WO2021066274A1 - Steel sheet having high strength and high formability and method for manufacturing same - Google Patents

Abstract

A steel sheet having high strength and high formability according to an aspect of the present invention comprises, by wt%, 0.05-0.15% of carbon (C), greater than 0 and 0.4% or less of silicon (Si), 4.0-9.0% of manganese (Mn), greater than 0 and 0.3% or less of aluminum (Al), 0.02% or less of phosphorus (P), 0.005% or less of sulfur (S), 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 more, a tensile strength (TS) of 980 Mpa or more, an elongation (EL) of 25% or more, and a hole expansion ratio (HER) of 20% or more.

Description

Steel plate having high strength and high formability and manufacturing method thereof

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

In recent years, from the viewpoint of safety and weight reduction of automobiles, the high strength of automobile steel sheets is progressing more rapidly. In order to ensure the safety of passengers, the steel plate used as a structural member of automobiles must be increased in strength or thickness to secure sufficient impact toughness. In addition, sufficient formability is required in order to be applied to automobile parts, and since it is essential to reduce the weight of the vehicle body to improve fuel efficiency of automobiles, research is underway to continuously increase the strength of the steel sheet for automobiles and increase the formability.

Currently, as high-strength steel sheets for automobiles having the above-described characteristics, dual-phase steel securing strength and elongation in two phases of ferrite and martensite, and through phase transformation of residual austenite in the final structure during plastic deformation. Transformation induced plasticity steel to secure strength and elongation has been proposed.

As a related technology, there is Patent Application No. 10-2016-0077463 (title of the invention: ultra-high strength, high ductility steel sheet with excellent yield strength and a manufacturing method thereof).

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

The steel sheet having high strength and high formability according to an aspect of the present invention is in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 0.4% or less, manganese (Mn): 4.0 to 9.0%, Aluminum (Al): greater than 0 and 0.3% or less, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.006% or less, balance iron (Fe) and other inevitable impurities included do. It has a microstructure made of ferrite and retained austenite. The grain size of the microstructure is 3 μm or less. Yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, hole expandability (HER): 20% or more.

In one embodiment, the steel sheet includes at least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), and the at least one may be greater than 0 and less than 0.02% by weight. .

In one embodiment, the steel sheet may further include boron (B): greater than 0 and less than or equal to 0.001%.

In one embodiment, the volume fraction of the retained austenite in the microstructure may be 10 to 30% by volume.

The method of manufacturing a steel sheet having high strength and high formability according to another aspect of the present invention is (a) in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 and not more than 0.4%, manganese (Mn) : 4.0 to 9.0%, aluminum (Al): more than 0 and 0.3% or less, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.006% or less, balance iron (Fe) And manufacturing a hot-rolled sheet material using a steel slab containing other inevitable impurities; (b) cold rolling the hot-rolled sheet to manufacture a cold-rolled sheet; (c) subjecting the cold-rolled sheet to a first heat treatment at a temperature of AC3 to AC3 + 15°C; And (d) subjecting the cold-rolled sheet material subjected to the first heat treatment to a second heat treatment at an ideal temperature. After step (d), the cold-rolled sheet has a microstructure made of ferrite and retained austenite.

In one embodiment, the steel slab includes at least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), wherein the at least one is greater than 0 and less than 0.02% by weight, respectively. have.

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

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

In an embodiment, step (d) may include cooling the heat-treated cold-rolled sheet material to 350 to 450°C or less at 4 to 10°C/s.

In one embodiment, the step (a) comprises the steps of (a1) reheating the steel slab to a temperature of 1150 ~ 1250 ℃; (a2) hot rolling the reheated steel slab at a finish rolling temperature of 925 to 975°C; And (a3) cooling the hot-rolled steel to 700°C to 800°C at a cooling rate of 10 to 30°C/s and then winding it up.

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

In one embodiment, after step (d), the cold-rolled sheet has yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, hole expandability (HER): It can have more than 20%.

In one embodiment, after step (d), the crystal grain size of the cold-rolled sheet may be 3 μm or less.

According to the present invention, a steel sheet having a microstructure made of ultrafine ferrite and retained austenite can be manufactured through component system control and process condition control. Due to the fine grain ferrite, the steel sheet has high strength, and due to the residual austenite present in the microstructure in 10 to 30% by volume, it has high strength and elongation, and by controlling the shape of the microstructure, high It can function to have hole expandability (HER). As a result, it is possible to effectively secure a steel sheet having high formability and high strength.

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

2 is a high temperature tensile test result of the comparative component-based specimen of the present invention.

3 is a high-temperature tensile test result of the actual component-based specimen of the present invention.

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

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily implement the present invention. The present invention may be implemented in various different forms, and is not limited to the embodiments described herein. The same reference numerals are assigned to the same or similar components throughout the present specification. In addition, detailed descriptions of known functions and configurations that may unnecessarily obscure the subject matter of the present invention will be omitted.

According to an embodiment of the present invention, a steel sheet having high strength and high formability may have a fine grain ferrite and residual austenite present in an amount of 10 to 30% by volume as a final microstructure. Through this, the steel sheet may have high strength, high elongation, and high hole expandability (HER).

First, in order for the steel sheet to have a high elongation, the steel sheet is made to sufficiently contain residual austenite at a level of 10 to 30% by volume. The retained austenite can improve the elongation of the steel sheet in substantially the same manner as in the conventional metamorphic organic plastic steel. In order to secure the fraction of the retained austenite, an austenite stabilizing element may be appropriately added to the steel sheet as described later. In addition, as will be described later, the first and second annealing heat treatment may be continuously performed, and the second annealing heat treatment may be performed in an ideal area.

Next, in order for the steel sheet to have high hole expandability, the interface between the hard phase and the soft phase, which can function as a crack formation point in the steel sheet, is reduced. To this end, it is possible to prevent hard phases such as martensite and bainite from being included in the final microstructure. In addition, in order for the steel sheet to have high hole expandability, the boundary surface between precipitates and crystal grains is reduced. To this end, the content of a precipitate generating element such as titanium, niobium, vanadium, and the like and a precipitate growth inhibiting element such as molybdenum may be controlled. In addition, in order for the steel sheet to have high hole expandability, the fraction of High Angle Grain Boundaries (HAGBs) in the final structure may be increased. As an example, the high-angle grain boundary may mean a grain boundary having an angle of 15° or more between neighboring grains. In addition, the shape of the microstructure may be controlled so that the steel sheet has high hole expandability. In order to increase the fraction of the high-angle grain boundaries and control the shape of the microstructure, as described later, the annealing heat treatment may be divided into a first and a second heat treatment in a two-stage heat treatment process.

Next, in order for the steel sheet to have high strength, the crystal grains of the final microstructure are refined. Through the annealing heat treatment proceeding in the above-described two steps, the grain size of ferrite and retained austenite can be controlled to be 3 μm or less. In addition, the primary annealing temperature can proceed at AC3 ~ AC3 +15 ℃.

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

Steel plate with high strength and high formability

The high-strength steel sheet according to an embodiment of the present invention is in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 0.4%, manganese (Mn): 4.0 to 9.0%, aluminum (Al) : More than 0 0.3%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Nitrogen (N): 0.006% or less The balance contains iron (Fe) and other unavoidable impurities. In addition, the high-strength steel sheet further includes at least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), wherein each of the at least one may exceed 0 and be 0.02% or less. In addition, the high-strength steel sheet may further include boron (B): greater than 0 and less than or equal to 0.001% by weight.

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

Carbon (C): 0.05% ~ 0.15%

Carbon (C) is the most important alloying element in steel making, and in the present invention, the main purpose is to play a basic reinforcing role and to stabilize austenite. The high carbon (C) concentration in austenite improves austenite stability, making it easy to secure appropriate austenite for material improvement. However, an excessively high carbon (C) content may lead to a decrease in weldability due to an increase in carbon equivalent, and since a large number of cementite precipitated structures such as pearlite may be formed during cooling, carbon (C) is 0.05 to 0.15% of the total weight of the steel sheet. It is preferable to add. When the carbon is included in an amount of less than 0.05%, it is difficult to secure the strength of the steel sheet, and when it is included in an amount exceeding 0.15%, toughness and ductility may be deteriorated.

Silicon (Si): greater than 0 and less than 0.4%

Silicon (Si) is an element that suppresses the formation of carbides in ferrite, and increases the activity of carbon (C) to increase the diffusion rate of austenite. Silicon (Si) is also well known as a ferrite stabilizing element and is known as an element that increases the ferrite fraction during cooling to increase ductility. In addition, since the suppression of the formation of carbide is very high, it is a necessary element to secure the TRIP effect by increasing the carbon concentration in the retained austenite during the formation of bainite. However, when silicon (Si) is added in excess of 0.4%, oxide (SiO2) may be formed on the surface of the steel sheet during the process, the rolling load may be increased during hot rolling, and a large amount of red scale may be generated. Therefore, it is preferable to add silicon (Si) in an amount of 0.4% or less of the total weight of the steel sheet.

Manganese (Mn): 4.0% ~ 9.0%

Manganese (Mn) is an austenite stabilizing element, and as manganese (Mn) is added, the starting temperature of martensite, Ms, gradually decreases, thereby increasing the residual austenite fraction after heat treatment.

Manganese is contained in 4.0 to 9.0% of the total weight of the steel sheet. When manganese is added in an amount of less than 4.0%, the above-described effect cannot be sufficiently secured. Conversely, when manganese is added in excess of 9.0%, weldability decreases due to an increase in carbon equivalent and oxides (MnO) are formed on the surface of the steel sheet during processing, resulting in a decrease in plating properties due to poor wettability.

Aluminum (Al): more than 0 and less than 0.3%

Aluminum (Al), like silicon (Si), is known as an element that stabilizes ferrite and suppresses the formation of carbides. In addition, since there is an effect of increasing the equilibrium temperature, there is an advantage in that an appropriate heat treatment temperature range is widened when aluminum (Al) is added. However, if aluminum exceeds 0.3% and is excessively added, problems may occur in playing due to AlN precipitation. Therefore, aluminum may be added in an amount greater than 0 and 0.3% or less 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 0.2% each

Niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo) may be selectively included in the steel. First, niobium (Nb), titanium (Ti), and vanadium (V) are elements precipitated in the form of carbides in steel, and are elements added to secure strength through precipitation of carbides. In the case of titanium (Ti), it is possible to suppress the formation of AlN to suppress the formation of cracks during playing. However, when niobium (Nb), titanium (Ti), and vanadium (V) are added in excess of 0.2%, respectively, by forming coarse precipitates, the amount of carbon in the steel is reduced to deteriorate the material, and niobium (Nb) , Titanium (Ti), and vanadium (V) There is a disadvantage of increasing the manufacturing cost according to the input. In addition, excessive addition of titanium may cause clogging of the nozzle during playing. Accordingly, when at least one of niobium (Nb), titanium (Ti), and vanadium (V) is added, each may be added in an amount of more than 0 and 0.2% or less of the total weight of the steel sheet.

Next, molybdenum (Mo) may play a role of controlling the size of the carbide by suppressing the growth of the carbide. However, when molybdenum is added in excess of 0.2%, the above effect is saturated and there is a disadvantage of an increase in manufacturing cost.

Boron (B)

Boron (B) may be selectively 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 0.001% or less of the total weight of the steel sheet. When boron is added in excess of 0.001%, high-temperature ductility can be reduced by forming nitrides such as BN.

Other elements

Phosphorus (P), sulfur (S) and nitrogen (N) may inevitably be added into the steel during the steelmaking process. That is, ideally, it is preferable not to include it, but it is difficult to completely remove it due to the process technology, so a certain small amount may be included.

Phosphorus (P) can play a similar role to silicon in steel. However, when phosphorus is added in excess of 0.02% of the total weight of the steel sheet, it may reduce the weldability of the steel sheet and increase brittleness, resulting in material degradation. Therefore, phosphorus can be controlled to be added to 0.02% or less of the total weight of the steel sheet.

Since sulfur (S) may impair toughness and weldability in the steel, it may be controlled to be contained in 0.005% or less of the total weight of the steel sheet.

When nitrogen (N) is excessively present in the steel, a large amount of nitride may be precipitated and ductility may be deteriorated. Therefore, nitrogen (N) can be controlled to be contained in 0.006% or less of the total weight of the steel sheet.

The high-strength steel sheet of the present invention having the above alloying components has a microstructure made of ferrite and retained austenite. In this case, the volume fraction of the retained austenite in the microstructure may be 10 to 30% by volume. The crystal grains of the high-strength steel sheet may be fine grains having a size of 3 μm or less. Among the crystal grains, a fraction of an elevation grain boundary may be 70% or more.

The high-strength steel sheet may have material properties such as yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, and hole expandability (HER): 20% or more. Accordingly, the high-strength steel sheet according to an embodiment of the present invention can be applied to fields requiring high strength and high formability.

The high-strength steel sheet according to the exemplary embodiment of the present invention described above may be manufactured by the method of an exemplary embodiment as follows. The present invention intends to present a steel sheet having excellent elongation, hole expandability and strength by performing a two-stage annealing heat treatment after proceeding with an alloy component of an appropriately controlled composition ratio, a hot rolling process and a cold rolling process, and a manufacturing method thereof.

Method for manufacturing a steel plate having high strength and high formability

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

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

First, the steel slab reheating step (S110) is, in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 0.4%, manganese (Mn): 4.0 to 9.0%, aluminum (Al) : More than 0 0.3%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Nitrogen (N): 0.006% or less Prepare a steel slab containing the balance iron (Fe) and other inevitable impurities And, it is a step of reheating the steel slab to re-dissolve the segregated components during casting and homogenize the components at the time of casting. Meanwhile, the steel slab further includes at least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), wherein each of the at least one may exceed 0 and be 0.02% or less. In addition, the steel slab may further include boron (B): greater than 0 and not more than 0.001% by weight.

The steel slab reheating temperature is preferably about 1150 to 1250°C so as to secure a normal hot rolling temperature. If the reheating temperature is less than 1150°C, the hot rolling load may rapidly increase, and if it exceeds 1250°C, it may be difficult to secure the strength of the final produced steel sheet due to coarsening of initial austenite grains.

Next, the hot rolling step (S120) is a step of forming a hot-rolled sheet material by performing hot rolling in a conventional method after reheating the slab, and performing finish rolling at a temperature of 925 to 975°C. In view of the fact that the steel slab of the present invention has a high content of an alloy component 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 sheet is cooled to 700 to 800°C at a cooling rate of 10 to 30°C/s, and then wound up. The cooling method may be applied to the non-injection cooling method. The hot-rolled sheet may have a full martensite structure after cooling.

According to some embodiments, before cold rolling a hot-rolled sheet material having a full martensite structure, softening heat treatment may be performed to reduce a rolling load during cold rolling. The softening heat treatment may be performed at 550 to 650°C. When the temperature of the softening heat treatment is less than 550° C., recrystallization does not occur for martensite generated after the hot rolling, and only tempering proceeds, so that supersaturated carbon in the structure may be formed in the form of cementite and spheroidized. In this case, the brittleness of the martensite may be expressed, and thus the plate may be fractured during cold rolling. On the other hand, when the temperature of the softening heat treatment exceeds 650° C., austenite is excessively formed, and martensite is formed from the austenite during cooling, so that the effect of the softening heat treatment may not occur effectively. By the softening heat treatment in the above temperature range, the martensite structure after hot rolling can be converted into a composite structure of ferrite and retained austenite.

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

Next, the annealing heat treatment step (S140) includes a step of primary heat treatment at a temperature of AC3 ~ AC3 + 15 °C for the cold rolled sheet material and a second heat treatment of the cold rolled sheet material subjected to the first heat treatment at an ideal temperature. Can proceed. 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 ideal temperature in the second heat treatment step may be, for example, a temperature of 640 to 660°C.

In an embodiment, the first heat treatment may convert a composite structure of ferrite and martensite of a sheet material after cold rolling into a structure of martensite. In the first heat treatment, first, the cold-rolled sheet is heated to a target temperature of 735 to 750°C at a temperature increase rate of 1 to 3°C/s, and a heat treatment for maintaining 40 to 120 seconds is performed.

When the heat treatment temperature is less than 735°C, austenite grains of sufficient size cannot be secured at the target temperature, and by forming a composite structure of martensite and ferrite after heat treatment, strength and ductility are reduced in the final structure following the annealing heat treatment. can do. On the other hand, when the heat treatment temperature exceeds 750°C, the size of the austenite grains at the target temperature increases excessively, which is disadvantageous in securing the stabilization of austenite in the final structure following the annealing heat treatment, and thus may be inferior in terms of strength. have.

In addition, when the temperature increase rate is less than 1°C/s, the time to stay at the target temperature of 735 to 750°C exceeds the range of 40 to 120 seconds, so that the austenite grain size at the target temperature may increase excessively. On the other hand, when the heating rate exceeds 3°C/s, the time to stay at the target temperature of 735 to 750°C is less than the range of 40 to 120 seconds, so that the austenite grains of sufficient size at the target temperature are not secured. It may not be possible.

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

The second heat treatment may be continuously performed on the cold-rolled sheet material on which the above-described first heat treatment has been completed. In an embodiment, in the second heat treatment, first, the cold-rolled sheet is heated to a target temperature of 640 to 660°C at a temperature increase rate of 1 to 3°C/s, and heat treatment is performed for 40 to 120 seconds. The secondary heat treatment may be performed at an ideal temperature within the target temperature range, so that the structure of martensite after the first heat treatment may be changed into a structure of ferrite and retained austenite. In this case, the volume fraction of retained austenite may be 10 to 30% by volume.

When the heat treatment temperature is less than 640°C, too little austenite structure is formed at the target temperature, resulting in increased austenite stability, and as a result, the austenite on the microstructure after cooling does not develop a phase transformation during plastic deformation, resulting in strength and ductility. Can decrease. On the other hand, when the heat treatment temperature exceeds 660°C, too much austenite structure is formed at the target temperature, resulting in a decrease in austenite stability, which results in the formation of martensite on the microstructure after cooling, resulting in ductility and hole expandability. This can be reduced.

When the heating rate is less than 1°C/s, the cold-rolled sheet may not be secured by deteriorating material properties by forming or spheroidizing unnecessary cementite before reaching the abnormal temperature. When the temperature increase rate exceeds 3°C/s, it may not be maintained in the target temperature range for 40 to 120 seconds, and thus a sufficient fraction of retained austenite may not be secured in the final tissue.

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

Through the above-described method, it is possible to manufacture a steel sheet having high strength and high formability according to an embodiment of the present invention.

The steel sheet of the present invention manufactured by the above-described process has yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, and hole expandability (HER): 20% or more. I can.

As described above, in the manufacturing method according to the embodiment of the present invention, a predetermined amount of an austenite stabilizing element may be added to the steel slab as described above. In addition, by continuously performing the first and second annealing heat treatments, the steel sheet may have a composite structure of ferrite of fine grains and 10 to 30% by volume of retained austenite as a final microstructure. Since the steel sheet retains a sufficient fraction of retained austenite, it may have a high elongation of 25% or more due to the metamorphic organic plastic properties.

In addition, as described above, by controlling the final microstructure so that hard phases such as martensite and bainite are not included, the interface between the hard phase and the soft phase can be reduced. In addition, in the component system of the steel slab, by controlling the content of the precipitate forming element such as titanium, niobium, vanadium, and the precipitate growth inhibiting element such as molybdenum, the boundary between the precipitate and the crystal grains can be reduced. The annealing heat treatment is performed as a two-stage heat treatment in which the annealing heat treatment is divided into the first and the second in a predetermined temperature range, thereby increasing the fraction of High Angle Grain Boundaries (HAGBs) in the final structure. 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 reverse transformed into austenite, and the second heat treatment is generated through the first heat treatment. By heat treatment of the martensite, recrystallization is relatively suppressed before the martensite is reversely transformed into austenite, so that the fraction of the high-angle grain boundary in the final microstructure can be increased to 70% or more of the grains. As a result, the steel sheet may have a high hole expandability of 20% or more.

Next, in order for the steel sheet to have high strength, crystal grains of the final microstructure may be refined. In particular, it is possible to optimize the crystal grain size of initial austenite by proceeding the temperature of the primary heat treatment at a temperature of AC3 to AC3 + 15°C. In addition, through the secondary heat treatment conducted in an ideal temperature range, the grain size of ferrite and retained austenite in the final microstructure can be controlled to be 3 μm or less.

Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, the following examples are for aiding understanding of the present invention, and the scope of the present invention is not limited to the following examples.

Example 1

Through the playing process, a steel slab having a comparative component system and an actual component system in Table 1 was manufactured. A specimen was prepared from the steel slab, and a high temperature tensile test was performed. In the case of the comparative component system, the content range of silicon and aluminum exceeded the upper limit of the content range of silicon and aluminum according to an embodiment of the present invention.

[Table 1]

2 is a high-temperature tensile test result of the comparative component-based specimen of the present invention, and Figure 3 is a high-temperature tensile test result of the actual component-based specimen of the present invention. Specifically, in the high-temperature tensile test, the sample of the comparative component system and the sample of the actual component system are respectively 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, and 1100°C. After heating to a temperature, it is a result of performing a tensile test at the temperature. In Figure 3, the high-temperature tensile test is a graph 201 and -20°C/ when the specimen is heated to a temperature exceeding 1100°C and then cooled to each tensile test temperature at a cooling rate of -1°C/s. A graph 202 in the case of cooling at each tensile test temperature at a cooling rate of s is shown together. Typically, when the area reduction rate is 50% or more at a predetermined temperature, it can be determined that ductility is secured at the predetermined temperature.

Referring to FIG. 2, in the case of the comparative component-based specimen, the area reduction rate is 55% at 1100°C, the area reduction rate is 50% at 700 to 800°C, and the area reduction rate is less than the target value of 50% in the temperature range of 800 to 1050°C. I did. On the other hand, referring to FIG. 3, in the temperature range of 800 to 1100° C., the area reduction rate exceeded the target value of 50%.

2 and 3, in the case of a comparative component-based specimen, in a high-temperature section at a temperature of 800° C. or higher in which continuous playing is performed according to an embodiment of the present invention, as compared to the specimen of the actual component-based specimen of the embodiment of the present invention, As it cannot be secured, it may not be possible to secure a sound slab due to the occurrence of cracks during continuous casting.

Table 2 is a chart showing the rolling force per pass calculated by simulating hot rolling according to an embodiment of the present invention for a comparative component specimen and an implementation component specimen.

[Table 2]

Referring to Table 2, it can be seen that in order to generate the same reduction ratio for each rolling pass, the specimen of the comparative component system should be applied with a large reduction force compared to the specimen of the actual component system. That is, it can be seen that a relatively large load is applied to the rolling mill when the specimen of the comparative component system is hot-rolled.

Example 2

The first and second annealing heat treatment processes were each performed according to Table 3 for the specimens of the implementation component system of Table 1. In the case of Comparative Example 1 and Comparative Example 3, the secondary annealing temperature was lower than the lower limit of the secondary annealing temperature according to the embodiment of the present invention 640 ℃. In the case of Comparative Example 2 and Comparative Example 4, the secondary annealing temperature was higher than the upper limit of 660 °C of the secondary annealing temperature according to the embodiment of the present invention. In Comparative Examples 5 to 7, the primary annealing temperature was higher than the upper limit of 750°C of the primary annealing temperature according to an embodiment of the present invention. In addition, in Comparative Example 7, the secondary annealing temperature was higher than the upper limit of 660°C of the secondary annealing temperature according to an embodiment of the present invention. In the case of Comparative Examples 8 to 11, the primary annealing heat treatment was not performed, and only the secondary annealing heat treatment was performed. In addition, in Comparative Example 11, the secondary annealing temperature was higher than the upper limit of 660°C of the secondary annealing temperature according to an embodiment of the present invention.

[Table 3]

Table 4 is a chart 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]

Target values of the material properties of the high-strength steel sheet according to an embodiment of the present invention, yield strength 800 MPa or more, tensile strength 980 MPa or more, elongation 25% or more, residual austenite volume fraction 10 to 30%, high angle grain boundaries (HAGBs) fraction More than 70% and more than 20% of hole expandability. 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 less than the target values. In the case of Comparative Example 2, the elongation was less than the target value. In the case of Comparative Example 3, tensile strength, elongation, and fraction of high-angle grain boundaries (HAGBs) were less than the target values. In the case of Comparative Example 4, the elongation, tensile strength x elongation, average grain size, and high-angle grain boundary (HAGBs) fraction were less than the target values. In the case of Comparative Example 5, the tensile strength, elongation, average grain size, and high angle grain boundaries (HAGBs) fraction were less than the target values. In the case of Comparative Examples 6 and 7, the yield strength, tensile strength, elongation, average grain size, and high angle grain boundaries (HAGBs) fraction were less than the target values. In the case of Comparative Example 8, the tensile strength, elongation, and fraction of high-angle grain boundaries (HAGBs) were less than 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 expandability were less than the target values.

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

Example 3

The first and second annealing heat treatment processes were each performed according to Table 5 for the specimens of the implementation component system of Table 1.

[Table 5]

Referring to Table 5, in Comparative Example 12, the temperature increase rate during the first annealing heat treatment exceeded the upper limit of the temperature increase rate of 3°C/s according to an embodiment of the present invention, and the first annealing holding time was not satisfied more than 40 seconds. I couldn't. In the case of Comparative Example 13, the lower limit of the temperature increase rate during the first annealing heat treatment according to an embodiment of the present invention was less than 1°C/s, and the upper limit of the first annealing holding time exceeded 120 seconds. In the case of Comparative Example 14, the lower limit of the heating rate according to an embodiment of the present invention was less than 1° C./s, and the upper limit of the first annealing holding time exceeded 120 seconds. Moreover, it was less than 4 degreeC/s of the lower limit of a cooling rate. In the case of Examples 7 to 10, both the primary and secondary annealing heat treatment conditions according to an embodiment of the present invention were satisfied.

Table 6 is a chart evaluating the material properties of the specimens of Comparative Examples 12 to 14 and Examples 13 to 16 subjected to annealing heat treatment according to Table 5.

[Table 6]

Referring to Table 6, 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. In the case of Examples 7 to 10, all of the target values of the material according to the embodiment of the present invention were satisfied.

Example 4

The first and second annealing heat treatment processes were each performed according to Table 7 for the specimens of the implementation component system of Table 1.

[Table 7]

Referring to Table 7, in Comparative Example 15, the temperature increase rate during the secondary annealing heat treatment exceeded the upper limit of the temperature increase rate of 3°C/s according to an embodiment of the present invention, and the secondary annealing holding time was not satisfied more than 40 seconds. I couldn't. In the case of Comparative Example 16, the lower limit of the heating rate during the secondary annealing according to an embodiment of the present invention was less than 1°C/s, and the upper limit of the secondary annealing holding time exceeded 120 seconds. In the case of Examples 11 to 14, both the primary and secondary annealing heat treatment conditions according to an embodiment of the present invention were satisfied.

Table 8 is a chart 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]

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 the elongation was not achieved. In the case of Examples 11 to 14, all of the target values of the materials according to the embodiments of the present invention were satisfied.

In the above, the embodiments of the present invention have been described mainly, but various changes or modifications may be made at the level of those skilled in the art. These changes and modifications can be said to belong to the present invention as long as they do not depart from the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Simple modifications or changes of the present invention can be easily implemented by those of ordinary skill in the art, and all such modifications or changes can be considered to be included in the scope of the present invention.

Claims (14)

1. By weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 0.4% or less, manganese (Mn): 4.0 to 9.0%, aluminum (Al): more than 0 0.3% or less, phosphorus (P) : 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.006% or less, the balance contains iron (Fe) and other inevitable impurities,

   It has a microstructure consisting of ferrite and retained austenite,

   The grain size of the microstructure is 3 μm or less,

   Yield strength (YS): 800 MPa or more, Tensile strength (TS): 980 MPa or more, Elongation (EL): 25% or more, Hole expandability (HER): 20% or more,

   Steel plate with high strength and high formability.
2. The method of claim 1,

   At least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo) is included, wherein the at least one is greater than 0 and less than 0.02% by weight, respectively.

   Steel plate with high strength and high formability.
3. The method of claim 1,

   Boron (B): more than 0 and containing 0.001% or less

   Steel plate with high strength and high formability.
4. The method of claim 1,

   The volume fraction of the residual austenite in the microstructure is 10 to 30% by volume.

   Steel plate with high strength and high formability.
5. (a) By weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 0.4% or less, manganese (Mn): 4.0 to 9.0%, aluminum (Al): more than 0 0.3% or less, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.006% or less, the balance of iron (Fe) and other unavoidable impurities containing steel slab using a steel slab to manufacture a hot-rolled sheet material ;

   (b) cold rolling the hot-rolled sheet to manufacture a cold-rolled sheet;

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

   (d) including the step of secondary heat treatment of the cold-rolled sheet material subjected to the first heat treatment at an ideal temperature,

   After step (d), the cold-rolled sheet material has a microstructure consisting of ferrite and retained austenite.

   A method of manufacturing a steel sheet having high strength and high formability.
6. The method of claim 5,

   The river slab is

   At least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo) is included, wherein the at least one is greater than 0 and less than 0.02% by weight, respectively.

   A method of manufacturing a steel sheet having high strength and high formability.
7. The method of claim 5,

   The river slab is

   Boron (B): more than 0 and containing 0.001% or less

   A method of manufacturing a steel sheet having high strength and high formability.
8. The method of claim 5,

   The volume fraction of the residual austenite in the microstructure is 10 to 30% by volume.

   A method of manufacturing a steel sheet having high strength and high formability.
9. The method of claim 5,

   Step (c) is

   Comprising the step of cooling the heat-treated cold-rolled sheet material to 350 ~ 450 ℃ at a cooling rate of 4 ~ 10 ℃ / s

   A method of manufacturing a steel sheet having high strength and high formability.
10. The method of claim 9,

    Step (d) is

    Comprising the step of cooling the heat-treated cold-rolled sheet material to 350 ~ 450 ℃ at 4 ~ 10 ℃ / s

    A method of manufacturing a steel sheet having high strength and high formability.
11. The method of claim 5,

    Step (a)

    (a1) reheating the steel slab to a temperature of 1150 to 1250°C;

    (a2) hot rolling the reheated steel slab at a finish rolling temperature of 925 to 975°C; And

    (a3) comprising the step of cooling the hot-rolled steel material to 700 ℃ ~ 800 ℃ at a cooling rate of 10 ~ 30 ℃ / s, and then winding

    A method of manufacturing a steel sheet having high strength and high formability.
12. The method of claim 5,

    Between steps (a) and (b),

    Further comprising the step of softening heat treatment of the hot-rolled sheet material at 550 ℃ ~ 650 ℃

    A method of manufacturing a steel sheet having high strength and high formability.
13. The method of claim 5,

    after step (d),

    The cold rolled sheet has yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, hole expandability (HER): 20% or more.

    A method of manufacturing a steel sheet having high strength and high formability.
14. The method of claim 5,

    after step (d),

    The crystal grain size of the cold-rolled sheet is 3 μm or less.

    A method of manufacturing a steel sheet having high strength and high formability.

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