KR102440772B1 - High strength steel sheet having excellent workability and manufacturing method for the same - Google Patents

Abstract

High-strength steel sheet having excellent formability according to an aspect of the present invention, by weight, carbon (C): 0.13 to 0.2%, manganese (Mn): 2.5 to 3%, silicon (Si): 0.4 to 0.6%, acid Soluble aluminum (sol.Al): 0.02~0.05%, phosphorus (P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, remaining iron (Fe) and unavoidable impurities And, as a microstructure, including fresh martensite and tempered martensite in a total fraction of 80 area% or more, wherein the ratio of the fraction of tempered martensite to the total fraction of fresh martensite and tempered martensite is 70~ It can be 90%.

Description

High strength steel sheet having excellent workability and manufacturing method for the same}

The present invention relates to a high-strength steel sheet having excellent formability and a method for manufacturing the same, and more particularly, to a cold-rolled steel sheet having physical properties suitable for a structural member of an automobile and a method for manufacturing the same.

Recently, as the need to improve fuel efficiency and durability increases due to various environmental regulations and energy use regulations, automobile companies are continuously pursuing the use of high-strength steel sheets for vehicle weight reduction. Adoption of high-strength steel is essential in order to reduce the thickness of the steel plate to reduce the body weight and at the same time secure the safety of passengers.

In particular, as the impact stability regulation of automobiles has been expanded recently, the application of high-strength steel to structural members such as members, seat rails and A/B/C pillars is increasing in order to improve the impact resistance of the vehicle body. Automotive structural members have advantageous characteristics in absorbing impact energy as the yield strength compared to tensile strength, that is, the yield ratio (yield strength/tensile strength) is high. However, as the strength of the steel sheet increases, the problem of deterioration of formability due to a decrease in elongation is accompanied, and therefore, the development of a material having a high yield ratio and improved formability at the same time is required.

As a representative manufacturing method for increasing yield strength, there is a technology using water cooling during continuous annealing. That is, a steel sheet having a tempered martensite structure in which the microstructure is tempered martensite by tempering by immersion in water after cracking in the annealing process can be manufactured. In Patent Document 1, after continuous annealing of steel with a carbon (C) content of 0.18% or more, water cooling to room temperature, and overaging treatment at a temperature of 120 to 300 ° C. for 1 to 15 minutes, the volume ratio of martensite is 80 We propose a technology for manufacturing steel that is ~97%. When high-strength steel is manufactured by applying tempering after water cooling as in Patent Document 1, although the yield ratio is very high, there may be a problem in that the shape quality of the coil is deteriorated due to the temperature deviation in the width and length directions. In addition, there may be problems such as material deviation for each part during roll forming or workability deterioration.

Patent Document 2 discloses a technique for improving workability in a high-strength steel sheet. Patent Document 2 proposes a method of manufacturing a high-tensile steel sheet in which fine copper (Cu) precipitated particles having a particle diameter of 1 to 100 nm are dispersed inside the structure to improve workability, as a steel sheet having a composite structure mainly composed of martensite. However, in Patent Document 2, in order to precipitate good fine copper (Cu) particles, since the copper (Cu) content is excessively added to 2 to 5%, red heat brittleness may occur, and there is a problem in that the manufacturing cost increases.

Therefore, there is an urgent need to develop a high-strength steel sheet capable of solving the above-described problems while simultaneously securing a high yield ratio and formability.

Japanese Patent Laid-Open No. 1992-289120 (published on Oct. 14, 1992) Japanese Laid-Open Patent Publication No. 2005-264176 (published on September 29, 2005)

According to one aspect of the present invention, a steel sheet having excellent formability and strength characteristics and a method for manufacturing the same can be provided.

The subject of the present invention is not limited to the above. A person of ordinary skill in the art will have no difficulty in understanding the further problems of the present invention from the general contents of the present specification.

High-strength steel sheet having excellent formability according to an aspect of the present invention, by weight, carbon (C): 0.13 to 0.2%, manganese (Mn): 2.5 to 3%, silicon (Si): 0.4 to 0.6%, acid Soluble aluminum (sol.Al): 0.02~0.05%, phosphorus (P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, remaining iron (Fe) and unavoidable impurities And, including fresh martensite and tempered martensite as a microstructure in a total fraction of 80 area% or more, the ratio of the fraction of tempered martensite to the total fraction of fresh martensite and tempered martensite is 70~ It may be in the range of 90%.

The steel sheet may further include 20 area% or less of ferrite and 5 area% or less of bainite as a microstructure.

The steel sheet may have a tensile strength of 1300 MPa or more, a yield ratio of 0.7 or more, an elongation of 8.0% or more, and a hole expandability (HER) of 30% or more.

The steel sheet may have a yield strength of 1000 MPa or more, and a three-point bending angle of 55° or more.

The steel sheet is, by weight%, molybdenum (Mo): 0.3% or less, chromium (Cr): 0.7% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V): 0.1% or less and boron (B): a high-strength steel sheet with excellent formability, further comprising at least one selected from 0.002% or less.

The steel sheet may further include Ti, Nb, V or Mo-based fine precipitates, the average diameter of the fine precipitates is 50 nm or less, and the number density of the fine precipitates is 10 ¹² pieces/m ² or more.

The method for manufacturing a high-strength steel sheet having excellent formability according to an aspect of the present invention, in weight %, carbon (C): 0.13 to 0.2%, manganese (Mn): 2.5 to 3%, silicon (Si): 0.4 to 0.6 %, acid soluble aluminum (sol.Al): 0.02~0.05%, phosphorus (P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, remaining iron (Fe) and unavoidable preparing a cold-rolled steel sheet containing impurities; continuously annealing the cold-rolled steel sheet in a temperature range of 700 to 810°C; primary cooling of the annealed steel sheet at an average cooling rate of 1 to 10° C./s to a primary cooling stop temperature of 620 to 700° C.; Secondary cooling of the first cooled steel sheet to a secondary cooling stop temperature of 300 ~ 580 ℃ at an average cooling rate of 5 ~ 20 ℃ / s; Thirdly cooling the secondary cooled steel sheet at an average cooling rate of 5°C/s or more to a third cooling stop temperature of 100°C or less; And after raising the temperature of the tertiary cooled steel sheet to a temperature range of 300 ~ 450 ℃ may include the step of holding for 15 ~ 40 seconds tempering treatment.

It may further include the step of overaging by maintaining the secondary cooled steel sheet at the secondary cooling stop temperature.

temper rolling the secondary cooled steel sheet at an elongation of 2% or less; hot-dip galvanizing the temper-rolled steel sheet; and selectively alloying the hot-dip galvanized steel sheet.

The cold-rolled steel sheet, in wt%, molybdenum (Mo): 0.3% or less, chromium (Cr): 0.7% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V) : 0.1% or less and boron (B): may further include one or more selected from 0.002% or less.

The cold-rolled steel sheet, after reheating the steel slab, by finishing hot rolling in a temperature range of Ar3 ~ Ar3 + 50 ℃ to provide a hot-rolled steel sheet; winding the hot-rolled steel sheet in a temperature range of 400 to 700°C; and cold rolling the wound hot-rolled steel sheet at a reduction ratio of 40 to 70%.

The means for solving the above problems do not enumerate all the features of the present invention, and various features of the present invention and its advantages and effects may be understood in more detail with reference to the following specific examples.

According to one aspect of the present invention, it is possible to provide a steel sheet having excellent formability and strength characteristics and a method for manufacturing the same.

The effect of the present invention is not limited to the above, and it may be construed as including the effect that those skilled in the art can infer from the description described below.

1 is a photograph of observing the microstructure of a steel sheet according to an aspect of the present invention using a scanning electron microscope (SEM).

The present invention relates to a high-strength steel sheet having excellent formability and a method for manufacturing the same. Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided in order to further detailed the present invention to those of ordinary skill in the art to which the present invention pertains.

Hereinafter, a high-strength steel sheet having excellent formability according to an aspect of the present invention will be described in more detail.

High-strength steel sheet having excellent formability according to an aspect of the present invention, by weight, carbon (C): 0.13 to 0.2%, manganese (Mn): 2.5 to 3%, silicon (Si): 0.4 to 0.6%, acid Soluble aluminum (sol.Al): 0.02~0.05%, phosphorus (P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, remaining iron (Fe) and unavoidable impurities And, including fresh martensite and tempered martensite as a microstructure in a total fraction of 80 area% or more, the ratio of the fraction of tempered martensite to the total fraction of fresh martensite and tempered martensite is 70~ It may be in the range of 90%.

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise indicated, % indicating the content of each element is based on weight.

High-strength steel sheet having excellent formability according to an aspect of the present invention, by weight, carbon (C): 0.13 to 0.2%, manganese (Mn): 2.5 to 3%, silicon (Si): 0.4 to 0.6%, acid Soluble aluminum (sol.Al): 0.02~0.05%, phosphorus (P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, remaining iron (Fe) and unavoidable impurities do.

Carbon (C): 0.13-0.2%

Carbon (C) is an effective component in manufacturing high-strength steel because it is an element advantageous for securing strength by forming martensite. In general, as the content of carbon (C) increases, the formation of martensite is easier, which is advantageous for the formation of a complex structure required for manufacturing high strength steel, but it is necessary to control the content to an appropriate level to simultaneously control the intended strength and elongation. have. Accordingly, 0.13% or more of carbon (C) may be added to secure the strength targeted by the present invention and to form martensite at an appropriate level. On the other hand, when the carbon (C) content is excessive, a problem of poor weldability and formability may occur, and the present invention may limit the carbon (C) content to 0.2% or less. More preferably, the upper limit of the carbon (C) content may be 0.18%.

Manganese (Mn): 2.5~3%

Manganese (Mn) is an element that improves the hardenability of steel, and in particular, is an element that plays an important role in forming martensite. Manganese (Mn) contributes to the increase in strength of steel by the solid solution strengthening effect, and by precipitating sulfur (S), which is unavoidably added in steel, as MnS, it suppresses the occurrence of plate breakage and high temperature embrittlement caused by sulfur (S) during hot rolling. serves to make Therefore, in the present invention, 2.5% or more of manganese (Mn) may be added to secure such an effect. More preferably, the lower limit of the manganese (Mn) content may be 2.6%. However, when the content of manganese (Mn) exceeds a certain level, not only the weldability is inferior, but also martensite is excessively formed, which makes the material unstable, and an oxide band in the form of a band is formed, resulting in processing cracks and plates. There is a problem in that the risk of occurrence of breakage increases. In addition, when the manganese (Mn) content is excessive, there is a problem that manganese (Mn) oxide is eluted on the surface of the steel sheet during annealing to impair plating properties, so the present invention can limit the manganese (Mn) content to 3.0% or less. have. More preferably, the upper limit of the manganese (Mn) content may be 2.8%.

Silicon (Si): 0.4-0.6%

Silicon (Si) is a useful element capable of securing strength without degrading the ductility of the steel sheet. Silicon (Si) is also an element that promotes the formation of ferrite and promotes the formation of martensite by promoting carbon (C) concentration in untransformed austenite. Therefore, in the present invention, 0.4% or more of silicon (Si) may be added to secure such an effect. A more preferable lower limit of the content of silicon (Si) may be 0.45%. On the other hand, when the content of silicon (Si) is excessive, the present invention may limit the content of silicon (Si) to 0.6% or less, since plating properties as well as hydrogen embrittlement and weldability may be deteriorated. More preferably, the upper limit of the content of silicon (Si) may be 0.55%.

Aluminum for acid value (sol.Al): 0.02~0.05%

Aluminum for acid value (sol.Al) is an element added for grain size refinement and deoxidation of steel, and is an element contributing to the production of aluminum killed steel in a stable state. Accordingly, in the present invention, 0.02% or more of aluminum (sol.Al) for acid value may be added to secure such an effect. On the other hand, if the content of aluminum (sol.Al) for acid value is excessive, there is a possibility that the surface defect of the plated steel sheet may occur due to excessive formation of inclusions during the steel making operation. It can be limited to 0.05% or less. More preferably, the upper limit of the content of aluminum (sol.Al) for acid value may be 0.04%.

Phosphorus (P): 0.05% or less (excluding 0%)

Phosphorus (P) is the most advantageous element for securing the strength of steel without greatly increasing the formability, but when it is added excessively, the possibility of brittle fracture increases significantly, and it is also an element that increases the possibility of plate fracture of the slab during hot rolling. . In addition, since there is a problem that phosphorus (P) also acts as an element that inhibits the plating surface characteristics, the present invention can limit the content of phosphorus (P) to 0.05% at most. However, 0% may be excluded from the lower limit of the phosphorus (P) content in consideration of the level that is unavoidably added to steel.

Sulfur (S): 0.01% or less (excluding 0%)

Sulfur (S) is an impurity element that is unavoidably added to steel, and it is desirable to manage its content as low as possible. In particular, since sulfur (S) in steel can increase the possibility of occurrence of red hot brittleness, it is preferable to control the content to 0.01% or less. However, 0% may be excluded from the lower limit of the content of sulfur (S) in consideration of the level that is unavoidably added during the manufacturing process.

Nitrogen (N): 0.01% or less

Nitrogen (N) is an impurity element that is unavoidably added to steel, and it is important to manage its content as low as possible. However, for this, there is a problem in that the refining cost of the steel rises rapidly, so it is preferable to control the nitrogen (N) content to 0.01% or less, which is a range where operating conditions are possible. However, 0% may be excluded from the lower limit of the nitrogen (N) content in consideration of the level that is unavoidably added to steel.

On the other hand, the high-strength steel sheet excellent in the standing property according to an aspect of the present invention has an alloy composition that may be additionally included in addition to the above-described alloy components, which will be described in detail below.

High-strength steel sheet having excellent formability according to an aspect of the present invention, by weight%, molybdenum (Mo): 0.3% or less, chromium (Cr): 0.7% or less, niobium (Nb): 0.1% or less, titanium (Ti) ): 0.1% or less, vanadium (V): 0.1% or less, and boron (B): 0.002% or less may be further included.

Molybdenum (Mo): 0.3% or less

Molybdenum (Mo) improves the hardenability of steel, and since it is an element that improves strength through miniaturization of ferrite, molybdenum (Mo) may be added to secure the strength characteristics of the present invention. However, since molybdenum (Mo) is a relatively expensive element, there is a problem that the manufacturing cost becomes disadvantageous as the content increases. As such, the content of molybdenum (Mo) may be limited to 0.3% or less.

Chromium (Cr): 0.7% or less

Chromium (Cr) is an element added to secure high strength of steel by improving hardenability. Chromium (Cr) is not only effective in forming martensite, but also minimizes the decrease in elongation compared to the increase in strength, so it is advantageous in manufacturing high-strength steel having high ductility. Therefore, in the present invention, chromium (Cr) may be added to secure such an effect, and the lower limit of the preferred chromium (Cr) content may be 0.1%. However, chromium (Cr) facilitates martensite formation by improving hardenability, but when the content exceeds a certain level, the formation rate of martensite is excessively increased, and the fraction of coarse chromium (Cr)-based carbides As it increases, there is a problem of causing a decrease in elongation. In addition, since excessive addition of chromium (Cr) may cause hydrogen embrittlement and inferior weldability, the content of chromium (Cr) may be limited to 0.7% or less. More preferably, the upper limit of the chromium (Cr) content may be 0.6%.

Niobium (Nb): 0.1% or less

Niobium (Nb) is segregated at the austenite grain boundary and suppresses coarsening of austenite grains during annealing heat treatment, and is also an element contributing to strength improvement by forming fine carbides. Therefore, in the present invention, niobium (Nb) may be added in order to secure such an effect, and the lower limit of the preferable niobium (Nb) content may be 0.02%. However, when niobium (Nb) is excessively added, strength and elongation may be reduced by precipitation of coarse carbides and reduction of carbon content in steel, and the manufacturing cost may increase, resulting in inferior economic feasibility. ) may be limited to 0.1% or less. More preferably, the upper limit of the niobium (Nb) content may be 0.07%.

Titanium (Ti): 0.1% or less

Titanium (Ti) not only contributes to securing yield and tensile strength by forming fine carbides, but also inhibits the precipitation of AlN by precipitating nitrogen (N) in the steel as TiN, so it is an element that can effectively reduce the risk of cracking during playing. . Therefore, in the present invention, titanium (Ti) may be added in order to secure such an effect, and the lower limit of the preferable titanium (Ti) content may be 0.01%. However, when titanium (Ti) is excessively added, strength and elongation may be reduced by precipitation of coarse carbides and reduction of carbon content in steel, and nozzle clogging may also be caused during playing, the content of titanium (Ti) may be limited to 0.1% or less. More preferably, the upper limit of the content of titanium (Ti) may be 0.05%.

Vanadium (V): 0.1% or less

Vanadium (V) is an element that reacts with carbon (C) or nitrogen (N) to form carbon nitride, and is an effective element for increasing the yield strength of steel by forming fine precipitates at low temperatures. Therefore, in the present invention, vanadium (V) may be added to secure such an effect. However, when vanadium (V) is excessively added, coarse carbides are precipitated, and strength and elongation can be reduced by reducing the amount of carbon in the steel, and the manufacturing cost can be increased to deteriorate economic feasibility. The content of (V) may be limited to 0.1% or less. More preferably, the upper limit of the content of vanadium (V) may be 0.05%.

Boron (B): 0.002% or less

Boron (B) is a component that delays the transformation of austenite into pearlite during the cooling process during annealing. However, when boron (B) is excessively added, boron (B) is concentrated on the surface of the steel sheet, which may lead to deterioration of plating adhesion, and thus the content of boron (B) may be limited to 0.002% or less.

The high-strength steel sheet having excellent formability according to an aspect of the present invention may include remaining Fe and other unavoidable impurities in addition to the above-described components. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, it cannot be entirely excluded. Since these impurities are known to those of ordinary skill in the art, all contents thereof are not specifically mentioned in the present specification. In addition, additional addition of effective ingredients other than the above-mentioned ingredients is not entirely excluded.

The high-strength steel sheet having excellent formability according to an aspect of the present invention may include martensite of 80 area% or more, ferrite of 20 area% or less, and bainite of 5 area% or less. Here, martensite is a concept including fresh martensite and tempered martensite, and the fraction of martensite means the total fraction of fresh martensite and tempered martensite. In addition, 0 area % may be excluded from the lower limit of the fractions of ferrite and bainite in terms of formability, and the preferred fraction of ferrite in terms of securing elongation may be 10 area % or more.

In addition, in the high-strength steel sheet with excellent formability according to an aspect of the present invention, the total fraction of fresh martensite and tempered martensite is limited to 80 area% or more, but the fraction of tempered martensite with respect to the total martensite fraction Since the ratio is limited to a range of 70 to 90%, it is possible to effectively achieve both formability and high strength properties. That is, in the present invention, since the fraction of total martensite is limited to 80 area% or more, tensile strength of 1300 MPa or more can be secured, and the ratio of tempered martensite to total martensite is limited to 70% or more. Yield ratio and hole expandability (HER) of 30% or more can be secured, and since the ratio of tempered martensite to total martensite is limited to 90% or less, elongation of 8.0% or more can be secured.

In addition, the high-strength steel sheet excellent in formability according to an aspect of the present invention may include Ti, Nb, V or Mo-based fine precipitates having an average diameter of 50 nm or less at a number density of 10 ¹² pieces/m ² or more. That is, since it contains a large amount of fine fine precipitates, it is possible to more effectively secure the desired high strength characteristics.

The steel sheet having excellent formability according to an aspect of the present invention may have a tensile strength of 1300 MPa or more, a yield ratio of 0.7 or more, an elongation of 8.0% or more, and a hole expandability (HER) of 30% or more. In addition, the steel sheet excellent in formability according to an aspect of the present invention may have a yield strength of 1000 MPa or more and a three-point bending angle of 55° or more.

Hereinafter, a method for manufacturing a high-strength steel sheet having excellent formability according to an aspect of the present invention will be described in more detail.

According to an aspect of the present invention, there is provided a method for manufacturing a high-strength steel sheet having excellent formability, comprising: preparing a cold-rolled steel sheet having predetermined components; continuously annealing the cold-rolled steel sheet in a temperature range of 700 to 810°C; primary cooling of the annealed steel sheet at an average cooling rate of 1 to 10° C./s to a primary cooling stop temperature of 620 to 700° C.; Secondary cooling of the first cooled steel sheet to a secondary cooling stop temperature of 300 ~ 580 ℃ at an average cooling rate of 5 ~ 20 ℃ / s; Thirdly cooling the secondary cooled steel sheet at an average cooling rate of 5°C/s or more to a third cooling stop temperature of 100°C or less; And after raising the temperature of the tertiary cooled steel sheet to a temperature range of 300 ~ 450 ℃ may include the step of holding for 15 ~ 40 seconds tempering treatment.

In addition, the cold-rolled steel sheet of the present invention comprises the steps of reheating a steel slab having a predetermined component and then performing finish hot-rolling in a temperature range of Ar3 to Ar3+50° C. to provide a hot-rolled steel sheet; winding the hot-rolled steel sheet in a temperature range of 400 to 700°C; and cold rolling the wound hot-rolled steel sheet at a reduction ratio of 40 to 70%.

Preparation and reheating of steel slabs

A steel slab having a predetermined component is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the alloy composition of the steel sheet described above, the description of the alloy composition of the steel slab is replaced with the description of the alloy composition of the steel sheet described above.

The prepared steel slab may be heated to a certain temperature range, and the heating temperature of the steel slab at this time may be in the range of 1000 to 1350 °C. If the heating temperature of the steel slab is less than 1000℃, it may be hot rolled in the temperature range below the target finish hot rolling temperature range. If the heating temperature of the steel slab exceeds 1350℃, it will reach the melting point of the steel because it has potential.

hot rolled and wound

The heated steel slab may be hot rolled to provide a hot rolled steel sheet. During hot rolling, the finish hot rolling temperature is preferably in the range of Ar3 to Ar3+50°C based on the exit side temperature of the finish rolling mill. If the outlet temperature is less than Ar3, the hot deformation resistance is highly likely to increase rapidly. This is because it may cause deterioration of the physical properties of the final steel sheet.

The hot-rolled steel sheet after the hot rolling has been completed may be cooled and wound in a temperature range of 400 to 700°C. If the coiling temperature is less than 400℃, excessive martensite or bainite is generated, which may cause manufacturing problems such as shape defects due to load during cold rolling. This is because it deteriorates.

cold rolled

Cold rolling may be performed after pickling to remove scale generated on the surface of the hot-rolled steel sheet. Although pickling and cold rolling conditions are not particularly limited, cold rolling is preferably performed at a reduction ratio of 40 to 70%. When the reduction ratio of cold rolling is less than 40%, the recrystallization driving force is weakened, and there is a high possibility that a problem may occur in obtaining good recrystallized grains, and shape correction may be very difficult. On the other hand, when the reduction ratio of cold rolling exceeds 70%, the possibility of cracking in the edge portion of the steel sheet is high, and the rolling load may increase rapidly.

continuous annealing

Continuous annealing is performed after cold rolling, and through continuous annealing, it is possible to prepare the basis of the microstructure desired in the present invention. The continuous annealing is preferably performed in a temperature range of 700°C or higher and 810°C or lower. When the annealing temperature is less than 700℃, it is difficult to secure elongation because recrystallization of ferrite is not sufficiently performed. because it is difficult to obtain.

primary cooling

The steel sheet on which continuous annealing has been completed can be first cooled to a primary cooling stop temperature of 650 to 700°C at an average cooling rate of 1 to 10°C/s. This primary cooling step has an aspect of suppressing the inferiority of the plate shape due to a sudden temperature drop in the rapid cooling section by performing slow cooling prior to the secondary rapid cooling to be described later.

Secondary cooling and overaging

The primary cooled steel sheet can be secondary cooled at an average cooling rate of 5-20°C/s to the secondary cooling stop temperature of 300-580°C. At this time, the optimum plate shape can be secured by appropriately adjusting the cooling rate and cooling stop temperature according to the width and thickness of the steel plate to be manufactured. If the secondary cooling stop temperature is less than 300℃, there is a possibility that a cooling deviation occurs in the width or length direction of the steel plate, resulting in inferior plate shape. There is a possibility that it cannot. By performing the overaging treatment of maintaining the secondary cooling stop temperature for a certain period of time, it is possible to secure the optimal shape.

temper rolling

The overaged steel sheet may be selectively temper rolled, and the elongation of the temper rolling may be preferably in the range of 0.1 to 2.0%. In general, in the case of performing temper rolling, the effect of increasing the yield strength by about 500 MPa or more can be obtained without accompanying an increase in tensile strength. However, when the elongation of the temper rolling is less than 0.1%, the effect of increasing the yield strength is not sufficient as well as difficulty in controlling the shape. On the other hand, when the elongation of the temper rolling exceeds 2.0%, the operability may be greatly deteriorated by the high drawing operation.

Hot-dip galvanizing or alloying hot-dip galvanizing

After temper rolling, hot-dip galvanizing may be performed, or the hot-dip galvanized steel sheet may be selectively alloyed. In addition, the present invention may include a case in which hot-dip galvanizing or alloyed hot-dip galvanizing is performed after over-aging treatment, and then the above-described temper rolling is performed. For the hot-dip galvanizing or hot-dip galvanizing of the present invention, the composition of the plating layer and the plating method are not particularly limited, and the process conditions of conventionally applied hot-dip galvanizing or hot-dip galvanizing may be applied. The steel sheet on which hot-dip galvanizing or alloying hot-dip galvanizing has been completed may be thirdly cooled at an average cooling rate of 5°C/s or more to a third cooling stop temperature of 100°C or less.

tempering treatment

Tempered martensite can be introduced into the steel sheet by the tempering treatment. Tempering temperature and holding time are major process factors that control the proportion of tempered martensite in the total martensite. The tertiary cooled steel sheet may be heated to a temperature range of 300 to 450° C. and maintained in the temperature range for 15 to 40 seconds. When the tempering temperature is low or the holding time at the temperature is short, the ratio of the tempered martensite fraction to the total martensite fraction is less than the desired level, so the present invention sets the lower limit of the tempering temperature and the lower limit of the holding time, respectively. It can be limited to 300°C and 15 seconds. A more preferable lower limit of the tempering temperature may be 310°C. On the other hand, if the tempering temperature is high or the holding time at the temperature is long, the ratio of the tempered martensite fraction to the total martensite fraction exceeds the desired level, or strength reduction due to tissue coarsening may be induced. Therefore, the present invention can limit the upper limit of the tempering temperature and the upper limit of the holding time to 450 ℃ and 40 seconds, respectively.

The high-strength steel sheet with excellent formability manufactured by the above-mentioned manufacturing method contains martensite of 80 area% or more, ferrite of 20 area% or less, and bainite of 5 area% or less, and tempered martens with respect to the total martensite fraction The site fraction ratio satisfies the range of 70-90%, and the tensile strength of 1300 MPa or more, and the yield strength of 1000 MPa or more. It may have a yield ratio of 0.7 or more, an elongation of 8.0% or more, a hole expandability (HER) of 30% or more, and a three-point bending angle of 55° or more.

Hereinafter, a high-strength steel sheet having excellent formability and a manufacturing method thereof of the present invention will be described in more detail through specific examples. It should be noted that the following examples are only for the understanding of the present invention, and are not intended to specify the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

The steel slab having the alloy composition shown in Table 1 below (the remainder is Fe and unavoidable impurities) was vacuum melted, heated in a temperature range of 1200 ° C. It was wound in the temperature range of ℃. After pickling to remove surface scale, cold rolling was performed at a cold rolling reduction of 50%. Thereafter, continuous annealing was performed at the annealing temperature of Table 2, primary cooling was performed to 650°C at an average cooling rate of 3°C/s, and secondary cooling was performed to 500°C at an average cooling rate of 11°C/s. Thereafter, hot-dip galvanizing was performed and cooled to room temperature at an average cooling rate of 10°C/s. Thereafter, a steel sheet was manufactured by performing tempering maintained at the tempering temperature of Table 2 for 25 seconds.

The microstructure of the thus-prepared steel sheet was observed and shown in Table 2. Among the microstructures, ferrite (F), bainite (B), fresh martensite (FM), and tempered martensite (TM) were observed through SEM after nital etching of the polished specimen cross section. In Table 2, M _t means the total fraction of fresh martensite (FM) and tempered martensite (TM), and TM/M _t is the sum of fresh martensite (FM) and tempered martensite (TM) It means the ratio of the fraction of tempered martensite (TM) to the fraction.

The physical property values for each specimen were measured, and the results are shown in Table 2. Yield strength (YS), tensile strength (TS) and elongation (El) were evaluated through a tensile test. Strength (YS), tensile strength (TS) and elongation (El) were measured. The hole expansion rate (HER) was evaluated through the hole expansion test, and after forming a 10mmØ punching hole (die inner diameter 10.3mm, clearance 12.5%), a conical punch with an apex angle of 60° was used. After inserting into the punching hole in the desired direction, pressing and expanding the peripheral portion of the punching hole at a moving speed of 12 mm/min, it was calculated using the following [Relational Expression 1]. The three-point bending angle was evaluated through a three-point bending test, and on two lower rolls with a diameter of 30 mm and a roll spacing of (2*specimen thickness + 0.5) mm, a punch with a radius of 0.4 mm was moved at 20 mm/min. During the process of bending the specimen up to 140° by moving at a speed, the bending angle just before cracks occurred in the bending part was evaluated as a three-point bending angle.

[Relational Expression 1]

Hole expansion rate (HER, %) = {(D - D ₀ ) / D ₀ } x 100

In Relation 6, D means the hole diameter (mm) when the crack penetrates the steel plate along the thickness direction, and D ₀ means the initial hole diameter (mm).

steel grade Ingredients (wt%) C Mn Si Mo Cr Nb Ti V B Al P S N A 0.13 2.6 0.5 0.1 0.5 0.05 0.02 0.04 0.0014 0.03 0.01 0.002 0.003 B 0.16 2.6 0.5 0.1 0.5 0.05 0.02 0.04 0.0014 0.03 0.01 0.002 0.003 C 0.16 2.6 0.5 0.1 0.5 0.03 0.02 - 0.0014 0.03 0.01 0.002 0.003 D 0.16 2.9 0.5 0.1 0.5 0.03 0.02 - 0.0014 0.03 0.01 0.002 0.003 E 0.16 2.6 0.5 0.1 0.5 - 0.02 - 0.0014 0.03 0.01 0.002 0.003 F 0.16 2.6 0.5 0.1 0.5 0.03 - - 0.0014 0.03 0.01 0.002 0.003

steel grade Annealing
temperature
(℃) tempering
temperature
(℃) Properties tissue fraction
(area%)

(area %)
One note YS
(MPa) ts
(MPa) El
(%) yield ratio 3 point bend
( ^o ) HER
(%) F FM TM B M _t TM/M _r
(%) A 790 - 845 1428 11.0 0.59 52 28 15 82 - 3 82 0 Comparative Example 1 A 790 150 841 1420 9.8 0.59 58 19 15 70 12 3 82 14.6 Comparative Example 2 A 790 200 850 1400 8.6 0.61 58 27 15 61 21 3 82 25.6 Comparative Example 3 A 790 250 946 1392 8.0 0.68 60 27 15 33 49 3 82 59.8 Comparative Example 4 A 790 500 1187 1309 7.5 0.91 66 55 15 6 76 3 82 92.7 Comparative Example 5 A 790 310 1055 1370 9.2 0.77 63 35 15 20 62 3 82 75.6 Invention Example 1 A 790 350 1102 1315 8.6 0.84 65 53 15 13 69 3 82 84.1 Invention Example 2 A 790 400 1166 1337 8.2 0.87 64 47 15 10 72 3 82 87.8 Invention example 3 A 770 - 764 1426 9.2 0.54 44 26 18 80 - 2 80 0 Comparative Example 6 A 770 310 1008 1389 9.8 0.73 59 32 18 19 61 2 86 76.3 Invention Example 4 B 790 - 854 1327 10.0 0.64 40 23 11 86 - 3 85 0 Comparative Example 7 C 790 - 927 1551 10.3 0.60 41 25 12 85 - 3 85 0 Comparative Example 8 C 790 150 918 1531 9.8 0.60 47 17 12 75 10 3 85 11.8 Comparative Example 9 C 790 200 914 1505 8.5 0.61 52 23 12 66 19 3 85 22.4 Comparative Example 10 C 790 250 999 1489 8.1 0.67 54 27 12 41 44 3 85 51.8 Comparative Example 11 C 790 310 1122 1433 8.0 0.78 55 33 12 25 60 3 85 70.6 Invention Example 5 D 790 - 1016 1504 8.0 0.68 36 20 10 87 - 3 87 0 Comparative Example 12 E 790 310 1092 1393 8.6 0.78 56 31 14 22 61 3 83 73.5 Invention example 6 F 790 310 1101 1401 8.4 0.79 59 33 13 20 64 3 84 76.2 Invention Example 7

Invention examples satisfying the alloy composition and tempering conditions limited by the present invention include martensite of 80 area% or more, ferrite of 20 area% or less, and bainite of 5 area% or less, and tempered with respect to the total martensite fraction The ratio of martensite fraction satisfies the range of 70-90%, tensile strength of 1300 MPa or more, and yield strength of 1000 MPa or more. It can be seen that a yield ratio of 0.7 or more, an elongation of 8.0% or more, a hole expandability (HER) of 30% or more, and a three-point bending angle of 55° or more are satisfied. 1 is a photograph of the microstructure of Inventive Example 1 observed using a scanning electron microscope (SEM). In FIG. 1, TM means tempered martensite. On the other hand, comparative examples that do not satisfy the tempering conditions limited by the present invention can be confirmed that the present invention does not simultaneously secure the desired microstructure and physical properties.

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (11)

By weight%, carbon (C): 0.13 to 0.2%, manganese (Mn): 2.5 to 3%, silicon (Si): 0.4 to 0.6%, acid soluble aluminum (sol.Al): 0.02 to 0.05%, phosphorus ( P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, molybdenum (Mo): 0.3% or less, chromium (Cr): 0.7% or less, niobium (Nb) ): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V): 0.1% or less, and boron (B): 0.002% or less, including at least one selected from the group consisting of the remaining iron (Fe) and unavoidable impurities. including,
As a microstructure, fresh martensite and tempered martensite are included in a total fraction of 80% by area or more, but the ratio of the fraction of tempered martensite to the total fraction of fresh martensite and tempered martensite is 70 to 90% ego,
A high strength steel sheet excellent in formability, comprising Ti, Nb, V or Mo-based fine precipitates, the average diameter of the fine precipitates is 50 nm or less, and the number density of the fine precipitates is 10 ¹² pieces/m ² or more.
According to claim 1,
The steel sheet is a high-strength steel sheet with excellent formability, further comprising 20 area% or less of ferrite and 5 area% or less of bainite as a microstructure.
According to claim 1,
The steel sheet has a tensile strength of 1300 MPa or more, a yield ratio of 0.7 or more, an elongation of 8.0% or more, and a hole expandability (HER) of 30% or more, a high-strength steel sheet with excellent formability.
According to claim 1,
The steel sheet has a yield strength of 1000 MPa or more, a three-point bending angle of 55° or more, a high-strength steel sheet excellent in formability.
delete delete By weight%, carbon (C): 0.13 to 0.2%, manganese (Mn): 2.5 to 3%, silicon (Si): 0.4 to 0.6%, acid soluble aluminum (sol.Al): 0.02 to 0.05%, phosphorus ( P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, molybdenum (Mo): 0.3% or less, chromium (Cr): 0.7% or less, niobium (Nb) ): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V): 0.1% or less, and boron (B): 0.002% or less, including at least one selected from the group consisting of the remaining iron (Fe) and unavoidable impurities. Preparing a cold-rolled steel sheet comprising;
continuously annealing the cold-rolled steel sheet in a temperature range of 700 to 810°C;
primary cooling of the annealed steel sheet at an average cooling rate of 1 to 10° C./s to a primary cooling stop temperature of 620 to 700° C.;
Secondary cooling of the first cooled steel sheet to a secondary cooling stop temperature of 300 ~ 580 ℃ at an average cooling rate of 5 ~ 20 ℃ / s;
tertiary cooling of the secondary cooled steel sheet at an average cooling rate of 5° C./s or more to a third cooling stop temperature of 100° C. or less; and
A method of manufacturing a high-strength steel sheet with excellent formability, comprising the step of heating the tertiarily cooled steel sheet to a temperature range of 300 to 450° C. and then maintaining the temperature for 15 to 40 seconds to perform a tempering treatment.
8. The method of claim 7,
The method of manufacturing a high-strength steel sheet having excellent formability, further comprising the step of over-aging by maintaining the secondary cooled steel sheet at the secondary cooling stop temperature.
8. The method of claim 7,
temper rolling the secondary cooled steel sheet at an elongation of 2% or less;
hot-dip galvanizing the temper-rolled steel sheet; and
The method of manufacturing a high-strength steel sheet having excellent formability, further comprising the step of selectively alloying the hot-dip galvanized steel sheet.
delete 8. The method of claim 7,
The cold-rolled steel sheet,
providing a hot-rolled steel sheet by reheating the steel slab and then finishing hot rolling in a temperature range of Ar3 to Ar3+50°C;
winding the hot-rolled steel sheet in a temperature range of 400 to 700°C; and
A method of manufacturing a high-strength steel sheet having excellent formability, which is provided through the step of cold-rolling the wound hot-rolled steel sheet at a reduction ratio of 40 to 70%.

Images