KR102468051B1 - Ultra high strength steel sheet having excellent ductility and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a steel sheet suitable as a material for automobiles, and more particularly, to an ultra-high strength steel sheet having excellent ductility.

Description

Ultra high strength steel sheet with excellent ductility and its manufacturing method {ULTRA HIGH STRENGTH STEEL SHEET HAVING EXCELLENT DUCTILITY AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a steel sheet suitable as a material for automobiles, and more particularly, to an ultra-high strength steel sheet having excellent ductility.

In recent years, the use of high-strength steel sheets has been required in the automobile industry to improve fuel efficiency or durability due to various environmental regulations and energy use regulations.

However, when the strength of the steel sheet is increased, it has been found that the ductility is relatively reduced, and thus, many studies have been conducted to improve the relationship between strength and ductility. As a result, transformation steels utilizing the retained austenite phase along with martensite and bainite, which are low-temperature structures, have been developed and applied.

Transformation steels include ferrite-martensite dual phase (DP) steels in which a hard martensite phase is formed on a ferrite base, TRIP (Transformation Induced Plasticity) steels using transformation-induced plasticity of retained austenite, and ferrite and hard steels. CP (Complexed Phase) steel composed of a bainite or martensite structure is distinguished, and each of these steels has different levels of mechanical properties, that is, tensile strength and elongation, depending on the type and fraction of the mother phase and the second phase.

In particular, TRIP steel containing a large amount of retained austenite has the highest tensile strength and elongation balance (TS×El) value.

As an example, Patent Document 1 includes about 10% of retained austenite phase in addition to ferrite and martensite, so that the product of tensile strength and elongation is 21000 MPa% or more, and a steel capable of securing a tensile strength of 780 MPa or more is disclosed. . However, since the steel has a carbon (C) content of about 0.2% and a silicon (Si) content of about 1.5% or more, there is a concern that spot weldability and hot-dip galvanizing property may be inferior. In addition, since annealing is performed twice to implement high physical properties, there is a problem in that the manufacturing cost of the steel sheet increases.

On the other hand, in Patent Document 2, in order to secure good plating and spot weldability, the content of Si is lowered to 1%, and it is composed of martensite, bainite and ferrite without including the retained austenite phase as a microstructure, and the tensile strength of 980 MPa or more A technique capable of securing strength and elongation of 15% or more is disclosed. However, with the recent expansion of automobile impact stability regulations, high-strength steel with excellent yield strength is being used for structural members such as members, seat rails, and pillars to improve the impact resistance of the body. However, the yield strength of the steel is 700 MPa or less, so there is a limit to the application target.

Korean Patent Publication No. 2015-0130612 Korean Patent Publication No. 2013-0106142

One aspect of the present invention is to provide a steel sheet suitable for structural members of automobiles, etc., excellent in tensile strength as well as yield strength, and improved ductility, and a method for manufacturing the same.

The object of the present invention is not limited to the above. The subject of the present invention will be understood from the entire contents of this specification, and those skilled in the art will have no difficulty in understanding the additional subject of the present invention.

One aspect of the present invention, in weight%, carbon (C): 0.1 ~ 0.2%, silicon (Si): 0.1 ~ 1.0%, manganese (Mn): 2.0 ~ 3.0%, aluminum (Al): 1.0% or less ( 0% or less), Chromium (Cr): 1.0% or less, Molybdenum (Mo): 0.5% or less, Titanium (Ti): 0.1% or less, Niobium (Nb): 0.1% or less, Antimony (Sb): 0.1% or less ( except for 0%), phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, including the remaining Fe and other unavoidable impurities,

Provided is an ultra-high strength steel sheet with excellent ductility, characterized in that it satisfies the following relational expressions 1 to 3.

[Relationship 1]

1110[C] + 41.5[Si] + 575[Mn] - 1092[Al] - 3590[Nb] - 5181[Ti] + 258[Cr] + 664[Mo] ≥ 1380

[Relationship 2]

2853[C] + 95[Si] + 309[Mn] - 153[Al] + 4661[Nb] - 780[Ti] + 210[Cr] + 457[Mo] ≥ 1300

[Relationship 3]

-29[C] + 0.6[Si] - 7.3[Mn] + 7.8[Al] - 145.2[Nb] + 62.6[Ti] - 3.3[Cr] - 2.2[Mo] ≥ -24

(In relational expressions 1 to 3, each element means a weight content)

Another aspect of the present invention comprises the steps of preparing a steel slab that satisfies the above-described alloy composition and relational expressions 1 to 3; heating the steel slab in a temperature range of 1050 to 1300 °C; manufacturing a hot-rolled steel sheet by hot-rolling the heated steel slab at a temperature range of 800 to 1000° C.; winding the hot-rolled steel sheet in a temperature range of 400 to 700° C.; manufacturing a cold-rolled steel sheet by cold-rolling the rolled hot-rolled steel sheet at a total reduction ratio of 20 to 70%; annealing the cold-rolled steel sheet in a temperature range of 800 to 900° C.; cooling the continuously annealed cold-rolled steel sheet to a temperature range of 250 to 400° C.; And reheating and maintaining the cooled cold-rolled steel sheet,

The reheating and maintaining step provides a method for manufacturing an ultra-high strength steel sheet having excellent ductility, which is performed for 0.1 to 60 minutes in the temperature range of the cooled temperature +50 ° C. or more and the cooled temperature + 200 ° C. or less.

According to the present invention, it is possible to provide a steel sheet having excellent yield strength as well as tensile strength and improved ductility, and the steel sheet of the present invention has the advantage of guaranteeing formability and crash stability required for a steel sheet for cold forming.

1 shows a photograph of a microstructure of an inventive steel according to an embodiment of the present invention measured by SEM.
2 shows a photograph of a microstructure of a comparative steel according to an embodiment of the present invention measured by SEM.

The inventors of the present invention conducted extensive research to provide a steel sheet as an automotive material that has excellent tensile strength and ductility as well as yield strength, thereby guaranteeing formability and crash stability, and thus applicable to structural members requiring processing into complex shapes. .

As a result, it was confirmed that a steel sheet having a structure advantageous to securing target physical properties could be provided by optimizing the alloy component system and manufacturing conditions, and the present invention was completed.

In particular, the present invention controls the content relationship of specific elements among alloy components and optimizes the process conditions of steel sheets manufactured through a series of processes, thereby properly dispersing the soft phase and the hard phase. There is a feature of providing a steel sheet having a composite structure.

Hereinafter, the present invention will be described in detail.

The ultra-high strength steel sheet with excellent ductility according to one aspect of the present invention contains, by weight, carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 3.0%, aluminum ( Al): 1.0% or less (excluding 0%), Chromium (Cr): 1.0% or less, Molybdenum (Mo): 0.5% or less, Titanium (Ti): 0.1% or less, Niobium (Nb): 0.1% or less, Antimony ( Sb): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less.

Hereinafter, the reason for limiting the alloy composition of the steel sheet provided in the present invention as above will be described in detail.

Meanwhile, in the present invention, unless otherwise specified, the content of each element is based on weight, and the ratio of tissue is based on area.

Carbon (C): 0.1~0.2%

Carbon (C) is an element that greatly contributes to enhancing the strength of the steel sheet, and the C precipitates in the crystal grains of the steel sheet to induce solid solution strengthening and promotes the formation of martensite in the steel to strengthen the steel. In addition, C is an austenite stabilizing element and plays an important role in forming retained austenite. Specifically, as the amount of carbon (C) dissolved in austenite increases, austenite stability increases, and the fraction of austenite in the steel increases. This induces an increase in the fraction of martensite formed due to the transformation of the austenite, thereby obtaining an effect of improving the strength of the steel sheet, and some austenite remains at room temperature to remain as retained austenite.

In order to sufficiently obtain the above-mentioned effect, C can be added in an amount of 0.1% or more, but when the content exceeds 0.2%, the fraction of martensite phase excessively increases, and the fraction of ferrite phase having relatively excellent elongation and impact absorption energy decreases. do. This causes the ductility of the steel sheet to decrease and the possibility of occurrence of brittleness to increase.

Therefore, the C may be included in 0.1 to 0.2%, more advantageously, 0.12% or more and 0.18% or less.

Silicon (Si): 0.1 to 1.0%

Silicon (Si) is an element contributing to stabilization of retained austenite by suppressing precipitation of carbides in ferrite and inducing diffusion of carbon in ferrite to austenite.

In order to obtain the above-mentioned effect, it is advantageous to include Si at 0.1% or more, but when the content exceeds 1.0%, Si oxide is formed on the steel surface, which may hinder the effects of hot-dip plating and chemical conversion coating. there is

Accordingly, the Si may be included in an amount of 0.1 to 1.0%, more advantageously in an amount of 0.2% or more, and even more advantageously in an amount of 0.4% or more. On the other hand, the Si may be more advantageously included in 0.9% or less.

Manganese (Mn): 2.0 to 3.0%

Manganese (Mn) may act as an austenite stabilizing element similarly to C. Specifically, the Mn may contribute to increasing the fraction of martensite in the steel by reducing the critical cooling rate at which martensite is formed in the composite structure steel.

In order to sufficiently obtain the above-mentioned effects, it is advantageous to contain Mn at 2.0% or more, but when the content exceeds 3.0%, the weldability of the steel sheet decreases and there is a risk of deterioration in hot rolling properties. In addition, there is a problem of forming striped bands called Mn-Band to impair moldability and increase the risk of machining cracks.

Therefore, the Mn may be included in 2.0 to 3.0%, more advantageously, 2.2% or more and 2.8% or less.

Aluminum (Al): 1.0% or less

Aluminum (Al) is an element added for deoxidation of steel, and is a ferrite stabilizing element similar to Si. Al is an element useful for improving martensitic hardenability by distributing carbon in ferrite into austenite and improving ductility of steel sheet by effectively suppressing the precipitation of carbides in bainite when maintained in the bainite region. to be.

When the Al content exceeds 1.0%, continuous castability is deteriorated during steel casting operation, and inclusions are excessively formed, increasing the possibility of material defects of the annealed material.

Therefore, the Al may be included in an amount of 1.0% or less, excluding 0%. More advantageously, the Al may be included in 0.01% or more.

In the present invention, Al means soluble aluminum (Sol.Al).

Chromium (Cr): 1.0% or less

Chromium (Cr) is an element added to improve the hardenability of steel and secure high strength, and plays an important role in the formation of martensite. In addition, it is advantageous to manufacture a composite structure steel having high ductility by minimizing a decrease in elongation compared to an increase in strength.

When the content of Cr exceeds 1.0%, not only the above-mentioned effect is saturated, but also the hot rolling strength excessively increases, resulting in deterioration of cold rolling properties, and the martensite fraction after annealing greatly increases, resulting in a decrease in elongation. There is a problem with

Therefore, the Cr may be included in an amount of 1.0% or less, and it is revealed that there is no difficulty in securing the intended physical properties even if the Cr is not intentionally added.

Molybdenum (Mo): 0.5% or less

Molybdenum (Mo) is an element that forms carbides in steel, and can contribute to improving yield strength and tensile strength by forming fine carbides in steel by combining with Ti, Nb, etc. in steel. When the content of Mo exceeds 0.5%, there is a problem in that the elongation of the steel decreases and the manufacturing cost increases.

Therefore, the Mo may contain 0.5% or less, and it is revealed that there is no difficulty in securing the intended physical properties even if the Mo is not intentionally added.

Titanium (Ti): 0.1% or less

Titanium (Ti) may contribute to securing the yield strength and tensile strength of the steel by forming fine carbides in the steel, similar to the above Mo. In addition, by forming Ti nitride, N contained in the steel is precipitated as TiN, and the N is combined with Al to suppress precipitation as AlN, which has the effect of reducing the risk of cracking in the casting process.

When the content of Ti exceeds 0.1%, coarse carbides are precipitated, and there is a risk that the strength of the steel sheet may decrease due to the reduction of C in the steel. In addition, there is a possibility of causing nozzle clogging in the playing process due to the coarse carbide.

Therefore, the Ti may be included in an amount of 0.1% or less, and it is revealed that there is no difficulty in securing intended physical properties even if the Ti is not intentionally added.

Niobium (Nb): 0.1% or less

Niobium (Nb) is segregated at austenite grain boundaries to suppress coarsening of austenite crystal grains during annealing heat treatment, and may contribute to increasing the strength of a steel sheet by precipitating fine carbides on the grains.

When the content of Nb exceeds 0.1%, the content of C in the steel is reduced due to the formation of coarse carbides, which causes a problem in that the strength and elongation of the steel sheet are reduced, and the manufacturing cost of the steel is increased.

Therefore, the Nb may be included in an amount of 0.1% or less, and it is revealed that there is no difficulty in securing intended physical properties even if the Nb is not intentionally added.

Antimony (Sb): 0.1% or less

Antimony (Sb) is distributed on grain boundaries to delay the diffusion of oxidizing elements such as Mn, Si, and Al in steel through grain boundaries, thereby suppressing the surface thickening of oxides, and coarsening the surface thickening due to temperature rise and changes in the hot rolling process. has a beneficial effect on suppressing

When the content of Sb exceeds 0.1%, there is a problem in that workability is deteriorated and manufacturing cost increases.

Therefore, the Sb may be included in an amount of 0.1% or less, excluding 0%. More advantageously, the Sb may be included at 0.01% or more.

Phosphorus (P): 0.05% or less

Phosphorus (P) is segregated at grain boundaries and becomes a major cause of temper brittleness, and there is a problem of impairing weldability and toughness. Therefore, it is advantageous to control the content of P as low as possible to be as close to 0% as possible, but it is inevitably contained in the steel manufacturing process, and the process for reducing the content of P is difficult and the production cost increases due to the additional process. Therefore, it is effective to manage the upper limit.

Accordingly, the P may be limited to 0.05% or less, more advantageously limited to 0.03% or less. However, it should be noted that 0% can be excluded considering the level that is unavoidably added.

Sulfur (S): 0.02% or less

Sulfur (S) is an impurity that is unavoidably contained in steel together with the above-mentioned P, and has a problem of inhibiting ductility and weldability of the steel sheet. Therefore, it is advantageous to control the S content as low as possible to be as close to 0% as possible, but considering the cost and time consumed in the process for reducing the S content, it is effective to manage the upper limit.

Accordingly, the S may be limited to 0.02% or less, more advantageously, it may be limited to 0.01% or less. However, it should be noted that 0% can be excluded considering the level that is unavoidably added.

Nitrogen (N): 0.02% or less

Nitrogen (N) may combine with Al in steel to form an alumina-based non-metallic inclusion of AlN. The AlN deteriorates the playing quality and increases the brittleness of the steel sheet, thereby increasing the risk of fracture defects.

Accordingly, the N may be limited to 0.02% or less, more advantageously, it may be limited to 0.01% or less. However, considering the unavoidable inflow level, 0% can be excluded.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

In the steel sheet of the present invention having the above-described alloy composition, it is preferable that the content relationship between specific elements in the steel satisfies all of the following relationship expressions 1 to 3.

[Relationship 1]

1110[C] + 41.5[Si] + 575[Mn] - 1092[Al] - 3590[Nb] - 5181[Ti] + 258[Cr] + 664[Mo] ≥ 1380

[Relationship 2]

2853[C] + 95[Si] + 309[Mn] - 153[Al] + 4661[Nb] - 780[Ti] + 210[Cr] + 457[Mo] ≥ 1300

[Relationship 3]

-29[C] + 0.6[Si] - 7.3[Mn] + 7.8[Al] - 145.2[Nb] + 62.6[Ti] - 3.3[Cr] - 2.2[Mo] ≥ -24

(In relational expressions 1 to 3, each element means a weight content)

The relational expressions 1 and 2 above are component relations derived by quantifying the degree of contribution to reinforcing the yield strength and tensile strength of the steel sheet by controlling the microstructural phase fraction constituting the steel sheet and improving the solid solution strengthening effect. .

In the relational expressions 1 and 2, the C has a relatively large coefficient compared to the Si and Mn, and this is because the C is dissolved in the grains of the steel sheet and greatly contributes to the improvement of strength. On the other hand, Si has a relatively smaller coefficient than C, which is due to a smaller effect contributing to solid solution strengthening than C. In addition, the Al has a negative modulus value, which contributes to solid solution strengthening, but causes a decrease in strength by retaining dual phase region ferrite during annealing or promoting ferrite transformation during subsequent cooling. This is due to the greater effect of On the other hand, the Cr and Mo are representative hardenable elements, and suppress ferrite transformation during cooling after annealing, so they have an effect of improving strength, and are represented by positive values.

On the other hand, since Ti and Nb are elements that contribute to the improvement of strength by forming fine carbides, they may have positive coefficient values in the strength relationship expression according to the component elements. However, since fine carbides are formed and the amount of solid-solution carbon is reduced, the solid-solution strengthening effect of carbon is reduced. Accordingly, Ti and Nb have a positive coefficient value when the precipitation strengthening effect is dominant due to their addition, whereas a negative coefficient value is obtained when the solid solution strengthening effect of carbon due to the precipitation of carbides is dominant. .

Relational Expression 3 is a component relational expression derived by quantifying the degree of contribution to improving the elongation of the steel sheet as well as the enhancement of the solid solution strengthening effect by specific elements.

In general, considering that the elongation tends to decrease as the strength of the steel sheet increases, the coefficients of each element in the above relational expression 3 tend to be opposite to the above relational expressions 1 and 2.

Specifically, the C and Mn are advantageous for strength improvement due to the solid solution strengthening effect, but since the elongation tends to decrease due to the strength improvement, they have negative coefficient values. On the other hand, Al has a positive coefficient value because it is effective in increasing the elongation. On the other hand, in the case of Si, since it contributes to securing the retained austenite as well as the strength improvement effect by solid solution strengthening, it also has a positive coefficient value in relational expression 3.

If any one of the relational expressions 1 to 3 proposed in the present invention is not satisfied, there is a problem in that any one or more of the physical properties of the steel sheet, in particular, tensile strength, yield strength, and elongation are inferior. It should be noted that this is proven from the examples described later.

The steel sheet of the present invention having the above-described alloy component system includes a soft phase and a hard phase appropriately dispersed in a microstructure, in particular, 3 to 20% area fraction of ferrite, 1 to 10% of retained austenite, and 1 to 30% of bay. It is characterized by containing nite, 30-70% tempered martensite and the balance fresh martensite.

The ferrite (Ferrite) is an allotrope of iron (Fe) having a body centered cubic structure (BCC), and unlike martensite and bainite, it has a soft structure. Accordingly, the elongation is higher than that of the bainite and martensite phases, and the impact absorption energy is excellent.

When the fraction of such ferrite exceeds 20%, a soft tissue in the steel sheet is excessively formed to promote plastic deformation, which causes a decrease in the yield strength of the steel sheet. On the other hand, if the fraction of the ferrite is less than 3%, there is a problem in that the elongation of the steel sheet decreases and formability deteriorates.

Therefore, the ferrite may be included in an area fraction of 3 to 20%, more advantageously, 5 to 15%.

The retained austenite remains in the steel without being transformed into martensite or bainite in a series of heat treatment processes (in the present invention, [annealing-cooling-reheating and maintenance] processes) during the manufacturing process of the steel sheet. It means the night structure and plays a role in adjusting the balance between the strength and elongation of the steel sheet.

In general, when the strength of the steel sheet increases, the elongation rate decreases, resulting in lower formability, and when the elongation rate of the steel sheet increases, the strength decreases, making it difficult to secure physical properties required as structural members. TS) × Elongation (El) value is increased, so it is useful for improving the balance between strength and elongation.

In order to sufficiently obtain the above-described effect, the retained austenite phase may be included in an area fraction of 1% or more, but when the fraction exceeds 10%, the sensitivity of liquid metal embrittlement increases, resulting in poor spot weldability.

Therefore, the retained austenite may be included in an area fraction of 1 to 10%, more advantageously, 3 to 9%.

The bainite may contribute to improving workability by reducing the difference in strength between structures in steel. That is, it serves to prevent cracks, defects and fractures from occurring in the steel sheet due to the difference in hardness between ferrite and retained austenite phases having relatively low hardness and tempered martensite and fresh martensite having relatively high hardness.

In order to sufficiently obtain the above-mentioned effect, the area fraction may be included in an area fraction of 1% or more, more advantageously, 5% or more. However, when the fraction exceeds 30%, the fraction of fresh martensite is reduced, making it difficult to secure the target level of strength.

Therefore, the bainite may be included in an area fraction of 1 to 30%.

The tempered martensite refers to a structure obtained by tempering a martensite phase obtained by quenching austenite at a temperature of about 500° C. to soften it. This tempered martensite phase has higher strength than the aforementioned structures, and thus greatly contributes to improving the yield strength and tensile strength of the steel sheet. In addition, since carbon in the martensite obtained by quenching is distributed to surrounding austenite during the tempering process to increase the thermal stability of austenite so that it can remain at room temperature, there is an effect of improving the elongation of the steel sheet.

In order to sufficiently obtain the above effects, it is preferable to include the tempered martensite phase at an area fraction of 30% or more. However, when the fraction exceeds 70%, there is a problem in that the fraction of the retained austenite phase is relatively reduced.

Therefore, the tempered martensite may be included in an area fraction of 30 to 70%.

The remaining structure other than the ferrite, retained austenite, bainite, and tempered martensite phase may include a fresh martensite phase.

The fresh martensite phase is a structure obtained in the process of final cooling to room temperature, and has the highest strength, so it greatly contributes to improving the yield strength and tensile strength of the steel sheet. The fraction of the fresh martensite phase is not particularly limited, but as an example, it should be noted that the area fraction may include 3% or more.

As described above, the steel sheet of the present invention is characterized by excellent tensile strength, yield strength and elongation due to the appropriate formation of the soft phase and the hard phase. have.

Meanwhile, the steel sheet of the present invention may be a cold-rolled steel sheet, and may be a hot-dip galvanized steel sheet including a zinc-based plating layer on at least one surface of the cold-rolled steel sheet, and an alloyed hot-dip galvanized steel sheet obtained by alloying the hot-dip galvanized steel sheet.

Although not particularly limited, the zinc-based plating layer may be a zinc plating layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and/or magnesium in addition to zinc.

Hereinafter, a method for manufacturing an ultra-high strength steel sheet having excellent ductility provided by the present invention, which is another aspect of the present invention, will be described in detail.

Briefly, the present invention can manufacture a desired steel sheet through the process of [steel slab reheating - hot rolling - winding - cold rolling - continuous annealing - cooling - reheating and holding], and then [dip galvanizing - alloying heat treatment] Further steps can be performed.

Conditions for each step are described in detail below.

[Heating of steel slabs]

First, after preparing a steel slab that satisfies all of the above-mentioned alloy components, it can be heated. This process is performed to smoothly perform the subsequent hot rolling process and to sufficiently obtain target physical properties of the steel sheet.

The heating process may be performed in a temperature range of 1050 to 1300 °C. If the heating temperature is less than 1050 ° C., there is a problem in that the load applied to the roller during hot rolling increases rapidly due to increased friction between the steel sheet and the rolling mill. On the other hand, when the temperature exceeds 1300 ° C., energy costs required for temperature increase increase, and the amount of surface scale increases, which can lead to material loss.

Therefore, the heating process can be carried out in a temperature range of 1050 ~ 1300 ℃, more advantageously can be carried out in a temperature range of 1090 ~ 1250 ℃.

[Hot rolling]

The steel slab heated according to the above may be hot-rolled to manufacture a hot-rolled steel sheet, and at this time, finish hot-rolling may be performed in a temperature range of 800 to 1000 ° C.

By performing finish hot rolling in the above-mentioned temperature range, the effect of simultaneously improving the rigidity and formability of the steel sheet can be obtained. However, if the temperature is less than 800 ° C., there is a problem in that the load due to rolling greatly increases due to increased friction between the steel sheet and the rolling mill due to rolling in the ferrite region. This causes the formation of coarse crystal grains on the surface of the steel sheet in the subsequent coiling or cold rolling process by forming excessive dislocations, which causes a decrease in strength. On the other hand, when the temperature exceeds 1000 ° C., the size of ferrite crystal grains increases and strength also decreases. In addition, scale is generated on the surface of the hot-rolled steel sheet, which may cause surface defects and shorten the life of the rolling roll.

Therefore, during the hot rolling, the finish hot rolling may be performed in a temperature range of 800 to 1000 ° C, more advantageously in a temperature range of 850 to 950 ° C.

[wind up]

The hot-rolled steel sheet manufactured according to the above may be wound, and at this time, it may be performed in a temperature range of 400 to 700 ° C.

If the coiling temperature is less than 400 ° C., the strength of the hot-rolled steel sheet may be excessively high, causing a rolling load during subsequent cold rolling. In addition, excessive cost and time are required to cool the hot-rolled steel sheet to the coiling temperature, which causes an increase in process cost. On the other hand, when the temperature exceeds 700 ° C., scale is excessively generated on the surface of the hot-rolled steel sheet, which is likely to cause surface defects and deteriorates plating properties.

Therefore, the winding process may be performed in a temperature range of 400 to 700°C, and more advantageously, in a temperature range of 500 to 700°C.

[Cooling]

The coiled hot-rolled steel sheet may be cooled to room temperature. At this time, the cooling rate is not particularly limited, but may be air-cooled.

[Cold Rolling]

Thereafter, the hot-rolled steel sheet may be cold-rolled to produce a cold-rolled steel sheet, and at this time, it may be performed at a cold rolling reduction of 20 to 70%.

If the cold rolling reduction is less than 20% during the cold rolling, it is difficult to obtain a steel sheet having a target thickness and it is difficult to correct the shape of the steel sheet. On the other hand, if it exceeds 70%, cracks are likely to occur at the edge of the steel sheet, and there is a problem of bringing a cold rolling load. In addition, there is a concern that coarse ferrite may be formed during subsequent continuous annealing due to excessive load on the surface of the steel sheet.

Therefore, the cold rolling can be performed at a cold rolling reduction of 20 to 70%, more advantageously at a cold rolling reduction of 30 to 60%.

Meanwhile, prior to performing the cold rolling, pickling treatment may be performed on the hot-rolled steel sheet. The pickling treatment is a process of removing the scale formed on the surface of the hot-rolled steel sheet using hydrochloric acid (HCl) or the like, and can be performed under normal conditions, so the conditions are not particularly limited.

[annealing]

The cold-rolled steel sheet manufactured according to the above may be annealed, and as one example, a continuous annealing process may be performed, but is not limited thereto, and any of the known annealing methods may be used.

In the present invention, ferrite formed in the cold-rolled steel sheet may be recrystallized through the annealing process, and the fractions of ferrite and austenite in the steel may be adjusted. At this time, the strength of the steel sheet manufactured after the final heat treatment (referred to as a reheating process to be described later) is determined by the fraction of each phase formed. In general, as the fraction of austenite increases, the fraction of martensite or bainite transformed from austenite increases. The increase in strength tends to improve the strength of the steel sheet. However, in the present invention, the strength can be additionally controlled by a series of heat treatment conditions to be described later.

In addition, carbon (C) in the steel can be distributed through the annealing process, thereby increasing the amount of carbon (C) contained in austenite to have an austenite phase of up to 10 area% even at room temperature.

The annealing process may be performed in a temperature range of 800 ~ 900 ℃.

If the temperature during the annealing is less than 800 ° C., the fraction of austenite formed through the annealing process decreases, so there is a concern that the fraction of tempered martensite, bainite, and fresh martensite formed during heat treatment described later may not be sufficient. This may cause the yield strength and tensile strength of the final steel sheet to decrease. On the other hand, when the temperature exceeds 900 ° C., the fraction of austenite in the steel sheet becomes excessively high, and there is a problem that some austenite is transformed into ferrite during a heat treatment process described later. In addition, there is a concern that the carbon concentration of retained austenite is lowered and the mechanical stability is reduced, and in this case, the elongation of the steel sheet is reduced. In addition, moisture generated as Fe in the steel is oxidized during the annealing process reacts with Si, Mn, and Al in the steel to increase the possibility of forming an oxide film on the steel sheet. The oxide film inhibits the wettability of Zn during hot-dip galvanizing, so that the surface quality of the steel sheet may be deteriorated.

Therefore, the annealing process can be carried out in a temperature range of 800 ~ 900 ℃, more advantageously can be carried out in a temperature range of 820 ~ 870 ℃.

[Cooling]

As described above, the cold-rolled steel sheet after the annealing process may be cooled.

In the present invention, quenched martensite can be formed by performing cooling on the annealed cold-rolled steel sheet, and for this purpose, the cooling is preferably performed below the martensitic transformation initiation temperature (Ms). More preferably, it can be carried out to the temperature range of 250-400 degreeC.

During the cooling, the lower the temperature is, the higher the fraction of quenched martensite is, thereby improving the strength of the steel sheet. In addition, supersaturated carbon in martensite is distributed to surrounding austenite in a subsequent heat treatment process to increase the stability of retained austenite, and as a result, elongation can be improved.

However, when the cooling temperature is less than 250° C., the fraction of quenched martensite excessively increases, so that the fraction of retained austenite decreases and the shape of the steel sheet deteriorates. On the other hand, when the temperature exceeds 400 ° C., quenching martensite is not sufficiently formed, making it difficult to expect the above-described effect.

When cooling to the above-described temperature range, it may be performed at an average cooling rate of 2 to 50 ° C./s. If the cooling rate is less than 2 ° C / s, ferrite is additionally transformed during cooling to cause a decrease in strength, while if the rate exceeds 50 ° C / s and rapidly cooled, cooling deviation occurs by position of the steel sheet. There is a problem that the shape of the steel sheet is inferior. In performing cooling at the cooling rate described above, the cooling method is not particularly limited. As one example, the cooling may be a single cooling method of cooling to the cooling end temperature at the initially set cooling rate, and as another example, step-by cooling performed by slow cooling to a certain section and then hard cooling to the cooling end temperature. step cooling) method, but it should be noted that it is not limited thereto.

Meanwhile, a process of maintaining the cooled temperature for a certain period of time may be performed, and during this process, an isothermal transformation phase may be additionally introduced to obtain an effect of accelerating the transformation of bainite in a subsequent process. To this end, the holding process may be performed for 0.1 to 60 minutes.

[Reheat and maintain]

The cooled cold-rolled steel sheet, and furthermore, the cooled and maintained cold-rolled steel sheet may be tempered by reheating the cooled and maintained cold-rolled steel sheet to a temperature range of about 50 to 200 ° C. higher than the cooling temperature, and then holding it for a predetermined time.

By reheating the cooled cold-rolled steel sheet, the quenched martensite phase formed in the cooling process is tempered and transformed into tempered martensite, and the tempered martensite has an advantage of high yield strength because carbon is fixed to dislocations. In addition, in the tempering process, supersaturated carbon (C) in the quenched martensite is redistributed into surrounding austenite, or bainite transformation is induced to improve the stability of retained austenite, thereby obtaining an effect of improving elongation.

Since the fixation of the dislocations and the distribution of carbon into austenite occur more smoothly as the tempering temperature is higher, it is necessary to reheat at a temperature 50 ° C. or more higher than the cooling temperature (cooled temperature + 50 ° C. or more). However, if the temperature is excessively high, cementite is generated in the quenched martensite, and the strength of the steel sheet is reduced due to coarsening, and the carbon redistribution effect to austenite is reduced, so it is difficult to expect an improvement in elongation. Considering this, the reheating may be limited to the cooled temperature + 200 ° C or less.

After reheating the cold-rolled steel sheet cooled to the above-mentioned temperature range, it is preferable to sufficiently implement the above-mentioned effect by maintaining it at that temperature for 0.1 to 60 minutes.

If the holding time is excessive and exceeds 60 minutes, equilibrium ferrite and cementite are formed at the holding temperature to reduce the strength of the steel sheet, and if the holding time is less than 0.1 minute, the intended effect cannot be obtained.

After completing the process of reheating and maintaining the cooled cold-rolled steel sheet as described above, it can be cooled to room temperature under normal conditions, and finally a steel sheet having a structure in which a certain fraction of the soft phase and the hard phase are properly distributed can be obtained. .

Specifically, it consists of 3-20% area fraction of ferrite, 1-10% of retained austenite, 1-30% of bainite, 30-70% of tempered martensite and the remainder of fresh martensite. A steel sheet having a microstructure can be obtained, and the steel sheet of the present invention has excellent yield strength and tensile strength, and has improved ductility.

The process of cooling to room temperature is not particularly limited, but may be performed by air cooling as an example. However, it will be obvious that it can be replaced with known cooling methods such as water cooling, oil cooling, and furnace cooling.

Meanwhile, a plated steel sheet having a plating layer on at least one side thereof may be manufactured by plating the cold-rolled steel sheet after completing a series of heat treatment processes as described below.

[Hot-dip galvanizing]

A hot-dip galvanized steel sheet may be manufactured by immersing the steel sheet manufactured through the above-described series of processes in a hot-dip zinc-based plating bath.

At this time, the hot-dip galvanizing may be performed under normal conditions, but may be performed in a temperature range of 430 to 490 ° C as an example. In addition, the composition of the hot-dip zinc-based plating bath is not particularly limited during the hot-dip galvanizing, and may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, and the like.

[Alloy heat treatment]

If necessary, an alloyed hot-dip galvanized steel sheet may be obtained by subjecting the hot-dip galvanized steel sheet to alloying heat treatment.

In the present invention, the alloying heat treatment process conditions are not particularly limited, and may be ordinary conditions. As an example, the alloying heat treatment process may be performed in a temperature range of 480 to 600 °C.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating 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 the matters reasonably inferred therefrom.

(Example)

After heating 30 kg of the slab having the alloy composition of Table 1 at a temperature of 1200 ° C. for 1 hour, the heated slab was hot-rolled at 900 ° C. to prepare a hot-rolled steel sheet. Thereafter, each hot-rolled steel sheet was charged into a furnace preheated to 600° C., maintained for 1 hour, and then hot-rolled coiling in which the furnace was cooled was simulated. Thereafter, after cooling (air cooling) to room temperature, cold rolling was performed at a cold rolling reduction ratio of 45% to prepare a cold rolled steel sheet.

Each of the cold-rolled steel sheets manufactured according to the above was continuously annealed for 1 minute at the temperature T1 (° C.) shown in Table 2, cooled to the temperature T2 (° C.), maintained for 10 seconds, and maintained at the temperature T3 (° C.). ), maintained for 1 minute, and then cooled (air-cooled) to room temperature to prepare a final steel sheet.

Mechanical properties and internal structure were measured for each steel sheet manufactured through all of the above processes, and the results are shown in Table 3 below.

As the mechanical properties, yield strength (YS), tensile strength (TS), and elongation (El) were measured, and measured through a universal tensile tester using an ASTM tensile test piece.

For the internal structure, the specimen was polished, nital etched, and then the area of each phase was calculated using a scanning electron microscope (SEM).

steel grade Alloy composition (% by weight) Relation 1 Relation 2 Relation 3 C Si Mn Sol. Al Nb Ti Cr Mo Sb P S N river 1 0.14 0.6 2.5 0.035 0.02 0.04 0.2 0.2 0.03 0.010 0.005 0.002 1485 1419 -23.2 river 2 0.14 0.6 2.4 0.035 0.02 0.04 0.4 0.2 0.03 0.008 0.005 0.003 1479 1430 -23.1 river 3 0.14 0.6 2.4 0.039 0.02 0.04 0.2 0.2 0.03 0.009 0.006 0.004 1423 1387 -22.4 river 4 0.14 0.6 2.5 0.038 0.02 0.04 0 0.21 0.03 0.011 0.009 0.002 1437 1381 -22.5 river 5 0.14 0.5 2.4 0.30 0 0 0.6 0.2 0.03 0.011 0.004 0.003 1516 1360 -21.4 river 6 0.14 0.5 2.4 0.40 0 0 0.7 0.2 0.03 0.012 0.009 0.003 1433 1366 -20.9 river 7 0.14 0.6 2.4 0.40 0 0 0.7 0.2 0.03 0.009 0.007 0.002 1437 1375 -20.9 river 8 0.16 0.6 2.3 0.40 0 0 0.7 0.2 0.03 0.012 0.004 0.004 1402 1401 -20.7 river 9 0.10 0.5 2.4 0.50 0 0 1.0 0.1 0.03 0.009 0.006 0.004 1290 1254 -19.7 river 10 0.14 0.5 2.4 0.30 0.02 0.04 0.6 0.2 0.03 0.009 0.006 0.004 1237 1422 -21.8 River 11 0.18 0.8 2.3 0.025 0.02 0.015 0.5 0.2 0.03 0.008 0.006 0.005 1640 1574 -25.4

steel grade annealing temperature
T1 (℃) cooling temperature
T2 (℃) reheat temperature
T2 (℃) division river 1 850 300 450 Invention example 1 river 2 830 320 460 Invention example 2 river 3 870 300 430 Inventive Example 3 river 4 830 280 460 Inventive example 4 river 4 850 320 470 Inventive Example 5 river 5 850 300 440 Inventive example 6 river 6 840 310 460 Inventive Example 7 river 6 860 300 460 Inventive Example 8 river 7 860 300 460 Inventive Example 9 river 8 850 300 460 Inventive Example 10 river 8 880 320 460 Inventive Example 11 river 9 850 300 460 Comparative Example 1 river 10 850 300 460 Comparative Example 2 river 5 780 300 460 Comparative Example 3 river 6 850 500 550 Comparative Example 4 river 6 850 300 300 Comparative Example 5 river 7 780 300 460 Comparative Example 6 River 11 850 300 460 Comparative Example 7

(In Table 2, steels 9, 10 and 11 are classified as comparative examples because the alloy composition system is out of the present invention.)

division mechanical properties microstructure (% area) YS(MPa) TS(MPa) El(%) F R-A T-M B F-M Invention Example 1 817 1064 13.4 19 5 54 17 5 Invention example 2 788 1052 13.9 15 6 61 8 10 Inventive Example 3 713 1045 13.2 11 4 49 16 20 Inventive Example 4 709 1016 13.5 15 7 65 10 3 Inventive Example 5 740 1032 13.2 10 7 55 17 11 Inventive example 6 862 1051 13.2 9 8 61 12 10 Inventive Example 7 780 1026 15.0 11 6 57 20 6 Inventive Example 8 776 1024 13.2 12 5 61 15 7 Inventive Example 9 800 1044 14.5 8 6 49 17 20 Inventive Example 10 736 1060 14.8 11 8 56 11 14 Inventive Example 11 789 1071 14.8 9 9 41 16 25 Comparative Example 1 520 886 15.7 31 One 22 11 35 Comparative Example 2 606 1106 12.6 25 3 23 15 34 Comparative Example 3 650 865 17.2 23 2 10 10 55 Comparative Example 4 900 1100 8.5 10 0 0 20 70 Comparative Example 5 801 1040 11.2 10 0 20 0 70 Comparative Example 6 670 875 16.5 22 One 11 14 52 Comparative Example 7 910 1207 9.5 7 0 75 8 10

As shown in Tables 1 to 3, inventive examples 1 to 11 satisfying all of the alloy component system and manufacturing conditions proposed in the present invention secured the target physical properties by forming the intended structure.

On the other hand, it can be seen that Comparative Examples 1 and 2, which do not satisfy the relational expressions 1 and 2 of the component relations proposed in the present invention, do not secure at least one physical property of yield strength and tensile strength at a target level. In addition, it can be confirmed that Comparative Example 7, which does not satisfy the relational expression 3 among the component relations, has a greatly inferior elongation.

From this, the relational expression 1 characterized in the present invention contributes to the enhancement of the yield strength by the fraction of the microstructure of the steel sheet and the solid solution strengthening effect, the relational expression 2 contributes to the improvement of the tensile strength of the steel sheet, and the relational expression 3 contributes to the improvement of the ductility of the steel sheet It has been proven to contribute to

In other words, when the relational expressions 1 and 2 of the present invention are not satisfied, the strength of the steel sheet is inferior, and when the relational expression 3 is not satisfied, the ductility of the steel sheet is inferior.

On the other hand, while the alloy component system proposed in the present invention is satisfied, in Comparative Examples 3 to 6 in which the heat treatment conditions deviate from the present invention, the soft phase and the hard phase were not properly formed as intended, and as a result, strength and ductility were improved in all examples. It was not possible to ensure excellent compatibility.

1 shows a photograph of the structure of Inventive Example 1, in which ferrite, retained austenite, tempered martensite, and bainite are formed within a target fraction range, and fresh martensite phase is formed as the rest of the structure. It can be confirmed that .

Figure 2 shows a photograph of the structure of Comparative Example 6, and it can be confirmed that the tempered martensite phase was not formed in a target fraction, the retained austenite phase was not sufficiently secured, and the fraction of the fresh martensite phase was formed relatively high. can

Claims (10)

In % by weight, carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 3.0%, aluminum (Al): 1.0% or less (excluding 0%), chromium ( Cr): 1.0% or less, Molybdenum (Mo): 0.5% or less, Titanium (Ti): 0.1% or less, Niobium (Nb): 0.1% or less, Antimony (Sb): 0.1% or less (excluding 0%), Phosphorus ( P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, including the remaining Fe and other unavoidable impurities,
An ultra-high strength steel sheet having excellent ductility, characterized in that it satisfies the following relational expressions 1 to 3.

[Relationship 1]
1110[C] + 41.5[Si] + 575[Mn] - 1092[Al] - 3590[Nb] - 5181[Ti] + 258[Cr] + 664[Mo] ≥ 1380
[Relationship 2]
2853[C] + 95[Si] + 309[Mn] - 153[Al] + 4661[Nb] - 780[Ti] + 210[Cr] + 457[Mo] ≥ 1300
[Relationship 3]
-29[C] + 0.6[Si] - 7.3[Mn] + 7.8[Al] - 145.2[Nb] + 62.6[Ti] - 3.3[Cr] - 2.2[Mo] ≥ -24
(In relational expressions 1 to 3, each element means a weight content)
According to claim 1,
The steel sheet has a microstructure, with an area fraction of 3 to 20% ferrite, 1 to 10% retained austenite, 1 to 30% bainite, 30 to 70% tempered martensite, and the remainder fresh martensite. Ultra-high-strength steel sheet with excellent ductility comprising a.
According to claim 2,
The steel sheet is an ultra-high strength steel sheet having excellent ductility including a fresh martensite phase in an area fraction of 3% or more.
According to claim 1,
The steel sheet is an ultra-high strength steel sheet having a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 13% or more.
According to claim 1,
The steel sheet is a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet, which is any one of ultra-high strength steel sheet with excellent ductility.
In % by weight, carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 3.0%, aluminum (Al): 1.0% or less (excluding 0%), chromium ( Cr): 1.0% or less, Molybdenum (Mo): 0.5% or less, Titanium (Ti): 0.1% or less, Niobium (Nb): 0.1% or less, Antimony (Sb): 0.1% or less (excluding 0%), Phosphorus ( P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, including the remaining Fe and other unavoidable impurities, and satisfying the following relations 1 to 3 Preparing a steel slab;
heating the steel slab in a temperature range of 1050 to 1300 °C;
manufacturing a hot-rolled steel sheet by hot-rolling the heated steel slab at a temperature range of 800 to 1000° C.;
winding the hot-rolled steel sheet in a temperature range of 400 to 700° C.;
manufacturing a cold-rolled steel sheet by cold-rolling the rolled hot-rolled steel sheet at a total reduction ratio of 20 to 70%;
annealing the cold-rolled steel sheet in a temperature range of 800 to 900° C.;
cooling the annealed cold-rolled steel sheet to a temperature range of 250 to 400° C.; and
Reheating and maintaining the cooled cold-rolled steel sheet,
The reheating and maintaining step is performed for 0.1 to 60 minutes in the temperature range of the cooled temperature +50 ° C. or more to the cooled temperature + 200 ° C. or less.

[Relationship 1]
1110[C] + 41.5[Si] + 575[Mn] - 1092[Al] - 3590[Nb] - 5181[Ti] + 258[Cr] + 664[Mo] ≥ 1380
[Relationship 2]
2853[C] + 95[Si] + 309[Mn] - 153[Al] + 4661[Nb] - 780[Ti] + 210[Cr] + 457[Mo] ≥ 1300
[Relationship 3]
-29[C] + 0.6[Si] - 7.3[Mn] + 7.8[Al] - 145.2[Nb] + 62.6[Ti] - 3.3[Cr] - 2.2[Mo] ≥ -24
(In relational expressions 1 to 3, each element means a weight content)
According to claim 6,
The cooling of the cold-rolled steel sheet is a method for producing an ultra-high strength steel sheet having excellent ductility to be performed at a cooling rate of 2 to 50 ° C / s.
According to claim 6,
Method for producing an ultra-high strength steel sheet with excellent ductility further comprising the step of maintaining the cooled cold-rolled steel sheet in the cooled temperature range for 0.1 to 60 minutes before reheating.
According to claim 6,
A method for producing an ultra-high strength steel sheet with excellent ductility further comprising the step of hot-dip galvanizing after the reheating and maintenance.
According to claim 9,
Method for producing an ultra-high strength steel sheet with excellent ductility further comprising the step of alloying heat treatment after the hot-dip galvanizing.

Images