Steel material for hot forming, hot-formed member, and manufacturing method therefor

Abstract

The present invention relates to: a steel material for hot forming, used for a vehicle and the like; a hot-formed member; and a method for manufacturing same. In the method, when the formed steel material is temper rolled, rolling pressure and roughness of temper-rolling rolls are controlled so that bendability can be improved.

Description

Technical Field

[0001] The present disclosure relates to a steel materialfor hot forming, used for a vehicle, and the like, a hot-formed member, and a manufacturing method therefor.

[0002] Background Art

[0003] Recently, efforts have been made to improve fuelefficiency by reducing the weight of vehicles. To this end,a thickness of a steel material may be reduced, but if thethickness thereof is reduced, a problem may occur instability of the vehicles, so improvement in the strength ofthe steel material should be supported. For this reason, ademand for high-strength steel sheets has been continuouslygenerated, and various types of steel materials have beendeveloped. However, since these steel materials have highstrength, there is a problem in that workability is poor.

[0004]

[0005] In order to solve this problem, a hot forming methodhas been proposed. The hot forming method is a method ofprocessing the steel material at a high temperature, suitablefor processing the steel material, and then rapidly coolingthe same to a low temperature to form a low-temperaturestructure such as martensite, or the like, in the steelmaterial, thereby increasing strength of a final product. Inthis case, it is possible to minimize a problem ofworkability when manufacturing a member having high strength.

[0006]

[0007] As a technology related to such hot forming, providedis Patent Document 1. Patent Document 1 proposes a technologyof securing ultra-high strength having a tensile strength of1600MPa or more, by heating an Al-Si plated steel sheet to850°C or higher and then forming a member structure intomartensite by hot forming and rapid cooling by pressing.

[0008]

[0009] Meanwhile, the hot-formed member used for the purposeof protecting passengers should have excellent impactresistance, and bendability is widely used as arepresentative index for evaluating such impact resistance.For example, in the case of a vehicle B-pillar, when a hot-formed member is bent by a side impact of a vehicle,characteristics (bendability) capable of withstanding acertain distance (angle) or more without breaking arerequired.

[0010]

[0011] Patent Document 2 proposes a method of controlling aferrite structure of a surface layer of a hot-formed member,and in addition thereto, in order to supplement therelatively poor energy absorption capacity, a technology toincorporate a blank (tailor welded blank, TWB) having acombination of different materials or different thicknessesinto hot forming has been suggested, and various studies arebeing conducted.

[0012]

[0013] However, structural control of ferrite of the surfacelayer by optimizing hot forming conditions has limitationsin improving bendability. In addition, even in theimprovement of impact resistance through TWB, there has beena limitation in improving the characteristics of partsrequiring impact resistance, such as rather inferiorbendability due to deterioration of welded parts.

[0014] (Patent Document 1) US Patent No. 6296805

[0015] (Patent Document 2) Korea Patent Registration No. 10-1569508

[0016] Summary of InventionTechnical Problem

[0017] An aspect of the present disclosure is to provide asteel material for hot forming in which a hot-formed memberhas high strength and simultaneously having excellentbendability, a hot-formed member manufactured using the same,and a manufacturing method therefor.

[0018]

[0019] The subject of the present invention is not limitedto the above. The subject of the present invention will beunderstood from the overall content of the presentspecification, and those of ordinary skill in the art towhich the present invention pertains will have no difficultyin understanding the additional subject of the presentinvention.

[0020] Solution to Problem

[0021] According to an aspect of the present disclosure,provided is a steel material for hot forming, the steelmaterial for hot forming comprising, by weight: 0.04 to 0.45%of carbon (C); 1.5% or less of silicon (Si) (excluding 0%);0.2 to 2.5% of manganese (Mn); 0.05% or less of phosphorous(P); 0.02% or less of sulfur (S); 0.01 to 0.1% of aluminum(Al); 0.01 to 5.0% of chromium (Cr); 0.02% or less ofnitrogen (N), and a balance of Fe and inevitable impurities,

[0022] wherein a surface roughness factor calculated by thefollowing [Relationship 1] is 1.8 μm or less,

[0023] [Relationship 1]Surface Roughness Factor= Rt / 100 + 10 x Rdq

[0024] where, Rt is defined as a vertical distance betweenthe highest peak and the deepest valley in a randommeasurement section on a surface of a steel sheet, and Rdqis a root mean square of a slope of the peak in a randommeasurement section on the surface of the steel sheet.

[0025]

[0026] According to another aspect of the present disclosure,provided is a method for manufacturing a steel material forhot forming, the method comprising operations of: obtaininga cold-rolled steel sheet using a steel slab comprising, byweight: 0.04 to 0.45% of carbon (C); 1.5% or less of silicon(Si) (excluding 0%); 0.2 to 2.5% of manganese (Mn); 0.05% orless of phosphorous (P); 0.02% or less of sulfur (S); 0.01to 0.1% of aluminum (Al); 0.01 to 5.0% of chromium (Cr);0.02% or less of nitrogen (N), and a balance of Fe andinevitable impurities; and

[0027] skin pass rolling the cold-rolled steel sheet tosatisfy the following [Relationship 2],

[0028] [Relationship 2]

[0029] Equation

[0030] where P is rolling force during skin pass rolling,and Raroll is arithmetic average roughness (Ra) of a skin passrolling roll.

[0031]

[0032] According to another aspect of the present disclosure,provided is a hot-formed member, the hot-rolled membercomprising by weight: 0.04 to 0.45% of carbon (C); 1.5% orless of silicon (Si) (excluding 0%); 0.2 to 2.5% of manganese(Mn); 0.05% or less of phosphorous (P); 0.02% or less ofsulfur (S); 0.01 to 0.1% of aluminum (Al); 0.01 to 5.0% ofchromium (Cr); 0.02% or less of nitrogen (N), and a balanceof Fe and inevitable impurities,

[0033] wherein a change amount of a maximum bending angle is5% or less.

[0034]

[0035] According to another aspect of the present disclosure,provided is a method for manufacturing a hot-formed member,the method comprising: obtaining a blank using the steelmaterial for hot forming described above;

[0036] heating the blank to a temperature of Ac3 to 980°C,and then maintaining the temperature for 1 to 1000 seconds;and

[0037] hot forming the heated and maintained blank and thencooling the blank.

[0038] Advantageous Effects of Invention

[0039] As set forth above, according to the presentdisclosure, it is possible to manufacture a hot-formed memberhaving high strength after hot forming and simultaneouslyhaving excellent bendability and excellent impact resistance.It is possible to provide a steel material for hot formingfor this purpose, a hot-formed member manufactured therefrom,and a manufacturing method thereof.

[0040]

[0041] The means for solving the above problems do notenumerate all the features of the present invention, and thevarious features of the present invention and the advantagesand effects thereof will be understood in more detail withreference to the specific embodiments and examples below.

[0042] Brief description of drawings

[0043] FIG. 1 simply illustrates a concept of surfaceroughness factors of [Relationship 1] proposed in the presentdisclosure.

[0044] FIG. 2 simply illustrates the concept of crackinitiation energy (CIE), which is a criterion for evaluatinga impact energy absorption capacity in the present disclosure.

[0045] Best Mode for Invention

[0046] The terms used herein are intended to describe thepresent disclosure and are not intended to limit the presentinvention. In addition, the singular forms used hereincomprise the plural forms unless the related definitionclearly dictates the contrary.

[0047] The meaning of "comprising" as used in thespecification specifies a component, and does not excludethe presence or addition of other components

[0048] Unless otherwise defined, all terms comprisingtechnical terms and scientific terms used in thisspecification have the same meaning as commonly understoodby a person of ordinary skill in the art to which the presentinvention belongs. The terms defined in the dictionary areinterpreted to have a meaning consistent with the relatedtechnical literature and the currently disclosed content.

[0049]

[0050] First, an embodiment of a steel material for hotforming of the present disclosure will be described in detail.The steel material of the present disclosure may comprise,by weight: 0.04 to 0.45% of carbon (C); 1.5% or less ofsilicon (Si) (excluding 0%); 0.2 to 2.5% of manganese (Mn);0.05% or less of phosphorous (P); 0.02% or less of sulfur(S); 0.01 to 0.1% of aluminum (Al); 0.01 to 5.0% of chromium(Cr); 0.02% or less of nitrogen (N), and a balance of Fe andinevitable impurities. Hereinafter, each alloy compositionwill be described in detail. Hereinafter,% herein represents%by weight.

[0051]

[0052] Carbon (C) : 0.04 to 0.45%

[0053] Carbon (C) is an essential element added for improvingstrength of a member. If a content of C is less than 0.04%,it is difficult to secure sufficient strength, and ultimately,even if bendability is high, an impact energy absorptioncapacity is rather low, so carbon (C) is preferably added in0.04% or more. On the other hand, when the C content is morethan 0.45%, the strength is increased, but the bendabilityis lowered, so the impact energy absorption capacity islowered, so the content of C is preferably less than 0.45%.

[0054]

[0055] Silicon (Si): 1.5% or less (excluding 0%)

[0056] Silicon (Si) should be added as a deoxidizer insteelmaking, as well as a solid solution and a carbideformation inhibiting element, contributing to an increase instrength of a hot-formed member and being added as aneffective element for material uniformity. When a content ofSi is more than 1.5%, plating properties may be deteriorateddue to Si oxides generated on a surface of the steel sheetduring annealing. Accordingly, Si is preferably comprised inan amount of 1.5% or less (excluding 0%).

[0057]

[0058] Manganese (Mn): 0.2 to 2.5%

[0059] Manganese (Mn) needs to be added not only for securinga solid solution strengthening effect, but also suppressingferrite formation during hot forming through improvinghardenability. If a content of Mn is less than 0.2%, thereis a limitation in obtaining the above effect, and otherexpensive alloy elements are excessively required to improveinsufficient hardenability, which may cause a problem ofgreatly increasing manufacturing costs. On the other hand,when the content of Mn is more than 2.5%, the strength ofthe steel sheet before the hot forming process may decreasedue to an increase in cold rolling properties, and a bandlike structure arranged in a rolling direction deepens, andthe impact energy absorbing capacity may become inferior.Therefore, the content of Mn is preferably 0.2 to 2.5%.

[0060]

[0061] Phosphorous (P): 0.05% or less

[0062] Phosphorous (P)is present as impurities in a steel,and when a content of P is more than 0.05%, weldability ofa hot-formed member may be greatly weakened. Meanwhile, P isan inevitable impurity in manufacturing a steel material,and a lower limit thereof may not be particularly limited.However, since a lot of manufacturing costs may be requiredto control the content of P to be less than 0.001%, thecontent of P may be 0.001% or more.

[0063]

[0064] Sulfur (S): 0.02% or less

[0065] Sulfur (S) present as an impurity in steel, and anelement which impairs ductility, impact properties, andweldability of the hot-formed member, so a content of S ispreferably limited to 0.02% at most. Meanwhile, S is aninevitable impurity, and a lower limit thereof may not beparticularly limited, but in order to control the content ofS to less than 0.0001 %, it may take much manufacturing costs,and thus, the content of S may be 0.0001% or more %.

[0066]

[0067] Aluminum (Al): 0.01 to 0.1%

[0068] Aluminum (Al) is an element acting as a deoxidizer insteelmaking to increase cleanliness of a steel, togetherwith Si. When a content of Al is less than 0.01%, it isdifficult to obtain the above effect, and when the contentof Al is more than 0.1%, there is a problem in that hightemperature ductility due to excessive AlN formed during acasting process deteriorates and slab cracks occur.Therefore, the content of Al is preferably 0.01 to 0.1%.

[0069]

[0070] Chromium (Cr): 0.01 to 5.0%

[0071] Cr is added for securing hardenability of steel likeMn, and to secure a beautiful surface during an HPF process.If a content of Cr is less than 0.01%, it may be difficultto secure sufficient hardenability. On the other hand, ifthe content of Cr is more than 5.0%, an effect of improvingthe hardenability compared to an addition amount isinsignificant, and it may promote formation of coarse Crbased carbides to deteriorate impact energy absorptionability, so it is preferable that the content of Cr does notexceed 5.0%.

[0072]

[0073] Nitrogen (N): 0.02% or less

[0074] Nitrogen (N) is comprised as an impurity in steel.When a content of N is more than 0.02%, there is a problemin that slab cracks due to AlN formation easily occur, as inthe case of Al above. N is an impurity, and a lower limitthereof may not be particularly limited, but may be 0.001%or more because a lot of manufacturing costs may be requiredto manage the content of N to be less than 0.001%.

[0075]

[0076] Meanwhile, the steel material may further compriseone or more of 0.5% or less of Mo, 0.5% or less of Ni, 0.1%or less of Nb, 0.1% or less of Ti, and 0.01% or less of B,in addition to the above-described alloy components.

[0077]

[0078] Molybdenum (Mo): 0.5% or less

[0079] Molybdenum (Mo) not only has an effect of improvinghardenability of steel, such as Cr and Mn, but also has aneffect of increasing bendability by refining crystal grainsthrough formation of fine precipitates. However, when thecontent of Mo is more than 0.5%, it causes an excessiveincrease in ferroalloy cost compared to the effect, so it ispreferable that the content thereof does not exceed 0.5%.The content of Mo is more preferably 0.45% or less, morepreferably 0.4% or less, and still more preferably 0.35% orless.

[0080]

[0081] Nickel (Ni): 0.5% or less

[0082] Nickel (Ni) is an austenite stabilizing element, andhardenability of steel can be improved through addition ofNi. However, since Ni is an expensive alloy element,considering an increase in manufacturing cost compared to aneffect of improving hardenability, an upper limit of thecontent of Ni is preferably set to 0.5%. Meanwhile, in orderto sufficiently obtain the effect of improving hardenabilityaccording to the addition of Ni, the content of Ni ispreferably at least 0.01% or more, more preferably 0.03% ormore, and more preferably 0.05% or more. The upper limit ofNi is more preferably 0.45%, still more preferably 0.4%, andmost preferably 0.35%.

[0083]

[0084] Niobium (Nb): 0.1% or less

[0085] Niobium (Nb) is element capable of obtaining aprecipitation hardening effect through formation of fineprecipitates, and thereby, an effect of improvingbendability by increasing strength and refining crystalgrains can be obtained. In addition, by suppressing excessivecrystal grain growth during heating for hot forming, it ispossible to promote robustness against variations in heattreatment conditions. However, when a content of Nb is morethan 0.1%, not only the effect is saturated, but alsorelatively coarse precipitates increase due to an increasein precipitation temperature, which may reduce costeffectiveness. Therefore, the content of Nb is preferably0.1% or less. A lower limit of the content of Nb is preferably0.005%, more preferably 0.01%, and still more preferably0.015%. An upper limit of the content of Nb is morepreferably 0.09%, still more preferably 0.08%, and mostpreferably 0.07%.

[0086]

[0087] Titanium (Ti): 0.1% or less

[0088] Titanium (Ti) is an element that is also addedtogether when B is added to secure hardenability by combiningwith nitrogen remaining as an impurity in steel to produceTiN. In addition, through formation of TiC precipitates,precipitation strengthening and grain refinement effects maybe expected. However, if a content of Ti is more than 0.1%,a large amount of coarse TiN is formed to deteriorate animpact energy absorption capacity, so an upper limit thereofis preferably 0.1%. A lower limit of Ti is preferably 0.005%,more preferably 0.01%, and still more preferably 0.015%. Anupper limit of Ti is more preferably 0.08%, more preferably0.06%, and most preferably 0.05%.

[0089]

[0090] Boron (B) : 0.01% or less

[0091] Boron (B) is an element that can improve hardenabilityeven with addition of a small amount and is segregated at aprior austenite grain boundary and can effectively suppressbrittleness of a hot-formed member due to grain boundarysegregation of P and / or S. however, if a content of B ismore than 0.01%, it is preferable that an upper limit thereofis 0.01%, because it caused brittleness in hot rolling, dueto formation of a Fe23CB6 composite compound. Meanwhile, alower limit of the content of B is preferably 0.0001%, morepreferably 0.0003%, and still more preferably 0.0005%. Anupper limit of the content of B is more preferably 0.009%,still more preferably 0.007%, and most preferably 0.005%.

[0092]

[0093] A remainder of the present disclosure may be iron(Fe). However, in a general manufacturing process,inevitable impurities may be inevitably added from rawmaterials or an ambient environment, and thus, impuritiesmay not be excluded. A person skilled in the art of a generalmanufacturing process may be aware of the impurities, andthus, the descriptions of the impurities may not be providedin the present disclosure.

[0094]

[0095] The steel material for hot forming of the presentdisclosure, and preferably has a surface roughness factor of1.8 μm or less, which is defined by the following[Relationship 1]. Surface roughness is expressed in variousmanners (Ra, Rt, Rsk, and the like), but it is difficult toimprove bendability of a hot-formed member by simply changingRa, Rsk, and the like, of a steel material. As a result ofstudying improvement of the bendability of the hot-formedmember, the inventors of the present disclosure haverecognized that the bendability of the hot-formed member maybe improved when the surface roughness of the steel materialis constantly managed. In order to secure the bendability ofthe hot-formed member, rather than simply measuring Rt andRdq, the technical relationship between Rt and Rdq wasderived to derive a surface roughness factor of [Relationship 1] below. Accordingly, in order to improve the bendabilityfor increasing the impact energy absorption capacity of thehot-formed member, it is preferable that the surfaceroughness factor of the hot-formed steel material is 1.8 μmor less. When the surface roughness factor is more than 1.8μm, a slope of a peak increases and the bendability maydeteriorate due to maximization of a surface notch effectduring bending.

[0096] [Relationship 1]Surface Roughness Factor= Rt / 100 + 10 x Rdy

[0097] where Rt is defined as a vertical distance between ahighest peak and a deepest valley in any measurement sectionon a surface of the steel sheet, and Rdq refers to a rootmean square of the slope of the peak in any measurementsection on the surface of the steel sheet. An example of acalculation method of Rt and Rdq according to theRelationship 1 is shown in FIG. 1, and a person skilled inthe art has no difficulty in deriving the Rt and Rdq thereby.

[0098]

[0099] A microstructure of the steel material for hot formingof the present disclosure may comprise, by area fraction:one or more of 50 to 90% of ferrite, 30% or less of pearlite,20% or less of bainite, and 20% or less of martensite.

[00100] The ferrite is a soft phase and a structure effectivefor reducing a blanking process load of a steel sheet inblank manufacture, and in order to obtain the effect, it ispreferable to secure 50% by area or more of ferrite. However,when ferrite is more than 90% by area, carbon is undulydistributed as a structure other than ferrite in blankmanufacturing, so that carbon may be unevenly distributedeven after hot forming. Therefore, it is preferable that theferrite is in a range of 50 to 90% by area.

[00101] When the pearlite is more than 30% by area, cementiteis incompletely dissolved after hot forming to decreasestrength or cause non-uniformity of a material. When thebainite and the martensite are more than 20% by area,respectively, the strength of the steel sheet is excessivelyincreased to cause problems such as mold wear in blankmanufacturing.

[00102]

[00103] Meanwhile, the steel material for hot forming of thepresent disclosure may compris a plating layer on at leastone surface, and the type of plating layer, such as azinc(Zn)-based plating layer, an aluminum (Al)-basedplating layer, and the like, is not particularly limited,and a method thereof such as hot-dip plating, electroplating,and the like, is not particularly limited. Preferably, forexample, an Al-based plating layer may be formed. The Albased plating is not particularly limited, but as an example,the Al-based plating layer may comprise, by weight: 6 to 12%of Si, 1 to 4% of Fe, and a balance of Al and inevitableimpurities.

[00104]

[00105] Next, an embodiment of a steel material for hotforming of the present disclosure will be described in detail.A method for manufacturing the same described below is onlyone embodiment of all possible embodiments, and does notmean that the steel material for hot forming of the presentdisclosure must be manufactured using only the followingmanufacturing method.

[00106]

[00107] A cold-rolled steel sheet is manufactured andobtained by using a steel slab satisfying the above-describedalloy composition, and the cold-rolled steel sheet is skinpass rolled to satisfy the following [Relationship 2] tomanufacture a steel material.

[00108] [Relationship 2]

[00109] Equation

[00110] where P is rolling force during skin pass rolling,and Raroll is arithmetic average roughness (Ra) of a skin passrolling roll.

[00111] By performing skin pass rolling on the cold-rolledsteel sheet, the surface roughness of the steel material iscontrolled. In the present disclosure, it is recognized thata surface of the steel material may be optimized inconsideration of technical action of rolling force (P) andthe arithmetic mean roughness (Raroll) of the roll during skinpass rolling, and [Relationship 2] is derived. The rollingforce (P) during the skin pass rolling is an important factor,but an upper or lower limit thereof is not particularlylimited in the present disclosure. However, for example,when rolling force is not applied, there may be issues suchas coiling failure, so the rolling force may be 100 tons ormore, and more preferably 150 tons or more. In addition,when the rolling force is too high, there may be a phenomenonin which a surface plating layer cracks, and when the rollingforce is more than 40 tons according to [Relationship 2],the upper limit thereof may be limited. For example, whenthe arithmetic average roughness of the skin pass rollingroll is 4 μm, the rolling force is preferably 400 tons orless in order to satisfy the above [Relationship 2].

[00112]

[00113] The cold-rolled steel sheet may be obtained througha process such as heating, hot rolling, coiling, cooling,cold rolling, and annealing of the steel slab. Hereinafter,each process is explained.

[00114]

[00115] Heating steel slab

[00116] The steel slab is heated at 1050 to 1300°C. When aheating temperature of the steel slab is lower than 1050°C,not only it may be difficult to homogenize a structure ofthe steel slab, but also it may be difficult to re-dissolvethe same when using precipitated elements. On the other hand,when the heating temperature thereof is more than 1300°C, anexcessive oxide layer is formed, which may increase thepossibility of causing surface defects after hot rolling.Therefore, the heating temperature of the steel slab ispreferably 1050 to 1300°C. A lower limit of the heatingtemperature of the steel slab is more preferably 1070°C, andstill more preferably 1100°C. An upper limit of heatingtemperature of the steel slab is more preferably 1280°C, andstill more preferably 1250°C.

[00117]

[00118] Hot rolling

[00119] The heated steel slab is hot-rolled and finish hotrolled at 800 to 950°C, to obtain a hot-rolled steel sheet.When the finish hot rolling temperature is lower than 800°C,it may be difficult to control a plate shape due tooccurrence of a mixed structure on a surface layer portionof the steel sheet due to two-phase region rolling. On theother hand, when the finish hot rolling temperature is higherthan 950°C, there may be a problem in that crystal graincoarsening by hot rolling easily occurs. Therefore, thefinish hot rolling temperature is preferably 800 to 950°C.A lower limit of the finish hot rolling temperature is morepreferably 810°C, and still more preferably 820°C. An upperlimit of the finish hot rolling temperature is morepreferably 940°C, and still more preferably 930°C.

[00120]

[00121] Coiling

[00122] The hot-rolled steel sheet is coiled at 500 to 700°C.If the coiling temperature is lower than 500°C, martensiteis formed wholly or partially on the steel sheet, so thatnot only it is difficult to control a plate shape, but alsoa problem of poor rollability in a subsequent cold rollingprocess may occur due to an increase in strength of the hotrolled steel sheet. If the coiling temperature is less than500°C, martensite is formed wholly or partially on a steelsheet, so that not only it is difficult to control a plateshape, but also a problem of poor rollability in a subsequentcold rolling process may occur due to an increase in strengthof the hot-rolled steel sheet. On the other hand, if thecoiling temperature is higher than 700°C, coarse carbidesmay be formed, so that the impact energy absorption capacityof the hot-formed member may be deteriorated. Therefore, thecoiling temperature is preferably 500 to 700°C. A lowerlimit of the coiling temperature is more preferably 520°C,and still more preferably 550°C. An upper limit of thecoiling temperature is more preferably 680°C, and still morepreferably 650°C.

[00123]

[00124] Cooling

[00125] The coiled hot-rolled steel sheet is cooled (hotrolled cooled) at a cooling rate of 10°C / Hr or more from acoiling temperature to 400°C, but when the cooling rate isless than 10°C / Hr, a disadvantage in that a large number ofcoarse carbides may be formed during cooling of a hot-rolledcoil due to a sufficient time to grow carbides. Therefore,the cooling rate is preferably 10°C / Hr or more, morepreferably 12°C / Hr or more, and still more preferably 15°C / Hror more. Meanwhile, in the present disclosure, as long asthe cooling rate is 10°C / Hr or more, the effect to beobtained in the present disclosure may be obtained, and thus,an upper limit thereof is not particularly limited.

[00126] Meanwhile, after the cooling, a process of picklingbefore cold rolling may be added. Surface quality of aproduct may be improved by removing scales formed on asurface of the steel sheet through the pickling process.

[00127]

[00128] Cold rolling

[00129] After the above process, the hot-rolled steel sheetis cold-rolled to obtain a cold-rolled steel sheet. In thepresent disclosure, a reduction rate during the cold rollingis not particularly limited, but a reduction rate of 30 to80% may be applied to obtain a target thickness of the steelsheet.

[00130]

[00131] Annealing and cooling

[00132] Annealing is performed on the cold-rolled steelsheet, and for this purpose, the cold-rolled steel sheet isheated, and in this case, it is preferable to heat atemperature range from 400°C to an annealing temperature ata rate of 20°C / s or less. When a heating rate from 400°C tothe annealing temperature is more than 20°C / s, there is notsufficient time for carbides precipitated in the hot-rollingstep to be solid-solubilized again, so that a coarse carbideremains, and impact energy absorbing capacity of the finallyobtained hot-formed member may be poor. Therefore, it ispreferable that the heating rate from 400°C to an annealingtemperature is 20°C / s or less. The heating rate is morepreferably 18°C / s or less, and still more preferably 15°C / sor less. Meanwhile, in the present disclosure, as long asthe heating rate is 20°C / s or less, the effect to be obtainedin the present disclosure may be obtained, and thus, a lowerlimit of the heating rate is not particularly limited.However, considering annealing productivity, the heatingrate may be 0.5°C / s or more, more preferably 1°C / s or more,and still more preferably 1.5°C / s or more. Meanwhile, in thepresent disclosure, in the temperature range of a coldrolling temperature to less than 400°C, the heating rate isnot particularly limited, since the effect of carbide resolid solubilization is insignificant even when the heatingrate is controlled.

[00133]

[00134] It is preferable that the heated cold-rolled steelsheet is annealed at an annealing temperature of 740 to 860°C.when the annealing temperature is lower than 740°C,recrystallization of the cold rolled structure isinsufficient, a plate shape becomes poor or strength afterplating is excessively increased, and thus, mold wear duringa blanking process may be caused. On the other hand, if theannealing temperature is higher than 860°C, since surfaceoxides such as Si and Mn may be formed during the annealingprocess and a problem of poor plating surface may occur, theannealing temperature is preferably 740 to 860°C. On theother hand, if the annealing temperature is higher than 860°C,since surface oxides such as Si and Mn may be formed duringthe annealing process and a problem of poor plating surfacemay occur, the annealing temperature is preferably 740 to860°C. A lower limit of the annealing temperature is morepreferably 750°C, and still more preferably 760°C. An upperlimit of the annealing temperature is more preferably 850°C,and still more preferably 840°C.

[00135]

[00136] It is preferable that the atmosphere during theannealing is preferably a non-oxidizing atmosphere. Forexample, a hydrogen-nitrogen mixture gas may be used, and inthis case, a dew point temperature of the atmospheric gasmay be -70 to -30°C. In order for the dew point temperatureto be less than -70°C, additional equipment for controllingthe same is required, so that there is a problem ofincreasing manufacturing costs, and if the dew point ishigher than -30°C, annealing oxides are excessively formedon a surface of the steel sheet during annealing, which maycause defects such as non-plating. Therefore, during thecontinuous annealing, the dew point temperature of theatmospheric gas is preferably -70 to -30°C. A lower limit ofthe dew point temperature of the atmospheric gas is morepreferably -65°C, and still more preferably -60°C. An upperlimit of the dew point temperature of the atmospheric gas ismore preferably -35°C, and still more preferably -40°C.

[00137]

[00138] The annealed cold-rolled steel sheet is cooled(annealed cooled) from an annealing temperature to 660°C ata cooling rate of 1°C / s or more. When the cooling rate isless than 1°C / s, a large amount of coarse carbides is formed,and thus the impact energy absorbing capacity of the finallyobtained hot-formed member may be deteriorated. Therefore,the cooling rate is preferably 1°C / s or more. The coolingrate is more preferably 1.5°C / s or more, and still morepreferably 2°C / s or more. An upper limit of the cooling rateis not particularly limited. However, in terms of suppressingdefects in a steel sheet shape, the cooling rate may be50°C / s or less, more preferably 45°C / s or less, and stillmore preferably 40°C / s or less.

[00139]

[00140] Meanwhile, before performing the skin pass rolling,plating may be additionally performed on the annealed coldrolled steel sheet. In the present disclosure, the type andmethod of plating are not particularly limited, but anexample of Al-based plating will be described. In the plating,the annealed cold-rolled steel sheet is cooled, and dippedin an Al-based plating bath to form an aluminum-based platinglayer. The composition and plating conditions of the Al10 based plating bath are not particularly limited.

[00141] However, as a non-limiting example, a composition ofa plating bath may comprise, by weight: 6 to 12% of Si, 1 to4% of Fe, and a balance of Al and other inevitable impurities,and a plating amount may be 30 to 130 g / m2 based on onesurface which is commonly applied in the art. When thecontent of Si in the plating bath composition is less than6% by weight, there may be a disadvantage in that the platingbath temperature rises excessively to deteriorate theequipment, and when the content of Si thereof is more than12% by weight, there is a disadvantage in that a heatingtime for hot forming must be lengthened by excessivelydelaying alloying. If the content of Fe is less than 1% byweight, plating adhesion or spot weldability may be inferior,and if the content of Fe is more than 4% by weight, drossgeneration in the plating bath may be excessive, causingpoor surface quality. If the plating adhesion amount is lessthan 30 g / m2 based on one surface, it may be difficult tosecure the desired corrosion resistance of the hot-formedmember, and if the plating adhesion amount is more than 130g / m2, excessive plating adhesion amount not only increasesmanufacturing costs, but also makes it difficult to uniformlyplate the steel sheet with a plating adhesion amountin anentire width and length direction of the coil.

[00142]

[00143] Meanwhile, according to another aspect of thepresent disclosure, as described above, continuous annealingand aluminum-based plating may be performed on a cold-rolledsteel sheet, but aluminum-based plating may be performedimmediately after pickling on the cooled hot-rolled steelsheet.

[00144]

[00145] Hereinafter, an embodiment of a hot-formed member ofthe present disclosure will be described in detail. The hotformed member of the present disclosure can be manufacturedby hot press-forming the above-described steel material forhot forming.

[00146]

[00147] A microstructure of the hot-formed member may havea martensite single-phase structure or a mixed structurecomprising martensite and 40% by area or less of bainite.Since the martensite is a structure which is effective forsecuring strength targeted in the present disclosure, themicrostructure of the present disclosure may be a martensitesingle-phase structure. Meanwhile, bainite is a structurehaving a somewhat lower strength than martensite, but is astructure which does not greatly lower bendability whenformed in a martensite base and favorable for securingstrength, and thus, in the present disclosure, the membermay have a mixed structure comprising less than 40% by areaof bainite with the martensite. However, when a fraction ofthe bainite is more than 40% by area, it may be difficult tosecure strength targeted in the present disclosure.

[00148]

[00149] Meanwhile, the microstructure may further compriseone or more of 10% by area or less of ferrite and 5% by areaor less of retained austenite. The ferrite and retainedaustenite may inevitably be formed in the manufacturingprocess. When the ferrite structure is comprised at morethan 10% by area, strength is lowered and bending propertiesmay be greatly deteriorated, and when the retained austenitestructure is comprised at more than 5% by area, strength maybe lowered or hydrogen incorporated from atmospheric gasduring hot forming is increased to cause hydrogen brittleness.

[00150]

[00151] The hot-formed member may have a yield strength (YS)of 800 MPa or more, a tensile strength (TS) of 1000 MPa ormore, and an elongation (El) of 3.5% or more.

[00152] A change amount of a maximum bending angle of thehot-rolled member of the present disclosure may be 5% orless. The maximum bending angle may be confirmed through athree-point bending test according to a VDA standard (VDA238-100). When the change amount of the maximum bending angle ismore than 5%, bendability or impact characteristics may beinferior even with similar physical properties.

[00153]

[00154] Next, an embodiment of a method for manufacturingthe hot-formed member of the present disclosure will bedescribed in detail. The manufacturing method describedbelow is only one embodiment outside of all possibleembodiments, and does not mean that the hot-formed member ofthe present disclosure must be manufactured only by thefollowing manufacturing method.

[00155]

[00156] The above-described steel material for hot formingor the steel material for hot forming produced by the abovemethod is prepared, a blank is manufactured, and the blankis heated to a temperature, equal to or higher than anaustenite single-phase temperature range, more specifically,to a temperature of Ac3 to 980°C, and then maintaining thetemperature for 1 to 1000 seconds.

[00157] If the blank heating temperature is lower than theAc3 temperature, it may be difficult to secure apredetermined strength due to existence of untransformedferrite. On the other hand, when the heating temperature ishigher than 980°C, it may be difficult to secure spotweldability due to excessive oxide formation on a surface ofthe member. Therefore, the blank heating temperature ispreferably Ac3 to 980°C. A lower limit of the blank heatingtemperature is more preferably Ac3+5°C, and still morepreferably Ac3+10°C. An upper limit of the blank heatingtemperature is more preferably 970°C, and still morepreferably 960°C.

[00158] If the retention time is less than 1 second, thetemperature is not uniform throughout the blank, which cancause material differences for each part, and if theretention time is more than 1000 seconds, it may be difficultto secure spot weldability due to excessive oxide formationon the surface of the member, similarly to excessive heatingtemperature. Therefore, the retention time is preferably 1to 1000 seconds. A lower limit of the retention time is morepreferably 30 seconds, and still more preferably 60 seconds.An upper limit of the retention time is more preferably 900seconds, and still more preferably 800 seconds.

[00159]

[00160] Thereafter, the heated and retained blank is hotformed and then cooled to room temperature, thereby finallymanufacturing a hot-formed member. In the present disclosure,specific conditions in the hot forming are not particularlylimited, and a hot forming method which is commonly known inthe art to which the present disclosure pertains is appliedas it is. As a preferred example, a mold cooling method maybe used.

[00161] Mode for Invention

[00162] Next, examples of the present disclosure will bedescribed.

[00163] The following examples may be modified in variousways without departing from the scope of the presentdisclosure to those skilled in the art. The followingexamples are for understanding of the present disclosure,and the scope of the present disclosure should not be limitedto the following examples and should not be defined, butshould be defined by the claims described later as well asthose equivalent thereto.

[00164]

[00165] (Example)

[00166] A steel slab having a thickness of 40 mm having thecomposition listed in Table 1 below (by weight%, a balancebeing Fe and inevitable impurities) was manufactured byvacuum melting. After heating the steel slab to 1250°C, hotrolling the slab at a finish hot rolling temperature of 900°C,and coiling the slab at a coiling temperature of 640°C, ahot-rolled steel sheet having a final thickness of 2.5 mmwas manufactured. After pickling the hot-rolled steel sheet,cold-rolling was performed at a cold rolling reduction rateof 45% to manufacture a cold-rolled steel sheet. Afterannealing the cold-rolled steel sheet at a temperature of780°Ca common annealing temperature, in a 5% hydrogen-95%nitrogen atmosphere, the cold-rolled steel sheet was cooled,and Al-based plating was performed.

[00167] In this case, the Al-based plating bath compositionwas formed of Al-9%Si-2%Fe with a remainder of inevitableimpurities, and the plating adhesion amount was 70g / m2 basedon one surface. In order to impart roughness to a surface ofthe steel sheet, skin pass rolling was additionally performed,and skin pass rolling roll roughness and rolling force werevaried in order to impart deviation of roughness. The rollroughness and rolling force applied to each specimen areshown in Table 2.

[00168]

[00169] After the steel sheet manufactured as such wasmanufactured into a blank, and was hot formed using a moldfor hot forming, to manufacture a hot-formed member. In thiscase, a heating temperature of the blank was 930°C, aretention time thereof was 5 minutes, and a transfer timefrom a heating furnace to molding was all 10 seconds, whichwere identically applied.

[00170]

[00171] A yield strength (YS), a tensile strength (TS), andan elongation (EI) were measured by collecting a specimen ofan ASTM standard in a direction perpendicular to the rollingdirection of the steel sheet, and then performing a tensiletest.

[00172]

[00173] Bendability, which is an important index for animpact energy absorption capacity, was evaluated byperforming a three-point bending test according to a VDAstandard (VDA238-100). By calculating an area until a maximumload is reached from the load-displacement curve obtainedfrom the three-point bending test (crack initiation energy,CIE), the impact energy absorption capacity of the materialcan be evaluated. FIG. 2 simply illustrates a concept of theCIE, which is a criterion for evaluating the impact energyabsorption capacity.

[00174]

[00175] [Table 1]

[00176]

[00177] [Table 2]

[00178]

[00179] In Table 2, Relationship 1 represents a surfaceroughness factor,

[00180] Surface Roughness Factor =Rt / 100+ 10 x Rdq

[00181]

[00182] where Rt is defined as a vertical distance betweena highest peak and a deepest valley in a random measurementsection on a surface of a steel sheet, and Rdq is a rootmean square of a slope of the peak in a random measurementsection on the surface of the steel sheet.

[00183] Relationship 2 represents Equation.

[00184] where P is rolling force during skin pass rolling,and Raroll is arithmetic mean roughness (Ra) of a skin passrolling roll.

[00185]

[00186] As can be seen from Tables 1 and 2, it could beconfirmed that excellent bendability can be secured when asurface roughness factor of 1.8 μm or less is secured bysatisfying both the alloy composition and skin pass rollingconditions proposed by the present disclosure.

[00187]

[00188] Specifically, comparing Inventive Example 1 withComparative Examples 1 to 2, they were all manufactured usingthe same A steel type, and in Inventive Example 1 satisfyingthe conditions of the present disclosure, it can be seenthat excellent bendability and impact resistancecharacteristics were secured by securing a maximum bendingangle of 60.14° and CIE 29692 Nm. However, ComparativeExamples 1 and 2 have strength after hot forming similar toInventive Example 1, but [Relationship 2], a skin passrolling condition, was more than an upper limit of 40, sothat the surface roughness factor is outside of the scope ofthe present disclosure. As a result, it was confirmed thata change amount of a bending angle compared to InventiveExample 1 finally was more than 5% due to a surface notcheffect, and it was confirmed that there was a decrease inbendability.

[00189]

[00190] Inventive Examples 2 to 4 and Comparative Example 3were all manufactured using the same B steel type, butInventive Example 2, satisfying the conditions of the presentdisclosure, secured excellent bendability and impactresistance, and compared to Inventive Example 2, in all ofInventive Examples 3 to 4, a decrease in maximum bendingangle was only 5% or less. However, in Comparative Example3, the skin pass rolling condition [Relational Expression 2]was more than an upper limit of 40, the surface roughnessfactor was outside of the scope of the present disclosure,and compared to Inventive Example 3, it is confirmed thatthe bendability and the impact resistance were significantlyreduced.

[00191] Inventive Example 5 and Comparative Examples 4 to 5were manufactured of the same type of C steel, and InventiveExample 5 had a maximum bending angle of 42° and was able tosecure CIE 39566 Nm. However, in Comparative Examples 4 to5, the skin pass rolling condition [Relational Expression 2]was more than the upper limit of 40, the surface roughnessfactor is outside of the scope of the present disclosure,and compared to Inventive Example 5, and it is confirmedthat bendability and impact resistance characteristics werereduced by checking a change amount of the bending angle ofmore than 5%.

[00192]

[00193] (Example 2)

[00194] Steel having steel components shown in Table 3 belowwas manufactured through the same steelmaking, hot rolling,cold rolling, and annealing processes as in Example 1, andno additional plating was performed. Skin pass rolling wasperformed to impart roughness to the annealed steel sheetthat had passed through the annealing process, and additionalelectroplating was performed on the skin pass rolled annealedsteel sheet to prevent decarburization of a surface layerthat may occur during the hot forming process. The steelsheet thus prepared was manufactured to form a blank, andthen was hot formed using a mold for hot forming, tomanufacture a hot-formed member. In this case, a heatingtemperature of the blank was 900°C, a retention time was 6minutes, and a transfer time from a heating furnace toforming was 10 seconds.

[00195]

[00196] [Table 3]

[00197]

[00198] [Table 4]

[00199]

[00200] In Table 4, Relationships 1 and 2 are the same asthose of Table 2 in Example 1 described above.

[00201]

[00202] In Table 4, looking at Inventive Examples 6 to 8 andComparative Example 6 made of steel type D in Table 3, itwas confirmed that Inventive Example 6 had very goodbendability with a maximum bending angle of 58.5°. InventiveExamples 7 and 8 also satisfy the conditions of the presentdisclosure, and have a change amount of a bending anglecompared to Inventive Example 6, but it can be confirmedthat they all have good bendability or impact resistance of5% or less.

[00203]

[00204] On the other hand, in the case of Comparative Example6, it was manufactured using the same D steel type, anddespite showing strength after hot forming, the alloycomposition satisfied the scope of the present invention,but the value according to Relationship 2 was more than 40,and as a result, the surface roughness factor was outside ofthe scope of the present invention, resulting in a changeamount of the bending angle exceeding 5% compared toInventive Example 6 due to a surface notch effect.

[00205] While example embodiments have been shown anddescribed above, it will be apparent to those skilled in theart that modifications and variations could be made withoutdeparting from the scope of the present disclosure as definedby the appended claims.

Claims

1. A steel material for hot forming comprising, by weight: 0.04 to 0.45% of carbon (C); 1.5% or less of silicon (Si) (excluding 0%); 0.2 to 2.5% of manganese (Mn); 0.05% or less of phosphorous (P); 0.02% or less of sulfur (S); 0.01 to 0.1% of aluminum (Al); 0.01 to 5.0% of chromium (Cr); 0.02% or less of nitrogen (N), and a balance of iron (Fe) and inevitable impurities, wherein a surface roughness factor calculated by the following [Relationship 1] is 1.8 μm or less, [Relationship 1] Surface Roughness Factor =Rt / 100+ 10 x Rdq where Rt is defined as a vertical distance between a highest peak and a deepest valley in a random measurement section on a surface of a steel sheet, and Rdq is a root mean square of a slope of the peak in a random measurement section on the surface of the steel sheet.

2. The steel material for hot forming of claim 1, wherein the steel material further comprises: one or more of 0.5% or less of Mo; 0.5% or less of Ni; 0.1% or less of Nb, 0.1% or less of Ti, and 0.01% or less of B.

3. The steel material for hot forming of claim 1, wherein a microstructure of the steel material comprises, by area fraction, one or more of 50 to 90% of ferrite; 30% or less of pearlite; 20% or less of bainite, and 20% or less of martensite.

4. The steel material for hot forming of claim 1, wherein the steel material further comprises a plating layer.

5. The steel material for hot forming of claim 4, wherein the plating layer comprises, by weight: 6 to 12% of Si; 1 to 4% of Fe, and a balance of Al and inevitable impurities.

6. A method for manufacturing a steel material for hot forming, the method comprising operations of: obtaining a cold-rolled steel sheet using a steel slab comprising, by weight: 0.04 to 0.45% of carbon (C); 1.5% or less of silicon (Si) (excluding 0%); 0.2 to 2.5% of manganese (Mn); 0.05% or less of phosphorous (P); 0.02% or less of sulfur (S); 0.01 to 0.1% of aluminum (Al); 0.01 to 5.0% of chromium (Cr); 0.02% or less of nitrogen (N), and a balance of Fe and inevitable impurities; and skin pass rolling the cold-rolled steel sheet to satisfy the following [Relationship 2], [Relationship 2] Equation where P is rolling force during skin pass rolling, and Raroll is arithmetic average roughness (Ra) of a skin pass rolling roll.

7. The method for manufacturing a steel material for hot forming of claim 6, wherein the cold-rolled steel sheet further comprises at least one of 0.5% or less of Mo, 0.5% or less of Ni, 0.1% or less of Nb, 0.1% or less of Ti, and 0.01% or less of B.

8. The method for manufacturing a steel material for hot forming of claim 6, wherein the operation of obtaining the cold-rolled steel sheet comprises operations of: heating the steel slab at 1050 to 1300°C; subjecting the heated steel slab to finish hot rolling at 800 to 950°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 500 to 700°C; cooling the coiled hot-rolled steel sheet from a coiling temperature to 400°C at a cooling rate of 10°C / Hr or more; cold rolling the cooled hot-rolled steel sheet at a reduction rate of 30 to 80% to obtain a cold-rolled steel sheet; heating the cold-rolled steel sheet in a temperature range of 400°C to an annealing temperature at a rate of 20°C / s or less; annealing the heated cold-rolled steel sheet at an annealing temperature of 740 to 860°C; and cooling the annealed cold-rolled steel sheet from the annealing temperature to 660°C at a cooling rate of 1°C / s or more.

9. The method for manufacturing a steel material for hot forming of claim 8, wherein a dew point temperature of atmospheric gas during the annealing is -70 to -30°C.

10. The method for manufacturing a steel material for hot forming of claim 8, further comprising: cooling the annealed cold-rolled steel sheet, and then dipping the cold-rolled steel sheet in an Al-based plating bath to form an aluminum plating layer.

11. The method for manufacturing a steel material for hot forming of claim 10, wherein the Al-based plating bath comprises, by weight: 6 to 12% of Si, 1 to 4% of Fe, and a balance of Al and inevitable impurities.

12. A hot-formed member, comprising by weight: 0.04 to 0.45% of carbon (C); 1.5% or less of silicon (Si) (excluding 0%); 0.2 to 2.5% of manganese (Mn); 0.05% or less of phosphorous (P); 0.02% or less of sulfur (S); 0.01 to 0.1% of aluminum (Al); 0.01 to 5.0% of chromium (Cr); 0.02% or less of nitrogen (N), and a balance of Fe and inevitable impurities, wherein a change amount of a maximum bending angle is 5% or less.

13. The hot-formed member of claim 12, wherein the hot-formed member has a yield strength (YS) of 800 MPa or more, a tensile strength (TS) of 1000 MPa or more, and an elongation (EI) of 3.5% or more.

14. The hot-formed member of claim 12, wherein the hot-formed member has a microstructure comprising a martensite single phase or a mixed structure comprising martensite and 40% by area or less of bainite.

15. The hot-formed member of claim 12, wherein the hot-formed member is manufactured using the steel material for hot forming in any one of claims 1 to 5.

16. A method for manufacturing a hot-formed member, comprising operations of: obtaining a blank using the steel material for hot forming of any one of claims 1 to 5; heating the blank to a temperature of Ac3 to 980°C, and then maintaining the temperature for 1 to 1000 seconds; and hot forming the heated and maintained blank, and then cooling the blank.

17. The method for manufacturing a hot-formed member of claim 16, wherein the cooling is performed by a mold cooling method.