KR20230056822A - Ultra-high strength steel sheet having excellent ductility and mathod of manufacturing the same - Google Patents

Abstract

The present invention relates to an ultra-high strength steel sheet having excellent ductility and a method of manufacturing the same, and more specifically, to a steel sheet having excellent strength and excellent ductility and used in vehicles and a method of manufacturing the same. The ultra-high strength steel sheet of the present invention comprises 0.15 to 0.22 wt% of carbon (C), 2.6 to 3.2 wt% of manganese (Mn), 0.3 to 0.7 wt% of silicon (Si), 0.3 to 0.7 wt% of acid soluble aluminum (sol.Al), 0.05 wt% or less of phosphorus (P), 0.01 wt% or less of sulfur (S), 0.01 wt% or less of nitrogen (N), and the balance iron (Fe) and other inevitable impurities.

Description

Ultra high strength steel sheet with excellent ductility and its manufacturing method {ULTRA-HIGH STRENGTH STEEL SHEET HAVING EXCELLENT DUCTILITY AND MATHOD OF MANUFACTURING THE SAME}

The present invention relates to an ultra-high strength steel sheet with excellent ductility and a method for manufacturing the same, and more particularly, to a steel sheet with excellent strength and ductility that can be used for automobiles and a method for manufacturing the same.

Recently, as carbon dioxide emission standards have become more stringent, the need for fuel efficiency improvement has increased, and automobile companies are making great efforts to reduce weight. In addition, as the market share of electric vehicles gradually increases, the application rate of high-strength steel, such as battery cases, in accordance with safety regulations, in addition to structural members that have been inherited from existing internal combustion engines, continues to increase. Therefore, there is a need for an alternative to secure passenger safety at the same time by reducing the weight of the steel plate for passenger cars and improving crash resistance.

In order to reduce the weight of the car body, the thickness of the steel plate must be reduced within the range of securing passenger safety. To this end, the yield strength to tensile strength of structural members such as seat rails and members The application rate of excellent high-strength steel is increasing. On the other hand, in general, as the strength increases, formability decreases due to a decrease in elongation. However, since most of these structural member parts pass through a molding process, not only crash stability but also moldability must be secured, so it is essential to improve ductility as well as a high yield ratio.

During continuous annealing, water cooling to a low temperature is a typical manufacturing method for increasing yield strength. After cracking in the annealing process, martensite is tempered by immersion in water. At this time, through the tempered martensite structure generated in the steel sheet, not only high yield strength, but also improvement in formability due to the decrease in hardness difference between phases can be promoted. can

Patent Document 1 proposes a technique for manufacturing a steel material having 80 to 97 area% or more of martensite by continuously annealing a steel material having 0.18% or more of carbon, water-cooling to room temperature, and overaging at 120 to 300 ° C. for 1 to 15 minutes. As such, when high-strength steel is manufactured by tempering after water cooling, the yield ratio is high, but during water cooling, cracks occur during molding due to deterioration of shape quality of the coil and material deviation due to temperature deviation in the width/length direction. sexuality may deteriorate.

Patent Document 2 proposed an ultra-high strength steel sheet having a tensile strength of 1400 MPa or more and a yield ratio of 0.7 or more and excellent crash characteristics. However, since a large amount of alloy is added, such as 0.1 to 0.3% of carbon and 6 to 12% of manganese, playability and spot weldability are inferior.

In addition, Patent Document 3 proposed an ultra-high strength steel sheet having a yield strength of 1300 MPa or more and a tensile strength of 1500 MPa or more. Such a steel plate has the advantage of excellent crash resistance properties for automobiles due to its excellent strength, but since the elongation is less than 8%, there is a limit in that it can be used only for manufacturing simple shaped parts such as roll forming.

Therefore, it is required to develop a high-strength steel sheet capable of solving the above-mentioned problems and simultaneously securing a high yield ratio and ductility.

Japanese Unexamined Patent Publication No. 1992-289120 Republic of Korea Patent Publication No. 2020-0027387 Republic of Korea Patent Publication No. 2014-0097332

According to one aspect of the present invention, it is intended to provide an ultra-high strength steel sheet with excellent ductility and a manufacturing method thereof.

The object of the present invention is not limited to the above. A person skilled in the art will have no difficulty understanding the further subject matter of the present invention from the general content of this specification.

One aspect of the present invention, by weight, carbon (C): 0.15 ~ 0.22%, manganese (Mn): 2.6 ~ 3.2%, silicon (Si): 0.3 ~ 0.7%, aluminum for acid value (sol.Al): 0.3~0.7%, phosphorus (P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the balance including iron (Fe) and other unavoidable impurities,

The R1 value defined in the following relational expression 1 is 0.7 to 1.1,

The R2 value defined in the following relational expression 2 is 4.0 to 6.0,

The microstructure contains 80 area% or more of martensite and less than 20 area% of ferrite, bainite and retained austenite, and the ratio of the tempered martensite fraction to the total martensite fraction is 92 to 97%,

A steel sheet having an elongation of 10.0% or more can be provided.

[Relationship 1]

R1 = [Si]+[Al]+([P]/[Mn])

(Where [Si], [Al], [P] and [Mn] are the weight percent of each element.)

[Relationship 2]

R2 = [C]+1.5[Mn]+2[Cr]+[B]

(Where [C], [Mn], [Cr] and [B] are the weight percent of each element.)

The steel sheet contains, in weight percent, chromium (Cr): 0.7% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V): 0.1% or less, and boron (B): 0.0025 It may further include one or more selected from % or less.

The steel sheet may include 3 area% or more of ferrite as a microstructure.

The steel sheet may include 3 area% or more of retained austenite as a microstructure.

The steel sheet includes, in weight percent, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, and includes Ti or Nb-based fine precipitates having an average diameter of 50 nm or less, 10 ¹² / m ² or more.

The steel sheet may have a tensile strength of 1500 MPa or more, a yield strength of 1000 MPa or more, a yield ratio of 0.68 or more, a product of yield strength and elongation of 10000 MPa %, and hole expandability (HER) of 35% or more.

Another aspect of the present invention, in weight percent, carbon (C): 0.15 ~ 0.22%, manganese (Mn): 2.6 ~ 3.2%, silicon (Si): 0.3 ~ 0.7%, aluminum for acid value (sol.Al) : 0.3 to 0.7%, phosphorus (P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the balance including iron (Fe) and other unavoidable impurities, in the following relational expression 1 Preparing a cold-rolled steel sheet having a defined R1 value of 0.7 to 1.1 and an R2 value defined in the following relational expression 2 of 4.0 to 6.0;

Continuous annealing step of maintaining the cold-rolled steel sheet in a temperature range of 750 to 900 ° C. for 30 to 300 seconds;

firstly cooling the continuously annealed steel sheet to a temperature range of 600 to 720°C at an average cooling rate of 10°C/s or less;

Secondary cooling of the primary cooled steel sheet to a temperature range of 50 to 300 °C at an average cooling rate of 30 to 70 °C/s;

An overaging treatment step of maintaining the secondary cooled steel sheet in a temperature range of 100 to 400 ° C for 450 to 650 seconds; and

It is possible to provide a method for manufacturing a steel sheet comprising the step of cooling the overaged steel sheet to room temperature.

[Relationship 1]

R1 = [Si]+[Al]+([P]/[Mn])

(Where [Si], [Al], [P] and [Mn] are the weight percent of each element.)

[Relationship 2]

R2 = [C]+1.5[Mn]+2[Cr]+[B]

(Where [C], [Mn], [Cr] and [B] are the weight percent of each element.)

The steel sheet contains, in weight percent, chromium (Cr): 0.7% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V): 0.1% or less, and boron (B): 0.0025 It may further include one or more selected from % or less.

The step of preparing the cold-rolled steel sheet,

Reheating the steel slab to a temperature range of 1000 to 1350 ° C;

Hot rolling the reheated steel slab at a finish hot rolling temperature of Ar3 ~ Ar3 + 50 ° C;

Cooling the hot-rolled steel sheet to a temperature range of 400 to 700° C. and winding it; and

A step of cold rolling the cooled and wound steel sheet at a reduction ratio of 40 to 70% may be included.

A step of temper rolling the steel sheet cooled to room temperature at a reduction ratio of 0.1 to 2.0% may be further included.

According to one aspect of the present invention, it is possible to provide an ultra-high strength steel sheet with excellent ductility and a manufacturing method thereof.

According to one aspect of the present invention, it is possible to provide a steel sheet used as a steel sheet for automobiles having excellent strength and excellent ductility and a manufacturing method thereof.

1 (a) and (b) are photographs of microstructures (1000 times and 5000 times, respectively) of an example according to an embodiment of the present invention observed using a scanning electron microscope (SEM).

Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

Conventional high-strength steel sheets with a tensile strength of 1500 MPa or more have an elongation of 5 to 8%, and it is difficult to cold form relatively complex parts such as battery cases due to insufficient elongation and consequently low formability. Therefore, an elongation of 10% or more is required to facilitate cold forming by relieving local stress.

Therefore, the inventors of the present invention have studied in depth to solve the above problems and manufacture a high-strength steel sheet having a tensile strength of 1500 MPa or more and at the same time securing an elongation of 10% or more.

In the case of steel with increased C content, it is possible to secure the desired level of tensile strength and elongation, but since weldability and mass productivity are greatly inferior, there is a limit to the application of mass-produced parts. Therefore, in order to secure all of the desired physical properties and mass productivity, various review work through changes in alloy composition, process conditions, etc. was required.

As a result, it was confirmed that strength and ductility can be secured at the same time by optimizing the alloy composition through the content and relational expression of alloy elements and controlling the cooling and overaging treatment, thereby completing the present invention.

Hereinafter, the present invention will be described in detail.

Hereinafter, the steel composition of the present invention will be described in detail.

In the present invention, unless otherwise specified, % indicating the content of each element is based on weight.

Steel according to one aspect of the present invention, by weight%, carbon (C): 0.15 ~ 0.22%, manganese (Mn): 2.6 ~ 3.2%, silicon (Si): 0.3 ~ 0.7%, acid soluble aluminum (sol.Al ): 0.3 to 0.7%, phosphorus (P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the balance may include iron (Fe) and other unavoidable impurities.

Carbon (C): 0.15 to 0.22%

Carbon (C) is an element advantageous in securing strength by forming martensite, and is an effective component in manufacturing high-strength steel. In general, as the carbon (C) content increases, the formation of martensite becomes easier, which is advantageous in forming the complex structure required for manufacturing high-strength steel. However, it is necessary to control the content at an appropriate level to simultaneously control the intended strength and elongation there is. Accordingly, carbon (C) may be added in an amount of 0.15% or more in order to secure the strength targeted by the present invention and form an appropriate level of martensite. A more preferred lower limit may be 0.17%. On the other hand, if the content is excessive, problems such as poor weldability and formability may occur, so in the present invention, the upper limit of the carbon (C) content may be limited to 0.22%. A more preferable upper limit may be 0.2%.

Manganese (Mn): 2.6 to 3.2%

Manganese (Mn) is an element that improves the hardenability of steel and, in particular, plays an important role in forming martensite. Manganese (Mn) contributes to the increase in strength of steel by the solid solution strengthening effect, and serves to suppress the occurrence of plate breakage and high-temperature embrittlement caused by S during hot rolling by precipitating S, which is inevitably added in steel, as MnS. In order to secure the above effect, manganese (Mn) may be added in an amount of 2.6% or more, and a more preferable lower limit may be 2.8%. However, if the content exceeds a certain level, not only does weldability deteriorate, martensite is excessively formed, making the material unstable, and a band-shaped oxide band is formed, resulting in the risk of processing cracks and plate breakage. There is a problem with this rising. In addition, during annealing, manganese (Mn) oxide is eluted on the surface of the steel sheet, and there is a possibility of impairing plating properties. Therefore, the content of manganese (Mn) may be limited to 3.2% or less. A more preferable upper limit may be 3%.

Silicon (Si): 0.3 to 0.7%

Silicon (Si) is a useful element capable of securing strength without reducing the ductility of the steel sheet. Silicon (Si) is also an element that promotes the formation of ferrite and promotes the formation of martensite by promoting the enrichment of C into untransformed austenite. In order to secure the above effect, 0.3% or more of silicon (Si) may be added. More preferably, 0.4% or more can be added. On the other hand, if the content is excessive, it may cause deterioration in plating properties as well as hydrogen embrittlement and weldability, and the upper limit of the content may be limited to 0.7%. More preferably, it can be limited to 0.6% or less.

Acid soluble aluminum (sol.Al): 0.3 to 0.7%

Acid-soluble aluminum (sol.Al) is an element added for refining and deoxidation of steel, and is a component that can improve ductility by delaying the precipitation of carbides in bainite, especially when maintaining the bainite region. In order to secure such an effect, 0.3% or more of acid soluble aluminum (sol.Al) may be added. A more preferred lower limit may be 0.4%. On the other hand, if the content is excessive, not only the castability deteriorates during continuous casting, but also the possibility of defects in the annealed material may increase due to the excessive formation of inclusions. The upper limit can be limited to 0.7%, More preferably, it can be limited to 0.6%.

Phosphorus (P): 0.05% or less

Phosphorus (P) is the most advantageous element for securing the strength of steel without significantly impairing formability, but when added excessively, the possibility of brittle fracture greatly increases, thereby increasing the possibility of plate breakage of the slab during hot rolling. In addition, since phosphorus (P) has a problem of inhibiting plating surface characteristics, the upper limit of the content may be limited to 0.05%. However, 0% can be excluded in consideration of the level inevitably added to steel.

Sulfur (S): 0.01% or less

Sulfur (S) is an impurity element that is inevitably added to steel, and it is desirable to manage its content as low as possible. In particular, since sulfur (S) in steel can increase the possibility of glowing brittleness, it is preferable to limit its content to 0.01% or less. However, 0% can be excluded in consideration of the level inevitably added during the manufacturing process.

Nitrogen (N): 0.01% or less

Nitrogen (N) is an impurity element that is inevitably added to steel, and it is important to manage its content as low as possible. However, since there is a problem in that the refining cost of steel rises rapidly for this purpose, it is preferable to control the content to 0.01% or less, which is a range in which operating conditions are possible. 0% can be excluded considering the level that is unavoidably added to steel.

The steel of the present invention may include remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing field, not all of them are specifically mentioned in this specification.

Steel according to one aspect of the present invention contains chromium (Cr): 0.7% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V): 0.1% or less, and boron (B) : It may further include one or more selected from 0.0025% or less.

Chromium (Cr): 0.7% or less

Chromium (Cr) is an element added to secure high strength of steel because it improves hardenability. Chromium (Cr) is not only effective in forming martensite, but also advantageous in manufacturing high-strength steel having high ductility by minimizing the decrease in elongation compared to the increase in strength. Chromium (Cr) may be added to secure the above-mentioned effect. However, chromium (Cr) facilitates the formation of martensite through improved hardenability, but when its content exceeds a certain level, the formation rate of martensite is excessively increased and the formation of coarse chromium (Cr)-based carbides Since the fraction is increased, there may be a problem of causing a decrease in elongation. In addition, since excessive addition may cause hydrogen embrittlement and poor weldability, the content may be limited to 0.7% or less. A more preferred upper limit may be 0.6%, more preferably 0.4%.

Niobium (Nb): 0.1% or less

Niobium (Nb) is an element that is segregated at austenite grain boundaries to suppress coarsening of austenite crystal grains during annealing heat treatment and to contribute to strength improvement by forming fine carbides. Therefore, in order to secure such an effect, niobium (Nb) may be added, preferably 0.02% or more. However, when niobium (Nb) is excessively added, the strength and elongation may be reduced due to the precipitation of coarse carbides and the reduction of the carbon content in the steel, and the manufacturing cost may increase, resulting in poor economic performance. Bar, Its content can be limited to 0.1% or less. More preferably, it can be added at 0.07% or less, more preferably at 0.04% or less. However, it may be included in an amount of 0.005% or more without limitation.

Titanium (Ti): 0.1% or less

Titanium (Ti) not only contributes to securing yield and tensile strength by forming fine carbides, but also suppresses the precipitation of AlN by precipitating N in steel as TiN, thereby effectively reducing the risk of cracking during playing. It is an element that can effectively reduce. In order to secure such an effect, titanium (Ti) may be added, preferably 0.02% or more. However, if the content is excessive, the strength and elongation may be reduced due to the precipitation of coarse carbides and the reduction of the carbon content in the steel, and nozzle clogging may also be caused during playing. Therefore, the content of titanium (Ti) may be limited to 0.1% or less. A more preferred upper limit may be 0.07%, and a more preferred upper limit may be 0.04%. However, it may be included in an amount of 0.005% or more without limitation.

Vanadium (V): 0.1% or less

Vanadium (V) is an element that reacts with C or N to form carbon and nitride, and plays an important role in increasing the yield strength of steel by forming fine precipitates at low temperatures. In order to secure the above effect, a certain amount or more of vanadium (V) may be added, preferably 0.02% or more. However, if the content is excessive, coarse carbides are precipitated, strength and elongation may be reduced by reducing the amount of carbon in steel, and manufacturing costs increase, so the content should be limited to 0.1% or less. can More preferably, it can be limited to 0.04% or less.

Boron (B): 0.0025% or less

Boron (B) is a component that retards the transformation of austenite to pearlite during cooling during annealing. However, when boron (B) is excessively added, boron (B) is concentrated on the surface of the steel sheet and may cause deterioration of coating adhesion, so the content may be limited to 0.0025% or less.

The steel according to one aspect of the present invention may have an R1 value of 0.7 to 1.1 defined in the following relational expression 1.

Relational Equation 1 is obtained as an empirical value of a component relationship that must be satisfied to secure the surface quality of the steel grade according to the present invention. If the R1 value defined in relational expression 1 exceeds 1.1, it means that the content of alloying elements such as Si and Al is relatively high, which causes not only a decrease in surface quality due to the concentration of alloying elements on the surface of the steel sheet, but also a decrease in hardenability. Due to this, some ferrite may be introduced during the first cooling. On the other hand, if the value is less than 0.7, the surface quality of the steel sheet becomes good, but when the bainite region is maintained, the content of Al and Si components, which promote ductility improvement through the delay of carbide precipitation in bainite, is reduced below the standard, making the steel sheet There is a risk of deterioration in ductility. A more preferable lower limit of the R1 value may be 0.8, more preferably 0.9.

[Relationship 1]

R1 = [Si]+[Al]+([P]/[Mn])

(Where [Si], [Al], [P] and [Mn] are the weight percent of each element.)

The steel according to one aspect of the present invention may have an R2 value of 4.0 to 6.0 defined in the following relational expression 2.

Relational Expression 2 is obtained as an empirical value of the component relationship for securing the basic material required in the present invention. If the value of R2 defined in relational expression 2 is less than 4.0, since the hardenability of the steel is lowered, the strength required in the present invention cannot be secured as the phase fraction of the low-temperature transformation structure such as martensite or bainite is lowered. On the other hand, if the value exceeds 6.0, the hardenability of the steel is excessively high, so that a material having desired formability cannot be obtained. A more preferred lower limit may be 4.6, and a more preferred upper limit may be 5.2.

[Relationship 2]

R2 = [C]+1.5[Mn]+2[Cr]+[B]

(Where [C], [Mn], [Cr] and [B] are the weight percent of each element.)

Hereinafter, the steel microstructure of the present invention will be described in detail.

In the present invention, % representing the fraction of the microstructure is based on the area unless otherwise specified.

Martensite in the present invention is a concept including fresh martensite and tempered martensite, and the martensite fraction means the total fraction of fresh martensite and tempered martensite.

The steel according to one aspect of the present invention contains 80 area% or more of martensite and 20 area% or less of ferrite, bainite and retained austenite as a microstructure, and the ratio of the tempered martensite fraction to the total martensite fraction may be 92-97%.

In the present invention, in order to effectively achieve both high ductility and high strength properties, the total fraction of fresh martensite and tempered martensite is limited to 80% or more, but the ratio of the fraction of tempered martensite to the total martensite fraction is 92 to limited to 97%. If the total martensite fraction is less than 80%, the desired strength cannot be effectively secured. In addition, if the ratio of tempered martensite to total martensite is less than 92%, the desired yield ratio and hole expandability cannot be secured, and if the ratio exceeds 97%, the desired strength and elongation are simultaneously secured. There are difficulties.

In the present invention, in order to simultaneously secure the desired strength and elongation, ferrite, bainite, and retained austenite may be included at 20 area% or less. When ferrite, bainite and retained austenite exceed 20%, it may be difficult to secure not only the desired elongation but also the hole expandability. More preferably, ferrite, bainite, and retained austenite may be included at 17% or less, and a more preferable lower limit may be 6%. In the present invention, more preferably 3% or more of ferrite and 3% or more of retained austenite may be included in order to simultaneously secure desired ductility. A more preferable upper limit of the ferrite fraction may be 6%, and a more preferable upper limit of the retained austenite fraction may be 5%. In the present invention, the fraction of bainite may include 0%, but may inevitably include 1% or more during manufacture.

Steel according to one aspect of the present invention is titanium (Ti): 0.1% or less, niobium (Nb): when containing one or more selected from among 0.1% or less, Ti or Nb-based fine precipitates having an average diameter of 50 nm or less 10 ¹² /m ² or more may be included.

In the present invention, by including a large amount of fine precipitates, it is possible to more effectively secure the desired high-strength characteristics.

Hereinafter, the steel manufacturing method of the present invention will be described in detail.

Steel according to one aspect of the present invention can be produced by continuous annealing, primary cooling, secondary cooling, and over-aging of a cold-rolled steel sheet satisfying the above-described alloy composition.

Cold rolled steel sheet preparation

A cold-rolled steel sheet satisfying the alloy composition of the present invention can be prepared.

The cold-rolled steel sheet of the present invention can be manufactured under normal processing conditions, preferably by reheating, hot rolling, cooling, winding and cold rolling a steel slab under the conditions described below.

reheat

A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1000 to 1350 ° C.

If the reheating temperature is less than 1000 ° C, there is a possibility of hot rolling in a temperature range below the target finishing hot rolling temperature range, and if the reheating temperature exceeds 1350 ° C, the melting point of the steel is reached and productivity may decrease. .

hot rolled

The reheated steel slab may be hot rolled at a finish hot rolling temperature of Ar3 ~ Ar3 + 50 ° C.

During hot rolling, the finishing hot rolling temperature may be a reference temperature at the exit side of the finishing mill. If the finish hot rolling temperature is less than Ar3, the hot deformation resistance is likely to increase rapidly, whereas if the temperature exceeds Ar3+50℃, an excessively thick oxide scale occurs and the crystal grains of the hot-rolled steel sheet are formed coarsely. This can cause deterioration of physical properties of the final steel sheet.

cooling and winding

The hot-rolled steel sheet may be cooled after cooling to a temperature range of 400 to 700° C.

If the coiling temperature is less than 400℃, excessive martensite or bainite is generated, which may cause manufacturing problems such as shape defects due to load during cold rolling, and if the temperature exceeds 700℃, pickling due to an increase in surface scale Sex may deteriorate.

In addition, in the present invention, pickling may be performed to remove scale generated on the surface of the steel sheet prior to cold rolling, which is a subsequent process.

cold rolled

The cooled and wound steel sheet may be cold rolled at a reduction ratio of 40 to 70%.

In the present invention, cold rolling conditions are not particularly limited, but cold rolling can be preferably performed at a reduction ratio of 40 to 70%. During cold rolling, when the reduction ratio is less than 40%, the recrystallization driving force is weakened, making it difficult to secure good recrystallized grains and shape correction. On the other hand, if the reduction ratio exceeds 70%, cracks are likely to occur at the edge of the steel sheet, and there may be problems in that the rolling load increases rapidly.

continuous annealing

In the present invention, continuous annealing may be performed by holding the prepared cold-rolled steel sheet at a temperature range of 750 to 900° C. for 30 to 300 seconds.

In the present invention, it is possible to prepare the basis for the desired microstructure through continuous annealing. When the continuous annealing temperature is less than 750 ° C., the ferrite fraction is increased. As a result, not only the strength is lowered, but also the hole expandability may be deteriorated due to the increase in the hardness difference between phases. On the other hand, if the temperature exceeds 900 ° C., there is a concern that the formation of annealed oxide on the surface of the steel sheet may progress in addition to the decrease in ductility due to the decrease in the ferrite fraction. A more preferable upper limit of the continuous annealing temperature of the present invention may be 830 ℃, more preferably may be 820 ℃. In addition, a more preferable lower limit may be 800°C, more preferably 810°C.

During continuous annealing, if the holding time is less than 30 seconds, ferrite recrystallization is not sufficiently achieved, making it difficult to secure a desired elongation rate, and there is a high risk of material deviation in the length / width direction of the steel sheet. On the other hand, if the time exceeds 300 seconds, the annealing effect is already saturated, and thus a problem of reduced productivity may occur.

1st cooling

The continuously annealed steel sheet may be first cooled at an average cooling rate of 10° C./s or less to a temperature range of 600 to 720° C.

In primary cooling, slow cooling is performed prior to performing secondary cooling, which is a subsequent process, so that plate-shaped inferiority due to rapid temperature drop in the rapid cooling section can be suppressed. If the primary cooling end temperature is less than 600 ° C or exceeds 720 ° C, it is out of the appropriate temperature gradient range with secondary cooling, making it difficult to secure the above effect, and as a result, it may be difficult to secure a stable material.

If the primary average cooling rate exceeds 10 ° C / s, there is a concern that C and Mn concentration in austenite may not sufficiently occur. In the present invention, the lower limit of the primary average cooling rate is not particularly limited, but since cooling is performed at the cooling rate of normal slow cooling, it can be preferably set to 1 ° C. / s or more.

secondary cooling

The primary cooled steel sheet may be secondary cooled at an average cooling rate of 30 to 70 °C/s to a temperature range of 50 to 300 °C.

If the secondary cooling end temperature is less than 50 ° C., the martensitic transformation end temperature (Mf) is passed, so even after overaging at an appropriate temperature, the effect of concentrating C and Mn into retained austenite is reduced. As a result, there is a problem in that the stability of retained austenite is reduced so that a desired elongation cannot be secured. On the other hand, if the temperature exceeds 300 ° C., when the secondary cooling is completed, the generated martensite fraction is greatly reduced, and there is a concern that the microstructure desired in the present invention may not be implemented. A more preferable upper limit of the secondary cooling temperature in the present invention may be 200°C, more preferably 150°C. A more preferable lower limit of the secondary cooling temperature may be 100°C.

If the secondary average cooling rate is less than 30 ℃ / s, cementite precipitation occurs in the steel sheet, and there is a concern that the retained austenite fraction may be insufficient. If the rate exceeds 70 ℃ / s, C and Mn as retained austenite There may be difficulty in obtaining a thickening effect, and the flatness of the steel sheet may also be inferior.

overaging treatment

Overaging treatment may be performed by holding the secondary cooled steel sheet at a temperature range of 100 to 400° C. for 450 to 650 seconds.

The overaging treatment induces a tempering effect of martensite, thereby effectively securing the desired strength and ductility in the present invention at the same time. In the present invention, martensite can be appropriately tempered by controlling the steel sheet to be maintained at a temperature range of 100 to 400 ° C. for a certain period of time after secondary cooling, and the effect of concentrating C and Mn into retained austenite can be obtained.

If the overaging treatment temperature is less than 100 ° C, there is a problem that the effect of concentrating C and Mn into retained austenite is insignificant, and if the temperature exceeds 400 ° C, cementite in the retained austenite is precipitated and ductility may decrease. A more preferable upper limit of the overaging treatment temperature may be 300°C, more preferably 250°C. A more preferable lower limit of the overaging treatment temperature may be 150°C.

In the overaging treatment, if the holding time is less than 450 seconds, sufficient tempering effect cannot be obtained, and if it exceeds 650 seconds, there may be a problem in that ductility is reduced due to precipitation of carbides such as cementite.

In the present invention, after the overaging treatment, the steel sheet may be cooled to room temperature. Cooling after overaging treatment is not particularly limited, and normal conditions can be applied.

temper rolling

According to one aspect of the present invention, temper rolling may be performed on the overaged steel sheet at a reduction ratio of 0.1 to 2.0%.

In the present invention, temper rolling can be selectively performed on the overaged steel sheet, and the conditions are not particularly limited. Usually, when temper rolling is performed, the effect of increasing the yield strength can be obtained without increasing the tensile strength. However, when the rolling reduction of temper rolling is less than 0.1%, the effect of increasing the yield strength is insufficient, and there is difficulty in shape control. On the other hand, when the reduction ratio exceeds 2.0%, operability may be greatly deteriorated due to high elongation work. Therefore, in the present invention, during temper rolling, the reduction ratio may be limited to 0.1 to 2.0%.

The steel of the present invention thus prepared has a tensile strength of 1500 MPa or more, a yield strength of 1000 MPa or more, a yield ratio of 0.68 or more, an elongation of 10.0% or more, a product of yield strength and elongation of 10000 MPa %, and hole expandability. (HER) of 35% or more, it is possible to have excellent ductility properties while having high strength.

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.

(Example)

The steel slab having the alloy composition of Table 1 below was vacuum-melted, heated at 1200 ° C, finish rolling was terminated at an outlet temperature of 880 to 920 ° C, and winding was performed at 600 ° C. Thereafter, after pickling to remove surface scale, cold rolling was performed at a cold rolling reduction of 50%, and continuous annealing, cooling, and overaging were performed under the conditions shown in Table 2 below, respectively.

In addition, in Table 1 below, R1 and R2 values of Relational Expression 1 and Relational Expression 2 according to the alloy composition of each steel type are calculated and shown.

steel grade Alloy composition (wt%) R1 R2 C Mn Si Cr Nb Ti V B Al P S N A 0.2 2.9 0.5 0.3 0.03 0.03 - 0.002 0.5 0.008 0.002 0.004 1.00 5.2 B 0.2 2.9 0.5 0.3 0.03 0.03 0.03 0.002 0.5 0.008 0.002 0.004 1.00 5.2 C 0.2 2.9 0.5 - 0.03 0.03 0.03 0.002 0.5 0.008 0.002 0.004 1.00 4.6 D 0.17 2.9 0.5 0.3 0.03 0.03 0.03 0.002 0.5 0.008 0.002 0.004 1.00 5.1 E 0.2 2.9 0.5 0.3 0.03 0.03 0.03 - 0.5 0.008 0.002 0.004 1.00 5.2 F 0.14 2.9 0.5 0.3 0.03 0.03 - 0.002 0.5 0.008 0.002 0.004 1.00 5.1 G 0.2 2.5 0.5 0.3 0.03 0.03 - 0.002 0.5 0.008 0.002 0.004 1.00 4.6 H 0.2 2.9 0.7 0.3 0.03 0.03 - 0.002 0.5 0.008 0.002 0.004 1.20 5.2 I 0.2 3.0 0.5 0.7 0.03 0.03 - 0.002 0.5 0.008 0.002 0.004 1.00 6.1

[Relationship 1]

R1 = [Si]+[Al]+([P]/[Mn])

(Where [Si], [Al], [P] and [Mn] are the weight percent of each element.)

[Relationship 2]

R2 = [C]+1.5[Mn]+2[Cr]+[B]

(Where [C], [Mn], [Cr] and [B] are the weight percent of each element.)

Psalter
number river
bell Annealing 1st cooling secondary cooling overaging treatment temperature
(℃) temperature
(℃) speed
(℃/s) temperature
(℃) speed
(℃/s) temperature
(℃) hour
(candle) One A 820 690 2.2 100 54.9 200 570 2 A 820 690 2.2 150 50.2 200 570 3 A 820 550 4.6 100 41.9 200 570 4 A 820 750 1.2 100 60.5 200 570 5 A 820 690 2.2 25 61.9 200 570 6 A 820 690 2.2 350 31.6 350 570 7 A 820 690 2.2 400 27.0 400 570 8 A 820 690 2.2 100 54.9 200 400 9 A 820 690 2.2 100 54.9 200 700 10 A 820 690 2.2 100 54.9 450 570 11 B 810 690 2.1 100 54.9 200 570 12 B 820 690 2.2 100 54.9 150 570 13 B 820 690 2.2 100 54.9 200 570 14 C 810 690 2.1 150 50.2 200 570 15 D 830 690 2.4 100 54.9 200 570 16 E 830 690 2.4 100 54.9 200 570 17 F 820 690 2.2 100 54.9 200 570 18 G 820 690 2.2 100 54.9 200 570 19 H 820 690 2.2 100 54.9 200 570 20 I 820 690 2.2 100 54.9 200 570

The microstructure of the manufactured steel sheet was observed and shown in Table 3 below, and fresh martensite (FM), tempered martensite (TM), ferrite (F), bainite (B) and retained austenite (retained γ) The fraction was measured and expressed. The microstructure was observed using SEM after nital etching of the polished specimen cross section, and the retained austenite was measured through XRD analysis. Mt in Table 2 means the total fraction of fresh martensite (FM) and tempered martensite (TM), and TM / Mt is tempered martensite for the total fraction (Mt) of fresh martensite and tempered martensite (TM) means the ratio of fractions.

In addition, Table 3 shows the measured physical property values for each of the prepared specimens. Yield strength (YS), tensile strength (TS), and elongation (El) were evaluated through a tensile test, and evaluated with test pieces taken in accordance with the JIS-5 standard based on the direction of 90 degrees with respect to the rolling direction of the rolled sheet. was measured. In addition, the yield ratio and the product of yield strength and elongation (YSxEl) were calculated and displayed through the above physical property values. The hole expandability (HER) was evaluated through a hole expansion test, and after forming a punched hole of 10 mmΨ (die inner diameter 10.3 mm, clearance 12.5%), a conical punch with an apex angle of 60 degrees was used, and the burr of the punched hole became the outside. It was inserted into the punched hole in the same direction, and the area around the punched hole was compressed and expanded at a moving speed of 12 mm/min, and then calculated using the following relational expression.

[relational expression]

Hole expandability (HER, %) = {(D - D ₀ ) / D ₀ } x 100

(Here, D means the hole diameter (mm) when the crack penetrates the steel sheet along the thickness direction, and D ₀ means the initial hole diameter (mm).)

city
side
th
like river
bell microstructure (% area) Properties division FM TM mt TM/Mt F B residue
γ YS
(MPa) TS
(MPa) El
(%) yield ratio YSxEl
(MPa %) HER
(%) One A 4 84 88 0.95 3 5 4 1125 1577 10.3 0.71 11588 38 Invention example 1 2 A 4 84 88 0.95 3 5 4 1062 1550 10.2 0.69 10832 37 Invention example 2 3 A 4 86 90 0.96 2 5 3 1129 1536 9.4 0.74 10613 38 Comparative Example 1 4 A 4 85 89 0.96 2 6 3 1133 1529 9.5 0.74 10764 39 Comparative Example 2 5 A 3 88 91 0.97 3 3 3 1138 1554 9.2 0.73 10470 40 Comparative Example 3 6 A 17 53 70 0.76 3 25 2 985 1530 9.1 0.64 8964 31 Comparative Example 4 7 A 42 - 42 - 3 54 One 941 1547 9.0 0.61 8469 28 Comparative Example 5 8 A 7 82 89 0.92 3 5 3 1068 1584 9.3 0.67 9932 36 Comparative Example 6 9 A 2 88 90 0.98 3 5 2 1075 1588 9.4 0.68 10105 35 Comparative Example 7 10 A 3 87 90 0.97 3 3 4 1142 1537 9.0 0.74 10278 41 Comparative Example 8 11 B 3 80 83 0.96 5 9 3 1052 1542 10.3 0.68 10836 36 Inventive example 3 12 B 4 84 88 0.95 3 5 4 1035 1532 10.3 0.68 10661 35 Inventive example 4 13 B 4 84 88 0.95 3 5 4 1086 1546 10.3 0.70 11186 39 Inventive Example 5 14 C 3 80 83 0.96 5 9 3 1018 1507 10.1 0.68 10282 35 Inventive example 6 15 D 4 87 91 0.96 3 2 4 1024 1517 10.4 0.68 10650 36 Inventive Example 7 16 E 3 88 91 0.97 3 2 4 1026 1520 10.2 0.68 10465 37 Inventive Example 8 17 F 3 85 88 0.97 4 5 3 940 1436 10.0 0.65 9400 35 Comparative Example 9 18 G 3 86 89 0.97 3 5 3 927 1422 9.8 0.65 9085 33 Comparative Example 10 19 H 3 83 86 0.97 4 6 4 1032 1553 10.4 0.66 10733 38 Comparative Example 11 20 I 3 84 87 0.97 3 5 5 1138 1601 9.4 0.71 10697 33 Comparative Example 12

* FM: fresh martensite, TM: tempered martensite, Mt: fresh martensite + tempered martensite, F: ferrite, B: bainite, retained γ: retained austenite

As shown in Table 3, in the case of the inventive example satisfying the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the desired physical properties were secured in the present invention.

1 (a) and (b) are photographs of microstructures (1000 times and 5000 times, respectively) of an example according to an embodiment of the present invention observed using a scanning electron microscope (SEM).

On the other hand, Comparative Examples 1 and 2 are cases in which the cooling end temperature did not satisfy the conditions of the present invention during the first cooling, and it was not possible to secure the elongation at the desired level because an appropriate temperature gradient with the second cooling was not obtained. did

Comparative Examples 3 to 5 are examples in which the secondary cooling conditions do not satisfy the present invention. In Comparative Example 3, the end temperature of the secondary cooling was excessively low, so that the martensitic transformation end temperature was passed, and the effect of concentrating C and Mn into retained austenite was reduced despite the subsequent overaging treatment. As a result, the stability of retained austenite decreased, and the desired elongation was not secured. In Comparative Examples 4 and 5, as examples in which the secondary cooling end temperature exceeds the range of the present invention, tempered martensite was not sufficiently formed as the final microstructure, and bainite was excessively formed, so that the fraction of total martensite was insufficient. did As a result, it was not possible to secure the desired physical properties other than tensile strength.

Comparative Examples 6 to 8 are examples that do not satisfy the overaging treatment conditions of the present invention. In Comparative Example 6, the target elongation was not secured due to a decrease in the effect of concentrating C and Mn into retained austenite due to insufficient overaging treatment time. In Comparative Example 7, the target elongation was not secured due to precipitation of carbides such as cementite due to excessive overaging treatment time. In Comparative Example 8, the overaging treatment temperature exceeded the temperature range proposed in the present invention, and the desired elongation was not secured due to the precipitation of carbides such as cementite in the retained austenite.

Comparative Examples 9 to 12 are examples that do not satisfy the alloy composition or relational expression proposed in the present invention. In Comparative Examples 9 and 10, the content of C and Mn, respectively, did not reach the range proposed in the present invention, and thus did not secure the desired strength. In Comparative Example 11, the value of Relational Equation 1 is outside the range suggested by the present invention, and alloy elements are concentrated on the surface of the steel sheet, resulting in a decrease in surface quality, as well as a decrease in yield strength due to a decrease in hardenability, resulting in a desired yield ratio. could not secure In Comparative Example 12, the value of Relational Equation 2 exceeded the range of the present invention, and the hardenability was excessively high, so that the desired level of elongation could not be secured.

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (10)

By weight, carbon (C): 0.15 to 0.22%, manganese (Mn): 2.6 to 3.2%, silicon (Si): 0.3 to 0.7%, acid soluble aluminum (sol.Al): 0.3 to 0.7%, phosphorus ( P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the balance including iron (Fe) and other unavoidable impurities,
The R1 value defined in the following relational expression 1 is 0.7 to 1.1,
The R2 value defined in the following relational expression 2 is 4.0 to 6.0,
The microstructure contains 80 area% or more of martensite and less than 20 area% of ferrite, bainite and retained austenite, and the ratio of the tempered martensite fraction to the total martensite fraction is 92 to 97%,
Steel sheet with an elongation of 10.0% or more.
[Relationship 1]
R1 = [Si]+[Al]+([P]/[Mn])
(Where [Si], [Al], [P] and [Mn] are the weight percent of each element.)
[Relationship 2]
R2 = [C]+1.5[Mn]+2[Cr]+[B]
(Where [C], [Mn], [Cr] and [B] are the weight percent of each element.)
According to claim 1,
The steel sheet contains, in weight percent, chromium (Cr): 0.7% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V): 0.1% or less, and boron (B): 0.0025 A steel sheet further comprising one or more selected from % or less.
According to claim 1,
The steel sheet is a steel sheet containing 3 area% or more of ferrite as a microstructure.
According to claim 1,
The steel sheet is a steel sheet containing 3 area% or more of retained austenite as a microstructure.
According to claim 2,
The steel sheet includes, in weight percent, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, and includes Ti or Nb-based fine precipitates having an average diameter of 50 nm or less, 10 ¹² / Steel sheet containing more than m ² .
According to claim 1,
The steel sheet has a tensile strength of 1500 MPa or more, a yield strength of 1000 MPa or more, a yield ratio of 0.68 or more, a product of yield strength and elongation of 10000 MPa %, and a hole expandability (HER) of 35% or more.
By weight, carbon (C): 0.15 to 0.22%, manganese (Mn): 2.6 to 3.2%, silicon (Si): 0.3 to 0.7%, acid soluble aluminum (sol.Al): 0.3 to 0.7%, phosphorus ( P): 0.05% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the balance including iron (Fe) and other unavoidable impurities, and the R1 value defined by the following relational expression 1 is 0.7 to 1.1 And, preparing a cold-rolled steel sheet having an R2 value of 4.0 to 6.0 defined in the following relational expression 2;
Continuous annealing step of maintaining the cold-rolled steel sheet in a temperature range of 750 to 900 ° C. for 30 to 300 seconds;
firstly cooling the continuously annealed steel sheet to a temperature range of 600 to 720°C at an average cooling rate of 10°C/s or less;
Secondary cooling of the primary cooled steel sheet to a temperature range of 50 to 300 °C at an average cooling rate of 30 to 70 °C/s;
An overaging treatment step of maintaining the secondary cooled steel sheet in a temperature range of 100 to 400 ° C for 450 to 650 seconds; and
A method of manufacturing a steel sheet comprising the step of cooling the overaged steel sheet to room temperature.
[Relationship 1]
R1 = [Si]+[Al]+([P]/[Mn])
(Where [Si], [Al], [P] and [Mn] are the weight percent of each element.)
[Relationship 2]
R2 = [C]+1.5[Mn]+2[Cr]+[B]
(Where [C], [Mn], [Cr] and [B] are the weight percent of each element.)
According to claim 7,
The steel sheet contains, in weight percent, chromium (Cr): 0.7% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, vanadium (V): 0.1% or less, and boron (B): 0.0025 Method for producing a steel sheet further comprising one or more selected from % or less.
According to claim 7,
The step of preparing the cold-rolled steel sheet,
Reheating the steel slab to a temperature range of 1000 to 1350 ° C;
Hot rolling the reheated steel slab at a finish hot rolling temperature of Ar3 ~ Ar3 + 50 ° C;
Cooling the hot-rolled steel sheet to a temperature range of 400 to 700° C. and winding it; and
A method of manufacturing a steel sheet comprising the step of cold rolling the cooled and wound steel sheet at a reduction ratio of 40 to 70%.
According to claim 7,
Method for producing a steel sheet further comprising the step of temper rolling the steel sheet cooled to room temperature at a reduction ratio of 0.1 to 2.0%.

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