Ultra-high strength steel sheet with excellent bendability and stretch flangeability, and method for manufacturing same

Abstract

The present invention relates to an ultra-high strength steel sheet excellent in bendability and stretch-flange formability, and a manufacturing method thereof. More specifically, the present invention relates to a steel sheet having excellent bendability and stretch-flange formability, a high yield ratio, and ultra-high strength, which utilizes rapid low-temperature tempering, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to an ultra-high strength steel sheet excellent in bendability and stretch-flange formability, and a manufacturing method thereof, and more particularly to a steel sheet which utilizes rapid low-temperature tempering and has excellent bendability and stretch-flange formability, a high yield ratio, and ultra-high strength, and which has excellent bendability and stretch-flange formability, and a manufacturing method thereof. [Background technology]

[0002] In order to meet the difficult goals of reducing weight and ensuring crashworthiness, various types of automotive steel sheets have been developed, including dual phase steel (hereinafter referred to as "DP steel"), transformation induced plasticity steel (hereinafter referred to as "TRIP steel"), and complex phase steel (hereinafter referred to as "CP steel").

[0003] While these advanced high-strength steels can be made stronger by increasing the carbon content, when practical aspects such as spot weldability are taken into consideration, the tensile strength that can be achieved is limited to approximately 1200 MPa. Regarding their application to structural components to ensure collision safety, a method that secures final strength by quenching the steel through direct contact with a water-cooled die after forming at high temperatures has been gaining attention, but its application has not been widespread due to the high capital investment costs and the high costs involved in heat treatment and other processes.

[0004] As an alternative to the water-quenching method, slow cooling is generally used. However, in continuous annealing furnaces and continuous annealing-type hot-dip galvanizing lines that have a slow cooling section, martensitic steel with a fine structure fraction of 90% or more after annealing heat treatment has the disadvantage of a reduced yield strength, with the yield strength to tensile strength ratio being less than 0.75.

[0005] To improve the resistance of automobiles to collisions, it is desirable to increase the yield strength, and a method for improving this is required. Tempering of martensitic steel is usually performed to improve the insufficient ductility and toughness of martensitic steel, but a method for increasing the yield strength while minimizing the decrease in tensile strength is needed.

[0006] Furthermore, excellent bendability and stretch-flangeability are essential for processing martensitic steels through roll forming or press forming, etc. However, because ordinary martensitic steels often do not have sufficient bendability and stretch-flangeability for forming due to their extremely high strength, research is also needed to improve these properties.

[0007] In Patent Document 1 (Japanese Patent Publication No. 2528387), since it is necessary to rapidly cool the steel sheet to room temperature after annealing, there is a problem that the steel sheet cannot be produced unless it is produced on a line that has special equipment that can rapidly cool the steel sheet between the annealing furnace and the overaging furnace.

[0008] Furthermore, in Patent Document 2 (Korean Patent Publication No. 10-2010-0116608), high strength can be obtained by auto-tempering a steel sheet that has reached the Ms point, i.e., the martensitic transformation start temperature, to cause martensitic transformation and, at the same time, tempering the martensite after transformation. However, strict control of the heat treatment conditions at a temperature just below Ms is required, which poses a problem in terms of manufacturing stability.

[0009] In addition, Patent Document 3 (Korean Patent Publication No. 10-2014-0030970) suggests performing an additional heat treatment to achieve target physical properties, but this takes too long, significantly reducing productivity, and it is difficult to set efficient conditions to achieve the target physical properties. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2528387 [Patent Document 2] Korean Patent Publication No. 10-2010-0116608 [Patent Document 3] Korean Patent Publication No. 10-2014-0030970 Summary of the Invention [Problem to be solved by the invention]

[0011] According to one embodiment of the present invention, there are provided an ultra-high strength steel sheet having excellent bendability and stretch flangeability, and a method for producing the same.

[0012] The object of the present invention is not limited to the above-mentioned content, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0013] One embodiment of the present invention comprises: In weight percent, 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%, Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other impurities. The microstructure contains, by area%, martensite: 90% or more, and the total of ferrite and bainite: 10% or less, The present invention provides an ultra-high strength steel sheet in which the value of M defined by the following relational expression 1 falls within the range of 100 to 500. [Equation 1] M=P size ×P number ×[C] 0.5 ×[Mn] 2 ×[S] (In the above relational expression 1, the above P sizeindicates the average diameter of inclusions with a diameter of 1 μm or more, and the above P nember indicates the average number of inclusions with a diameter of 1 μm or more. The above [C] and [Mn] indicate the average weight percent content of the element in parentheses in the steel sheet, and [S] indicates the average ppm content of the element in parentheses in the steel sheet.

[0014] According to another embodiment of the present invention, preparing a steel sheet having a microstructure containing, by weight percent, 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%, Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other impurities, and containing, by area percent, martensite: 90% or more, and the sum of ferrite and bainite: 10% or less; and tempering the steel sheet; The present invention provides a method for producing an ultra-high strength steel sheet in which the value of P defined by the following relational expression 2 falls within the range of 1.5 to 77.0.

[0015]

number

[0016] (In the above relational expression 2, T indicates the maximum tempering temperature, and its unit is °C. Also, eff indicates the effective heat treatment time, in seconds.) [Effects of the Invention]

[0017] According to one embodiment of the present invention, it is possible to provide an ultra-high strength steel sheet having excellent bendability and stretch flangeability, and a method for producing the same.

[0018] Alternatively, according to one embodiment of the present invention, the yield strength of martensitic steel with a martensite fraction of 90% or more can be improved or one or more of the properties of bendability and stretch flangeability can be improved by subjecting a steel sheet with low yield strength produced in a continuous annealing furnace or a continuous annealing-type hot-dip galvanizing line having a slow cooling section to additional heat treatment.

[0019] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0020] [Figure 1] 1 shows photographs taken with a scanning electron microscope (SEM) to observe the microstructure of cross-sectional test pieces obtained by cutting the steel plates obtained from Comparative Example 2 and Invention Examples 1 to 3 in the thickness direction. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into several other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0022] Meanwhile, the terms used in this specification are intended to describe specific embodiments and are not intended to limit the present invention. For example, the singular form used in this specification includes the plural form unless a related definition clearly indicates a contrary meaning. Furthermore, the meaning of "comprises" used in this specification embodies a configuration and does not exclude the presence or addition of other configurations.

[0023] Conventionally, when a slow cooling method without quenching equipment is used, the slow cooling conditions in a continuous annealing furnace or continuous annealing-type hot-dip galvanizing line having a slow cooling section generally involve cooling to 650°C or 460°C, the deposition temperature of the hot-dip galvanizing bath, at a cooling rate of 3°C / s after annealing. Steel sheets having the composition of the present invention manufactured under the above conditions have a martensite fraction of 90% or more in the microstructure, but have the drawback of having an initial yield strength of 1000 to 1250 MPa, an initial tensile strength of 1200 to 1700 MPa, and a yield ratio of less than 0.75, resulting in a deteriorated yield strength.

[0024] In order to increase the resistance to automobile collisions, not only is it necessary to improve yield strength, but it is also necessary to improve bendability and stretch flangeability in order to process the steel through roll forming or press forming.

[0025] Therefore, an object of the present invention is to improve the yield strength of such ultra-high strength steel sheets having low yield strength while minimizing the decrease in tensile strength.

[0026] The present inventors have conducted extensive research to obtain a steel sheet that not only improves bendability and stretch flangeability but also satisfies the above-mentioned properties. As a result, they have found that it is effective to adjust the inclusion properties in the steel while controlling the C, Mn, and S content in the steel within limited ranges, and have thus completed the present invention.

[0027] Hereinafter, the ultra-high strength steel sheet having excellent bendability and stretch flangeability according to the present invention will be described in detail.

[0028] The high-strength steel plate according to the present invention contains, 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% (where [N] indicates the weight percent content of nitrogen (N) in the steel), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), with the balance being Fe and other impurities.

[0029] The reasons for adding elements to the steel sheet and the reasons for limiting the content thereof in the present invention will be specifically explained below. In this specification, the content of each element is expressed in wt% unless otherwise specified.

[0030] C: 0.12 to 0.4% Carbon (C) is an essential element for ensuring the strength of martensite and must be added in an amount of 0.12% or more. However, if the C content exceeds 0.4%, weldability deteriorates, so the upper limit of the C content is set to 0.4%. Meanwhile, in an embodiment that further improves the above-mentioned effects, the lower limit of the C content may be 0.15%, or the upper limit of the C content may be 0.30%.

[0031] Si: 0.5% or less (excluding 0%) Silicon (Si) is an element added to stabilize ferrite and must be present in an amount greater than 0% to achieve the above-mentioned effects. However, Si has the drawback of promoting ferrite formation during slow cooling after annealing in a conventional continuous annealing-type hot-dip galvanizing heat treatment furnace that has a slow cooling section, thereby reducing strength. Furthermore, when a large amount of Mn is added to suppress phase transformation as in the present invention, there is a risk of deterioration of hot-dip galvanizing properties due to the formation of surface oxides by Si during annealing, and of dent defects due to surface segregation and oxidation of Si. Therefore, the upper limit of the Si content is limited to 0.5%. Meanwhile, in an embodiment that further improves the above-mentioned effects, the lower limit of the Si content may be 0.1%, or the upper limit of the Si content may be 0.45%.

[0032] Mn: 2.5 to 4.0% Manganese (Mn) is an element that inhibits the formation of ferrite in steel and facilitates the formation of austenite. To ensure the above-mentioned effects, 2.5% or more of Mn is added. If the Mn content in steel is less than 2.5%, there is a problem that ferrite is easily formed during slow cooling in a continuous annealing type hot-dip galvanizing heat treatment furnace. Furthermore, if the Mn content exceeds 4.0%, excessive band formation due to segregation occurs in the slab and hot rolling processes, and there is a problem of increased costs for ferroalloys due to excessive alloy input during converter operation. Therefore, in the present invention, the Mn content is limited to 2.5 to 4.0%. In an embodiment that further improves the above-mentioned effects, the lower limit of the Mn content can be 2.7%, or the upper limit of the Mn content can be 3.8%.

[0033] P: 0.03% or less (excluding 0%) Phosphorus (P) is an unavoidable impurity element in steel and is present at more than 0%. However, if the P content exceeds 0.03%, the weldability deteriorates, the risk of steel embrittlement increases, and the possibility of dent defects increases. Therefore, the upper limit of the P content is limited to 0.03%. Meanwhile, in an embodiment that further improves the above-mentioned effects, the upper limit of the P content may be 0.012%, or the lower limit of the P content may be 0.0005%.

[0034] S: 0.012% or less (excluding 0%) Sulfur (S), like P, is an unavoidable impurity element contained in steel, and is present in amounts exceeding 0%. However, since S is an element that impairs the ductility and weldability of steel sheets, if the S content exceeds 0.012%, the ductility and weldability of steel sheets are likely to be impaired. Therefore, it is preferable to limit the upper limit of the S content to 0.012%. Meanwhile, in an embodiment that further improves the above-mentioned effects, the upper limit of the S content may be 0.009%, or the lower limit of the S content may be 0.0001%.

[0035] Al: 0.1% or less (excluding 0%) Aluminum (Al) is an alloying element that expands the ferrite region. When a continuous annealing-type hot-dip galvanizing heat treatment process with slow cooling is used as in the present invention, Al has the drawback of promoting ferrite formation, and the formation of AlN can lead to a decrease in high-temperature hot rollability. Therefore, the upper limit of the Al content is limited to 0.1%. To further improve the above-mentioned effects, the lower limit of the Al content may be 0.01%, or the upper limit of the Al content may be 0.08%.

[0036] Cr: 1% or less (excluding 0%) Chromium (Cr) is an alloying element that inhibits ferrite transformation, thereby facilitating the formation of a low-temperature transformation structure, and is included in an amount exceeding 0% to achieve the above-mentioned effects. The Cr content has the advantage of inhibiting ferrite formation when a continuous annealing type hot-dip galvanizing heat treatment process with slow cooling is used, as in the present invention. However, if the Cr content exceeds 1%, there is a problem of increased costs for the ferroalloy due to excessive alloy input. Therefore, the upper limit of the Cr content is limited to 1%. Meanwhile, in an embodiment that further improves the above-mentioned effects, the lower limit of the Cr content may be 0.01%, or the upper limit of the Cr content may be 0.5%.

[0037] Ti: 48 / 14 × [N] ~ 0.1% (where [N] above indicates the weight percent content of nitrogen (N) in the steel) Titanium (Ti) is a nitride-forming element and scavenges N in steel by precipitating TiN. Furthermore, if Ti is not added, AlN may form, causing cracks during continuous casting. Therefore, to achieve the above-mentioned effects, Ti must be added in a chemically equivalent amount of 48 / 14 × [N]% or more. However, if the Ti content exceeds 0.1%, not only will solute nitrogen (N) be removed but additional carbide precipitation will reduce martensite strength. Therefore, the upper limit of the Ti content is limited to 0.1%. To further improve the above-mentioned effects, the lower limit of the Ti content may be 0.01%, or the upper limit of the Ti content may be 0.08%.

[0038] Nb: 0.1% or less (excluding 0%) Niobium (Nb) is an element that segregates at austenite grain boundaries and suppresses coarsening of austenite grains during annealing heat treatment, so it must be added in an amount exceeding 0%. However, if the Nb content exceeds 0.1%, there is a problem that the cost of the 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 an embodiment that further improves the above-mentioned effects, the lower limit of the Nb content may be 0.01%, or the upper limit of the Nb content may be 0.06%.

[0039] B: 0.005% or less (excluding 0%) Boron (B) is an element that suppresses the formation of ferrite, and has the advantage of suppressing the formation of ferrite, especially during cooling after annealing. Therefore, it is contained in an amount exceeding 0%. However, if the B content exceeds 0.005%, the Fe content will increase. 23 Since the precipitation of (C, B)6 causes a problem of promoting ferrite formation, the upper limit of the B content is limited to 0.005%. On the other hand, in an embodiment that further improves the above-mentioned effects, the upper limit of the B content can be 0.003%, or the lower limit of the B content can be 0.0005%.

[0040] N: 0.01% or less (excluding 0%) Nitrogen (N) is an unavoidable impurity element in steel and is present in amounts exceeding 0%. However, if the N content exceeds 0.01%, the risk of cracks occurring during continuous casting due to the formation of AlN increases significantly. Therefore, in the present invention, it is preferable to limit the upper limit of the N content to 0.01%. Meanwhile, in an embodiment that further improves the above-mentioned effects, the upper limit of the N content can be 0.008%, or the lower limit of the N content can be 0.0005%.

[0041] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities are inevitably mixed in due to variables in the raw materials and the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal steel manufacturing process, the details of all of them will not be specifically mentioned in this specification.

[0042] The ultra-high strength steel sheet according to the present invention has a microstructure that contains, by area percentage, 90% or more of martensite and 10% or less of the total of ferrite and bainite. Because it is not easy to measure the microstructure in terms of volume fraction, which is a three-dimensional concept, the microstructure is measured in terms of area fraction through observation of a cross section cut in the thickness direction, a method commonly used in observing microstructures. It is important to note, however, that the microstructure of the ultra-high strength steel sheet has the same microstructure before and after the heat treatment (tempering) described below.

[0043] The microstructure of the steel sheet preferably contains 90% or more martensite, since having martensite, a hard phase, as the main phase is advantageous for ensuring ultra-high strength. In other words, if the microstructure of the ultra-high strength steel sheet contains less than 90% martensite, the target strength may not be achieved. To maximize the above-described effects, the lower limit of the martensite area ratio in the microstructure may be 94%.

[0044] On the other hand, from the viewpoint of ensuring ultra-high strength, a higher fraction of the hard phase martensite is advantageous for ensuring strength, so the upper limit of the martensite area fraction is not particularly limited, although, as an example of the present invention, the upper limit of the martensite area fraction may be 99%.

[0045] Furthermore, if the total area ratio of ferrite and bainite in the microstructure of the ultra-high strength steel sheet exceeds 10%, a problem may occur in that the target strength cannot be ensured. From the viewpoint of further maximizing the above-mentioned effect, the lower limit of the total area ratio of ferrite and bainite in the microstructure may be 1% or 2%, and the upper limit of the total area ratio of ferrite and bainite may be 6%.

[0046] Alternatively, although not particularly limited, according to one embodiment of the present invention, the microstructure of the ultra-high strength steel plate may further include, by area %, 1 to 5% ferrite and 1% or less (including 0%) bainite.

[0047] The ultra-high strength steel sheet according to the present invention satisfies the value of M defined by the following relational expression 1, which is 100 to 500. If the value of M is less than 100, a problem may arise in that the target strength cannot be ensured. On the other hand, if the value of M exceeds 500, a problem may arise in that the impact properties and bendability of the steel material deteriorate. Here, since the following relational expression 1 is an empirically obtained value, it is not necessary to define a separate unit, and it is sufficient that only the units of each variable defined below are satisfied. [Equation 1] M=P size ×P number ×[C] 0.5 ×[Mn] 2 ×[S] (In the above relational expression 1, the above P size indicates the average diameter of inclusions with a diameter of 1 μm or more, and the above P nemberindicates the average number of inclusions with a diameter of 1 μm or more. The above [C] and [Mn] indicate the average weight percent contents of the elements in parentheses in the steel sheet, and [S] indicates the average ppm content of the elements in parentheses in the steel sheet.

[0048] The present inventors have conducted extensive research to provide an ultra-high strength steel material that improves yield strength while minimizing the decrease in tensile strength, and at the same time improves stretch flangeability and bendability. As a result, they have discovered that it is important to minimize inclusions in the steel to the greatest extent possible while setting the C, Mn, and S content of the steel within limited ranges.

[0049] Specifically, to manufacture an ultra-high strength steel sheet according to the present invention, it is necessary to first combine the contents of the elements C, Mn, and S in a heat-treated steel sheet in an optimized form. Therefore, in the above Relational Equation 1, [C] represents the average carbon (C) content (wt%) in the steel sheet, [Mn] represents the average manganese (Mn) content (wt%) in the steel sheet, and [S] represents the average sulfur (S) content (ppm) in the steel sheet. However, when the [S] value is less than 30 ppm (0.003 wt%), the effect of sulfur (S) is similar to that of 30 ppm, so the [S] value is defined as 30 when calculating the value of M.

[0050] Meanwhile, all of the elements mentioned above are elements that form inclusions in steel, examples of which include sulfides such as MnS and carbides such as (Nb,Ti)C. The term "inclusions" is a broader concept that encompasses all of the sulfides and carbides mentioned in the present invention to explain the optimal tempering effect. To suppress the formation of such inclusions, it is necessary to optimally combine the components of the elements mentioned above and control the size and number of the inclusions formed so as to satisfy the above-mentioned Relational Formula 1. Inclusions formed in steel act as starting points for crack initiation, which reduces the impact properties of the steel and causes a decrease in bendability. Therefore, by controlling the contents of the above-mentioned elements and the properties of the inclusions, as shown in Relational Formula 1, it is possible to ensure the strength properties and stretch flangeability of the steel sheet, as well as improve bendability.

[0051] In this specification, the inclusions refer to sulfides and carbides such as MnS and (Nb, Ti)C. Although nitrides are also commonly known types of inclusions, in the present invention, the inclusions that have a significant effect on strength and bendability are those formed from Mn, C, and S. Therefore, in this specification, the inclusions include only sulfides and carbides (including carbonitrides), but do not include nitrides.

[0052] In addition, the average diameter [μm] of the above inclusions with a diameter of 1 μm or more is defined as P size The inclusions can be in various forms, such as MnS and carbides. If the inclusions are spherical, those with a diameter of 1 μm or more are considered to be major inclusions. If the inclusions are not spherical, they are assumed to be spheres with the same area and their diameters are measured. If the diameter is 1 μm or more, the inclusions are considered to be effective inclusions. While there are no particular limitations on the measurement method, it is preferable to measure using a high-performance microscope with a magnification of 3000x or more for accurate determination.

[0053] In addition, the average number of inclusions with a diameter of 1 μm or more among the above inclusions is defined as P number The method for measuring the average number of inclusions is not particularly limited, but it is preferable to measure using a high-performance microscope with a magnification of 3000 times or more, as in the examples of the present invention. 2 It can mean the average number of inclusions with a diameter of 1 μm or more present within the range. Meanwhile, in this specification, when the average number of inclusions with a diameter of 1 μm or more is less than 1, the value of Relation 1 is defined as 1. In order to increase the statistical accuracy of the numerical value of the number of inclusions present per unit area, the average value of at least three measurements can be used.

[0054] On the other hand, although not particularly limited, according to one embodiment of the present invention, in order to further improve the above-mentioned effects, the lower limit of the value of M may be 103, or the upper limit of the value of M may be 441.

[0055] According to one embodiment of the present invention, 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, although this is not particularly limited. This is because, in terms of the properties of steel sheets used in collision members, having strengths within these numerical values ​​is appropriate when considering strength, weight reduction, formability, and productivity. Meanwhile, although not particularly limited, more preferably, the ultra-high strength steel sheet may have a lower limit of the yield strength of 1250 MPa or an upper limit of the yield strength of 1350 MPa. Furthermore, the ultra-high strength steel sheet may have a lower limit of the tensile strength of 1480 MPa or an upper limit of the tensile strength of 1600 MPa.

[0056] Furthermore, 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 a high yield strength relative to tensile strength is advantageous in terms of the properties of steel sheets used in collision members. Meanwhile, although not particularly limited, from the viewpoint of further improving the above-mentioned effects, preferably, the lower limit of the yield ratio of the ultra-high strength steel sheet may be 0.84, or the upper limit of the yield ratio may be 0.90.

[0057] Furthermore, according to one embodiment of the present invention, although not particularly limited, the ultrahigh strength steel sheet may have a stretch flangeability (HER) of 25% or more. This is because excellent stretch flangeability is preferred for processing ultrahigh strength steel sheets through roll forming, press forming, or the like. On the other hand, although not particularly limited, from the viewpoint of further improving the above-mentioned effects, preferably, the lower limit of the stretch flangeability (HER) of the ultrahigh strength steel sheet may be 28%, or the upper limit of the stretch flangeability (HER) may be 40%.

[0058] Furthermore, 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 excellent bending properties are preferable for processing the ultra-high strength steel sheet through roll forming, press forming, or the like. Meanwhile, although not particularly limited, from the viewpoint of further improving the above-mentioned effects, preferably, in the ultra-high strength steel sheet, the lower limit of the bendability (R / t) may be 2.6, or the upper limit of the bendability (R / t) may be 3.8.

[0059] Furthermore, according to one embodiment of the present invention, the ultra-high strength steel sheet may have an elongation (El) in the range of 3 to 13%, although this is not particularly limited. If the elongation is less than 3%, a problem of insufficient formability may occur, and if it exceeds 13%, a large amount of soft phase other than martensite is formed in the steel, which may cause a problem in operability for ensuring a stable target strength.

[0060] Next, a "method for manufacturing an ultra-high strength steel sheet" according to still another embodiment of the present invention will be described in detail below. However, this does not necessarily mean that the ultra-high strength steel sheet of the present invention needs to be manufactured by the manufacturing method described below.

[0061] First, a steel sheet is prepared having a microstructure containing, by weight, 0.12 to 0.4% C, 0.5% or less (excluding 0%) Si, 2.5 to 4.0% Mn, 0.03% or less (excluding 0%) P, 0.012% or less (excluding 0%) S, 0.1% or less Al, 1% or less Cr, 48 / 14 × [N] to 0.1%, 0.1% or less Nb, 0.005% or less (excluding 0%) B, 0.01% or less N, the balance being Fe and other impurities, and having, by area, 90% or more martensite and 10% or less of the total of ferrite and bainite. The above-mentioned explanations are similarly applicable to the alloy composition and microstructure of the steel sheet.

[0062] In this case, the steel sheet before the heat treatment (tempering) described below may be a cold-rolled steel sheet, a hot-dip galvanized steel sheet, a hot-dip zinc-alloy plated steel sheet, an electro-galvanized steel sheet, or the like, and during or after the heat treatment, the properties of the cold-rolled steel sheet, the hot-dip galvanized steel sheet, the hot-dip zinc-alloy plated steel sheet, the electro-galvanized steel sheet, or the like may be maintained as they are, or may be changed into a steel sheet of a new form.

[0063] Subsequently, the steel sheet is tempered (or rapid tempered) using an induction heater, etc. At this time, the tempering is controlled so that the value of P defined by the following relational expression 2 falls within the range of 1.5 to 77.0.

[0064]

number

[0065] (In the above relational expression 2, T indicates the maximum tempering temperature, and its unit is °C. Also, eff indicates the effective heat treatment time, in seconds.)

[0066] Ultra-high-strength steel sheets with a yield ratio of less than 0.75, which are produced by passing through a continuous annealing furnace or a continuous annealing alloy plating furnace with a slow cooling section, have solute carbon fixed at the potential introduced during martensite formation. Rapid low-temperature tempering using an induction heater allows the fixed carbon to diffuse freely, thereby increasing the yield strength to tensile strength ratio. Once the fixed carbon is free to diffuse, the potential is fixed, suppressing material deformation and ultimately increasing yield strength. Like normal diffusion behavior, the freeing of the fixed carbon is a function of temperature and time. The higher the temperature and the longer the time, the more free the carbon can diffuse. However, if the temperature is too high and the time is too long, the yield strength and tensile strength decrease due to the formation of carbides.

[0067] Furthermore, this increase in yield strength results in improved stretch flangeability of the material. Generally, stretch flangeability tends to increase as the yield strength increases for the same tensile strength grade, due to increased toughness. Furthermore, stretch flangeability tends to increase as the difference in strength between the microstructure phases within the material decreases. However, tempering heat treatment can reduce the difference in strength between phases due to differences in cooling at different locations within the material.

[0068] However, if the tempering temperature is too high or the tempering time is too long, the carbides that are formed become excessively coarse, which induces crack formation at the corresponding locations and has the adverse effect of reducing stretch flangeability. For the same tensile strength, the higher the yield strength, the higher the toughness of the material tends to increase as well.

[0069] Therefore, it is possible to improve bending properties through tempering heat treatment under appropriate conditions as proposed in the present invention. However, if the heat treatment temperature is high or the heat treatment time is long, the generated carbides tend to become excessively coarse, which can become the starting point for crack generation during bending experiments, resulting in poor bending properties.

[0070] Therefore, the present inventors have conducted extensive research to provide an ultra-high strength steel material that improves yield strength while minimizing the decrease in tensile strength, and at the same time improves stretch flangeability and bendability. As a result, they have confirmed that the above-mentioned object can be achieved by controlling the tempering conditions so that the value of P defined in the above Relational Formula 2 falls within the range of 1.5 to 77.0.

[0071] On the other hand, although not particularly limited, from the viewpoint of further improving the above-mentioned effect, the lower limit of the value of P defined in the above relational formula 2 may be 15.8, or the upper limit of the value of P defined in the above relational formula 2 may be 54.7.

[0072] In this specification, the above t effis the effective heat treatment time, which indicates the residence time [sec] in the section where the tempering temperature reaches 90% or more of the maximum tempering temperature. Whether or not the tempering temperature reaches 90% or more of the maximum tempering temperature is determined based on the absolute temperature [K].

[0073] Furthermore, according to one embodiment of the present invention, although not particularly limited, the T (maximum tempering temperature) may be in the range of 100 to 300°C. If the T is less than 100°C, it may be difficult to induce the above-mentioned carbon diffusion behavior, and if the T exceeds 300°C, the carbides may become excessively coarse, making it difficult to achieve the target physical properties. Meanwhile, although not particularly limited, from the viewpoint of further improving the above-mentioned effects, preferably, the lower limit of the T may be 200°C, or the upper limit of the T may be 250°C.

[0074] Furthermore, according to one embodiment of the present invention, although not particularly limited, the above-mentioned t eff The above t can be in the range of 1 to 120 seconds. eff If the time is less than 1 second, the target strength cannot be stably secured due to the extremely short effective heat treatment time. eff If the heat treatment time exceeds 120 seconds, not only will the heat treatment time be prolonged, which may cause productivity problems, but the carbides may become coarse, which may result in a decrease in bendability.

[0075] Furthermore, according to one embodiment of the present invention, although not particularly limited, the tempering can be performed so as to satisfy the following relational expression 3. [Equation 3] 5≦t total ≦120 (In the above relational expression 3, the above t total indicates the total heat treatment time for tempering, in seconds.)

[0076] That is, the total heat treatment time for tempering (t totalIf the total heat treatment time for tempering (t ) is less than 5 seconds, it is difficult to ensure sufficient time for the carbon diffusion behavior to occur, and equipment limitations may arise even when the target heat treatment temperature is reached. total ) is one of the key control conditions of the present invention, and the total heat treatment time (t total If the heat treatment time exceeds 120 seconds, the carbides become coarse, making it difficult to achieve the desired physical properties, and in particular, the adverse effect on bending properties is significant. In addition, as the heat treatment time increases, productivity decreases significantly, and an additional process may be required. On the other hand, although not particularly limited, in order to further improve the above-mentioned effects, it is preferable to set the total heat treatment time (t total The lower limit of the total heat treatment time (t total ) can have an upper limit of 30 seconds.

[0077] Furthermore, according to one embodiment of the present invention, although not particularly limited, the tempering can be performed so as to satisfy the following relational expression 4. [Equation 4] 1≦t heat ≦119 (In the above relational expression 4, the above t heat indicates the tempering temperature rise time, in seconds.)

[0078] According to one embodiment of the present invention, the temperature rise time (t heat If the temperature rise time (t) is less than 1 second, the heating equipment may be overloaded due to a very short temperature rise time, or the steel may not be uniformly heated during heat treatment. heat If the time exceeds 119 seconds, productivity may decrease and it may become difficult to maintain a sufficient time. On the other hand, although not particularly limited, from the viewpoint of further improving the above-mentioned effects, From the above, the temperature rise time of the tempering (t heat The lower limit of the tempering temperature rise time (t heat ) can have an upper limit of 50 seconds.

[0079] Furthermore, according to one embodiment of the present invention, although not particularly limited, the tempering can be performed so as to satisfy the following relational expression 5. That is, the tempering maintenance time (t hold If the tempering time (t) is less than 1 second, the target strength cannot be secured, and the same physical properties cannot be secured at all positions of the steel material. hold If the tempering time (t ) exceeds 119 seconds, not only will productivity decrease, but the carbides will become coarse and the bendability will decrease. On the other hand, although not particularly limited, from the viewpoint of further improving the above-mentioned effects, it is preferable to set the tempering time (t hold The lower limit of the tempering time (t hold ) can be 30 seconds. [Equation 5] 1≦t hold ≦119 (In the above relational expression 5, the above t hold indicates the tempering duration, in seconds.)

[0080] In this specification, the above-mentioned relations 4 and 5 refer to the conditions that are satisfied when tempering is performed in a general heating-maintaining-cooling manner. Therefore, if the heat treatment process of the steel does not take the heating-maintaining-cooling form, it is sufficient not to satisfy the conditions of the above-mentioned relations 4 and 5, and in this case it is sufficient to satisfy only the above-mentioned relation 3. Meanwhile, examples of cases where the heat treatment process of the steel does not take the heating-maintaining-cooling form include cases where heating-maintaining-cooling is repeated several times during heat treatment, or where the maintaining or cooling step is omitted. [Example]

[0081] (Example) The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0082] Steel sheets having the compositions shown in Table 1 below and the microstructures shown in Table 2 below were prepared, and then the steel sheets were subjected to rapid tempering so as to satisfy the conditions shown in Table 3 below.

[0083] [Table 1]

[0084] However, the unit of S*:S content is ppm

[0085] [Table 2]

[0086] [Table 3]

[0087] T* = Maximum tempering temperature [℃] t eff * = Residence time in the section where the tempering temperature reaches 90% or more of the maximum temperature [sec] t heat * = Tempering temperature rise time [sec] t hold *=Tempering duration [sec] t total *=Total heat treatment time [sec]

[0088]

number

[0089] The steel sheets obtained from the examples and comparative examples in Table 3 above were cut in the thickness direction to prepare cross-sectional test pieces, and the average diameter (P size ) and the average number of inclusions with a diameter of 1 μm or more (P number ) to 400 μm 2 The results are shown in Table 4 below. However, if there are no inclusions with a diameter of 1 μm or more, the above P size and P number are indicated by "1" respectively.

[0090] In addition, room temperature tensile tests were conducted, and the yield strength (YS), tensile strength (TS) and yield ratio (yield strength / tensile strength; YR) were calculated in accordance with the ISO-6892 standard, and are shown in Table 5 below.

[0091] Furthermore, for each of the comparative examples and invention examples described below, the yield strength (YS) and tensile strength (TS) values ​​of each test specimen were measured before the tempering heat treatment, and then, based on the above measured values, the change in yield strength (ΔYS) and the change in tensile strength (ΔTS) of each test specimen after the tempering heat treatment were measured, and the results are shown in Table 5 below.

[0092] The elongation (El) was measured according to ISO-6892, and the stretch flangeability (HER) was measured by drilling a 10 mm hole in the steel and expanding the hole at a constant speed. The bendability (R / t) was also measured by pressing the steel with an indenter having a constant R value, and the results are shown in Table 5 below.

[0093] In addition, the steel material was cut into pieces 1000 mm or longer in length, placed on a flat surface, and the wave height was measured. The flatness of the steel material was evaluated based on the maximum wave height. If the maximum wave height was less than 10 mm, the shape was evaluated as "good," and if the maximum wave height was 10 mm or greater, the shape was evaluated as "poor," as shown in Table 5 below.

[0094] [Table 4]

[0095] M*=P size ×P number ×[C] 0.5 ×[Mn] 2 ×[S]

[0096] [Table 5]

[0097] As can be seen from the experimental results in Table 5 above, in the case of Invention Examples 1 to 4, which satisfy the alloy composition and manufacturing conditions of the present invention and the value of M defined by Relational Formula 1 is in the range of 100 to 500, it was confirmed that while high yield strength and tensile strength are ensured, the yield ratio, bendability, and stretch flangeability are excellent, and at the same time, flatness is also excellent.

[0098] On the other hand, in Comparative Examples 1 to 4, which satisfy the alloy composition of the present invention but have a P value defined by Relational Formula 2 of less than 1.5 or more than 77.0, it was confirmed that the tempering conditions were inappropriate, resulting in deterioration of one or more of the properties of strength, yield ratio, bendability, stretch flangeability, and flatness.

[0099] On the other hand, in the case of Comparative Examples 5 to 7, which do not satisfy the alloy composition of the present invention, the strength, bendability, stretch flangeability and flatness were all deteriorated.

[0100] Specifically, Comparative Example 5 is a steel type that does not satisfy the alloy composition of the present invention, specifically, the carbon content is insufficient. Carbon, an interstitial strengthening element, significantly contributes to increasing the strength of steel types, and the lack of carbon resulted in the tensile strength and yield strength not reaching the values ​​targeted by the present invention. Furthermore, Comparative Example 5 was unable to ensure sufficient time and temperature in the tempering process, and the P value in formula (2) was less than the value targeted by the present invention. As a result, a sufficient increase in yield strength could not be ensured in the tempering process, and the yield strength after the tempering process was insufficient.

[0101] Comparative Example 6 is a case where a steel type with a sulfur content exceeding the alloy composition targeted in the present invention was used. When the sulfur concentration in steel is high, sulfur reacts with manganese to form inclusions such as manganese sulfides, which significantly degrade the bending properties and stretch-flange formability of the steel. Therefore, in Comparative Example 6, the M value in Equation (1), which was calculated by taking these factors into consideration, exceeded the target value targeted in the present invention. As a result, the R / t, an index indicating bending properties, and the HER, an index indicating stretch-flange formability, of Comparative Example 6 did not meet the target values ​​targeted in the present invention.

[0102] Comparative Example 7 is a steel grade with a manganese content exceeding the alloy composition targeted in the present invention. When the manganese concentration in steel is high, manganese reacts with sulfur to form inclusions such as manganese sulfides, which significantly degrade the bending properties and stretch-flangeability of the steel. Therefore, the M value of Comparative Example 7, calculated by taking the above factors into consideration, exceeded the target value targeted in the present invention. As a result, the R / t, an index of bending properties, and the HER, an index of stretch-flangeability, of Comparative Example 7 did not meet the target values ​​targeted in the present invention. Furthermore, when the manganese concentration in the steel is high, manganese forms a band structure within the steel. This structural characteristic of manganese causes deterioration of the bending properties and shape properties of the steel. Furthermore, an increase in the manganese content improves the hardenability of the steel, increasing its tensile strength. However, if the tensile strength exceeds the target value targeted in the present invention, the shape of the steel deteriorates during production, and correcting this deteriorated shape becomes difficult, resulting in poor shape. As a result, Comparative Example 7 did not satisfy the target values ​​of the present invention for any of the tensile strength, HER, bendability, and flatness of the steel type.

Claims

1. The alloy contains, 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%, Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), with the balance being Fe and other impurities; The microstructure contains, in area percentage, martensite: 90% or more, and the total of ferrite and bainite: 10% or less, An ultra-high strength steel plate in which the value of M defined by the following relational expression 1 falls within the range of 100 to 500. [Relationship 1] M=P ｓｉｚｅ ×P ｎｕｍｂｅｒ ×[C] ０．５ ×[Mn] ２ ×[S] (In the above-mentioned relational expression 1, the P ｓｉｚｅ indicates the average diameter of inclusions with a diameter of 1 μm or more, and ｎｕｍｂｅｒ indicates the average number of inclusions with a diameter of 1 μm or more. The [C] and [Mn] represent the average weight percent contents of the elements in parentheses in the steel sheet, and [S] represents the average ppm content of the elements in parentheses in the steel sheet.

2. 2. The ultra-high strength steel plate according to claim 1, wherein the yield strength is 1140 to 1500 MPa and the tensile strength is 1470 to 1700 MPa.

3. The ultra-high strength steel plate according to claim 2, wherein the yield ratio is 0.8 or more.

4. 2. The ultra-high strength steel sheet according to claim 1, wherein the stretch flangeability HER is 25% or more and the bendability R / t is 4 or less.

5. preparing a steel sheet having a microstructure containing, 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%, Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other impurities, and containing, by area %, martensite: 90% or more, and the sum of ferrite and bainite: 10% or less; and tempering the steel plate; The method for producing an ultrahigh strength steel plate according to claim 1, wherein the value of P defined by the following relational expression 2 satisfies the range of 1.5 to 77.0: [Equation 1] (In the above-mentioned relational expression 2, the T indicates the maximum tempering temperature, and its unit is ° C. ｅｆｆ indicates the effective heat treatment time, and its unit is seconds. t eff is the effective heat treatment time, and indicates the residence time [sec] in the section where the temperature reaches 90% or more of the maximum tempering temperature. Whether or not the temperature reaches 90% or more of the maximum tempering temperature is determined based on the absolute temperature [K].

6. The method for producing an ultra-high strength steel plate according to claim 5, wherein the T satisfies the range of 100 to 300°C.

7. Said t ｅｆｆ The method for producing an ultra-high strength steel plate according to claim 5, wherein the time satisfies the range of 1 to 120 seconds.

8. The method for producing an ultra-high strength steel plate according to claim 5, wherein the following relational expression 3 is satisfied: [Relationship 3] 5≦t ｔｏｔａｌ ≦120 (In the above-mentioned relational expression 3, the t ｔｏｔａｌ indicates the total heat treatment time for tempering, in seconds.)

9. The method for producing an ultra-high strength steel plate according to claim 5, wherein the following relational expression 4 is satisfied: [Relationship 4] 1≦t ｈｅａｔ ≦119 (In the above-mentioned relational expression 4, the t ｈｅａｔ indicates the tempering temperature rise time, in seconds.)

10. The method for producing an ultra-high strength steel plate according to claim 5, wherein the following relational expression 5 is satisfied: [Relationship 5] 1≦t ｈｏｌｄ ≦119 (In the above-mentioned relational expression 5, the t ｈｏｌｄ indicates the tempering maintenance time, and the unit is sec.)

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