CN118019873A

CN118019873A - Ultrahigh-strength steel sheet excellent in bendability and stretch flangeability and method for producing same

Info

Publication number: CN118019873A
Application number: CN202280065147.1A
Authority: CN
Inventors: 金炵完; 具民书
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-11-12
Filing date: 2022-10-27
Publication date: 2024-05-10
Also published as: WO2023085660A1; KR20230069426A; US20240175103A1

Abstract

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

Description

Ultrahigh-strength steel sheet excellent in bendability and stretch flangeability and method for producing same

Technical Field

Background

In order to meet the contradictory goals of ensuring weight reduction and collision safety, various steel sheets for automobiles, such as Dual-phase steel (Dual PHASE STEEL, hereinafter referred to as "DP steel"), transformation-induced plasticity steel (Transformation Induced PLASTICITY STEEL, hereinafter referred to as "TRIP steel"), and Complex-phase steel (Complex PHASE STEEL, hereinafter referred to as "CP steel"), have been developed.

In such advanced high-strength steels, the strength can be further increased by increasing the carbon content, but the achievable tensile strength is limited to about 1200MPa level in view of practicality such as spot weldability. Among applications in structural members for ensuring collision safety, a method of ensuring final strength by rapid cooling in direct contact with a mold (Die) for water cooling after molding at high temperature has been attracting attention, but the applications cannot be expanded due to excessive equipment investment costs and high heat treatment and process costs.

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

In order to increase the resistance against an automobile collision, it is preferable to further increase the yield strength, for which an improvement is required. Tempering of martensitic steels is generally performed to improve insufficient ductility and toughness of martensitic steels, and thus a scheme for improving yield strength while suppressing the decrease in tensile strength to the maximum is required.

In addition, in order to process martensitic steel by roll forming, press forming, or the like, excellent bendability and stretch flangeability are required. However, since the conventional martensitic steel has very high strength, sufficient bendability and stretch flangeability required for forming cannot be ensured in most cases, and thus studies for improving bendability and stretch flangeability have been conducted.

In patent document 1 (japanese patent publication No. 2528387), since rapid cooling to room temperature is required after annealing, there is a problem that it is impossible to manufacture a steel sheet without providing a special equipment line between an annealing furnace and an overaging furnace, which can rapidly cool the steel sheet.

In addition, in patent document 2 (korean laid-open patent publication No. 10-2010-016608), for a steel sheet reaching the Ms point, that is, the martensite transformation start temperature, high strength can be obtained by an autotempering treatment in which martensite after transformation is tempered while the steel sheet is caused to undergo martensite transformation, but heat treatment conditions for strictly controlling the temperature of Ms or less are required, and thus there is a problem in terms of manufacturing stability.

Further, patent document 3 (korean laid-open patent publication No. 10-2014-0030970) proposes performing additional heat treatment to achieve the target physical properties, but has a problem in that productivity is excessively lowered due to an excessively long time or it is difficult to set effective conditions to achieve the target physical properties.

(Patent document 1) Japanese patent publication No. 2528387

(Patent document 2) korean laid-open patent publication No. 10-2010-016608

(Patent document 3) Korean laid-open patent publication No. 10-2014-0030970

Disclosure of Invention

Technical problem to be solved

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

The technical problems of the present invention are not limited to the above. Additional technical problems of the present invention will be readily apparent to one of ordinary skill in the art from the present description.

Technical proposal

One aspect of the present invention provides an ultra-high strength steel sheet comprising, in weight percent: c:0.12-0.4%, si: less than 0.5% (except 0%), mn:2.5-4.0%, P: less than 0.03% (except 0%), S:0.012% or less (except 0%), al: less than 0.1% (except 0%), cr: less than 1% (except 0%), ti:48/14 x [ N ] to 0.1%, nb:0.1% or less (except 0%), B: less than 0.005% (except 0%), N:0.01% or less (excluding 0%), the balance of Fe and other impurities, as a microstructure, comprising in area%: martensite: 90% or more, sum of ferrite and bainite: 10% or less, wherein the M value defined by the following relation 1 satisfies the range of 100 to 500.

[ Relation 1]

M＝P_{Size of the device (size)}×P_{Quantity of (number)}×[C]^0.5×[Mn]²×[S]

(In the above-mentioned relation 1, P _{Size of the device} represents the average diameter of inclusions having a diameter of 1 μm or more, P _{Quantity of} represents the average number of inclusions having a diameter of 1 μm or more, the [ C ] and the [ Mn ] represent the average weight% content of the element in brackets in the steel sheet, respectively, and the [ S ] represents the average ppm content of the element in brackets in the steel sheet.)

Further, according to another aspect of the present invention, there is provided a method of manufacturing an ultra-high strength steel sheet, the method comprising the steps of: preparing a steel sheet comprising, in weight percent: c:0.12-0.4%, si: less than 0.5% (except 0%), mn:2.5-4.0%, P: less than 0.03% (except 0%), S:0.012% or less (except 0%), al: less than 0.1% (except 0%), cr: less than 1% (except 0%), ti:48/14 x [ N ] to 0.1%, nb:0.1% or less (except 0%), B: less than 0.005% (except 0%), N:0.01% or less (excluding 0%), the balance of Fe and other impurities, and as a microstructure, the steel sheet contains, in area%: martensite: 90% or more, sum of ferrite and bainite: less than 10 percent; and tempering the steel sheet, wherein a P value defined by the following relation 2 satisfies a range of 1.5 to 77.0.

[ Relation 2]

( In the relation 2, T represents the highest temperature of tempering in ℃. The t _eff represents an effective heat treatment time, and the unit is seconds (sec). )

Advantageous effects

According to one aspect of the present invention, an ultra-high strength steel sheet excellent in bendability and stretch flangeability and a method for manufacturing the same can be provided.

Or according to an aspect of the present invention, by additionally heating a steel sheet having a low yield strength manufactured on a continuous annealing furnace or a continuous annealing type hot dip plating line in which a slow cooling zone exists, the yield strength of martensitic steel having a martensite fraction of 90% or more can be improved, or one or more of the characteristics of bendability and stretch flangeability can be improved.

The various advantageous advantages and effects of the present invention are not limited to the above, and will be more readily understood in describing particular embodiments of the present invention.

Drawings

Fig. 1 is a view showing photographs taken by a Scanning Electron Microscope (SEM) in order to observe a microstructure of a cross-sectional test piece taken in the thickness direction of the steel sheet obtained in comparative example 2 and invention examples 1 to 3 of the present invention.

Best mode for carrying out the invention

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified in various forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, embodiments of the present invention are provided to more fully illustrate the invention to those skilled in the art.

In addition, the terminology used in the description presented herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. For example, as used in this specification, the singular forms also include the plural unless the context clearly indicates to the contrary. Furthermore, the use of "including" or "comprising" in the specification is intended to embody the components and does not exclude the presence or addition of other components.

In the case of the conventional slow cooling method using a rapid cooling-free facility, the slow cooling conditions of the continuous annealing furnace or continuous annealing type hot dip plating line having a slow cooling section are usually configured such that the continuous annealing furnace or continuous annealing type hot dip plating line is cooled to 650 ℃ at a cooling rate of 3 ℃/sec after annealing or 460 ℃ as the immersion temperature of the hot dip plating bath. In the steel sheet having the component system of the present invention produced under the above-described conditions, the martensite fraction as a microstructure is 90% or more, the initial yield strength is at a level of 1000 to 1250MPa, the initial tensile strength is at a level of 1200 to 1700MPa, and the yield ratio is less than 0.75, which has a disadvantage of poor yield strength.

However, in order to improve the resistance against collision of the automobile, not only the yield strength but also the bendability and the stretch flangeability are required to be improved for processing by roll forming or press forming.

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

The present inventors have conducted intensive analysis to obtain a steel sheet satisfying the above characteristics while improving the bendability, stretch flangeability, etc., and have found that it is effective to adjust the characteristics of inclusions in steel while controlling the content of C, mn and S in the steel within a limited range, and have completed the present invention.

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

The high-strength steel sheet according to the present invention comprises, in weight percent: c:0.12-0.4%, si: less than 0.5% (except 0%), mn:2.5-4.0%, P: less than 0.03% (except 0%), S:0.012% or less (except 0%), al: less than 0.1% (except 0%), cr: less than 1% (except 0%), ti:48/14 x [ N ] to 0.1% (wherein [ N ] represents the weight% content of nitrogen (N) in the steel), nb:0.1% or less (except 0%), B: less than 0.005% (except 0%), N: less than 0.01% (except 0%), and the balance of Fe and other impurities.

The reasons for adding components and the reasons for limiting the content of the steel sheet according to the present invention will be described in detail below. In this case, unless otherwise specified, the content of each element is expressed as weight% in the present specification.

C：0.12-0.4％

Carbon (C) is an element necessary for securing the strength of martensite, and should be added at 0.12% or more. However, when the C content exceeds 0.4%, the weldability is deteriorated, and thus the upper limit thereof is limited to 0.4%. In addition, in terms of further improving the above-described 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: less than 0.5% (except 0%)

Silicon (Si) is an element added to stabilize ferrite, and needs to be present in an amount exceeding 0% for the above effect. However, si has a disadvantage in that ferrite formation is promoted when slow cooling is performed after annealing in a conventional continuous annealing type hot dip coating heat treating furnace having a slow cooling section, thereby deteriorating strength. Further, when a large amount of Mn is added in order to suppress phase transition as described in the present invention, there are a risk of deterioration of hot dip plating characteristics due to formation of surface oxide of Si at the time of annealing and a risk of dent defects due to surface enrichment and oxidation of Si, so that the upper limit of Si content is limited to 0.5%. In addition, 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：2.5-4.0％

Manganese (Mn) is an element that suppresses the formation of ferrite in steel and easily forms austenite, and 2.5% or more of Mn is added to ensure the above effects. When the Mn content in the steel is less than 2.5%, ferrite is easily formed at the time of slow cooling in the case of the continuous annealing type hot dip plating heat treatment furnace. Further, when the Mn content exceeds 4.0%, excessive formation of bands is caused due to segregation caused in slab and hot rolling processes, and there is a problem in that the cost of alloyed iron is increased due to excessive addition of the alloy at the time of converter operation. Therefore, in the present invention, the Mn content is limited to 2.5 to 4.0%, and in terms of further improving the above effects, the lower limit of the Mn content may be 2.7%, or the upper limit of the Mn content may be 3.8%.

P: less than 0.03 percent (except 0 percent)

Phosphorus (P) is an impurity element inevitably contained in steel, and is present in an amount exceeding 0%. However, when the P content exceeds 0.03%, weldability decreases, the risk of occurrence of brittleness of the steel increases, and the possibility of causing dent defects increases, so that the upper limit of the P content is limited to 0.03%. In addition, 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: less than 0.012% (except 0%)

Like P, sulfur (S) is an impurity element inevitably contained in steel, and is present in an amount exceeding 0%. However, S is an element that inhibits ductility and weldability of the steel sheet, and when the S content exceeds 0.012%, the possibility of inhibiting ductility and weldability of the steel sheet is high, so that the upper limit of the S content is preferably limited to 0.012%. In addition, 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: less than 0.1% (except 0%)

Aluminum (Al) is an alloying element that enlarges the ferrite region. Such Al has a disadvantage of promoting ferrite formation when using a continuous annealing type hot dip plating heat treatment process with slow cooling as in the present invention, and high temperature hot rolling property may be lowered due to AlN formation, thus limiting the upper limit of Al content to 0.1%. In addition, in terms of further improving the above-described 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: less than 1% (except 0%)

Chromium (Cr) is an alloy element that easily secures a low-temperature transformation structure by suppressing ferrite transformation, and is contained in an amount exceeding 0% for the above effect. When the continuous annealing type hot dip plating heat treatment process with slow cooling is utilized as in the present invention, the Cr has an advantage of suppressing the formation of ferrite, but when the Cr content exceeds 1%, there is a problem in that the cost of the alloy iron increases due to the excessive addition amount of the alloy, so that the upper limit of the Cr content is limited to 1%. In addition, in terms of further improving the above-described 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 x [ N ] to 0.1% (wherein [ N ] represents the weight% content of nitrogen (N) in steel)

Titanium (Ti) is an element forming a nitride, and N in steel is precipitated as TiN and removed (scavenging). In addition, when Ti is not added, cracks may occur during continuous casting because AlN is formed, and therefore, 48/14× [ N ]% or more of Ti is required to be added in terms of chemical equivalent to the above effects. However, when the Ti content exceeds 0.1%, the martensite strength is lowered due to further precipitation of carbides in addition to removal of solid solution nitrogen (N), and therefore the upper limit of the Ti content is limited to 0.1%. In addition, in terms of further improving the above-described 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: less than 0.1% (except 0%)

Niobium (Nb) is an element that segregates at austenite grain boundaries and suppresses coarsening of austenite grains during annealing heat treatment, and therefore it is necessary to add more than 0% niobium. However, when the Nb content exceeds 0.1%, there is a problem in that the cost of alloyed iron increases due to an excessive amount of added alloy, and thus the upper limit of the Nb content is limited to 0.1%. In addition, 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: less than 0.005% (except 0%)

Boron (B) is an element that suppresses the formation of ferrite, and particularly has an advantage of suppressing the formation of ferrite when cooling is performed after annealing, and is contained in an amount exceeding 0%. However, when the B content exceeds 0.005%, the problem of promoting ferrite formation due to precipitation of Fe ₂₃(C,B)₆ instead occurs, and therefore the upper limit of the B content is limited to 0.005%. In addition, in terms of further improving the above-described 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: less than 0.01 percent (except 0 percent)

Nitrogen (N) is an impurity element inevitably contained in steel, and is present in an amount exceeding 0%. However, when the N content exceeds 0.01%, the risk of occurrence of cracks upon continuous casting by formation of AlN or the like greatly increases. Therefore, in the present invention, the upper limit of the N content is preferably defined as 0.01%. In addition, in terms of further improving the above-described 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 remainder of the invention is iron (Fe). However, in the usual production process, unwanted impurities are inevitably mixed in due to raw materials or environmental variables, and therefore cannot be excluded. These impurities are well known to the skilled person of the usual steel manufacturing process and are therefore not specifically mentioned in the present description in their entirety.

As a microstructure, the ultra-high strength steel sheet according to the present invention comprises, in area%: martensite: 90% or more, sum of ferrite and bainite: less than 10%. The method of measuring the microstructure using a volume fraction as a three-dimensional concept is not easy, and thus the microstructure is measured by a method generally used when observing the microstructure, that is, by observing an area fraction of a cross section taken in a thickness direction. Note that the microstructure of the ultra-high strength steel sheet has the same microstructure before and after heat treatment (tempering) described later.

The microstructure has a structure in which martensite as a hard phase (HARD PHASE) is a main phase, and thus it is advantageous to ensure ultra-high strength, and therefore, the present invention includes 90% or more of martensite. That is, when the martensite in the microstructure of the super-strength steel sheet is less than 90%, there may occur a problem that the desired strength cannot be ensured. In order to further maximize the above effect, the lower limit of the area ratio of the martensite in the microstructure may be 94%.

In order to ensure ultra-high strength, the higher the fraction of martensite as the hard phase, the more advantageous the strength is to be ensured, and therefore the upper limit of the area ratio of martensite is not particularly limited. But as an example of the present invention, the upper limit of the area ratio of the martensite may be 99%.

Further, when the sum of ferrite and bainite in the microstructure of the super-strength steel sheet exceeds 10%, there is a possibility that a problem may arise in that the desired strength cannot be ensured. In further maximizing the above effect, the lower limit of the total area ratio of ferrite and bainite in the microstructure may be 1% or 2%, or the upper limit of the total area ratio of ferrite and bainite may be 6%.

Or, although not particularly limited, according to an aspect of the present invention, the microstructure of the super strength steel sheet may further include, in area%: ferrite: 1-5% and bainite: less than 1% (including 0%).

In the ultra-high strength steel sheet according to the present invention, the M value defined by the following relation 1 satisfies 100 to 500. When the following M value is less than 100, there is a possibility that a problem may arise in that the desired strength cannot be ensured. On the other hand, when the following M value exceeds 500, there may occur problems that impact properties and bendability of the steel material are deteriorated. Since the following relation 1 is an empirically obtained value, the unit may not be defined alone, and the unit of each variable defined below may be satisfied.

[ Relation 1]

M＝P_{Size of the device}×P_{Quantity of}×[C]^0.5×[Mn]²×[S]

( In the relation 1, P _{Size of the device} represents the average diameter of the inclusions having a diameter of 1 μm or more, and P _{Quantity of} represents the average number of the inclusions having a diameter of 1 μm or more. The terms [ C ] and [ Mn ] each denote the average weight% content of the element in brackets in the steel sheet, and [ S ] denotes the average ppm content of the element in brackets in the steel sheet. )

The present inventors have conducted intensive studies to provide an ultra-high strength steel product which is improved in yield strength while minimizing the decrease in tensile strength, and which is improved in stretch flangeability and bendability, and as a result, have found that it is important to minimize inclusions in steel within a possible range while setting the component contents of C, mn and S in the steel grade to a limited range.

Specifically, in order to manufacture the ultra-high strength steel sheet according to the present invention, it is first necessary to combine the contents of C, mn, S components of the heat-treated steel sheet in an optimized form. Therefore, in the above-mentioned relation 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. In addition, [ S ] represents the average ppm content of sulfur (S) in the steel sheet. But when the S value has a value of less than 30ppm (0.003 wt%), the effect caused by sulfur (S) is similar to that when the S value is 30ppm, and therefore the S value is defined as 30 when the M value is calculated.

In addition, the above-mentioned elements are elements forming inclusions in steel, and include sulfides such as MnS and carbides such as (Nb, ti) C as examples. In the present invention, the generic concept, which includes both sulfides and carbides, is referred to as inclusion for the purpose of explaining the best tempering effect. In order to suppress the generation of such inclusions, it is necessary to optimally combine the components of the mentioned elements and control the size and the number of the generated inclusions so as to satisfy the above-mentioned relational expression 1. Since the inclusions formed in the steel become starting points for the occurrence of cracks, the formation of the inclusions reduces the impact properties of the steel grade and causes a phenomenon of reduced bendability, as shown in relation 1, not only the strength properties and the stretch flangeability of the steel sheet but also the bendability can be improved by controlling the content of the above components and the properties of the inclusions.

In the present specification, the inclusions refer to sulfides and carbides such as MnS and (Nb, ti) C. In general, the known inclusions are of the type of nitrides and the like, but in the present invention, the inclusions formed of Mn, C and S have a large influence on strength and bendability, and thus the inclusions in the present specification include only sulfides, carbides (including carbonitrides), and do not include nitrides.

Among the inclusions, the average diameter [ μm ] of the inclusions having a diameter of 1 μm or more was defined as P _{Size of the device}. In this case, the inclusions may be composed of various forms such as MnS and carbide. If the shape of the inclusions is spherical, the inclusions having a diameter of 1 μm or more are judged as main inclusions, and if the inclusions are not spherical, the inclusions are assumed to have a spherical shape having the same area and the diameters are measured, and if the values are 1 μm or more, the inclusions are judged as effective inclusions. The measurement method is not limited to a single method, but for accurate determination, it is preferable to measure with a high-performance microscope having a magnification of 3000 times or more.

Among the inclusions, the average number [ number ] of inclusions having a diameter of 1 μm or more was defined as P _{Quantity of}. Although the method of measuring the average number of the inclusions is not particularly limited, it is preferable to measure the inclusions using a high-performance microscope having a magnification of 3000 times or more as in the embodiment of the present invention, and as an example, it may be an average number of inclusions having a diameter of 1 μm or more existing in a range of 100 to 600 μm ² per unit area. In the present specification, when the average number of inclusions having a diameter of 1 μm or more is smaller than 1, the value of the relation 1 is defined as 1. In order to improve the statistical accuracy of the number of inclusions present per unit area, an average value of the measured values of at least 3 times or more may be used.

In addition, although not particularly limited, according to an aspect of the present invention, in an aspect of further improving the above-described effects, the lower limit of the M value may be 103, or the upper limit of the M value may be 441.

According to an embodiment of the present invention, although not particularly limited, the ultra-high strength steel sheet may have a Yield Strength (YS) of 1140-1500MPa and a Tensile Strength (TS) of 1470-1700MPa. This is because, in terms of the characteristics of the steel sheet used for the collision member, the strength having such a value is suitable in view of strength, weight saving, formability, and productivity. In addition, although not particularly limited, more preferably, in the ultra-high strength steel sheet, the lower limit of the yield strength may be 1250MPa, or the upper limit of the yield strength may be 1350MPa. Further, in the ultra-high strength steel sheet, the lower limit of the tensile strength may be 1480MPa, or the upper limit of the tensile strength may be 1600MPa.

In addition, according to one embodiment of the present invention, although not particularly limited, the yield ratio of the ultra-high strength steel sheet may be 0.8 or more. This is because it is advantageous that the yield strength is higher than the tensile strength in terms of the characteristics of the steel sheet for the collision member. In the ultra-high strength steel sheet, the lower limit of the yield ratio may preferably be 0.84 or the upper limit of the yield ratio may preferably be 0.90, although not particularly limited thereto, in order to further enhance the above-described effects.

In addition, according to an embodiment of the present invention, although not particularly limited, the stretch flangeability (HER) of the ultra-high strength steel sheet may be 25% or more. This is because it is preferable to have excellent stretch flangeability in order to process an ultra-high strength steel sheet by roll forming, press forming, or the like. In the ultra-high strength steel sheet, the lower limit of the stretch flangeability (HER) may be 28% or the upper limit of the stretch flangeability (HER) may be 40%, although not particularly limited, in order to further improve the above-described effects.

Further, according to an embodiment of the present invention, although not particularly limited, the bending R/t of the ultra-high strength steel sheet may be 4 or less. This is because it is preferable to have excellent bending characteristics in order to process an ultra-high strength steel sheet by roll forming, press forming, or the like. In addition, although not particularly limited, in order to further improve the above-described effects, it is preferable that the lower limit of the bendability (R/t) in the ultra-high strength steel sheet is 2.6 or the upper limit of the bendability (R/t) is 3.8.

Further, according to an embodiment of the present invention, although not particularly limited, the elongation (El) of the ultra-high strength steel sheet may be in the range of 3 to 13%. When the elongation is less than 3%, there may occur a problem of insufficient formability, and when the elongation exceeds 13%, a large amount of soft phases other than martensite are formed in the steel, and thus there may occur a problem of operability for securing stable target strength.

Next, a detailed description will be given of [ a method of manufacturing an ultra-high strength steel sheet ] according to another aspect of the present invention. However, the ultra-high strength steel sheet of the present invention is not necessarily produced by the following production method.

First, a steel sheet is prepared, which comprises, in weight%: c:0.12-0.4%, si: less than 0.5% (except 0%), mn:2.5-4.0%, P: less than 0.03% (except 0%), S:0.012% or less (except 0%), al: less than 0.1% (except 0%), cr: less than 1% (except 0%), ti:48/14 x [ N ] to 0.1%, nb:0.1% or less (except 0%), B: less than 0.005% (except 0%), N:0.01% or less (excluding 0%), the balance of Fe and other impurities, and as a microstructure, the steel sheet contains, in area%: martensite: 90% or more, sum of ferrite and bainite: less than 10%. In this case, the above description can be similarly applied to the alloy composition and microstructure of the steel sheet.

In this case, as the steel sheet before heat treatment (tempering) described later, a cold-rolled steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, an electrogalvanized steel sheet, or the like may be used, and properties of the cold-rolled steel sheet, the hot-dip galvanized steel sheet, the alloyed hot-dip galvanized steel sheet, the electrogalvanized steel sheet, or the like may be maintained during or after the heat treatment, or may be converted into a new steel sheet.

Next, the steel sheet is tempered (or flash tempered) using an induction heater or the like. At this time, the tempering control is such that the P value defined by the following relation 2 satisfies the range of 1.5 to 77.0.

[ Relation 2]

( In the relation 2, T represents the highest temperature of tempering in ℃. And, t _eff represents an effective heat treatment time in seconds. )

In the case of an ultra-high strength steel sheet having a yield ratio of less than 0.75 manufactured by a continuous annealing furnace or a continuous annealing alloy plating furnace having a slow cooling zone, solid solution carbon is fixed to dislocations introduced when martensite is formed. At this time, the fixed carbon is freely subjected to diffusion behavior by the rapid low-temperature tempering heat treatment of the induction heater, so that the ratio of yield strength to tensile strength can be increased. When the fixed carbon freely undergoes diffusion behavior, deformation of the material is suppressed by fixing dislocations, ultimately increasing yield strength. In the same way as in the usual diffusion behavior, the fixed carbon is also allowed to freely diffuse as a function of temperature and time, and when the temperature is too high and the time is too long, the yield strength and tensile strength are reduced due to carbide formation.

In addition, this increase in yield strength has the result of improving the stretch flangeability of the material. In general, the elongation flangeability tends to increase as the yield strength increases and the toughness increases at the same level of tensile strength. Further, the smaller the inter-phase strength difference between the fine tissues in the material, the more the stretch flangeability tends to increase, and by tempering heat treatment, the inter-phase strength difference due to the difference in cooling at different positions in the material can be reduced.

However, when the tempering temperature is high or the time is excessively long, the generated carbide is excessively coarse, thereby inducing the generation of cracks at this position, thereby adversely affecting the reduction of stretch flangeability. As for the bending property, the higher the yield strength is at the same level of tensile strength, the toughness of the material increases, and thus the bending property tends to be improved.

Accordingly, the bending property can be improved by 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 carbide is too coarse, and therefore becomes a starting point of cracking during the bending test, and the bending property tends to be deteriorated.

Accordingly, the present inventors have conducted intensive studies to provide an ultra-high strength steel material that can maximize the control of the decrease in tensile strength and the increase in yield strength, and that can simultaneously increase the stretch flangeability and bendability, and as a result, have confirmed that the above object can be achieved by controlling the tempering conditions such that the P value defined by the above-mentioned relational expression 2 satisfies the range of 1.5 to 77.0.

In addition, although not particularly limited, in terms of further improving the above-described effects, the lower limit of the P value defined by the above-described relation 2 may be 15.8, or the upper limit of the P value defined by the above-described relation 2 may be 54.7.

In the present specification, t _eff is an effective heat treatment time, and represents a residence time [ seconds ] in a range of 90% or more of the highest temperature at which the tempering is performed. At this time, it is judged whether or not 90% or more of the highest temperature of the tempering is reached based on the absolute temperature [ K ].

Furthermore, according to one embodiment of the present invention, although not particularly limited, the T (highest temperature of tempering) may satisfy a range of 100 to 300 ℃. When the T is lower than 100 ℃, it may be difficult to induce the diffusion behavior of the carbon, and when the T exceeds 300 ℃, carbide is too coarse, and it may be difficult to achieve desired physical properties. In addition, although not particularly limited, in terms of further improving the above-described effects, the lower limit of T may be preferably 200 ℃, or the upper limit of T may be 250 ℃.

Further, according to an embodiment of the present invention, the t _eff may satisfy the range of 1 to 120 seconds, although not particularly limited. When the t _eff is less than 1 second, a problem may occur in that the target strength cannot be stably ensured due to the effective heat treatment time being too short. Further, when t _eff exceeds 120 seconds, problems of productivity may occur due to the long heat treatment time, and the bendability may be reduced due to coarsening of carbide.

Further, according to an embodiment of the present invention, although not particularly limited, the tempering may satisfy the following relation 3.

[ Relation 3]

5≤t_{Total (S) (total)}≤120

(In the above-mentioned relation 3, t _{Total (S)} represents the total heat treatment time of tempering, the unit is seconds.)

That is, when the total heat treatment time (t _{Total (S)}) of tempering is less than 5 seconds, it is difficult to secure a sufficient time to induce the diffusion behavior of carbon, and there may occur restrictions on equipment in reaching the target heat treatment temperature. On the other hand, controlling the upper limit of the total heat treatment time (t _{Total (S)}) of tempering to 120 seconds or less is one of the core control conditions of the invention, and when the total heat treatment time (t _{Total (S)}) of tempering exceeds 120 seconds, it is difficult to achieve desired physical properties due to coarsening of carbide, and particularly, adverse effects on bending characteristics are very large. In addition, as the heat treatment time becomes longer, productivity is greatly reduced, and a case may occur in which an additional process is required. In addition, although not particularly limited, in terms of further improving the above-described effects, the lower limit of the total heat treatment time (t _{Total (S)}) of tempering may be 10 seconds, or the upper limit of the total heat treatment time (t _{Total (S)}) of tempering may be 30 seconds.

Further, according to a specific embodiment of the present invention, although not particularly limited, the tempering may be performed so as to satisfy the following relation 4.

[ Relation 4]

1≤t_{Heating (heat)}≤119

(In the above-mentioned relational expression 4, t _Heating represents the temperature rise time of tempering, and the unit is seconds.)

According to an embodiment of the present invention, when the tempering temperature rise time (t _Heating) is less than 1 second, an overload problem of the heating apparatus occurs due to an excessively short temperature rise time, or a problem that the steel material cannot be uniformly heat-treated and heated may occur. Further, when the temperature rise time (t _Heating) of the tempering exceeds 119 seconds, there may occur problems in that productivity is lowered and it is difficult to secure a sufficient holding time. In addition, although not particularly limited, in terms of further improving the above-described effects, the lower limit of the tempering temperature rise time (t _Heating) may be 30 seconds, or the upper limit of the tempering temperature rise time (t _Heating) may be 50 seconds.

Further, according to an embodiment of the present invention, although not particularly limited, the tempering may satisfy the following relation 5. That is, when the holding time (t _{Holding (hold)}) of tempering is less than 1 second, there may occur a problem that the desired strength cannot be ensured and the same physical properties of all positions of the steel cannot be ensured. Further, when the tempering retention time (t _Holding) exceeds 119 seconds, not only productivity is lowered, but also carbide becomes coarse, and thus there is a possibility that a problem of lowering of bendability occurs. In addition, although not particularly limited, in order to further improve the above-described effects, the lower limit of the holding time (t _Holding) for tempering may be preferably 15 seconds, or the upper limit of the holding time (t _Holding) for tempering may be preferably 30 seconds.

[ Relation 5]

1≤t_Holding≤119

(In the above-mentioned relation 5, t _Holding represents the holding time of tempering, and the unit is seconds.)

In the present specification, the above-mentioned relational expressions 4 and 5 refer to conditions that are satisfied when tempering is performed in a conventional temperature raising-maintaining-cooling manner. Therefore, when the heat treatment process of the steel material is not in the form of temperature increase-holding-cooling, the conditions of the above-mentioned relational expression 4 and relational expression 5 may not be satisfied, and in this case, only the above-mentioned relational expression 3 may be satisfied. In addition, examples of the case where the heat treatment process of the steel material is not in the form of temperature rising-holding-cooling include a case where temperature rising-holding-cooling is repeated a plurality of times at the time of heat treatment, a case where the holding or cooling step is omitted, or the like.

Detailed Description

Example (example)

Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted, however, that the following examples are provided by way of illustration only and are not intended to limit the scope of the claims. This is because the scope of the invention is determined by what is recited in the claims and what is reasonably derived from this disclosure.

A steel sheet having the composition described in table 1 below and the microstructure described in table 2 below was prepared, and then the steel sheet was flash tempered so as to satisfy the conditions described in table 3 below.

TABLE 1

[ Wt.%)	C	Mn	S*	Si	P	Al	Cr	Ti	Nb	B	N
												Inventive steel A	0.18	3.6	36	0.11	0.012	0.022	0.05	0.02	0.039	0.0016	0.004
Inventive 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
Inventive 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

Wherein S: the unit of S content is ppm.

TABLE 2

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TABLE 3

	T*	t_eff*	t_Heating*	t_Holding*	t_{Total (S)}*	Tempering index, P
							Inventive example 1	200	25.4	20	20	40	15.8
Inventive 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 temperature of tempering [ DEGC ]

T _eff x = residence time in the interval above 90% of the maximum temperature of tempering [ seconds ]

T _Heating x = tempering ramp time [ seconds ]

T _Holding x = hold time for tempering [ seconds ]

T _{Total (S)} x = total heat treatment time [ seconds ]

The steel sheets obtained from each of the invention examples and comparative examples of Table 3 were cut in the thickness direction to produce cross-section test pieces, and then the average diameter (P _{Size of the device}) of inclusions having a diameter of 1 μm or more and the average number (P _{Quantity of}) of inclusions having a diameter of 1 μm or more in the cross section were measured by the same method as described above in the specification on the basis of the unit area of 400 μm ², and are shown in Table 4 below. However, in the case where no inclusion having a diameter of 1 μm or more is present, the P _{Size of the device} and P _{Quantity of} are each represented by "1".

Further, yield Strength (YS), tensile Strength (TS) and yield ratio (yield strength/tensile strength; YR) were calculated by room temperature tensile experiments and according to ISO-6892 standard and are shown in Table 5 below.

Further, for each of the following comparative examples and examples, the Yield Strength (YS) and Tensile Strength (TS) values of each test piece before the tempering heat treatment were measured, and then the amount of change in yield strength (Δys) and the amount of change in tensile strength (Δts) of each test piece after the tempering heat treatment were measured based on the above measured values, and are shown in table 5 below.

In addition, elongation (El) was measured according to ISO-6892, and stretch flangeability (HER) was measured by drilling a Hole (Hole) of 10mm in size in steel and reaming at a constant rate. Further, the bendability (R/t) was measured in a method of pressing a steel material with a press head having an R value of a prescribed size, and is shown in table 5 below.

Further, the steel material was cut into a size of 1000mm or more in length, placed in a flat place, and the wave height was measured, and the flatness of the steel material was evaluated with reference to the maximum value of the wave height. At this time, the shape was evaluated as "good" when the maximum value of the wave height was less than 10mm, and as "bad" when the maximum value of the wave height was 10mm or more, and is shown in table 5 below.

TABLE 4

Remarks	P_{Size of the device}[μm]	P _{Quantity of} [ number ]	M*
				Inventive example 1	1	1	198
Inventive example 2	1	1	441
				Inventive example 3	1	1	103
Inventive example 4	1.4	2	287
				Comparative example 1	1	1	664
Comparative example 2	1	1	26
				Comparative example 3	1.2	3	1786
Comparative example 4	1.5	4	1188
				Comparative example 5	1	1	83
Comparative example 6	1.4	8	13094
				Comparative example 7	2.1	5	7047

M*＝P_{Size of the device}×P_{Quantity of}×[C]^0.5×[Mn]²×[S]

TABLE 5

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

As can be seen from the experimental results of table 5, in the cases of inventive examples 1 to 4 in which the alloy composition and the manufacturing conditions of the present invention were satisfied and the M value defined by the relation 1 satisfied the range of 100 to 500, it was confirmed that the alloy composition was excellent in yield ratio, bendability, and stretch flangeability while securing high yield strength and tensile strength, and also excellent in flatness.

On the other hand, in the case of comparative examples 1 to 4, which satisfy the alloy composition of the present invention but have a P value defined by the relation 2 of less than 1.5 or more than 77.0, it was confirmed that one or more of the strength, yield ratio, bendability, stretch flangeability and flatness were poor due to unsuitable tempering conditions.

In addition, in the case of comparative examples 5 to 7, which did not satisfy the alloy composition of the present invention, the strength, the bendability, the stretch flangeability and the flatness were poor.

Specifically, comparative example 5 is a steel grade that does not satisfy the alloy composition of the present invention, specifically, the carbon content is insufficient. Carbon, which is an invasive strengthening element, is an element that greatly contributes to the increase in strength of the steel grade, and due to the shortage of this carbon, the tensile strength and yield strength do not reach the values targeted by the present invention. Further, in comparative example 5, sufficient time and temperature were not ensured in the tempering process so that the P value of formula (2) was lower than the target value of the present invention. Therefore, a sufficient increase in yield strength cannot be ensured in the tempering process, resulting in insufficient yield strength after the tempering process.

Comparative example 6 is a case where a steel grade having a sulfur content exceeding that of the target alloy composition of the present invention was used. When the concentration of sulfur in steel is high, sulfur reacts with manganese to form inclusions such as manganese sulfide, and these inclusions greatly reduce the bending property and stretch flangeability of steel. Therefore, in comparative example 6, the value of M of the numerical formula (1) exceeds the target value of the present invention in consideration of these factors. Therefore, the index R/t indicating bending characteristics and the index HER indicating stretch flangeability of comparative example 6 cannot satisfy the values targeted by the present invention.

Comparative example 7 is a steel grade having a manganese content exceeding that of the target alloy composition of the present invention. When the concentration of manganese in steel is high, manganese reacts with sulfur to form inclusions such as manganese sulfide, and these inclusions greatly reduce the bending property and stretch flangeability of steel. Therefore, in comparative example 7, the numerical value of M of the formula (1) exceeds the value aimed at by the present invention in consideration of these factors. Therefore, it was confirmed that the index R/t indicating bending characteristics and the index HER indicating stretch flangeability of comparative example 7 did not satisfy the target values of the present invention. In addition, when the concentration of manganese in steel is high, manganese forms a band structure (band structure) in steel. The structural characteristics of this manganese are responsible for the reduced bending and shape characteristics of the steel grade. In addition, an increase in manganese content increases hardenability of the steel grade to increase tensile strength of the steel grade, and when the tensile strength exceeds the value targeted by the present invention, the shape of the steel at the time of production becomes poor, and such poor shape is difficult to correct, and a problem of shape deterioration of the steel grade occurs. Therefore, the tensile strength, HER, bendability, and flatness of the steel grade of comparative example 7 do not satisfy the values targeted by the present invention.

Claims

1. An ultra-high strength steel sheet comprising, in weight percent: c:0.12-0.4%, si:0.5% or less and 0% or less, mn:2.5-4.0%, P:0.03% or less except 0%, S:0.012% or less except 0%, al:0.1% or less except 0%, cr: less than 1% and excluding 0%, ti:48/14 x [ N ] to 0.1%, nb:0.1% or less and 0% or less, B:0.005% or less except 0%, N:0.01% or less except 0% Fe and other impurities in balance,

As a microstructure, it contains, in area%: martensite: 90% or more, sum of ferrite and bainite: the content of the catalyst is less than 10 percent,

Wherein the value of M defined by the following relation 1 satisfies the range of 100 to 500,

[ Relation 1]

M＝P_{Size of the device}×P_{Quantity of}×[C]^0.5×[Mn]²×[S]

In the relation 1, P _{Size of the device} represents the average diameter of inclusions having a diameter of 1 μm or more, P _{Quantity of} represents the average number of inclusions having a diameter of 1 μm or more, C and Mn represent the average weight% content of the element in brackets in the steel sheet, respectively, and S represents the average ppm content of the element in brackets in the steel sheet.

2. The ultra-high strength steel sheet according to claim 1, wherein the ultra-high strength steel sheet has a yield strength of 1140-1500MPa and a tensile strength of 1470-1700MPa.

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

4. The ultra-high strength steel sheet according to claim 1, wherein the ultra-high strength steel sheet has an elongation flange formability HER of 25% or more and a bendability R/t of 4 or less.

5. A method of manufacturing an ultra-high strength steel sheet, comprising the steps of:

Preparing a steel sheet comprising, in weight percent: c:0.12-0.4%, si:0.5% or less and 0% or less, mn:2.5-4.0%, P:0.03% or less except 0%, S:0.012% or less except 0%, al:0.1% or less except 0%, cr: less than 1% and excluding 0%, ti:48/14 x [ N ] to 0.1%, nb:0.1% or less and 0% or less, B:0.005% or less except 0%, N:0.01% or less and 0% excluding, the balance being Fe and other impurities, and as a microstructure, the steel sheet contains, in area%: martensite: 90% or more, sum of ferrite and bainite: less than 10 percent; and

The steel sheet is tempered in such a way that,

Wherein the P value defined by the following relation 2 satisfies the range of 1.5 to 77.0,

[ Relation 2]

In the relation 2, the T represents the highest temperature of tempering in units of c, and the T _eff represents the effective heat treatment time in units of seconds.

6. The method of manufacturing an ultra-high strength steel sheet according to claim 5, wherein said T satisfies a range of 100-300 ℃.

7. The method of manufacturing an ultra-high strength steel sheet according to claim 5, wherein said t _eff satisfies the range of 1-120 seconds.

8. The method for manufacturing an ultra-high strength steel sheet according to claim 5, wherein the method satisfies the following relation 3,

[ Relation 3]

5≤t_{Total (S)}≤120

In the relation 3, t _{Total (S)} represents the total heat treatment time of tempering, and the unit is seconds.

9. The method for manufacturing an ultra-high strength steel sheet according to claim 5, wherein said method satisfies the following relation 4,

[ Relation 4]

1≤t_Heating≤119

In the above-described relation 4, t _Heating represents the temperature rise time of tempering, and the unit is seconds.

10. The method of manufacturing an ultra-high strength steel sheet according to claim 5, wherein the method satisfies the following relation 5,

[ Relation 5]

1≤t_Holding≤119

In the relation 5, t _Holding represents a holding time of tempering, and the unit is seconds.