Environmentally friendly high-strength, highly formable steel plate and its manufacturing method

Abstract

The present invention relates to a steel sheet for use in automobiles and the like, which not only has the characteristics of high strength and high formability but also is produced in an environmentally friendly manner, and to a method for producing the same.

Description

[Technical field]

[0001] The present invention relates to a steel sheet for use in automobiles and the like, which not only has the characteristics of high strength and high formability but also is produced in an environmentally friendly manner, and to a method for producing the same. [Background technology]

[0002] Improving fuel efficiency and durability are important issues that automakers must address. To achieve this, the use of thin, high-strength steels can simultaneously improve various issues such as the environment, fuel efficiency, crashworthiness, and durability. For example, the Insurance Institute for Highway Safety in the United States has been gradually tightening regulations on crash stability for passenger protection, and since 2013 has required severe crash components such as a 25% small overlap. One solution to this problem is to reduce the weight of automobiles, but to achieve weight reduction, high strength steel is required, and high formability is also required.

[0003] In particular, as regulations on the impact stability of automobiles become more stringent, steel with excellent strength is being used for structural members such as members, seat rails, pillars, etc., which are used to improve the impact resistance of the vehicle body. These parts have complex shapes according to stability and design, and are mainly manufactured by forming them using press dies, so they require high strength as well as a high level of formability.

[0004] However, while high strength steel has the advantage of absorbing impact energy, high strength generally comes with the problem of low elongation and poor formability. Furthermore, if the yield strength is excessively high, the flow of material into the die during forming is reduced, resulting in poor formability. Therefore, the automotive industry is calling on the steel industry to develop steel materials with excellent strength and formability, i.e., a good balance between strength and elongation (TS x El).

[0005] Steel manufacturers are developing various products to meet these needs, such as dual phase steel (DP steel), transformation induced plasticity steel (TRIP steel), complex phase steel (CP steel), and ferrite-bainite steel (FB steel), and these products are manufactured through the processes of ironmaking, steelmaking, continuous casting, hot rolling, and cold rolling and annealing.

[0006] Meanwhile, with the increasing demand for environmental protection around the world, companies are concentrating their efforts on ESG (Environment, Social, Governance) and reducing carbon dioxide (CO2) emissions. Many steelmakers have announced plans to achieve zero carbon dioxide emissions, and as the conventional method of coke reduction has the problem of emitting large amounts of carbon dioxide, there is growing interest in environmentally friendly technologies such as DRI technology, which uses hydrogen for reduction.

[0007] Normally, the energy required for cold rolling and annealing is generated using by-product gases from the ironmaking process, but when environmentally friendly technologies such as DRI are applied, the by-product gases are reduced and energy costs increase, so technology to lower the heat treatment temperature is necessary to save energy. Furthermore, using by-product gases to create a heat source also generates CO2, so it is necessary to minimize its usage. Summary of the Invention [Problem to be solved by the invention]

[0008] An aspect of the present invention is to provide a steel sheet having high strength and formability that can be produced in an environmentally friendly manner by reducing carbon dioxide (CO2) emissions, and a method for producing the same.

[0009] The object of the present invention is not limited to the above. Further object of the present invention is described in the overall content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no problem in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0010] One aspect of the present invention is a steel sheet having, by weight percent, C: 0.05 to 0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0 to 2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1% or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, S: 0.01% or less, and the remainder including Fe and unavoidable impurities, The microstructure of the steel sheet is, in terms of area fraction, 15 to 35% of a hard phase and 65 to 85% of a soft phase, and the steel sheet is an environmentally friendly, high-strength, high-formability steel sheet.

[0011] Another aspect of the present invention is a method for manufacturing a hot-rolled steel sheet using a steel slab containing, by weight percent, C: 0.05 to 0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0 to 2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1% or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, S: 0.01% or less, and the remainder being Fe and unavoidable impurities; cold rolling the hot rolled steel sheet at a rolling reduction of 70 to 90% to produce a cold rolled steel sheet; Heating the cold-rolled steel sheet to a temperature range of Ac1 to Ac1+50° C. and maintaining the temperature; and a cooling step of slowly cooling the cold-rolled steel sheet to a temperature range of 650 to 700°C at an average cooling rate of 1 to 10°C / s, and then rapidly cooling the cold-rolled steel sheet to a temperature range of 300 to 580°C at an average cooling rate of 5 to 50°C / s. Effect of the Invention

[0012] The present invention provides a steel sheet with high strength and formability, particularly with an excellent balance between strength and ductility (TS×El), which can prevent processing defects such as cracks or wrinkles during press forming, and is therefore suitable for use in structural parts that require processing into complex shapes. It is also effective in producing automobile parts with excellent crash resistance that are less susceptible to cracks in the event of an unavoidable automobile collision.

[0013] In addition, by lowering the annealing heat treatment temperature during the manufacturing process, it is possible to provide a steel sheet and its manufacturing method that are manufactured in an eco-friendly manner by reducing the generation of carbon dioxide (CO2).

[0014] The various and beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief description of the drawings]

[0015] [Figure 1] 1 is a graphical representation of the heat treatment steps of a continuous annealing process. [Diagram 2] An arbitrary Fe-C phase diagram to illustrate the relationship between composition and annealing temperature. [Diagram 3] FIG. 2 is a schematic diagram illustrating an example of a method for measuring the aspect ratio of a hard phase. [Figure 4] 1 is a photograph showing the microstructure of Example 1 of the embodiments. [Diagram 5] 1 is a photograph showing the microstructure of Comparative Example 1 among the Examples. [Figure 6] 1 is a photograph showing the microstructure of Comparative Example 4 among the Examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The terms used herein are for the purpose of describing the invention and are not intended to be limiting of the invention. Additionally, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the relevant definition clearly indicates otherwise.

[0017] The meaning of "comprise" as used herein is to embody features and does not exclude the presence or addition of other features.

[0018] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the present disclosure.

[0019] Representative high-strength steels used as automotive materials include dual phase steel (DP steel), transformation induced plasticity steel (TRIP steel), complex phase steel (CP steel), and ferrite-bainite steel (FB steel).

[0020] Among these, DP steels contain a soft phase and a hard phase, and may contain some retained austenite. Such DP steels have low yield strength, high tensile strength, low yield ratio (YR), high work hardening rate, high ductility, continuous yield behavior, room temperature aging resistance, bake hardenability, and in some cases excellent hole expandability.

[0021] However, in order to secure ultra-high strength of tensile strength of 780 MPa or more, the fraction of hard phases such as martensite, which is advantageous for improving strength, must be increased, but in this case, the yield strength increases and defects such as cracks may occur during press forming. Therefore, it is important to secure excellent properties in both strength and elongation.

[0022] Therefore, the inventors of the present invention have increased the reduction ratio of cold rolling performed at room temperature to finely disperse the structure, and adjusted the heat treatment temperature to increase the driving force for recrystallization, thereby inducing sufficient recrystallization of the soft phase, which affects the ductility of the steel. In addition, they have confirmed that it is possible to ensure a good balance between strength and elongation by uniformly ensuring the refinement and distribution of the hard phase, which is advantageous for ensuring strength, and have thus completed the present invention.

[0023] Meanwhile, in order to manufacture the DP steel, a steel slab is prepared, and then the steel slab is manufactured through hot rolling, cold rolling, and annealing processes. The cold rolling process is a process mainly performed in the manufacture of cold-rolled steel sheets, and means that a hot-rolled coil is rolled at a certain rolling reduction at room temperature. Usually, the cold rolling is performed by reversible rolling using a TCM (Tandum Cold Rolling Mill). The TCM has the advantage of mass production due to low manufacturing costs. Meanwhile, the annealing process reduces hardness and improves workability through recrystallization and phase transformation phenomena by heating and holding a steel sheet (cold-rolled steel sheet) at a certain temperature range in a heating furnace. A steel sheet that has not undergone the annealing process has high hardness, especially surface hardness, and insufficient workability, whereas a steel sheet that has undergone the annealing process has a recrystallized structure, thereby obtaining the effect of reducing hardness, yield point, and tensile strength.

[0024] The annealing process requires a large amount of energy because the steel sheet must be heated at room temperature and then further heated to a high temperature, which not only increases energy costs and the cost of purifying the gas generated after combustion, but also inevitably increases the generation of pollutants such as carbon dioxide (CO2), making it unfriendly to the environment. Therefore, the inventors of the present invention have studied a method for lowering the heating temperature of the annealing process and a method for minimizing the generation of pollutants such as carbon dioxide (CO2) during the energy production process and the post-combustion treatment process. Therefore, in order to save energy in the heat treatment process, which generates a high rate of CO2 and requires high energy costs, the inventors have developed a technology for ensuring excellent material properties even when the heat treatment temperature is low by increasing the cold reduction rate during cold rolling after hot rolling, and have completed the present invention.

[0025] That is, the present invention provides a steel sheet and a manufacturing method thereof that not only saves energy and minimizes pollutants by applying the above-mentioned low annealing temperature, but also has an excellent balance between strength and elongation and is environmentally friendly.

[0026] First, the steel sheet according to one embodiment of the present invention will be described in detail. The alloy composition of the steel sheet is, in weight percent, C: 0.05-0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0-2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1% or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, S: 0.01% or less, and the remainder includes Fe and inevitable impurities. The alloy composition is described in detail below. Unless otherwise specified in the present invention, the content of each element is based on weight percent.

[0027] Carbon (C): 0.05~0.10% The C is an important element added for solid solution strengthening, and such C combines with precipitate elements to form fine precipitates, thereby contributing to improving the strength of the steel. If the C content exceeds 0.10%, the hardening ability increases, and when manufacturing the steel, martensite is formed during cooling, which increases the strength excessively, but may lead to a decrease in elongation. In addition, the weldability decreases, and there is a risk of welding defects occurring during processing into parts. If the C content is less than 0.05%, it may be difficult to ensure the target level of strength. More advantageously, it is preferably 0.06 to 0.08%.

[0028] Silicon (Si): 0.3% or less (excluding 0) The above-mentioned Si is a ferrite stabilizing element, and is advantageous in securing a target level of ferrite fraction by promoting ferrite transformation. In addition, it has excellent solid solution strengthening ability, is effective in increasing the strength of ferrite, and is a useful element in securing strength without reducing the ductility of steel. If the above-mentioned Si content exceeds 0.3%, the solid solution strengthening effect becomes excessive, and rather the ductility decreases, and it may induce surface scale defects, adversely affect the surface quality of the plating, and inhibit the chemical conversion treatability. More advantageously, it is preferable that the content is 0.1% or less.

[0029] Manganese (Mn): 2.0-2.5% The Mn element prevents hot embrittlement caused by the formation of FeS by precipitating sulfur (S) in steel as MnS, and is advantageous for solid solution strengthening of steel. If the Mn content is less than 2.0%, not only is the above effect not obtained, but it is also difficult to ensure the target level of strength. On the other hand, if the Mn content exceeds 2.5%, problems such as weldability and hot rolling are likely to occur, and martensite is more easily formed due to the increased hardenability, so that ductility may decrease. In addition, there is a problem that Mn oxide bands are excessively formed in the structure, increasing the risk of defects such as processing cracks. And there is a problem that Mn oxide dissolves on the surface during annealing, greatly impairing platability. More advantageously, the Mn content is preferably 2.2 to 2.4%.

[0030] Titanium (Ti): 0.05% or less (excluding 0) The Ti is an element that forms fine carbides, and contributes to ensuring yield strength and tensile strength. In addition, Ti has the effect of precipitating N in steel as TiN and suppressing the formation of AlN from Al that is inevitably present in steel, and has the effect of reducing the possibility of cracks occurring during continuous casting. If the Ti content exceeds 0.05%, coarse carbides may precipitate, and the strength and elongation may decrease due to the reduction in the carbon content in the steel. In addition, there is a problem that nozzle clogging may be induced during continuous casting, and manufacturing costs may increase. Therefore, the Ti content is preferably 0.05% or less, and more than 0%.

[0031] Niobium (Nb): 0.1% or less (excluding 0) The above-mentioned Nb is an element that segregates at the austenite grain boundaries, suppresses the coarsening of austenite grains during annealing heat treatment, and forms fine carbides to contribute to improving strength. If the content of Nb exceeds 0.1%, coarse carbides are precipitated, and the strength and elongation may decrease due to the reduction in carbides in the steel, resulting in a problem of increased manufacturing costs. The above-mentioned Nb is preferably 0.1% or less, and more preferably 0% or more.

[0032] Chromium (Cr): 1.5% or less (excluding 0) The Cr element facilitates the formation of bainite, inhibits the formation of martensite during annealing heat treatment, and forms fine carbides to contribute to improving strength. If the Cr content exceeds 1.5%, bainite is excessively formed, reducing elongation, and if carbides are formed at grain boundaries, there is a possibility that strength and elongation will decrease, resulting in an increase in manufacturing costs. Therefore, the Cr content is preferably 1.5% or less, and more preferably more than 0%.

[0033] Phosphorus (P): 0.1% or less The above P is a substitutional element with the greatest effect of solid solution strengthening, and is an element advantageous in improving in-plane anisotropy and ensuring strength without significantly reducing formability. However, when the above P is added in excess, there is a problem that the possibility of brittle fracture occurring increases significantly, the possibility of slab breakage occurring during hot rolling increases, and the coating surface properties are impaired. Therefore, the content of the above P is preferably 0.1% or less, and 0% can be excluded in consideration of the level of unavoidable inclusion.

[0034] Sulfur (S): 0.01% or less The above-mentioned S is an element that is inevitably added as an impurity element in steel, and since it impairs ductility, it is preferable to control its content as low as possible. In particular, since S has the problem of increasing the possibility of generating red shortness, it is preferable to control its content to 0.01% or less. However, 0% can be excluded in consideration of the level of unavoidable inclusion.

[0035] The remainder includes iron (Fe), and in the normal manufacturing process, unintended impurities from the raw materials or the surrounding environment are inevitably mixed in, and cannot be excluded. Since these impurities are known to anyone with ordinary skill in the manufacturing process, the entire contents of these impurities are not specifically mentioned in this specification.

[0036] The high-strength steel sheet of the present invention preferably has a microstructure composed of a hard phase and a soft phase, and in particular, by maximizing ferrite recrystallization through an optimized annealing process, the microstructure ultimately includes a structure in which hard phases, bainite and martensite, are uniformly distributed in the recrystallized ferrite matrix. The hard phase in the microstructure is mainly martensite, and refers to a phase in which a small amount of bainite is partially contained and mixed, and the soft phase refers to a ferrite phase. With regard to the deformation characteristics in a structure composed of a soft phase and a hard phase, the soft phase determines the formability, and the hard phase determines the strength.

[0037] The hard phase is preferably contained in an area fraction of 15 to 35%. If the hard phase fraction is too high, the strength is high but the elongation is low, and if the soft phase fraction is high, the elongation is high but the strength is low. In order to ensure the strength of 780 MPa or more provided by the present invention, the hard phase is preferably contained in an area fraction of 15% or more, and preferably does not exceed 35% in order to ensure formability.

[0038] In order to ensure appropriate strength and formability, the soft phase is preferably 65 to 85% in terms of area fraction. The ferrite of the soft phase can be divided into recrystallized ferrite and non-recrystallized ferrite. As shown in FIG. 3, the difference between recrystallized ferrite and non-recrystallized ferrite can be divided by the aspect ratio of the grain size to the rolling direction. Non-recrystallized ferrite has a large aspect ratio as shown in FIG. 3(b), and when analyzed in detail, linear deformation structures are observed within the ferrite grains. On the other hand, since recrystallized ferrite is advantageous for ensuring formability, it is preferable that the recrystallized ferrite of the soft phase is 60% or more, and although non-recrystallized ferrite is a soft phase, it reduces formability when its fraction is high, so it is preferable that the recrystallized ferrite of the soft phase is 5% or less.

[0039] On the other hand, the aspect ratio of the hard phase is preferably 1.2 or less. The aspect ratio means the ratio (b / a) of the major axis (b) and minor axis (a) of the crystal grain size in the rolling direction, as shown in (a) and (b) of FIG. 3, and the aspect ratio of the hard phase is the aspect ratio of the structure formed by stretching the hard phase in the rolling direction. If the aspect ratio of the hard phase increases, it adversely affects bending, which is important for deformation resistance in the thickness direction. In addition, if the aspect ratio of the hard phase increases, it reduces the hole expandability. Therefore, it is important to control the aspect ratio of the hard phase as low as possible, so it is preferable that it does not exceed 1.2.

[0040] The steel sheet of the present invention has a high strength of tensile strength (TS) of 780 MPa or more and an elongation of 18% or more, and can ensure excellent strength and formability.

[0041] Next, one embodiment of the method for manufacturing a steel sheet according to the present invention will be described in detail. The steel sheet according to the present invention can be manufactured by first preparing a steel slab, heating it, hot rolling it, coiling and cooling it, cold rolling it, and then continuous annealing it. Each step will be described in detail below.

[0042] Steel slab heating A steel slab containing the above-mentioned alloy composition, i.e., by weight percent, C: 0.05-0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0-2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1% or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, S: 0.01% or less, and the remainder being Fe and inevitable impurities, is prepared and then heated. This is for smoothly performing the subsequent hot rolling process and ensuring the target physical properties of the steel sheet, and the heating process conditions are not particularly limited, and may be any method and conditions normally used in the technical field to which the present invention belongs. As an example, it is preferable to heat the slab to a temperature range of 1100 to 1300°C.

[0043] Hot rolling The heated steel slab is hot-rolled to produce a hot-rolled steel sheet. At this time, the finish hot rolling is preferably performed at an outlet temperature in the range of Ar3 to 1000°C. If the outlet temperature during the finish hot rolling is less than Ar3, the hot deformation resistance increases rapidly, and the top, tail and edge of the hot rolled coil become single-phase regions, which may increase the in-plane anisotropy and reduce the formability. On the other hand, if the temperature exceeds 1000°C, the rolling load is relatively reduced, which is advantageous for productivity, but there is a risk of thick oxide scale being generated. More preferably, the process may be performed in the temperature range of 760 to 940°C.

[0044] Winding and cooling The hot-rolled steel sheet produced by the hot rolling can be wound into a coil. The winding can be performed at a temperature range of 400 to 700°C. If the winding temperature is less than 400°C, excessive martensite or bainite is formed, which leads to an excessive increase in strength of the hot-rolled steel sheet, and problems such as defective shape due to load may occur during subsequent cold rolling. On the other hand, if the winding temperature exceeds 700°C, the surface scale may increase, which may deteriorate the pickling property.

[0045] Meanwhile, it is preferable to cool the coiled hot-rolled steel sheet to room temperature at an average cooling rate of 0.1°C / s or less (excluding 0). The coiled hot-rolled steel sheet may be cooled after undergoing processes such as transportation and storage, and the process before cooling is not limited thereto. By cooling the coiled hot-rolled steel sheet at a constant rate, it is possible to obtain a hot-rolled steel sheet in which carbides, which become austenite nucleation sites, are finely dispersed.

[0046] Thereafter, before the subsequent cold rolling, a process of removing surface scale by pickling the surface of the hot-rolled steel sheet may be further performed. The pickling method is not particularly limited, and may be a method commonly used in the technical field to which the present invention belongs.

[0047] Cold rolling The hot-rolled steel sheet coiled as described above can be cold-rolled at room temperature with a constant rolling reduction to produce a cold-rolled steel sheet.

[0048] During the cold rolling, it is preferable to perform the cold rolling at a reduction ratio of 70 to 90%. If the reduction ratio of the cold rolling is less than 70%, the driving force for recrystallization is reduced, resulting in the formation of coarse ferrite, and the formation of austenite is also reduced, so that the austenite fraction can be sufficiently secured only by increasing the soaking temperature of the annealing furnace. On the other hand, if the cold rolling ratio exceeds 90%, there is a high possibility that cracks will occur at the edge of the steel sheet, and the initial thickness before rolling must be made excessively thick, resulting in an increase in the number of rolling passes and a decrease in productivity.

[0049] The method of performing the cold rolling is not particularly limited in the present invention, and any method that is performed in the technical field to which the present invention belongs can be applied. For example, there are a TCM (Tandum Cold Rolling Mill) method and a ZRM (Sendzimir Rolling Mill) method. To explain these methods briefly, TCM is a reversible rolling method, and has the advantage of being excellent in productivity because of low manufacturing costs and the possibility of mass production, but has the disadvantage of being somewhat restricted in applying a rolling force. ZRM is a reversible arrangement type, and has the disadvantage of being low in productivity, but has the advantage of being somewhat easy to apply a rolling force.

[0050] The reduction ratio of the cold rolling is an important operational factor for improving the phase transformation of steel and enhancing various physical properties, and therefore, controlling the reduction ratio is particularly important for ensuring quality. In the present invention, it is preferable to adopt an appropriate method in consideration of the material, size, operating environment, etc. of the product.

[0051] Continuous Annealing It is preferable to continuously anneal the cold rolled steel sheet thus manufactured. The continuous annealing process can be performed, for example, in a continuous annealing furnace (CAL). An example of the heat treatment steps of the continuous annealing process is shown in the graph of FIG. 1. As shown in FIG. 1, the heat treatment steps in the annealing furnace may include a heating section (HS), a soaking section (SS), a slow cooling section (SCS), a rapid cooling section (RCS), and an over aging section (OAS). In general, the temperature of each section is measured at the end of each section, so the temperature means the temperature at the end of each section. For example, the rapid cooling section (RCS) temperature is the temperature of the section where the rapid cooling section ends, and is represented by 4 in the case of FIG. 1.

[0052] In the heating zone (HS), the steel sheet is heated at a constant heating rate, and as the temperature of the steel sheet increases, dislocation recovery, cementite precipitation, ferrite recrystallization, and reverse transformation in the two-phase region occur. The sheet threading speed varies depending on the thickness and width of the steel sheet, and the microstructural changes in each temperature range may vary depending on the initial hot rolling structure and cold rolling reduction rate.

[0053] When entering the soaking zone (SS), the material is held at a certain temperature for a certain time, during which reverse transformation to austenite in the two-phase region or single-phase region is observed depending on the annealing temperature. The soaking zone (SS) is known to be one of the zones that consume the most energy in an annealing furnace. In the slow cooling zone (SCS), the material is usually cooled at a low cooling rate, and after the SCS, the material is continuously cooled at a high cooling rate in the rapid cooling zone (RCS). Depending on the RCS set temperature and the degree of hardening ability, some bainite may be generated during cooling.

[0054] Meanwhile, FIG. 2 is an arbitrary Fe-C phase diagram, and when the temperature T1 is determined for an arbitrary component, the ratio of austenite and ferrite corresponding to the temperature can be determined by the lever rule. That is, the temperature of the soaking zone (SS) is closely related to phase transformation. Factors that affect phase transformation and changes in the state of a material include temperature, pressure, composition, etc., and when the composition is determined, it can be adjusted by temperature and pressure. In particular, the higher the temperature and pressure, the faster the phase transformation during heating in the annealing furnace can proceed, but the higher the temperature, the higher the energy cost consumed and the higher the carbon emissions such as carbon dioxide after combustion, which is not environmentally friendly. In the steel manufacturing process, the variable compared to pressure is the cold reduction rate, and if the cold reduction rate is increased at the same temperature, the phase transformation proceeds rapidly, but if the cold reduction rate is increased in the opposite concept, phase transformation can be made even at a low temperature. Using this principle, in the present invention, the cold reduction rate is set to 70 to 90%, which is higher than the conventional method.

[0055] The soaking zone temperature in a normal annealing process is generally in the range of Ac1+30°C to Ac3-30°C. However, as described above, in the present invention, even if the cold rolling reduction is increased and heat treatment is performed at a low temperature, ferrite recrystallization and austenite formation are possible, so in the annealing process of the present invention, it is preferable to heat to and hold a temperature range of Ac1 to Ac1+50°C. Even in the above temperature range, the present invention can reduce hardness and improve workability through recrystallization and phase transformation phenomena.

[0056] The cold-rolled steel sheet heat-treated in the above temperature range can be cooled to form a target structure, and stepwise cooling is preferably performed. In the present invention, the stepwise cooling can be performed in a slow cooling zone (SCS) and a rapid cooling zone (RCS). For example, it is preferable to perform slow cooling at an average cooling rate of 1 to 10°C / s to a temperature range of 650 to 700°C, and then rapid cooling at an average cooling rate of 5 to 50°C / s to a temperature range of 300 to 580°C. By slowing down the cooling rate during slow cooling, it is possible to suppress defects in the sheet shape due to a sudden temperature drop during subsequent rapid cooling.

[0057] If the end temperature of the slow cooling is less than 650°C, the carbon diffusion activity is low due to the low temperature, and the carbon concentration in ferrite is high, while the carbon concentration in austenite is low, and the fraction of the hard phase becomes excessive, which increases the yield ratio and increases the tendency for cracks to occur during processing. In addition, the temperature difference from the soaking zone becomes too large, and the shape of the plate may become uneven. If the end temperature exceeds 700°C, there is a disadvantage that an excessively high cooling rate is required during subsequent cooling (quenching). In addition, if the average cooling rate during the slow cooling exceeds 10°C / s, carbon diffusion cannot occur sufficiently, and it is preferable to cool at an average cooling rate of 1°C / s or more in consideration of productivity.

[0058] After the slow cooling is completed, rapid cooling is performed. If the rapid cooling end temperature is less than 300°C, the cooling variation occurs in the width and length directions of the steel sheet, which may cause deterioration of the sheet shape, and if it exceeds 580°C, the hard phase may not be sufficiently secured, which may cause a decrease in strength. On the other hand, if the average cooling rate during the rapid cooling is less than 5°C / s, the fraction of the hard phase may be excessive, and if it exceeds 50°C / s, the hard phase may be insufficient.

[0059] Meanwhile, in the annealing process, after the cooling is completed, overaging treatment (OAS) can be performed as necessary. The overaging treatment is a process of holding the steel for a certain period of time after the quenching end temperature. The overaging treatment does not require any separate treatment and can be regarded as a type of air cooling treatment. The overaging treatment has the effect of improving the shape quality by homogenizing the coil in the width direction and length direction of the coil. For this reason, the overaging treatment can be performed for 200 to 800 seconds. EXAMPLES

[0060] Hereinafter, the embodiments of the present invention will be described. It goes without saying that the following embodiments can be modified in various ways by those having ordinary skill in the art to which the present invention belongs, without departing from the scope of the present invention. The following embodiments are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be defined by the claims below as well as equivalents thereto.

[0061] (Example) Steel slabs having the alloy composition shown in Table 1 below (units are weight %, and the remainder not shown in Table 1 is Fe and unavoidable impurities) were prepared, and then each steel slab was heated at 1200°C for 1 hour, and then finish hot-rolled at a finish rolling temperature of 800 to 920°C to produce hot-rolled steel sheets. The hot-rolled steel sheets were cooled at a cooling rate of 0.1°C / s and coiled at 650°C. The coiled hot-rolled steel sheets were then cold-rolled at reductions of 40% and 80% to produce cold-rolled steel sheets.

[0062] The manufactured cold-rolled steel sheets were heated to an annealing temperature in the range of 730 to 860°C, and heat treated under the annealing temperature conditions in Table 2. The temperatures of each stage of the annealing heat treatment in the heating zone (HS), soaking zone (SS), slow cooling zone (SCS), rapid cooling zone (RCS), and overaging zone (OAS) in Fig. 1 are shown in Table 2. Meanwhile, slow cooling (SCS section in Table 2) was performed at an average cooling rate of 3°C / s, and rapid cooling (RCS section in Table 2) was performed at an average cooling rate of 20°C / s.

[0063] [Table 1]

[0064] [Table 2]

[0065] The microstructure of each of the steel sheets produced by the above method was observed, and the mechanical properties and plating properties were evaluated. The results are shown in Table 3 below.

[0066] In this case, the tensile test for each test piece was carried out by taking a JIS No. 5 size tensile test piece in the direction perpendicular to the rolling direction and then carrying out the tensile test at a strain rate of 0.01 / s.

[0067] Among the structural phases, the unrecrystallized ferrite was observed using a SEM at a magnification of 5000 times after nital etching. From the crystal grain shape of the observed ferrite phase, subgrains observed in normal unrecrystallized ferrite or particles elongated in the rolling direction were analyzed as unrecrystallized ferrite and their fractions were measured. For the other phases, their fractions were also measured using a SEM and an image analyzer after nital etching. The aspect ratio of the hard phase was measured by measuring the ratio of the horizontal (a) and vertical (b) in the rolling direction as shown in Figure 3, which is a commonly used method.

[0068] [Table 3]

[0069] In the above Table 3, F means ferrite, YS means yield strength, and TS means tensile strength.

[0070] As shown in Tables 1 to 3 above, in Examples 1 to 3, the steel alloy composition and manufacturing conditions, particularly the continuous annealing process, satisfy all of the conditions proposed in the present invention, and therefore the required fine structure can be obtained even at a low annealing temperature, the physical properties are good, and the elongation is excellent while having high strength, and since the annealing temperature is low, an environmentally friendly manufacturing process is provided.

[0071] FIG. 4 is an SEM photograph of the microstructure of Example 1, in which 60% or more of the ferrite is recrystallized, the hard phase has a rounded shape, and the aspect ratio is 1.2 or less.

[0072] In Comparative Examples 1 and 2, since the reduction ratio is low, when the annealing temperature is lowered, ferrite recrystallization is insufficient and austenite is rapidly formed during heating, and although strength is ensured, there is a problem that the elongation is low. In particular, in Comparative Example 2, the same heat treatment as in Invention Example 2 was performed, but recrystallization did not occur smoothly due to the low reduction ratio, and the fraction of recrystallized ferrite was low, resulting in a decrease in elongation. Figure 5 is an SEM photograph of the microstructure of Comparative Example 1, and a large amount of non-recrystallized ferrite was observed.

[0073] In Comparative Examples 3 to 7, the elongation does not satisfy the target physical properties. In particular, there are problems in that the elongation is low due to the formation of coarse austenite due to the low rolling reduction, and the energy efficiency is low due to the high annealing temperature. Fig. 6 is an SEM photograph of the microstructure of Comparative Example 4, which shows the characteristics of a large hard phase fraction and a large aspect ratio, and it was confirmed that the formability such as the elongation was reduced.

[0074] Comparative Examples 8 to 11 have the problems of low energy efficiency due to high annealing temperatures, and low elongation due to a high fraction of secondary phases.

Claims

1. In weight percent, it contains C: 0.05-0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0-2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1% or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, and S: 0.01% or less, with the remainder being Fe and unavoidable impurities. The microstructure consists of a hard phase of 15-35% and a soft phase of 65-85% in terms of area fraction. The aforementioned soft phase is ferrite, and includes a surface fraction in which recrystallized ferrite accounts for 60% or more and unrecrystallized ferrite accounts for 5% or less. The aforementioned hard phase is a mixed structure in which bainite is mixed with martensite or martensite matrix. An environmentally friendly, high-strength, and highly formable steel sheet with a tensile strength (TS) of 780 MPa or higher and an elongation (El) of 18% or higher.

2. The environmentally friendly, high-strength, and highly formable steel sheet according to claim 1, including one in which the aspect ratio of the hard phase is 1.2 or less.

3. A step of manufacturing a hot-rolled steel sheet using a steel slab containing, by weight percent, C: 0.05-0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0-2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1% or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, and S: 0.01% or less, with the remainder being Fe and unavoidable impurities. The steps include: cold rolling the aforementioned hot-rolled steel sheet at a reduction ratio of 70-90% to produce a cold-rolled steel sheet; The steps include heating and holding the cold-rolled steel sheet to a temperature range of Ac1 to Ac1+50°C, A method for producing an environmentally friendly high-strength, highly formable steel sheet according to claim 1 or 2, comprising a cooling step of slowly cooling the cold-rolled steel sheet to a temperature range of 650 to 700°C at an average cooling rate of 1 to 10°C / s, and then rapidly cooling it to a temperature range of 300 to 580°C at an average cooling rate of 5 to 50°C / s.

4. The aforementioned hot-rolled steel sheet is The steps include heating the steel slab to a temperature range of 1100 to 1300°C, The process involves hot rolling, which is performed by finishing rolling a heated steel slab at a temperature range of Ar3 to 1000°C, The process involves hot rolling followed by winding at a temperature range of 400 to 700°C and cooling at a cooling rate of 0.1°C / s or less. A method for manufacturing an environmentally friendly, high-strength, and highly formable steel sheet according to claim 3, including the method described in claim 3.

5. The method for manufacturing an environmentally friendly high-strength, highly formable steel sheet according to claim 3, further comprising the step of performing an overaging treatment for 200 to 800 seconds after the rapid cooling.

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