steel, TRIP (transformation-induced plasticity) steel sheet is known, which utilizes the transformation-induced plasticity of retained austenite. [5] PTL 1 to 3 describe the technique related to L LCRnn / Lznz / E / YIL is a controlled high-strength TRIP steel sheet with structural component fractions at predetermined intervals and improved elongation and hole expansion rates. Furthermore, PTL 4 describes a high-strength steel sheet having a predetermined chemical composition, and including, in terms of volume fraction, 15% or less ferrite with an average crystal grain diameter of 2 µm or less, 2 to 15% retained austenite with an average crystal grain diameter of 2 pm or less, 10% or less martensite with an average crystal grain diameter of 3 pm or less, and a balance of bainite and tempered martensite with an average crystal grain diameter of 6 pm or less, wherein the average number of cementite grains having a grain diameter of 0.04 pm or more existing in the grains of tempered bainite and martensite is 10 or more, and describes that this high-strength steel sheet has a tensile strength of 1180 MPa or more and has a high elongation and hole expansion capacity and excellent bending workability that accompanies it. [6] In addition, the TRIP type high-strength hot-dip galvanized steel sheet is described in several publications. [7] Normally, to produce hot-dip galvanized steel sheet in a continuous annealing furnace, it is necessary to heat the steel sheet to the reverse transformation temperature region (> Acl) and soak it, L LCRnn / Lznz / E / YIL then, halfway through the cooling process to room temperature, the steel sheet is immersed in a hot-dip galvanizing bath at approximately 460°C. Alternatively, after heating and soaking, then cooling to room temperature, the steel sheet must be reheated to the temperature of the hot-dip galvanizing bath and immersed in the bath. Furthermore, to produce hot-dip galvanized steel sheet, it is generally necessary to perform an alloying treatment after immersing the steel sheet in the coating bath, and then reheat the steel sheet to a temperature range of 460°C or higher.For example, PTL 5 describes how the steel sheet is heated to Acl or higher, then rapidly cooled to or below the martensite transformation start temperature (Ms), then reheated to the bainite transformation temperature region and held in the temperature region to stabilize the austenite (austemper it), and finally reheated to the coating bath temperature or the alloy treatment temperature for galvanizing-annealing. However, with such a production method, because the martensite is over-quenched in the coating and alloying steps, there was a problem that the material quality deteriorated. [8] PTLs 6 to 10 describe a method for producing L LCRnn / Lznz / E / YIL hot-dip galvanized steel sheet comprising cooling the steel sheet after coating and alloy treatment, and then reheating it to temper the martensite. List of appointments Patent literature [9] [PTL 1] WO 2013 / 051238 [PTL 2] Unexamined Japanese Patent Publication No. 2006-104532 [PTL 3] Unexamined Japanese Patent Publication No. 2011-184757 [PTL 4] WO 2017 / 179372 [PTL 5] WO 2014 / 020640 [PTL 6] Unexamined Japanese Patent Publication No. 2013-144830 [PTL 7] WO 2016 / 113789 [PTL 8] WO 2016 / 113788 [PTL 9] WO 2016 / 171237 [PTL 10] Unexamined Japanese Patent Publication No. 2017-48412 Summary of the invention Technical problem

[10] On the other hand, the steel sheet is required Hot-dip galvanized steel sheets used for automotive applications in cold regions should not only be pressed formable but also resistant to brittle fracture in low-temperature environments. However, prior art has not necessarily conducted sufficient research on both improving pressed formability and enhancing low-temperature toughness. Therefore, there is still room for improvement in the properties of hot-dip galvanized steel sheets, particularly those used for automotive components.

[11] Therefore, an object of the present invention is to provide a hot-dip galvanized steel sheet with excellent press formability and low-temperature toughness and having a tensile strength of 980 MPa or more and a method for producing the same. L LCRnn / Lznz / E / YIL Solution to the problem

[12] The inventors conducted intensive studies to solve this problem and, as a result, obtained the following findings: (i) In the continuous heat treatment step of hot-dip galvanizing, martensite is formed by cooling to Ms or lower after coating or coating and alloying. Furthermore, the steel can then be reheated and held isothermally to adequately quench the martensite and, in the case of a steel sheet containing retained austenite, further stabilize the retained austenite. Through such heat treatment, the martensite is no longer excessively quenched by the coating or coating and alloying, and the balance of strength and ductility is thus improved. (ii) Furthermore, the inventors discovered that if the concentration of Mn at the interfaces of different composite phases of quenched martensite and soft structures in contact with it (i.e., ferrite and bainite) reaches a certain value or higher, low-temperature toughness is improved. The detailed mechanism is unclear, but it is generally believed that brittle fracture occurs due to the formation and propagation of cleavage cracks by stacked dislocations at the crystal grain boundaries and the consequent stress concentration at the grain boundaries. In composite steel, the regions where dislocations accumulate and stress concentrates are thought to be the interfaces between the different composite phases of soft structures (ferrite and bainite) and hard structures (quenched martensite).If the Mn concentration at the interfaces of different phases reaches a certain value or higher, it is believed that some type of LCRnn / ίζηζ / E / γΐΛ interaction occurs between the dislocation group and the Mn atoms deposited at the interfaces, suppressing the formation of cleavage cracks. Furthermore, the inventors discovered that the aforementioned Mn concentration profile is achieved by isothermal retention in the temperature region of 480–600°C or lower during a heat treatment step in continuous hot-dip galvanizing. However, this isothermal retention must be performed before the austenite transforms into martensite. Moreover, if this isothermal retention is performed in this temperature region after coating, the sprayability of the coating layer deteriorates, etc. Therefore, isothermal retention must be carried out before the coating and alloying treatments. (iii) Furthermore, the inventors discovered that the effect of the above (ii) becomes more pronounced due to the limitation of casting conditions during continuous casting. That is, they found that, due to the early formation of segregated Mn regions during casting, the Mn concentration in the interphase interface regions described in (ii) increased. However, if the Mn concentration is advanced excessively, it was found that low-temperature toughness deteriorates. This is believed to be because, if the Mn concentration is advanced excessively, in the final structure, the concentration The concentration of Mn in Mη increases not only in the interface regions of different phases, but also within the grains of quenched martensite. Quenched martensite in which Mn is concentrated over the grain as a whole is thought to have low toughness.

[13] The present invention was made based on the above findings and is specifically as follows: (1) A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized coating on at least one surface of the base steel sheet, wherein the base steel sheet has a chemical composition comprising, in % by mass L LCRnn / Lznz / E / YIL C: 0.050% to 0.350%, Si: : 0.10% to 2.50%, Mn: : 1.00% to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: : 0.001% to 1.500% N: 0.0100% or less, 0: 0.0100% or less, Ti: : 0% to 0.200%, B: 0% to 0.0100%, V: 0% to 1.00%, Nb: : 0% to 0.100%, 0% to 1.00% to 1.00%, 0% to 1.00%, 0% to 0.0100%, 0% to 0.0100%, 0% to 0.0100%, 0% to 0.0100%, 0% to 0.0100%, 0% to 0.0100%, 0% to 0.0100%, other than Ce and La: 0% to 0 rest of Fe e impurities, steel microstructure in .0100% and a thickness range to 3 / 8 thickness centered around a 20 of 1 / 4 base thickness contains, by volume fraction, the sheet 1 / 8 ferrite steel position: 0% to 50%, L LPRnn / Lznz / E / YIA retained austenite: 0% to 30%, quenched martensite: 5% or more, fresh martensite: 0% to 10%, and total pearlite and cementite: 0% to 5%, when structures remain, the remaining structures consist of bainite, and a numerical ratio of quenched martensite with a Mn concentration profile that satisfies the following formulas (1) and (2) is 0.2 or more with respect to the total number of quenched martensite: [Mn]b / [Mn]a>1.2 ... (1) [Mn]a / [Mn] <2.0 ... (2) where [Mn] is the Mn content (% by mass) in the base steel sheet, [Mn]a is the average concentration of Mn (% by mass) in the quenched martensite and [Mn]bes is the concentration of Mn (% by mass) at the interfaces of the different phases of the quenched martensite and ferrite and the bainite phase. (2) Hot-dip galvanized steel sheet in accordance with (1), wherein the steel microstructure also contains, by volume fraction, retained austenite: 6% to 30%. (3) A method for producing hot-dip galvanized steel sheet according to (1) or (2) above, comprising a continuous casting step for continuously casting a plate having the chemical composition according to (1) above, a hot rolling step for hot rolling the cast plate, and a hot-dip galvanizing step for hot-galvanizing the resulting steel sheet, wherein (A) the continuous casting step satisfies the conditions of (A1) and (A2) below: (A1) the surface temperature of the plate at the end of secondary cooling is from 500 to 1100°C and (A2) a pouring speed is from 0.4 to 3.0 m / s, and (B) the hot-dip galvanizing step comprises heating the steel sheet to first soak it, first cooling and then soaking the first soaked steel sheet a second time, immersing the second soaked steel sheet in a hot-dip galvanizing bath, second cooling the coated steel sheet, and heating the cooled steel sheet a second time and then soaking it a third time, and further satisfies the following conditions of (B1) to (B6): (B1) In the heating of the steel sheet before the first soak, the average heating rate from 650°C to a maximum heating temperature of Acl + 30°C or more and 950°C or less is 0.5°C / s to 10.0°C / s, (B2) the steel sheet is held at the maximum heating temperature for 1 second to 1000 seconds (first soak), (B3) an average cooling rate in a L LCRnn / Lznz / E / YIL temperature range of 700 to 600°C in the first cooling is 10 to 100°C / s, (B4) the first cooled steel sheet is held in a range of 480 to 600°C for 80 seconds to 500 seconds (second soak), (B5) the second cooling is carried out to Ms50°C or less, and (B6) the second steel sheet is heated to a temperature region of 200 to 420°C, then held in the temperature region for 5 to 500 seconds (third soak). Advantageous effects of the invention

[14] In accordance with the present invention, it is possible to obtain a hot-dip galvanized steel sheet with excellent press forming capacity, specifically ductility and hole expansion capacity and greater low-temperature toughness. Brief description of the drawings

[15] Figure 1 shows a reference view of a secondary electron SEM image. Figure 2 is a temperature-thermal expansion curve when simulating a heat cycle corresponding to the hot-dip galvanizing treatment of L LCRnn / Lznz / E / YIL in accordance with the modality of the present invention by means of a thermal expansion measuring apparatus. L LCRnn / Lznz / E / YIL Description of the modalities

[16] Hot-dip galvanized steel sheet The hot-dip galvanized steel sheet according to the embodiment of the present invention comprises a base steel sheet and a hot-dip galvanized layer on at least one surface of the base steel sheet, wherein the base steel sheet has a chemical composition comprising, in % by mass, C: 0.050% to 0.350%, Si: 0.10% to 2.50%, Mn: 1.00% to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001% to 1.500%, N: 0.0100% or less, O: 0.0100% or less, Ti: 0% to 0.200%, B: 0% to 0.0100%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 1.00%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to 0.0100%, Hf: 0% to 0.0100%, Bi: 0% to 0.0100%, REM other than Ce and La: 0% to 0.0100% and a remainder of Fe and impurities, A steel microstructure in a thickness range of 1 / 8 to 3 / 8 of a thickness centered around a position 1 / 4 of the thickness from a surface of the base steel sheet contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 0% to 30%, quenched martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: at 5% when structures remain, the remaining structures consist of bainite, and a numerical ratio of quenched martensite with an Mn concentration profile that satisfies the following formulas (1) and (2) is 0.2 or more with respect to the total number of quenched martensite: [Mn]b / [Mn]a>1.2 ... (1) [Mn]a / [Mn] <2.0 ... (2) where [Mn] is the Mn content (% by mass) in the base steel sheet, [Mn]a is the average concentration of Mn (% by mass) in the quenched martensite and [Mn]b is the concentration of Mn (% by mass) at the interfaces of the different phases of the quenched martensite and ferrite and the bainite phase. L LCRnn / Lznz / E / YIL

[17] Chemical composition First, the reasons for limiting the chemical composition of the base steel sheet according to the embodiment of the present invention (hereinafter also referred to simply as steel sheet) as described above will be explained. In this Description, the % used to prescribe the chemical composition is % by mass unless otherwise stated. Furthermore, in this Description, when numerical value ranges are shown, unless otherwise stated, 'a' will be used to include the lower and upper limits of the numerical values ​​described above and below.

[18] C: 0.050% to 0.350% Carbon (C) is an essential element for ensuring the strength of steel sheets. If it is less than 0.050%, the required high strength cannot be achieved, and therefore the carbon content is 0.050% or higher. The carbon content can also be 0.070% or higher, 0.080% or higher, or 0.100% or higher. On the other hand, if it is greater than 0.350%, workability or weldability decreases, and therefore the carbon content is 0.350% or lower. The carbon content can be 0.340% or lower, 0.320% or lower, or even 0.300% or lower.

[19] Yes: 0.10% to 2.50% Silicon (Si) is an element that suppresses the formation of iron carbides and contributes to improved strength and formability, but excessive addition impairs the weldability of the steel sheet. Therefore, the content is 0.10 to 2.50%. The Si content can be 0.20% or more, 0.30% or more, 0.40% or more, or 0.50% or more, and / or it can be 2.20% or less, 2.00% or less, or 1.90% or less.

[20] Mn: 1.00% to 3.503% Manganese (Mn) is a powerful austenite stabilizing element and an effective element for increasing the strength of steel sheets. Excessive addition makes L LCRnn / Lznz / E / YIL that deteriorates weldability or toughness at low temperatures. Therefore, the content is from 1.00 to 3.50%. The Mn content can be 1.10% or more, 1.30% or more or 1.50% or more also and / or it can be 3.30% or less, 3.10% or less, or 3.00% or less also.

[21] P: 0.050% or less Phosphorus (P) is a strengthening element in the solution and an effective element for increasing the strength of steel sheets. Excessive addition impairs weldability and toughness. Therefore, the P content is limited to 0.050% or less. Preferably, it is 0.045% or less, 0.035% or less, or 0.020% or less. However, since an extreme reduction in P content would result in high dephosphorization costs, from an economic standpoint, a lower limit of 0.001% is preferable.

[22] S: 0.0100% or less Sulfur (S) is an element present as an impurity in steel, forming MnS, which impairs toughness and hole expansion capacity. Therefore, the S content is restricted to 0.0100% or less as a range in which toughness and hole expansion capacity are not noticeably impaired. Preferably, it is 0.0050% or less, 0.0040% or less, or 0.0030% or less. However, since an extreme reduction in S content would result in high desulfurization costs, from an economic standpoint, a lower limit of 0.0001% is preferable.

[23] Al: 0.001% to 1.500% Aluminum (Al) is added at least 0.001% for steel deoxidation. However, even if added in excess, not only does the effect become saturated, leading to increased costs, but it also raises the steel's transformation temperature and increases the load during hot rolling. Therefore, an Al content of 1.500% is the upper limit. Preferably, it should be 1.200% or less, 1.000% or less, or 0.800% or less.

[24] N: 0.0100% or less Nitrogen (N) is an element present as an impurity. If its content exceeds 0.0100%, it forms coarse nitrides in the steel, impairing its bending capacity and hole expansion capacity. Therefore, the N content is limited to 0.0100% or less. Preferably, it is 0.0080% or less, 0.0060% or less, or 0.0050% or less. However, since an extreme reduction in N content would result in high denitrification costs, a lower limit of 0.0001% is preferable from an economic standpoint.

[25] O: 0.0100% or less Oxygen (O₂) is an element present as an impurity. If its content exceeds 0.0100%, it forms coarse oxides in steel and causes a deterioration of its strength. L LCRnn / Lznz / E / YIL bending and hole expansion capacity. Therefore, the O content is limited to 0.0100% or less. Preferably it is 0.0080% or less, 0.0060% or less, or 0.0050% or less. However, from the point of view of production costs, a lower limit of 0.0001% is preferable.

[26] The basic chemical composition of the base steel sheet according to the embodiment of the present invention is as explained above. The base steel sheet may further contain the following elements as required.

[27] Ti: 0% to 0.200%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, B: 0% to 0.0100%, W: 0% to 1.00%, Sn: 0% to 1.00%, and Sb: 0% to 1.00% Titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), nickel (Ni), copper (Cu), cobalt (Co), molybdenum (Mo), boron (B), tungsten (W), tin (Sn), and antimony (Sb) are effective elements for increasing the strength of steel sheets. For this reason, one or more of these elements can be added as needed. However, excessive addition of these elements can saturate the effect and, in particular, lead to increased costs. Therefore, the contents are Ti: 0% to 0.200%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, B: 0% to 0.0100%, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 1.00%. The elements may also be LfRnn / LZnZ / E / YIL of 0.005% or more or 0.010% or more. In particular, the content of B may be 0.0001% or more or 0.0005% or more.

[28] Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to 0.0100%, Hf: 0% to 0.0100%, Bi: 0% to 0.0100%, and REM that different of Ce and La: 0% to 0.0100% Calcium (Ca), magnesium (Mq), cerium (Ce), zirconium (Zr), lanthanum (La), hafnium (Hf), and rare earth elements (REM) other than Ce and La are elements that contribute to the microdispersion of inclusions in steel. Bismuth (Bi) is an element that reduces the microdispersion of manganese (Mn), silicon (Si), and other substitution-type alloying elements in steel. Since these elements contribute to improving the workability of the steel sheet, one or more of these elements can be added as needed. However, excessive addition leads to a deterioration of ductility. Therefore, a content of 0.0100% is the upper limit. Elements can also be added at 0.0005% or more, or 0.0010% or more.

[29] In the base steel sheet according to the embodiment of the present invention, the remainder other than the aforementioned elements is composed of Fe and impurities. The impurities are components that enter due to various factors in the production process, primarily the raw materials such as ore and scrap metal, when the base steel sheet is industrially produced, and encompass all L LCRnn / Lznz / E / YIL refers to components not intentionally added to the base steel sheet according to the embodiment of the present invention. Furthermore, impurities encompass all elements other than the constituents explained above contained in the base steel sheet at levels where the distinct actions and effects of those elements do not affect the properties of the hot-dip galvanized steel sheet according to the embodiment of the present invention. L LCRnn / Lznz / E / YIL

[30] Steel structures within the steel sheet The reasons for the limitation of the internal structure of the base steel sheet in accordance with the embodiment of the present invention will now be explained.

[31] Ferrite: 0 to 50% Ferrite is a soft structure with excellent ductility. It can be included to improve the elongation of steel sheets according to the required strength or ductility. However, if it is contained in excess, it becomes difficult to ensure the desired strength of the steel sheet. Therefore, the content is a volume fraction of 50% as an upper limit and can be 45% or less, 40% or less, or 35% or less. The ferrite content can be a volume fraction of 0%. For example, it can be 3% or more, 5% or more, or 10% or more.

[32] Tempered martensite: 5% or more Tempered martensite is a tough, high-strength structure and is an essential metallic structure in the present invention. To balance strength, ductility, and bore expansion capacity to a high level, it is included in a volume fraction of at least 5% or more. Preferably, it is a volume fraction of 10% or more. It may be 15% or more or 20% or more as well. For example, the tempered martensite content may be a volume fraction of 90% or less, 85% or less, 80% or less, or 70% or less.

[33] Fresh martensite: 0 to 10% In the present invention, fresh martensite means unquenched martensite, i.e., martensite that does not contain carbides. This fresh martensite is a brittle structure, thus becoming a point of fracture initiation during plastic deformation and causing local ductility deterioration of the steel sheet. Therefore, the content is a volume fraction of 0 to 10%. Very preferably, it is 0 to 8% or 0 to 5%. The fresh martensite content may be a volume fraction of 1% or more or 2% or more.

[34] Retained austenite: 0% to 30% Retained austenite improves the ductility of the steel sheet due to the TRIP effect of the transformation into martensite due to the work-induced transformation L LCRnn / Lznz / E / YIL during the deformation of the steel sheet. On the other hand, to obtain a large amount of retained austenite, C and other alloying elements must be included in large quantities. For this reason, the upper limit for retained austenite is a volume fraction of 30%. It can be 25% or less, or 20% or less as well. However, if it is desired to improve the ductility of the steel sheet, the content is preferably a volume fraction of 6% or more, 8% or more, or 10% or more. Furthermore, if the retained austenite content is 6% or more, the Si content in the base steel sheet is preferably, by mass percent, 0.50% or more.

[35] Pearlite and cementite in total: 0 to 5% Pearlite contains hard, coarse cementite and forms a fracture initiation point during plastic deformation, thus causing local ductility deterioration in the steel sheet. Therefore, its content, along with the cementite, is a volume fraction of 0 to 5%. It can also be 0 to 3% or 0 to 2%.

[36] The remaining structures besides the above structures may be 0%, but if present, they are bainite. The remaining bainite structures may be upper bainite or lower bainite or they may be mixed structures of both. L LCRnn / ίZΖΠZ / Β / YΙΛ

[37] Numerical relationship of quenched martensite with Mn concentration profile that satisfies formulas (1) and (2) of 0.2 or more with respect to the total number of quenched martensite [Mn]b / [Mn]a>1.2 ... (1) [Mn]a / [Mn] <2.0 ... (2) where [Mn] is the Mn content (% by mass) in the base steel sheet, [Mn]a is the average concentration of Mn (% by mass) in the quenched martensite and [Mn]b is the concentration of Mn (% by mass) at the interfaces of the different phases of the quenched martensite and ferrite and the bainite phase.

[38] In the present invention, the above conditions must be met to obtain the desired low-temperature toughness. To achieve the effect of increasing toughness by strengthening the interfaces of the different phases, [Mn]b / [Mn]a must be greater than 1.2. On the other hand, if [Mn]a / [Mn] becomes 2.0 or more, the hardness of the quenched martensite itself deteriorates. If the numerical ratio of the quenched martensite that simultaneously satisfies the above conditions becomes 0.2 or more with respect to the total number of quenched martensite, the low-temperature toughness increases to the desired level. The numerical ratio of the quenched martensite can be 0.3 or more, 0.4 or more, 1.0 or less, or 0.9 or less.

[39] The fractions of the steel structures of the hot-dip galvanized steel sheet are evaluated using the SEM-EBSD method (Electron Electromagnetic Dispersion Diffraction Method) L LCRnn / Lznz / E / YIL electron backscattering) and observation of secondary electron images SEM.

[40] First, a cross-sectional sample of the steel sheet thickness parallel to the rolling direction is taken so that the cross-section of the thickness at the center of the width becomes the observed surface. The observed surface is machine-polished to a mirror finish, then electropolished. Next, in one or more observation fields ranging from 1 / 8 to 3 / 8 of a thickness, centered around 1 / 4 of the thickness from the base steel sheet surface on the observed surface, a total surface area of ​​2.0 x 10⁻⁹ m² or more is analyzed for crystal structures and orientations using the SEM-EBSD method. The data obtained by the EBSD method are analyzed using the OIM Analysis 6.0 developed by TSL. Furthermore, the distance between the evaluation points (steps) is 0.03 to 0.20 pm.Regions identified as FCC iron based on observational results are considered retained austenite. Additionally, boundaries with crystal orientation differences of 15 degrees or more are considered grain boundaries for the purpose of generating a crystal grain boundary map.

[41] Next, the same sample observed by EBSD is corroded by Nital and observed by imaging of Secondary electrons are used for the same fields as the EBSD observation. Since the same fields are observed as at the time of the EBSD measurement, Vickers notches and other visual markings can be provided in advance. From the resulting secondary electron image, the area ratios of ferrite, retained austenite, bainite, quenched martensite, fresh martensite, and pearlite are measured respectively, and the results are considered volume fractions. Regions with lower grain structure and several cementite variants, more specifically two or more, that precipitate are judged to be quenched martensite (e.g., see the reference drawing in Figure 1). Regions where cementite precipitates in a lamellar form are considered to be pearlite (or pearlite and cementite combined).Regions with low brightness and no underlying structures are considered ferrite (e.g., see the reference drawing in Figure 1). Regions with high brightness and where underlying structures are not revealed by etching are considered fresh martensite and retained austenite (e.g., see the reference drawing in Figure 1). Regions that do not correspond to either of the above are considered bainites. Their volume ratios are calculated using the point-counting method and used as the volume ratios for the structures. The volume ratio of fresh martensite can be found by subtracting the volume ratio of retained austenite found by X-ray diffraction.

[42] The retained austenite volume ratio is measured using the X-ray diffraction method. In a thickness range of 1 / 8 to 3 / 8 inch centered approximately 1 / 4 inch thick from the surface of the base steel sheet, a surface parallel to the sheet surface is polished to a mirror finish and the FCC iron area ratio is measured using the X-ray diffraction method. This is used as the retained austenite volume fraction.

[43] In the present invention, the concentration of Mn at the interfaces of the different phases of quenched martensite, ferrite, and bainite is determined using the STEM-EELS method. Specifically, for example, it is determined using the method described in METALLURGICAL AND MATERIALS TRANSACTIONS A: vol. 45A, pp. 1877-1888. First, a sample is taken from the cross-section of the thickness of the steel sheet parallel to the rolling direction so that the cross-section of the thickness at the center position in the width direction becomes the observed surface. The observed surface is machine-polished and finished to a mirror surface, then electropolished. Next, in one or more fields of observation, within a thickness range of 1 / 8 to 3 / 8 A thickness of LCRnn / Lznz / E / YIL, centered on 1 / 4 of the thickness from the surface of the base steel sheet on the observed surface, totaling 2.0 x 10⁻² sm, is analyzed for crystal structures and orientations using the SEMEBSD method to identify quenched martensite, ferrite, and bainite. The region encompassing the interfaces of the different phases is then extracted by FIB in the SEM. Following this, Ar ion milling, etc., is used to reduce the thickness to approximately 70 nm. The electron energy loss spectrum (EELS) is obtained along a line traversing the interfaces of the different phases using the STEM aberration spectrum of the reduced-thickness test piece. The above measurement is performed for individual interfaces for at least five, preferably ten, quenched martensite samples.The maximum value of the Mn concentration in the Mn concentration profile measured across the interfaces of the different phases is [Mn]b. The value obtained by averaging the Mn concentration profile on the quenched martensite side, disregarding the interface regions of different phases, is [Mn]a. [Mn] is the same as the Mn content in the steel composition. The scan steps at the time of line analysis are preferably approximately 0.1 nm. In the present invention, for example, if 10 different quenched martensites are measured, if the number of... L LCRnn / Lznz / E / YIL tempered martensite satisfying the following formulas (1) and (2) is two or more, it is judged whether the numerical ratio of the tempered martensite where the concentration profile of Mn satisfying formulas (1) and (2) is 0.2 or more with respect to the total number of tempered martensite: [Mn]b / [Mn]a>1.2 ... (1) [Mn]a / [Mn] <2.0 ... (2) L LCRnn / Lznz / E / YIL

[44] Hot-dip galvanized coating The base steel sheet according to the embodiment of the present invention has a hot-dip galvanized coating on at least one surface, preferably on both surfaces. This coating layer may be a hot-dip galvanized layer or a hot-dip galvanized-annealed layer having any composition known to the art and may include aluminum and other additive elements in addition to zinc. Furthermore, the amount of coating deposition is not particularly limited and may be a general amount.

[45] Method of manufacturing hot-dip galvanized steel sheet The method for producing hot-dip galvanized steel sheet according to the embodiment of the present invention will now be explained. The following explanation is intended to illustrate the characteristic method for producing hot-dip galvanized steel sheet according to the embodiment of the present invention and is not intended to limit the hot-dip galvanized steel sheet to one produced by the production method explained below.

[46] The method for producing hot-dip galvanized steel sheet comprises a continuous casting step for continuously casting a plate having the same chemical composition as the chemical composition explained above in relation to the base steel sheet, a hot rolling step for hot rolling the cast plate, and a hot-dip galvanizing step for hot galvanizing the resulting steel sheet, wherein (A) the continuous casting step satisfies the conditions of the following (A1) and (A2): (A1) the surface temperature of the plate at the end of secondary cooling is 500 to 1100°C and (A2) a pouring speed is 0.4 to 3.0 m / s, and (B) the hot-dip galvanizing step comprises heating the steel sheet to first soak it, first cooling it, and then soaking it a second time. L LCRnn / įZРZ / B / YIL soaked steel sheet, immerse the second soaked steel sheet in a hot-dip galvanizing bath, cool the coated steel sheet a second time, and heat the cooled steel sheet a second time and then soak it a third time, and further satisfies the following conditions (B1) to (B6): (B1) In the heating of the steel sheet before the first soak, the average heating rate from 650°C to a maximum heating temperature of Acl + 30°C or more and 950°C or less is 0.5°C / s to 10.0°C / s, (B2) the steel sheet is held at the maximum heating temperature for 1 second to 1000 seconds (first soak), (B3) an average cooling rate in a temperature range of 700 to 600°C in the first cooling is 10 to 100°C / s, (B4) the first-cooled steel sheet is held in a range of 480 to 600°C for 80 seconds to 500 seconds (second soak), (B5) the second cooling is carried out to Ms50°C or less, and (B6) the steel sheet is heated second by second to a temperature region of 200 at 420°C, then it is kept in the temperature region for 5 to 500 seconds (third soak). L LCRnn / Lznz / E / YIL

[47] The method for producing hot-dip galvanized steel sheet will now be explained in detail.

[48] ​​(A) Continuous conversion step Plate surface temperature at the end of secondary cooling: 500 to 1100°C and casting speed: 0.4 to 3.0 m / min The steel plate used in the present invention is cast by the continuous casting method. The plate thickness is generally 200 to 300 mm, for example, 250 mm. If the plate surface temperature at the end of secondary water quenching during continuous casting (the time of the end of cooling in the secondary quenching zone on the casting exit side) rises above 1100°C or the casting speed falls below 0.4 m / min, the degree of Mn segregation increases excessively, and [Mn]a / [Mn] tends to become 2.0 or higher. Conversely, if the plate surface temperature falls below 500°C or the casting speed increases above 3.0 m / min, the degree of Mn segregation becomes insufficient, and [Mn]b / [Mn] tends to become 1.2 or lower.The surface temperature of the plate at the end of secondary cooling can be 600°C or higher, 700°C or higher, and / or 1050°C or lower. Furthermore, the pouring speed can be 0.6 m / min or higher, 0.8 m / min or higher, and / or 2.5 m / min or lower, or 2.0 m / min or lower. The surface temperature of the plate is measured using a radiation thermometer. L LCRnn / Lznz / E / YIL

[49] Hot rolling pass In this method, the hot rolling step is not particularly restricted and can be performed under any suitable conditions. Therefore, the following explanation regarding the hot rolling step is intended as a simple illustration and is not intended to limit the hot rolling step in this method to one performed under the specific conditions explained below.

[50] First, in the hot rolling step, a plate having the same chemical composition as the chemical composition explained above in relation to the base steel sheet before hot rolling is heated. The heating temperature of the plate is not particularly limited, but for sufficient dissolution of the borides, carbides, etc., 1150°C or higher is generally preferable.

[51] Rough rolled In this method, for example, the heated plate can be rough-rolled before the finish rolling to adjust the sheet thickness, etc. Such rough rolling is not particularly limited; it is preferable to perform it to achieve a total rolling reduction of 60% or more at 1050°C or higher. If the total rolling reduction is less than 60%, since recrystallization during hot rolling becomes insufficient, this sometimes leads to irregularities in the structure of the hot-rolled sheet. The total previous rolling reduction can be, for example, 90% or less. L LCRnn / Lznz / E / YIL

[52] Inlet temperature of the finishing laminate: 900 to 1050°C, outlet temperature of the finishing laminate: 850°C to 1000°C and total laminate reduction: 70 to 95% The finishing roll is performed within a range that satisfies the conditions of an entry-side temperature of 900 to 1050°C, an exit-side temperature of 850°C to 1000°C, and a total rolling reduction of 70 to 95%. If the entry-side temperature falls below 900°C, the exit-side temperature falls below 850°C, or the total rolling reduction exceeds 95%, the hot-rolled steel sheet develops texture, sometimes resulting in anisotropy in the final product sheet.On the other hand, if the inlet temperature of the finishing roll rises above 1050°C, the outlet temperature rises above 1000°C, or the total rolling reduction falls below 70%, the hot-rolled steel sheet becomes coarser in crystal grain size. This sometimes leads to a thickening of the sheet structure in the final finished product and, in turn, a deterioration in workability. For example, the inlet temperature of the finishing roll might be 950°C or higher. The outlet temperature might be 900°C or higher. The total rolling reduction might be 75% or higher, or 80% or higher. l LfRnn / LZnZ / E / YIL

[53] Winding temperature: 450 to 680°C The rolling temperature is 450–680°C. If the rolling temperature is below 450°C, the strength of the hot-rolled sheet becomes excessive, and sometimes the ductility of the cold-rolled sheet deteriorates. On the other hand, if the rolling temperature exceeds 680°C, the cementite becomes thicker, and some cementite remains undissolved, which can sometimes affect workability. The rolling temperature can be 470°C or higher and / or 650°C or lower.

[54] In the present method, the hot-rolled steel sheet obtained (hot-rolled coil) can be pickled or otherwise treated as required. The hot-rolled coil can be pickled by any ordinary method. In addition, the hot-rolled coil can be wound with skin to correct its shape and improve its pickling capacity. L LCRnn / Lznz / E / YIL

[55] Cold rolling step In this method, after hot rolling and / or pickling, the steel sheet can be heat-treated as is by a continuous hot-dip galvanizing line or it can be cold-rolled and then heat-treated on a continuous hot-dip galvanizing line. If cold rolling is performed, the cold-roll reduction is preferably 25% or more, or 30% or more. On the other hand, since excessive cold-roll reduction results in excessive rolling force and leads to increased loads on the cold-rolling mill, the upper limit is preferably 75% or 70%.

[56] (B) Hot-dip galvanizing step Average heating rate from 650°C to the maximum heating temperature of Acl + 30°C or more and 950°C or less: 0.5 to 10.0°C / s In this method, after the hot-rolling step, the resulting steel sheet undergoes a hot-dip galvanizing process. In this process, the steel sheet is first heated and then subjected to an initial soaking treatment. During heating, the average heating rate, from 650°C to the maximum heating temperature of Acl + 30°C or higher and 950°C or lower, is limited to 0.5 to 10.0°C / s. If the heating rate exceeds 10.0°C / s, ferrite recrystallization is insufficient, sometimes resulting in poor elongation of the steel sheet. Conversely, if the average heating rate falls below 0.5°C / s, the austenite becomes coarse, sometimes leading to thick steel structures. This average heating rate can be 1.0°C or more and / or can be 8.0°C or less, or 5.0°C or less.In the present invention, the average heating rate means the value obtained by dividing the difference between 650°C and the maximum heating temperature by the time elapsed from 650°C to the maximum heating temperature. L LCRnn / Lznz / E / YIL

[57] First soaking treatment: holding at Acl's maximum heating temperature + 30 °C to 950 °C for 1 second to 1000 seconds To achieve sufficient austenite transformation, the steel sheet is heated to at least Acl + 30°C or higher and held at that temperature (maximum heating temperature) as a soaking treatment. However, excessively high heating temperatures not only encourage a deterioration of toughness due to coarsening of the austenite grain size but also damage the annealing equipment. For this reason, the upper limit is 950°C, preferably 900°C. If the soaking time is too short, the austenite transformation does not proceed sufficiently, so the time is at least 1 second or more. Preferably, it is 30 seconds or more, or 60 seconds or more. On the other hand, if the soaking time is too long, productivity is compromised, so the upper limit is 1000 seconds, preferably 500 seconds.During soaking, the steel sheet does not necessarily have to be kept at a constant temperature. It can also fluctuate within a range that satisfies the above conditions. Maintaining the temperature in the first, second, and third soaking treatments, as explained below, means keeping the temperature within a predetermined range of ±20°C, preferably ±10°C, within a range that does not exceed the upper and lower limits prescribed for the soaking treatments. Therefore, for example, a heating or cooling operation that gradually heats or gradually cools, whereby the temperature fluctuates by more than 40°C, preferably 20°C, does not fall within this range. L LCRnn / Lznz / E / YIL with the prescribed temperature intervals in the soaking treatments are not included in the first, second and third soaking treatments in accordance with the modality of the present invention. L LCRnn / ίZΖΠZ / Β / YΙΛ

[58] First cooling: average cooling rate over a temperature range of 700 to 600°C: 10 to 100°C / s After being held at the maximum heating temperature, the steel sheet undergoes initial quenching. The quenching stop temperature is between 480°C and 600°C, which then becomes the subsequent second quenching temperature. The average cooling rate over a temperature range of 700°C to 600°C is 10 to 100°C / s. If the average cooling rate is less than 10°C / s, the desired ferrite fraction may not be achieved. The average cooling rate can be 15°C / s or more, or even 20°C / s or less. It can also be 80°C / s or less, or even less. Furthermore, in the present invention, the average cooling rate in a temperature range of 700 to 600°C means the value obtained by dividing the temperature difference between 700°C and 600°C, i.e., 100°C, by the time elapsed from 700°C to 600°C.

[59] Second soaking treatment: holding at a range of 480°C to 600°C for 80 to 500 seconds The second soaking treatment, which holds the steel sheet at 480°C to 600°C for 80 to 500 seconds, causes Mn to segregate at the interfaces of the different phases consisting of austenite, ferrite, and bainite. The austenite at this stage later transforms into quenched martensite. In other words, due to the second soaking treatment, Mn segregates at the interfaces of the different phases composed of austenite, ferrite, and bainite. Due to the subsequent second cooling and third soaking treatment, the austenite transforms into martensite and is quenched, resulting in an increased concentration of Mn at the interfaces of these different phases. If the temperature of the second soaking treatment falls below 480°C or becomes greater than 600°C or if the retention time is less than 80 seconds, the segregation of Mn does not proceed sufficiently.On the other hand, if the holding time exceeds 500 seconds, the excessive bainite transformation will prevent the metal structures from being obtained according to the method of the present invention. The temperature of the second immersion treatment can be 500°C or higher and / or 570°C or lower. Furthermore, the holding time can be 95 seconds or higher. 460 seconds or less. In this regard, even if the second soaking treatment is properly performed, if the segregated Mn regions are not adequately formed in the continuous casting step, the Mn concentration at the interfaces of the different phases decreases. Therefore, in the method for producing hot-dip galvanized steel sheet according to one embodiment of the present invention, to increase the Mn concentration at the interface of the different phases, it is important to satisfy the conditions of (A1) and (A2) explained above in the continuous casting step while properly performing the second soaking treatment in the hot-dip galvanizing step. In the present method for producing hot-dip galvanized steel sheet according to one embodiment of the present invention, a predetermined coating treatment is to be performed after the second soaking treatment. However, if the second soaking treatment is performed after immersion in the coating bath, the dust resistance of the coated layer is sometimes significantly impaired. This is because if the heat treatment is performed after immersion in a coating bath at 480°C or higher for 80 seconds or more, the alloying reaction between the coating sheet and the coating material is impaired. L LCRnn / Lznz / E / YIL the steel proceeds excessively and the internal structure of the coating film changes from ductility-excellent δ phases to ductility-poor Γ phases.

[60] After the second soaking treatment, the steel sheet is immersed in a hot-dip galvanizing bath. The temperature of the steel sheet at this time has little effect on its performance, but if the difference between the steel sheet temperature and the coating bath temperature is too large, as the coating bath temperature will fluctuate and sometimes hinder operation, providing a cooling step for the steel sheet to a coating bath temperature range of -20°C to +20°C is desirable. Hot-dip galvanizing can be carried out using a standard method. For example, the coating bath temperature can be 440 to 460°C and the immersion time can be 5 seconds or less. The coating bath is preferably one containing 0.08 to 0.08% Al.2%, but impurities may also include Fe, Si, Mg, Mn, Cr, Ti, and Pb. Furthermore, it is preferable to control the coating basis weight by gas cleaning or another known method. The basis weight is preferably 25 to 75 g / m² per side. L LCRnn / Lznz / E / YIL

[61] Alloy treatment For example, hot-dip galvanized steel sheet, formed with the hot-dip galvanized coating, can be alloyed as required. In this case, if the alloying treatment temperature is below 460°C, not only does the alloying rate slow down and productivity suffer, but uneven alloying also occurs. Therefore, the alloying treatment temperature should be 460°C or higher. On the other hand, if the alloying treatment temperature is above 600°C, excessive alloying sometimes occurs, and the coating's adhesion deteriorates the steel sheet. Furthermore, pearlite transformation sometimes occurs, preventing the desired metallic structure from being achieved. Therefore, the alloying treatment temperature should be 600°C or lower. L LCRnn / Lznz / E / YIL

[62] Second cooling: cooling to Ms-50°C or less The steel sheet, after coating or alloying treatment, is cooled by a second quench to the martensitic transformation start temperature (Ms) of -50°C or lower. This cools the steel to cause some or most of the austenite to transform into martensite. The martensite produced here is then quenched by subsequent reheating and a third soaking treatment to become quenched martensite. If the quench stop temperature is above Ms -50°C, the martensitic transformation is insufficient, resulting in inadequate quenched martensite formation and the resulting undesired metal structure. If the retained austenite is to be used to improve the ductility of the steel sheet, it is desirable to set a lower limit on the quench stop temperature.Specifically, the cooling stop temperature is desirablely controlled in a range of Ms-50°C to Ms-130°C.

[63] The martensite transformation in the present invention occurs after the ferrite and bainite transformations. Along with the ferrite and bainite transformations, carbon diffuses into the austenite. For this reason, this does not coincide with the Ms when single-phase austenite is heated and rapidly cooled. The Ms in the present invention is found by measuring the thermal expansion temperature during the second cooling. For example, the Ms in the present invention can be found using a Formastor tester or other apparatus capable of measuring the amount of thermal expansion during continuous heat treatment, reproducing the heating cycle of the hot-dip galvanizing line from the start of the hot-dip galvanizing heat treatment (corresponding to ambient temperature) to the second L LCRnn / Lznz / E / YIL previous cooling, and measuring the thermal expansion temperature during that second cooling. However, in the actual heat treatment of hot-dip galvanizing, the cooling sometimes stops between Ms and ambient temperature, but at the time of the thermal expansion measurement, the cooling is carried out to ambient temperature. Figure 2 is a temperature-thermal expansion curve that simulates, using a thermal expansion measuring device, a heat cycle corresponding to the hot-dip galvanizing treatment according to an embodiment of the present invention. The steel sheet thermally contracts linearly in the second cooling step, but deviates from a linear relationship at a certain temperature. The temperature at this point is Ms in the present invention. L LCRnn / Lznz / E / YIL

[64] Third soaking treatment: holding in a temperature region of 200°C to 420°C for 5 to 500 seconds After the second cooling, the steel sheet is reheated to between 200°C and 420°C for the third soak treatment. In this step, the martensite produced during the second cooling is quenched. If the holding temperature is below 200°C or the holding time is less than 5 seconds, the quenching is insufficient. On the other hand, if the holding temperature is above 420°C or the holding time is greater than 500 seconds, the martensite is excessively quenched and the bainite transformation progresses too rapidly, making it difficult to achieve the desired strength and metal structure. The temperature of the third soak treatment can be 240°C or higher and can be 400°C or lower. Furthermore, the holding time can be 15 seconds or more, 100 seconds or more, and 400 seconds or less.

[65] After the third soaking treatment, the steel sheet is cooled to room temperature to obtain the final finished product. The steel sheet can also be surface rolled to correct flatness and adjust surface roughness. In this case, to avoid ductility deterioration, the elongation rate is preferably 2% or less. L LCRnn / ίZΖΠZ / Β / YΙΛ Examples

[66] Examples of the present invention will now be explained. The conditions in the examples are illustrations of the conditions employed to confirm the workability and effects of the present invention. The present invention is not limited to these illustrations of conditions. The present invention may employ various conditions, provided that it does not depart from the essence of the present invention and achieves the object of the present invention. L LCRnn / Lznz / E / YIL

[67] Example A Steels with the chemical compositions shown in Tables 1-1 and 1-2 were cast to prepare plates. The remainder, in addition to the constituents shown in Tables 1-1 and 1-2, comprised iron and impurities. These plates were hot-rolled under the conditions shown in Tables 2-1 to 2-6 to produce hot-rolled steel sheets. The hot-rolled steel sheets were then pickled to remove surface inclusions and subsequently cold-rolled. The resulting steel sheets were then continuously hot-dip galvanized under the conditions shown in Tables 2-1 to 2-6 and treated appropriately for alloying. In the soaking treatments shown in Tables 2-1 to 2-6, the temperatures were maintained within the range shown in Tables 2-1 to 2-6 ± 10°C.The chemical compositions of the base steel sheets obtained by analyzing samples taken from the hot-dip galvanized steel sheets produced were equal to the chemical compositions of the steels shown in Tables 1-1 and 1-2.

[68] Table

[69] Table | PV | or to Cü I 719 1 | 760 | 1 752 1 718 | CP to to I 736 I 731 | 716 719 | 720 | 5 706 | 2g to 708 | 772 | 1 6«¿ I 707 | Others | to g ¿S Ca:0.0043 O to O © tea to © © OX go Éji s ci ω üt ÍM to © φ o Os © CD i Sb | 800 j 1 Sn | | gold 2» 0.17 | i °0 0.39 | z 1..........0.7..........1 1 cu ! | 720 1........W6.......3 00 0.0020 | | OíOO'O 0.0008 | | tzooo 0.0012 | | 0100Ό 0.0028 | 0.0028 | 0.002Z | K 0.017 I | zoo you> or 0.030 | 0.040 | | 0100 0.021 1 0.0Z6 1 >o steel ¡ < 00 o co UJ LL Φ X -> XO Cu <3 Oí > £ G +> Ü o •a ü tí 5 tí •o •HO tí tí $ » & o* <3 Or £ £ to you? G * t7 a» © O' w Five and & & oh oh Λ and >1 © ¡3 β tí t* GG tí Ώ $ o rí (3 o yf¥ tí O Λ Oh wow WHO? Blackboard CM Cold rolling pass Cold rolling reduction % <*7 IL <7 J47 07 W7 s ií7 <n un un Γ7 U7 Γ7 «7 C7 lO 07 io C7 U7 <O U7 ίΛ m «7 07 ίΛ en un m un Paso de laminada en caliente Temp. de enrollamiento C I 0L9 | I 560 I 1 580 1 | 580 | I 560 | I 530 | I 550 I 1 580 | I 570 I 1 550 | I 550 I I 570 I I 560 I O LD I 570 I I O‘s I O xt m 1 520 | I 590 I 580 | Reducción de laminado total de laminado de acabado % £ 5^ es 51 5 5 o- 5- 5* 5^ es £ £ Temp. del ledo de salida de acabado C o ©s co Otó 950 o O OJ 086 O o o- 910 o 880 o 890 076 o o 920 0Z6 © 910 Temp. del lado de entrada de acabado °C s 1 1010 1 1030 | icio otoi | | 1030 066 | | 1010 | 1010 046 | I 1040 086 | I 1020 1 1030 I icio | 0101 | I 1020 ] oooi | I 1020 ooot | Reducción de laminado total de laminado en bruto a 1050°C o más % I 85 j I 58 I ! 85 | I 85 ¡ 1 85 1 un «i 1 85 1 85 1 85 ¡ 1 85 ¡ 1 85 ¡ 08 lO <0 85 un Cü 85 85 1 85 ¡ U7 co 85 Temp.plate heating °C 1250 [ ! 1250! 1250 ] O or Oj 1260 ] 1240 | 1270 [ 1240 [ 1270 l [ OSZL 1210 ] 1220 ] O CN I 1280' ! 1250 ] 1270 [ 1210 j 1250 j Continuous flow rate Flow rate m / min TI 1.3 joa ro | ει ! 51 91 r- | 6-0 a 5T O Ό | yes 60 Plate surface temperature at the final time of secondary cooling 3C 980 | © I 068 i © es I 006 | 066 or © 1030 | © co oo σ oo O es I 0Í6 1030 | 930 | 850 | I °'6 I Type of maple < < < < < < < < m CD CD CQ CD CD u CJ o £3 No. - O) <7 m O oo oo ξΤ Oí ro ir? so £ co P· O Oí. a 'tí n ir· 21 ΰ o «i « & tí t( 1 « ü tí £ tí tí tí u 21 $ H tí +J «I 8 tí tí l LCRnn / Lznz / E / YiAi Table ) 04 Cold rolling pass Cold rolling reduction íi 07 U7 07 ιO 07 L!7 07 1Í7 C7 iD 07 iÓ 07 ÍÍ7 σ> ΙL 07 U7 07 1X7 07 1X7 O> U7 07 1X7 07 1X7 07 1X7 07 Lrt 07 U7 07 1X7 Hot rolling pass Temp. of winding ”0 560 510 570 540 490 550 520 530 520 510 580 I 560 610 520 CP CN 600 600 08S 580 550 Total laminate reduction of finish laminate and / or £ £ 5^ 5 <> <T g: es íT £ s θ' θ' 51 Temp. del lado de salida de acabado =C 930 | | otó 920 | I 0i6 1 016________ o O I 068 930 | 970 | o θ' 910 | a 920 | 920 | o θ' | otó 960 | 096 Temp. del lado de entrada de acabado “C icio | I 010L toso | | osol j ooot ! o o 1030 | 970 | wio | 1050 1 1050 j 1010 | 1020 1010 | 1020 | 1020 | 1020 | 1010 | I OSOL (0901 Reducción de laminado total de laminado en bruto a IQ5O”C o más % m co I 85 1 85 j 85 j 85 ! 85 j ΙΛ CO I 85 j i s® 1 85 j 85 j 85 j ¡ 85 j I sg ¡ «7 00 I________________85________________I U7 1 85 ¡ 1 88 85 Temp.de calentamiento de la placa -jc OtZl 1210 1230 1280 cS 1250 1270 1240 1240 1 1190 1240 1230 1260 1260 1270 I 1260 1220 1260 1230 1230 Paso de colada continua Velocidad de colada m / mín ¡ '·* 1 I 21 ¡ VI I 91 O ! θ·8 I ¡ 0·® ¡ I n ! O 1.5 ¡ I·8 ¡ 1 8·1 1 ¡ 1-5 ! I 91 ¡ θ' £23 1 n ! <¡ 1.3 Plate surface temperature at the final time of secondary cooling 'C © oa CP CO: 1010 066 a U7 o 920 U7 θ' I 910 jo ! 1020! 970 O OTol | 970 j 970 a o- oo 970 3 Maple type □ ω 04 04 04 Oí ai U4 w U- Ll_ LL o OXX -7 “7 ó CN CN CN 07 CN CN CN CN c~ o 07 07 07 07 O> 117 07 07 ¢0 07 j 40. ω or •u > aa or τι í or P Φ 3 w rt o >4 L LCRnn / Lznz / Ε / ΥΙΛΙ Table C: Hey L LCRnn / Lznz / Ε / ΥΙΛΙ

[73] Table <N Ms in the galvanizing step by hot dip ℃ | 322 | so cc 04 04 04 04 CM 05 Yes 0¿ 04 1 286 1 04 04 co 07 co I 408 | On page 04 s in I 350 1 Go to: CO | 1 Γ4 with W c cu c Φ -o ai £ £ oo re § *<0 cr wo <n <c £1 Tercer remojo Tiempo de mantenimiento s 300 1 300 i 300 300 300 300 [ 300 300 o o co 300 300 300 00£ 300 300 300 008 300 o í¿ c * o 068 O o 088 D8£ 380 380 i o O 380 400 390 400 3 380 088 400 088 ¡ 350 | o o co Segundo enfriamiento Temp. de detención de enfriamiento C co α co Q es 00 CO O 04 | va o oo 04 250 | Oí Oí O LO 04 04 230 | 200 | <N 200 | O 0J Segundo remojo Aleación Tiempo de Temp. de mantenimiento aleación s C 069 | Si § id 95 | 480 j 95 | 480 j o -3 ΙΛ CO 460 | 480 | 95 I 480 j 95 | 480 uo 95 | 480 | I 061? | 56 o xr id 95 | 480 | 95 | 500 j S O: O í» O 00 Temp.500 580 USE O tñ 500 500 a 04 m 500 1 500 540 550 550 550 540 1 550 ΙΛ OO· 500 530 1 530 Primer enfriamiento Velocidad de enfriamiento “C / s O co Ld oo <O co O <c <n 5 co O 04 04 041 s 5 OÓ 00 00 07 00 00 Primer remojo Tiempo de mantenimiento s o C o a o a a o O o a £ a CT O O a O a O £X e 9 φ 820 c DO 0Z8 1 820 ! 820 820 820 ! 820 O 04 CO 880 930 Oí U> i 098 1 870 860 C3 C0 CD 890 890 1 890 088 Calentamiento VeL of heating from 65073 to temp. of heating max. °C / s C0 -4 04 o 00 tn 'T 'r 04 04 04 oi <* C0 ÚO Oí 04 04 04 r·: ó - 04 00 <r ΙΛ o 09 ¢0 O 04 CJ 3 5Λ O £ O 0J. ii «I O ¢) tí O s ü o 4J Ü O<n ΰ A «i o > <4 *1 $ l LfRnn / LZnZ / Ε / ΥΙΛΙ Ms in the hot-dip galvanizing step 'C 07 04 358 | to 07 in O 1 $42 O O 07 327 § So 07 355 § C Lí7 07 ¡D 07 rj 349 1 334 | <r 07 07 04 04 333 § 335 j ?rcer remojo Tiempo de mantenimiento s 300 300 300 300 300 1 300 00E 07 300 300 © 300 g O o <n 300 1 300 300 300 1 300 I 003 £ φ f— 350 06£ 06£ § 300 400 © © 390 § -9 ¡ 009 1 © © <r © © © 400 1 400 g 350 350 I 370 380 Segundo enfriamiento Temp. de detención de enfriamiento “C s 04 04 o 07 04 O 04 .3.50 230 1 230 | g © 04 o OQ © os S 04 O © 04 1 240 1 240 © 04 OOÍ 200 1 200 1 ente Aleación Temp. de aleación ’C 1 .480 o oí o LD I 540 1 500 I 550 1 © lñ © o LD © o íD O LD © o 1Λ • © iD © 07 LD • © 04 <D i ' inmersión en cali gundo remojo Tiempo de mantenimiento s g g g U7 θ' g ÍD θ' s LD θ' g g g g ID a- g g U7 □*· LD θ' g '0 H O o <0 U7 o 3 1 550 1 550 I 550 1 550 I 550 1 550 1 550 <>1L 0VS 1 © O*· U7 1 550 1 550 1 550 1 530 OES | 095' | ovs | 1 550 oss | out of the presea! Galvanizing step First cooling Cooling rate 'C / s 04 O 07 Ot O 07 07 ID 07 O 07 O 07 07 3 o ex a 07 a LD 07 07 • First soak Holding time so O o OOO oo © O © OO o O © © w 0 íS £ g £ •rf M aa ε w0 880 098 © <2D © g 850 850 088 870 © e 850 o ÍÍT <o 350 830 O 07 <0 088 880 870 870 870 So 0 U 6 Tabla 2-5 Calentamiento Vel. de calentamiento desde 650°C hasta temp. de calentamiento máx. C / s DG 'T -- <D 3 © OJ 04 OJ sí 04 Oí © CJ W7 ¡47 04 04 C4 c-i subrayado en negrita;

[74] O z 04 oí 04 07 04 £M íD C-4 Oí 04 © o04 O 07 n oí 07 07 07 oí iD 07 -O 07 07 DO 07 07 o 0. L LCRnn / Lznz / E / YILI

[75] Tabla Ms in the hot-dip galvanizing step “C | 607 or 407 | or 397 | CD O <<1 395 | un 363 | 365 | 375 | 375 1 $ ce | os» <5 372 | CÑ cn 223 1 1 Hot-dip galvanizing step | Third soak T Temp. time t . x maintenance sooc? oo ca 280 | 20 280 | 20 280 j 20 280 | 20 02 | 00£ 300 | 20 300 | 20 300 | 20 280 [ 20 280 5 20 290 | 20 02 | 062 390 | 330 400 | 330 380 | 330 370 | 330 390 | 330 400 5 330 Second cooling Cooling stop temperature °C s 5 o OO s un un un O CN O CN 100 100 250 100 081 250 200 OSL Alloy Temp.of alloy °C or U> 530 | 530 | io a > 500 | ? Or a * 530 | • Or a 500 I 530 | 580 | 520 | 520 | Second stirring Maintenance time s U3 U3 a CN a o· in a a a CN a a a a a a a a a a a a a E θ j oss | bone and 540 | 550 | 550 and bones 550 | 540 | O and C2 and oo and | bone | oss 540 | 550 ! bone | ovs 550 550 First cooling Cooling speed “C / s 3 co CN CN <n cn cn -5 CQ có 3 CO o CN un CN un <O *2 o un un CN OO CN Primer remojo Tiempo de mantenimiento s o O O O O o O O O o o o O C3 en o O 2 E a o CN CC 1 028 820 | 023 1 028 820 | O CD E O CO E 850 | 850 | I 098 860 | I 088 I 088 © 03 900 | 850 | 900 | Calentamiento Vel. de calentamiento desde 650°C hasta temp. de calentamiento máx. “C / s CN CN CO Oí <7 CN 49 cq CN CN «t Cí oi un o ÍN 7.1 £ ¡Ni ó Nt <N 5 3 3 3 s o·* <í a un lo OI un un un un un o un un ¢0 un un CJ. φ φ φ φ Μ Λ Ο ¡H d η Φ and L LCRnn / Lznz / Ε / ΥΙΛΙ

[76] A JIS No. 5 tensile test piece was taken from each of the steel sheets thus obtained in a direction perpendicular to the rolling direction and subjected to a tensile test based on JIS Z2241:2011 to measure tensile strength (TS) and total elongation (El). In addition, each test piece was tested by the JES T 1001 Hole Expansion Test Method of the Japan Iron and Steel Federation Standards to measure the hole expansion rate (λ). A test piece with a TS of 980 MPa or more and a T3χE1χλ°·5 / 1000 of 80 or more was considered good in terms of mechanical properties and preferable for use as an automotive part.

[77] Low-temperature toughness was evaluated using a Charpy impact test. A Charpy test piece was obtained by stacking and bolting a plurality of steel sheets together, confirming that there was no gap between the steel sheets, and then fabricating a test piece with 2 mm deep V-notches. The number of steel sheets stacked was set so that the thickness of the test piece after stacking was as close to 10 mm as possible. For example, if the thickness is 1.2 mm, eight sheets are stacked to give a test piece thickness of 9.6 mm. The stacked Charpy test piece was obtained using the width direction of the sheet as the long direction. Low-temperature toughness when the test temperature was between -50°C and +20°C and the ratio of L LCRnn / įZРZ / B / YIL impact absorption energy vE -50 / VE20 at -50°C and +20°C was 0.6o higher and was considered excellent (in Tables 3-1 to 3-3, very good). Conditions other than the above were based on JIS Z 2242: 2018.

[78] In addition, the steel sheets thus obtained were examined for microstructures. The procedures for examining the microstructures were those explained above.

[79] The results are shown in Tables 3-1 to 3-3. In Tables 3-1 to 3-3, GA means hot-dip galvanized, while GI means hot-dip galvanized without alloy treatment. The numerical ratio that satisfies formulas (1) and (2) in Tables 3-1 to 3-3 is obtained by following the measurement method explained earlier in this description to measure, in particular, 10 different particles of quenched martensite. l LfRnn / LZnZ / E / YIL en

[80] Table Observations ¡IT 1 εί 1 1 £j. comp. 1 E o U ΰΓ | Ej. comp. | uZ | Ej. comp. | | Ej. comp. | ω iZT UJ | Ej. comp, | E o ¡t | Ej. comp. | ül uT | Ej. comp. | ÜJ ¡IT LU ! Mechanical properties 1 v£W vEsú | Very good | | Very good | | Poor | | Poor | | Poor | I Very good | Very good | | Poor | | Very good | | Very good | | Very good | | Very good | I Very good I Poor Very good | Very good | | Very good | 1 Very good 1 | Very good | | Very good | o Ό Π3 (Λ e φ o sx 3? ro EO λ TS*El*D5 % / 1000 31 | 140 671 | 125 j 129 CG 45 CD O CM 16 | IM 23.1 19.2 [íOój 23 7 22.8 20.7 07 ΓΜ LLl 061 TS MPa 1028 i¿0t ό o 1003 1059 988 ÍM 1022 s 0ZLI 07 C5 <x>| 913. ! p057| wire CM OO o 816 986 o 1103 Microstructure I Number density that satisfies formulas (1) and (2) 1 only ! £3 1 « aa O ¡ °7 1 0.8 | 00 1………07………1 a eq 0.8 | > 1 80 O·7 | I 0-7 I Cainita » / / » 07 CM £ L„ A2 J C4 o CM 1......36.......1 1 AS 1 en <M ?M co 5 CN CM CM £ 45 03 CM CM CM Perlitat cementita % O O O O O O O O O o o O O O O a O O O Martensita fresca % 07 tM CM CM 04 o 5Z CM CM CM a> yes 04 04 m and ΙΛ - or? Tempered martensite % 30 | 34 |<n o> o> 03 CQ o 03 07 i» o in 40 | ÍM 03 CM <□ o 50 IO Retained authenticity % $2 TI' - CM CM with CM o -o in a X / a 07 Ό 07 CM 03 is 03 33 in 33 04 03 in CD S 04 CM C3 5 with tr? U7> C5 CM Coating 1 GA | 1 GA ¡ vo I GA | 1 ga ¡ ga V9 1 GA ¡5 GA I GA ¡ I GA ¡ GA 1 1 ga J <5 ga 1 ga ¡ O 1 ga ¡ I ga ¡ Type of steel < < < < < < < < m 02 02 m 02 02 a ω OQ o ó 2 - CM Q00-o CM<r in o £ CD © CM Λ! «í M 0 R ü £ •H U C & > c M 0' 0 £ $ The T3 >1 £ « gold d F uc L LCRnn / Lznz / E / YILI «TQBJG

[18] cM Observations ¡ Ex. comp. | Ui ¡ Ex. comp. j UJ | -duJOO la [ Ex. comp. ] | Ex. comp. | ΰΓ ÜJ ir LU ΰΓ tu UJ uT uT id f±í LÜ Mechanical properties | a W 1 Poor 1 1 Very good 1 | Poor | | Very good | 1 Very good 1 1 Poor 1 Poor | | Poor | | Very good | | Very good | I Very good I Very good 1 Very good 1 Very good | | Very good | | Very good | | Very good | | Very good | | Very good | Very good 0 T3 TO <Λ C Φ CL σ >r £ £ TSsEI» / ..'5 / mo 1 π Ϊ có 145 | 1 m 1 o O 1 ιει ¡ 51 1 «o 1 126 IF £21 1__________.........i CM I 821 | 6£l 127 | 126 | $ 881 o CM § íñ CM co 07 © 10 Me 5 10 5 CM 07 07 O © o ω yes CO £ 1 165 -r su 1 16.5 | 12.2 9" I w·5 07 ιει 1 18.2 I 4« 5 I 18.0 10 I 150 16.2 TS MPa CM Yes 1207 07 CM 1279 1314 o CM 10 CM 1377 s CM m CM 1228 1267 £0Zl 9611 1479 1486 co ΜΓ 1461 1567 1554 | 0.7 | 1 0.8 | 0.6 Bainite % o cm CM 04 n ΓΜ oa CM 07 CM © CM CM cí en CM 04 CN © 0- PeritB+ cementite % OOOO o O © © © oooo O © © © CM7 © CM δ % Martensite £1 CM r- CM Xt 07 07 Tempered martensite % O <0<g 10 Ό CM © R m> 07 © © 5 m CM CM © a 45 CM m5 S 10 © m Retained autenite % C- - Γ4 O m c- CM © 00 F- cü CM CM CM <M CM L0 CM Ferrita % £ s O CM CM CM oq «? ά CM O O o © O in ñecubrímtento 1 GA i I GA I 1 vs 1 1 GA 1 1 GA 1 1........GA........1 1 GAJ 1 GA | I VG I 1 1 I GA ] <3 I GA I <3 1 GA 1 <5 «í Φ O Tipo de acero Q UJ Ld IXl U1 UJ UJ UJ UJ u_ Ll Ll O O X X - -7 No. CM CM CM o-j ÍM CM J0 tM CM 04 © CM © R cm co <0 co in O co co «? 07 o. ñ & W Φ a A$ M Φ <u *4 g>$ a ñ & G τ' <9 >tas M « the L LCRnn / Lznz / E / YILI

[82] Tabla Observations LO ϊΞΓ uT tu ÜT LLJ LU LL LU ilT LU ¿7 Ex. comp. | Ex. comp. | I Ex. comp. | | Ex. comp. | I Ex. comp. I | Ex. comp. | I Mechanical properties ! Very good j Very good Very good j Very good j Very good | Very good j Very good j : Very good | Very good Very good | Very good | Very good j Very good ¡ Very good j Very good Very good j Poor ¡ Poor | Poor j Very good j í Formability by pressing | w O * d tn «0 o 5 & CN co s O 00 co 60 Os 851 123 | un Nf NT -•¿ LO om co s CN LO S cr cc en 5 O < LA 5 Í2 CÑ S © LZí I o tn | CN no | N 25.6 1545 ) cof 1412 1367 1 1382 8 | Microstructure I Numerical density satisfying formulas (1) and (2} O LO 0.6 O 0.8 ¡ O o 08 | £GO 0.8 0.8 0.8 ¡ 0.8 0.7 j I 0.6 j 90 I 40 0.8 ¡ O 05 ¡ Bainite % CN O CN LO m Ό cn CN cn CN CN O oog 0 CN — σ- CN CN Perlite* cementite % O <=> oo ODO => o C3 0 O ooooo O GJ o Fresh martensite % <5 un cn <r <r CN CN cn -o N2 a a u~i «43j a Martensita templada % cn cn IO cn rñ O cn m s co un οχ ΙΛ S <n tO 2 LO OD o co S Q LO un in cn o~ Autenita retenida % CN - - CN rw o o CN CN CN CN NT NT CN O cn n- esc a Ferrits % co cn CD en en O cn LO LO £ £ CN CN nj a o o o LO cn g cn o a Recubrimiento I GA j O GA O GA S I GA ¡ S < O o 1 GA 1 <3 GA o GA GA I GA | < 1.......GA.......! 1 GA ¡ Tipo de acero r£ 2 o o 0. 0. σ σ ir ΙΛ >Not with cn Nt 5 5 5 § 5 o iñ LO cn LO ΙΛ JO a co LO in «j £ «j £4 ΰ Φ * U $ 3 th & § í? hsco •H ac & > § >T The U L LCRnn / Lznz / Ε / ΥΙΛΙ

[83] Comparative Examples 3 and 21 had second soak treatment temperatures in the hot-dip galvanizing step of less than 480°C and more than 600°C, respectively. As a result, the quenched martensite ratios satisfying formulas (1) and (2) became less than 0.2, and the low-temperature toughness was poor. Comparative Examples 4 and 23 had surface plate temperatures at the end of the secondary cooling of the continuous casting step of less than 500°C and more than 1100°C, respectively. As a result, the quenched martensite ratios satisfying formulas (1) and (2) became less than 0.2, and the low-temperature toughness was poor. Comparative Examples 5 and 14 had pouring speeds in the continuous casting step of less than 0.4 m / min and more than 3.0 m / min, respectively.As a result, the quenched martensite ratios satisfying formulas (1) and (2) fell below 0.2, and the low-temperature toughness was poor. Comparative Examples 7, 12, 17, and 26 had second-cooling stop temperatures in the hot-dip galvanizing pass exceeding Ms-50°C and a quenched martensite ratio below 5%. Furthermore, Comparative Example 26 had a fresh martensite ratio exceeding 10%. Consequently, Comparative Examples 7, 12, and 17 had tensile strengths below 980 MPa. Comparative Example 26. L LCRnn / Lznz / E / YIL contained large amounts of hard fresh martensite, so while tensile strength was ensured, press formability and low-temperature toughness became poor. Comparative Example 8 had a third soaking treatment temperature in the hot-dip galvanizing step of less than 200°C. As a result, the fresh martensite content increased by more than 10%, and press formability and low-temperature toughness were poor.

[84] Comparative Example 13 had an average cooling rate for the first quench in the hot-dip galvanizing step of less than 10°C / s. As a result, ferrite was converted by more than 50%, total pearlite and cementite were converted by more than 5%, and press formability was poor. Comparative Example 27 had a holding time for the second soak treatment in the hot-dip galvanizing step of less than 80 seconds. As a result, the ratio of quenched martensite satisfying formulas (1) and (2) was less than 0.2, and low-temperature toughness was poor. Comparative Example 28 had a holding time for the third soak treatment in the hot-dip galvanizing step of less than 5 seconds. As a result, fresh martensite was converted by more than 10%, and both press formability and low-temperature toughness were poor.Comparative Examples 55 to 60 had chemical compositions no. L LCRnn / įZРZ� / B / YIL controlled within predetermined intervals, so the desired metallic structure was not obtained and the formability by pressing and / or low temperature toughness were poor.

[85] In contrast, the hot-dip galvanized steel sheets of the examples have a tensile strength of 980 MPa or more and TSxElxÁ°-5 / 1000 of 80 or more and also have a VE-50 / VE20 of 0.6 or more, so they are known to be excellent in terms of press formability and low-temperature toughness.

[86] Example B In this example, the inventors studied the presence or absence of a specific soaking treatment. First, they prepared a plate with the chemical composition shown in Tables 1-1 and 1-2. Then, as shown in Table 4, they performed the first gradual cooling to eliminate the second soaking treatment. Aside from this, the same procedure as in Example A was followed to obtain a hot-dip galvanized steel sheet. The steel structure and mechanical properties of the resulting hot-dip galvanized steel sheet were investigated using methods similar to those in Example A. The results are shown in Table 5. In the different soaking treatments shown in Table 4, the temperature was maintained within the temperature range shown in Table 4 ± 10°C. L LCRnn / ίZΖΠZ / Β / YΙΛ

[87] Table l LCRnn / Lznz / E / YiAi

[89] As can be seen from the results in Table 5, if the first cooling was done by gradual cooling to eliminate the second soaking treatment, the desired metallic structure could not be obtained, the quenched martensite ratio satisfying formulas (1) and (2) became less than 2.0, and the press formability and low-temperature toughness were poor.

[90] Example C In this example, the inventors similarly studied the relationship between the soaking and coating treatments. First, they prepared a plate with the chemical composition shown in Tables 1-1 and 1-2. Then, as shown in Table 6, they performed the same procedure as in Example A to obtain a hot-dip galvanized steel sheet, except that they carried out the coating and alloying treatments not after the second soaking treatment, but after the third. The steel structure and mechanical properties of the resulting hot-dip galvanized steel sheet were investigated using methods similar to those in Example A. The results are shown in Table 7. In the different soaking treatments shown in Table 6, the temperature was maintained within the temperature range shown in Table 6 ± 10°C. L LCRnn / Lznz / E / YIL

[91] Table l LCRnn / Lznz / E / YiAi

[93] As can be seen from the results in Table 7, if the coating and alloying treatment was carried out after the third soaking treatment, the desired metallic structures could not be obtained and the formability by pressing and the low temperature toughness were poor. ​< / x>

Claims

1. A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized coating on at least one surface of the base steel sheet, wherein the base steel sheet has a chemical composition comprising, in % by mass, C: 0.050% to 0.350%, Si: 0.10% to 2.50%, Mn: 1.00% to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001% to 1.500%, N: 0.0100% or less, O: 0.0100% or less, Ti: 0% to 0.200%, B: 0% to 0.0100%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1. 00%, Sn: 0% to 1.00%, Sb: 0% to 1.00%, Ca: 0% to 0.0100% Mg: 0% to 0.0100% Ce: 0% to 0.0100% Zr: 0% to 0.0100% La: 0% to 0.0100% Hf: 0% to 0.0100% Bi: 0% to 0.0100% REM other than Ce and La: to 0.0100% and L LCRnn / Lznz / E / YIL a residue of Fe and impurities a steel microstructure in a thickness range of 1 / 8 to 3 / 8 thickness centered around a position 1 / 4 of the thickness from a surface of the base steel sheet contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 0% to 30%, quenched martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, when structures remain, the remaining structures consist of bainite, and a numerical ratio of quenched martensite with a concentration profile of Mn that satisfies the following formulas (1) and (2) is 0.2 or more with respect to the total number of quenched martensite: [Mn]b / [Mn]a >1.2 ... (1) [Mn]a / [Mn] <2.0 ...(2) wherein [Mn] is the Mn content (% by mass) in the base steel sheet, [Mn]a is the average concentration of Mn (% by mass) in the quenched martensite and [Mn]b is the concentration of Mn (% by mass) at the interfaces of the different phases of the quenched martensite and ferrite and the bainite phase.

2. The hot-dip galvanized steel sheet according to claim 1, wherein the steel microstructure further contains, by volume fraction, retained austenite: 6% to 30%.

3. A method for producing hot-dip galvanized steel sheet according to claim 1 or 2, comprising a continuous casting step for continuously casting a plate having the chemical composition according to claim 1, a hot rolling step for hot rolling the cast plate, and a hot-dip galvanizing step for hot galvanizing the resulting steel sheet, wherein (A) the continuous casting step satisfies the conditions of the following (A1) and (A2): (A1) the surface temperature of the plate at the time of the end of a secondary cooling is from 500 to 1100°C and (A2) a casting rate is from 0.4 to 3.0 m / s, and (B) the hot-dip galvanizing step comprises heating the steel sheet to soak it first, first cooling and then soaking the first soaked steel sheet a second time, immersing the second soaked steel sheet in a hot-dip galvanizing bath, cooling the coated steel sheet a second time, and heating the cooled steel sheet a second time and then soaking it a third time, and further satisfies the following conditions of (B1) to (B6): (B1) in heating the steel sheet before the first soak, the average heating rate from 650°C to a maximum heating temperature of Acl + 30°C or more and 950°C or less is 0.5°C / s at 10.(B2) the steel sheet is held at the maximum heating temperature for 1 second to 1000 seconds (first soak), (B3) an average cooling rate in a temperature range of 700 to 600°C in the first cooling is 10 to 100°C / s, (B4) the first cooled steel sheet is held in a range of 480 to 600°C for 80 seconds to 500 seconds (second soak), (B5) the second cooling is carried out to Ms50°C or less, and (B6) the second steel sheet is heated to a temperature region of 200 to 420°C, then held in the temperature region for 5 to 500 seconds (third soak).