KR20220041506A - Steel sheet having high strength and high formability and method for manufacturing the same - Google Patents

Abstract

The present invention provides a steel sheet having high strength and high formability and a method for manufacturing the same. The steel sheet having high strength and high formability according to one aspect of the present invention comprises, in weight percent, carbon (C): 0.12% to 0.22%, silicon (Si): 1.6% to 2.4%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): 0.01% to 0.05%, a sum of titanium (Ti), niobium (Nb), and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.015%, sulfur (S): greater than 0% to 0.003%, nitrogen (N): greater than 0% to 0.006%, and the balance being iron (Fe) and other inevitable impurities, has a mixed structure in which austenite, ferrite, and tempered martensite are mixed, and satisfies a yield strength (YS): 600 MPa or more, a tensile strength (TS): 980 MPa or more, an elongation (EL): 20% or more, and a product of the tensile strength and elongation: 20,000 MPa% or more.

Description

Steel sheet having high strength and high formability and method for manufacturing the same

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a high strength and high formability steel sheet and a method for manufacturing the same.

In recent years, from the viewpoints of safety and weight reduction of automobiles, high strength of automobile steel sheets is progressing more rapidly. In order to secure passenger safety, steel sheets used as structural members of automobiles must have sufficient impact toughness by increasing the strength or thickness. In addition, sufficient formability is required to be applied to automobile parts, and weight reduction is essential to improve fuel efficiency of automobiles.

In order to achieve a weight reduction of a steel sheet applied as a structural member of a vehicle such as a vehicle, many attempts have been made to improve the strength and elongation of the existing steel material. Among the parts used for automobiles, the members and fillers that influence collision safety are complex shapes, so it is appropriate for the mechanical properties of dual-phase steel, which secures elongation in two phases of ferrite and martensite. Formability cannot be ensured. Therefore, as a high-strength steel sheet exhibiting superior ductility than DP steel, transformation induced plasticity steel is attracting attention. These TRIP steels use polygonal ferrite as the main phase and TRIP type composite structure steel (TPF steel) containing retained austenite, and TRIP containing retained austenite using bainitic ferrite as the mother phase. It is classified into several types such as type bainite steel (TBF steel). However, because the general TRIP steel currently used cannot escape the limits of the Rule of mixture (ROM), the tensile strength of 980 MPa or more, the elongation of 20% or more, and the product of the tensile strength and the elongation satisfy the mechanical properties of 20,000 MPa% or more. hard to do

Korean Patent Application No. 10-2016-0077463

The problem to be solved by the present invention is to provide a high strength and high formability steel sheet 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, there is provided a high strength and high formability steel sheet and a method for manufacturing the same.

According to an embodiment of the present invention, the high strength and high formability steel sheet, by weight, carbon (C): 0.12% to 0.22%, silicon (Si): 1.6% to 2.4%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): 0.01% to 0.05%, sum of titanium (Ti), niobium (Nb) and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.015% , sulfur (S): greater than 0% to 0.003%, nitrogen (N): greater than 0% to 0.006%, and the balance including iron (Fe) and other unavoidable impurities, including austenite, ferrite, and tempered martensite has a mixed structure, yield strength (YS): 600 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 20% or more, and the product of tensile strength and elongation: 20,000 MPa% or more can be satisfied,

According to an embodiment of the present invention, the fraction of austenite is 11% to 15%, the fraction of ferrite is 25% to 35%, and the fraction of tempered martensite is 50% to 64%. there is.

According to an embodiment of the present invention, the size of the austenite is in the range of 0.1 μm to 3 μm, the size of the ferrite is in the range of 1 μm to 5 μm, and the size of the tempered martensite is in the range of 1 μm to 5 μm. can be

According to an embodiment of the present invention, the average long axis/short axis ratio of the austenite may be 2.0 or more.

According to an embodiment of the present invention, the manufacturing method of the high strength and high formability steel sheet is (a) by weight, carbon (C): 0.12% to 0.22%, silicon (Si): 1.6% to 2.4%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): 0.01% to 0.05%, sum of titanium (Ti), niobium (Nb) and vanadium (V): greater than 0% to 0.05%, phosphorus (P): More than 0% to 0.015%, Sulfur (S): More than 0% to 0.003%, Nitrogen (N): More than 0% to 0.006%, and the remainder by hot rolling steel containing iron (Fe) and other unavoidable impurities manufacturing a hot-rolled steel sheet; (b) cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; (c) annealing the cold-rolled steel sheet at 820°C to 850°C; and (d) cooling the cold-rolled steel sheet in multiple stages.

According to an embodiment of the present invention, the step (a) comprises the steps of: (a-1) preparing a steel material having the alloy composition; (a-2) reheating the steel material at 1,150 to 1,250°C; (a-3) preparing a hot-rolled steel sheet by hot finish rolling the reheated steel at 900°C to 950°C; and (a-4) winding the hot-rolled steel sheet at 550°C to 650°C.

According to an embodiment of the present invention, in step (c), the temperature of the cold-rolled steel sheet is raised at a temperature increase rate of 3° C./sec to 8° C./sec, and can be maintained at 820° C. to 850° C. for 60 seconds to 300 seconds. .

According to an embodiment of the present invention, the step (d) includes: primary cooling of the cold-rolled steel sheet to 730°C to 770°C at a cooling rate of 5°C/sec to 10°C/sec; and performing secondary cooling of the cold-rolled steel sheet to 200° C. to 300° C. at a cooling rate of 50° C./sec to 100° C./sec, and maintaining the cold-rolled steel sheet for 5 seconds to 20 seconds.

According to an embodiment of the present invention, after performing step (d), (e) heating the cold-rolled steel sheet at a temperature increase rate of 10° C./sec to 20° C./sec, and 10 at a temperature of 400° C. to 460° C. It may further include; a post-heat treatment step of maintaining for seconds ~ 240 seconds.

According to an embodiment of the present invention, after performing step (d), (e) immersing the cold-rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanizing layer; and (f) heat-treating the cold-rolled steel sheet on which the hot-dip galvanizing layer is formed.

According to an embodiment of the present invention, after step (d), the steel sheet has a mixed structure in which austenite, ferrite, and tempered martensite are mixed, yield strength (YS): 600 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 20% or more, and product of tensile strength and elongation: 20,000 MPa% or more.

According to the present invention, it has a microstructure composed of austenite, ferrite, and tempered martensite through component system control and process condition control, yield strength (YS): 600 MPa or more, tensile strength (TS): 980 MPa or more , elongation (EL): 20% or more, and product of tensile strength and elongation: 20,000 MPa% or more, a high strength and high formability steel sheet can be manufactured.

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 flowchart schematically illustrating a method of manufacturing a high strength and high formability steel sheet according to an embodiment of the present invention.
2 is a photograph showing the microstructure of the high strength and high formability steel sheet according to an embodiment of the present invention.
3 is a graph showing the number of grains with respect to the major axis/short axis ratio in austenite of the high strength and high formability steel sheet according to an embodiment 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 this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

In order to overcome the limitations of the mechanical properties of the existing transformation organic plastic steel, the development direction of the next-generation ultra-high-strength automotive steel sheet that secures high strength and appropriate elongation by replacing the main matrix with martensite instead of bainite is attracting attention from each steelmaking company. . As a prior art, when a composite structure of ferrite, annealed martensite, and retained austenite is formed, high strength and high elongation are secured, but the yield ratio is high due to a low ferrite fraction, and workability is deteriorated. As another prior art, the volume fraction of ferrite was increased in order to secure workability, but the tensile strength of 1000 MPa or more and the elongation of 20% or more and the product of tensile strength and elongation of 20,000 MPa% were not satisfied. In other prior art, high strength and suitable high formability and workability were ensured, but weldability was deteriorated due to high carbon content. As another prior art, a high-strength cold-rolled steel sheet with excellent burring properties was implemented with a composite structure of ferrite, annealed martensite, retained austenite, and bainite, but it is difficult to produce in a general CGL (Continuous Galvanized Line) due to restrictions in heat treatment conditions. , for example, the time of the overaging section requires a long time compared to general CGL.

Hereinafter, a high strength and high formability steel sheet, which is an aspect of the present invention, will be described.

The high strength and high formability steel sheet according to the technical idea of the present invention can stably secure high tensile strength and elongation by controlling the final microstructure through mass-production process conditions, and has excellent elongation despite high strength. .

High strength and high formability steel sheet

The high strength and high formability steel sheet according to an aspect of the present invention is, by weight, carbon (C): 0.12% to 0.22%, silicon (Si): 1.6% to 2.4%, manganese (Mn): 2.0% to 3.0%, Aluminum (Al): 0.01% to 0.05%, the sum of titanium (Ti), niobium (Nb) and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.015%, sulfur (S) ): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.006%, and the balance contains iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the high strength and high formability steel sheet according to the present invention will be described as follows. At this time, the content of the component elements all mean wt%.

Carbon (C): 0.12% to 0.22%

Carbon is the most important alloying element in steelmaking, and has a primary purpose of strengthening and stabilizing austenite. High carbon concentration in austenite improves austenite stability, making it easy to secure proper austenite for material improvement. When the carbon content is less than 0.12%, it is difficult to secure the desired yield strength and elongation. When the carbon content exceeds 0.22%, the weldability may decrease due to the increase in carbon equivalent, and a large number of cementite precipitated structures such as pearlite may be generated during cooling. Accordingly, the carbon content is preferably 0.12% to 0.22% of the total weight of the steel sheet.

Silicon (Si): 1.6% to 2.4%

Silicon is an element that inhibits the formation of carbides (eg, Fe ₃ C) in ferrite and increases the activity of carbon to increase the diffusion rate of austenite. Silicon is also known as a ferrite stabilizing element, which increases ductility by increasing the ferrite fraction during cooling. When the content of silicon is less than 1.6%, the effect of adding silicon is insufficient. When the content of silicon exceeds 2.4%, oxide (SiO ₂ ) is formed on the surface of the steel sheet during the process, which may lead to a decrease in plating property according to the inferior wettability of the part. Therefore, the content of silicon is preferably 1.6% to 2.4% of the total weight of the steel sheet.

Manganese (Mn): 2.0% to 3.0%

Manganese is an austenite stabilizing element, and as manganese is added, Ms, which is the starting temperature of martensite formation, is gradually lowered, thereby increasing the retained austenite fraction during the continuous annealing heat treatment process. When the manganese content is less than 2.0%, it is difficult to secure strength. When the manganese content exceeds 3.0%, the carbon equivalent is increased to greatly decrease the weldability, and oxide (MnO) is formed on the surface of the steel sheet during the process, which may lead to a decrease in plating properties according to the inferior wettability of the part. Accordingly, the content of manganese (Mn) is preferably 2.0% to 3.0% of the total weight of the steel sheet.

Aluminum (Al): 0.01% to 0.05%

Like silicon, aluminum stabilizes ferrite and retained austenite, and also mainly serves to strengthen solid solution and inhibit carbide formation. When the content of aluminum is less than 0.01%, the effect of adding aluminum is insufficient. If the content of aluminum exceeds 0.05%, a problem may occur in the continuous casting process. Accordingly, the content of aluminum is preferably 0.01% to 0.05% of the total weight of the steel sheet.

Sum of titanium (Ti), niobium (Nb) and vanadium (V): >0% to 0.05%

Titanium, vanadium, and niobium are the main elements that precipitate in the form of carbides in steel. The purpose of adding titanium, vanadium, and niobium is to secure retained austenite stability and improve strength through initial austenite grain refinement according to the formation of precipitates, refine ferrite grains, and precipitation hardening by the presence of precipitates in ferrite. When the total content of titanium, vanadium, and niobium exceeds 0.05%, material deterioration and manufacturing cost may increase. Therefore, it is preferable that the sum of the contents of titanium (Ti), niobium (Nb) and vanadium (V) is more than 0% to 0.05% of the total weight of the steel sheet.

Phosphorus (P): >0% to 0.015%

Phosphorus can play a role similar to silicon in steel. However, when phosphorus is added in excess of 0.015% of the total weight of the steel sheet, it may deteriorate the weldability of the steel sheet and increase brittleness, thereby causing material deterioration. Therefore, the content of phosphorus is preferably limited to more than 0% ~ 0.015% of the total weight of the steel sheet.

Sulfur (S): >0% to 0.003%

Sulfur is an element that is unavoidably contained in the manufacture of steel, and inhibits the toughness and weldability of the steel, and forms MnS by combining with manganese (Mn), thereby reducing the corrosion resistance and impact properties of the steel. Therefore, it is preferable to limit the sulfur content to more than 0% to 0.003% of the total weight of the steel sheet.

Nitrogen (N): >0% to 0.006%

Sulfur is an element that is unavoidably contained in the manufacture of steel, and if it is present in excess, nitrides may precipitate in a large amount, which may deteriorate ductility. Therefore, the content of nitrogen is preferably limited to more than 0% to 0.006% of the total weight of the steel sheet.

The remaining component of the high strength and high formability steel sheet is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal steelmaking process, it cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

High strength and high formability steel sheet according to the technical idea of the present invention, by adding alloying elements such as titanium (Ti), niobium (Nb), and vanadium (V) to induce the formation of an appropriate amount of carbide, formability and elongation It is possible to refine the crystal grains of retained austenite without significant degradation of In addition, by appropriately securing the stability of retained austenite, it is possible to secure the strength, elongation, and formability of the transformation induced firing mechanism. In addition, it is possible to prevent a decrease in yield strength and tensile strength that occurs when the fraction of ferrite increases by inducing precipitation hardening by refining the crystal grains of ferrite and forming precipitates inside the ferrite. However, it is necessary to adjust the total of titanium (Ti), niobium (Nb), and vanadium (V) to 0.05% by weight or less.

A high strength and high formability steel sheet manufactured through a method for manufacturing a steel sheet to be described later by controlling specific components of the alloy composition and their content ranges, yield strength (YS): 600 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 20% or more, and product of tensile strength and elongation: 20,000 MPa% or more may be satisfied. The high strength and high formability steel sheet, yield strength (YS): 600 MPa ~ 850 MPa, tensile strength (TS): 980 MPa ~ 1180 MPa, elongation (EL): 20% ~ 30%, product of tensile strength and elongation: 20,000 MPa% to 25,000 MPa% may be satisfied.

As factors affecting the material of the high strength and high formability steel sheet, the strength and elongation of the retained austenite are secured by the phase transformation of the retained austenite according to the transformation induced plasticity phenomenon, the stability of the retained austenite and the elongation by the ferrite are secured, and the basic base is tempered There is an increase in strength due to martensite itself, and an increase in strength due to grain refinement and precipitation hardening. The high strength and high formability steel sheet has a product of tensile strength and elongation of 20,000 or more, which is generally superior to the value suggested by the corresponding ultra-high strength level.

The high strength and high formability steel sheet may have a mixed structure in which austenite, ferrite, and tempered martensite are mixed. The fraction of the austenite may be, for example, 11% to 15%. The fraction of ferrite may have a great effect on the overall material, for example, may be 25% to 35%, and may be 28% to 32%. When the fraction of the ferrite is less than 25%, the yield ratio is high, the workability is lowered, and it may be disadvantageous in securing the elongation. When the fraction of ferrite exceeds 35%, it may be difficult to secure sufficient yield strength and tensile strength because the fraction of tempered martensite, which is a matrix structure, is reduced. The fraction of the tempered martensite may be, for example, 50% to 64%. The fraction means an area ratio derived from a microstructure photograph through an image analyzer.

The size of the austenite may be, for example, in the range of 0.1 μm to 3 μm. The size of the ferrite may be, for example, in the range of 1 μm to 5 μm. The size of the tempered martensite may be, for example, in the range of 1 μm to 5 μm.

The austenite is retained austenite, and is a core structure capable of securing both the strength and elongation of the steel sheet. The average long axis/short axis ratio of the austenite may be, for example, 2.0 or more, for example, 2.0 to 4.0. When the major axis/short axis ratio is less than 2.0, stability of the austenite may be low, which may be disadvantageous in securing high elongation.

Hereinafter, a method for manufacturing a high strength and high formability steel sheet according to the present invention will be described with reference to the accompanying drawings.

Manufacturing method of high strength and high formability steel sheet

1 is a flowchart schematically illustrating a method of manufacturing a high strength and high formability steel sheet according to an embodiment of the present invention.

Referring to Figure 1, the manufacturing method of the high-strength and high formability steel sheet comprises the steps of: manufacturing a hot-rolled steel sheet by hot-rolling a steel material (S110), cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet (S120); annealing the cold-rolled steel sheet (S130); and cooling the cold-rolled steel sheet in multiple stages (S140).

The method of manufacturing the high strength and high formability steel sheet may further include a step (S150) of post-heating the cold rolled steel sheet.

In addition, if necessary, the method for manufacturing the high strength and high formability steel sheet may further include a step of immersing the cold rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanizing layer (S160). In addition, if necessary, the method of manufacturing the high strength and high formability steel sheet may further include the step (S170) of alloying and heat-treating the cold-rolled steel sheet on which the hot-dip galvanizing layer is formed.

In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling and cold rolling processes may be, for example, a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process.

Hot-rolled steel sheet manufacturing step (S110)

In the hot-rolled steel sheet manufacturing step (S110), by weight, carbon (C): 0.12% to 0.22%, silicon (Si): 1.6% to 2.4%, manganese (Mn): 2.0% to 3.0%, aluminum (Al) : 0.01% to 0.05%, the sum of titanium (Ti), niobium (Nb) and vanadium (V): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.015%, sulfur (S): 0% Exceeds to 0.003%, nitrogen (N): more than 0% to 0.006%, and the balance prepares steel containing iron (Fe) and other unavoidable impurities.

The steel is reheated to a temperature of Ac3 or higher, at a reheating temperature (Slab Reheating Temperature, SRT) in the range of 1,150°C to 1,250°C. Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur. When the reheating temperature is less than 1,150° C., there may be a problem in that the hot rolling load rapidly increases. When the reheating temperature exceeds 1,250 ℃, it may be difficult to charge and discharge in the heating furnace due to the bending of the slab, and it may be difficult to secure the strength of the final produced steel sheet due to the coarsening of the initial austenite grains.

Subsequently, the reheated steel material may be hot rolled, and finish rolling may be performed at a finish delivery temperature (FDT) in the range of 900°C to 950°C. When the finish rolling end temperature exceeds 950° C., there is a fear that the quality of the steel sheet may be deteriorated due to the generation of scale on the surface of the steel sheet. In addition, when the finish rolling end temperature is less than 900 ℃, the crystal grains are refined to increase the strength, but may cause an increase in the rolling load and a decrease in productivity.

Then, the hot-rolled steel is cooled to a predetermined coiling temperature. The cooling may be both air cooling or water cooling, and cooling may be performed at a cooling rate of 10° C./sec to 30° C./sec. The cooling is preferably cooled to a coiling temperature of 550 °C to 650 °C. Then, the hot-rolled steel sheet is wound at a coiling temperature (CT) in the range of 550°C to 650°C.

Cold-rolled steel sheet manufacturing step (S120)

In the cold-rolled steel sheet manufacturing step (S120), pickling and cold rolling are performed. A pickling treatment of washing the hot-rolled steel sheet with an acid may be performed. Subsequently, cold rolling may be performed on the pickling-treated hot-rolled steel sheet at an average reduction ratio of 40% to 60%. Cold rolling is performed to match the thickness of the finally produced steel sheet. The microstructure of the cold-rolled steel sheet has a shape in which the structure of the hot-rolled steel sheet is stretched, and the microstructure of the finally produced steel sheet is determined in the subsequent heat treatment.

Annealing heat treatment step (S130)

In the annealing heat treatment step (S130), the cold rolled steel sheet may be heat treated in a continuous annealing furnace having a normal slow cooling section. The annealing heat treatment may be performed in an ideal temperature region of austenite and ferrite. This is to obtain the target final material of the steel sheet through a microstructure in which the ideal residual austenite, ferrite, and tempered martensite are mixed in the final microstructure by securing ferrite of an appropriate shape and fraction.

The annealing heat treatment is heated at a temperature increase rate of 3° C./sec to 8° C./sec, and maintained at a temperature of 820° C. to 850° C. for 60 seconds to 300 seconds. When the annealing heat treatment temperature is less than 820° C., it may be difficult to form sufficient austenite, and thus it may be difficult to secure target strength and elongation. When the annealing heat treatment temperature exceeds 850° C., the fraction of ferrite decreases and sufficient elongation cannot be obtained.

Multi-stage cooling step (S140)

In the multi-stage cooling step (S140), primary cooling is performed on the annealed cold-rolled steel sheet to 730°C to 770°C at a cooling rate of 5°C/sec to 10°C/sec. The primary cooling is performed to ensure plasticity by securing a certain amount of ferrite in the final microstructure. In addition, through the shape and fraction of the ferrite formed during the heat treatment process, it is performed to secure an appropriate fraction of retained austenite and a major/short axis ratio.

Then, the cold-rolled steel sheet is subjected to secondary cooling at a cooling rate of 50° C./sec to 100° C./sec to 200° C. to 300° C., and maintained for 5 seconds to 20 seconds. The secondary cooling is performed to control the quenching end temperature to transform austenite in the microstructure into martensite after slow cooling, thereby facilitating securing the final material. In addition, in order to suppress the phase transformation that may occur during the quenching process, it is performed at a cooling rate of 50° C./sec or more.

Post-heat treatment step (S150)

In the post-heat treatment step (S150), the cold-rolled steel sheet is heated at a temperature increase rate of 10° C./sec to 20° C./sec, and maintained at a temperature of 400° C. to 460° C. for 10 seconds to 240 seconds. By the post-heat treatment, carbon is concentrated in retained austenite, and tempered martensite is formed through martensite tempering, so that high strength and high elongation can be secured.

The above-described steps (S110 to S150) may be performed to form a cold-rolled steel sheet. In addition, if necessary, the following steps may be performed to manufacture a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet. In the case of a continuous process, after performing the multi-stage cooling step (S140), the post-heat treatment step (S150) may be omitted, and the following hot-dip galvanizing layer forming step (S160) may be directly performed. In addition, only the following hot-dip galvanized layer forming step (S160) may be performed to manufacture a hot-dip galvanized steel sheet.

Hot-dip galvanizing layer forming step (S160)

In the hot-dip galvanizing layer forming step (S160), the step of forming a hot-dip galvanizing layer by immersing the post-heat-treated cold-rolled steel sheet in a hot-dip galvanizing bath is performed. The temperature of the plating bath may be in the range of 430° C. to 470° C. depending on the type and ratio of alloying elements constituting the plating layer and the composition of the cold-rolled steel sheet. The cold-rolled steel sheet is immersed in the plating bath, heated at a temperature increase rate of 10° C./sec to 20° C./sec, and maintained at a temperature of 430° C. to 470° C. for 30 seconds to 100 seconds. While the hot-dip galvanizing layer is easily formed on the surface of the cold-rolled sheet under the plating bath conditions, the adhesion of the plating layer may be excellent. The post-heat treatment step may be performed when the hot-dip galvanizing layer is formed.

alloying heat treatment step (S160)

If necessary, an alloying heat treatment step of the cold-rolled steel sheet having the hot-dip galvanized layer formed thereon may be further performed. The alloying heat treatment may be performed at a temperature in the range of 490 ℃ ~ 530 ℃. Under the above conditions, while the hot-dip galvanized layer is stably grown during the alloying heat treatment, the adhesion of the plating layer may be excellent. When the alloying heat treatment temperature is less than 490° C., alloying may not proceed sufficiently, and thus the health of the hot-dip galvanizing layer may be deteriorated. When the alloying heat treatment temperature exceeds 530° C., a change in material may occur while passing to an abnormal temperature range.

The cold-rolled steel sheet, the hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized steel sheet manufactured in this way are cooled to room temperature immediately after completing the process.

The cold-rolled steel sheet, the hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized steel sheet manufactured by the method of the present invention may have a mixed structure in which austenite, ferrite, and tempered martensite are mixed.

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 is a table showing the composition of the high strength and high formability steel sheet according to an embodiment of the present invention.

Steel having the composition (unit: weight %) shown in Table 1 below is prepared, and a cold-rolled steel sheet manufactured through predetermined hot rolling and cold rolling processes is prepared. The remainder is iron (Fe) and other unavoidable impurities. Both Examples and Comparative Examples have the same alloy composition.

C Si Mn Al Ti+Nb+V P S N ingredient system 0.18 1.7 2.3 0.03 0.04 0.008 0.001 0.003

Table 2 is a table showing separately the process conditions of the manufacturing method of high strength and high formability steel sheet according to an embodiment of the present invention.

division Annealing heat treatment primary cooling secondary cooling post heat treatment Temperature (℃) maintain
hour
(candle) Cooling
speed
(℃/s) end
temperature
(℃) Cooling
speed
(℃/s) end
temperature
(℃) temperature
(℃) maintain
hour
(candle) Comparative Example 1 815 60 7 750 100 240 430 60 Comparative Example 2 830 60 7 725 100 240 430 60 Example 830 60 7 750 100 240 430 60

Referring to Table 2, Comparative Example 1 has an annealing heat treatment temperature of 815° C., which is higher than the range suggested by the present invention, and Comparative Example 2 has an end temperature of primary cooling of 725° C., which is lower than the range suggested by the present invention.

Table 3 is a table showing the mechanical properties of the high strength and high formability steel sheet according to an embodiment of the present invention.

yield strength
(MPa) tensile strength
(MPa) yield ratio elongation
(%) Seal
elongation
(%) Tensile strength x elongation
(MPa%) Comparative Example 1 587 1050 0.56 18.3 14.2 19215 Comparative Example 2 603 1040 0.58 19.2 13.9 19968 Example 659 1092 0.60 21.9 16 23915

Table 4 is a table showing the microstructure of the high strength and high formability steel sheet according to an embodiment of the present invention.

austenite
(%) ferrite
(%) tempered martensite
(%) Long/Short Ratio
(%) Comparative Example 1 14.9 35 50.1 1.77 Comparative Example 2 14.5 34 52.5 1.86 Example 13.6 29 57.4 2.45

Comparative Example 1 had a relatively high retained austenite fraction, but exhibited a low yield strength of 587 MPa and a low elongation of 18.3. Although a certain level of strength and elongation were secured by the high retained austenite fraction, it is analyzed that the strength and elongation did not achieve the target values because the stability of retained austenite was lowered due to an average long/short axis ratio of less than 2.0.

In the case of Comparative Example 2, the yield strength and tensile strength were satisfactory, but the elongation rate of 20% or more was not achieved. It is analyzed that the elongation did not achieve the target value due to the presence of retained austenite with an average long/short axis ratio of less than 2.0 with low stability.

In the case of Examples, the yield strength, tensile strength, and total elongation satisfied the target range. In particular, the tensile strength x elongation was 23915 MPa%, which achieved more than the target value of 20,000 MPa%. The reason why high strength and high formability can be secured as described above is considered to be due to the ideal retained austenite, ferrite, and tempered martensite microstructure shapes and fractions. In addition, it is analyzed that excellent elongation is secured due to the presence of retained austenite having an average major/short axis ratio of 2.0 or more.

2 is a photograph showing the microstructure of the high strength and high formability steel sheet according to an embodiment of the present invention.

Referring to FIG. 2 , in the high strength and high formability steel sheet, austenite, ferrite, and tempered martensite were observed. The austenite fraction was about 13.6%, the ferrite fraction was 29%, and the tempered martensite fraction was measured to be 57.4%.

3 is a graph showing the number of grains with respect to the major axis/short axis ratio in austenite of the high strength and high formability steel sheet according to an embodiment of the present invention.

Referring to FIG. 3 , the long axis/short axis ratio is in the range of 1 to 7, and most of the grains were observed to have a long axis/short axis ratio of 1 to 2.5. In addition, grains having a long axis/short axis ratio of 1.5 to 2 occupied the largest number. The average of the major axis/short axis ratio showed a value of 2 or more.

The high strength and high formability steel sheet according to the technical idea of the present invention was manufactured by controlling the fraction and shape of retained austenite, ferrite, and tempered martensite. In the high strength and high formability steel sheet, since retained austenite is a key structure that can secure both the strength and elongation of the steel sheet, it is present in a fraction of 11% to 15%, and it is preferable to have an average major/short axis ratio of 2.0 or more. Do. When the average long/short axis ratio of the retained austenite is less than 2.0, it is difficult to secure a high elongation because the stability of the retained austenite is low.

Although the above description has been focused on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. As long as such changes and modifications do not depart from the scope of the present invention, it can be said that they belong to the present invention. Accordingly, the scope of the present invention should be judged by the claims set forth below.

Claims (11)

By weight%, carbon (C): 0.12% to 0.22%, silicon (Si): 1.6% to 2.4%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): 0.01% to 0.05%, titanium ( Ti), sum of niobium (Nb) and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.015%, sulfur (S): greater than 0% to 0.003%, nitrogen (N) : More than 0% ~ 0.006%, and the balance contains iron (Fe) and other unavoidable impurities,
It has a mixed structure of austenite, ferrite, and tempered martensite,
yield strength (YS): 600 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 20% or more, and product of tensile strength and elongation: 20,000 MPa% or more;
High strength and high formability steel sheet. The method of claim 1,
The austenite fraction is 11% to 15%,
The fraction of ferrite is 25% to 35%, and
The fraction of tempered martensite is 50% to 64%,
High strength and high formability steel sheet. The method of claim 1,
The size of the austenite ranges from 0.1 μm to 3 μm,
The size of the ferrite is in the range of 1 μm to 5 μm,
The size of the tempered martensite is in the range of 1 μm to 5 μm,
High strength and high formability steel sheet. The method of claim 1,
The average long/short axis ratio of the austenite is 2.0 or more,
High strength and high formability steel sheet. (a) by weight, carbon (C): 0.12% to 0.22%, silicon (Si): 1.6% to 2.4%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): 0.01% to 0.05% , the sum of titanium (Ti), niobium (Nb) and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.015%, sulfur (S): greater than 0% to 0.003%, nitrogen (N): manufacturing a hot-rolled steel sheet by hot-rolling a steel material containing more than 0% to 0.006%, and the remainder being iron (Fe) and other unavoidable impurities;
(b) cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet;
(c) annealing the cold-rolled steel sheet at 820°C to 850°C; and
(d) step of cooling the cold-rolled steel sheet in multiple stages; including,
Method for manufacturing high strength and high formability steel sheet. 6. The method of claim 5,
The step (a) is,
(a-1) preparing a steel material having the alloy composition;
(a-2) reheating the steel material at 1,150 to 1,250°C;
(a-3) preparing a hot-rolled steel sheet by hot finish rolling the reheated steel at 900°C to 950°C; and
(a-4) comprising the step of winding the hot-rolled steel sheet at 550 ℃ ~ 650 ℃,
Method for manufacturing high strength and high formability steel sheet. 6. The method of claim 5,
Step (c) is,
raising the temperature of the cold-rolled steel sheet at a temperature increase rate of 3° C./sec to 8° C./sec, and maintaining at 820° C. to 850° C. for 60 seconds to 300 seconds,
Method for manufacturing high strength and high formability steel sheet. 6. The method of claim 5,
Step (d) is,
first cooling the cold-rolled steel sheet to 730°C to 770°C at a cooling rate of 5°C/sec to 10°C/sec; and
Secondary cooling of the cold-rolled steel sheet to 200° C. to 300° C. at a cooling rate of 50° C./sec to 100° C./sec, and maintaining for 5 seconds to 20 seconds;
Method for manufacturing high strength and high formability steel sheet. 6. The method of claim 5,
After performing step (d),
(e) heating the cold-rolled steel sheet at a temperature increase rate of 10° C./sec to 20° C./sec, and a post-heat treatment step of maintaining the cold-rolled steel sheet at a temperature of 400° C. to 460° C. for 10 seconds to 240 seconds; further comprising,
Method for manufacturing high strength and high formability steel sheet. 6. The method of claim 5,
After performing step (d),
(e) immersing the cold-rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanizing layer; and
(f) heat-treating the cold-rolled steel sheet with the hot-dip galvanized layer formed thereon; further comprising,
Method for manufacturing high strength and high formability steel sheet. 6. The method of claim 5,
After step (d),
The steel plate
It has a mixed structure of austenite, ferrite, and tempered martensite,
yield strength (YS): 600 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 20% or more, and product of tensile strength and elongation: 20,000 MPa% or more;
Method for manufacturing high strength and high formability steel sheet.

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