KR20230069426A - High strength steel sheet having excellent bendablilty and stretch-flangeability and manufacturing method of the same - Google Patents

Abstract

The present invention relates to an ultra-high strength steel plate with excellent bendability and elongation flangeability and a manufacturing method thereof, and more specifically to the steel plate with excellent bendability and elongation flangeability, a high yield ratio, and ultra-high strength using rapid low-temperature tempering, and the manufacturing method thereof. The ultra-high strength steel plate of the present invention includes, in weight %, 0.12 to 0.4% of C, 0.5% or less (excluding 0%) of Si, 2.5 to 4.0% of Mn, 0.03% or less (excluding 0%) of P, 0.012% or less (excluding 0%) of S, 0.1% or less (excluding 0%) of Al, 1% or less (excluding 0%) of Cr, 48/14×[N] to 0.1% of Ti, 0.1% or less (excluding 0%) of Nb, 0.005% or less (excluding 0%) of B, 0.01% or less (excluding 0%) of N, and the remaining Fe and other impurities.

Description

Ultra high strength steel sheet with excellent bendability and elongation flangeability and method for manufacturing the same

The present invention relates to an ultra-high strength steel sheet excellent in bendability and elongation flangeability and a method for manufacturing the same, and more particularly, to a steel sheet having excellent bendability and elongation flangeability using rapid low-temperature tempering and having a high yield ratio and ultra-high strength. and a method for producing the same.

In order to satisfy the contradictory goals of lightweighting and securing crash safety, automotive steel sheets are called Dual Phase Steel (hereinafter referred to as 'DP Steel') and Transformation Induced Plasticity Steel (hereinafter referred to as 'TRIP Steel'). ), and complex phase steel (hereinafter referred to as 'CP steel'), various steel sheets for automobiles are being developed.

Although the strength can be further increased by increasing the amount of carbon in the advanced high-strength steel, the tensile strength that can be realized is limited to about 1200 MPa level when considering practical aspects such as spot weldability. As for the application to structural members to secure collision safety, the method of securing final strength by rapid cooling through direct contact with a die that is water-cooled after being molded at high temperature is in the limelight, but excessive investment in equipment and heat treatment and process costs This is high, so the spread of application is not large.

As an alternative to rapid cooling through water cooling, slow cooling is generally used. However, in a continuous annealing furnace and a continuous annealing hot-dip plating line in which a slow cooling section exists, martensitic steel having a microstructure fraction of 90% or more after annealing heat treatment has a disadvantage in that the ratio of yield strength to tensile strength is less than 0.75, and the yield strength is inferior. there is.

In order to increase the resistance in the event of a car crash, it is desirable to increase the yield strength, and an improvement plan for this is required. In general, tempering of martensitic steel is performed to improve insufficient ductility and toughness of martensitic steel, but it is necessary to increase yield strength while suppressing the decrease in tensile strength as much as possible.

In addition, in order to process martensitic steel through roll forming or press forming, excellent bendability and stretch flangeability are essential. However, since conventional martensitic steels often fail to secure sufficient bendability and stretch flangeability for forming due to their very high strength, research to increase them is also required.

In Patent Document 1 (Japanese Patent Publication No. 2528387), since it is necessary to quench to room temperature after annealing, there is a problem that it cannot be manufactured unless it is a line with special equipment capable of quenching the steel sheet between the annealing furnace and the overaging furnace. there is.

In addition, in Patent Document 2 (Korean Laid-Open Patent Publication No. 10-2010-0116608), martensite transformation is caused to the steel sheet that has reached the Ms point, that is, the martensite transformation start temperature, and martensite after transformation is Although high strength can be obtained by tempering auto-temper treatment, there is a problem in manufacturing stability because strict control of heat treatment conditions at a temperature directly below Ms is required.

In addition, Patent Document 3 (Korean Laid-Open Patent Publication No. 10-2014-0030970) suggests that additional heat treatment is performed to achieve target properties, but the time is too long, so productivity is excessively lowered, or target properties are not achieved. There is a problem in that it is difficult to set conditions that are efficient to achieve.

Japanese Patent Publication No. 2528387 Korean Patent Publication No. 10-2010-0116608 Korean Patent Publication No. 10-2014-0030970

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

The object of the present invention is not limited to the foregoing. Anyone with ordinary knowledge in the technical field to which the present invention belongs will have no difficulty in understanding the additional objects of the present invention from the contents throughout the present specification.

One aspect of the present invention,

In % by weight, C: 0.12 to 0.4%, Si: 0.5% or less (excluding 0%), Mn: 2.5 to 4.0%, P: 0.03% or less (excluding 0%), S: 0.012% or less (excluding 0%) excluding), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14×[N] to 0.1%, Nb: 0.1% or less (0% is excluding excluding), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance including Fe and other impurities,

As a microstructure, in area%, martensite: 90% or more, the sum of ferrite and bainite: 10% or less,

Provided is an ultra-high strength steel sheet in which the value of M defined from the following relational expression 1 satisfies the range of 100 to 500.

[Relationship 1]

M = P _size × P _number × [C] ^0.5 × [Mn] ² × [S]

(In the above relational expression 1, the P _size represents the average diameter of inclusions having a diameter of 1 μm or more, and the P _number represents the average number of inclusions having a diameter of 1 μm or more. [C] and [Mn] are brackets in the steel sheet, respectively. Indicates the average weight % content of the elements in the inside, and [S] shows the average ppm content of the elements in parentheses in the steel sheet.)

According to another aspect of the present invention,

In % by weight, C: 0.12 to 0.4%, Si: 0.5% or less (excluding 0%), Mn: 2.5 to 4.0%, P: 0.03% or less (excluding 0%), S: 0.012% or less (excluding 0%) excluding), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14×[N] to 0.1%, Nb: 0.1% or less (0% is excluding excluding), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), balance including Fe and other impurities, by area%, martensite: 90% or more, ferrite and Total of bainite: preparing a steel sheet having a microstructure containing 10% or less; and

Including; tempering the steel sheet;

Provided is a method for manufacturing an ultra-high strength steel sheet in which the value of P defined by the following relational expression 2 satisfies the range of 1.5 to 77.0.

[Relationship 2]

P =

(In the relational expression 2, the T represents the highest tempering temperature, and the unit is ° C. In addition, the t _eff represents the effective heat treatment time, and the unit is sec.)

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

Or, according to one aspect of the present invention, the yield strength of martensitic steel having a martensite fraction of 90% or more through additional heat treatment on a steel sheet having a low yield strength manufactured in a continuous annealing furnace or a continuous annealing hot-dip plating line in which a slow cooling section exists. Strength can be improved, or one or more properties of bendability and stretch flangeability can be improved.

Various and beneficial advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the process of describing specific embodiments of the present invention.

1 shows pictures taken with a scanning electron microscope (SEM) to observe microstructures of cross-sectional specimens cut in the thickness direction of steel sheets obtained from Comparative Example 2 and Inventive Examples 1 to 3 of the present invention.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified in many different forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Meanwhile, terms used in this specification are for describing specific embodiments and are not intended to limit the present invention. For example, the singular forms used herein include the plural forms unless the relevant definition clearly dictates the contrary. Also, the meaning of "comprising" used in the specification specifies a component, and does not exclude the presence or addition of other components.

Conventionally, in the case of using a slow cooling method without a rapid cooling facility, the slow cooling condition of a continuous annealing furnace or a continuous annealing hot-dip plating line with a slow cooling section is generally 650 ° C or a hot-dipping bath with a cooling rate of 3 ° C / s after annealing. It consists of cooling down to the immersion temperature of 460°C. The steel sheet having the component system of the present invention manufactured under the above conditions has a martensite fraction of 90% or more as a microstructure, an initial yield strength of 1000 to 1250 MPa, an initial tensile strength of 1200 to 1700 MPa, and a yield ratio Less than 0.75, there is a disadvantage that the yield strength is inferior.

By the way, in order to increase the resistance upon collision of a vehicle, not only the improvement of the yield strength is required, but also the bendability and stretch flangeability need to be improved in order to process through roll forming or press forming.

Accordingly, an object of the present invention is to improve the yield strength of an ultra-high strength steel sheet having such resistance to yield strength while maximally suppressing the decrease in tensile strength.

As a result of an intensive study to obtain a steel sheet that not only improves bendability and stretch flangeability, but also satisfies the above characteristics, the inventors of the present invention found that while controlling the component content of C, Mn, and S in the steel within a limited range, It was found that controlling the properties of inclusions in steel was effective, and the present invention was completed.

Hereinafter, the ultra-high strength steel sheet excellent in bendability and stretch flangeability according to the present invention will be described in detail.

In the high-strength steel sheet according to the present invention, by weight, C: 0.12 to 0.4%, Si: 0.5% or less (excluding 0%), Mn: 2.5 to 4.0%, P: 0.03% or less (excluding 0%), S: 0.012% or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14×[N] to 0.1% (however, , [N] represents the weight % content of nitrogen (N) in steel), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less ( 0%), balance Fe and other impurities.

Hereinafter, the reason for adding the components of the steel sheet and the reason for limiting the content of the steel sheet in the present invention will be described in detail. At this time, when the content of each element is indicated in the present specification, unless otherwise specified, weight % is indicated.

C: 0.12-0.4%

Carbon (C) is an essential element to secure martensite strength, and should be added in an amount of 0.12% or more. However, if the C content exceeds 0.4%, weldability deteriorates, so the upper limit is limited to 0.4%. Meanwhile, in terms of further improving the above effects, the lower limit of the C content may be 0.15%, or the upper limit of the C content may be 0.30%.

Si: 0.5% or less (excluding 0%)

Silicon (Si) is an element added to stabilize ferrite, and needs to be present in an amount greater than 0% for the above effect. However, Si has the disadvantage of deteriorating strength by promoting ferrite generation during slow cooling after annealing in a conventional continuous annealing type hot-dip plating heat treatment furnace in which a slow cooling section exists. In addition, in the case of adding a large amount of Mn to suppress phase transformation as in the present invention, there is a risk of deterioration of hot-dip plating characteristics due to surface oxide formation by Si during annealing, surface enrichment of Si, and dent defects caused by oxidation. , limiting the upper limit of the Si content to 0.5%. Meanwhile, in terms of further improving the above effects, the lower limit of the Si content may be 0.1%, or the upper limit of the Si content may be 0.45%.

Mn: 2.5 to 4.0%

Manganese (Mn) is an element that suppresses the formation of ferrite in steel and facilitates the formation of austenite, and 2.5% or more of Mn is added to secure the above-mentioned effects. If the Mn content in the steel is less than 2.5%, there is a problem in that ferrite is easily generated during slow cooling in the case of a continuous annealing hot-dip galvanizing heat treatment furnace. In addition, when the Mn content exceeds 4.0%, band formation due to segregation caused in the slab and hot rolling process becomes excessive, and the problem of increasing the cost of ferroalloy due to the excessive amount of alloy input during converter operation occurs. Therefore, in the present invention, the Mn content is limited to 2.5 to 4.0%, and in terms of further improving the above-described effect, the lower limit of the Mn content may be 2.7%, or the upper limit of the Mn content may be 3.8%. .

P: 0.03% or less (excluding 0%)

Phosphorus (P) is an impurity element unavoidably contained in steel, and exists in excess of 0%. However, if the P content exceeds 0.03%, the weldability is lowered, the risk of brittleness of the steel increases, and the possibility of causing dent defects increases, so the upper limit of the P content is limited to 0.03%. Meanwhile, in terms of further improving the above effects, the upper limit of the P content may be 0.012%, or the lower limit of the P content may be 0.0005%.

S: 0.012% or less (excluding 0%)

Sulfur (S), like P, is an impurity element inevitably included in steel and exists in excess of 0%. However, since S is an element that inhibits the ductility and weldability of the steel sheet, if the S content exceeds 0.012%, it is highly likely to inhibit the ductility and weldability of the steel sheet, so it is preferable to limit the upper limit of the S content to 0.012%. Meanwhile, in terms of further improving the above effects, the upper limit of the S content may be 0.009%, or the lower limit of the S content may be 0.0001%.

Al: 0.1% or less (excluding 0%)

Aluminum (Al) is an alloying element that expands the ferrite range. Such Al has the disadvantage of promoting the formation of ferrite when using the continuous annealing type hot-dip plating heat treatment process in which slow cooling exists, as in the present invention, and since it is possible to lower the high-temperature hot rolling property by the formation of AlN, the upper limit of the Al content is limited to 0.1%. Meanwhile, in terms of further improving the above effects, the lower limit of the Al content may be 0.01%, or the upper limit of the Al content may be 0.08%.

Cr: 1% or less (excluding 0%)

Chromium (Cr) is an alloying element that facilitates securing a low-temperature transformation structure by inhibiting ferrite transformation, and contains more than 0% for the above-mentioned effect. The Cr has the advantage of suppressing the formation of ferrite when using the continuous annealing type hot-dip plating heat treatment process in which slow cooling exists, as in the present invention, but if it exceeds 1%, the cost of ferroalloy increases due to the excessive amount of alloy input. Since there is, the upper limit of the Cr content is limited to 1%. Meanwhile, in terms of further improving the above effects, the lower limit of the Cr content may be 0.01%, or the upper limit of the Cr content may be 0.5%.

Ti: 48/14 × [N] ~ 0.1% (however, the [N] represents the weight % content of nitrogen (N) in steel)

Titanium (Ti), as a nitride-forming element, performs scavenging by precipitating N in steel as TiN. In addition, if Ti is not added, there is concern about cracking during continuous casting due to AlN formation, so it is necessary to add 48/14×[N]% or more of Ti in chemical equivalent for the above effect. However, if the Ti content exceeds 0.1%, the martensite strength is reduced by additional carbide precipitation in addition to the removal of dissolved nitrogen (N), so the upper limit of the Ti content is limited to 0.1%. Meanwhile, in terms of further improving the above effects, the lower limit of the Ti content may be 0.01%, or the upper limit of the Ti content may be 0.08%.

Nb: 0.1% or less (excluding 0%)

Since niobium (Nb) is an element that is segregated at austenite grain boundaries and suppresses coarsening of austenite crystal grains during annealing heat treatment, it is necessary to add more than 0% of niobium (Nb). However, if the Nb content exceeds 0.1%, there is a problem in that the cost of ferroalloy increases due to an excessive amount of alloy input, so the upper limit of the Nb content is limited to 0.1%. Meanwhile, in terms of further improving the above effects, the lower limit of the Nb content may be 0.01%, or the upper limit of the Nb content may be 0.06%.

B: 0.005% or less (excluding 0%)

Boron (B) is an element that suppresses the formation of ferrite, especially since it has an advantage of suppressing the formation of ferrite during cooling after annealing, it is included in an amount greater than 0%. However, if the B content exceeds 0.005%, the formation of ferrite is promoted by the precipitation of Fe ₂₃ (C, B) ₆ rather, so the upper limit of the B content is limited to 0.005%. Meanwhile, in terms of further improving the above effects, the upper limit of the B content may be 0.003%, or the lower limit of the B content may be 0.0005%.

N: 0.01% or less (excluding 0%)

Nitrogen (N) is an impurity element that is unavoidably included in steel and exists in excess of 0%. However, if the N content exceeds 0.01%, the risk of cracking during playing through AlN formation or the like greatly increases. Therefore, in the present invention, it is preferable to limit the upper limit of the N content to 0.01%. Meanwhile, in terms of further improving the above effects, the upper limit of the N content may be 0.008%, or the lower limit of the N content may be 0.0005%.

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

The ultra-high strength steel sheet according to the present invention includes a microstructure, in area%, of martensite: 90% or more, and the total of ferrite and bainite: 10% or less. Since it is not easy to measure the microstructure in terms of volume fraction, which is a three-dimensional concept, the microstructure is measured in area fraction through cross-sectional observation cut in the thickness direction, which is a method used for normal microstructure observation. On the other hand, it is necessary to note that the microstructure of the ultra-high strength steel sheet has the same microstructure before and after heat treatment (tempering) described later.

As the configuration of the microstructure, since it is advantageous to secure ultra-high strength by having martensite, which is a hard phase, as the main phase, in the present invention, martensite is included at 90% or more. That is, if martensite is less than 90% in the microstructure of the super-strength steel sheet, a problem of not securing the target strength may occur. In terms of further maximizing the above effects, in the microstructure, the lower limit of the martensite area ratio may be 94%.

On the other hand, in terms of securing ultra-high strength, the higher the fraction of hard phase martensite is, the more advantageous it is to secure strength, so the upper limit of the martensite area ratio is not particularly limited. However, as an example of the present invention, the upper limit of the martensite area ratio may be 99%.

In addition, if the total amount of ferrite and bainite in the microstructure of the super-strength steel sheet exceeds 10%, a problem of not securing a target strength may occur. In terms of further maximizing the above effects, in the microstructure, the lower limit of the total area ratio of ferrite and bainite may be 1%, or the upper limit of the total area ratio of ferrite and bainite may be 6%.

In the ultra-high strength steel sheet according to the present invention, the value of M defined by the following relational expression 1 satisfies 100 to 500. If the value of M described below is less than 100, a problem of not securing the target strength may occur. On the other hand, if the value of M below exceeds 500, a problem of deteriorating impact properties and bendability of the steel may occur. Here, since the following relational expression 1 is an empirically obtained value, a unit may not be separately defined, and it is sufficient to satisfy only the unit of each variable defined below.

[Relationship 1]

M = P _size × P _number × [C] ^0.5 × [Mn] ² × [S]

(In the above relational expression 1, the P _size represents the average diameter of inclusions having a diameter of 1 μm or more, and the P _number represents the average number of inclusions having a diameter of 1 μm or more. [C] and [Mn] are brackets in the steel sheet, respectively. Indicates the average weight % content of the elements in the inside, and [S] shows the average ppm content of the elements in parentheses in the steel sheet.)

The inventors of the present invention, as a result of intensive research to provide an ultra-high strength steel material in which the yield strength is improved while maximally controlling the decrease in tensile strength, and at the same time, the extension flangeability and bendability are improved, as a result of It has been found that it is important to minimize the inclusions in the steel within a possible range while setting the component content to a limited range.

Specifically, in order to manufacture the ultra-high strength steel sheet according to the present invention, first, it is necessary to combine the contents of C, Mn, and S components of the heat-treated steel sheet in an optimized form. Therefore, in the relational expression 1, [C] represents the average weight % content of carbon (C) in the steel sheet, and [Mn] represents the average weight % content of manganese (Mn) in the steel sheet. On the other hand, [S] represents the average ppm content of sulfur (S) in the steel sheet. However, when the value of [S] is less than 30 ppm (0.003 wt%), the effect of sulfur (S) is similar to the case of 30 ppm, so when calculating the value of M, the value of [S] is defined as 30.

Meanwhile, all of the above-mentioned elements are elements that generate inclusions in steel, and examples thereof include sulfides such as MnS and carbides such as (Nb,Ti)C. In the present invention, an upper concept including both sulfide and carbide mentioned in order to explain the optimal tempering effect is inclusion. In order to suppress the formation of such inclusions, it is necessary to optimally combine the components of the mentioned elements, and manage the size and number of the generated inclusions to satisfy the relational expression 1 above. Inclusions generated in the steel become the starting point of crack generation, and because of this, the generation of inclusions lowers the impact properties of the steel type and causes a decrease in bendability. By controlling it, not only the strength characteristics and stretch flangeability of the steel sheet can be secured, but also the bendability can be improved.

In the present specification, the inclusion means sulfides and carbides such as MnS and (Nb,Ti)C. Commonly known types of inclusions include nitrides and the like, but in the present invention, inclusions formed from Mn, C, and S have a large effect on strength and bendability, so inclusions in this specification include sulfides and carbides (including carbonitrides) contains bay, but does not contain nitride.

In addition, among the inclusions, the average diameter [μm] of inclusions having a diameter of 1 μm or more is defined as P _size . At this time, the aforementioned inclusions may be composed of various forms such as MnS and carbides. When the shape is spherical, a diameter of 1 μm or more is determined as a major inclusion, and when it is not spherical, the diameter is measured assuming a sphere having the same area, and when the value is 1 μm or more, it is judged to be an effective inclusion. On the other hand, the measurement method is not particularly limited, but it is preferable to measure using a high-performance microscope with a magnification of 3000 times or more in order to accurately determine.

In addition, among the above inclusions, the average number [pieces] of inclusions having a diameter of 1 μm or more is defined as P _number . The method of measuring the average number of inclusions is not particularly limited, but, as in the embodiment of the present invention, it is preferable to measure using a high-performance microscope with a magnification of 3000 times or more, and as an example, a unit area of 100 to 600 μm ² It may mean the average number of inclusions having a diameter of 1 μm or more existing within the range. Meanwhile, in the present specification, if the average number of inclusions having a diameter of 1 μm or more is less than 1, the value of relational expression 1 is defined as 1. In order to increase the statistical accuracy of the number of inclusions per unit area, an average value of at least three measurements may be used.

According to one embodiment of the present invention, although not particularly limited, the ultra-high strength steel sheet may have a yield strength (YS) of 1140 to 1500 MPa and a tensile strength (TS) of 1470 to 1700 MPa. This is because, given the characteristics of the steel plate applied to the collision member, having a strength of the corresponding numerical value is appropriate when considering strength, weight reduction, formability, and productivity. Meanwhile, although not particularly limited, more preferably, in the ultra-high strength steel sheet, the lower limit of the yield strength may be 1250 MPa, or the upper limit of the yield strength may be 1350 MPa. In addition, in the ultra-high strength steel sheet, the lower limit of the tensile strength may be 1480 MPa, or the upper limit of the tensile strength may be 1600 MPa.

In addition, according to one embodiment of the present invention, although not particularly limited, the ultra-high strength steel sheet may have a yield ratio of 0.8 or more. This is because it is advantageous to have a high yield strength compared to tensile strength due to the characteristics of the steel sheet applied to the collision member. On the other hand, it is not particularly limited, but preferably in terms of further improving the above-described effect, in the ultra-high strength steel sheet, the lower limit of the yield ratio may be 0.84, or the upper limit of the yield ratio may be 0.90. .

In addition, according to one embodiment of the present invention, although not particularly limited, the ultra-high strength steel sheet may have an elongational flangeability (HER) of 25% or more. This is because it is desirable to have excellent stretch flangeability in order to process the ultra-high strength steel sheet through roll forming or press forming. On the other hand, although not particularly limited, in terms of further improving the above-mentioned effect, preferably, in the ultra-high strength steel sheet, the lower limit of the elongation flangeability (HER) may be 28%, or the elongation flangeability (HER) ) may be 40%.

In addition, according to one embodiment of the present invention, although not particularly limited, the ultra-high strength steel sheet may have a bendability R/t of 4 or less. This is because it is desirable to have excellent bending properties in order to process the ultra-high strength steel sheet through roll forming or press forming. On the other hand, although not particularly limited, in terms of further improving the above-mentioned effect, preferably, in the ultra-high strength steel sheet, the lower limit of the bendability (R / t) may be 2.6, or the bendability (R /t) may be 3.8.

In addition, according to one embodiment of the present invention, although not particularly limited, the ultra-high strength steel sheet may have an elongation (El) in the range of 3 to 13%. If the elongation is less than 3%, a problem of insufficient formability may occur, and if it exceeds 13%, a large amount of soft phases other than martensite in the steel may be formed, resulting in problems in workability to secure a stable target strength.

Next, a [method for manufacturing an ultra-high-strength steel sheet] according to another aspect of the present invention will be described in detail. However, this does not mean that the ultra-high strength steel sheet of the present invention must be manufactured by the following manufacturing method.

First, in terms of weight%, C: 0.12 to 0.4%, Si: 0.5% or less (excluding 0%), Mn: 2.5 to 4.0%, P: 0.03% or less (excluding 0%), S: 0.012% or less ( 0% or less), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14×[N] to 0.1%, Nb: 0.1% or less (0.1% or less) %), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance including Fe and other impurities, in area%, martensite: 90% or more, A steel sheet having a microstructure containing 10% or less of the total of ferrite and bainite is prepared. At this time, the above description can be equally applied to the alloy composition and microstructure of the steel sheet.

At this time, as the steel sheet before heat treatment (tempering) described later, cold-rolled steel sheet, hot-dip galvanized steel sheet, hot-dip galvanized steel sheet, electro-galvanized steel sheet, etc. can be used, and cold-rolled steel sheet, hot-dip galvanized steel sheet during or after heat treatment The properties of a steel sheet, hot-dip galvanized steel sheet, electro-galvanized steel sheet, etc. may be maintained as they are, or may be changed into a new type of steel sheet.

Subsequently, tempering (or rapid tempering) is performed on the steel sheet using an induction heater or the like. At this time, the tempering is controlled so that the value of P defined by the following relational expression 2 satisfies the range of 1.5 to 77.0.

[Relationship 2]

P =

(In the relational expression 2, the T represents the highest tempering temperature, and the unit is ° C. In addition, the t _eff represents the effective heat treatment time, and the unit is sec.)

In the ultra-high strength steel sheet having a yield ratio of less than 0.75, which is manufactured by passing through a continuous annealing furnace or a continuous annealing alloy plating furnace in which a slow cooling section exists, solid-solution carbon is fixed to dislocations introduced during martensite formation. At this time, the fixed carbon can be made to freely diffuse by rapid low-temperature tempering heat treatment through an induction heater, thereby increasing the ratio of yield strength to tensile strength. When the fixed carbon freely diffuses, the deformation of the material is suppressed by fixing the dislocation, and as a result, the yield strength is increased. Freeing the fixed carbon is a function of temperature and time like normal diffusion behavior. The higher the temperature and the longer the time, the more freely it can diffuse. However, if the temperature is too high and the time is too long, carbides are formed. Rather, the yield strength and tensile strength are reduced.

In addition, this increase in yield strength has the result of improving the stretch flangeability of the material. In general, elongational flangeability tends to increase as the yield strength increases at the same grade of tensile strength as the toughness increases. In addition, the strength difference between phases between microstructures in the material tends to increase as the difference in strength between phases decreases. Through tempering heat treatment, the difference in strength between phases due to the difference in cooling by position in the material can be reduced.

However, when the tempering temperature is high or the time is excessively long, the produced carbide is excessively coarsened, causing cracks to be generated at the corresponding location, thereby having a negative effect of reducing the stretch flangeability. Bending properties also tend to increase as the yield strength increases at the same grade of tensile strength, as the toughness of the material increases.

Therefore, it is possible to increase the bending properties through the tempering heat treatment under the appropriate conditions proposed in the present invention. However, when the heat treatment temperature is high or the heat treatment time is long, the produced carbide is excessively coarsened, and it becomes the starting point of crack generation during a bending test, and bending properties tend to deteriorate.

Therefore, the inventors of the present invention, as a result of intensive research to provide an ultra-high strength steel material in which the yield strength is improved while maximally controlling the decrease in tensile strength, and at the same time, the extension flangeability and bendability are improved, P defined by the above relational expression 2 It was confirmed that the above object can be achieved by controlling the tempering conditions so that the value of is in the range of 1.5 to 77.0. On the other hand, although it is not particularly limited, in terms of further improving the above-mentioned effect, the lower limit of the value of P defined by the relational expression 2 may be 15.8, or the upper limit of the value of P defined by the relational expression 2 may be 54.7 days. can

In the present specification, t _eff is an effective heat treatment time, and represents a residence time [sec] in a section reaching 90% or more of the highest temperature of the above-described tempering. At this time, whether or not 90% or more of the highest temperature of the tempering is reached is determined based on the absolute temperature [K].

In addition, according to one embodiment of the present invention, although not particularly limited, the T (maximum temperature of tempering) may satisfy the range of 100 to 300 ° C. If the T is less than 100 ° C, it may be difficult to induce the above-described diffusion behavior of carbon, and if the T exceeds 300 ° C, it may be difficult to achieve target physical properties due to excessive coarsening of carbides. On the other hand, it is not particularly limited, but preferably in terms of further improving the above-mentioned effect, the lower limit of the T may be 200 ℃, or the upper limit of the T may be 250 ℃.

In addition, according to one embodiment of the present invention, although not particularly limited, the t _eff may satisfy the range of 1 to 120 sec. If the t _eff is less than 1 second, there may be a problem in that the target strength cannot be stably secured due to an excessively short effective heat treatment time. In addition, when the t _eff exceeds 120 seconds, the heat treatment time may increase, which may cause productivity problems, and carbides may become coarse, resulting in deterioration in bendability.

In addition, according to one embodiment of the present invention, although not particularly limited, the tempering may satisfy the following relational expression 3.

[Relationship 3]

5 ≤ t _total ≤ 120

(In the relational expression 3, the t _total represents the total heat treatment time of tempering, and the unit is sec.)

That is, if the total heat treatment time (t _total ) of tempering is less than 5 seconds, it is difficult to secure enough time to induce carbon diffusion behavior, and even when the target heat treatment temperature is reached, facility restrictions may occur. On the other hand, controlling the upper limit of the total heat treatment time (t _total ) of tempering to 120 seconds or less is one of the key control conditions of the invention, and when the total heat treatment time (t _total ) of tempering exceeds 120 seconds, carbides are coarsened Therefore, it is difficult to achieve the target physical properties, and in particular, the adverse effect on the bending properties is very large. In addition, as the heat treatment time increases, productivity significantly decreases, and a separate additional process may be required. On the other hand, although not particularly limited, in terms of further improving the above-described effect, the lower limit of the total heat treatment time (t _total ) of the tempering may be 10 seconds, or the upper limit of the total heat treatment time (t _total ) of the tempering is 30 can be seconds

In addition, according to one embodiment of the present invention, although not particularly limited, the tempering may satisfy the following relational expression 4.

[Relationship 4]

1 ≤ t _heat ≤ 119

(In the relational expression 4, the t _heat represents the temperature rising time of tempering, and the unit is sec.)

According to one embodiment of the present invention, if the temperature increase time (t _heat ) of the tempering is less than 1 second, an excessively short temperature increase time may cause an overload problem of the heating facility or a problem that the steel material may not be heated evenly during heat treatment. there is. In addition, when the temperature rise time (t _heat ) of the tempering exceeds 119 seconds, productivity may decrease and it may be difficult to secure a sufficient holding time. On the other hand, although not particularly limited, in terms of further improving the above-mentioned effect, the lower limit of the temperature increase time (t _heat ) of the tempering may be 30 seconds, or the upper limit of the temperature increase time (t _heat ) of the tempering may be 50 seconds there is.

In addition, according to one embodiment of the present invention, although not particularly limited, the tempering may satisfy the following relational expression 5. That is, if the holding time of tempering (t _hold ) is less than 1 second, a problem may arise in which the target strength cannot be secured and the same physical properties cannot be secured at all positions of the steel. In addition, when the tempering holding time (t _hold ) exceeds 119 seconds, not only productivity is lowered, but also carbide is coarsened and bendability may be deteriorated. On the other hand, it is not particularly limited, but preferably in terms of further improving the above-mentioned effect, the lower limit of the holding time (t _hold ) of the tempering may be 15 seconds, or the upper limit of the holding time (t _hold ) of the tempering is It can be 30 seconds.

[Relationship 5]

1 ≤ t _hold ≤ 119

(In the relational expression 5, the t _hold represents the holding time of tempering, and the unit is sec.)

On the other hand, in the present specification, the relational expressions 4 and 5 refer to conditions that are satisfied when tempering is performed in the form of a general heating-holding-cooling. Therefore, when the heat treatment process of the steel material is not in the form of heating-holding-cooling, it is sufficient not to satisfy the conditions of the above relational expressions 4 and 5, and in this case, only the above-mentioned relational expression 3 is sufficient. On the other hand, examples of cases in which the above-described heat treatment process of steel materials is not in the form of heating-holding-cooling include repeating heating-holding-cooling several times during heat treatment, or omitting a holding or cooling step.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for explaining the present invention through examples, 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 preparing a steel sheet having the composition shown in Table 1 and the microstructure shown in Table 2 below, rapid tempering was performed on the steel sheet to satisfy the conditions shown in Table 3 below.

[wt%] C Mn S* Si P Al Cr Ti Nb B N Invention Steel A 0.18 3.6 36 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 Invention Steel B 0.16 3.5 90 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 Inventive Steel C 0.22 2.7 5 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 Invention Steel D 0.22 2.7 5 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 Inventive Steel E 0.29 3.7 90 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 Inventive Steel F 0.15 2.6 10 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 Inventive Steel G 0.26 3.2 95 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 comparative steel H 0.11 2.5 40 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 Comparative steel I 0.27 3 250 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004 comparative steel J 0.26 4.5 65 0.11 0.012 0.022 0.05 0.02 0.039 0.0016 0.004

However, S*: unit of S content is ppm

steel grade martensite
[area%] ferrite
[area%] bainite
[area%] Invention Example 1 Invention Steel A 96 4 0 Invention example 2 Invention Steel B 94 5 One Inventive example 3 Inventive Steel C 98 2 0 Inventive example 4 Invention Steel D 98 2 0 Comparative Example 1 Inventive Steel E 99 One 0 Comparative Example 2 Inventive Steel F 90 8 2 Comparative Example 3 Inventive Steel G 97 3 0 Comparative Example 4 Invention Steel A 96 4 0 Comparative Example 5 comparative steel H 80 5 15 Comparative Example 6 Comparative steel I 95 One 4 Comparative Example 7 comparative steel J 97 0 3

T* t _eff * t _heat * don't _hold * t _total * Tempering index, P* Invention example 1 200 25.4 20 20 40 15.8 Invention example 2 200 25.4 20 20 40 15.8 Inventive example 3 200 25.4 20 20 40 15.8 Inventive Example 4 250 24.7 20 20 40 54.7 Comparative Example 1 250 139.5 600 600 1200 90.7 Comparative Example 2 100 30.0 20 20 40 1.4 Comparative Example 3 250 139.5 600 600 1200 90.7 Comparative Example 4 300 24.2 20 20 40 189.4 Comparative Example 5 100 30.0 20 20 40 1.4 Comparative Example 6 200 25.4 20 20 40 15.8 Comparative Example 7 200 25.4 20 20 40 15.8

T* = highest tempering temperature [°C]

t _eff * residence time in the section reaching 90% or more of the highest tempering temperature [sec]

t _heat * = heating time of tempering [sec]

t _hold * = holding time of tempering [sec]

t _total * = total heat treatment time [sec]

P*=

After preparing cross-sectional specimens by cutting the steel sheets obtained from each of the invention examples and comparative examples in Table 3 in the thickness direction, the average diameter (P _size ) of inclusions having a diameter of 1 μm or more in the cross section and the average diameter of inclusions having a diameter of 1 μm or more in the cross section The number (P _number ) was measured in the same manner as described in the specification based on a unit area of 400 μm ² , and is shown in Table 4 below. However, when there were no inclusions having a diameter of 1 μm or more, the P _size and P _number were indicated as '1', respectively.

In addition, through a tensile test at room temperature, yield strength (YS), tensile strength (TS) and yield ratio (yield strength / tensile strength; YR) were calculated according to the ISO-6892 standard and are shown in Table 5 below.

In addition, for each comparative example and invention example, after measuring the yield strength (YS) and tensile strength (TS) values for each specimen before tempering heat treatment, based on the measured values, each after tempering heat treatment The amount of change in yield strength (ΔYS) and the amount of change in tensile strength (ΔTS) of the specimens were measured and shown in Table 5 below.

In addition, the elongation (El) was measured according to the ISO-6892 standard, and the elongation flangeability (HER) was measured by drilling a hole of 10 mm in a steel material and expanding the hole at a constant speed. In addition, the bendability (R / t) was measured by a method of pressing the steel material with an indenter having an R value of a certain size, and is shown in Table 5 below.

In addition, after cutting the steel material to a size of 1000 mm or more in length, placing it in a flat place and measuring the wave height, the flatness of the steel material was evaluated based on the maximum value of the wave height. In this case, when the maximum value of the crest height was less than 10 mm, the shape was evaluated as 'good', and when the maximum value of the crest height was 10 mm or more, it was evaluated as 'poor' and shown in Table 5 below.

note P _size [㎛] P _number [pcs] M* Invention example 1 One One 198 Invention example 2 One One 441 Inventive example 3 One One 103 Inventive example 4 1.4 2 287 Comparative Example 1 One One 664 Comparative Example 2 One One 26 Comparative Example 3 1.2 3 1786 Comparative Example 4 1.5 4 1188 Comparative Example 5 One One 83 Comparative Example 6 1.4 8 13094 Comparative Example 7 2.1 5 7047

M* = P _size × P _number × [C] ^0.5 × [Mn] ² × [S]

note YS [MPa] TS [MPa] YR El [%] HER [%] R/t flatness Invention Example 1 1311 1540 0.85 8.7 38 2.6 Good Invention example 2 1284 1498 0.86 9.1 40 2.8 Good Inventive example 3 1271 1511 0.84 8.5 37 3.3 Good Inventive example 4 1291 1484 0.87 8.3 28 3.8 Good Comparative Example 1 1387 1611 0.86 6.2 20 4.5 error Comparative Example 2 1071 1443 0.74 8.2 32 2.5 Good Comparative Example 3 1411 1615 0.87 6.5 21 4.5 error Comparative Example 4 1208 1425 0.85 10.5 24 5.3 Good Comparative Example 5 917 1221 0.75 13.1 28 2.8 error Comparative Example 6 1377 1594 0.86 6.1 18 5.7 error Comparative Example 7 1421 1657 0.86 5.4 17 5.8 error

As can be seen from the experimental results of Table 5, in the case of Inventive Examples 1 to 4 satisfying the alloy composition and manufacturing conditions of the present invention and satisfying the value of M defined from Relational Equation 1 in the range of 100 to 500, high yield strength And while securing the tensile strength, it was confirmed that the yield ratio, bendability, and stretch flangeability were excellent, and at the same time, the flatness was also excellent.

On the other hand, the alloy composition of the present invention is satisfied, but in the case of Comparative Examples 1 to 4 in which the value of P defined from relational expression 2 is less than 1.5 or exceeds 77.0, the tempering conditions are not appropriate, so that strength, yield ratio, bendability, It was confirmed that one or more properties of stretch flangeability and flatness were inferior.

On the other hand, in the case of Comparative Examples 5 to 7 that do not satisfy the alloy composition of the present invention, strength, bendability, stretch flangeability and flatness were all inferior.

Specifically, Comparative Example 5 is a steel grade that does not satisfy the alloy composition of the present invention, and specifically, the carbon content is insufficient. Carbon, an interstitial reinforcing element, is an element that greatly contributes to the increase in strength of steel, and due to the lack of such carbon, the tensile strength and yield strength do not reach the target values in the present invention. In Comparative Example 5, sufficient time and temperature were not secured in the tempering process, so the P value of Equation (2) was less than the target value in the present invention. Due to this, it was not possible to secure a sufficient increase in yield strength in the tempering process, and the yield strength after the tempering process was insufficient.

Comparative Example 6 is a case in which a steel grade in which the sulfur content exceeds the alloy composition targeted in the present invention is used. When the concentration of sulfur in the steel is high, sulfur reacts with manganese to generate inclusions such as manganese sulfide, and these inclusions greatly deteriorate the bending properties and stretch flangeability of the steel. Therefore, in Comparative Example 6, the M value of Equation (1), which was digitized in consideration of these factors, exceeded the target value in the present invention. For this reason, R/t, an index representing the bending properties, and HER, an index representing the stretch flangeability of Comparative Example 6, did not satisfy the target values of the present invention.

Comparative Example 7 is a steel grade in which the content of manganese exceeds that of the alloy composition targeted in the present invention. When the concentration of manganese in the steel is high, manganese reacts with sulfur to form inclusions such as manganese sulfide, and these inclusions greatly deteriorate the bending properties and stretch flangeability of the steel. Therefore, in Comparative Example 7, the M value of Equation (1), which was digitized in consideration of the above factors, exceeded the target value in the present invention. For this reason, it was confirmed that R/t, an index representing the bending properties, and HER, an index representing the stretch flangeability of Comparative Example 7, did not satisfy the target values in the present invention. In addition, when the concentration of manganese in the steel is high, manganese forms a band structure in the steel. This structural feature by manganese causes deterioration of bending and shape characteristics of steel. In addition, the increase in the content of manganese increases the tensile strength of the steel by improving the hardenability of the steel, and when the tensile strength exceeds the target value in the present invention, the shape is inferior during production of the steel. Since it is also difficult to correct the inferior shape, a problem occurs that the shape of the steel grade deteriorates. As a result, Comparative Example 7 did not satisfy the target values of the present invention in terms of tensile strength, HER, bendability, and flatness of the steel.

Claims (10)

In % by weight, C: 0.12 to 0.4%, Si: 0.5% or less (excluding 0%), Mn: 2.5 to 4.0%, P: 0.03% or less (excluding 0%), S: 0.012% or less (excluding 0%) excluding), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14×[N] to 0.1%, Nb: 0.1% or less (0% is excluding excluding), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance including Fe and other impurities,
As a microstructure, in area%, martensite: 90% or more, the sum of ferrite and bainite: 10% or less,
An ultra-high strength steel sheet in which the value of M defined from the following relational expression 1 satisfies the range of 100 to 500.
[Relationship 1]
M = P _size × P _number × [C] ^0.5 × [Mn] ² × [S]
(In the above relational expression 1, the P _size represents the average diameter of inclusions having a diameter of 1 μm or more, and the P _number represents the average number of inclusions having a diameter of 1 μm or more. [C] and [Mn] are brackets in the steel sheet, respectively. Indicates the average weight % content of the elements in the inside, and [S] shows the average ppm content of the elements in parentheses in the steel sheet.)
The method of claim 1,
Yield strength is 1140 ~ 1500MPa, tensile strength is 1470 ~ 1700MPa, ultra-high strength steel sheet.
The method of claim 2,
An ultra-high-strength steel sheet with a yield ratio of 0.8 or more.
The method of claim 1,
An ultra-high strength steel sheet having an elongational flange property of 25% or more and a bendability R/t of 4 or less.
In % by weight, C: 0.12 to 0.4%, Si: 0.5% or less (excluding 0%), Mn: 2.5 to 4.0%, P: 0.03% or less (excluding 0%), S: 0.012% or less (excluding 0%) excluding), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14×[N] to 0.1%, Nb: 0.1% or less (0% is excluding excluding), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), balance including Fe and other impurities, by area%, martensite: 90% or more, ferrite and Total of bainite: preparing a steel sheet having a microstructure containing 10% or less; and
Including; tempering the steel sheet;
A method for manufacturing an ultra-high strength steel sheet in which the value of P defined by the following relational expression 2 satisfies the range of 1.5 to 77.0.
[Relationship 2]
P =

(In the relational expression 2, the T represents the highest tempering temperature, and the unit is ° C. In addition, the t _eff represents the effective heat treatment time, and the unit is sec.)
The method of claim 5,
Wherein T satisfies the range of 100 to 300 ° C., a method for manufacturing an ultra-high strength steel sheet.
The method of claim 5,
The t _eff is a method for manufacturing an ultra-high strength steel sheet that meets the range of 1 to 120 sec.
The method of claim 5,
A method for manufacturing an ultra-high strength steel sheet that satisfies the following relational expression 3.
[Relationship 3]
5 ≤ t _total ≤ 120
(In the relational expression 3, the t _total represents the total heat treatment time of tempering, and the unit is sec.)
The method of claim 5,
A method for manufacturing an ultra-high strength steel sheet that satisfies the following relational expression 4.
[Relationship 4]
1 ≤ t _heat ≤ 119
(In the relational expression 4, the t _heat represents the temperature rising time of tempering, and the unit is sec.)
The method of claim 5,
A method for manufacturing an ultra-high strength steel sheet that satisfies the following relational expression 5.
[Relationship 5]
1 ≤ t _hold ≤ 119
(In the relational expression 5, the t _hold represents the holding time of tempering, and the unit is sec.)

Images