Hot-dip galvanized steel sheet and its manufacturing method

Abstract

A hot-dip galvanized steel sheet and a method for manufacturing the same are provided. The present invention relates to the production of high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or higher and used for automotive structural members, and provides a hot-dip galvanized steel sheet having a hole expandability (HER) of 45% or higher, a relationship between yield strength (YS) and hole expandability (HER) of 5 or higher, HER*100 / YS, a relationship between elongation (EL) and hole expandability (HER) of 700 or higher, and a yield ratio (YS / TS) of 0.80 or higher, which is free from surface and internal LME cracking in welds.

Description

[Technical Field]

[0001] The present invention relates to the production of hot-dip galvanized steel sheets used for automotive structural members, and more specifically to high-strength hot-dip galvanized steel sheets that are free from surface and internal LME cracking in welds and have excellent ductility, hole expandability, and weldability, and a method for producing the same. [Background technology]

[0002] Recently, the automotive industry has been facing increasingly stringent regulations for environmental conservation. Accordingly, carbon reduction and fuel efficiency regulations have become increasingly stringent, leading to the increased adoption of high-strength steel to ensure passenger stability in the event of a collision or other accident. Furthermore, to improve the crashworthiness of vehicle bodies, high-strength steels with excellent yield strength are being used for structural components such as members, seat rails, and pillars. However, increasing the strength of steel sheets can result in reduced ductility and formability. To address this issue, the development of materials that simultaneously achieve high strength and high formability is needed. Generally, increasing the strength of steel sheets leads to a decrease in elongation, which can lead to reduced formability. Therefore, the development of materials that can address this issue is needed. Conventional steel strengthening methods, such as solid solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening, have been studied. However, among the above methods, solid solution strengthening and grain refinement steels have the drawback of being extremely difficult to produce high-strength steels with tensile strengths of 490 MPa or higher.

[0003] Precipitation-hardened high-strength steel is a technology that ensures strength by precipitating carbon and nitrides through the addition of carbon and nitride-forming elements such as Nb, Ti, and V. The fine precipitates inhibit grain growth, resulting in finer grains and increased strength. While this technology has the advantage of easily achieving high strength at low manufacturing costs, it has the disadvantage of requiring high-temperature annealing to induce sufficient recrystallization and ensure ductility because the fine precipitates cause the recrystallization temperature to rise sharply. Another problem with precipitation-hardened steel, which is strengthened by the precipitation of carbon and nitrides in the ferrite matrix, is that it is difficult to obtain high-strength steel of 600 MPa or higher.

[0004] Meanwhile, various transformation-strengthened high-strength steels have been developed, including dual-phase (DP) steel, which is composed of a soft ferrite matrix and a hard martensite matrix; transformation-induced plasticity (TRIP) steel, which achieves high ductility by utilizing the transformation-induced plasticity of retained austenite; and complex-phase (CP) steel, which is composed of a composite structure of ferrite and hard bainite or martensite. Recently, automotive steel sheets have been required to achieve higher strength to improve fuel efficiency and durability. This has led to increased demand for ultra-high-strength steel sheets with tensile strengths of 1180 MPa or more for body structures and reinforcements to ensure crash safety and passenger protection. Among these, DP steels are the most commonly used automotive steel sheets due to their excellent ductility. However, they suffer from low yield ratios (YR) and poor formability and workability. Furthermore, the gradual trend toward higher strength steel sheets has led to cracks and wrinkles during press forming of automotive parts, making it difficult to manufacture complex parts. Among ultra-high-strength steels, TRIP steel and XF steel have higher ductility, superior yield ratios, and good workability compared to conventional DP steel. However, they suffer from poor weldability due to the large amounts of Si and Al added to ensure high elongation. To overcome these drawbacks of conventional ultra-high-strength steels, reducing the Si and Al content can create a composition with good weldability while still maintaining a certain level of yield ratio. This allows for the expanded application of ultra-high-strength steels to more complex parts. This can be achieved by utilizing Quenching and Partitioning (Q&P) heat treatment, a cutting-edge heat treatment technology that ensures retained austenite.

[0005] An example of a conventional technique for simultaneously ensuring the ductility and workability of the high-tensile steel sheet is the invention disclosed in Patent Document 1. In the invention disclosed in Patent Document 1, it is necessary to precisely control the slow cooling and rapid cooling temperatures to form retained austenite, and to control the heat treatment temperature so as to ensure a high elongation percentage.

[0006] Another prior art is the invention disclosed in Patent Document 2. The invention disclosed in Patent Document 2 is characterized by producing a steel sheet having a high yield ratio by a Q&P process and a painting treatment.

[0007] Another example of the prior art is the invention disclosed in Patent Document 3. The invention disclosed in Patent Document 3 provides a method for producing a high-strength cold-rolled steel sheet with a high bainite fraction by cooling to the bainite region, but has the problem of a lower carbon partitioning effect and a lower elongation rate compared to the Q&P process. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 2018-0165176 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-090432 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-106696 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a hot-dip galvanized steel sheet that is mainly used for automobile structural members, and a method for producing the same.

[0010] Furthermore, the technical problems that the present invention aims to solve are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by a person having ordinary knowledge in the technical field to which the present invention pertains from the following description. [Means for solving the problem]

[0011] Thus, one aspect of the present invention is In weight percent, carbon (C): 0.08 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 2.0 to 3.0%, molybdenum (Mo): 0.4% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), aluminum (sol.Al): 0.6% or less (excluding 0%), titanium (Ti): 0.001 to 0.04%, niobium (Nb): 0.001 to 0.04%, nitrogen (N): 0.01% or less (excluding 0%), boron (B): 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities, wherein the C, Si, Mn, Cr, and Mo satisfy the following relational expression 1, A microstructure containing, by area, 70% or more of bainite and tempered martensite in total, 10% or less of ferrite, and the remainder being fresh martensite and retained austenite; and This relates to hot-dip galvanized steel sheets having a hole expandability (HER) of 45% or more, a relationship between yield strength (YS) and hole expandability (HER) of 5 or more (HER*100 / YS), and a yield ratio (YS / TS) of 0.80 or more. [Equation 1] C+Si / 30+Mn / 20+(Cr+Mo) / 5≧0.3 Here, C, Si, Mn, Cr and Mo refer to the content % of the steel components in the base structure at the 1 / 4t point in the base steel sheet thickness.

[0012] Another aspect of the present invention is a step of preparing a steel slab that satisfies the steel composition and Relational Formula 1, and then reheating the steel slab; hot rolling the reheated steel slab so that the temperature at the outlet of the finish rolling is Ar3 to Ar3 + 50°C, followed by coiling at 400 to 650°C, and then cooling to room temperature at an average cooling rate of 0.1°C / s or less; cold rolling the cooled hot-rolled steel sheet at a reduction rate of 40 to 70% to produce a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature of Ac3-20°C to Ac3+20°C; a step of primarily cooling the continuously annealed cold-rolled steel sheet to a temperature range of 500 to 550°C at an average cooling rate of 10°C / s or less, and secondarily cooling to a temperature range of 300 to 340°C at an average cooling rate of 5°C / s or more; Reheating the secondarily cooled cold-rolled steel sheet to a temperature of Ms or higher and then holding the temperature for 60 seconds or more; and and a step of cooling the cold-rolled steel sheet held as described above to a temperature of 150°C or less at an average cooling rate of 5°C / s or more. [Effects of the Invention]

[0013] As described above, the present invention optimizes the chemical composition and manufacturing process to produce high-strength hot-dip galvanized steel sheets that have excellent yield strength / tensile strength (YS / TS) and hole expandability compared to conventional DP steels while still maintaining the high ductility characteristic of DP steels. Specifically, it provides high-strength hot-dip galvanized steel sheets with a hole expandability (HER) of 45% or more, a relationship between yield strength (YS) and hole expandability (HER) of 5 or more (HER*100 / YS), a relationship between elongation (EL) and hole expandability (HER*EL) of 700 or more, and a yield strength / tensile strength (YS / TS) of 0.80 or more. This prevents processing defects such as cracks that occur during press forming, enabling the steel sheets to be used in a variety of automotive structural components with complex shapes that require high formability. Furthermore, the reduced Si and Al content provides the advantage of better weldability compared to conventional TRIP steels. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a diagram showing changes in the relationship HER*100 / YS between yield strength (YS) and hole expandability (HER) according to Relationship 1 between an invention steel and a comparative steel in an example of the present invention. [Figure 2] 1 is a diagram showing the change in hole expandability (HER) according to Relational Formula 1 between an inventive steel and a comparative steel in an example of the present invention. [Figure 3] 1 is a graph showing the change in yield ratio according to Relational Formula 1 between an inventive steel and a comparative steel in an example of the present invention. [Figure 4] 1 is a diagram showing changes in the relationship HER*EL between elongation (EL) and hole expandability (HER) according to Relationship 1 for an inventive steel and a comparative steel in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. The singular forms used herein include the plural forms unless the context clearly dictates otherwise. The term "comprises" as used herein embodies certain properties, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other specific properties, regions, integers, steps, operations, elements, components, and / or groups.

[0016] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted as having an ideal or very formal meaning unless defined.

[0017] The inventors have experimentally confirmed that by optimizing the components and manufacturing process to introduce 70% or more tempered martensite or bainite, the remainder fresh martensite, and retained austenite into the final microstructure, the yield ratio is higher than that of conventional DP steel, improving workability, and that less than 5% retained austenite is formed during final cooling, further improving ductility. Based on these experimental results, the present invention was completed.

[0018] First, the inventors produced a hot-rolled steel sheet in which carbides that become austenite nucleation sites during annealing are finely dispersed, by hot-rolling a slab in which the base steel composition C+Si / 30+Mn / 20+(Cr+Mo) / 5 in the base structure at the 1 / 4t point of the base steel sheet thickness was controlled to 0.3 or less so that the exit temperature of the finish rolling was Ar3 to Ar3+50°C, and coiling was performed. The hot-rolled steel sheet was then cold-rolled at a reduction rate of 40 to 70% to produce a cold-rolled steel sheet, which was then continuously annealed at a temperature of Ac3-20°C to Ac3+20°C. The continuously annealed steel sheet was then primarily cooled to a temperature range of 500°C to 550°C at an average cooling rate of 10°C or less, and then secondarily cooled to a temperature range of 300°C to 400°C at an average cooling rate of 5°C or more to introduce fresh martensite. The steel sheet was then reheated to a temperature of Ms or higher and held for 60 seconds or more to form tempered martensite and bainite, concentrating carbon in the surrounding untransformed austenite. Fine fresh martensite was then introduced into the remainder by cooling to a temperature of 150°C or lower at an average cooling rate of 5°C or more.

[0019] By controlling the chemical composition and manufacturing process described above, the proportions of ferrite and fresh martensite are reduced compared to conventional DP steel, and tempered martensite, bainite, and retained austenite are introduced, thereby increasing the yield ratio and ensuring workability compared to conventional DP steel. Furthermore, a large amount of mobile potential is formed around the retained austenite during plastic deformation, which helps improve ductility. This precisely controlled dual-phase steel ensures ductility while maintaining a high yield ratio compared to conventional DP steel. This allows for the production of ultra-high-strength hot-dip galvanized steel sheets with excellent ductility, hole expandability, and weldability.

[0020] The technical configuration according to the present invention will be described in more detail below with reference to various embodiments and accompanying drawings.

[0021] First, the hot-dip galvanized steel sheet of the present invention, which is excellent in ductility, hole expandability, and weldability, contains, by weight, carbon (C): 0.08 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 2.0 to 3.0%, molybdenum (Mo): 0.4% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), aluminum (sol.Al): 0.6% or less (excluding 0%), titanium (Ti): 0.001 to 0.04%, niobium (Nb): 0.001 to 0.04%, nitrogen (N): 0.01% or less (excluding 0%), boron (B): 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities. The properties of each alloying element and the critical meaning of the composition range will be briefly explained below, and unless otherwise specified, the contents of each component are all expressed in weight percent.

[0022] C: 0.08 to 0.16% Carbon (C) is a very important element added to strengthen the transformed structure. Carbon promotes the formation of hard martensite in dual-phase steels, improving strength. As the carbon content increases, the amount of martensite also increases. However, if the carbon content exceeds 0.16%, the strength of martensite increases, but the strength difference between martensite and ferrite, which has a low carbon concentration, becomes significant. This strength difference facilitates fracture at the interphase interface during plastic deformation, reducing ductility and work hardening rate. Furthermore, weldability deteriorates, resulting in welding defects during customer part processing. However, if the carbon content is lower than 0.08%, it becomes difficult to achieve the desired strength. Therefore, it is preferable to limit the carbon content to 0.08 to 0.16%. More preferably, the carbon content can be limited to the range of 0.09 to 0.15%.

[0023] Si: 0.8% or less (excluding 0%) Silicon (Si) is a ferrite-stabilizing element that promotes ferrite transformation and contributes to the formation of retained austenite by promoting carbon enrichment in untransformed austenite during the Q&P process. It also effectively increases the strength of ferrite through solid solution strengthening and reduces interphase hardness differences, making it a valuable element that ensures strength without reducing the ductility of steel sheets. However, if Si exceeds 0.8%, it can induce surface scale defects, adversely affecting surface quality and reducing weldability and phosphatability. Therefore, its addition amount should be limited to 0.8% or less, and more preferably 0.6% or less.

[0024] Mn: 2.0 to 3.0% Manganese (Mn) refines grain size without reducing ductility, completely precipitates sulfur (S) in steel as MnS, and strengthens the steel. At the same time, it lowers the critical cooling rate for the formation of martensite in dual-phase steels, facilitating martensite formation. A manganese content of less than 2.0% makes it difficult to achieve the strength targeted by the present invention. On the other hand, a manganese content exceeding 3.0% increases the likelihood of problems with weldability and hot rolling, leading to unstable material properties due to excessive martensite formation and the formation of Mn-bands (bands of Mn oxides) within the structure, increasing the risk of processing cracks and sheet fractures. Furthermore, Mn oxides dissolve on the surface during annealing, significantly impairing surface quality. Therefore, in the present invention, the Mn content is preferably limited to a range of 2.0 to 3.0%, more preferably 2.2 to 2.8%.

[0025] Mo: 0.4% or less (excluding 0%) Molybdenum (Mo) is an element that delays the transformation of austenite to pearlite while also refining ferrite and improving strength. Mo has the advantages of improving the hardenability of steel and finely forming martensite at grain boundaries, allowing for control of the yield ratio. However, as an expensive element, a higher Mo content increases manufacturing costs, resulting in a cost disadvantage. Therefore, it is preferable to appropriately control the Mo content. To achieve the above-mentioned effects, a maximum of 0.4% is preferably added. However, if the Mo content exceeds 0.4%, the alloy cost increases sharply, reducing economic viability, and the excessive grain refinement and solid solution strengthening effects can actually reduce the ductility of the steel. Therefore, in the present invention, the Mo content is preferably limited to 0.4% or less, more preferably 0.2% or less. Note that 0% Mo is excluded in the present invention to take into account the amount of Mo that is inevitably added during manufacturing.

[0026] Cr: 1.0% or less (excluding 0%) Chromium (Cr) is an element added to improve the hardening ability of steel and ensure high strength. It also plays a very important role in the formation of martensite, minimizing the decrease in elongation rate while increasing strength, making it advantageous for the production of dual-phase steel with high ductility. In particular, Cr is used in the hot rolling process. 23 Cr forms Cr-based carbides such as C6, some of which dissolve during annealing and some of which remain undissolved. This allows the amount of solute C in martensite after cooling to be controlled below an appropriate level, making it an advantageous element for producing dual-phase steels with low yield ratios by suppressing the occurrence of elongation at the yield point. However, if the Cr content exceeds 1.0%, not only does this effect saturate, but there are also problems with the deterioration of cold rolling due to an excessive increase in hot rolling strength, and the high and coarsening fraction of Cr-based carbides leads to coarsening of the martensite after annealing, resulting in a decrease in elongation. Therefore, in the present invention, the Cr content is preferably limited to 1.0% or less, and more preferably to 0.8% or less. Meanwhile, the Cr content in the present invention excludes 0%, taking into account the amount of Cr that is inevitably added during manufacturing.

[0027] P: 0.1% or less (excluding 0%) Phosphorus (P) is a substitutional element with the greatest solid solution strengthening effect, improving in-plane anisotropy and ensuring strength without significantly impairing formability. However, excessive addition of P above a certain level can significantly increase the likelihood of brittle fracture, potentially causing slab breakage during hot rolling, and can also act as an element that impairs the coating surface properties. Therefore, in the present invention, P is limited to a maximum of 0.1%, but 0% is excluded to take into account unavoidable addition levels.

[0028] S: 0.02% or less (excluding 0%) Sulfur (S) is an unavoidably added impurity element in steel that reduces ductility and weldability, so it is important to keep its content as low as possible. In particular, because it increases the likelihood of red shortness, it is preferable to control its content to 0.02% or less. However, 0% is excluded in consideration of the level of sulfur that is unavoidably added during the manufacturing process.

[0029] Sol.Al: 0.6% or less (excluding 0%) Acid-soluble aluminum (sol. Al) is an element added to refine the grain size and deoxidize steel. Similar to silicon, it is a ferrite stabilizer. It also distributes carbon from ferrite to austenite, improving the hardening ability of martensite and forming retained austenite. It is also a valuable element that effectively inhibits carbide precipitation in bainite during annealing, thereby improving the ductility of steel sheets. However, although its content exceeds 0.6%, it is beneficial for increasing strength through its grain refinement effect, but it also increases the likelihood of surface defects in plated steel sheets due to excessive inclusion formation during continuous steel casting operations, resulting in increased production costs. Therefore, in the present invention, the sol. Al content is preferably limited to 0.6% or less, more preferably 0.4% or less.

[0030] Ti, Nb: 0.001~0.04% each Titanium (Ti) and niobium (Nb) are elements that are effective in increasing the strength of steel sheets and refining grain size through the formation of nano-precipitates. When these elements are added, they combine with carbon to form very fine nano-precipitates, which strengthen the matrix and reduce the hardness difference between phases. If the Ti and Nb contents are less than 0.001%, it is difficult to ensure these effects. However, if the Ti and Nb contents exceed 0.04%, manufacturing costs increase and excessive precipitates can significantly reduce ductility. Therefore, the Ti and Nb contents should be limited to 0.001-0.04%.

[0031] N: 0.01% or less (excluding 0%) Nitrogen (N) is an element that effectively stabilizes austenite, but if it exceeds 0.01%, there are problems such as a sharp increase in steel refining costs and a significant increase in the risk of cracks occurring during continuous casting due to the formation of AlN, so it is preferable to limit the upper limit to 0.01%. However, 0% is excluded in consideration of the level that is unavoidably added.

[0032] B: 0.01% or less (excluding 0%) Boron (B) is a component that delays the transformation of austenite to pearlite during the cooling process during annealing, and is a hardening element that inhibits the formation of ferrite and promotes the formation of martensite. However, if the content exceeds 0.01%, excessive B will concentrate on the surface, which can lead to deterioration of plating adhesion. Therefore, the content should be controlled to 0.01% or less, and more preferably, 0.005% or less. Meanwhile, in the present invention, 0% B is excluded in consideration of the level of unavoidable addition of B.

[0033] Sb: 0.05% or less (excluding 0%) The present invention may also optionally contain antimony (Sb) in the range of 0.05% or less. Antimony (Sb) is distributed at grain boundaries and delays the diffusion of oxidizing elements such as Mn, Si, and Al through the grain boundaries, thereby suppressing surface oxide segregation and further suppressing coarsening of surface segregated oxides due to temperature increases and changes in the hot rolling process. However, if the content exceeds 0.05%, not only does this effect saturate but manufacturing costs and workability deteriorate. Therefore, it is preferable to limit the content to 0.05% or less, and more preferably to 0.03% or less. Meanwhile, in the present invention, 0% Sb is excluded in consideration of the level at which Sb is inevitably added. [Equation 1] C+Si / 30+Mn / 20+(Cr+Mo) / 5≧0.3 Here, C, Si, Mn, Cr and Mo refer to the content % of the steel components in the base structure at the 1 / 4t point in the base steel sheet thickness.

[0034] In the present invention, it is important to control the contents of C, Si, Al, Cr, and Mo among the alloy components in the base structure at the 1 / 4t point of the base steel sheet thickness so as to satisfy the above relational expression 1.

[0035] Si and Al are ferrite stabilizing elements that promote ferrite transformation and contribute to the formation of retained austenite and martensite by promoting the enrichment of carbon in untransformed austenite. Carbon also contributes to the formation and fraction adjustment of martensite by promoting the enrichment of carbon in untransformed austenite. However, C, Si, and Al have a negative effect on weldability, causing cracks on the surface and inside of the weld.

[0036] On the other hand, Mn, Cr, Mo, and B are elements that contribute to improving hardenability, but their effect on enriching C in austenite is relatively low compared to C, Si, and Al. Therefore, it is very important to properly adjust the ratios of C, Si, Al, and the other hardenable elements Mn, Cr, Mo, and B.

[0037] When the value defined by the above relational expression 1 is 0.3 or more, the relational expression HER*100 / YS between yield strength (YS) and hole expandability (HER) can be ensured to be 5 or more. Furthermore, the fraction of bainite and tempered martensite can be ensured to be 70% or more, and at the same time, the interphase hardness difference can be reduced to ensure a hole expandability value of 45% or more. Furthermore, the yield ratio (YS / TS) can be ensured to be 0.8 or more. However, when the value defined by the above relational expression 1 is less than 0.3, the above-mentioned effects cannot be expected.

[0038] In addition to the above components, the present invention may also comprise the balance Fe and other unavoidable impurities.

[0039] On the other hand, the hot-dip galvanized steel sheet of the present invention is a dual-phase steel sheet that can improve workability by increasing the yield ratio compared to conventional DP steels while still maintaining a certain degree of ductility. To achieve this, in addition to the above-mentioned alloy composition, it is necessary to satisfy the following microstructure and phase fraction control conditions.

[0040] Specifically, the hot-dip galvanized steel sheet of the present invention has a microstructure containing, in area %, 70% or more of bainite and tempered martensite in total, 10% or less of ferrite, and the remainder being fresh martensite and retained austenite.

[0041] To achieve the lean chemical composition and high ductility characteristics of DP steel while improving the yield ratio (YS / TS) compared to conventional DP steel, careful control of the structure and composition and careful heat treatment are essential. First, it is important to introduce a small amount of retained austenite. Retained austenite helps improve the ductility of steel sheets by inducing transformation-induced plasticity. To introduce this retained austenite, the steel undergoes a quenching and partitioning (Q&P) process, in which the steel is rapidly cooled to a temperature below Ms to form fresh martensite, and then reheated to a temperature above Ms. This process results in the formation of large amounts of tempered martensite and bainite, and the stable distribution of carbon contributes to the formation of a small amount of retained austenite in the final structure. To ensure high hole expandability, it is also important to minimize the formation of structures other than tempered martensite and bainite. Annealing in the single-phase region prevents the formation of ferrite during annealing, and the addition of hardening elements prevents ferrite formation even in the slow cooling region. In addition, ductility is improved by partially introducing bainite during the slow cooling period. Furthermore, the Q&P process, which involves rapid cooling below the Ms temperature and reheating above the Ms temperature, minimizes the formation of fresh martensite. Furthermore, the precipitation of fine nano-precipitates within ferrite further reduces the interphase hardness difference, improving workability. Finally, the introduction of a small amount of fresh martensite during final cooling ensures the desired strength.

[0042] Therefore, in the microstructure of the hot-dip galvanized steel sheet of the present invention, the total of bainite and tempered martensite is controlled to 70 area % or more. If the total of bainite and tempered martensite is less than 70 area %, there is a problem that the desired yield ratio and hole expandability cannot be ensured. More preferably, the total of bainite and tempered martensite can be limited to 75 area % or more.

[0043] In the present invention, the ferrite fraction in the microstructure of the hot-dip galvanized steel sheet is preferably limited to 10% or less. If the fraction exceeds 10%, there is a problem that the desired yield ratio and hole expandability cannot be ensured. More preferably, the proportion of the ferrite structure can be limited to 8% or less by area.

[0044] The hot-dip galvanized steel sheet of the present invention is composed of a balance of fresh martensite and retained austenite.

[0045] In this case, in the present invention, it is preferable to limit the area fraction of the retained austenite to less than 5% (excluding 0%). In order to increase the area fraction of the retained austenite to 5% or more, it is necessary to increase the Si content, which may cause the LME problem. More preferably, the area fraction of the retained austenite can be limited to the range of 1 to 4%.

[0046] The hot-dip galvanized steel sheet of the present invention having the above-described microstructure can achieve a superior yield ratio and improved workability and formability compared to conventional DP steels due to the effect of structural homogenization. Specifically, this can provide a high-strength hot-dip galvanized steel sheet having a hole expandability (HER) of 45% or more, a relationship between yield strength (YS) and hole expandability (HER) of 5 or more, a relationship between elongation (EL) and hole expandability (HER) of 700 or more, and a yield ratio (YS / TS) of 0.80 or more, while also having excellent ductility, hole expandability, and weldability, preventing surface and internal LME cracking in welds, and a tensile strength of 980 MPa or more.

[0047] Next, the method for producing a hot-dip galvanized steel sheet of the present invention will be described in detail.

[0048] The method for producing a hot-dip galvanized steel sheet of the present invention comprises the steps of: preparing a steel slab that satisfies the above-mentioned steel composition and Relational Formula 1, and then reheating the steel slab; hot-rolling the reheated steel slab so that the temperature at the outlet of finish rolling is Ar3 to Ar3 + 50°C, followed by coiling at 400 to 650°C, and then cooling to room temperature at an average cooling rate of 0.1°C / s or less; cold-rolling the cooled hot-rolled steel sheet at a reduction ratio of 40 to 70% to produce a cold-rolled steel sheet; and The process includes a step of continuously annealing at a temperature of Ac3-20°C to Ac3+20°C; a step of primarily cooling the continuously annealed cold-rolled steel sheet to a temperature range of 500 to 550°C at an average cooling rate of 10°C / s or less, and secondarily cooling to a temperature of 300 to 340°C at an average cooling rate of 5°C / s or more; a step of reheating the secondarily cooled cold-rolled steel sheet to a temperature of Ms or more and holding it for 60 seconds or more; and a step of cooling the held cold-rolled steel sheet to a temperature of 150°C or less at an average cooling rate of 5°C or more.

[0049] First, in the present invention, the steel slab constructed as described above is reheated under normal conditions. The slab reheating process is a process of heating the steel slab to smoothly carry out the subsequent rolling process and to sufficiently obtain the target physical properties of the steel sheet. The present invention is not particularly limited to these reheating conditions, and normal reheating conditions may be used. As an example, the slab is reheated in a temperature range of 1100 to 1300°C.

[0050] Next, in the present invention, the reheated steel slab is hot rolled so that the exit temperature of the finish rolling is Ar3 to Ar3 + 50°C, and then coiled at 400 to 650°C, and then cooled to room temperature at an average cooling rate of 0.1°C / s or less.

[0051] The reheated steel slab is finish hot rolled under normal conditions at a temperature above the Ar3 transformation point. The present invention is not limited to specific hot rolling conditions, and normal hot rolling temperatures can be used. For example, finish hot rolling can be performed in the temperature range of 800 to 1000°C.

[0052] In the present invention, the finish hot-rolled steel sheet is then coiled in a temperature range of 400 to 650°C, and then cooled to room temperature at an average cooling rate of 0.1°C / s or less to produce a hot-rolled steel sheet in which carbides that serve as austenite nucleation sites are finely dispersed. By uniformly dispersing fine carbides during this hot rolling process, the carbides dissolve in the subsequent annealing process to form finely dispersed austenite, resulting in uniformly dispersed fine martensite after annealing.

[0053] In the present invention, the cooled hot-rolled steel sheet is cold-rolled at a reduction ratio of 40 to 70% to produce a cold-rolled steel sheet.

[0054] If the cold reduction is less than 40%, it is difficult to obtain the target thickness and also difficult to correct the shape of the steel sheet, whereas if it exceeds 70%, there is a high possibility of cracks occurring at the edge of the steel sheet, which increases the cold rolling load. Therefore, in the present invention, it is preferable to limit the cold reduction to 40 to 70%.

[0055] Next, in the present invention, the cold-rolled steel sheet is continuously annealed at a temperature of Ac3-20°C to Ac3+20°C.

[0056] In the present invention, the cold-rolled steel sheet is subjected to continuous annealing in a temperature range of Ac3-20°C to Ac3+20°C. The purpose of this continuous annealing step is to form a single austenite phase and achieve a uniform microstructure. If the continuous annealing temperature is below Ac3-20°C, it is difficult to ensure a sufficient austenite fraction, resulting in an increased ferrite fraction after annealing and making it difficult to ensure a uniform microstructure. On the other hand, if the continuous annealing temperature exceeds Ac3+20°C, productivity decreases, and excessive austenite is formed, resulting in excessively coarse grains, making it difficult to ensure the desired strength. Furthermore, surface segregation of elements that deteriorate surface quality, such as Si, Mn, and B, becomes severe, which can degrade surface quality. In consideration of this, in the present invention, it is preferable to limit the continuous annealing temperature to Ac3-20°C to Ac3+20°C. More preferably, continuous annealing can be performed in a temperature range of 810 to 850°C.

[0057] In the present invention, fresh martensite is introduced by primarily cooling the continuously annealed cold-rolled steel sheet to a temperature range of 500 to 550°C at an average cooling rate of 10°C / s or less, and then secondary cooling to a temperature of 300 to 340°C at an average cooling rate of 5°C / s or more. In this case, in the present invention, the steel sheet can also be cooled using H2 gas during the secondary cooling.

[0058] In the present invention, it is very important to control the slow cooling temperature during the primary cooling to 500 to 550°C. If the temperature exceeds 550°C, bainite is not introduced in the slow cooling section, making it impossible to ensure the desired ductility. Also, if the temperature is controlled to less than 500°C, the proportion of bainite increases excessively, making it impossible to ensure the desired strength.

[0059] It is very important to control the quenching temperature during the secondary cooling to 300 to 340°C, which is below the martensite formation temperature Ms. If the temperature exceeds 340°C, the fraction of martensite initially formed will be very small, or martensite formation will be difficult, making it difficult to form the desired fractions of tempered martensite and bainite during final cooling, and the desired hole expandability will not be obtained. If the temperature is lowered below 300°C, the fractions of tempered martensite and bainite will be excessive, making it difficult to form the desired fraction of fresh martensite during final cooling, and the desired strength will not be obtained.

[0060] Subsequently, in the present invention, the secondarily cooled cold-rolled steel sheet is reheated to a temperature of Ms or higher, and then held for 60 seconds or longer.

[0061] In the present invention, it is important to control the slow cooling temperature, quenching temperature, and reheating temperature in the Q&P process, in which the steel sheet is reheated to above Ms after the above-mentioned first slow cooling process and second rapid cooling process in which the steel sheet is cooled to below Ms, to form the desired microstructure. By holding the reheated cold-rolled steel sheet for 60 seconds or more, tempered martensite and bainite are formed, and carbon is concentrated in the surrounding untransformed austenite.

[0062] Thereafter, in the present invention, the cold-rolled steel sheet thus held is cooled to a temperature of 150°C or less at an average cooling rate of 5°C / s or more to introduce fine fresh martensite.

[0063] In the present invention, the cooled steel sheet can be subjected to hot-dip galvanizing treatment as necessary to produce a hot-dip galvanized steel sheet. Specifically, the cooled cold-rolled steel sheet can be immersed in a zinc pot at 440 to 480°C, heated in a GA furnace at 500 to 540°C, and then cooled to produce a galvannealed steel sheet, or the hot-dip galvanized steel sheet can be produced by immersing the steel sheet in a zinc pot and then cooling it without alloying treatment.

[0064] In the present invention, the cold-rolled steel sheet or hot-dip galvanized steel sheet may be subjected to temper rolling of less than 1%. [Example]

[0065] The present invention will be described in detail below with reference to preferred examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0066] (Example) Steel slabs with the chemical compositions shown in Table 1 below were prepared and reheated to a temperature range of 1050-1250°C. They were then finish hot-rolled at a temperature range of Ar3 + 50°C, above the Ar3 transformation point. The hot-rolled steel sheets were then coiled at 400-650°C and cooled at a cooling rate of 0.1°C per second or less to produce hot-rolled steel sheets. The hot-rolled steel sheets were pickled and cold-rolled at a reduction of 40-70%. Subsequently, they were subjected to continuous annealing and Q&P heat treatment under the conditions shown in Table 2 below. They were then cooled to a temperature of 150°C or less at an average cooling rate of 5°C or more, immersed in a molten zinc pot at 460°C for 5 seconds, and subsequently heated to 520°C for approximately 2 seconds in a GA furnace and cooled to produce galvannealed steel sheets. These galvannealed steel sheets were then subjected to temper rolling of less than 1%. Meanwhile, in this experiment, the Q&P heat treatment was carried out with a cooling rate of 6°C / s for the first cooling, a cooling rate of 15°C / s for the second cooling, and a holding time of 200 seconds after reheating.

[0067] The mechanical properties and microstructural characteristics of each steel sheet manufactured as described above were evaluated, and the results are shown in Table 3 below. Tensile tests were performed on each specimen in the C direction according to JIS standards to evaluate tensile properties. The microstructural fraction was determined by analyzing the matrix structure at 1 / 4 of the thickness of the annealed steel sheet. Specifically, after Nital corrosion, the fractions of ferrite, bainite, martensite, and austenite were measured using an FE-SEM and an image analyzer. Hole expandability was also measured using a hole expandability tester. The occurrence of spot weld LME cracking was also evaluated.

[0068] [Table 1]

[0069] [Table 2]

[0070] [Table 3]

[0071] As shown in Tables 1-3 above, in the case of Invention Examples 1-4, in which the ranges of the steel composition, manufacturing conditions, and steel microstructure satisfy the ranges of the present invention, the hole expandability (HER) was 45% or more, the relationship between yield strength (YS) and hole expandability (HER) (HER*100 / YS) was 5 or more, and the relationship between elongation (EL) and hole expandability (HER) (HER*EL) was 700 or more. Furthermore, no surface or internal LME cracking occurred in the weld, and the yield ratio (YS / TS) was 0.80 or more, which shows that the material properties, hole expandability, and weldability of the steel sheet targeted by the present invention can be ensured.

[0072] In contrast, in Comparative Examples 1 to 10, in which the steel composition (Relationship 1) and / or steel manufacturing process are outside the range of the present invention, or the steel microstructure fraction is outside the range of the present invention, the relationship between yield strength (YS) and hole expandability (HER) (HER*100 / YS) is less than 5, or the relationship between elongation (EL) and hole expandability (HER) (HER*EL) is less than 700, the yield ratio (YS / TS) is less than 0.80, and the hole expandability is less than 45%. Therefore, it is clear that the strength, ductility, hole expandability, and weldability of the steel sheet targeted by the present invention cannot be simultaneously ensured.

[0073] Specifically, in Comparative Example 1, the annealing temperature was too high, resulting in a low elongation rate and making it impossible to ensure hole expandability, while in Comparative Example 2, the annealing temperature was too low, resulting in excessive formation of two-phase ferrite and making it impossible to obtain the target strength.

[0074] In Comparative Example 3, the primary cooling end temperature was too high, so bainite could not be obtained, and the YS was too high, so the elongation was poor.

[0075] In Comparative Example 4, the primary cooling end temperature was too low to ensure the target strength, and in Comparative Example 5, the secondary cooling end temperature was too high to ensure sufficient tempered martensite, and hole expandability was not ensured.

[0076] In Comparative Example 6, the secondary cooling end temperature was too low, resulting in the formation of excessive tempered martensite and bainite, resulting in a decrease in strength.

[0077] Meanwhile, Fig. 1 shows the change in the relational expression HER*100 / YS between yield strength (YS) and hole expandability (HER) according to relational expression 1 for the invention steel and the comparative steel in the examples of the present invention, Fig. 2 shows the change in hole expandability (HER) according to relational expression 1 for the invention steel and the comparative steel in the examples of the present invention, Fig. 3 shows the change in yield ratio according to relational expression 1 for the invention steel and the comparative steel in the examples of the present invention, and Fig. 4 is a diagram showing the change in the relational expression HER*EL between elongation (EL) and hole expandability (HER). Note that the invention steels in Figs. 1 to 4 refer to steels corresponding to invention examples 1 to 4, and the comparative steels refer to steels corresponding to comparative examples 7 to 10.

[0078] As described above, the detailed description of the present invention has been given with reference to the preferred embodiment of the present invention, but it is obvious that various modifications can be made by a person skilled in the art without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiment, but should be determined by the claims below as well as their equivalents.

Claims

1. The alloy contains, by weight, carbon (C): 0.08 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 2.0 to 3.0%, molybdenum (Mo): 0.4% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), aluminum (sol. Al): 0.6% or less (excluding 0%), titanium (Ti): 0.001 to 0.04%, niobium (Nb): 0.001 to 0.04%, nitrogen (N): 0.01% or less (excluding 0%), boron (B): 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities, and the C, Si, Mn, Cr, and Mo satisfy the following relational expression 1, The microstructure contains, by area percentage, 70% or more of bainite and tempered martensite in total, 10% or less of ferrite, and the remainder being fresh martensite and retained austenite; and A hot-dip galvanized steel sheet having a hole expandability (HER) of 45% or more, a relationship between yield strength (YS) and hole expandability (HER) of 5 or more, HER*100 / YS, a relationship between elongation (EL) and hole expandability (HER) of 700 or more, and a yield ratio (YS / TS) of 0.80 or more. [Relationship 1] C+Si / 30+Mn / 20+(Cr+Mo) / 5≧0.3 Here, C, Si, Mn, Cr and Mo refer to the content percentage of the steel components in the base structure at the point of 1 / 4t of the thickness of the base steel sheet.

2. The hot-dip galvanized steel sheet according to claim 1, wherein the retained austenite fraction is less than 5% (excluding 0%) in terms of area percentage.

3. The hot-dip galvanized steel sheet according to claim 2, wherein the retained austenite fraction satisfies 1 to 4% in terms of area %.

4. The hot-dip galvanized steel sheet according to claim 1, further comprising antimony (Sb): 0.05% or less (excluding 0%).

5. a step of preparing a steel slab containing, by weight, 0.08 to 0.16% carbon (C), 0.8% or less (excluding 0%) silicon (Si), 2.0 to 3.0% manganese (Mn), 0.4% or less (excluding 0%) molybdenum (Mo), 1.0% or less (excluding 0%) chromium (Cr), 0.1% or less (excluding 0%) phosphorus (P), 0.02% or less (excluding 0%) sulfur (S), 0.6% or less (excluding 0%) aluminum (sol. Al), 0.001 to 0.04% titanium (Ti), 0.001 to 0.04% niobium (Nb), 0.01% or less (excluding 0%) nitrogen (N), 0.01% or less (excluding 0%) boron (B), the balance being Fe and other inevitable impurities, wherein the C, Si, Mn, Cr, and Mo satisfy the following relational expression 1, and then reheating the steel slab; a step of hot rolling the reheated steel slab so that the exit temperature of the finish rolling is Ar3 to Ar3 + 50°C, subsequently coiling the slab at 400 to 650°C, and then cooling the slab to room temperature at an average cooling rate of 0.1°C / s or less; cold rolling the cooled hot-rolled steel sheet at a reduction ratio of 40 to 70% to produce a cold-rolled steel sheet; A step of continuously annealing the cold-rolled steel sheet at a temperature of Ac3-20°C to Ac3+20°C; a step of primarily cooling the continuously annealed cold-rolled steel sheet to a temperature range of 500 to 550°C at an average cooling rate of 10°C / s or less, and secondarily cooling to a temperature of 300 to 340°C at an average cooling rate of 5°C / s or more; Reheating the secondarily cooled cold-rolled steel sheet to a temperature of Ms or higher and then holding the temperature for 60 seconds or more; and a step of cooling the held cold-rolled steel sheet to a temperature of 150°C or less at an average cooling rate of 5°C / s or more. [Relationship 1] C+Si / 30+Mn / 20+(Cr+Mo) / 5≧0.3 Here, C, Si, Mn, Cr and Mo refer to the content percentage of the steel components in the base structure at the point of 1 / 4t of the thickness of the base steel sheet.

6. The method for producing a hot-dip galvanized steel sheet according to claim 5, wherein continuous annealing is performed in a temperature range of 810 to 850°C.

7. The method for producing a hot-dip galvanized steel sheet according to claim 5, wherein the steel sheet cooled to a temperature of 150°C or less at an average cooling rate of 5°C / s or more is subjected to an additional temper rolling of less than 1%.

8. The method for producing a hot-dip galvanized steel sheet according to claim 5, further comprising antimony (Sb): 0.05% or less (excluding 0%).

9. The method for producing a hot-dip galvanized steel sheet according to claim 5, further comprising the step of immersing the steel sheet cooled to a temperature of 150°C or less at an average cooling rate of 5°C / s or more in a zinc pot at 440 to 480°C, and then cooling the steel sheet.

10. The method for producing a hot-dip galvanized steel sheet according to claim 5, further comprising the steps of immersing the steel sheet cooled to a temperature of 150°C or less at an average cooling rate of 5°C / s or more in a zinc pot at 440 to 480°C, heating it in a GA furnace at 500 to 540°C, and then cooling it.

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