High-strength hot-dip galvanized steel sheet with excellent ductility and formability, and manufacturing method for the same

Abstract

To provide a high-strength hot-dip galvanized steel sheet with excellent ductility and formability, and a manufacturing method for the same.SOLUTION: A hot-dip galvanized steel sheet comprises, in weight%, carbon: 0.06 to 0.16%, silicon: 0.8% or less, manganese: 2.1 to 2.7%, molybdenum: 0.4% or less, chromium: 1% or less, phosphorus: 0.1% or less, sulfur: 0.02% or less, aluminum: 1% or less, titanium: 0.001 to 0.04%, niobium: 0.001 to 0.04%, nitrogen: 0.01% or less, boron: 0.01% or less, and antimony: 0.05% or less, the balance being Fe and other inevitable impurities, wherein the microstructure of the base steel sheet is composed of, in area%, 70% or more of the sum of bainite and tempered martensite, 10% or less of ferrite, and the balance being fresh martensite and retained austenite, and the fraction of the retained austenite is 5% or less in area%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a high-strength welding material having a tensile strength of 980 MPa or more, which is mainly used for automotive structural members. Regarding the production of hot dip galvanized steel sheets, more specifically, the yield strength (YS) and elongation (EL) The relationship YS × EL is 9000 or more, but the yield ratio (YS / TS) is 0.65 or more. The present invention relates to a hot-dip galvanized steel sheet having excellent ductility and formability, and a method for producing the same. [Background technology]

[0002] Recently, regulations in the automotive industry to protect the global environment have become increasingly strict. Therefore, fuel efficiency regulations are being strengthened, and in order to solve this, weight reduction and high The use of high-strength steel plates is required. In addition, regulations for impact safety to protect passengers are being implemented. In order to improve the impact resistance of the vehicle body, the members, seats, etc. Structural members such as seat rails and pillars are subject to yield stress. High-strength steel with excellent strength is used. However, increasing the strength of steel plates leads to the deterioration of ductility and formability. To solve this problem, we have developed a method to simultaneously achieve high strength and high formability. Generally, as the strength of steel plate increases, the elongation rate decreases. This leads to the problem of reduced workability, so there is a need to develop materials that can compensate for this. Generally, methods for strengthening steel include solid solution strengthening, precipitation strengthening, and grain strengthening. Strengthening by refinement and transformation strengthening have been studied. However, among the above methods, solid solution strengthening Steel materials using refined grains can be manufactured into high-strength steel with a tensile strength of 490 MPa or more. The problem is that it is extremely difficult to

[0003] On the other hand, precipitation-strengthened high-strength steels contain carbonitride-forming elements such as Nb, Ti, and V. By this, carbonitrides are precipitated, and the growth of crystal grains is inhibited by fine precipitates. This technology ensures strength by refining the crystal grains. The above technology is expensive despite its low manufacturing cost. It has the advantage of being easy to secure strength, but the recrystallization temperature rises rapidly due to fine precipitates. Therefore, high-temperature annealing is required to induce sufficient recrystallization and ensure ductility. In addition, the precipitation of carbonitrides in the ferrite matrix to strengthen the alloy has the disadvantage of being unable to be achieved. The problem with the strengthened steel is that it is difficult to obtain high strength steel of 600 MPa or more. .

[0004] Transformation-strengthened high-strength steel is a two-phase steel consisting of a soft ferrite matrix and a hard martensite. DP (Dual Phase) steel, which uses the transformation-induced plasticity of retained austenite TRIP (Transformation Induced Plastic) technology ensures high ductility. sticity steel, or a composite of ferrite and hard bainite or martensite Various steels have been developed, including CP (Complex Phase) steel, which is made up of a composite structure. Recently, automotive steel sheets have been made even stronger to improve fuel efficiency and durability. Steel plates are required for the vehicle body structure and reinforcement in terms of collision safety and passenger protection. Demand for high-strength steel plates with tensile strengths of 780 to 980 MPa or more is increasing. Among these, DP steel It has excellent ductility and is the most widely used steel sheet for automobiles. o, YR) is low, and the steel sheet has the disadvantage of being poor in formability and workability. Due to the trend towards higher strength, cracks and wrinkles occur during the press forming of automotive parts, resulting in complex It is difficult to manufacture complex parts using TRIP steel. It has excellent mechanical properties and good workability, but because a large amount of Si and Al is added to ensure high elongation, Therefore, it has the disadvantage of being poor in weldability.

[0005] In order to overcome these shortcomings of existing DP steels, careful heat treatment has been carried out to improve the properties of existing DP steels. By manufacturing steel that meets a certain level of yield ratio while maintaining high ductility, This allows for the expansion of high strength steels to more complex parts. Q&P (Quenching and Partitioning) is the latest heat treatment technology that can ensure This can be achieved by utilizing heat treatment (or "tioning").

[0006] As a conventional technique for simultaneously ensuring the ductility and workability of the high-tensile steel sheet, the following patent document is mentioned: The invention disclosed in the above technology is an example of an invention disclosed in the above technology. A significant amount of martensite is present, and fresh martensite (FM) is formed during the final cooling stage. However, fresh martensite has a high carbon content, which inhibits hole expansion, so heat treatment is required. The temperature must be carefully selected.

[0007] Another prior art is the invention disclosed in Patent Document 2. High strength and high ductility are achieved by utilizing tempered martensite produced by quenching heat treatment. At the same time, the present invention provides a method for producing a cold-rolled steel sheet that has an excellent sheet shape after continuous annealing. However, the above technology has a high carbon content of 0.2% or more, which results in poor weldability, and the amount of Si added is also limited to 1.0% or less. Because the temperature is high, there is a problem that depressions may occur inside the furnace during annealing.

[0008] The prior art disclosed in Patent Document 3 involves hole expansion by quenching and reheating. However, the amount of Si added is still 1.3 %, which is high, and this may cause dents inside the furnace. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-177278 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-090432 [Patent Document 3] Korean Patent Publication No. 2016-0173006 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention provides a steel sheet having a yield strength (YS) and elongation (EL) relationship of YS × EL of 9000 or more. It is used for structural components of automobiles, with a yield ratio (YS / TS) of 0.65 or more. The present invention aims to provide a hot-dip galvanized steel sheet having excellent ductility and formability, and a manufacturing method thereof. The target.

[0011] On the other hand, the object of the present invention is not limited to the above-mentioned contents. A person who can understand the present invention from the entire contents and has ordinary skill in the art to which the present invention pertains. If so, there is no difficulty in understanding the further object of the present invention. [Means for solving the problem]

[0012] One aspect of the present invention is By weight, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (0% excluding manganese (Mn): 2.1-2.7%, molybdenum (Mo): 0.4% or less ( 0% except), Chromium (Cr): 1% or less (0% except), Phosphorus (P): 0.1% or less ( 0%, sulfur (S): 0.02% or less, aluminum (sol.Al): 1% or less (excluding 0%), titanium (Ti): 0.001-0.04%, niobium (Nb): 0.00 1-0.04%, Nitrogen (N): 0.01% or less (excluding 0%), Boron (B): 0.01 %, antimony (Sb): 0.05% or less, balance Fe and other unavoidable impurities fruit, Among the steel components in the base structure at the 1 / 4t point of the base steel plate thickness, C, Si, Al, Mn, Cr , Mo and B components satisfy the following relational formula 1, The microstructure of the base steel sheet is the sum of bainite and tempered martensite by area %: 7 0% or more, ferrite: 10% or less, balance fresh martensite and retained austenite The fraction of the retained austenite is 5% or less by area. The present invention relates to a hot-dip galvanized steel sheet having excellent properties.

[0013] [Equation 1] (4×C+Si+Al) / (Mn+Cr+5×Mo+200×B)≦0.35

[0014] The above hot-dip galvanized steel sheets have a high hole expansion ratio (HOR). HER) is 30% or more, and the relationship between yield strength (YS) and elongation (EL) is YS × EL The yield ratio (YS / TS) can be 0.65 or more while the strain rate is 9000 or more. .

[0015] Another aspect of the present invention is a step of providing a steel slab that satisfies the above compositional components and Relational Formula 1, and then reheating the steel slab; The reheated slab is then rolled so that the exit temperature of the finish rolling mill is Ar3 to Ar3 + 50°C. Then, the steel is hot-rolled at 400 to 650°C, and then coiled at an average cooling rate of 0.1°C or less. and cooling the mixture to room temperature. A process of cold-rolling the cooled hot-rolled steel sheet at a reduction ratio of 40 to 70% to produce a cold-rolled steel sheet. and, A step of continuously annealing the cold-rolled steel sheet at a temperature of 820 to 860 ° C.; The above continuously annealed steel sheet is cooled to a temperature range of 630 to 680°C at an average cooling rate of 10°C or less. First cooling is performed using hydrogen gas to a temperature of 300-350°C at an average cooling rate of 5°C or more. After cooling, it is reheated to a temperature of 400-480°C and held for 60 seconds or more. , The steel sheet is subjected to hot-dip galvanizing at a temperature of 400 to 450°C, and then subjected to Ms to 10 and cooling the alloy to a temperature of 0°C or less at an average cooling rate of 5°C or more. The present invention relates to an excellent method for manufacturing hot-dip galvanized steel sheets.

[0016] The microstructure of the hot-dip galvanized steel sheet is composed of bainite and tempered martensite in terms of area percentage. Total of sites: 70% or more, ferrite: 10% or less, balance fresh martensite, It is composed of retained austenite, and the fraction of retained austenite is 5% or less by area. This can be done.

[0017] The method may further include a step of subjecting the manufactured hot-dip galvanized steel sheet to alloying heat treatment. . [Effects of the Invention]

[0018] As described above, the present invention optimizes the components and manufacturing process to achieve the high strength of DP steel. High strength weld metal with excellent yield strength / tensile strength (YS / TS) compared to conventional DP steels while still satisfying high ductility. This has a beneficial effect on the production of hot-dip galvanized steel sheets. By preventing processing defects such as cracks, it is possible to manufacture complex shapes that require high formability. It can be used in a variety of automotive structural components, and the material and plating properties can be improved at the same time. This has the advantage of being able to ensure [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 2 is a diagram showing the change in the relational expression YS×EL between yield strength (YS) and elongation (EL) depending on the yield ratio (YS / TS) in an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing changes in the relational expression YS×EL between yield strength (YS) and elongation (EL) according to Relational Expression 1 in an embodiment of the present invention. [Figure 3] FIG. 2 is a diagram showing changes in hole expandability value according to Relational Formula 1 in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described below.

[0021] The inventors have optimized the steel composition and manufacturing process to achieve a final microstructure containing retained austenite. By introducing ferrite, bainite, and fresh martensite, the conventional D It was confirmed that the yield ratio can be increased compared to P steel, improving workability. Such microstructural changes relieve local stress and deformation concentrations after necking. Improved ductility by delaying the generation, growth, and coalescence of voids that cause ductile fracture Furthermore, during final cooling, 5% or less of retained austenite is formed, improving ductility. It was confirmed through experiments that the present invention can be further improved. I began to complete the following.

[0022] That is, the present invention reduces the fraction of ferrite and martensite compared to existing DP steel. By reducing the amount of annealing and introducing retained austenite and bainite, the steel has a higher strength than existing DP steels. The yield ratio can be increased to ensure workability. This helps to improve ductility by forming a large number of mobile dislocations around the nitrides. Compared to existing DP steels, alloyed structure steels can maintain a high yield ratio while also ensuring ductility. This allows the production of high-tensile hot-dip galvanized steel sheets with excellent ductility and workability. It is possible.

[0023] The hot-dip galvanized steel sheet of the present invention, which is excellent in ductility and formability, has a carbon ( C): 0.06 to 0.16%, Silicon (Si): 0.8% or less (excluding 0%), Manganese Mn: 2.1-2.7%, Molybdenum (Mo): 0.4% or less (excluding 0%), Cr: 1% or less (excluding 0%), Phosphorus (P): 0.1% or less (excluding 0%), Sulfur Yellow (S): 0.02% or less, Aluminum (sol.Al): 1% 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, Antimony Sb: 0.05% or less, balance Fe and other unavoidable impurities. Among the steel components in the base structure at the 1 / 4t point, C, Si, Al, Mn, Cr, Mo, and The composition of B satisfies the following relational expression 1, and the microstructure of the base steel sheet is composed of bainite and tungsten in area percent. Total of bumped martensite: 70% or more, ferrite: 10% or less, balance fresh It is composed of martensite and retained austenite, and the fraction of the retained austenite is % is less than 5%.

[0024] First, the alloy composition of the base steel sheet constituting the hot-dip galvanized steel sheet of the present invention and its The reasons for limiting the content will be explained below. Here, "%" means % by weight unless otherwise specified. show.

[0025] C: 0.06~0.16% Carbon (C) is a very important element added to strengthen the transformed structure. Promotes the formation of hard martensite in woven steel, improving strength. Increased carbon content However, when the content exceeds 0.16%, the amount of martensite increases. Although the strength of rutensite increases, the difference in strength between it and ferrite, which has a low carbon concentration, becomes large. Due to this difference in strength, fracture is likely to occur at the interphase interface during plastic deformation, resulting in poor ductility and The work hardening rate is reduced. Also, due to poor weldability, welding defects occur when customers process parts. On the other hand, if the carbon content is lower than 0.06%, it is difficult to ensure the desired strength.

[0026] Therefore, in consideration of this, the carbon content is set to the range of 0.06 to 0.16% in the present invention. It is preferable to limit it to 0.07 to 0.15%, and more preferably to control it to the range of 0.07 to 0.15%.

[0027] Si: 0.8% or less (excluding 0%) Silicon (Si) is a ferrite stabilizing element that promotes ferrite transformation and contributes to the Q&P process. During this process, carbon enrichment in untransformed austenite is promoted, contributing to the formation of retained austenite. It is also effective in increasing the strength of ferrite through solid solution strengthening and reducing the hardness difference between phases. It is a useful element that is effective in ensuring strength without reducing the ductility of the steel sheet. However, if it exceeds 0.8%, it will induce surface scale defects and adversely affect the surface quality of the plating. Furthermore, the upper limit of the amount of addition is limited to 0.8% because it reduces weldability and phosphatability. More preferably, it is controlled to 0.7% or less.

[0028] Mn: 2.1-2.7% Manganese (Mn) refines grains without reducing ductility, and completely converts sulfur (S) in steel to Mn. S precipitates, preventing hot embrittlement due to the formation of FeS, and also strengthening the steel. At the same time, in dual-phase steels, it plays a role in lowering the critical cooling rate at which the martensite phase is obtained. Its content below 2.1% It is difficult to secure the strength targeted by the present invention, and if it exceeds 2.7%, the weldability and heat resistance will be deteriorated. There is a high possibility that problems such as cold rolling may occur, and excessive martensite may be formed, resulting in poor material quality. It is unstable, and Mn-Bands (bands of Mn oxides) are formed in the structure, causing processing cracks and There is a problem that the risk of breakage of the plate increases. Also, Mn oxides form on the surface during annealing. Therefore, in the present invention, Mn is dissolved into the It is preferable to limit the content to 2.1 to 2.7%, more preferably 2.3 to 2.5%. Control within the range.

[0029] Mo: 0.4% or less (excluding 0%) Molybdenum (Mo) retards the transformation of austenite to pearlite and Mo is an element that refines ferrite and improves strength. It is said that this improves the yield ratio and allows fine martensite to form at the grain boundaries. However, it is an expensive element, and the higher its content, the higher the production cost. However, since there is a problem that it is disadvantageous in terms of cost, it is preferable to appropriately control the content. To obtain the above-mentioned effects, it is preferable to add up to 0.4%. If the content exceeds 0.4%, the alloy cost will rise sharply, reducing its economic viability. The grain refinement effect and solid solution strengthening effect actually result in a decrease in the ductility of the steel. Therefore, in the present invention, the content of Mo is limited to 0.4% or less, and the addition of Mo that is unavoidable in manufacturing is Taking into consideration the amount of Mo to be used, 0% is excluded. More preferably, the Mo content is controlled to 0.3% or less.

[0030] Cr: 1% or less (excluding 0%) Chromium (Cr) is an element added to improve the hardenability of steel and ensure high strength. It is an element that plays a very important role in the formation of martensite, and it is also an element that contributes to the increase in strength. This minimizes the decrease in elongation compared to conventional methods, and is also advantageous for producing dual-phase steels with high ductility. In particular, Cr 23 Cr-based carbides such as C6 are formed, but these carbides are sintered. During the annealing process, some of the carbon dissolves and some remains undissolved. After cooling, the amount of solute carbon in the martensite is Since it is possible to limit it to an appropriate level or higher, it is possible to suppress the occurrence of yield point elongation and reduce the yield ratio. It is an element that is advantageous for the production of dual phase steels. However, if its content exceeds 1%, the effect is reduced. Not only does this cause saturation, but the excessive increase in hot rolling strength also leads to poor cold rolling properties. The fraction of Cr-based carbides increases and coarsens, resulting in a decrease in the size of martensite after annealing. Therefore, in the present invention, the content of Cr is It is preferable to limit the amount to 1% or less, and considering the amount that is inevitably added in manufacturing, 0% More preferably, the Cr content is controlled to 0.6% or less.

[0031] P: 0.1% or less (excluding 0%) Phosphorus (P) is a substitutional element with the greatest effect on solid solution strengthening, improving in-plane anisotropy and facilitating forming. It is the most advantageous element for ensuring strength without significantly impairing the properties. However, adding too much In this case, the possibility of brittle fracture increases significantly, and the slab may break during hot rolling. This has the problem that it acts as an element that inhibits the functionality and surface properties of plating. , limited to a maximum of 0.1%, except for 0% to take into account unavoidable levels of addition.

[0032] S: 0.02% or less (excluding 0%) Sulfur (S) is an impurity element that is inevitably added to steel and reduces ductility and weldability. It is important to keep it as low as possible because it is an element that causes red shortness. Therefore, its content must be controlled to 0.02% or less. However, 0% is excluded in consideration of the level that is inevitably added during the manufacturing process.

[0033] Sol.Al: 1.0% or less (excluding 0%) Acid-soluble aluminum (sol.Al) is an element added to refine the grain size of steel and to deoxidize it. It is a ferrite stabilizing element, just like Si. Partitions into austenite to improve martensite hardening ability and form retained austenite In addition, when the steel is held in the bainite region during annealing, It is a useful element that can effectively suppress the precipitation of carbides in steel sheets and improve the ductility of steel sheets. However, if its content exceeds 1.0%, the grain refinement effect reduces the strength. However, excessive formation of inclusions during continuous casting of steel can cause surface imperfections in the plated steel sheet. This not only increases the possibility of defects occurring, but also leads to an increase in manufacturing costs. Therefore, in the present invention, it is preferable to control the sol. Al content to 1.0% or less. .

[0034] ·Ti, Nb: 0.001~0.04% each Titanium (Ti) and niobium (Nb) increase the strength of steel sheets and reduce the crystallinity by forming nano-precipitates. These elements are effective in grain refinement. When these elements are added, they combine with carbon to form very fine grains. These nano-precipitates strengthen the matrix structure and form interphases. The Ti and Nb contents are less than 0.001% each. If the content is less than 0.04%, it is difficult to ensure such effects. This increases the manufacturing cost and can significantly reduce ductility due to excessive precipitates. Therefore, it is preferable to limit the contents of Ti and Nb to 0.001 to 0.04% each. More preferably, each of them is controlled to be in the range of 0.005 to 0.02%.

[0035] N: 0.01% or less (excluding 0%) Nitrogen (N) is an element that has an effective effect on stabilizing austenite, but 0.01% If the temperature exceeds this limit, the cost of refining steel will rise sharply, and the formation of AlN will cause problems. The risk of cracks occurring during continuous casting increases significantly, so the upper limit is set at 0.01%. However, 0% is excluded in consideration of the level that is inevitably added.

[0036] ·B: 0.003% or less Boron (B) prevents austenite from transforming into pearlite during annealing and cooling. It is a hardening element that retards the formation of ferrite and promotes the formation of martensite. However, if the content exceeds 0.003%, excessive B will be concentrated on the surface. Since this can cause deterioration of plating adhesion, its content is controlled to 0.003% or less. More preferably, the B content is controlled to 0.002% or less.

[0037] ·Sb: 0.05% or less Antimony (Sb) is distributed at the grain boundaries and acts as a barrier against the grains of oxidizing elements such as Mn, Si, and Al. By slowing down the diffusion through the boundary, the surface concentration of oxides is suppressed, and the temperature rise and It has an excellent effect in suppressing the coarsening of surface precipitates due to changes in the hot rolling process. If the content exceeds 0.05%, not only does the effect saturate, but the manufacturing cost and workability also increase. Therefore, the Sb content is limited to 0.05% or less. Control it to below 3%.

[0038] In addition to the above components, the present invention preferably comprises the balance Fe and other unavoidable impurities. stomach.

[0039] Next, the hot-dip galvanized steel sheet of the present invention has a higher yield ratio than existing DP steel, improving workability. To this end, in addition to the above alloy composition, The following conditions for controlling the microstructure and phase fraction of the base steel sheet must be satisfied. The fraction, distribution and concentration of components in the microstructure are described.

[0040] The hot-dip galvanized steel sheet of the present invention has a microstructure that is composed of bainite and tempered steel in area percentage. Total of fresh martensite: 70% or more, ferrite: 10% or less, balance fresh martensite The area percentage of the retained austenite is 5%. If the sum of bainite and tempered martensite is less than 70%, If the ferrite content is too low or exceeds 10%, the desired yield ratio cannot be secured. In addition, in order for the retained austenite to exceed 5%, the contents of Si and Al must be increased. There is a problem that must be solved.

[0041] The hot-dip galvanized steel sheet of the present invention has a structure of the steel components in the base structure at the 1 / 4t point of the thickness of the base steel sheet. Among these, the components C, Si, Al, Mn, Cr, Mo and B satisfy the following relational expression 1.

[0042] [Equation 1] (4×C+Si+Al) / (Mn+Cr+5×Mo+200×B)≦0.35

[0043] The present invention provides a steel sheet having a yield strength (YS) and elongation (EL) relationship of YS × EL of 9000 or more. We manufacture hot-dip galvanized steel sheets with a yield ratio (YS / TS) of 0.65 or more. For this purpose, the steel composition in the base structure at the 1 / 4t point of the base steel plate thickness can be The components of C, Si, Al, Mn, Cr, Mo, and B are controlled to satisfy the above relational expression 1. It is important to note that Si and Al are ferrite stabilizing elements and This promotes the formation of austenite and promotes the concentration of carbon in untransformed austenite, thereby reducing the amount of retained austenite. C is also an element that contributes to the formation of untransformed austenite and martensite. It is an element that contributes to the formation of martensite and adjustment of its fraction by promoting shrinkage. On the other hand, Mn, Cr, Mo, and B are elements that contribute to improving hardening ability, but C, Si, and Al The effect of C on the enrichment of austenite is relatively low compared to that of C, S. It is important to properly adjust the ratio of Al and other hardening elements Mn, Cr, Mo, and B. It's always important.

[0044] If the value defined by the above relational expression 1 is 0.35 or less, as described above, The relationship between yield strength (YS) and elongation (EL) based on the yield ratio (YS / TS) is expressed as YS × EL. 000 or more, and the distribution of bainite and tempered martensite The hardness difference between the phases is reduced to ensure a hole expandability value of 30% or more. On the other hand, if the value defined by the above relational expression 1 exceeds 0.35, The effect of the above will be lost.

[0045] In this way, ferrite, bainite, martensite, and retained austenite are formed simultaneously. The composite structure thus formed disperses each phase finely and uniformly, and minimizes the hardness difference between the phases. By reducing the yield strength, the yield ratio is superior to conventional DP steel, and workability and formability are improved. Furthermore, such changes in the microstructure can be attributed to local stresses after necking. Relieves the concentration of forces and deformations, slowing the formation, growth and coalescence of voids that cause ductile fracture This has the effect of improving ductility.

[0046] This resulted in a hole expansion ratio (HER) of 30 % or more, and the relationship between yield strength (YS) and elongation (EL) is YS × EL of 9000 or more. and provide hot-dip galvanized steel sheets with a yield ratio (YS / TS) of 0.65 or more. It is possible.

[0047] Next, a method for producing a hot-dip galvanized steel sheet having excellent ductility and formability according to the present invention will be described. do.

[0048] The present invention satisfies the lean chemical composition and high ductility characteristics of DP steel, while In order to improve the yield strength ratio (YS / TS) compared to conventional DP steels, it is necessary to control the structure and components and First, it is important to introduce a small amount of retained austenite. The retained austenite induces transformation-induced plasticity and helps improve the ductility of the steel plate. In order to introduce such retained austenite, the steel is cooled to a temperature below Ms during quenching. After forming some martensite, it is immediately reheated to a temperature above Ms to form the partition. At this time, a large amount of bainite is formed, C is stably distributed, and Bainite contributes to the formation of retained austenite in the final structure. This has the effect of reducing the hardness difference between the phases of steel and martensite. The process of partially forming ferrite is also important. Annealing in the single-phase region or just below the single-phase region The ferrite fraction is controlled to 10% or less, and a small amount of additional ferrite is formed in the slow cooling section. This will further improve ductility. By precipitating it within the phase, the hardness difference between the phases can be further reduced, improving workability. Finally, by introducing a small amount of fresh martensite during the final cooling, This makes it possible to ensure the desired strength.

[0049] To achieve this, the method for producing a hot-dip galvanized steel sheet of the present invention is based on the above-mentioned compositional components. a step of providing a steel slab that satisfies Equation 1 and then reheating the steel slab; The finish rolling is hot rolled so that the exit temperature is Ar3 to Ar3 + 50°C, and then After coiling at 00-650℃, the wire is cooled to room temperature at an average cooling rate of 0.1℃ or less. The cooled hot-rolled steel sheet is then cold-rolled at a reduction rate 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 820 to 860°C; The steel plate is cooled to a temperature range of 630-680°C at an average cooling rate of 10°C or less, and then cooled with water. Secondary cooling is performed using nitrogen gas to a temperature of 300-350°C at an average cooling rate of 5°C or more. a step of reheating to a temperature of 400 to 480°C and then holding the temperature for 60 seconds or more; The steel sheet is hot-dip galvanized at a temperature of 400 to 450°C, and then heated to a temperature of Ms to 100°C or less. and cooling the mixture to 100°C at an average cooling rate of 5°C or more.

[0050] First, a steel slab having the above-mentioned composition is prepared, and then reheated. The process is carried out to ensure that the subsequent rolling process is carried out smoothly and that the steel slab is sufficiently thick to obtain the desired physical properties of the steel plate. The present invention is not particularly limited to such reheating conditions, and ordinary reheating conditions may be used. Any heating conditions are acceptable. For example, reheating in the temperature range of 1100 to 1300°C is acceptable. The following are some of the reasons:

[0051] Next, in the present invention, the reheated steel slab is subjected to finish rolling at an outlet temperature of Ar3 to Finish hot rolling is performed so that the temperature becomes Ar3+50°C. There are no restrictions on the rolling conditions, and ordinary hot rolling temperatures can be used.

[0052] Thereafter, in the present invention, the above-mentioned finish hot-rolled steel sheet is subjected to a temperature treatment in the range of 400 to 650°C. After winding, the wire is cooled to room temperature at an average cooling rate of 0.1°C or less to remove austenitic The hot rolling process produces a hot rolled steel sheet in which carbides are finely dispersed, which act as nucleation sites for iron oxide. By dispersing fine carbides uniformly through this process, the carbides dissolve and the o- As a result, fine martensite is formed uniformly after annealing. It can be dispersed.

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

[0054] The coiled hot-rolled steel sheet is pickled and then cold-rolled at a reduction ratio of 40 to 70%. However, if the cold rolling reduction rate is less than 40%, it is difficult to obtain the target thickness. On the other hand, if the thickness exceeds 70%, the edge of the steel plate will be damaged. There is a high possibility of cracks occurring, which causes a problem of load during cold rolling. Therefore, in the present invention, it is preferable to limit the cold rolling reduction to 40 to 70%.

[0055] Next, in the present invention, the cold-rolled steel sheet is subjected to continuous annealing in a temperature range of 820 to 860°C. Such a continuous annealing process forms ferrite and austenite simultaneously with recrystallization, and carbon If the continuous annealing temperature is less than 820°C, sufficient It is difficult to secure the austenite fraction, and it is difficult to obtain the desired martensite and vein structure after annealing. On the other hand, at temperatures above 860°C, the fraction of inite and retained austenite cannot be ensured. This leads to a decrease in productivity and the formation of excess austenite, which becomes bainite and mafic acid after cooling. The ruthenium fraction increases significantly, increasing yield strength and decreasing ductility. In addition, elements such as Si, Mn and B reduce the wettability of hot dip galvanizing. This can cause surface segregation of elements that cause plating to deteriorate, resulting in a deterioration in the surface quality of the plating.

[0056] In the present invention, the continuously annealed steel sheet is heated to a temperature range of 630 to 680°C for 10 minutes. Cooling is performed at an average cooling rate of 50°C or less, and then the temperature is lowered to 300-350°C by 5°C using hydrogen gas. After secondary cooling at an average cooling rate of 400-480°C, the material is reheated to a temperature of 400-480°C and then cooled for 60 minutes. Hold for more than a second.

[0057] The above continuously annealed steel sheet is cooled to a temperature range of 630 to 680°C at an average cooling rate of 10°C or less. Then, the temperature is cooled to 300-360°C using hydrogen gas quenching equipment. Secondary cooling is performed at an average cooling rate of 5°C / s or more to introduce some fresh martensite. Then, immediately reheat to a temperature of 400-480°C and hold for 60 seconds or more to This causes the formation of ferrite and concentrates carbon in the surrounding untransformed austenite.

[0058] In this case, the quenching temperature during the secondary cooling is set to 300 to 500°C below the martensite formation temperature Ms. It is very important to control the quenching temperature to 360°C. If the quenching temperature exceeds 360°C, , the fraction of martensite formed in the early stage is very small, or martensite formation is This makes it difficult to achieve smooth carbon partitioning and the desired fraction of residual carbon is not obtained during final cooling. On the other hand, if the temperature is below 300°C, the sheet shape will be deteriorated and the design will be deteriorated. This may cause a load on the equipment.

[0059] It is also important to control the reheating temperature to 400 to 480°C, which is above the Ms temperature. If the reheating temperature is less than 400°C, the formation of bainite does not occur quickly, and the carbon When the temperature exceeds 480°C, the bainite partitioning is not smooth and the bainite is small. As a result, the fraction of fresh martensite increases during final cooling. During Q&P annealing, the secondary quenching temperature and reheating temperature are carefully controlled to form the desired microstructure. It is very important to achieve this.

[0060] Next, in the present invention, the held steel sheet is hot-dip galvanized at a temperature of 400 to 450°C. After the treatment, the material is cooled to a temperature of Ms~100°C or less at an average cooling rate of 5°C or more. This produces a final product in which fresh martensite is formed adjacent to bainite. At this time, temper rolling can be performed at a reduction rate of less than 1% if necessary. .

[0061] In the present invention, the hot-dip galvanized steel sheet thus produced is further subjected to a step of alloying heat treatment. It can also be included in the above. [Example]

[0062] The present invention will be described in detail below with reference to examples.

[0063] (Example) A steel slab having the composition shown in Table 1 below was prepared. After reheating to a temperature range of 1250°C, the temperature was reduced to 950°C within the range of Ar3 to Ar3 + 50°C. The hot-rolled steel sheet was then coiled at 400 to 650°C. After that, the hot-rolled steel sheets were produced by cooling at a cooling rate of 0.1°C per second or less. After that, the cold rolled steel sheet was subjected to the following rolling reduction. After continuous annealing at the temperatures shown in Table 2, Q&P heat treatment was carried out under the conditions shown in Table 2 below. The QP heat-treated cold-rolled steel sheet is then hot-dip galvanized and then alloyed. After final cooling to introduce fresh martensite and retained austenite, less than 1% The steel sheets were then subjected to full temper rolling to produce hot-dip galvanized steel sheets.

[0064] The microstructure and mechanical properties of each of the steel sheets manufactured as described above were evaluated. The results are shown in Table 3 below. At this time, the tensile test for each test piece was carried out according to ASTM standards. The test was carried out in the L direction using a steel sheet, and the tensile properties (tensile strength (TS), yield strength (YS) and elongation ( The fraction of the microstructure was evaluated at the 1 / 4th thickness point of the annealed steel sheet. The base structure was analyzed using the results. Specifically, after Nital corrosion, FE-SEM and Image analyzer was used to analyze ferrite, bainite, and fresh martensite. The fractions of ferrite and austenite were measured. Also, the hole expandability was measured using a hole expandability tester. did.

[0065] [Table 1] *In the steel sheet composition in Table 1 above, N is contained as an impurity element within the range of 30 to 50 ppm. It has been done.

[0066] [Table 2]

[0067] [Table 3] *In Table 3, F is ferrite, B is bainite, TM is tempered martensite, FM stands for fresh martensite and RA stands for retained austenite.

[0068] As shown in Table 1-3 above, the compositional components of the steel and the conditions of the manufacturing process satisfy the requirements of the present invention. In the case of invention examples 1-6, the yield strength (YS) and elongation ratio (YS × EL) are 9000 or more. The yield ratio (YS / TS) is 0.65 or more, and the steel sheet has the quality and It is clear that workability can be ensured.

[0069] On the other hand, if the composition of the steel and the conditions of the manufacturing process are outside the scope of the present invention, or if the steel In the case of Comparative Examples 1 to 10, in which the fraction and occupancy ratio of the internal structure are outside the range of the present invention, the yield strength The relationship between the degree of elongation (YS) and the elongation rate (YS × EL) is less than 9000, or the yield ratio (YS / T Therefore, the strength of the steel sheet targeted in the present invention, It was not possible to simultaneously ensure ductility, workability, and weldability.

[0070] Specifically, in Comparative Example 1, the composition of the steel falls within the range of the present invention, but the secondary cooling temperature is The cooling rate is too low, with the residual austenite fraction exceeding 5%, and the excessive cooling rate places a burden on the equipment. A load was generated.

[0071] In Comparative Example 2, although the composition of the steel falls within the range of the present invention, the secondary cooling temperature was too high. The formation of bumped martensite is insufficient and carbon partitioning occurs. As a result, the target yield ratio cannot be achieved.

[0072] In Comparative Example 3, although the composition of the steel is within the range of the present invention, the reheating temperature is too low and the temper In Comparative Example 4, excessive permalloy martensite was formed, and the desired strength was not obtained. The temperature is too high, which reduces the proportion of bainite and increases the proportion of fresh martensite. As a result, the target yield ratio cannot be achieved.

[0073] Comparative Examples 5-10 are cases where the steel composition and manufacturing process conditions are outside the range of the present invention. Specifically, in Comparative Example 5-6, the composition of the steel is outside the range of the present invention, and the continuous annealing temperature is The reheating temperature is outside the range of the invention, and excessive ferrite is formed, resulting in a lower than desired yield ratio. In addition, in Comparative Examples 7 and 8, the composition of the steel was outside the range of the present invention, and The secondary cooling temperature and reheating temperature were too high to achieve the target yield ratio. The heat temperature was too high and the target yield ratio could not be achieved.

[0074] On the other hand, Fig. 1 shows the yield ratio (Y Graph showing the change in the yield strength (YS) and elongation (EL) relationship (YS × EL) due to the change in S / TS 2 shows that in the examples of the present invention (invention steel 1-6 and comparison steel 5-10), the relational expression 1 1 is a graph showing the change in the relationship between yield strength (YS) and elongation (EL) YS × EL due to the 3 shows that in the examples of the present invention (invention steel 1-6 and comparison steel 5-10), the 1-3 shows the change in hole expandability value due to the change in the steel sheet thickness. The invention steel corresponding to 1-6 is shown.

[0075] As mentioned above, the detailed description of the present invention describes the preferred embodiment of the present invention. However, a person having ordinary skill in the art to which the present invention pertains would be able to easily understand the above-mentioned invention from the perspective of the present invention. It goes without saying that various modifications are possible within the scope of the present invention. The scope of the invention should not be limited to the described embodiments, but should be understood as set forth in the following claims. and should be determined not only by the provisions of the Act but also by something equivalent thereto.

Claims

1. In weight percent, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (0 %, manganese (Mn): 2.1 to 2.7%, molybdenum (Mo): 0.4% or less (excluding 0%), Chromium (Cr): 1% or less (excluding 0%), Phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.02% or less, aluminum (sol. Al): 1% or less Below (excluding 0%), titanium (Ti): 0.001-0.04%, niobium (Nb): 0.0 0.01-0.04%, Nitrogen (N): 0.01% or less (excluding 0%), Boron (B): 0.0 1% or less, antimony (Sb): 0.05% or less, balance Fe and other unavoidable impurities Including, Among the steel components in the base structure at the 1 / 4t point of the thickness of the base steel plate, C, Si, Al, Mn, C The components r, Mo, and B satisfy the following relational formula 1: The microstructure of the base steel sheet is, in area%, the sum of bainite and tempered martensite: 7 0% or more, ferrite: 10% or less, balance fresh martensite and retained austenite The fraction of the retained austenite is 5% or less by area percent. Hot-dip galvanized steel sheet with excellent corrosion resistance. [Relationship 1] (4×C+Si+Al) / (Mn+Cr+5×Mo+200×B)≦0.35

2. The hot-dip galvanized steel sheet has a hole expansion ratio (Hole Expansion Ratio, HER) is 30% or more, and the relation between yield strength (YS) and elongation (EL) is YS × EL 10. The method according to claim 1, wherein the yield strength ratio (YS / TS) is 9000 or more and the yield strength ratio (YS / TS) is 0.65 or more. The hot-dip galvanized steel sheet has excellent ductility and formability.

3. In weight percent, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (0 %, manganese (Mn): 2.1 to 2.7%, molybdenum (Mo): 0.4% or less (excluding 0%), Chromium (Cr): 1% or less (excluding 0%), Phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.02% or less, aluminum (sol. Al): 1% or less Below (excluding 0%), titanium (Ti): 0.001-0.04%, niobium (Nb): 0.0 0.01-0.04%, Nitrogen (N): 0.01% or less (excluding 0%), Boron (B): 0.0 1% or less, antimony (Sb): 0.05% or less, balance Fe and other unavoidable impurities and a step of providing a steel slab that satisfies the following relational expression 1, and then reheating the steel slab. The reheated slab is rolled so that the exit temperature of the finish rolling is Ar3 to Ar3 + 50°C. Then, the steel sheet is hot-rolled at 400 to 650°C, and then coiled at an average cooling rate of 0.1°C or less. and cooling the mixture to room temperature. cold-rolling the cooled hot-rolled steel sheet at a reduction ratio of 40 to 70% to produce a cold-rolled steel sheet; and, A step of continuously annealing the cold-rolled steel sheet at a temperature of 820 to 860 ° C.; The continuously annealed steel sheet is cooled to a temperature range of 630 to 680°C at an average cooling rate of 10°C or less. First cooling is performed using hydrogen gas to a temperature of 300-350°C at an average cooling rate of 5°C or more in 2 minutes. After cooling, the material is reheated to a temperature of 400 to 480°C and held for 60 seconds or more. 、 The steel sheet is subjected to hot-dip galvanizing at a temperature of 400 to 450°C, and then subjected to Ms to 10 and cooling the mixture to a temperature of 0°C or less at an average cooling rate of 5°C or more. A manufacturing method for hot-dip galvanized steel sheets with excellent properties. [Relationship 1] (4×C+Si+Al) / (Mn+Cr+5×Mo+200×B)≦0.35

4. The microstructure of the hot-dip galvanized steel sheet is, in terms of area %, bainite and tempered martensite. total of sites: 70% or more, ferrite: 10% or less, balance fresh martensite, It is composed of retained austenite, and the fraction of retained austenite is 5% or less by area percent. The method for producing a hot-dip galvanized steel sheet having excellent ductility and formability according to claim 3.

5. The method according to claim 3, further comprising a step of subjecting the manufactured hot-dip galvanized steel sheet to an alloying heat treatment. A method for producing the hot-dip galvanized steel sheet having excellent ductility and formability described above.

6. The hot-dip galvanized steel sheet according to claim 3 is subjected to temper rolling at a reduction ratio of less than 1%. A method for producing the hot-dip galvanized steel sheet having excellent ductility and formability described above.

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