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 and a hot-dip galvanized coating on at least one surface of the base steel sheet, wherein the base steel sheet has a predetermined chemical composition and contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 6% to 30%, bainite: 5% or more, quenched martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, a number density of quenched martensite with an equivalent circle diameter of 5.0 µm or more is 20 / 1000 µm2 or less, and an area ratio of fresh martensite with an equivalent circle diameter of 2.0 µm or more after imparting 5% plastic deformation is 10% or less, and a method for producing the same.

Description

HOT-DIP GALVANIZED STEEL SHEET AND METHOD FOR PRODUCING THE SAME 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 types of workability necessary for forming parts. Specifically, from the point of view of press formability, excellent elongation (total elongation in tensile test: El) and stretch flanging ability (hole expansion rate: A) of the steel sheet are required. [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, TRIE (transformation-induced plasticity) steel sheet is known, which utilizes the transformation-induced plasticity of retained austenite. [5] PTLs 1 to 3 describe the technique related to controlled high-strength TRIP steel sheet in structural component fractions at predetermined intervals and improved elongation and hole expansion rates. In addition, 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 pm 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] Typically, to produce hot-dip galvanized steel sheet in a continuous annealing furnace, the steel sheet must be heated to the reverse transformation temperature region (> Acl). Then, halfway through the process, it is cooled to room temperature before being immersed in a hot-dip galvanizing bath at approximately 460°C. Alternatively, after cooling to room temperature, the steel sheet must be reheated to the hot-dip galvanizing bath temperature and then immersed in the coating bath. Furthermore, to produce hot-dip galvanized steel sheet, an alloying treatment is required after the steel sheet has been immersed in the coating bath.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 hot-dip galvanized steel sheet comprising cooling the steel sheet after coating and alloying and reheating it to temper the martensite. nfroonn / Lznz / E / YiAi 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] [PTL 5] [PTL 6] WO 2017 / 179372 WO 2014 / 020640 Unexamined Japanese Patent Publication No. 2013-144830 [PTL 7] [PTL 8] [PTL 9] [PTL 10] WO 2016 / 113789 WO 2016 / 113788 WO 2016 / 171237 Unexamined Japanese Patent Publication no. 2017-48412 nfroonn / Lznz / E / YiAi Summary of the invention Technical problem

[10] On the other hand, hot-dip galvanized steel sheet used for automotive components is required not only to be press-formable but also to resist brittle fracture upon deformation following a collision. Specifically, for steel sheet used in automotive components, it is not the toughness before press forming, but rather the toughness after the introduction of plastic deformation due to press forming, that must be excellent. However, in the prior art, the improvement of toughness after the introduction of plastic deformation has not necessarily been sufficiently studied. For this reason, there is still room for improvement in the properties of hot-dip galvanized steel sheet, particularly hot-dip galvanized steel sheet used for automotive components.

[11] An object of the present invention is to provide a hot-dip galvanized steel sheet with excellent press formability and toughness after press forming and a method for producing the same. 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 is then reheated and held isothermally to adequately quench the martensite and stabilize the retained austenite. Through this heat treatment, the martensite is no longer excessively quenched by the coating or coating and alloying, thus improving the balance of strength and ductility. (ii) Originally, quenched martensite is an excellent structure in terms of strength and toughness, but if it is large, it acts as a toughness-degrading factor. The inventors studied effective means of decreasing the amount of quenched martensite in the rough. As a result, they discovered that it is effective to hold the steel isothermally within a suitable temperature range before immersing it in the coating bath and the subsequent alloying treatment, and to partially advance the bainite transformation. By holding it isothermally, the untransformed austenite that will later become martensite is divided by the bainite. By dividing the austenite by the bainite, it is possible to reduce the size of the martensite formed from that austenite and, consequently, decrease the coarse quenched martensite in the final structure. As a result, toughness is greatly improved. (iii) To improve toughness after the introduction of plastic deformation, the metal structure at the time of plastic deformation should not contain a large amount of hard, brittle fresh martensite (unquenched martensite, i.e., martensite that does not contain carbides). To reduce such fresh martensite, in the production steps from hot rolling to continuous hot-dip galvanizing, it has been found effective to limit the production conditions so as to suppress the dispersion of Mn between ferrite and austenite, and then perform a continuous hot-dip galvanizing heat treatment that satisfies the above (i) and (ii).The details are not necessarily clear, but it is assumed that the source of fresh martensite formation seen in the metal structure at the time of plastic deformation is not only the fresh martensite present before the introduction of plastic deformation, but also unstable retained austenite that transforms into martensite through strain-induced transformation due to the introduction of slight plastic deformation. It is believed that such unstable retained austenite forms readily in the Mn-concentrated portion where it is difficult to proceed with austenite (austenite stabilization by carbon atom dispersion) in a continuous hot-dip coating step.The concentrated Mn portion can be considered to have originally been a segregated region formed during casting. However, if the steel subsequently remains in the ferrite-austenite two-phase temperature region, an alloy disperses between the two phases, and the concentrated Mn portion becomes more pronounced. In the two-phase temperature region present during the steps from hot rolling to continuous hot-dip galvanizing, controlling production conditions to minimize Mn dispersion reduces the formation of the concentrated Mn portion. Furthermore, the amount of unstable retained austenite that readily forms within the concentrated Mn portion can be decreased.As a result, since the amount of martensite resulting from the strain-induced transformation decreases from the unstable retained austenite at the time of the introduction of plastic deformation, it is assumed that the fresh martensite contained in the metallic structure at the time of the introduction of plastic deformation is reduced.

[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, C: 0.100% to 0.350%, Yes: 0.50% 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, 5 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%, 10 Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1.00%, B: 0% to 0.0100%, 15 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 residue of Fe and impurities, A steel microstructure in a thickness range of 1 / 8 to 3 / 8 of 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: 6% to 30%, bainite: 5% or more, quenched martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, a number density of quenched martensite with a circle equivalent diameter of 5.0 pm or more is 20 / 1000 pm² or less, and an area ratio of fresh martensite with a circle equivalent diameter of 2.0 pm or more after imparting 5% plastic strain is 10% or less. (2) A method for producing hot-dip galvanized steel sheet in accordance with (1), comprising: (A) a hot rolling step comprising heating a plate having the chemical composition in accordance with (1) and finishing rolling the heated plate by means of a plurality of rolling stands and then winding it, wherein the hot rolling step satisfies the conditions of the following (A1) to (A3): nfroonn / Lznz / E / YiAi (Al) an average heating rate from Acl to Acl + 30°C at the time of heating the plate is 2 to 50°C / min, (A2) in the finishing lamination by the plurality of lamination supports, a lamination reduction per pass is 37% or less, an entry side temperature of the first pass is 1000°C or more, an exit side temperature of the final pass is 900°C or more, an average time between supports is 0.20 seconds or more, and the time from the end of the finish rolling to the start of cooling is 1 second or more, and (A3) a winding temperature is 450 to 680°C, and (B) a hot-dip galvanizing step comprising heating the obtained steel sheet to first soak it, first cooling and then second soaking of the first soaked steel sheet, immersing the second soaked steel sheet in a hot-dip galvanizing bath, second cooling of the coated steel sheet, and heating the second cooled steel sheet and then soaking it a third time, wherein the hot-dip galvanizing step satisfies the following conditions of (B1) to (B6):. (Bl) In the heating of the steel sheet before the first soak, an average heating rate of nfroonn / Lznz / E / YiAi (B1) The cooling rate from Acl to Acl + 30°C is 0.5°C / s more, (B2) the steel sheet is held at a maximum heating temperature of Acl°C + 30°C to 950°C 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 250 to 480°C for 80 seconds to 500 seconds (second soak), (B5) the second cooling is carried out at 150°C or less, and (B6) the second cooled steel sheet is heated to a temperature region of 300 to 420°C, then held in the temperature region for 100 to 1000 seconds (third soak). nfroonn / Lznz / E / YiAi Advantageous effects of the invention

[14] In accordance with the present invention, it is possible to obtain hot-dip galvanized steel sheet of high strength excellent formability by pressing, specifically hot-dip galvanized steel sheet of high strength excellent ductility and hole expansion capacity and additional hardness after the introduction of plastic deformation. Brief description of the drawings

[15] Figure 1 shows a reference view of a secondary electron SEM image. 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: 0.100% to 0.350%, Yes: 0.50% 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%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, nfroonn / Lznz / E / YiAi Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, 5W: 0% to 1. 00%, B: 0% to 0.0100%, Sn: 0% to 1.00%, Sb: 0% to 1.00%, Ca: 0% to 0.0100%, 10 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%, 15 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 range of 1 / 8 A 3 / 8-thickness layer centered around a position 1 / 4 of the thickness from a base steel sheet surface contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 6% to 30%, bainite: 5% or more, quenched martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, a numerical density of quenched martensite with a circle equivalent diameter of 5.0 pm or more is 20 / 1000 pm2 or less, and an area ratio of fresh martensite with a circle equivalent diameter of 2.0 pm or more after imparting 5% plastic deformation is 10% or less. nfroonn / Lznz / E / YiAi

[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 mean both the lower and upper limits of the numerical values ​​described above and below.

[18] C: 0.100% to 0.350% Carbon (C) is an essential element for ensuring the strength of steel sheets. If it is less than 0.100%, the required high strength cannot be achieved, and therefore the carbon content is 0.100% or higher. The carbon content can be 0.120% or higher, or even 0.150% 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. nfroonn / Lznz / E / YiAi

[19] Yes: 0.50% 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 0.50 to 2.50%. The Si content can be 0.60% or more, or 0.80% or more, and / or it can be 2.40% or less, 2.20% or less, or 2.00% or less.

[20] Mn: 1.00% to 3.50% Manganese (Mn) is a powerful austenite stabilizing element and an effective element for increasing the strength of steel sheets. Excessive addition leads to a deterioration of weldability or low-temperature toughness. Therefore, the content is 1.00 to 3.50%. The Mn content can be 1.20% or more, or 1.50% or more, and / or it can be 3.40% or less, 3.20% or less, or 3.00% or less.

[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, but excessive addition causes weldability and toughness to deteriorate. 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 the 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 impurity found 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 significantly 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, a lower limit of 0.001% is preferable from an economic standpoint.

[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 impurity. If its content exceeds 0.0100%, it forms coarse nitrides in the steel, impairing its bending and hole expansion capabilities. 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 (O) is an element present as an impurity. If its content exceeds 0.0100%, it forms coarse oxides in the steel, leading to a deterioration of bending capacity 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 a production cost standpoint, 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%, W: 0% to 1.00%, B: 0% to 0.0100%, 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), tungsten (W), boron (B), 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%, W: 0% to 1.00%, B: 0% to 0.0100%, Sn: 0% to 1.00%, Sb: 0% to 1.00%. Elements may also be 0.005% or more or 0.010% or more. In particular, the B content 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 nfroonn / Lznz / E / YiAi 0.0100%, and REM other than Ce and La: 0% to 0.0100% Calcium (Ca), magnesium (Mg), cerium (Ce), zirconium (Zr), lanthanum (La), hafnium (Hf), and rare earth elements (REMs) other than Ce and La are elements that contribute to the microdispersion of inclusions in steel. Bismuth (Bi) is an element that reduces the microsegregation 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 raw materials such as ore and scrap, when the base steel sheet is industrially produced, and encompass all components not intentionally added to the base steel sheet according to the embodiment of the present invention. Furthermore, the impurities encompass all elements other than the constituents explained above (Fe / Izn / E / YiAi) 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.

[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 hole 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 can also be 15% or more or 20% or more. For example, the tempered martensite content can be a volume fraction of 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 has a brittle structure, thus becoming a point of origin for fracture during plastic deformation and causing a local reduction in the ductility 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: 6% to 30% Retained austenite improves the ductility of steel sheets due to the TRIP effect of the transformation to martensite caused by the work-induced transformation during deformation. For this reason, it is contained in a volume fraction of 6% or more. It can also be contained in 8% or more, or even 10% or more. The greater the retained austenite, the greater the increase in elongation, but to obtain a large amount of retained austenite, carbon 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 also be 25% or less, or even 20% or less.

[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] Bainite: 5% or more In the present invention, to suppress the formation of rough-hardened martensite, the bainite transformation is partially advanced before the martensite transformation. Therefore, to achieve this effect, the bainite content must be a volume fraction of 5% or more. The bainite content may also be a volume fraction of 8% or more, or 12% or more. The upper limit for the bainite content is not specifically defined, but, for example, it may be a volume fraction of 50% or less, 40% or less, or 35% or less.

[37] Total number density of tempered martensite with circle equivalent diameter of 5.0 pm or more than 20 / 1000 pm2 or less To improve toughness after the introduction of plastic deformation, the number density of coarse quenched martensite with an equivalent circle diameter of 5.0 pm or more is limited to 20 / 1000 pm² or less. Preferably, it is 15 / 1000 pm² or less, or 10 / 1000 pm² or less. The number density may also be 0 / 1000 pm² or 1 / 1000 pm² or more.

[38] Ratio of fresh martensite area to equivalent circle diameter of 2.0 pm or more after imparting 5% plastic strain: 10% or less For toughness after plastic deformation, it is important to minimize the amount of fresh martensite present. In particular, coarse fresh martensite with an equivalent circular diameter greater than 2.0 pm has a significant detrimental effect. Therefore, in the steel sheet according to the embodiment of the present invention, the proportion of fresh martensite with an equivalent circular diameter of 2.0 pm or more after 5% plastic deformation is limited to 10% or less. For example, the proportion of fresh martensite can be 8% or less, or 6% or less. Furthermore, the proportion of fresh martensite can be 0%, or it can be 1% or more.

[39] The fractions of steel structures of hot-dip galvanized steel sheet are evaluated by the SEM-EBSD method (electron backscatter diffraction method) and SEM secondary electron imaging observation.

[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 and then electropolished. Next, in one or more observation fields ranging from 1 / 8 to 3 / 8 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 secondary electron imaging 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 inferior grain structure and several cementite variants—more specifically, two or more variants—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 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] The numerical density of tempered martensite with an equivalent circle diameter of 5.0 pm or more is determined by calculating the equivalent circle diameters of tempered martensite in the fields observed by image processing for tempered martensite identified by the above EBSD observation and SEM observation and determining the frequency of presence of tempered martensite with an equivalent circle diameter of 5.0 pm or more.

[44] The ratio of fresh martensite area to an equivalent circle diameter of 2.0 pm or more after the introduction of plastic deformation is evaluated by the following method. First, a tensile test piece is taken using the width direction of the steel sheet as the long direction of the test piece and pre-stressed using a tensile testing machine so that the amount of plastic deformation is 5%. From the center of the parallel portion of the pre-stretched test piece, a sample is taken for microstructure observation so that the cross-section of thickness parallel to the rolling direction of the steel sheet becomes the observed surface. The observed surface is machine polished to a mirror finish, then electropolished.After that, the method mentioned above is used for EBSD observation and SEM observation to identify fresh martensite, then image processing is used to measure the area ratio of fresh martensite with a circle equivalent diameter of 2.0 pm or more.

[45] 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 Al and other additive elements besides Zn. Furthermore, the amount of coating deposition is not particularly limited and may be a general amount.

[46] 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.

[47] The method for producing hot-dip galvanized steel sheet comprises (A) a hot rolling step comprising heating a plate having the same chemical composition as the chemical composition explained above in relation to the base steel sheet and finishing rolling the heated plate by means of a plurality of rolling stands and then coiling it, wherein the hot rolling step satisfies the conditions of the following (A1) to (A3): (A1) an average heating rate from Acl to Acl + 30°C at the time of heating the plate is 2 to 50°C / min, (A2) in the finishing lamination by the plurality of lamination supports, a lamination reduction per pass is 37% or less, an entry side temperature of the first pass is 1000°C or more, an exit side temperature of the final pass is 900°C or more, an average time between supports is 0.20 seconds or more, and the time from the end of the final rolling to the start of cooling is 1 second or more, and (A3) a winding temperature is 450 to 680°C, and (B) a hot-dip galvanizing step comprising heating the obtained steel sheet to soak it first, first cooling and then secondly soaking the first soaked steel sheet, immersing the second soaked steel sheet in a hot-dip galvanizing bath, second cooling the coated steel sheet, and heating the second cooled steel sheet and then soaking it a third time, wherein the hot-dip galvanizing step satisfies the following conditions of (B1) to (B6):. (B1) In the heating of the steel sheet before the first soak, an average heating rate from Acl to Acl + 30°C is 0.5°C / s or more, (B2) the steel sheet is held at a maximum heating temperature of Acl°C + 30°C to 950°C for 1 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 250 to 480°C for 80 to 500 seconds (second soak), (B5) the second cooling is carried out at 150°C or less, and (B6) the second-cooled steel sheet is heated to a temperature range of 300 to 420°C, then held in the temperature of the region for 100 to 1000 seconds (third soak).

[48] ​​The method for producing hot-dip galvanized steel sheet will now be explained in detail. nfroonn / Lznz / E / YiAi

[49] (A) Hot rolling pass First, in the hot rolling step, a plate with the same chemical composition as the base steel sheet (as previously described) is heated. The heating temperature of the plate is not strictly limited, but 1150°C or higher is generally preferable for sufficient dissolution of borides, carbides, etc. The steel plate used is preferably produced by continuous casting for production capacity purposes, but it can also be produced by ingot casting or thin-plate casting.

[50] Average heating rate from Acl to Acl + 30°C: 2 to 50°C / min In this method, the average heating rate from Acl to Acl + 30°C during plate heating is controlled from 2 to 50°C / min. In the two-phase temperature region (austenite and ferrite) just above Acl, the alloying elements disperse particularly readily between the austenite and ferrite. For this reason, when reheating the plate, it is heated in this temperature region at a relatively rapid average rate of 2°C / min or more. If the heating rate falls below 2°C / min, the amount of coarse fresh martensite increases in the final structure after plastic deformation. On the other hand, if rapid heating at a rate exceeding 50°C / min is performed, the temperature distribution along the plate thickness becomes uneven, leading to thermal stress and sometimes resulting in heat deformation and other defects in the plate.For example, the above-average heating rate can be 4°C / min or more and / or it can be 40°C / min or less, 30°C / min or less, 20°C / min or less, or 10°C / min or less. The Acl point is calculated using the following formula: The mass percentage of the elements is entered for the element symbols in the following formula. For elements not contained, 0% by mass is entered. Acl (°C) = 723 a 10.7 x Mn-16.9 x Ni + 29.1 x Si + 16.9 x Cr Furthermore, in the present invention, the average heating rate from Acl to Acl + 30°C at the time of heating the plate means the value obtained by dividing the difference from Acl to Acl + 30°C, i.e., 30°C, by the time elapsed from Acl to Acl + 30°C.

[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 achieve a total rolling reduction of 60% or more at 1050°C or higher. If the total rolling reduction is less than 60%, recrystallization during hot rolling becomes insufficient, sometimes leading to irregularities in the structure of the hot-rolled sheet. The total rolling reduction above this point can be, for example, 90% or less. nfroonn / Lznz / E / YiAi

[52] Finishing laminate by a plurality of laminating supports The finishing roll is performed within a range that satisfies the following conditions: maximum roll reduction per pass of 37% or less, entry-side temperature of the first pass of 1000°C or higher, exit-side temperature of the final pass of 900°C or higher, average time between supports of 0.20 seconds or higher, and time from the end of the finishing roll to the start of cooling of 1 second or higher. In the finishing roll, the greater the strain energy accumulated in the austenite, the more readily the high-temperature ferrite transformation occurs after the end of the finishing roll. The lower the ferrite transformation temperature, the more the dispersion of alloying elements, particularly Mn, that occurs between the ferrite and austenite can be suppressed.Therefore, to reduce the strain energy accumulated in the austenite, the finishing roll is performed within a range that satisfies the above requirements. For example, the maximum reduction per roll can be 30% or less, 25% or less, 20% or less, and / or 5% or more. The initial entry-side temperature of the roll can be 1100°C or less. The exit-side temperature of the final roll can be 1000°C or less, or 990°C or less. The average time between supports can be 0.50 seconds or more, and / or 10 seconds or less. The time from the end of the finishing roll to the start of cooling can be 2 seconds or more, 3 seconds or more, and / or 10 seconds or less.

[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 is above 680°C, ferrite transformation occurs more readily at this high temperature, so the dispersion of alloying elements, particularly manganese, easily occurs between the ferrite and austenite. The rolling temperature can be 500°C or higher, 650°C or lower, or 600°C or lower.

[54] In the present method, the hot-rolled steel sheet obtained (hot-rolled coil) may be pickled or otherwise treated as required. The hot-rolled coil may be pickled by any ordinary method. In addition, the hot-rolled coil may be subjected to surface hardening to correct its shape and improve its pickling capacity.

[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 reduction in thickness is preferably 25% or more, or 30% or more. On the other hand, since excessive 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 nfroonn / Lznz / E / YiAi In this method, after the hot-rolling step, the resulting steel sheet is coated in a hot-dip galvanizing step. In the hot-dip galvanizing step, the steel sheet is first heated and then undergoes an initial soaking. Although not strictly limited to this, during heating the steel sheet, the average heating rate from 600°C to Acl is preferably limited to, for example, 10.0°C / s or less. If the average heating rate exceeds 10.0°C / s, ferrite recrystallization does not progress sufficiently, and the steel sheet may sometimes deteriorate due to elongation. The average heating rate can also be 6.0°C / s or less. The lower limit for the average heating rate is not strictly restricted but, for example, can be 1.0°C / s or more.In the present invention, the average heating rate from 600°C to Acl means a value obtained by dividing the difference between 600°C and Acl by the time elapsed from 600°C to Acl. nfroonn / Lznz / E / YiAi

[57] Average heating rate from Acl to Acl + 30°C: 0.5°C / so more The average heating rate from Acl to Acl + 30°C during the heating of the steel sheet is limited to 0.5°C / s or more. If the average heating rate from Acl to Acl + 30°C falls below 0.5°C / s, the dispersion of Mη between the ferrite and austenite becomes significant, resulting in an increased amount of coarse fresh martensite in the final structure after plastic deformation. This average heating rate can be 1.0°C / s or more. The upper limit for the average heating rate is not specifically defined, but, for example, it could be 10.0°C / s or less. In the present invention, the average heating rate from Acl to Acl + 30°C at the time of heating the steel sheet means a value obtained by dividing the difference from Acl to Acl + 30°C, i.e., 30°C, by the time elapsed from Acl to Acl + 30°C.

[58] 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 toughness deterioration 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 should be at least 1 second or more. Preferably, it should be 30 seconds or more, or 60 seconds or more. On the other hand, if the soaking time is too long, productivity is impaired, 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 it within a predetermined range of ±20°C, preferably ±10°C, within a range that does not exceed the upper and lower limits prescribed for each soaking treatment.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, with the temperature intervals prescribed in the soaking treatments not included in the first, second, and third soaking treatments according to the embodiment of the present invention. nfroonn / Lznz / E / YiAi

[59] 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 is cooled by the first quench. The quenching stop temperature is between 480°C and 600°C, which becomes the subsequent second soaking 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 20°C / s or less. It can also be 80°C / s or 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.

[60] Second soaking treatment: holding at a range of 250°C to 480°C for 80 to 500 seconds The second impregnation treatment is performed by holding the steel sheet at a temperature between 250°C and 480°C for 80 to 500 seconds to partially advance the bainite transformation. Due to this heat treatment, the untransformed austenite, which then becomes martensite, is separated by the bainite. Therefore, in the final structure, the coarse tempered martensite is reduced, thus improving toughness after plastic deformation. The temperature of the second soaking treatment can be 280°C or higher and 450°C or lower. Furthermore, the holding time can be 100 seconds or more and 400 seconds or less.In this regard, even if the second soaking treatment is simply carried out properly, if the dispersion of Mn between the ferrite and austenite is not sufficiently suppressed from the hot rolling step to the hot-dip galvanizing step, it will not be possible to reduce the amount of unstable retained austenite that readily forms in the concentrated Mn part and, as a result, in the structure after plastic deformation, the amount of coarse fresh martensite will increase and the toughness will decrease.Therefore, in the method for producing hot-dip galvanized steel sheet according to the embodiment of the present invention, to improve toughness after the introduction of plastic deformation, it is important to satisfy the conditions of (A1) to (A3) explained above in the hot rolling step while properly performing the second soaking treatment in the hot-dip galvanizing step.

[61] After the second soaking treatment, the steel sheet is immersed in a hot-dip galvanizing bath.The temperature of the steel sheet at this stage has little effect on its performance. However, 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 step for reheating 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 470°C, and the immersion time can be 5 seconds or less. The coating bath is preferably one containing 0.08 to 0.2% aluminum, but impurities such as iron, silicon, magnesium, manganese, chromium, titanium, and lead can also be present.Furthermore, it is preferable to control the basis weight of the coating by gas cleaning or another known method. The basis weight is preferably 25 to 75 g / m² per side. nfroonn / Lznz / E / YiAi

[62] Alloy treatment For example, hot-dip galvanized steel sheet with a 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 adhesion of the coating to the steel sheet deteriorates. Furthermore, pearlite transformation sometimes occurs, preventing the desired metallic structure from being achieved. Therefore, the alloying treatment temperature should be 600°C or lower.

[63] Second cooling: cooling to 150°C or less The steel sheet, after coating or coating and alloying treatment, is cooled by a second quench to or below the martensite transformation start temperature (Ms) to initiate some of the austenite-to-martensite transformation. 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 150°C, the quenched martensite is not sufficiently formed, and the desired metal structure is not achieved. Therefore, the quench stop temperature is 150°C or lower. It can also be 100°C or lower. The Ms point is calculated using the following formula: The mass percentage of the elements is entered for the element symbols in the following formula. For elements not contained, 0% by mass is entered: Ms (°C) = 550-361xC-39xMn-35xV-20xCr-17xNi-10xCu5xMo+30xAl nfroonn / Lznz / E / YiAi

[64] Third soaking treatment: holding at a range of 300°C to 420°C for 100 to 1000 seconds After the second cooling, the steel sheet is reheated to between 300°C and 420°C for the third soaking treatment. In this step, to obtain the desired amount of retained austenite, carbon is concentrated within the austenite to stabilize it (austenitize it). Additionally, the martensite produced during the second cooling is quenched. If the holding temperature is below 300°C or the holding time is less than 100 seconds, the bainite transformation does not proceed sufficiently, making it difficult to obtain the desired amount of retained austenite. Furthermore, the untransformed austenite that is subsequently converted to martensite is not sufficiently divided by the bainite, and as a result, a large amount of coarse, fresh martensite is sometimes produced after the introduction of plastic deformation.On the other hand, if the holding temperature exceeds 420°C or the holding time exceeds 1000 seconds, the martensite becomes excessively warmed and the bainite transformation proceeds excessively, making it difficult to obtain the desired strength and metallic structure. The temperature of the third soaking treatment can be 350°C or higher, or 400°C or lower. Furthermore, the holding time can be 150 seconds or higher, or 600 seconds or lower.

[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. 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. nfroonn / Lznz / E / YiAi

[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 2-4 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 2-4 and treated appropriately for alloying. In the soaking treatments shown in Tables 2-1 to 2-4, the temperatures were maintained within the range shown in Tables 2-1 to 2-4 ± 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 ον | ZLO 0 with CM OO CM © <r 08 íM CM O © © © Ι- Ο 1.08 IZO o 0005 g S o 0025 .0018 8Z00 © o 0010 § 0007 0022 CM S O © o SíGO 0005 O © © 0020 0020 9Z00 IZ0O 0028 8000 O 0 a O O O © © © © © 0 © z ¿£00 0024 0024 CO g £100' 9100 o o «'ó iC © o .0014 0043 oseo 0057 o o § 0035 oseo 0035 0034 .0051 5100 0060 ΙΛ © © O O O o O ó © © © © O © o © © © © O O O 0 < wo 510 οεο s en 076 652 120 .018 .151 800 056 *3f 037 592 042 014 .103 032 wo 005 ©! o O © a 0 O O o © a O O ω o CM g 0020 οεοο SUJO CN o 0Z00 0025 0005 0015 .0007 0024 © © 0029 OZOO 1100 0026 CM © © 0030 ¿ZOO .0012 .0021 © 0 O O O O o o © <5 © © © o © O © © 0 O α. O o 023 ZLO szo 007 .031 0L0 © © © 027 s © £10’ 006 © © .015 810 800 .014 oto 0 es o O O © O O © 0 © o O O o O © O o O Mn 00 CM CO in cg 0 CM oí CM © p CM m © <O «r>CMj 0>> with © © cmí with CM *-* CM CM CXÍ <M CM CM CM CM CM ©! CM CM CM o co co 3 O'' -O -o CM © 00 © IT! tb © CM CM CM .95 <O © © 0 S CM eS © CM oí ,163 230 -•O CM CM © CM 157 081 .132 206 339 .135 .193 2Θ5 177 CM 197 ISO CM o» OO Ó © 0 © O o O © © OO © © © © © © © 0 o ϕ !XJ w < ω O a UJ LL o X — £ z Ή a -J a εε in H* >► Type i. Fl

[69] Table Ms 381 396 373 © o? 384 430 416 íÑ 388 888 437 C0 CP 03 382 391 388 <r 381 452 333 o s en — 04 04 en U3 00 m ΐη O? <0 731 04 SJ ©- <r εο ít>< CP © in 04 o Í2 m 04 © IO O* U7 04 oowo 07 O cp .0029 ,Ζγ: 0. 0042 0068 ISOO O / *·> © © oh? but {_· © LÜ CE a 00 X ai 2E <31 ja <— U1 O o CP 5 o ω o .25 .59 £3 O __ <4 o © «O © U7 CN 04 07 m O O © ©i O p Φ u nj Φ < C3 QQ UJ ii. Φ X ..J Σ O 0. a ce ω Z> Ό O O. b− launch Q § § 0» PC Φ a OP ft ίβ u r-í ftf Ό flj MO flí ü •r| Λ «f P rt H S > 0 and § § >1 fv knot / Lznz / E / YiAi

[70] Table CM Laminating step 1 in cold 1 Reduction of laminating in cold % -O ΙΛ 10 Lf> L0 ic LO LO LO -0 L0 LO LO O LO ΙΛ 10 in LO LO •Ό LO *o tn ΙΛ Ό in Paso de laminado en caliente Theme. de enrollamiento ”C 6oo I § 540 I 550 1 600 | 500 | s LO 590 I 500 | 570 | 530 | 500 | | OiS 540 1 | 087 580 | 530 I 530 I 540 1 510 I s 600 I 510 | 500 | 590 | 3 uo | 09S Tiempo desde ei final del laminado de acabado liasta el inicio del enfriamiento s co CO o ro CO CO co co CO CO CO CO co CO co co ΓΟ CO co co en co CO CO ro 1 ro I Tiempo prom. entre soportes a ! SO I | SO I L... ! | SO 1 1 0-5 i I 0.5 ¡ 1 0.5 ¡ 1 0.5 ! LO © fío O 1 o.5 ! 1 0.5 ¡ 1 0.5 ! 1 0.5 i LO © tT> OI so | I 0.5 ¡ LO or WO © LO © I 007 ¡ «0 © I 0.5 ¡ I o.5 ! ! SO I 1 o.5 ! Maximum laminate reduction. % o oj © OI O Oí © 04 © 04 © o» O o; O OI O Oí o Oí © Oí © <N © o» © o; © OI © 04 © oí O 04 o © es © CN 3! o <N © <N © CM © 04 O 04 Temp.of the side of the final climb »C ¡ 096 920 | OO? O 930 i ¡ 096 © o 0^ 940 ; the 900! [ 006 920 i 940 ! ) 006 900 i 940 ! 930 | 940 | | 076 [ 006 § 970 i 920 i 970 i 960 i ¡ 0S6 940 ! ¡ 076 Final entrance side temp. 'C [ 1070 ! © © I 1090 I | 1040! | 1070 ¡ | 1010 1 § i [ 0101 1 I 1020 I | 1040 ¡ | 0SOI I [ OZOL I 1 1020 I ! 1040 ¡ § I 1060 ! The 1060! § © © | 1060 | ¡ 1020 i | 1070 S | 1060 j | 1050 ¡ I 1070 ! ¡ 1040 ¡ Total rolling reduction of raw rolling at 'OSO'Co más % 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 8 eo co 3 3 3 Speed.of plate heating (Act-Ac1+3ÍFQ °C / min ce | | 1240 i 11250 i | 1260 I 1 1210 ! ¡ 1230 i | 1240 i [ OiZl 1 os ¡ 1260 ! ¡ 1230 | i OiZl II 1270 ! 1 1210 1 | 1220 I | 1280 i ! 1260 1 i 1220 i | 1220 | ! 1230 i ¡ 0921 1 | 1220 I Tipo de acero < < < < < ω CQ CQ 00 ω ω O o Q C3 DQQD ÜJ LU Ld til lL· L1_ LL. u. No. - co mo r- CO o- © - CO jr uo •O o oo o- © Yes CJ C'-J with CN ΙΛ ti -o •ti O ti O > ti Ή ti 0 » 0 ti t & O you H you Ό to £44 to 0 −0 £ Ή 4} for -P 'H you © O & fly M « o Lt. nfroonn / Lznz / E / YiAi

[71] Table “Μ 1 CM Cold Rolling Step Cold Rolling Reduction A 8 O ro s LO 8 LO 8 8 LD lD LO 8 «Λ LO 8 8 40 ID LO 8 LO 8 irt •45 LÍJ 8 O ID LD Hot Rolling Ratio Temp. of winding eC | 085 II 550 | | 650 I 1 770 | ¡ 570 | ¡ 560 ¡ ¡ 510 ¡ 1 550 | | 530 1 | 520 1 | 09S ¡ | 520 ¡ | 08S I i 550 ¡ | 560 1 ¡ 540 ¡ ¡ 08S ¡ 1 510 ¡ i 590 ¡ | 520 I ί 490 ! i 570 ¡ I oís | ! 560 ! | 550 | | 06S I ¡ 560 ¡ Time from the end of finish rolling to the start of cooling s en nd CQ 4*1 O 09 L<9 co ιο C3 <*7 14*1 d 09 w> C9 LO d en tn 09 o 09 M*Í< CO md <o ΙΓ5 d CO d 09 CO CO ÍTJ ΓΟ o CO Tiempo i prom entre i soportes i s i LO d M*4 o tí LO tí d ir? w» d Lf> d lD dd to d LO ti ΙΛ in Laminate reduction max. % o CM 8 04 8 es CM w 04 ​​O CM 8 8 O CM 8 8 d CM 8 O ÍM 8 o CM R o CM o CM 8 O CM 8 ID d 04 Temp.of the side of the final exit “C | Oto 920 ! 940 | | 046 980 1 920 i 910 ¡ 940 ¡ 016 940 j 950 j | § 940 ¡ 970 1 910 | | 006 i 960 j Temp. del lado de la entrada final. C 1050 ! 1030 ¡ 1070 | 1060 ! Ϊ080 ! 1030 i ¡ 0801 1080 I 1030 ¡ í 0101 ioio ¡ 1010 1 1040 ¡ 1030 | 1040 |.0401 1040 j 0901 0601 1060 i 0 Reduction of total O lamin plus 10%. 00 ¢0 ÚO 00 00 00 00 00 OO CO 00 00 00 00 c© CO CQ CO CO 00 ÓG CG plate (Ac1-Ad+30°C) 'C / min CO CM o- CD ID<t Mí Γ- o Γ > » ό sr ID σχ 00 óo ID LO Mt Ό c- ID Ό Temp.of plate heating C 1280 | 1250 ¡ 1230 j 1260 1 1 1280 ] 1250 j 1210 ! [ 0121 1220 1 1250 j 1230 ¡ oszt i 1240 | 1250 ! [ 0S21 1270 j |1220¡ 1250 ! 1250 | 1220 j 1220 1260 | 0821 1240 1 1220 ! | 0721 1230 j Steel type OO <0 X - -Ϊ ΙΞ 2 Q £L ea: in b- □ > a Q to UJ CO 04 or CM in 07 LO<n OQ en Cb CQ CO LD 04 i?? LD CM LD 8 tí •o •d tí tí tí > one Ή one •one one «1 one d Λ «one one o. Ü H you you you w you Ψί “you ü you Ή tt “you tn § -you you á* you w V) o >4 units / Lznz / E / YiAi

[72] Table rtj £ Μ tn (*) φ co Third soaking Holding time yes 180 | | 081 I i | 081 ¡ I 180 I 180 I | 081 1 ί 180 I | 081 | 370 I | 081 | 081 j | 370 | | 081 | 180 | 1 081 ] 1 180 I ! 180 I | 081 j | 081 ! 180 | | 081 | 081 | 081 ¡ I 180 | | 081 ] Temp. 'C 390! 390 | 400 i ¡ 009 O co co ¡ 009 io 390 I i 400 1 410 j 25Ü ii 400 ! i 400 ¡ 410 j [ 009 i 451 ¡ 1 06£ i 400 ¡ [ 009 i 390 1 400 i 380 I 390 | 390 | 380 | 300 j 390 j Second cooling Cooling stop temperature °C g ί m ! O ¡O gg O o OOO Ό O un s O to o •Ό gg O io O 07 •5 OOS© es s 3 a 40 Alloy Temp.de aleación «C ¡ 485 | [ 495 | 1 495 | o CD < i 500 1 o co | 069 I 1 [ 505 | i 505 I ¡ 495 I ¡ 485 | 1 S69 ( [ 495 | i 495 1 i 500 1 < | 089 ] | S69 ] ! 480 I i ! 475 | [ 480 | íf? 00 í 470 | Second time of maintenance s 105 | ¡ 320 J 105 | 105 | | G | SOI 105 I _____105 J 105 1 ¡ 300 í 105 I 105 | 310 i 900 1 ! 520 i 105 1 í 310 j 105 | 105 IU>l 105 | i 350 i 105 I 105 | 105 | [ 390 ¡ 105 | [ 370 ¡ 105 | Temp. “C [ 320 ! | 320 i ! 320 ! OO <*> O o co | 300 i í 300 ¡ ¡ 330 ii 310 ! | oeε ¡ ¡ 3io ! | 350 | ! 330 ! | 330 j ! θιε ! i 450 ¡ Primer enfriamiento Vel. de enfriamiento promedio entre700600C “C / s CN Oí & có OI CN ro co m Oí CN Ol Oí CQ Ol Ol] OI Oí Ό CO oí OI Oí g Oí Oí O co O Oí Cü . <r>tn co CO co 40 Oí Primer remojo Tiempo de mantenimiento saog OO o O <h g O θ' o o O O o o- g a a g o s o 5 o g 3o duiai I 815 | I 815 | | 815 | I 810 I I 815 I I 840 I | 071 | I 820 I I 840 I | S08 1 ¡ SOS | | 008 I | 008 I | 820 1 | 820 | | 820 | 1 820 1 I 840 1 820 I 830 I I 830 | I 830 ! I 820 I | 098 1 | 860 | | 0S8 I I 098 J Calentamiento Vel. de calentamiento entre Ad-Ac1+30"C ’C / s -a eo t-. LO ¢-. CO O -O -o cq CO O; o. ΙΛ 1Λ ex? oí LO io «q Vet. de calentamiento entre 600’C-Acl =C / s id [ 5.3 1 [ 4.9 | I 99 5 ! 5.2 ! | ε1 j ! 5.0 Ϊ í 5.2 ! [ 4.5 | [ 5.0 | ¡ 4.5 i i 4.5 i [ 5.3 | ! 4.8 | ¡ 9'9 j L___É2__J ! 4.8 1 i 5.3 | I 5.2 | co ir? ί 4.5 ¡ ¡ 5.2 ¡ 1 4.6 | [ 4.7 | ¡ 6'9 j | 4.8 j No. — Oí ίΛ o· QQ O- o 5= ¢4 to £ co O CS Oí OI OI CO Oí oí ϊθ OI CM b O H The b & ► b •H F m b F «> F m L F O Tí $ □ β <0 o H <0 <0 M 0 3 rtj υ Η τ5 ñ Η V) ¢0 M F F C f ·§ <β >1 <5 M

[73] Table CM c gold ÍÑ the φ And φ -the Third soaking Holding time s | 180 | I SOI í 1 081 i | 180 | | 180 I s 1...........180___________1 I 540 I 1 180 I s ! 180 i | 081 | | 081 | | 180 | | 180 II 370 I | 180 | | 081 | I 180 | | 180 | | 081 1 | 081 1 ί 180 | | 081 | 1 081 1 i 180 I 3« diuai ¡ 370 | [370 | | 08£ | | 08£ | I 410 II 380 | 1 390 ] I 360 I ooaoa 5 i 380 1 | 088 1 | 300 | | 04£ l 380 II 400 II 380 | s | 390 i [ 410 | i 400 I | 400 | I 400 I > I 400 j [ 400 | Secure Cooling Temp. cooling stop temperature °C j 50 ¡ o OO £3 O o ! 80 ! 06 ____! | OS j | 40 | o ¡ 001 ¡ í 80 J ! 04 j ¡ 80 i ¡ 80 I L_________so_________1 ¡ 60 j ! 60 ¡ί 80 | s 0 | I heard ! os 1 os! 1..................1 1 0£ ! 0 10 Alloy Temp.of randomness °C | 490 I l 550 i | 500 | [ 490 ¡ [ 480 | ¡ S27 S ! 005 1 [ 480 ¡ | 495! I OES ¡ | 530! • | 530 | | 540 | • | 037 ¡ I 490 ¡ I 470 | í 490 | ¡ 495 i 1 475 ¡ i 500 | ! 069 I [ 480 i 1 470 j 1 480 ¡ Next time of maintenance s 105 | 105 | ¡ SOI 105 | [ SOI 105 | | SOI 315 j 105 | 105 I | SOI | 801 105 | 105 | THE SOI | SOI | SOI I sot | SOI [ 801 105 ¡ | SOI 105 ¡ i SOI | SOI | SOI t Temp. C [ 290 | | 017 j | 028 ¡ | 09E j [ 300 1 | 260 | L óóz i I 300 II 310 I | 098 1 rwj ! 360 1 ¡ 350 | § | 086 j 1 370 1 I 300 | The 340 | | OSÉ I | OVE j ¡ 340 | | 00£1 [ 320 | í 350 I | 088 1 I οεε | t Primer enfriamiento Vel.de enfriamiento prmedio entreTOO600'C 'JC / s I 38 ¡ O r*7 co [ ov 1 I 30 S Π 29 1 I 13 ! í 9ε 1 ÍM ! 09 | ¡ ?s 1 ! zs | ! zs | I 52 I L_________________91___________________I ! 97 | <*> I 37 ¡ 5 Cs» 1 44 S 1 44 ¡ ! οε 1 i SO | 1 37 | 1 32 1 Primer remojo Maintenance time s í I o O· o CF- o Q* O o- I 04 i ! 90! The 270! 90 1 1 06 i ¡ 06 1 ! «6 ) í 90 | I 06 ¡ 1............90.............1 ¡ 06 ¡ I 06 j ! 90 ¡ l 90 IO ί 06 j ¡ 06 ¡ 1 06 s 1 06 j ! 90 ! £ Temp. "C | 0é8 | | 068 1 | 088 1 | 088 | | 900 1 I 830 | 1 ÓZ8 1 | 088 | | 098 ¡ I 865 | 0 i 865 1 | 865 | | 098 | I 890 I 1 870 1 I osa | 1 815 | I 900 I | 810 | | 078 I 1 900 1 ¡ 900 | 0 026 1 830 I | 830 | Heating Heating rate between Ad-Acl+SO^C 'C / s I *1 I o ! 20 | 1 1 ¡ 17 1 í 1.4 II 0.6 J 1 43 1 1 2.2] [ 17 JI 1.3 I IΜ l [ 21 1 I 17 1 ΙΛ I 15 1 i 1.5 I -r 1 ¿i 1 í 16 1 CM a 1 si 1 1 ¿i 1 1 19 1 Vel. de calentamiento entre 600"C-Ad "C / s 5.2 ! OS fM 4.6 | 5.3 i OS 8 7 ii 61 i os 6'7 5 5 4.9 ¡ ¡ 9'7 ! 9'7 i 8 7 5.8 i____________4,9____________ 9 7 4 5 | ín 4,7 j 8'7 5.2 ¡ | 2'7 4.7 í C3 *0 ΐ rs No. 28 O ΓΟ OJ m 33 34 35 : 9E 37 38 L 39 J 07 17 04 A! 5 45 £ 49 50 LO 53 7 τη. ao •H » § P £ Φ M ft •ΰ M «fo Ή Tt C M tf ω co or >> ra M V) O »4 aai

[74] 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.

[75] Toughness after the introduction of plastic deformation (toughness after pressure forming) was evaluated using the following technique. A JIS No. 5 tensile test piece was taken perpendicular to the rolling direction and subjected to 5% plastic deformation by means of a tensile test. A 2 mm V-notch Charpy test piece was taken from a parallel portion of the tensile test piece after deformation was imparted. After this, both the deformed and undeformed material were subjected to a Charpy impact test at the test temperature and -20°C. A test piece with a Charpy absorption energy ratio after imparting strain absorption energy / Charpy absorption energy ratio before imparting plastic deformation of 0.7 or higher was considered very good, one of 0.5 to 0.7 was considered good, and one of 0.5 or lower was considered poor. Those rated as very good and good were considered to pass.

[76] The results are shown in Tables 3-1 and 3-2. In Tables 3-1 and 3-2, GA means hot-dip galvanized-annealed, while GI means hot-dip galvanized without alloy treatment.

[77] Table Observations u? E o ó üT Ex. Comp. Ex. Comp. uT ixi Ex. Comp. Ex. Comp. iXT LlT Ex. Comp. E o ó iZT u? ω Ex. Comp. Ex. Comp. I Mechanical properties I Toughness after pre-deformation Very good S CU Very good Poor Very good Very good Sounds o: Very good Sounds i Sounds Sounds Very good o a. É c Ό «3 ω SO φ C £ r? < o *. O LU O LC 116 o 119 125 OJ o 104 119 a> 31 UP <o 145 s -< CP cp cr- OJ <P o m Ό Oí o ΓΜ <Ñ LD OJ ?s o OJ C4 40 CP £ tn ω 19.7 16.8 o 20,0 22.5 29.7 22.0 22.1 17.0 00 19.9 17.0 24.3 22,6 20.8 tn CL S 1023 0S01 CQ <20 θ' 1058 ¿001 1087 s N» 1029 1078 mi QmO OJ up! coi Oí s 8001 Oí 1033 I Microestructura 1 Martensita templada f fe5 .united) / lOOOum2 OJ O OJ L0 CP cp O trt CP o OJ CP CP Fresh martensite after pre-strain (>2 pm) OZ zc 10 Oí O Π <r <T O UP 00 ;=i co ’-O <í O< tí C ’φ £0 s 3 OJ cp o <» O- O- <p o O) o Oí O; Perlita+cementita % O o O o o O o o o o <3! o O o o Martensita fresca % Cp co o a>04 OJ in o- OJ sO Fd 10 <0 04 tí Tempered martensite % es o OJ <5 CP O OY 04 CP O Oj <5 04 oí oi Retained austenite ®Z za o OJ O Oí 2 o rrt 40! t- y Oj o- Φ Έ u. <n OI IO 04 m 04 OG 04 10 Oí R o o o- CH 04 CP 10 L0 CP Recubrimiento GA GA GA GA ó GA GA GA ÍS GA < o GA e GA GA GA Tipo de acero < < < < GQ CQ ω CÜ O o O o O a Q No. OJ CP m | o o- CÜ eo o £3 Oj rp 2 40 *0. ηι?οοηη / Lznz / Β / γΐΛΐ Comp. E oo Ld td Comp. Comp. u? uT LO uT Comp. ÜJ ΪΣΓ uF uF LU Very good Poor Very good Very good ó! O! cd Poor Very good Very good Very good Good É «3: EEl 138 O 135 140 LEI Sil S7 cF CM CM CM CM 38 o i0 CM 17.7 24.8 25.4 21.2 00 26.0 23.1 o Fm «> 22.5 06¿ 61U 997 1140 9£lí 1138 § 982 8901 OUi O CM! CM CM cm¡ LO í= MG OM to ención. so CM 00 o- tí t- <T Ό o presente inv· 47 CO 00 OY CM Ό CM 25 O RJ H de O O O © O o O O O O o a» o ñ 3 Ü H in — — CM CC Mt ΓΌ CM o del Lea fues CM: © © CM c? 1O «i Ρλ CM - - <3 cm CM <N Cm CM <? E= o en rptix s^r 35 © CM co o- o CM CO 33 33 38 3 Ή & di β GA GA O GA GA GA © GA GA GA GA ua op«Ae Q O O Ui Ul LU L13 u. u_ u. Ll- M $ £ DD θ' 20 CM CJ CM CM 24 ur> Yes CM CM It η^οοηη / lzoz / b / yiai

[78] Table Observations c uT iu Ex. Comp. UJ LU lu u? ÜT üT LxT iu CU ll? LU ilT | Mechanical properties I Toughness after pre-defaming Very good Very good Very good 23 o EÜ Very good Very good Good Sounds Very good Very good Very good Very good Very good Very good Very good Very good O Ό TE tn c <D Ss UJ o * cr en 132 120 102 £ 118 139 mO 04 133 125 118 126 115 Z01 Z1L 129 | Formabilidad por ¡ << sí ce 04 04 04 04 07 147 05 04 U7 O <2 5 o 07 C5 U7 ÜJ 21.6 23.0 22.0 23.0 23.2 23.6 30.1 20.6 O ir? 14.2 04 15.0 15,5 14.8 14.9 ω mg 982 07 3 «7 s § 05 O» O co g 1236 s 1225 1198 zozt § 1221 i Microstructure | Tempered martensite)· (>5 jim) / lOOOjinf Λ o - 07 o O 04 OI co í= Fresh martensite after pre-strain (>2 pm) % ¡nm O' ao o- -c 04 o OJ 04 CQ Eainite % 07 OI0 « Sí Oí p 04 UD 04 04 Paper+cementite % O o O o O ooo es o OOOOO a Fresh martensite % 04 1Λ <” Oí Ό co 04 O 1X7 03 o Tempered martensite % 07 07 o 00 o 00 O*0 W7 or •o .. Ferrite austenite , , retained 0 / í 0 / Λ < 30 | STEEL TYPES STEEL type O and OOX - -7 X ς: 2: 2T ς: c No, 00 04 04 O F5 04 07 07 ro ιΛ 07 P> a oo oí. <O. nfroonn / Lznz / Ε / γΐΛΐ Yes. Comp Yes. Comp. Yes. Comp. Yes. Comp. Yes. Comp. Yes. Comp. Yes. Comp. Yes. Comp Yes. Comp. ÍJJ Yes. Comp. Underlined in bold indicates outside the scope of the present invention. Very good Very good Very good i<y £U Pobre u Pobre Pobre I Buena Pobre 153 3 116 ΓΟ 00 co co s S COL 108 a m CO a co o co o> 04 04 tet 12.0 31.5 12.0 04 SSL o* 04 04 18.3 LÓ 20.5 03 168 1097 roí 1374 1310 1233 i 1095 04 o 1611 ir O -4 o ;3 04 81 04 to know £js Yes Yes: 04 04 04 ooo> co Co O4| O aa O ooaoa © - 04 04 οή 'O 515 yes 00 is -o 00 a in 0? 3 CO CO 04 Ό oa: 04 sv O m3 04 C- o ÍM ΐ= a 04: £ 3 θ' with £0 a 04 en Ό <n 04 LO CQ 1Λ η^οοηη / lzoz / b / yiai

[79] In Comparative Example 2, the holding temperature of the second cooling in the hot-dip galvanizing step was greater than 150°C. As a result, the desired metal structure could not be obtained, and the press formability and toughness after pre-deformation were poor. In Comparative Example 3, the holding time of the third soak treatment in the hot-dip galvanizing step was more than 1000 seconds. As a result, the desired metal structure could not be obtained, and the press formability was poor. In Comparative Example 4, the plate heating rate was less than 2°C / min. As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor. In Comparative Example 7, the temperature of the first soak treatment in the hot-dip galvanizing step was less than Acl + 30°C (812°C).As a result, the desired metal structure could not be obtained, and the formability by pressing was poor. In Comparative Example 8, the first inlet temperature of the finish roll in the hot rolling pass was less than 1000°C, and the outlet temperature of the final pass was less than 900°C. As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor. In Comparative Example 11, the third soaking temperature in the hot-dip galvanizing pass was less than 300°C. As a result, the desired metal structure could not be obtained, and both the formability by pressing and the toughness after pre-deformation were poor. In Comparative Example 12, the average cooling rate of the first cooling in the hot-dip galvanizing pass was less than 10°C / s.As a result, the desired metal structure could not be obtained, and the formability by pressing was poor. In comparative example 15, the holding time of the third soaking treatment was less than 100 seconds. As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor.

[80] In Comparative Example 16, the temperature of the third soaking treatment in the hot-dip galvanizing step was above 420°C. As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor. In Comparative Example 17, the holding time of the second soaking treatment in the hot-dip galvanizing step was more than 500 seconds. As a result, the desired metal structure could not be obtained, and the formability by pressing was poor. In Comparative Example 18, the temperature of the second soaking treatment was above 480°C. As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor. In Comparative Example 21, the holding time of the second soaking treatment in the hot-dip galvanizing step was less than 80 seconds.As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor. In Comparative Example 22, the maximum rolling reduction of the finish roll in the hot rolling pass was more than 37%, and the average time between supports was less than 0.20 seconds. As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor. In Comparative Example 27, the time from the end of the finish roll to the start of cooling was less than 1 second. As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor. In Comparative Example 31, the coiling temperature in the hot rolling pass was more than 680°C. As a result, the desired metal structure could not be obtained, and the formability and press toughness after pre-deformation were poor.In comparative examples 44 to 50, the chemical compositions were not controlled within the predetermined ranges, resulting in poor formability and / or toughness after pre-deformation. In comparative example 51, the average heating rate from Acl to Acl + 30 °C in the hot-dip galvanizing step was less than 0.5 seconds. As a result, the desired metal structure could not be obtained, and the toughness after pre-deformation was poor. In comparative example 52, the third soaking treatment was omitted, resulting in the failure to obtain the desired metal structure, and the formability and toughness after pre-deformation were poor. In comparative example 54, the second soaking treatment was omitted, resulting in the failure to obtain the desired metal structure, and the toughness after pre-deformation was poor.

[81] In contrast, the hot-dip galvanized steel sheets of the examples have a tensile strength of 980 MPa or more and TS x El x λ°·5 / 1000 of 80 or more and, in addition, have excellent toughness after pre-deformation, so it is learned that they are excellent in press-forming ability and toughness after press forming.

[82] 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 Tables 4-1 and 4-2, 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 Tables 4-1 and 4-2, the temperature was maintained within the temperature range shown in Tables 4-1 and 4-2 (± 10°C). nfroonn / Lznz / E / YiAi

[83] Table

[84] Table 4-2 Φ c Φ 75 c Φ c Ό W Φ £ £ a £2 'O <C N C ra 7c O) g o ¢0 Tercer remojo I Tiempo de mantenimiento s 180 Lo subrayado en negritas indica fuera del alcance de la presente invención. Temp. 400 1 Aleación ! Temp. de aleación "C w> DO I First Cooling 1 Cooling Stop Temp. 'C w Average Cooling Rate between 700~cooling stop temp. 'C / s Cs¡f | First Soak Holding Time s OOE 9 820 I Heating I Heating Rate between Ad-Aci+SOX "C / s U7 Heating Rate between ÓOIT'C-Acl oC / s 4.8 No. mm η^οοηη / Lznz / Β / γΐΛΐ

[86] As can be seen from the results in Table 5, if the first cooling is done by gradual cooling to eliminate the second soaking treatment, the desired metallic structure could not be obtained and the toughness after pre-deformation would be poor.< / r>

Claims

1. A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer 5 on at least one surface of the base steel sheet, wherein the base steel sheet has a chemical composition comprising, in % by mass, C: 0.100% to 0.350%, Si: 0.50% 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%, 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%, 25 W: 0% to 1.00%, B: 0% to 0.0100%, Sn: 0% to 1.00%, Sb: 0% to 1.00%, Ca: 0% to 0.0100%, 5 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%, 10 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 thickness centered around a position 1 / 4 of the thickness from a base steel sheet surface contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 6% to 30%, bainite: 5% or more, quenched martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, a number density of quenched martensite with a circle equivalent diameter of 5.0 pm or more is 20 / 1000 pm2 or less, and an area ratio of fresh martensite with a circle equivalent diameter of 2.0 μη or more after imparting 5% of Plastic deformation is 10% or less.

2. A method for producing hot-dip galvanized steel sheet according to claim 1, comprising: (A) a hot rolling step comprising heating a plate having the chemical composition according to claim 1 and finishing rolling the heated plate by means of a plurality of rolling stands and then coiling it, wherein the hot rolling step satisfies the conditions of the following (A1) to (A3): (A1) an average heating rate from Acl to Acl + 30°C at the time of heating the plate is 2 to 50°C / min, (A2) in the finishing rolling by the plurality of rolling stands, a rolling reduction per pass is 37% or less, an entry-side temperature of the first pass is 1000°C or more, an exit-side temperature of the final pass is 900°C or more, and an average time between stands is 0.20 seconds or more, and the time from the end of the finish rolling to the start of cooling is 1 second or more, and (A3) a winding temperature is 450 to 680°C, and (B) a hot-dip galvanizing step comprising heating the obtained 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 second-cooled steel sheet and then soaking it a third time, wherein the hot-dip galvanizing step satisfies the following conditions of (B1) to (B6): (B1) in heating the steel sheet before the first soak, an average heating rate from A1c to A1c + 30°C is 0.5°C / s more, (B2) the steel sheet is held at a maximum heating temperature of Acl°C + 30°C to 950°C 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 250 to 480°C for 80 seconds to 500 seconds (second soak), (B5) the second cooling is carried out at 150°C or less, and (B6) the second cooled steel sheet is heated to a temperature region of 300 to 420°C, then held in the temperature region for 100 to 1000 seconds (third soak).