STEEL SHEET HAVING HIGH STRENGTH AND HIGH FORMABILITY AND METHOD FOR MANUFACTURING SAME

Abstract

Provided herein is a steel sheet having high strength and high formability according to an aspect of the present invention including, % by weight, an amount of 0.05 to 0.15% of carbon (C), an amount greater than 0 and 0.4% or less of silicon (Si), an amount of 4.0-9.0% of manganese (Mn), an amount of greater than 0 and 0.3% or less of aluminum (Al), an amount of 0.02% or less of phosphorus (P), an amount of 0.005% or less of sulfur (S), an amount of 0.006% or less of nitrogen (N), and the remainder of iron (Fe) and other inevitable impurities. The steel sheet has a microstructure consisting of ferrite and residual austenite. The grain size of the microstructure is 3 μm or less. The steel sheet has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

Description

TECHNICAL FIELD

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

BACKGROUND

In recent years, from the viewpoint of the safety and weight reduction of automobiles, high strengthening of automobile steel sheets has progressed more rapidly. In order to secure passenger safety, steel sheets that are used for automotive structural members need to have sufficient impact toughness by increasing the strengths or thicknesses thereof. In addition, these steel sheets need to have sufficient formability is required in order to be applied to automotive parts, and it is essential to reduce the weight of the automobile body in order to improve the fuel efficiency of the automobile. Therefore, studies have been conducted to substantially strengthen the automobile steel sheet continuously and to increase the formability thereof.

Currently, as high-strength steel sheets for automobiles having the above-described characteristics, a dual-phase steel, which has the strength and elongation secured by two phases, ferrite and martensite phases, and a transformation-induced plasticity steel which has the strength and elongation secured by phase transformation of retained austenite in the final structure during plastic deformation, have been proposed.

Technologies related thereto include Korean Patent Application No. 10-2016-0077463 (entitled “Ultra-High-Strength, High-Ductility Steel Sheet Having Excellent Yield Strength and Method for Manufacturing the Same”).

SUMMARY OF THE INVENTION

Technical Problem

A problem to be solved by the present invention is to provide a steel sheet having high formability and high strength and a method for manufacturing the same.

Technical Solution

In an aspect of the present invention, provided is a steel sheet having high strength and high formability including, % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities. The steel sheet has a microstructure consisting of ferrite and retained austenite. The microstructure has a grain size of 3 μm or less. The steel sheet has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

In an exemplary embodiment, the steel sheet may further include one or more of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which may be included in an amount of greater than 0 and less than or equal to 0.02 wt %.

In an exemplary embodiment, the steel sheet may further include more than 0 and less than or equal to 0.001 wt % boron (B).

In an exemplary embodiment, the volume fraction of the retained austenite in the microstructure may be 10 to 30 vol %.

In an aspect of the present invention, provided is a method for manufacturing a steel sheet having high strength and high formability including steps of: (a) manufacturing a hot-rolled steel sheet from a steel slab including, % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities; (b) manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; (c) subjecting the cold-rolled steel sheet to first heat treatment at a temperature of AC3 to (AC3+15) ° C.; and (d) subjecting the cold-rolled steel sheet, subjected to the first heat treatment, to second heat treatment at an intercritical temperature. The cold-rolled steel sheet after step (d) has a microstructure consisting of ferrite and retained austenite.

In an exemplary embodiment, the steel slab may further include one or more of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which may be included in an amount of greater than 0 and less than or equal to 0.02 wt %.

In an exemplary embodiment, the steel slab may further include an amount greater than 0 and less than or equal to 0.001 wt % boron (B).

In an exemplary embodiment, step (c) may include a step of cooling the heat-treated cold-rolled steel sheet to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s.

In an exemplary embodiment, step (d) may include a step of cooling the heat-treated cold-rolled steel sheet to a temperature of 350 to 450° C. at 4 to 10° C./s.

In an exemplary embodiment, step (a) may include steps of: (a1) reheating the steel slab to a temperature of 1,150 to 1,250° C.; (a2) hot-rolling the reheated steel slab to a finish delivery temperature of 925 to 975° C.; and (a3) cooling the hot-rolled steel sheet to a temperature of 700° C. to 800° C. at a cooling rate of 10 to 30° C./s, followed by coiling.

In an exemplary embodiment, the method may further include, between steps (a) and (b), a step of subjecting the hot-rolled steel sheet to softening heat treatment at a temperature of 550° C. to 650° C.

In an exemplary embodiment, the cold-rolled steel sheet after step (d) may have a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

In an exemplary embodiment, the cold-rolled steel sheet after step (d) may have a grain size of 3 μm or less.

Advantageous Effects

According to the present invention, it is possible to manufacture a steel sheet having a microstructure consisting of ultrafine grained ferrite and retained austenite through component system control and process condition control. Due to the fine grained ferrite, the steel sheet may have high strength, and due to the retained austenite present in an amount of 10 to 30 vol % in the microstructure, the steel sheet may have high strength and elongation. In addition, the steel sheet may have a high hole expansion ratio (HER) as a result of controlling the shape of the microstructure. As a result, it is possible to effectively obtain a steel sheet having high formability and high strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a process flow chart schematically showing a method for manufacturing a steel sheet having high strength and high formability according to an exemplary embodiment of the present invention.

FIG. 2 shows the results of a high-temperature tensile test for a comparative component system specimen of the present invention.

FIG. 3 shows the results of a high-temperature tensile test for an implementation component system specimen of the present invention.

FIG. 4 is a photograph showing the microstructure of a high-strength steel sheet according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that it can be easily carried out by those skilled in the art to which the present invention pertains. The present invention may be embodied in a variety of different forms, and is not limited to the embodiments described herein. Like reference numerals are given to the same or similar components throughout the present specification. In addition, detailed descriptions of known functions and configurations will be omitted when it may unnecessarily obscure the subject matter of the present invention.

According to an exemplary embodiment of the present invention, a steel sheet having high strength and high formability may have a final microstructure consisting of fine grained ferrite and a retained austenite present in an amount of 10 to 30 vol %. Thereby, the steel sheet may have a high strength, high elongation and high hole expansion ratio (HER).

First, in order for the steel sheet to have a high elongation, the steel sheet sufficiently contains retained austenite at a level of 10 to 30 vol %. The retained austenite may enhance the elongation of the steel sheet in substantially the same manner as in conventional transformation-induced plasticity steel. In order to ensure the fraction of the retained austenite, an austenite stabilizing element may be appropriately added into the steel sheet as described later. In addition, as described later, the first and second annealing heat treatment may be continuously performed, and the second annealing heat treatment may be performed at an intercritical temperature.

Next, in order for the steel sheet to have a high hole expansion ratio, the phase boundary between a hard phase and a soft phase, which can act as a crack formation site in the steel sheet, is reduced. To this end, the steel sheet may not contain hard phases such as martensite and bainite in the final microstructure thereof. In addition, in order for the steel sheet to have a high hole expansion ratio, the interfaces between precipitates and grains are reduced. To this end, the contents of precipitate forming elements, such as titanium, niobium and vanadium, and precipitate growth inhibiting elements such as molybdenum, may be controlled. In addition, in order for the steel sheet to have a high hole expansion ratio, the fraction of high-angle grain boundaries (HAGBs) in the final structure may be increased. As an example, the high-angle grain boundaries may refer to grain boundaries at which the angle between adjacent grains is 15° or greater. In addition, the shape of the microstructure may be controlled so that the steel sheet has a high hole expansion ratio. In order to increase the fraction of the high-angle grain boundaries and control the shape of the microstructure, as described later, annealing heat treatment may be performed in two steps consisting of first heat treatment and second heat treatment.

Next, in order for the steel sheet to have high strength, the grains of the final microstructure are refined. Through the above-described annealing heat treatment performed in two steps, the grains sizes of ferrite and retained austenite may be controlled to 3 μm or less. In addition, the first annealing heat treatment may be performed at a temperature of AC3 to (AC3+15) ° C.

Hereinafter, the steel sheet having high formability and high strength according to an exemplary embodiment of the present invention having the above-described characteristics will be described in more detail.

Steel Sheet Having High Strength and High Formability

A high-strength steel sheet according to an exemplary embodiment of the present invention includes, % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities. In addition. The high-strength steel sheet further includes one or more of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), each of which may be included in an amount of greater than 0 and less than or equal to 0.02 wt %. In addition, the high-strength steel sheet may further include more than 0 and less than or equal to 0.001 wt % boron (B).

Hereinafter, the role and content of each component included in the high-strength cold-rolled steel sheet according to an exemplary embodiment of the present invention will be described in detail (the content of each component is given in wt % based on the total weight of the steel sheet and will hereinafter be expressed in %).

Carbon (C): 0.05% to 0.15%

Carbon (C) is the most important alloying element in steel making, and is used for the main purpose of providing basic strengthening and stabilizing austenite in the present invention. High carbon (C) concentration in austenite improves austenite stability, making it easy to ensure proper austenite for material property improvement. However, an excessively high carbon (C) content may result in a decrease in weldability due to an increase in carbon equivalent, and a large number of precipitated cementite structures such as pearlite may be formed during cooling. For this reason, carbon (C) is preferably added in an amount of 0.05 to 0.15% of the total weight of the steel sheet. If the carbon content is less than 0.05%, it may be difficult to secure the strength of the steel sheet, and when the carbon content is more than 0.15%, the toughness and ductility of the steel sheet may deteriorate.

Silicon (Si): More than 0 and Less than or Equal to 0.4%

Silicon (Si) is an element that suppresses carbide formation in ferrite and increases the diffusion rate of austenite by increasing the activity of carbon (C). Silicon (Si) is also well known as a ferrite stabilizing element, which increases ductility by increasing the ferrite fraction during cooling. In addition, silicon has a very high ability to suppress the formation of carbides, and thus is a necessary element for securing the TRIP effect by increasing the carbon concentration in retained austenite during bainite formation. However, if silicon (Si) is added in an amount greater than 0.4%, it may form silicon oxide (SiO₂) on the surface of the steel sheet during the process, increase the rolling load during hot rolling, and generate a large amount of red scale. Thus, silicon (Si) is preferably added in an amount of 0.4% or less of the total weight of the steel sheet.

Manganese (Mn): 4.0% to 9.0%

Manganese (Mn) is an austenite stabilizing element. As manganese (Mn) is added, Ms, which is a martensite formation starting temperature, is gradually lowered, thereby exhibiting the effect of increasing the fraction of retained austenite after heat treatment.

Manganese is included in an amount of 4.0 to 9.0% of the total weight of the steel sheet. If manganese is added in an amount of less than 4.0%, the above-described effect cannot be sufficiently secured. On the other hand, if manganese is added in an amount greater than 9.0%, weldability may decrease due to an increase in carbon equivalent, and manganese oxide (MnO) may be formed on the surface of the steel sheet during the process, resulting in a decrease in platability due to a decrease in the wettability of the corresponding portion.

Aluminum (Al): More than 0 and Less than or Equal to 0.3%

Aluminum (Al) is known as an element that stabilizes ferrite and inhibits the formation of carbides, like silicon (Si). In addition, aluminum has the effect of increasing the equilibrium temperature, and thus when aluminum (Al) is added, there is an advantage in that an appropriate heat treatment temperature range is widened. However, if aluminum is excessively added in an amount greater than 0.3%, a problem in continuous casting may occur due to AlN precipitation. Accordingly, aluminum may be added in an amount of more than 0 and less than or equal to 0.3% of the total weight of the steel sheet.

At Least One of Niobium (Nb), Titanium (Ti), Vanadium (V) and Molybdenum (Mo): More than 0 and Less than or Equal to 0.2% for Each

Niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo) may optionally be included in the steel. First, niobium (Nb), titanium (Ti) and vanadium (V) are elements that are precipitated in the form of carbides in steel, and are added to secure strength through carbide precipitation. Titanium (Ti) may function to suppress the formation of cracks during continuous casting by suppressing the formation of AlN. However, if niobium (Nb), titanium (Ti) and vanadium (V) are each added in an amount greater than 0.2%, they may form coarse precipitates, which causes disadvantages in that the amount of carbon in the steel is reduced and the material properties thereof are degraded, and the manufacturing cost increases due to the addition of niobium (Nb), titanium (Ti) and vanadium (V). In addition, if titanium is added excessively, it may cause nozzle clogging during continuous casting. Accordingly, when at least one of niobium (Nb), titanium (Ti) and vanadium (V) is added, each of niobium (Nb), titanium (Ti) and vanadium (V) may be added in an amount greater than 0 and less than or equal to 0.2% of the total weight of the steel sheet.

In addition, molybdenum (Mo) may serve to control the size of carbides by suppressing the growth of the carbides. However, if molybdenum is added in an amount greater than 0.2%, there are disadvantages in that the above effect is saturated and the manufacturing cost increases.

Boron (B)

Boron (B) may optionally be added to the steel sheet, and may function as a grain boundary strengthening element. Boron may be added in an amount greater than 0 and less than or equal to 0.001% of the total weight of the steel sheet. If boron is added in an amount of more than 0.001%, it may lower the high-temperature ductility of the steel sheet by forming a nitride such as BN.

Other Elements

Phosphorus (P), sulfur (S) and nitrogen (N) may inevitably be added to the steel during the steelmaking process. That is, it is preferable that these elements are ideally not included, but they may be included in certain amounts because it is difficult to completely remove these elements in terms of process technology.

Phosphorus (P) may play a role similar to silicon in the steel. However, if phosphorus is added in an amount greater than 0.02% of the total weight of the steel sheet, it may reduce the weldability of the steel sheet and increase the brittleness thereof, thereby causing material property deterioration. Accordingly, the amount of phosphorus added may be controlled to 0.02% or less of the total weight of the steel sheet.

Sulfur (S) may inhibit the toughness and weldability of the steel, and hence the content thereof may be controlled to 0.005% or less of the total weight of the steel sheet.

If nitrogen (N) is excessively present in the steel, a large amount of nitride may be precipitated, resulting in deterioration in the ductility of the steel sheet. Accordingly, the content of nitrogen (N) may be controlled to 0.006% or less of the total weight of the steel sheet.

The high-strength steel sheet of the present invention, which has the above-described alloying components, has a microstructure consisting of ferrite and retained austenite. In this case, the volume fraction of the retained austenite in the microstructure may be 10 to 30 vol %. The grains of the high-strength steel sheet may be fine grains having a size of 3 μm or less. The fraction of high-angle grain boundaries among the grains may be 70% or greater.

The high-strength steel sheet may have material properties, including a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

Accordingly, the high-strength steel sheet according to the embodiment of the present invention may be applied to fields requiring high strength and high formability.

The above-described high-strength steel sheet according to the embodiment of the present invention may be manufactured by a method of an exemplary embodiment as follows. The present invention intends to provide a steel sheet having excellent elongation, hole expansion ratio and strength as a result of using alloying components having appropriately controlled composition ratios and performing two-step annealing heat treatment after performing a hot rolling process and a cold rolling process, and a method for manufacturing the same.

Method for Manufacturing Steel Sheet Having High Strength and High Formability

FIG. 1 is a process flow chart schematically showing a method for manufacturing a steel sheet having high strength and high formability according to an exemplary embodiment of the present invention.

Referring to FIG. 1, the method for manufacturing a steel sheet includes: steps of: (S110) reheating a steel slab; (S120) manufacturing a hot-rolled steel sheet by hot-rolling the steel slab; (S130) cold-rolling the hot-rolled steel sheet; and (S140) subjecting the cold-rolled steel sheet to annealing heat treatment.

First, step (S110) of reheating a steel slab is a step of preparing a steel slab including, % by weight an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities, and reheating the steel slab to re-dissolve components segregated during casting and homogenize as-cast components. Meanwhile, the steel slab may further include one or more of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which may be included in an amount of more than 0 and less than or equal to 0.02 wt %. In addition, the steel slab may further include more than 0 and less than or equal to 0.001 wt % boron (B).

The steel slab reheating temperature is preferably about 1,150 to 1,250° C. so that a normal hot delivery temperature may be ensured. If the reheating temperature is lower than 1,150° C., a problem may arise in that the hot rolling load increases rapidly, and if the reheating temperature is greater than 1,250° C., it may be difficult to secure the strength of the final manufactured steel sheet, due to the coarsening of initial austenite grains.

Next, hot-rolling step (S120) is performed after the steel slab reheating, and is a step of forming a hot-rolled steel sheet by performing hot rolling by a conventional method and performing finish rolling at a temperature of 925 to 975° C. Considering that the steel slab of the present invention has high contents of alloying elements such as manganese, the finish rolling may be performed at a high temperature of 925 to 975° C. After the finish rolling, the hot-rolled steel sheet is cooled to a temperature of 700 to 800° C. at a cooling rate of 10 to 30° C./s and then coiled. The cooling method may be performed using a water-free cooling method. The hot-rolled steel sheet may have a full martensitic structure after cooling.

According to some exemplary embodiments, before the hot-rolled steel sheet having a full martensitic structure is cold-rolled, softening heat treatment may be performed to reduce the rolling load during cold rolling. The softening heat treatment may be performed at a temperature of 550 to 650° C. If the temperature of the softening heat treatment is lower than 550° C., recrystallization of the martensite produced after the hot rolling may not occur, and only tempering may proceed, and thus supersaturated carbon in the structure may be formed in the form of cementite and spheroidized. In this case, since the brittleness of the martensite may be expressed, fracture of the steel sheet may occur during cold rolling. On the other hand, if the temperature of the softening heat treatment is higher than 650° C., austenite may be excessively formed, and martensite may be formed from the austenite during cooling, so that the effect of the softening heat treatment may not be effective. By the softening heat treatment performed in the above temperature range, the martensitic structure after the hot rolling may be transformed into a composite structure of ferrite and retained austenite.

Next, cold rolling step (S130) is a step of cold-rolling the hot-rolled steel sheet after pickling. The cold rolling may be performed under a condition where the hot-rolled steel sheet is cold-rolled at a reduction ratio of 40 to 60%. By the cold rolling, the composite structure of ferrite and retained austenite after the softening heat treatment may be transformed into a composite structure of ferrite and martensite.

Next, annealing heat treatment step (S140) may include a step of subjecting the cold-rolled steel sheet to first heat treatment at a temperature of AC3 to (AC3+15)° C., and a step of subjecting the cold-rolled steel sheet, subjected to the first heat treatment, to second heat treatment at an intercritical temperature. The temperature of AC3 to (AC3+15) ° C. in the first heat treatment step may be, for example, a temperature of 735 to 750° C. The intercritical temperature in the second heat treatment step may be, for example, a temperature of 640 to 660° C.

In an exemplary embodiment, the first heat treatment may transform a composite structure of ferrite and martensite in the steel sheet after cold rolling into a martensitic structure. In the first heat treatment, heat treatment is performed by heating the cold-rolled steel sheet to a target temperature of 735 to 750° C. at a heating rate of 1 to 3° C./s and holding the cold-rolled steel sheet at the target temperature for 40 to 120 seconds.

If the heat treatment temperature is lower than 735° C., it is not possible to secure austenite grains having a sufficient size at the target temperature, and a composite structure of martensite and ferrite may be formed after heat treatment, and thus the strength and ductility of the final structure following the annealing heat treatment may decrease. On the other hand, if the heat treatment temperature is higher than 750° C., the size of austenite grains at the target temperature may excessively increase, which is disadvantageous in securing the stabilization of austenite in the final structure following the annealing heat treatment, so that the steel sheet may have inferior strength.

In addition, if the heating rate is less than 1° C./s, the retention time at the target temperature of 735 to 750° C. may be longer than the upper limit of the range of 40 to 120 seconds, so that the austenite grain size at the target temperature may excessively increase. On the other hand, if the heating rate is greater than 3° C./s, the retention time at the target temperature of 735 to 750° C. may be shorter than the lower limit of the range of 40 to 120 seconds, so that it is impossible to secure austenite grains having a sufficient size at the target temperature.

Then, the heat-treated cold-rolled steel sheet is cooled to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s. In an exemplary embodiment, the cold-rolled steel sheet cooled to the above temperature may be aged for 120 to 330 seconds.

The cold-rolled steel sheet that has been subjected to the first heat treatment may be continuously subjected to second heat treatment. In an exemplary embodiment, in the second heat treatment, heat treatment is performed by heating the cold-rolled steel sheet to a target temperature of 640 to 660° C. at a heating rate of 1 to 3° C./s and holding the cold-rolled steel sheet at the target temperature for 40 to 120 seconds. As the second heat treatment is performed at an intercritical temperature corresponding to the target temperature range, the martensitic structure after the first heat treatment may be transformed into a structure consisting of ferrite and retained austenite. In this case, the volume fraction of the retained austenite may be 10 to 30 vol %.

If the second heat treatment temperature is lower than 640° C., excessively few austenite structures may be formed at the target temperature and the austenite stability may increase, and for this reason, the austenite in the microstructure after cooling may not exhibit phase transformation during plastic deformation, and thus the strength and ductility of the steel sheet may decrease. On the other hand, if the second heat treatment temperature is higher than 660° C., excessively many austenite structures may be formed at the target temperature and the austenite stability may be lowered, and for this reason, martensite may be formed in the microstructure after cooling, resulting in decreases in the ductility and hole expansion ratio of the steel sheet.

If the heating rate is less than 1° C./s, unnecessary cementite may be formed or spheroidized before the cold-rolled sheet material reaches the above-described intercritical temperature range, resulting in deterioration in the material properties of the steel sheet. If the heating rate is greater than 3° C./s, the steel sheet may not be held for 40 to 120 seconds in the target temperature range, so that it is not possible to secure a sufficient fraction of retained austenite in the final structure.

Then, the heat-treated cold-rolled steel sheet is cooled to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s. In an exemplary embodiment, the cold-rolled steel sheet cooled to the above temperature may be aged for 120 to 330 seconds.

Through the above-described method, a steel sheet having high strength and high formability according to an exemplary embodiment of the present invention may be manufactured.

The steel sheet of the present invention, manufactured by the above-described process, may have a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

As described above, in the manufacturing method according to an exemplary embodiment of the present invention, austenite stabilizing elements may be added into the steel slab in predetermined amounts as described above. In addition, as the first and second annealing heat treatments are continuously performed, the steel sheet may have a final microstructure as a composite structure consisting of fine grained ferrite and 10 to 30 vol % of retained austenite. Since the steel sheet has a sufficient fraction of retained austenite, it may have a high elongation of 25% or greater due to transformation-induced plasticity properties thereof.

In addition, the phase boundary between a hard phase and a soft phase may be reduced by controlling so that hard phases such as martensite and bainite are not contained in the final microstructure as described above. In addition, the interfaces between precipitates and grains may be reduced by controlling the contents of precipitate forming elements, such as titanium, niobium and vanadium and precipitate growth inhibiting elements such as molybdenum, in the component system of the steel slab. Furthermore, it is possible to increase the fraction of high-angle grain boundaries (HAGBs) in the final structure by performing annealing heat treatment in two divided steps, which consists of first and second heat treatment steps, in predetermined temperature ranges. In the first heat treatment, due to the high dislocation density present in the martensite formed by the cold rolling process, recrystallization may actively occur before the martensite is reversely transformed into austenite. In the second heat treatment, the martensite formed through the first heat treatment is heat-treated, and thus recrystallization is relatively suppressed before the martensite is reversely transformed into austenite, whereby the fraction of high-angle grain boundaries in the final microstructure may increase to 70% or greater of the grains. As a result, the steel sheet may have a high hole expansion ratio of 20% or greater.

Next, the grains of the final microstructure may be refined in order for the steel sheet to have high strength. In particular, it is possible to optimize the grain size of the initial austenite by performing the first heat treatment at a temperature of AC3 to (AC3+15) ° C. In addition, the grain sizes of ferrite and retained austenite in the final microstructure may be controlled to 3 μm or less through the second heat treatment performed in the intercritical temperature range.

EXAMPLE

Hereinafter, the configuration and effects of the present invention will be described in more detail with reference to preferred examples of the present invention. However, the following examples are provided to help understand the present invention, and the scope of the present invention is not limited to the following examples.

Example 1

Steel slabs having the comparative component system and implementation component system shown in Table 1 below were produced through a continuous casting process. A specimen was prepared from each of the steel slabs and subjected to a high-temperature tensile test. In the case of the comparative component system, the contents of silicon and aluminum were greater than the upper limits of the ranges of the contents of silicon and aluminum according to an exemplary embodiment of the present invention.

	TABLE 1
	Component system (wt %)
	C	Si	Mn	Al	P	S	N
Comparative	0.09	0.78	6.01	0.521	0.006	0.002	0.004
component
system
Implementation	0.0772	0.081	6.385	0.266	0.0066	0.0008	0.004
component
system

FIG. 2 shows the results of a high-temperature tensile test for the comparative component system specimen of the present invention, and FIG. 3 shows the results of a high-temperature tensile test for the implementation component system specimen of the present invention. Specifically, the results of the high-temperature tensile tests are the results obtained by heating each of the comparative component system specimen and the implementation component system specimen to temperatures of 700° C., 750° C., 800° C., 850° C., 900° C., 950° C., 1,000° C., and 1,100° C. and then subjecting each specimen to a tensile test at the above temperatures. Regarding the high-temperature tensile test, FIG. 3 shows a graph 201 obtained by heating the specimen to a temperature higher than 1,100° C. and then cooling the specimen to each of the tensile test temperatures at a cooling rate of −1° C./s, together with a graph 202 obtained by cooling the specimen to each of the tensile test temperatures at a cooling rate of −20° C./s. In general, when the area reduction rate at a predetermined temperature is 50% or greater, it may be determined that the ductility at the predetermined temperature is secured.

Referring to FIG. 2, in the case of the comparative component system specimen, the area reduction rate at 1,100° C. was 55%, the area reduction rate in the temperature range of 700 to 800° C. was 50%, and the area reduction rate in the temperature range of 800 to 1,050° C. was less than 50%, which is the target value. On the other hand, referring to FIG. 3, the area reduction rate in the temperature range of 800 to 1,100° C. exceeded 50%, which is the target value.

Referring to FIGS. 2 and 3, in the case of the comparative component system specimen, unlike the implementation component system specimen of the embodiment of the present invention, the high-temperature ductility is not ensured in the high temperature range equal to or higher than 800° C. in which continuous casting is performed according to the embodiment of the present invention, and thus cracks may occur during continuous casting, making it impossible to secure a good slab.

Table 2 below shows the rolling force for each pass calculated by simulating the hot rolling according to an exemplary embodiment of the present invention for each of the comparative component system specimen and the implementation component system specimen.

	TABLE 2
	Rolling pass
	No. 1	No. 2	No. 3	No. 4	No. 5	No. 6	No. 7
	Reduction	35.6%	34.5%	26.3%	30%	39.8%	50.8%	51.7%
	ratio
Comparative	Rolling	158.8	177.6	146.7	193.1	282.1	387.8	419.9
component	force
system	(ton)
Implementation	Rolling	92.6	147.6	128.0	166.7	235.8	320.8	334.6
component	force
system	(ton)

Referring to Table 2 above, it can be seen that a greater rolling force must be applied to the comparative component system specimen compared to that applied to the implementation component system specimen in order to generate the same reduction ratio for each rolling pass. That is, it can be confirmed that a relatively high load is applied to a rolling mill during hot rolling of the comparative component system specimen.

Example 2

The specimen prepared from the implementation component system shown in Table 1 above was subjected to each of first and second annealing heat treatment processes according to Table 3 below. In the case of Comparative Examples 1 and 3, the second annealing temperature was lower than 640° C., which is the lower limit of the second annealing temperature according to the embodiment of the present invention. In the case of Comparative Examples 2 and 4, the second annealing temperature was higher than 660° C., which is the upper limit of the second annealing temperature according to the embodiment of the present invention. In the case of Comparative Examples 5 to 7, the first annealing temperature was higher than 750° C., which is the upper limit of the first annealing temperature according to the embodiment of the present invention. In addition, in the case of Comparative Example 7, the second annealing temperature was higher than 660° C., which is the upper limit of the second annealing temperature according to the embodiment of the present invention. In the case of Comparative Examples 8 to 11, the first annealing heat treatment was not performed, and only the second annealing heat treatment was performed. In addition, in the case of Comparative Example 11, the second annealing temperature was higher than 660° C., which is the upper limit of the second annealing temperature according to the embodiment of the present invention.

	TABLE 3
	First annealing	Second annealing
			Cooling			Cooling
	Annealing	Cooling	termination	Annealing	Cooling	termination
	temperature	rate	temperature	temperature	rate	temperature
	(° C.)	(° C./s)	(° C.)	(° C.)	(° C./s)	(° C.)
Comparative	735	6	400	630	6	400
Example 1
Example 1	735	6	400	640	6	400
Example 2	735	6	400	650	6	400
Example 3	735	6	400	660	6	400
Comparative	735	6	400	670	6	400
Example 2
Comparative	750	6	400	630	6	400
Example 3
Example 4	750	6	400	640	6	400
Example 5	750	6	400	650	6	400
Example 6	750	6	400	660	6	400
Comparative	750	6	400	670	6	400
Example 4
Comparative	850	6	400	650	6	400
Example 5
Comparative	850	6	400	660	6	400
Example 6
Comparative	850	6	400	670	6	400
Example 7
Comparative	—	—	—	640	6	400
Example 8
Comparative	—	—	—	650	6	400
Example 9
Comparative	—	—	—	660	6	400
Example 10
Comparative	—	—	—	670	6	400
Example 11

Table 4 shows the results of evaluating the material properties of the specimens of Comparative Examples 1 to 11 and Examples 1 to 6, subjected to annealing heat treatment according to Table 3.

	TABLE 4
	Material properties
	Yield	Tensile		Tensile	Retained	Average	Fraction	Hole	Whether
	strength	strength	Elongation	strength ×	austenite	grain size	(%) of	expansion	material
	(MPa)	(MPa)	(%)	elongation	(%)	(μm)	HAGBs	ratio (%)	properties
	800 or	980 or	25 or	(MPa · %)	10 to	2 or	70 or	20 or	were
	greater	greater	greater	—	30	less	greater	greater	achieved
Comparative	915	988	23	22,724	17	2 or less	65	21	Not achieved
Example 1
Example 1	882	1,003	25	25,075	21	2 or less	72	28	Achieved
Example 2	871	1,031	26	26,806	24	2 or less	77	27	Achieved
Example 3	823	1,027	25	25,675	20	1 or less	70	23	Achieved
Comparative	808	1,071	21	22,491	18	1 or less	71	22	Not achieved
Example 2
Comparative	911	946	24	22,704	15	2 or less	62	22	Not achieved
Example 3
Example 4	921	990	26	25,740	23	2 or less	75	27	Achieved
Example 5	883	1,007	25	25,175	22	3 or less	73	30	Achieved
Example 6	851	1,022	25	25,550	21	2 or less	69	22	Achieved
Comparative	801	1,071	21	22,491	17	3 or less	68	21	Not achieved
Example 4
Comparative	813	882	23	20,286	20	6 or less	65	22	Not achieved
Example 5
Comparative	793	906	24	21,744	21	7 or less	61	21	Not achieved
Example 6
Comparative	732	942	21	19,782	14	7 or less	62	19	Not achieved
Example 7
Comparative	828	969	20	19,380	14	2 or less	45	21	Not achieved
Example 8
Comparative	1,045	1.019	24	24,456	22	2 or less	51	16	Not achieved
Example 9
Comparative	997	1,050	23	24,150	20	1 or less	47	15	Not achieved
Example 10
Comparative	976	1,115	18	20,070	18	2 or less	48	14	Not achieved
Example 11

The target values of the material properties of the high-strength steel sheet according to an exemplary embodiment of the present invention are a yield strength of 800 MPa or greater, a tensile strength of 980 MPa or greater, an elongation of 25% or greater, a retained austenite volume fraction of 10 to 30%, a high-angle grain boundary (HAGB) fraction of 70% or greater, and a hole expansion ratio of 20% or greater. The specimens of Examples 1 to 6 satisfied all of the above target values. In the case of Comparative Example 1, the elongation and the fraction of high-angle grain boundaries (HAGBs) were below the target values. In the case of Comparative Example 2, the elongation was below the target value. In the case of Comparative Example 3, the tensile strength, the elongation, and the fraction of high-angle grain boundaries (HAGBs) were below the target values. In the case of Comparative Example 4, the elongation, the tensile strength×elongation, the average grain size, and the fraction of high-angle grain boundaries (HAGBs) were below the target values. In the case of Comparative Example 5, the tensile strength, the elongation, the average grain size, and the fraction of high-angle grain boundaries (HAGBs) were below the target values. In the case of Comparative Examples 6 and 7, the yield strength, the tensile strength, the elongation, the average grain size, and the fraction of high-angle grain boundaries (HAGBs) were below the target values. In the case of Comparative Example 8, the tensile strength, the elongation, and the fraction of high-angle grain boundaries (HAGBs) were below the target values. In the case of Comparative Examples 9 to 11, the elongation, the fraction of high-angle grain boundaries (HAGBs), and the hole expansion ratio were below the target values.

FIG. 4 is a photograph showing the microstructure of the high-strength steel sheet according to an exemplary embodiment of the present invention. Specifically, FIG. 4 is a microstructure photograph of the specimen of Example 1. Referring to Table 4 and FIG. 4, retained austenite having a volume fraction of 17% and the remainder ferrite were observed in the specimen of Example 1.

Example 3

The specimen prepared from the implementation component system shown in Table 1 above was subjected to first and second annealing heat treatment processes according to Table 5 below.

	TABLE 5
	First annealing	Second annealing
					Cooling			Cooling
	Heating	Annealing	Holding	Cooling	termination	Annealing	Cooling	termination
	rate	temperature	time	rate	temperature	temperature	rate	temperature
	(° C./s)	(° C.)	(s)	(° C./s)	(° C.)	(° C.)	(° C./s)	(° C.)
Comp. Example 12	4.0	750	31	13	400	660	6	400
Example 7	3.0	750	43	10	400	660	6	400
Example 8	2.0	750	59	7	400	660	6	400
Example 9	1.5	750	80	6	400	660	6	400
Example 10	1.0	750	118	4	400	660	6	400
Comp. Example 13	0.5	750	236	2	400	660	6	400
Comp. Example 14	0.03	750	3,600	—	—	660	6	400

Referring to Table 5 above, in the case of Comparative Example 12, the heating rate during the first annealing heat treatment was greater than 3° C./s, which is the upper limit of the heating rate during the first annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing holding time did not satisfy 40 seconds or more. In the case of Comparative Example 13, the heating rate during the first annealing heat treatment was less than 1° C./s, which is the lower limit of the heating rate during the first annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing holding time exceeded 120 seconds, which is the upper limit. In the case of Comparative Example 14, the heating rate during the first annealing heat treatment was less than 1° C./s, which is the lower limit of the heating rate during the first annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing holding time exceeded 120 seconds, which is the upper limit. In addition, the cooling rate was less than 4° C./s, which is the lower limit. Examples 7 to 10 satisfied both the first and second annealing heat treatment conditions according to an exemplary embodiment of the present invention.

Table 6 below shows the results of evaluating the material properties of the specimens of Comparative Examples 12 to 14 and Examples 7 to 10, subjected to annealing heat treatment according to Table 5 above.

	TABLE 6
	Material properties
	Yield	Tensile		Tensile	Average	Whether
	strength	strength	Elongation	strength ×	grain size	material
	(MPa)	(MPa)	(%)	elongation	(μm)	properties
	800 or	980 or	25 or	(MPa · %)	2 or	were
	greater	greater	greater	—	less	achieved
Comp. Example 12	881	932	23	21,436	2 or less	Not achieved
Example 7	899	982	25	24,550	2 or less	Achieved
Example 8	874	1,011	25	25,275	2 or less	Achieved
Example 9	849	1,003	26	26,078	2 or less	Achieved
Example 10	865	993	26	25,818	3 or less	Achieved
Comp. Example 13	888	955	24	22,920	4 or less	Not achieved
Comp. Example 14	755	888	21	18,648	10 or less	Not achieved

Referring to Table 6 above, in the case of Comparative Example 12, the target values of tensile strength and elongation were not achieved. In the case of Comparative Example 13, the target values of tensile strength, elongation and average grain size were not achieved. In the case of Comparative Example 14, the target values of yield strength, tensile strength, elongation and average grain size were not achieved. Examples 7 to 10 satisfied all of the target values of material properties according to the embodiment of the present invention.

Example 4

The specimen prepared from the implementation component system shown in Table 1 above was subjected to each of first and second annealing heat treatment processes.

	TABLE 7
	First annealing	Second annealing
			Cooling					Cooling
	Annealing	Cooling	termination	Heating	Annealing	Holding	Cooling	termination
	temperature	rate	temperature	rate	temperature	time	rate	temperature
	(° C.)	(° C./s)	(° C.)	(° C./s)	(° C.)	(° C.)	(° C./s)	(° C.)
Comp. Example 15	735	6	400	4.0	660	31	13	400
Example 11	735	6	400	3.0	660	43	10	400
Example 12	735	6	400	2.0	660	59	7	400
Example 13	735	6	400	1.5	660	80	6	400
Example 14	735	6	400	1.0	660	118	4	400
Comp. Example 16	735	6	400	0.5	660	236	2	400

Referring to Table 7 above, in the case of Comparative Example 15, the heating rate during the second annealing heat treatment exceeded 3° C./s, which is the upper limit of the heating rate during the second annealing according to an exemplary embodiment of the present invention, and the second annealing holding time did not satisfy 40 seconds or more. In the case of Comparative Example 16, the heating rate during the second annealing was lower than 1° C./s, which is the lower limit of the heating rate during the second annealing according to an exemplary embodiment of the present invention, and the second annealing holding time exceeded 120 seconds, which is the upper limit. Examples 11 to 14 satisfied both the first and second annealing heat treatment conditions according to an exemplary embodiment of the present invention.

Table 8 below shows the results of evaluating the material properties of the specimens of Comparative Examples 15 and 16 and Examples 11 to 14, subjected to annealing heat treatment according to Table 7.

	TABLE 8
	Material properties
	Yield	Tensile		Tensile	Average	Whether
	strength	strength	Elongation	strength ×	grain size	material
	(MPa)	(MPa)	(%)	elongation	(μm)	properties
	800 or	980 or	25 or	(MPa · %)	2 or	were
	greater	greater	greater	—	less	achieved
Comp. Example 15	895	972	24	23,328	2 or less	Not achieved
Example 11	881	1,030	25	25,750	1 or less	Achieved
Example 12	900	1,028	25	25,700	2 or less	Achieved
Example 13	928	1,039	25	25,975	1 or less	Achieved
Example 14	915	1,016	26	26,416	2 or less	Achieved
Comp. Example 16	876	1,001	24	24,024	2 or less	Not achieved

Referring to Table 8, in the case of Comparative Example 15, the target values of tensile strength and elongation were not achieved. In the case of Comparative Example 16, the target value of elongation was not achieved. Examples 11 to 14 satisfied all of the target values of material properties according to the exemplary embodiment of the present invention.

Although the above description has been described with reference to the exemplary embodiments of the present invention, various changes or modifications may be made by those skilled in the art. These changes and modifications can be considered falling within the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the appended claims.

Simple modifications or changes of the present invention can be easily implemented by those skilled in the art, and these modifications or changes can be considered included within the scope of the present invention.

Claims

1. A steel sheet having high strength and high formability, comprising, % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount of greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities, wherein the steel sheet comprises a microstructure consisting of ferrite and retained austenite,

wherein the microstructure has a grain size of 3 μm or less, and the steel sheet has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

2. The steel sheet of claim 1, comprising one or more components of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which is included in an amount greater than 0 and less than or equal to 0.02 wt %.

3. The steel sheet of claim 1, further comprising an amount greater than 0 and less than or equal to 0.001 wt % boron (B).

4. The steel sheet of claim 1, wherein a volume fraction of the retained austenite in the microstructure is 10 to 30 vol %.

5. A method for manufacturing a steel sheet having high strength and high formability, the method comprising steps of:

(a) manufacturing a hot-rolled steel sheet from a steel slab comprising: % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities;

(b) manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet;

(c) subjecting the cold-rolled steel sheet to first heat treatment at a temperature of AC3 to (AC3+15) ° C.; and

(d) subjecting the cold-rolled steel sheet, subjected to the first heat treatment, to second heat treatment at an intercritical temperature,

wherein the cold-rolled steel sheet after step (d) has a microstructure consisting of ferrite and austenite.

6. The method of claim 5, wherein the steel slab includes one or more of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each which is included in an amount of greater than 0 and less than or equal to 0.02 wt %.

7. The method of claim 5, wherein the steel slab further comprises an amount greater than 0 and less than or equal to 0.001 wt % boron (B).

8. The method of claim 5, wherein a volume fraction of the retained austenite in the microstructure is 10 to 30 vol %.

9. The method of claim 5, wherein step (c) comprises a step of cooling the cold-rolled steel sheet, subjected to the heat treatment, to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s.

10. The method of claim 9, wherein step (d) comprises a step of cooling the cold-rolled steel sheet, subjected to the heat treatment, to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s.

11. The method of claim 5, wherein step (a) comprises steps of:

(a1) reheating the steel slab to a temperature of 1,150 to 1,250° C.;

(a2) hot-rolling the reheated steel slab to a finish delivery temperature of 925 to 975° C.; and

(a3) cooling the hot-rolled steel sheet to a temperature of 700° C. to 800° C. at a cooling rate of 10 to 30° C./s, followed by coiling.

12. The method of claim 5, further comprising, between steps (a) and (b), a step of subjecting the hot-rolled steel sheet to softening heat treatment at a temperature of 550° C. to 650° C.

13. The method of claim 5, wherein the cold-rolled steel sheet after step (d) has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

14. The method of claim 5, wherein the cold-rolled steel sheet after step (d) has a grain size of 3 μm or less.