KR20220133842A - Cold rolled plated steel sheet and method of manufacturing the same - Google Patents

Abstract

Provided in the present invention are a high-strength cold rolled plated steel sheet with an excellent yield ratio and bending property and a method for manufacturing the same. The cold rolled plated steel sheet according to one embodiment of the present invention comprises: 0.24-0.30wt% of C; 0.05-0.3wt% of Si; 1.0-2.5wt% of Mn; 0.01-0.05wt% of Al; more than 0 and equal to or less than 0.02wt% of P; more than 0 and equal to or less than 0.01wt% of S; 0.2-0.5wt% of Cr; 0.1-0.3wt% of Mo; 0.01-0.1wt% of at least one element selected from the group of Ti, Nb and V; more than 0 and equal to or less than 0.003wt% of B; and the rest being Fe and other inevitable impurities. The yield strength is 1200-1450MPa, and the tensile strength is 1500-1700MPa. The elongation percentage is 5% or more, and the yield ratio can be 75% or more.

Description

Cold rolled steel sheet and its manufacturing method

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet having excellent yield ratio and bendability and a method for manufacturing the same.

Among steel sheets for automobiles, materials for collision members require high strength and high collision absorption capacity. In addition, it is necessary to secure high bending workability for machining into parts with complex structures. This requires high strength and yield ratio (YP/TS) and high degree of bendability (R/t). However, it is difficult to implement this using a general continuous annealing facility.

Korean Patent Publication No. 10-2004-0111413

The technical problem to be achieved by the technical idea of the present invention is to provide a high-strength cold-rolled steel sheet having excellent yield ratio and bendability and a method for manufacturing the same. However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, a high-strength cold-rolled steel sheet having excellent yield ratio and bendability is provided.

According to an embodiment of the present invention, the cold rolled steel sheet is carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al) ): 0.01 to 0.05 wt%, phosphorus (P): more than 0, 0.02 wt% or less, sulfur (S): more than 0, 0.01 wt% or less, chromium (Cr): 0.2 to 0.5 wt%, molybdenum (Mo): 0.1 to 0.3% by weight, at least one element selected from the group consisting of titanium (Ti), niobium (Nb) and vanadium (V): 0.01 to 0.1% by weight, boron (B): greater than 0 0.003% by weight or less and the remainder iron (Fe) and other unavoidable impurities, the yield strength is 1200 ~ 1450 MPa, the tensile strength is 1500 ~ 1700 MPa, the elongation may be 5% or more, and the yield ratio may be 75% or more.

According to an embodiment of the present invention, the bending workability (R/t) of the cold rolled steel sheet may be 5.0 or less.

According to an embodiment of the present invention, the final microstructure of the cold-rolled steel sheet may be made of 70% or more and less than 100% tempered martensite, more than 0 and less than 20% bainite, and more than 0% and less than 10% of ferrite. .

According to another aspect of the present invention, there is provided a method for manufacturing a high-strength cold-rolled steel sheet having excellent yield ratio and bendability.

According to an embodiment of the present invention, the method of manufacturing the cold rolled steel sheet comprises (a) carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al): 0.01 to 0.05 wt%, phosphorus (P): more than 0, 0.02 wt% or less, sulfur (S): more than 0, 0.01 wt% or less, chromium (Cr): 0.2 to 0.5 wt%, molybdenum (Mo): 0.1 to 0.3% by weight, titanium (Ti), at least one element selected from the group consisting of niobium (Nb) and vanadium (V): 0.01 to 0.1% by weight, boron (B): more than 0 0.003% by weight or less And providing a steel material consisting of the remaining iron (Fe) and other unavoidable impurities; (b) hot-rolling the steel material under the condition that the reheating temperature (SRT) is 1150 to 1300 °C, and the hot rolling termination temperature (FDT) is 800 to 1000 °C; (c) winding the hot-rolled steel at 500 to 650°C; (d) cold rolling the steel; (e) annealing the cold-rolled steel at 800 to 900° C.; (f) plating the annealed heat-treated steel; And (g) tempering the plated steel at 100 ~ 250 ℃; includes

According to an embodiment of the present invention, the step (e) comprises the steps of annealing the cold rolled steel at 800 ~ 900 ℃; and cooling the steel material to 250 to 500° C. at a cooling rate of 3 to 20° C./s.

According to an embodiment of the present invention, the step (f) comprises the steps of plating the annealed heat-treated steel material in a plating bath of 450 ~ 550 ℃; and heat-treating the steel at 450 to 650°C.

According to an embodiment of the present invention, step (g) may include tempering the plated steel material at 100 to 250° C. for 3 to 12 hours.

According to an embodiment of the present invention, step (b) may include hot rolling at a reduction ratio of 90% or more, and step (d) may include cold rolling at a reduction ratio of 40 to 60%. can

According to the technical idea of the present invention, a high-strength cold-rolled steel sheet having excellent yield ratio and bendability and a manufacturing method thereof can be realized. The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a photograph of the final microstructure of a cold-rolled plated steel sheet according to an embodiment of the present invention.
FIG. 2 is a photograph of precipitates in martensite in the final microstructure of a cold-rolled plated steel sheet according to an embodiment of the present invention.
3 is a flowchart illustrating a method of manufacturing a cold rolled steel sheet according to an embodiment of the present invention.
4 and 5 are photographs of hot-rolled microstructures when the coiling temperatures are 620°C and 720°C, respectively, in the method for manufacturing a cold-rolled plated steel sheet according to an embodiment of the present invention.
6 is a diagram illustrating a heat treatment configuration after a cold rolling process in a method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention.
7 is a stress-strain diagram before and after heat treatment in the method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention.
8 is a graph showing the number of precipitates and the size of the precipitates in the cold rolled steel sheet according to the experimental example of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In the present specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Therefore, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

cold rolled steel sheet

Cold rolled steel sheet according to an embodiment of the present invention is carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al): 0.01 to 0.05% by weight, phosphorus (P): more than 0, 0.02% by weight or less, sulfur (S): more than 0, 0.01% by weight or less, chromium (Cr): 0.2 to 0.5% by weight, molybdenum (Mo): 0.1 to 0.3% by weight %, at least one element selected from the group consisting of titanium (Ti), niobium (Nb) and vanadium (V): 0.01 to 0.1% by weight, boron (B): greater than 0 and less than or equal to 0.003% by weight and the remainder iron (Fe) and others It consists of unavoidable impurities.

Hereinafter, the role and content of each component included in the steel sheet according to an embodiment of the present invention will be described.

carbon (C)

Carbon (C) is an element that improves the strength of the steel sheet and plays a major role in forming precipitates and bainite phases. In addition, it is an element that has the greatest influence on weldability. Carbon (C) may be added in a content ratio of 0.24 to 0.30 wt% based on the total weight of the cold-rolled steel sheet according to an embodiment of the present invention. When the content of carbon is less than 0.24% by weight of the total weight, it is difficult to secure strength because the formation of precipitates and formation of bainite phase for strength are not sufficient. Conversely, when the content of carbon exceeds 0.30% by weight of the total weight, the content is limited because cracks in the cast steel and poor workability and weldability in the steel making and casting process are performed.

Silicon (Si)

Silicon (Si) is well known as a ferrite stabilizing element and is well known as an element that increases ductility by increasing the ferrite fraction during cooling. On the other hand, silicon is added together with aluminum as a deoxidizer for removing oxygen in steel in the steelmaking process, and may also have a solid solution strengthening effect. The silicon may be added in a content ratio of 0.05 to 0.3% by weight based on the total weight of the steel sheet according to an embodiment of the present invention. When the content of silicon is less than 0.05% by weight of the total weight, it is not easy to secure ductility, and the effect of solid solution strengthening cannot be sufficiently realized. By creating (red scale), the plating property of the steel sheet is deteriorated and it may give a problem to the surface quality.

Manganese (Mn)

Manganese (Mn) is an element that plays a major role in forming an appropriate bainite phase fraction in the present invention. That is, it is used as an austenite stabilizing element to increase the fraction of the low-temperature phase and to increase the strength of steel due to the solid solution strengthening effect. Manganese is effective in solid solution strengthening and can increase the hardenability of steel. Manganese may be added in a content ratio of 1.0 to 2.5% by weight based on the total weight of the steel sheet according to an embodiment of the present invention. When the content of manganese is less than 1.0% by weight, it is difficult to implement the above-described effect and it is not easy to secure strength. In addition, when the content of manganese exceeds 2.5% by weight, the elongation is lowered, weldability is lowered, MnS inclusions and center segregation occur, so that the ductility of the steel sheet is lowered and corrosion resistance can be lowered, It causes cracks to occur and propagate, resulting in reduced workability.

Aluminum (Al)

Aluminum (Al), like silicon, is an element that stabilizes ferrite and suppresses the formation of carbides. That is, aluminum is an effective element for improving elongation by inducing the growth of ferrite present in the initial stage by enabling high-temperature heat treatment during annealing. In addition, aluminum is added to the steelmaking process as a deoxidizer for removing oxygen from the steel, and may be precipitated as AlN in the steel to contribute to grain refinement. The aluminum may be added in a content ratio of 0.01 to 0.05% by weight based on the weight of the steel sheet according to an embodiment of the present invention. If the content of aluminum is less than 0.01% by weight, the effect of adding aluminum is insufficient, and if it exceeds 0.05% by weight, the quality of the slab and hot-rolled material deteriorates, process loads such as increased steelmaking and annealing temperature occur, and it is difficult to play. Productivity is lowered, and alumina (Al ₂ O ₃ ), which is a non-metallic inclusion, is formed and concentrated on the surface of the steel sheet, so there may be problems in that plating properties are lowered and ductility and toughness are lowered.

Phosphorus (P)

Phosphorus (P) increases the strength of the strength by solid solution strengthening, and may perform a function of suppressing the formation of carbides. The phosphorus may be added in a content ratio of greater than 0 and 0.02 wt% or less of the total weight of the cold-rolled steel sheet according to an embodiment of the present invention. When the phosphorus content exceeds 0.02% by weight, grain boundaries and interphase grain boundaries segregate, so that the weld is brittle, the press formability is poor, hot brittleness and weldability may be impaired, and there is a problem in that the low-temperature impact value is lowered by the precipitation behavior.

Sulfur (S)

Sulfur (S) can improve processability by forming fine MnS precipitates. The sulfur may be added in a content ratio of greater than 0 and 0.01 wt% or less of the total weight of the steel sheet according to an embodiment of the present invention. When the sulfur content exceeds 0.01% by weight, MnS non-metallic inclusions in the steel segregate during casting and solidification, causing high-temperature cracks, causing surface defects and processing cracks, impairing toughness and weldability, and lowering the low-temperature impact value. .

Chromium (Cr)

Chromium (Cr) is an element that plays a major role in improving hardenability and increasing strength in the present invention. Chromium may be added in a content ratio of 0.2 to 0.5 wt% based on the total weight of the steel sheet according to an embodiment of the present invention. When the content of chromium is less than 0.2% by weight, it is difficult to implement the above-described effect. In addition, when the content of chromium exceeds 0.5% by weight, there is a problem in that a saturation effect occurs and ductility is lowered.

Molybdenum (Mo)

Molybdenum (Mo) is an element that plays a major role in improving hardenability and increasing strength in the present invention. Molybdenum may be added in a content ratio of 0.1 to 0.3% by weight of the total weight of the steel sheet according to an embodiment of the present invention. When the content of molybdenum is less than 0.1% by weight, it is difficult to implement the above-described effect and it is difficult to secure strength. In addition, when the content of molybdenum exceeds 0.3% by weight, there is a problem in that the production cost increases because it is more expensive than chromium.

Titanium (Ti), Niobium (Nb), Vanadium (V)

Titanium (Ti) is an element that serves to refine the crystal grains and suppress the formation of boron nitride in the present invention. Niobium (Nb) and vanadium (V) are elements that play a role in grain refinement and strength improvement. At least one element selected from the group consisting of titanium (Ti), niobium (Nb), and vanadium (V) may be added in a content ratio of 0.01 to 0.1 wt% based on the total weight of the steel sheet according to an embodiment of the present invention. When the content of at least one element selected from the group of titanium (Ti), niobium (Nb) and vanadium (V) is less than 0.01% by weight, the ductility of the cast slab is reduced due to excessive precipitation of AIN and BN precipitates. There is a problem of lowering, and it is difficult to expect the effect of grain refinement and strength improvement. In addition, when the content of at least one element selected from the group of titanium (Ti), niobium (Nb) and vanadium (V) is greater than 0.1 wt%, it is difficult to refine grains due to the formation of coarse TiN and TiC precipitates, and the recrystallization temperature There is a problem in that it rises too much and causes a non-uniform structure, and there is a problem in that a process load is induced by excessive precipitation.

Boron (B)

Boron (B) is an element that increases the hardenability of steel and may be an effective element for increasing the hardenability by suppressing the formation of ferrite. The phosphorus may be added in a content ratio of greater than 0 to 0.003% by weight or less based on the total weight of the cold-rolled steel sheet according to an embodiment of the present invention. When the content of boron exceeds 0.003% by weight, there is a problem in that weldability is deteriorated.

As described above, the cold rolled steel sheet according to an embodiment of the present invention having an alloying element composition has a yield strength of 1200 to 1450 MPa, a tensile strength of 1500 to 1700 MPa, an elongation of 5% or more, and a yield ratio of 75% or more. have. Furthermore, the bending workability (R/t) of the cold-rolled steel sheet may be 5.0 or less. That is, the cold-rolled steel sheet may have a minimum bending radius (R) of less than or equal to 5 times the thickness (t) of the steel sheet at 90° bending.

1 is a photograph of the final microstructure of a cold rolled steel sheet according to an embodiment of the present invention, and FIG. 2 is a photograph of precipitates in martensite in the final microstructure of a cold rolled steel sheet according to an embodiment of the present invention. It's a photo.

1 and 2, the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention is 70% or more and less than 100% tempered martensite, more than 0 and less than 20% bainite, and more than 0 10 % or less of ferrite. The volume fraction is based on a result of analyzing a 1/4 point in the thickness direction in a direction perpendicular to the rolling direction with a scanning electron microscope (SEM).

If the fraction of tempered martensite is less than 70%, the target strength cannot be obtained. Bainite is a microstructure that is inevitably generated due to an insufficient cooling rate, and is a major factor for lowering strength. The smaller the fraction, the better. If the fraction of bainite exceeds 20%, the target strength cannot be obtained. Ferrite is a microstructure that is inevitably generated during production, and the smaller the fraction, the more advantageous. When the fraction of ferrite exceeds 10%, strength and formability are reduced.

In addition, the density of carbides with a diameter of 10 nm or more in martensite in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention having the alloy element composition as described above is 1.0x10 ⁷ or more per 1 mm ² and the average diameter size is less than 50 nm. The generation of carbides reduces the dissolved carbon (C) in martensite, which reduces the strength, while restricting the movement of internal dislocations to increase the yield strength. In this case, in order to effectively prevent the movement of dislocations, it must have a size of 10 nm or more in diameter. Even when the number of carbides is small, it is difficult to see such an effect. On the other hand, if the carbide grows excessively, it acts as a source of generating fine cracks during deformation, and the workability deteriorates, and the solid solution carbon in martensite is greatly reduced, so that the target strength cannot be obtained. The size and density of the precipitates were observed through a transmission electron microscope (TEM), and the average size and distribution of the precipitates were calculated by photographing a 50-70 nm thin film sample at a magnification of 15000 times. Carbide is easily distinguished because it is expressed as dark particles in the image taken by a transmission electron microscope. The diameter was measured based on the shortest section of the precipitate. The average grain size of the old austenite is 10 μm or less. The finer the grain size of austenite, the finer the martensite grains, so it is advantageous to obtain high yield strength. The smaller the grain size, the more advantageous, and it can be strictly controlled to 5 μm or less.

Hereinafter, a method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention having the above-described alloying element composition will be described.

Manufacturing method of cold rolled steel sheet

3 is a flowchart schematically illustrating a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention. 4 and 5 are photographs of hot-rolled microstructures when the coiling temperatures are 620°C and 720°C, respectively, in the method for manufacturing a cold-rolled plated steel sheet according to an embodiment of the present invention.

Referring to FIG. 3 , the method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention includes (a) carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5% by weight, aluminum (Al): 0.01 to 0.05% by weight, phosphorus (P): more than 0, 0.02% by weight or less, sulfur (S): more than 0, 0.01% by weight or less, chromium (Cr): 0.2 to 0.5% by weight %, molybdenum (Mo): 0.1 to 0.3 wt%, titanium (Ti), at least one element selected from the group of niobium (Nb) and vanadium (V): 0.01 to 0.1 wt%, boron (B): greater than 0 0.003 Providing a steel material consisting of less than or equal to weight % and the remaining iron (Fe) and other unavoidable impurities (S100); (b) hot-rolling the steel material under the condition that the reheating temperature (SRT) is 1150 to 1300 °C, and the hot rolling termination temperature (FDT) is 800 to 1000 °C (S200); (c) winding the hot-rolled steel at 500 to 650° C. (S300); (d) cold rolling the steel (S400); (e) annealing the cold-rolled steel at 800 to 900° C. (S500); (f) plating the annealed heat-treated steel (S600); And (g) tempering the plated steel at 100 ~ 250 ℃ (S700); includes

The step of hot rolling (S200) in a broad sense includes reheating the steel sheet having the above-described predetermined composition. The steel sheet may be manufactured through a continuous casting process after obtaining molten steel of a desired composition through a steelmaking process.

In one embodiment, the steel may be reheated at a temperature of 1150 ~ 1300 ℃. When the steel sheet is reheated at the above-described temperature, the segregated components during the continuous casting process are re-dissolved, which affects the uniformity of the structure and the dissolution of precipitates in the slab. If the reheating temperature is lower than 1150 ℃, there may be a difficulty in completely re-dissolving the precipitates, the solid solution of various carbides may not be sufficient, and there may be a problem that the segregated components are not sufficiently evenly distributed during the continuous casting process. When the reheating temperature exceeds 1300°C, the austenite grain size becomes coarse, making it difficult to obtain a fine hot-rolled structure and it may be difficult to secure strength. In addition, when it exceeds 1300 ℃, heating cost increases and process time is added, which may result in an increase in manufacturing cost and a decrease in productivity.

The reheated steel material is hot rolled. The hot rolling is performed so that the hot rolling end temperature (FDT) is in the range of 800 ~ 1000 ℃. When the hot-rolling end temperature is less than 800° C., the rolling proceeds in the non-recrystallized region, thereby increasing the rolling addition. The hot rolling may have a rolling reduction of 90% or more. The hot rolled microstructure consists of martensite, bainite, and ferrite. After hot rolling, cooling proceeds at a cooling rate of 1 ~ 100°C/s, and the faster the cooling rate, the more advantageous it is to reduce the average grain size.

Subsequently, a step (S300) of winding the hot-rolled steel is performed. Coiling temperature (CT) may be performed at 500 ~ 650 ℃. When winding at a temperature of less than 500℃, it makes the shape of the hot-rolled coil non-uniform, causes a load during cold rolling, and it is difficult to form precipitates, making it difficult to obtain a sufficient precipitation strengthening effect. It causes a non-uniform microstructure. The non-uniform microstructure may cause flatness defects as well as a difference in strength between the center portion and the edge portion. In particular, when niobium (Nb) is added, the coiling temperature is preferably 600° C. or less, and when it exceeds 600° C., precipitates grow and the target strength cannot be achieved. Even when titanium (Ti) and vanadium (V) are added, precipitates and crystal grains are coarsened during winding at a high temperature exceeding 650° C., thereby reducing strength and bendability (see FIGS. 4 and 5 ).

The step (S400) of cold rolling the steel material may be performed at a reduction ratio of 40 to 60%, and as the reduction ratio increases within this range, there is an effect of increasing the formability due to the effect of refining the structure. If the cold reduction ratio is less than 40%, recrystallization does not occur sufficiently, so there is a concern that non-recrystallization occurs during annealing, and it is difficult to obtain a uniform microstructure.

6 is a diagram illustrating a heat treatment configuration after a cold rolling process in a method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention.

6 and 3 together, the annealing heat treatment step (S500) of the cold-rolled steel material at 800 ~ 900 ℃ is performed. Annealing temperature is a factor that affects recrystallization and coarsening of precipitates. When it is less than 800°C, there is concern about non-recrystallization, and when it exceeds 900°C, the precipitate becomes coarse and the precipitation strengthening effect is reduced, which makes it difficult to obtain the target strength.

As a specific example, in the annealing heat treatment process ('single-phase annealing process' in FIG. 6), the temperature may be increased to a temperature of Ac3 or higher at a temperature increase rate of 1 to 10°C. Preferably, it can be maintained at a temperature of 800 to 900° C. for 60 to 600 seconds. After that, it can be cooled to 250 ~ 500℃ which is the end temperature (RCS) of secondary cooling at an average cooling rate of 3 ~ 20℃/s and maintained for 10 ~ 100 seconds.

A step (S600) of plating the annealed heat-treated steel is performed. As a specific example, the step of plating ('plating' in FIG. 6) may include plating the annealed steel material in a plating bath of 450 to 550°C; and heat-treating the steel at 450 to 650°C.

Tempering the plated steel material at 100 to 250° C. (S700) is performed. As a specific example, the tempering process ('low temperature tempering' in FIG. 6) is performed at a temperature of 100 to 250° C. for 3 to 12 hours. When the temperature is 100°C or less, it is difficult to obtain the target yield strength, and when the temperature is 250°C or more, the tensile strength is excessively reduced and it is difficult to obtain the target bendability.

7 is a stress-strain diagram before and after heat treatment in the method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention. In FIG. 7 , 'one-stage heat treatment' corresponds to the above-described annealing heat treatment step (S500), and 'two-stage heat treatment' corresponds to the above-described tempering step (S700). Referring to FIG. 7 , it can be seen that the yield strength is increased after the tempering process.

The present invention includes a tempering process of 100 ~ 250 ℃ after the conventional continuous annealing process. During tempering in this temperature range, the dislocation inside the martensite decreases and the carbon sticks to the dislocation and lath boundary. Carbide is formed with tempering temperature and time, and the size increases proportionally as the temperature and time increase. As a result, the strength decreases due to the reduction of carbon dissolved in martensite, but the ductility and bendability are improved. In addition, the yield strength is improved due to the carbon adhering to dislocations and lath boundaries and the newly formed fine carbides.

The yield strength of the cold rolled steel sheet according to an embodiment of the present invention implemented by performing the above-described steps (S100) to (S700) is 1200 to 1450 MPa, the tensile strength is 1500 to 1700 MPa, and the elongation is 5% or more. , the yield ratio may be 75% or more, and the bending workability (R/t) may be 5.0 or less.

On the other hand, the final microstructure of the steel sheet according to an embodiment of the present invention implemented by performing the above-described steps (S100) to (S700) is 70% or more and less than 100% tempered martensite, 0 to 20% or less It may consist of bainite and greater than 0 and up to 10% ferrite.

1.5GPa grade cold-rolled steel sheet manufactured through the existing continuous annealing process adds a large amount of manganese (Mn) to secure a high martensite fraction, which makes it difficult to obtain excellent bending workability. In addition, it has a limited resistance to recovery ratio, so the ability to absorb impact energy is insufficient. According to the manufacturing method according to an embodiment of the present invention, there is provided a method for manufacturing a cold-rolled alloy hot-dip galvanized steel sheet having a tensile strength of 1.5 GPa class and a high yield ratio (YP/TS) of 75% or more. When the steel sheet realized by this manufacturing method is used as a vehicle collision member component, the collision performance is excellent, and it is expected that it will contribute to the improvement of fuel efficiency by reducing the weight of the vehicle body.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Table 1 shows the specific alloy element composition (unit: wt%) of the compositional component system in this experimental example, and Table 2 shows Comparative Examples 1-10 and Examples 1-10 implemented with the compositional component system and process conditions described in Table 1 The mechanical properties and analysis results of the specimen in Fig. 4 are shown.

ingredient system C Si Mn Al P S Cr Mo Ti Nb V B A 0.25 0.1 2.0 0.03 0.01 0.005 0.4 0.2 0.03 - - 0.0025 B 0.26 0.1 2.0 0.03 0.01 0.005 0.4 0.2 0.12 - - 0.0025 C 0.25 0.1 2.5 0.03 0.01 0.005 0.4 0.1 0.02 0.04 - 0.0025 D 0.24 0.1 2.0 0.03 0.01 0.005 0.4 0.2 0.02 - 0.05 0.0025

Referring to Table 1, the compositional component systems A, C, and D are carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al) ): 0.01 to 0.05% by weight, phosphorus (P): more than 0, 0.02% by weight or less, sulfur (S): more than 0, 0.01% by weight or less, chromium (Cr): 0.2 to 0.5% by weight, molybdenum (Mo): 0.1 to 0.3% by weight, titanium (Ti), at least one element selected from the group consisting of niobium (Nb) and vanadium (V): 0.01 to 0.1% by weight, boron (B): greater than 0 0.003% by weight or less and the remainder iron (Fe) satisfies the composition of one embodiment of the present invention.

On the other hand, the compositional system B of Table 1, unlike the composition of the steel sheet according to an embodiment of the present invention, at least one element selected from the group of titanium (Ti), niobium (Nb) and vanadium (V): 0.01 to 0.1 weight % out of range.

castle
minute
total winding temperature
(℃) tempering
temperature
(℃) tempering
hour
(h) Precipitate average size
(nm) number of precipitates YP
(MPa) ts
(MPa) EL
(%) yield ratio
(YP/TS)X100 R/t Example 1 A 560 150 4 41 2.1x10 ⁸ 1248 1515 6.3 0.82 3.1 Example 2 A 560 140 4 33 1.9x10 ⁸ 1252 1623 6.3 0.77 3.1 Example 3 A 560 160 4 41 6.1x10 ⁷ 1248 1626 6.9 0.77 3.4 Example 4 D 560 180 4 42 7.5x10 ⁷ 1277 1617 7.3 0.79 4.7 Comparative Example 1 A 560 80 4 31 2.3x10 ⁸ 1147 1569 7.7 0.73 3.4 Comparative Example 2 C 560 200 14 58 8.7x10 ⁷ 1308 1452 5.2 0.90 3.3 Comparative Example 3 A 560 260 2 61 5.4x10 ⁷ 1378 1453 5.6 0.95 4.2 Comparative Example 4 D 560 0 0 26 4.1x10 ⁵ 1109 1636 6.5 0.67 5.8 Comparative Example 5 C 560 120 2 27 4.6x10 ⁸ 1120 1604 7.3 0.75 3.2 Comparative Example 6 B 560 0 0 59 6.2x10 ⁴ 977 1529 6.8 0.64 7.2 Comparative Example 7 B 560 150 4 67 1.6x10 ⁷ 1126 1439 6.7 0.78 5.8 Comparative Example 8 C 680 150 6 67 6.3x10 ⁶ 1211 1489 6.9 0.81 5.2 Comparative Example 9 D 720 120 6 57 4.3x10 ⁶ 1123 1659 7.3 0.67 5.6 Comparative Example 10 A 700 180 6 41 9.1x10 ⁶ 1222 1465 6.5 0.83 4.2

Referring to Table 2, Examples 1 to 4 of the present invention are carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, Aluminum (Al): 0.01 to 0.05% by weight, phosphorus (P): more than 0, 0.02% by weight or less, sulfur (S): more than 0, 0.01% by weight or less, chromium (Cr): 0.2 to 0.5% by weight, molybdenum (Mo) : 0.1 to 0.3% by weight, at least one element selected from the group of titanium (Ti), niobium (Nb) and vanadium (V): 0.01 to 0.1% by weight, boron (B): more than 0 0.003% by weight or less and the remainder iron (Fe) satisfies the composition range, the coiling temperature satisfies the range of 500 ~ 650 ℃, the tempering temperature is 100 ~ 250 ℃, and satisfies that the tempering time is 3 ~ 12 hours.

The steel sheets of Examples 1 to 4 of the present invention satisfying these composition and process conditions have a yield strength of 1200 to 1450 MPa, a tensile strength of 1500 to 1700 MPa, an elongation of 5% or more, and a yield ratio of 75% or more. , it can be seen that the bending workability (R/t) satisfies the range of 5.0 or less. In addition, in the final microstructure, the density of carbides with a diameter of 10 nm or more inside the martensite is 1.0×10 ⁷ or more per 1 mm ² , and the average diameter size is 50 nm or less.

On the other hand, Comparative Example 1, Comparative Example 4, Comparative Example 5, Comparative Example 6 of the present invention, unlike the process conditions of the steel sheet manufacturing method according to an embodiment of the present invention, the tempering temperature was less than the range of 100 ~ 250 ℃, not satisfied Looking at these comparative examples, it can be seen that the yield strength is less than the range of 1200 to 1450 MPa appears.

Comparative Example 3 of the present invention, unlike the process conditions of the steel sheet manufacturing method according to an embodiment of the present invention, the tempering temperature exceeds the range of 100 ~ 250 ℃ is not satisfied. Looking at these comparative examples, it can be seen that the tensile strength is less than the range of 1500 ~ 1700 MPa appears.

In Comparative Example 2 of the present invention, unlike the process conditions of the method for manufacturing a steel sheet according to an embodiment of the present invention, the tempering time exceeds the range of 3 to 12 hours and is not satisfied. Looking at these comparative examples, it can be seen that the tensile strength is less than the range of 1500 ~ 1700 MPa appears.

Comparative Example 4, Comparative Example 5, and Comparative Example 6 of the present invention, unlike the process conditions of the steel sheet manufacturing method according to an embodiment of the present invention, the tempering time is not satisfied because the tempering time is less than the range of 3 to 12 hours. Looking at these comparative examples, it can be seen that the yield strength is less than the range of 1200 to 1450 MPa appears.

In Comparative Examples 6 and 7 of the present invention, at least one element selected from the group of titanium (Ti), niobium (Nb) and vanadium (V) is different from the composition of the method for manufacturing a steel sheet according to an embodiment of the present invention. Exceeding the range of 0.01 to 0.1 wt% is not satisfactory. Looking at these comparative examples, it can be seen that the amount of solid solution greatly reduced due to the excessive addition of titanium, so that the yield strength is less than the range of 1200 to 1450 MPa, and the bending workability (R/t) of the steel sheet does not satisfy the range of 5.0 or less. can

Comparative Example 8, Comparative Example 9, and Comparative Example 10 of the present invention, unlike the process conditions of the steel sheet manufacturing method according to an embodiment of the present invention, the coiling temperature exceeds the range of 500 ~ 650 ℃ is not satisfied. Looking at these comparative examples, it can be seen that there is a problem in that the precipitates grow excessively and the density of the precipitates falls below the range of 1.0x10 ⁷ or more per 1 mm ² , which is not satisfactory.

8 is a graph showing the number of precipitates and the size of the precipitates in the cold rolled steel sheet according to the experimental example of the present invention.

Referring to FIG. 8 , it can be seen that the number of precipitates located within the dotted rectangle is 1.0x10 ⁷ to 1.0x10 ⁹ per 1 mm ² , and the average diameter size satisfies the range of 10 nm to 50 nm. The formation of carbide as a precipitate reduces the solid solution carbon (C) in martensite, which reduces the strength, while limiting the movement of internal dislocations to increase the yield strength. In this case, in order to effectively prevent the movement of dislocations, it must have a size of 10 nm or more in diameter. Even when the number of carbides is small, it is difficult to see such an effect. On the other hand, if the carbide grows excessively, it acts as a source of generating fine cracks during deformation, deteriorates workability, and greatly reduces solid solution carbon in martensite, making it impossible to obtain the target strength.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (5)

(a) carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al): 0.01 to 0.05 wt%, phosphorus (P) ): more than 0 and less than 0.02 wt%, sulfur (S): more than 0 and less than or equal to 0.01 wt%, chromium (Cr): 0.2 to 0.5 wt%, molybdenum (Mo): 0.1 to 0.3 wt%, titanium (Ti), niobium ( Nb) and at least one element selected from the group of vanadium (V): 0.01 to 0.1% by weight, boron (B): more than 0 and 0.003% by weight or less, and the remaining iron (Fe) and other unavoidable impurities. ;
(b) hot-rolling the steel material under the condition that the reheating temperature (SRT) is 1150 to 1300 °C, and the hot rolling termination temperature (FDT) is 800 to 1000 °C;
(c) winding the hot-rolled steel at 500 to 650°C;
(d) cold rolling the steel;
(e) annealing the cold-rolled steel at 800 to 900° C.;
(f) plating the annealed heat-treated steel;
(g) cooling the plated steel; and
(h) heating the cooled steel material again and tempering at 100 to 250°C; including,
The step (e) is annealing the cold rolled steel at 800 ~ 900 ℃; And cooling the steel material to 350 ~ 500 ℃ at a cooling rate of 3 ~ 20 ℃ / s and holding for 10 ~ 100 seconds; Containing,
The step (h) is characterized in that the re-heated steel is maintained at a constant temperature at 100 to 250 ° C. for 3 to 12 hours and tempered to form carbides,
Martensite is formed by performing the step (g), and tempered martensite is formed by performing the step (h),
A method for manufacturing a cold rolled steel sheet. The method of claim 1,
The step (f) is a step of plating the annealed heat-treated steel in a plating bath of 450 ~ 550 ℃; And the step of alloying heat treatment at 450 ~ 650 ℃ the steel material; Containing,
A method for manufacturing a cold rolled steel sheet. The method of claim 1,
The step (b) includes the step of hot rolling at a reduction ratio of 90% or more,
The step (d) comprises the step of cold rolling at a reduction ratio of 40 to 60%,
A method for manufacturing a cold rolled steel sheet. The method of claim 1,
After performing the step (h), carbides, which are precipitates in the steel sheet, satisfy a range of 1.0x10 ⁷ to 1.0x10 ⁹ per 1 mm ² and an average diameter size of 10 nm to 50 nm, and the final microstructure of the steel sheet is 70% Characterized in that it consists of more than 100% tempered martensite, more than 0 and less than 20% bainite, and more than 0 and less than 10% of ferrite,
A method for manufacturing a cold rolled steel sheet. Carbon (C): 0.24 to 0.30 wt%, Silicon (Si): 0.05 to 0.3 wt%, Manganese (Mn): 1.0 to 2.5 wt%, Aluminum (Al): 0.01 to 0.05 wt%, Phosphorus (P): 0 More than 0.02% by weight, sulfur (S): more than 0, 0.01% by weight or less, chromium (Cr): 0.2 to 0.5% by weight, molybdenum (Mo): 0.1 to 0.3% by weight, titanium (Ti), niobium (Nb) and At least one element selected from the group of vanadium (V): 0.01 to 0.1% by weight, boron (B): more than 0 0.003% by weight or less, and a cold-rolled steel sheet consisting of iron (Fe) and other unavoidable impurities,
Yield strength is 1200 ~ 1450 MPa, tensile strength is 1500 ~ 1700 MPa, elongation is 5% or more, yield ratio is 75% or more, bending workability (R / t) is 5.0 or less,
The number of carbides as precipitates in the steel sheet is 1.0x10 ⁷ to 1.0x10 ⁹ per 1mm ² and the average diameter size satisfies the range of 10nm to 50nm,
The final microstructure of the steel sheet is characterized in that it consists of 70% or more and less than 100% tempered martensite, more than 0 and less than 20% bainite, and more than 0% and less than 10% of ferrite,
Cold rolled galvanized steel sheet.

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