KR102398151B1 - A method of preparing utlra high strength hot-rolled steel sheet having excellent ductility and utlra high strength hot-rolled steel sheet using the same - Google Patents

Abstract

By weight%, C: 0.17 to 0.3%, Si: 0.1 to 1%, Mn: 1.0 to 4.0%, Al: 1.0% or less, Mo: 0.5% or less, Ti: 0.003 to 0.1%, Nb: 0.01 to 0.1% Below, B: 0.01% or less, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the process of manufacturing a slab containing the remaining Fe and other unavoidable impurities, the slab is heated at 800 to 1,000 ° C. A process of manufacturing a hot-rolled steel sheet by rolling, a process of producing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction ratio of 20 to 70%, a process of annealing the cold-rolled steel sheet at 740 to 820°C, the annealed cold-rolled steel sheet A process of cooling to room temperature after the first heat treatment; And it relates to a method of manufacturing an ultra-high strength steel sheet having excellent ductility, characterized in that the secondary heat treatment is performed on the cold-rolled steel sheet cooled to room temperature.

Description

Method for manufacturing ultra-high-strength steel sheet with excellent ductility and ultra-high-strength steel sheet manufactured using the same

The present invention relates to a method for manufacturing an ultra-high-strength steel sheet having excellent ductility and to an ultra-high strength steel sheet manufactured using the same, more preferably a yield strength of 1,000 MPa or more, a tensile strength of 1,470 MPa and an elongation of 9% or more. It relates to a method for manufacturing a composite structural steel sheet and an ultra-high strength steel sheet manufactured using the same.

As environmental regulations related to carbon dioxide emissions are gradually strengthened, research is being actively conducted in automobile manufacturers to develop lightweight automobiles with good fuel efficiency and low smoke emission. To this end, there is an increasing demand for ultra-high-strength steel sheets having the same or higher strength with less thickness and non-systemic compared to conventional steel sheets.

As one of them, a hot stamping technique has been studied, but there is a problem in that the hot stamping requires a dedicated facility and increases the manufacturing cost of parts in that a hot stamping exclusive steel plate must be used.

In order to solve the above problems, research has been continuously made on steel materials capable of being high-strength and cold forming. As an example, Korean Patent Laid-Open Publication No. 2014-0097332 discloses a steel sheet having a yield strength of 1,344 MPa and a tensile strength of 1,520 MPa by adding 0.2 to 0.3% by weight of C and 2.0 to 3.5% by weight of Mn to the steel sheet, but the invention steel is Since the elongation is less than 8%, there is a disadvantage in that the workability is low. Korean Patent Application Laid-Open No. 2020-0027387 discloses a steel sheet having a tensile strength of 1400 MPa or more and a yield ratio of 0.7 or more by adding 0.1 to 0.3% by weight of C and 6 to 12% by weight of Mn, but a large amount of an alloy such as Mn is added It has the disadvantages of high alloy cost and low playability and spot weldability.

Therefore, there is a demand for development of a steel sheet having a predetermined strength and excellent workability at the same time.

Republic of Korea Patent Publication No. 2014-0097332 Republic of Korea Patent Publication No. 2020-0027387

Therefore, the present invention is to solve the above problems, and by controlling the content and manufacturing conditions of additional elements, an ultra-high-strength steel sheet with excellent formability having a yield strength of 1,000 MPa or more, a tensile strength of 1,470 MPa or more, and an elongation of 9% or more. and to provide a method for manufacturing the same.

In addition, an object of the present invention is to provide an ultra-high strength steel sheet excellent in formability and a method for manufacturing the same while limiting the relatively expensive Mn component to less than 5% by weight, but maintaining strength and elongation.

In addition, the subject of this invention is not limited to the above-mentioned content. The subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.

One aspect of the present invention for achieving the above object is by weight%, C: 0.17 to 0.3%, Si: 0.1 to 1%, Mn: 1.0 to 4.0%, Al: 1.0% or less, Mo: 0.5% or less, Ti : 0.003 to 0.1%, Nb: 0.01 to 0.1% or less, B: 0.01% or less, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, remaining Fe and other unavoidable impurities to prepare a slab a process of manufacturing a hot-rolled steel sheet by hot-rolling the slab at 800 to 1,000°C, a process of cold-rolling the hot-rolled steel sheet at a rolling reduction of 20 to 70% to produce a cold-rolled steel sheet, and 740 to 820 of the cold-rolled steel sheet Excellent ductility, characterized in that the annealing process at ℃, performing a primary heat treatment and a secondary heat treatment on the annealed cold-rolled steel sheet, and further comprising a step of cooling to room temperature between the primary heat treatment and the secondary heat treatment It relates to a method for manufacturing an ultra-high strength steel sheet.

In the above aspect, the steel sheet may satisfy the following relational expression (1).

[Relational Expression 1]

4204[C] + 16.3[Si] + 439[Mn] - 61[Al] + 21868[B] - 428 ≥ 1600

(Here, [C], [Si], [Mn], [Al] and [B] mean the weight percent of C, Si, Mn, Al and B, and 0 is substituted if not added.)

In one aspect, the first heat treatment may be maintained at 200 to 500 ℃ for 0.5 to 60 minutes.

In one aspect, the secondary heat treatment may be maintained at 100 to 390 °C for 0.1 to 60 minutes.

In the above aspect, a step of cooling to room temperature may be included between the first heat treatment and the second heat treatment.

In one aspect, the annealing process may be performed for 0.5 to 20 minutes.

According to another aspect of the present invention, by weight, C: 0.17 to 0.3%, Si: 0.1 to 1%, Mn: 1.0 to 4.0%, Al: 1.0% or less, Mo: 0.5% or less, Ti: 0.003 to 0.1%, Nb: 0.01 to 0.1% or less, B: 0.01% or less, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less In the ultra-high-strength steel sheet containing the remaining Fe and other unavoidable impurities, The steel sheet may satisfy Relation 1 below.

[Relational Expression 1]

4204[C] + 16.3[Si] + 439[Mn] - 61[Al] + 21868[B] - 428 ≥ 1600

(Here, [C], [Si], [Mn], [Al] and [B] mean the weight percent of C, Si, Mn, Al and B, and 0 is substituted if not added.)

In the one aspect, the steel sheet contains 3 to 14% of ferrite, 3 to 10% of retained austenite, 5 to 20% of tempered bainite, and the balance includes tempered martensite by area fraction of the steel sheet It relates to an ultra-high strength steel sheet excellent in ductility, characterized in that.

In the above aspect, the steel sheet may have an elongation of 9% or more.

In one aspect, the steel sheet may have a yield strength of 1,000 MPa or more and a tensile strength of 1,470 MPa or more.

According to the present invention, there is an effect that can provide an ultra-high strength steel sheet excellent in ductility in order to simultaneously satisfy the formability and collision safety required for an automobile steel sheet for cold forming. In addition, it is possible to reduce the manufacturing cost of parts by replacing hot press forming steel.

1 is a photograph taken with a scanning electron microscope (SEM) of the annealed tissue before the reheating process of the steel sheet manufactured in Example 1. FIG.
2 is a photograph taken with a scanning electron microscope (SEM) of the annealed tissue after the reheating process of the steel sheet manufactured in Example 1. Referring to FIG.

Features of the embodiments of the present invention, and methods of achieving them, will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and only these embodiments allow the disclosure of the present invention to be complete, and common knowledge in the art to which the present invention pertains It is provided to fully inform those who have the scope of the invention, and the present invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout.

In describing the embodiments of the present invention, if it is determined that a detailed description of a well-known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, the terms to be described later are terms defined in consideration of functions in an embodiment of the present invention, which may vary according to intentions or customs of users and operators. Therefore, the definition should be made based on the content throughout this specification. Hereinafter, embodiments of the present invention will be described in detail.

The present invention relates to an ultra-high strength steel sheet having a yield strength (YS) of 1,000 MPa and a tensile strength (TS) of 1,470 MPa or more, and an elongation of 9% or more by controlling the compositional components and heat treatment conditions of the steel. Through this, it is possible to provide an ultra-high-strength steel sheet capable of securing formability while having mechanical strength during press working.

According to an embodiment, the ultra-high strength steel sheet is, by weight, C: 0.17 to 0.3%, Si: 0.1 to 1%, Mn: 1.0 to 4.0%, Al: 1.0% or less, Mo: 0.5% or less, Ti: 0.003 to 0.1%, Nb: 0.01 to 0.1% or less, B: 0.01% or less, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the remaining Fe and other unavoidable impurities may be included. In addition, in order for the ultra-high strength steel sheet to have a yield strength (YS) of 1,000 MPa and a tensile strength (TS) of 1,470 MPa or more, t/4 of the steel sheet is based on the base structure at the point Relation 1 below may be satisfied.

[Relational Expression 1]

4204[C] + 16.3[Si] + 439[Mn] - 61[Al] + 21868[B] - 428 ≥ 1600

(Here, [C], [Si], [Mn], [Al] and [B] mean the weight percent of C, Si, Mn, Al and B, and 0 is substituted if not added.)

Relation 1 is a relational expression derived by quantifying the extent to which the composition contributes to strengthening the mechanical properties of the steel sheet, such as controlling the fraction of the microstructure in the steel sheet and improving the solid solution strengthening effect. Specifically, in the case of C, it can be seen that the coefficient is relatively larger than that of Si and Mn, because the C is solid-solution-strengthened in the grains of the steel sheet and greatly contributes to the improvement of strength. On the other hand, Si has a relatively small coefficient compared to C, because the effect of contributing to solid solution strengthening is smaller than that of C. The degree of contribution of B to solid solution strengthening is relatively smaller than that of C. However, since B suppresses ferrite transformation of the steel sheet in the cooling process after annealing and contributes to the improvement of strength, it can be confirmed that B has a high coefficient despite the addition of a relatively small amount compared to other components.

On the other hand, since Al has relatively little effect of solid-solution strengthening and acts as an austenite stabilizing element, it contributes to improving the fraction of retained austenite. , since it is an important component for ensuring proper ductility of the steel sheet, it is included in the above relational formula (1).

According to an embodiment, when the composition component of the steel sheet does not satisfy the above relational expression 1, any one or more of the tensile strength or the yield strength of the steel sheet may decrease, thereby reducing the strength of the steel sheet.

Hereinafter, the composition range of the present invention will be described in detail. Hereinafter, unless otherwise specified, the unit is % by weight.

C is included in an amount of 0.18 to 0.3% by weight.

C is an element that decisively contributes to strengthening the strength of the steel sheet. The C may be precipitated in the crystal grains of the steel sheet to induce solid solution strengthening, and promote the formation of martensite in the steel sheet to strengthen the steel sheet. In addition, C plays the most important role in forming retained austenite as an austenite stabilizing element. Specifically, when the C dissolved in the austenite increases, the austenite is stabilized and the fraction of the austenite in the steel sheet increases. This may improve the strength of the steel sheet by increasing the fraction of martensite formed due to the transformation of the austenite. In addition, some austenite may remain at room temperature after heat treatment. In order to make this action effective, the C may be added in an amount of 0.18% by weight or more. However, when the C exceeds 0.3% by weight, the fraction of martensite increases, but the fraction of ferrite having relatively excellent elongation and shock absorption energy decreases. This causes the ductility of the steel sheet to decrease and the brittleness to increase. In addition, the martensite has low weldability, which may cause welding defects during welding. For this reason, the C is preferably added in an amount of 0.18 to 0.3% by weight, and more preferably 0.18 to 0.2% by weight.

Si may be included in an amount of 0.1 to 1.0% by weight.

The Si suppresses the precipitation of carbides in the ferrite and induces diffusion of carbon in the ferrite into austenite, and may contribute to stabilization of retained austenite. In order to make this action effective, the Si content is preferably 0.1 wt% or more, and more preferably 0.4 wt% or more. However, when the Si content exceeds 1.0 wt%, the fraction of retained austenite remaining at room temperature after heat treatment may increase, thereby weakening the strength of the steel sheet. In addition, when the Si content exceeds 1.0 wt%, the effect of hot-dip plating and chemical conversion coating may be inhibited by forming Si oxide on the surface of the steel sheet. For this reason, the Si may be included in an amount of 0.1 to 1.0% by weight, more preferably 0.2 to 0.9% by weight.

Mn may be included in an amount of 1.0 to 4.0% by weight.

The Mn may act as an austenite stabilizing element similarly to the C. Specifically, the Mn may contribute to increasing the fraction of martensite in the steel sheet by reducing the critical cooling rate at which martensite is formed in the composite steel. In order to make this action effective, the Mn may be included in an amount of 1.0 wt% or more, and more preferably, it may be included in an amount of 2.5 wt% or more. However, when the Mn exceeds 4 wt %, the weldability of the steel sheet is reduced, and there is a high possibility that a problem in which the hot rolling property is deteriorated occurs. In addition, the martensite may be excessively formed to reduce ductility. In addition, when the Mn exceeds 4% by weight, MnO ₂ , Mn ₂ O ₃ and Manganese oxide formed of Mn ₂ SiO ₄ or the like may be eluted on the surface of the steel sheet. The eluted manganese oxide forms a striped band called Mn-Band, thereby inhibiting formability and increasing the risk of processing cracks and plate breakage. For this reason, Mn may be included in an amount of 1.0 to 4.0% by weight, more preferably 2.5 to 3.5% by weight.

Al may be included in an amount of 1.0% or less.

The Al may be added for deoxidation in steel. In addition, the Al acts as a ferrite stabilizing element similarly to the Si, and distributes carbon in the ferrite to austenite to improve hardenability of martensite formed by transformation of the austenite. In addition, when the steel sheet is maintained in the bainite region, it is possible to suppress the precipitation of carbides in the bainite to improve the ductility of the steel sheet. However, when the Al content exceeds 1.0 wt %, retained austenite may increase and the fraction of bainite or martensite may be relatively decreased. Continuous castability is lowered during the steel making operation, and the possibility of defective steel sheet can be increased by excessively forming alumina-based non-metallic inclusions. For this reason, it is preferable to limit the Al to 1.0 wt% or less.

Mo may be included in an amount of 0.5 wt% or less.

The Mo is an element that forms carbides in the steel sheet, and forms fine carbides in the steel sheet together with Ti and Nb to be described later, thereby contributing to improving the yield strength and tensile strength. However, when the Mo content exceeds 0.5% by weight, there is a problem in that the elongation is reduced and the manufacturing cost is increased. For this reason, it is preferable to limit the Mo to 0.5% by weight or less.

Ti may be included in an amount of 0.003 to 0.1 wt%.

The Ti is an element that forms fine carbide like Mo, and can contribute to securing the yield strength and tensile strength of the steel sheet. In addition, Ti forms a nitride to precipitate N contained in the steel sheet as TiN, thereby inhibiting the N from combining with Al and precipitating into AlN. This has the effect of reducing the risk of cracks occurring in the playing process. In order to make this action effective, the Ti content is preferably 0.003 wt% or more, and more preferably 0.03 wt% or more. However, when the content of Ti exceeds 0.1% by weight, coarse carbides may be precipitated, and the C in the steel sheet may be reduced. This becomes a cause of reducing the strength of the steel sheet. In addition, a phenomenon in which the nozzle is clogged in the playing process may occur due to the carbide. For this reason, Ti may be included in an amount of 0.003 to 0.1% by weight.

Nb may be included in an amount of 0.01 to 0.1% by weight.

The Nb segregates at the austenite grain boundary to suppress coarsening of austenite grains during annealing heat treatment, and may contribute to increasing the strength of the steel sheet by precipitating fine carbides on the grains. In order to make this action effective, the Nb content is preferably 0.01 wt% or more, and more preferably 0.03 wt% or more. However, when the Nb exceeds 0.1 wt%, the carbide becomes coarse like Ti, and the content of C in the steel sheet is reduced to reduce the strength and elongation of the steel sheet. In addition, when the content of Nb exceeds 0.1% by weight, there is a problem in that the manufacturing cost increases due to the excessive use of additives. For this reason, the Nb may be included in an amount of 0.01 to 0.1% by weight, more preferably 0.03 to 0.05% by weight.

B may be included in 0.01 wt% or less.

B is a component that delays the transformation of austenite into ferrite during the cooling process after the annealing process, and is a hardenable element that promotes martensite formation. However, when the content exceeds 0.01% by weight, excessive B is concentrated on the surface, which may lead to deterioration of plating adhesion. For this reason, B may be limited to 0.01 wt% or less.

P may be limited to 0.05% or less.

The P is segregated at the grain boundary and is a major cause of occurrence of temper brittleness, and may reduce weldability and toughness. Theoretically, it is advantageous to control the content of P to be low so that it is close to 0% by weight, but P is inevitably contained in the manufacturing process, and the process for reducing the content of P is difficult and production due to additional processes Since the cost increases, it is desirable to set and manage the upper limit. Therefore, it is preferable to manage the P to 0.05% by weight or less.

S may be limited to 0.02% by weight or less.

S is an impurity element unavoidably contained in the steel sheet like P described above, and is an element that inhibits the ductility and weldability of the steel sheet. Like the P, it is advantageous to control the content to be low to be close to 0% by weight, but it is preferable to set and manage the upper limit in consideration of the cost and time consumed for this. Accordingly, it is preferable to manage the S to 0.008% by weight or less.

N may be limited to 0.02% by weight or less.

The N may be combined with the Al to form an alumina-based non-metallic inclusion of AlN. The AlN may decrease the playing quality and increase the risk of fracture defects by increasing the brittleness of the steel sheet. For this reason, the Al content is preferably limited to 0.02 wt% or less, and more preferably limited to 0.005 wt% or less.

The remaining component of the present invention is Fe. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The composition, which is one characteristic of the present invention, has been described above. Hereinafter, another feature of the present invention will be described with respect to the microstructure. Hereinafter, unless otherwise indicated, % indicating the proportion of microstructure is based on the area.

The ultra-high strength steel sheet with excellent ductility according to an embodiment of the present invention may have a composite structure composed of two or more microstructures selected from ferrite, retained austenite, tempered bainite and tempered martensite, and more preferably ferrite. , retained austenite, tempered bainite and the remainder may have a composite structure formed of tempered martensite.

According to an embodiment, the steel sheet may include 3 to 14% of ferrite, 3 to 10% of retained austenite, 5 to 20% of tempered bainite, and the remainder tempered martensite as an area fraction.

Ferrite is an allotrope of iron having a body-centered cubic structure (BCC), and is characterized in that it has a soft structure unlike tempered bainite and tempered martensite, which will be described later. Therefore, the elongation is relatively high compared to the tempered bainite and the tempered martensite, and there is an advantage in that the shock absorption energy is excellent.

When the ferrite content exceeds 14%, the soft tissue in the steel sheet may be excessively formed to promote plastic deformation. This causes the yield strength (YS) of the steel sheet to decrease. On the other hand, when the ferrite content is less than 3%, the elongation of the steel sheet may decrease and formability may deteriorate. For this reason, the ferrite may be included in an area fraction of 3 to 14%, more preferably 6 to 12%.

The retained austenite refers to the austenite structure remaining in the steel sheet without being transformed into martensite or bainite in a heat treatment process to be described later, and serves to control the balance between the strength and elongation of the steel sheet. In general, when the strength of the steel sheet increases, the elongation decreases and formability decreases. When the elongation of the steel sheet increases, the strength decreases and the role as a structural material decreases, whereas the retained austenite is the tensile strength X elongation ( TS X EL), the balance between strength and elongation can be improved. In order to effect this action, the retained austenite may contain 3% or more. However, when the retained austenite exceeds 10%, there is a problem in that the sensitivity to hydrogen embrittlement increases and molding becomes difficult. For this reason, it is preferable that the retained austenite is contained in an amount of 3 to 10%, more preferably 3 to 6%.

The tempered bainite refers to a bainite structure formed through tempering. The tempered bainite may contribute to improving workability by reducing a strength difference between structures in the steel sheet. That is, it is possible to prevent cracks, defects, and fractures from occurring in the steel sheet due to a difference in hardness between the ferrite and retained austenite having relatively low hardness and the tempered martensite having relatively high hardness. In order to make this action effective, the tempered bainite may contain 5% or more, more preferably 8% or more. However, when the fraction of tempered bainite exceeds 20%, the strength of the steel sheet may be reduced by reducing the fraction of martensite. For this reason, the tempered bainite is preferably contained in an amount of 5 to 20%, and more preferably, may be included in an amount of 8 to 12%.

Hereinafter, the remaining microstructure is provided as tempered martensite. The tempered martensite refers to a structure in which martensite is softened by tempering a martensite structure obtained by quenching austenite at a temperature of 400° C. or less. The tempered martensite significantly contributes to improving the yield strength and tensile strength of the steel sheet as it has the highest strength compared to the above-mentioned structure.

Above, the ultra-high strength steel sheet excellent in ductility according to an embodiment of the present invention has been described. Hereinafter, a method for manufacturing an ultra-high strength steel sheet having excellent ductility according to an embodiment of the present invention will be described.

According to an embodiment, the method for manufacturing the ultra-high strength steel sheet excellent in ductility includes a step of reheating a slab having the above-described compositional components at 1,050 to 1,300°C; A process of manufacturing a hot-rolled steel sheet by winding the reheated slab after hot rolling at 800 to 1,000°C; a process of manufacturing a cold-rolled steel sheet by performing pickling treatment of the hot-rolled steel sheet and then cold-rolling the hot-rolled steel sheet at a reduction ratio of 20 to 70% at room temperature; continuously annealing the cold-rolled steel sheet for 10 to 900 seconds; cooling the continuously annealed cold-rolled steel sheet for 30 to 300 seconds; and reheating the cooled cold-rolled steel sheet for 10 to 3600 seconds.

First, in the present invention, a process of reheating the slab having the above composition can be performed, and through this, the above-described composition, that is, by weight%, C: 0.17 to 0.3%, Si: 0.1 to 1%, Mn: 1.0 to 4.0%, Al: 1.0% or less, Mo: 0.5% or less, Ti: 0.1% or less, Nb: 0.01 to 0.1% or less, B: 0.01% or less, P: 0.05% or less, S: 0.02% or less, N: It is possible to homogenize the slab containing 0.02% or less, the remaining Fe and other unavoidable impurities. At this time, the reheating process may be performed at 1,050 to 1,300 ° C. This is a problem in that when the reheating temperature is less than 1,050 ° C, friction between the steel sheet and the rolling mill increases, so that the load applied to the rollers in the hot rolling step to be described later increases rapidly. there is On the other hand, when the reheating temperature exceeds 1,300° C., not only the energy cost required for temperature increase increases, but also the amount of surface scale increases, which may lead to material loss. For this reason, the reheating process may be performed at 1,050 to 1,300 °C, and more preferably at 1,900 to 1,250 °C.

Thereafter, the reheated slab may be hot rolled at 800 to 1,000° C., and a hot rolled steel sheet may be manufactured by winding it.

First, the reheated steel slab may be hot rolled at 800 to 1,000° C. based on a Finish Rolling Temperature (FRT). When hot rolling at the finish rolling temperature, the rigidity and formability of the steel sheet may be improved at the same time. If the finish hot rolling temperature is less than 800, since rolling proceeds in the ferrite region, friction between the steel sheet and the rolling mill increases, and thus there is a problem in that the load due to rolling increases significantly. This may form excessive dislocations, which may reduce strength due to the formation of coarse grains on the surface of the steel sheet during winding or cold rolling, which will be described later. On the other hand, if the finish hot rolling temperature exceeds 1,000 ℃, the size of the ferrite grains may increase to decrease the strength. In addition, scale may occur on the surface of the hot-rolled steel sheet, which may cause surface defects and shorten the life of the rolling roll. For this reason, the hot rolling process is preferably performed at 800 to 1,000°C based on the finish rolling temperature (FRT), and more preferably, 850 to 950°C.

Thereafter, the hot-rolled steel sheet may be wound at 400 to 700° C. to manufacture a hot-rolled steel sheet. If the coiling temperature is less than 400 ℃, the strength of the hot-rolled steel sheet is excessively high may cause a rolling load during cold rolling. In addition, the cost and time for cooling the steel sheet to the coiling temperature are excessively consumed, which causes an increase in the process cost. On the other hand, when the coiling temperature exceeds 700° C., scale may be excessively generated on the surface of the hot-rolled steel sheet, which may cause surface defects and weaken the plating property. For this reason, the coiling temperature is preferably 400 to 700 °C, more preferably 500 to 700 °C.

According to an embodiment, after winding, the hot-rolled steel sheet may be cooled to room temperature.

After that, the hot-rolled steel sheet may be subjected to a pickling treatment and then cold-rolled at a reduction ratio of 20 to 70% at room temperature to manufacture a cold-rolled steel sheet.

The pickling treatment refers to a process of removing the scale formed on the surface of the hot-rolled steel sheet by using hydrochloric acid (HCl). Cold rolling may be performed after the pickling treatment. The pickling and cold rolling conditions are not particularly limited and may be carried out under normal process conditions, but the cold rolling reduction is preferably performed in a range of 20 to 70%. If the reduction ratio of the cold rolling is less than 20%, it is difficult to manufacture a steel sheet to a desired thickness, and it is difficult to correct the shape of the steel sheet. On the other hand, when the reduction ratio exceeds 70%, there is a high possibility that cracks in the edge portion of the steel sheet occur, and there is a problem of bringing a cold rolling load. In addition, when the reduction ratio exceeds 70%, there is a possibility that coarse ferrite may be formed in a continuous annealing process to be described later by applying an excessive load to the surface of the steel sheet. For this reason, the rolling reduction of the cold rolling may be performed in a range of 20 to 70%, more preferably 30 to 50%.

A method for manufacturing a cold-rolled steel sheet having the above-described composition has been described above. Hereinafter, a method for manufacturing an ultra-high strength steel sheet having a composite structure in which the cold-rolled steel sheet is heat-treated to form ferrite, retained austenite, tempered bainite and the remainder of tempered martensite will be described.

First, a process of annealing the cold-rolled steel sheet for 10 to 900 seconds may be performed, and preferably, a continuous annealing process may be performed, but is not limited thereto, and any of the known annealing methods may be performed. is also free Through the annealing process, the ferrite formed in the cold-rolled steel sheet may be recrystallized, and the fractions of the ferrite and the austenite may be adjusted. The strength of the steel sheet manufactured after the final heat treatment is determined due to the fraction. In general, as the austenite fraction increases, martensite or bainite transformed from the austenite increases, so that the strength of the final steel sheet tends to increase. , it is possible to additionally control the strength due to the heat treatment conditions to be described later.

In addition, by increasing the amount of C included in austenite through the annealing process, the C may be distributed to form a retained austenite fraction of 1 to 10% even at room temperature.

According to an embodiment, the annealing process may be performed at 740 to 820°C. If the temperature at which the continuous annealing process is performed is less than 740 ° C, the fraction of austenite formed due to the annealing process is reduced, and the fraction of tempered martensite and tempered bainite formed through a heat treatment process to be described later is reduced. can This may cause the yield strength and tensile strength of the steel sheet to be reduced . On the other hand, when the temperature at which the continuous annealing process is performed exceeds 820° C., the austenite fraction in the steel sheet is excessively increased, which may transform some austenite into ferrite in a heat treatment process to be described later. In addition, the carbon concentration of the retained austenite is low, and mechanical stability may be reduced. This causes the elongation of the steel sheet to decrease. In addition, in the continuous annealing process, as the Fe is oxidized, moisture is generated, and the moisture may react with the Si, Mn, and B to form an oxide film on the steel sheet. The oxide film may reduce the wettability of Zn during hot-dip galvanizing, thereby reducing the surface quality. For this reason, the continuous annealing process is preferably performed at 740 to 820 °C, and more preferably at 750 to 800 °C.

Thereafter, the annealed cold-rolled steel sheet may be subjected to a first heat treatment, a cooling process, and a second heat treatment. Among them, the first heat treatment will be described.

The primary heat treatment refers to a heat treatment comprising a step of cooling the annealed cold-rolled steel sheet to 100 to 500° C., more preferably 200 to 500° C., and a step of maintaining the annealed cold-rolled steel sheet at the cooled predetermined temperature for 30 to 3600 seconds. .

The first heat treatment temperature may be performed at 100 to 500 °C, and more preferably, a portion of the austenite may be transformed into the bainite through a martensite transformation initiation temperature (MS) treatment. In this process, some of the C dissolved in the transformed bainite may be diffused into the austenite to increase the amount of the C dissolved in the austenite. This can improve the stability of the austenite as described above.

and the bainite transformation onset temperature (BS). This is because when the temperature at which the primary heat treatment is performed exceeds the bainite transformation initiation temperature (BS), bainite may not be formed in the steel sheet structure or may be formed in a very low fraction. Conversely, if the temperature at which the primary heat treatment is performed is less than the martensitic transformation initiation temperature (MS), the austenite may not be transformed into martensite but may be transformed into ferrite and bainite. For this reason, the cooling and maintaining temperature may be performed at a martensitic transformation starting temperature (T _MS ) or higher, a bainite transformation starting temperature ( _TB ) or lower, and more preferably at 250 to 450°C.

In addition, if the time maintained at the corresponding temperature after cooling during the first heat treatment is less than 30 seconds, the fraction of bainite is reduced to reduce the C diffused into the austenite. This causes a decrease in the retained austenite fraction and consequently a decrease in elongation and formability of the steel sheet. Conversely, when the holding time exceeds 3,600 seconds, it is difficult to expect any further increase in the effect, and productivity may be reduced. For this reason, the holding time is preferably performed for 30 to 3,600 seconds, and more preferably 200 to 600 seconds.

According to an embodiment, after the cooling and maintenance, the steel sheet may be subjected to hot-dip galvanizing treatment and alloying heat treatment at 430 to 490° C. as needed. The hot-dip galvanizing treatment and alloying heat treatment conditions are not particularly limited, and may be performed according to known conditions.

After the first heat treatment, a process of cooling to room temperature may be performed. Some austenite that is not transformed into bainite in the primary heat treatment may be transformed into martensite through the cooling to room temperature. In addition, the remaining austenite that is not transformed into martensite may form retained austenite at room temperature.

According to an embodiment, the cooling to room temperature may be performed by air cooling, but is not limited thereto, and it is obvious that a known cooling method such as water cooling or oil cooling furnace cooling may be used.

Finally, secondary heat treatment may be performed on the steel sheet cooled to room temperature.

Referring to FIG. 1 , it can be confirmed that the steel sheet cooled to room temperature has a composite structure composed of ferrite, bainite, retained austenite and martensite. Specifically, the composite structure has a relatively high fraction of ferrite, and a movable dislocation is formed around martensite due to heat treatment, so that the structure is unstable and brittle. For this reason, cracks occur at the interface between ferrite and martensite during welding and forming.

In order to improve this, secondary heat treatment (tempering) may be performed on the steel sheet cooled to room temperature. Through this, the martensite and the bainite may be softened by reheating the steel sheet. Hereinafter, martensite softened by the secondary heat treatment will be described as tempered martensite, and bainite softened due to the secondary heat treatment will be described as tempered bainite. In the tempered bainite and tempered martensite, a difference in hardness from the ferrite may be reduced, so that ductility of the steel sheet may be improved and collision resistance may be improved. In addition, in the secondary heat treatment process, the C is fixed to the movable dislocation around martensite, so that the Cottrell effect can be implemented. The Correl effect refers to an effect in which impurity atoms are pulled close to the dislocation line to fix the dislocation. Due to this, the movable dislocation is fixed by the cottrell effect, so that the yield strength can be improved. That is, through the secondary heat treatment, the yield strength may be improved and the impact resistance property may also be improved. In addition, a portion of C dissolved in the martensite in the secondary heat treatment process is distributed to the retained austenite, thereby increasing the stability of the retained austenite and contributing to the improvement of ductility of the steel sheet.

According to an embodiment, the secondary heat treatment may be performed at 100 to 500 °C, more preferably at 100 to 390 °C. When the secondary heat treatment exceeds 390° C., the carbide precipitated in the heat treatment process may become coarse and the tensile strength may decrease. In addition, the retained austenite may be decomposed into a ferrite phase to decrease elongation. On the other hand, if the secondary heat treatment is less than 100 ℃, it is difficult to expect a softening effect due to the second heat treatment. For this reason, the tempering process may be performed at 100 to 390 °C, more preferably at 250 to 350 °C. In addition, even if the secondary heat treatment is performed in the above-mentioned temperature range, it is difficult to expect an effect when the second heat treatment time is performed less than 10 seconds, and when the second heat treatment time exceeds 3,600 seconds, the carbide becomes coarse and strength Since the tendency to decrease is confirmed, the second heat treatment is preferably performed for 10 to 3,600 seconds, more preferably 10 to 1,000 seconds.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[Production Example]

30 kg of the slab having the composition shown in Table 1 below was reheated at a temperature of 1,200 °C for 1 hour, and the reheated slab was hot-rolled at 900 °C based on the finish rolling temperature (FRT). Thereafter, it was charged into a furnace heated to 600° C., maintained for 1 hour, and then furnace cooled to simulate hot rolling winding. After cooling to room temperature, cold rolling was performed at a cold rolling reduction ratio of 40% to manufacture a cold rolled steel sheet. The component composition of the above-mentioned slab is described in Table 1 below. In this case, Inventive Steels 1 to 7 refer to slabs satisfying the following Relational Expression 1, and Comparative Steels 1 to 2 refer to slabs that do not satisfy the following Relational Expression 1.

[Relational Expression 1]

4204[C] + 16.3[Si] + 439[Mn] - 61[Al] + 21868[B] - 428 ≥ 1600

(Here, [C], [Si], [Mn], [Al] and [B] mean the weight percent of C, Si, Mn, Al and B, and 0 is substituted if not added.)

steel grade Ingredients (wt%) Relation 1 C Si Mn Al Mo Ti Nb B P S N Invention lecture 1 0.18 0.8 2.9 0 0.3 0.03 0.03 0 0.010 0.005 0.002 1615 Invention lecture 2 0.2 0.8 2.7 0 0.3 0.03 0.03 0 0.008 0.005 0.003 1611 Invention Lesson 3 0.18 0.4 2.9 0 0.3 0.03 0.03 0 0.009 0.006 0.004 1608 Invention lecture 4 0.18 0.4 2.9 0.2 0.3 0.03 0.03 0 0.011 0.009 0.002 1662 Invention Lesson 5 0.2 0.4 2.9 0 0.3 0.03 0.03 0.003 0.011 0.004 0.003 1692 Invention Lesson 6 0.2 0.4 2.9 0.2 0.3 0.03 0.03 0.003 0.012 0.009 0.003 1680 Invention Lesson 7 0.2 0.8 2.9 0 0 0.03 0.03 0 0.009 0.007 0.002 1699 Comparative Steel 1 0.16 0.4 2.9 0.2 0.3 0 0.03 0 0.012 0.004 0.004 1512 Comparative Steel 2 0.16 0.4 2.9 0 0.3 0 0.03 0 0.009 0.006 0.004 1524

[Example]

The cold-rolled steel sheet prepared according to Preparation Example was continuously annealed at T ₁ for 300 seconds. Thereafter, the first heat treatment was performed by maintaining the temperature of the cold-rolled steel sheet for 300 seconds after cooling to T ₂ . After lowering to room temperature, a secondary heat treatment was performed at T ₃ for 30 seconds.

The results of evaluation of the mechanical properties and internal structures of the T ₁ , T ₂ , T ₃ and the cold-rolled steel sheet manufactured by the above-described process are shown in Table 2 below. The mechanical properties refer to yield strength (YS), tensile strength (TS) and elongation (%), and were measured and evaluated using JIS No. 5 tensile test pieces. In addition, the internal structure means ferrite (F), retained austenite (A), tempered bainite (TB) and tempered martensite (TM), and after nital etching of the polished specimen, a scanning electron microscope ( SEM) was used to calculate the area fraction.

steel grade Heat treatment temperature (℃) Properties Tissue fraction (%) T ₁ T ₂ T ₃ YS
(MPa) ts
(MPa) El
(%) F A TB TM Example 1 Invention lecture 1 770 300 300 1020 1570 10 10 3 12 75 Example 2 Invention lecture 2 800 250 300 1003 1523 9 8 4 12 76 Example 3 Invention Lesson 3 770 300 300 1128 1576 9 12 4 9 75 Example 4 Invention lecture 4 770 300 300 1057 1588 11 10 3 10 77 Example 5 Invention lecture 4 800 400 300 1068 1524 9 8 5 8 79 Example 6 Invention Lesson 5 750 300 300 1036 1644 9 12 6 12 70 Example 7 Invention Lesson 5 800 300 300 1096 1515 10 6 4 11 79 Example 8 Invention Lesson 6 770 350 300 1083 1643 11 11 6 11 72 Example 9 Invention Lesson 7 770 300 300 1008 1472 9 12 4 12 72 Comparative Example 1 Comparative Steel 1 770 300 300 980 1460 10 10 2 10 80 Comparative Example 2 Comparative Steel 1 800 350 300 971 1357 10 9 2 11 80 Comparative Example 3 Comparative Steel 2 750 300 300 960 1459 9 16 2 8 76 Comparative Example 4 Comparative Steel 2 770 400 300 1003 1423 6 11 One 10 79 Comparative Example 5 Invention Lesson 5 770 560 300 1147 1629 6 11 3 0 86 Comparative Example 6 Invention Lesson 6 750 300 80 949 1603 7 15 5 10 70 Comparative Example 7 Invention Lesson 6 800 560 300 1081 1549 6 10 3 0 87 Comparative Example 8 Invention Lesson 7 730 350 300 939 1367 10 25 4 9 62

Referring to Table 2, it can be seen that Examples 1 to 9 satisfying all of the heat treatment conditions of the present invention are formed in a composite structure composed of ferrite, retained austenite, tempered bainite and tempered martensite.

In fact, referring to FIG. 2 taken by SEM of Example 1, it can be confirmed that it has a complex structure as described above, and specifically, its area fraction is 3 to 14% for ferrite, 3 to 10% for retained austenite, and temper It can be seen that de bainite is formed of 5 to 20% and the balance is tempered martensite.

As a result of the formation of a microstructure with the above-mentioned area fraction, Examples 1 to 9 prepared with the invention steels 1 to 9 had a yield strength (YS) of 1,000 MPa or more, a tensile strength (TS) of 1,470 MPa or more, and an elongation of 9 % or more.

On the other hand, Comparative Examples 1 to 4 prepared with Comparative Steels 1 and 2 that do not satisfy Relational Equation 1 have yield strength (YS) of 960 to 1003 MPa and tensile strength (TS) of 1,357 to 1,460 MPa, respectively, in Examples 1 to 9. compared to the decrease in strength. As described above, Relation 1 is a relational expression quantifying the degree of contribution to strengthening the mechanical properties of the steel sheet, such as controlling the fraction of microstructure in the steel sheet and improving the solid solution strengthening effect. If Relation 1 is not satisfied, the strength of the steel sheet decreases means it can be

On the other hand, Comparative Examples 5 to 8 satisfy Relational Expression 1, but the manufacturing conditions of the present invention, specifically, T ₁ T ₂ and T ₃ are less than or more than the temperature range defined in the present invention, so that the ferrite and retained austenite , it can be confirmed that at least one of tempered bainite and tempered martensite is insufficient or exceeded. For this reason, it can be confirmed that Comparative Examples 5 to 8 did not secure any one of the mechanical properties desired by the present invention, that is, yield strength (YS), tensile strength (TS), and elongation.

First, in Comparative Examples 5 and 7, it can be seen that the fraction of tempered bainite is 0 because T ₂ exceeds 500°C. This is because, as described above, the cooling and maintenance process exceeds the bainite transformation initiation temperature ( _TB ), and bainite is not generated. As a result, it can be confirmed that the fraction of martensite is increased to 86 to 87%, and the elongation is 6%.

Comparative Example 6 has a T ₃ of 80° C. and the secondary heat treatment temperature is less than 100° C. As a result, since tempering was not sufficiently performed in the secondary heat treatment, the stress in the steel sheet was not sufficiently relieved. For this reason, it can be seen that Comparative Example 6 has an elongation of 7%, which is less than 9%.

In Comparative Example 8, it can be seen that the continuous annealing was performed at less than 740° C., so that austenite was not sufficiently formed and the ferrite fraction was relatively improved. For this reason, the fractions of tempered bainite and tempered martensite formed by the transformation of the austenite were also reduced. As a result, it can be confirmed that the yield strength (YS) is 939 MPa, which is less than 1,000 MPa, and the tensile strength (TS) is 1,367 MPa, which is less than 1,470 MPa.

In the above description, various embodiments of the present invention have been presented and described, but the present invention is not necessarily limited thereto. It will be readily appreciated that branch substitutions, transformations and alterations are possible.

Claims (10)

By weight%, C: 0.17 to 0.3%, Si: 0.1 to 1%, Mn: 1.0 to 4.0%, Al: 1.0% or less, Mo: 0.5% or less, Ti: 0.003 to 0.1%, Nb: 0.01 to 0.1% or less, B: 0.01% or less, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the process of manufacturing a slab containing the remaining Fe and other unavoidable impurities;
a process of manufacturing a hot-rolled steel sheet by hot-rolling the slab at 800 to 1,000°C;
manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction ratio of 20 to 70%;
annealing the cold-rolled steel sheet at 740 to 820°C;
cooling the annealed cold-rolled steel sheet to room temperature after primary heat treatment at 200 to 500°C; and
Secondary heat treatment is performed on the cold-rolled steel sheet at 100 to 390°C,
The method for manufacturing an ultra-high strength steel sheet excellent in ductility, characterized in that the steel sheet satisfies the following Relational Equation 1
[Relational Expression 1]
4204[C] + 16.3[Si] + 439[Mn] - 61[Al] + 21868[B] - 428 ≥ 1600
(Here, [C], [Si], [Mn], [Al] and [B] mean the weight % of C, Si, Mn, Al and B, respectively, and 0 is substituted if not added. )
delete The method of claim 1,
The method of manufacturing an ultra-high strength steel sheet excellent in ductility, characterized in that the primary heat treatment is maintained for 0.5 to 60 minutes. The method of claim 1,
The second heat treatment is a method of manufacturing an ultra-high strength steel sheet excellent in ductility, characterized in that it is maintained for 0.1 to 60 minutes. The method of claim 1,
The method for producing an ultra-high strength steel sheet excellent in ductility, characterized in that the annealing process is performed for 0.5 to 20 minutes. The method of claim 1,
Between the first heat treatment and the second heat treatment,
A method of manufacturing an ultra-high strength steel sheet excellent in ductility, characterized in that it further comprises the step of hot-dip galvanizing the cold-rolled steel sheet. By weight%, C: 0.17 to 0.3%, Si: 0.1 to 1%, Mn: 1.0 to 4.0%, Al: 1.0% or less, Mo: 0.5% or less, Ti: 0.003 to 0.1%, Nb: 0.01 to 0.1% or less, B: 0.01% or less, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, in the ultra-high strength steel sheet containing the remaining Fe and other unavoidable impurities,
The steel sheet is characterized in that it satisfies the following Relational Expression 1,
The steel sheet is an area fraction, ferrite is 3 to 14%, retained austenite is 3 to 10%, tempered bainite 5 to 20%, characterized in that the balance comprises tempered martensite, ductility This excellent ultra-high-strength steel sheet..
[Relational Expression 1]
4204[C] + 16.3[Si] + 439[Mn] - 61[Al] + 21868[B] - 428

1600
(Here, [C], [Si], [Mn], [Al] and [B] mean the weight % of C, Si, Mn, Al and B, respectively, and 0 is substituted if not added. ) delete 8. The method of claim 7,
The steel sheet is an ultra-high strength steel sheet excellent in ductility, characterized in that the elongation is 9% or more. 8. The method of claim 7,
The steel sheet has a yield strength of 1,000 MPa or more and a tensile strength of 1,470 MPa or more, an ultra-high strength steel sheet with excellent ductility.

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