Title of Invention: Steel for hot forming, hot forming member and manufacturing method thereof
technical field
[One]
The present invention relates to a steel material for hot forming, a hot forming member, and a manufacturing method thereof.
[2]
background
[3]
Hot-formed ultra-high-strength members have recently been widely applied to structural members of automobiles for the purpose of improving fuel efficiency and protecting passengers by reducing the weight of automobiles.
[4]
[5]
Patent Document 1 is proposed as a representative technique related to such hot forming. Patent Document 1 proposes a technique for securing ultra-high strength with a tensile strength of over 1600 MPa by heating an Al-Si plated steel sheet to 850° C. or higher and then forming the structure of the member into martensite by hot forming and rapid cooling by pressing. are doing In the case of the technique proposed in Patent Document 1, since it is molded at a high temperature, complex shapes can be easily molded, and a weight reduction effect can be expected due to high strength through an increase in strength due to rapid cooling in the mold.
[6]
[7]
However, in the case of a hot-formed member manufactured through hot forming, the microstructure is made of martensite, so the strength at the time of collision is high, but the bendability is inferior, so there is a limit in terms of energy absorption ability. Accordingly, in order to supplement the energy absorbing ability for relatively heat, a technology grafting a blank (TWB, Tailor welded blank) having a combination of different materials or different thicknesses to hot forming has been proposed and various studies are being conducted.
[8]
[9]
However, as such, low-strength steel for hot forming has been developed and applied as a material for TWB hot forming, which is excellent in terms of energy absorption. It is not easy to secure uniform physical properties according to differences in cooling conditions, etc., and the tensile strength is only at the level of 500 to 600 MPa.
[10]
[11]
[Prior art literature]
[12]
(Patent Document 1) US Patent No. 6296805
[13]
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[14]
One aspect of the present invention is to provide a steel material for hot forming, a hot forming member, and a method for manufacturing the same, which can impart excellent collision energy absorption ability while having high strength to the member.
[15]
[16]
The object of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the general description of the present invention.
[17]
means of solving the problem
[18]
One embodiment of the present invention is by weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1% , Cr: 0.01 to 0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the remainder including Fe and unavoidable impurities, and an alloy factor expressed by the following relation 1 is 7 or more and 10 5 carbides /mm 2 or less of carbide having an equivalent circle diameter of 0.5 μm or more.
[19]
[Relational Expression 1] Alloy index = I(Mn)×I(Si)×I(Cr)×I(Mo)
[20]
(However, in Relation 1, the I value for each component is I(Mn) = 3.34×Mn+1, I(Si) = 0.7×Si+1, I(Cr) = 2.16×Cr+1, I(Mo ) = 3×Mo+1, and the content for each component is wt%)
[21]
[22]
Another embodiment of the present invention is by weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1% , Cr: 0.01 to 0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the remainder including Fe and unavoidable impurities, and an alloy factor expressed by the following relation 1 is 7 or more and 10 4 carbides /mm 2 or less of carbides having an equivalent circle diameter of 0.5 μm or more.
[23]
[Relational Expression 1] Alloy index = I(Mn)×I(Si)×I(Cr)×I(Mo)
[24]
(However, in Relation 1, the I value for each component is I(Mn) = 3.34×Mn+1, I(Si) = 0.7×Si+1, I(Cr) = 2.16×Cr+1, I(Mo ) = 3×Mo+1, and the content for each component is wt%)
[25]
[26]
Another embodiment of the present invention is by weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1 %, Cr: 0.01 to 0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the remainder including Fe and unavoidable impurities, the alloy factor represented by the following relation 1 is 7 Heating the above steel slab at 1050 ~ 1300 ℃; obtaining a hot-rolled steel sheet by hot-rolling the heated steel slab at 800 to 950°C; winding the hot-rolled steel sheet at 500 to 700°C; cooling the wound hot-rolled steel sheet from the coiling temperature to 400°C at a cooling rate of 10°C/Hr or more; obtaining a cold rolled steel sheet by cold rolling the cooled hot rolled steel sheet; heating the cold-rolled steel sheet in a temperature range from 400°C to annealing temperature at a rate of 20°C/s or less; annealing the heated cold-rolled steel sheet at 740 to 860°C; and cooling the annealed cold-rolled steel sheet from an annealing temperature to 660° C. at a cooling rate of 1° C./s or more.
[27]
[Relational Expression 1] Alloy index = I(Mn)×I(Si)×I(Cr)×I(Mo)
[28]
(However, in Relation 1, the I value for each component is I(Mn) = 3.34×Mn+1, I(Si) = 0.7ХSi+1, I(Cr) = 2.16×Cr+1, I(Mo) = 3×Mo+1, and the content for each component is wt%)
[29]
[30]
Another embodiment of the present invention comprises the steps of obtaining a blank using the above-described steel for hot forming; After heating the blank at Ac3 ~ 980 ℃, maintaining 1 ~ 1000 seconds; and cooling the heated and maintained blank to room temperature after hot forming.
[31]
Effects of the Invention
[32]
According to one aspect of the present invention, a steel material for hot forming capable of manufacturing a member having high strength of 1000 MPa or more based on tensile strength, high impact energy absorption, and excellent material uniformity, a hot forming member using the same, and manufacturing thereof method can be provided.
[33]
[34]
Various and beneficial advantages and effects of the present invention are not limited to the above, and will be more easily understood in the course of describing specific embodiments of the present invention.
[35]
Brief description of the drawing
[36]
1 is a simplified view of the concept of CIE (Crack initiation Energy), which is a criterion for evaluating collision energy absorption capacity in the present invention.
[37]
2 is a schematic view showing a hot-formed member manufactured according to an embodiment of the present invention.
[38]
3 is a graph showing the collision energy absorption capacity according to the carbon content and alloy index of Inventive Examples 1 to 7 and Comparative Examples 3 to 9 according to an embodiment of the present invention.
[39]
Best mode for carrying out the invention
[40]
The terminology used herein is for the purpose of referring to specific embodiments only, and is not intended to limit the invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite.
[41]
The meaning of "comprising," as used herein, specifies a particular characteristic, region, integer, step, operation, element and/or component, and other specific characteristic, region, integer, step, operation, element, component, and/or group. It does not exclude the existence or addition of
[42]
Although not defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Commonly used terms defined in the dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and unless defined, they are not interpreted in an ideal or very formal meaning.
[43]
[44]
The present inventors have studied in depth how to improve the collision energy absorption capacity of the hot-formed member. Accordingly, the present inventors used the energy (area of ​​the load-displacement curve) value up to the maximum load in the three-point bending test (VDA238-100) as an index that can well express the impact energy absorption capacity of the hot-formed member, and various components and manufacturing conditions , organization, etc. were evaluated.
[45]
As a result, if the range of carbon content in the steel for hot forming, hardenability of alloying elements, cooling conditions after hot rolling, heating and cooling conditions during annealing, and the number density of coarse carbides in the structure are controlled to a certain level, the hot forming member It was concluded that the ability to absorb collision energy of
[46]
[47]
Hereinafter, the steel material for hot forming according to an aspect of the present invention will be described in detail. In the present invention, when expressing the content of each element, it is necessary to note that unless otherwise specified, it means weight %. In addition, the ratio of crystals or tissues is based on the area unless otherwise indicated.
[48]
[49]
First, the component system of the steel for hot forming according to an aspect of the present invention will be described. The steel for hot forming according to the present invention is, by weight, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 0.8%, Mo: 0.01 to 0.5%, N: 0.02% or less, the remainder may include Fe and unavoidable impurities.
[50]
[51]
Carbon (C): 0.06-0.1%
[52]
Carbon (C) is an essential element added to increase the strength of the heat treatment member. In general, it is common to use a high C content of 0.2% or more in ultra-high-strength steel for hot forming, but an appropriate C content is required to achieve the maximum effect in terms of collision energy absorption capacity. If the C content is less than 0.06%, it is difficult to secure sufficient strength, and ultimately, the ability to absorb collision energy is rather decreased even if the bendability is high, so 0.06% or more should be added. On the other hand, when the C content exceeds 0.1%, the strength is increased, but the ability to absorb impact energy is lowered due to poor bendability, so it can be limited to 0.1% or less. Therefore, the content of C is preferably in the range of 0.06 to 0.1%. The lower limit of the C content is more preferably 0.065%, and even more preferably 0.07%. The upper limit of the C content is more preferably 0.095%, and even more preferably 0.09%.
[53]
[54]
Silicon (Si): 0.05-0.6%
[55]
Silicon (Si) not only has to be added as a deoxidizer in steelmaking, but also contributes to increase the strength of the hot-formed member as a solid solution strengthening element and a carbide generation inhibiting element, and is added as an effective element for material uniformity. When the content is less than 0.05%, the above effect cannot be expected. On the other hand, when the Si content exceeds 0.6%, the plating property is greatly reduced due to the Si oxide generated on the surface of the steel sheet during annealing. can be limited Therefore, the Si content is preferably in the range of 0.05 to 0.6%. The lower limit of the Si content is more preferably 0.1%, even more preferably 0.15%. The upper limit of the Si content is more preferably 0.55%, even more preferably 0.5%.
[56]
[57]
Manganese (Mn): 0.6~2%
[58]
Manganese (Mn) needs to be added to suppress the formation of ferrite during hot forming by improving hardenability as well as securing a solid solution strengthening effect. If the Mn content is less than 0.6%, there is a limit to obtaining the above effect, and when the Mn content is too low, other expensive alloying elements are excessively required for insufficient hardenability, thereby greatly increasing the manufacturing cost. On the other hand, when the Mn content exceeds 2%, not only the cold rolling property decreases due to the increase in strength of the steel sheet before the hot forming process, but also the band-like structure arranged in the rolling direction in the microstructure deepens, so that the collision energy absorption ability is inferior. can be done Accordingly, the Mn content is preferably in the range of 0.6 to 2%. The lower limit of the Mn content is more preferably 0.7%, even more preferably 0.8%, and most preferably 0.9%. The upper limit of the Mn content is more preferably 1.8%, even more preferably 1.6%, and most preferably 1.4%.
[59]
[60]
Phosphorus (P): 0.05% or less
[61]
Phosphorus (P) exists as an impurity in steel, and when its content exceeds 0.05%, it may greatly embrittle the weldability of the hot-formed member. On the other hand, the P as an impurity may not be particularly limited with respect to its lower limit, but may be limited to 0.001% or more because a large manufacturing cost may be required to control the P content to less than 0.001%.
[62]
[63]
Sulfur (S): 0.02% or less
[64]
Sulfur (S) exists as an impurity in steel, and since it is an element that inhibits ductility, impact properties and weldability of hot-formed members, the maximum content may be limited to 0.02%. On the other hand, S is an impurity, and the lower limit thereof may not be particularly limited, but may be limited to 0.0001% or more because a large manufacturing cost may be required to control the S content to less than 0.0001%.
[65]
[66]
Aluminum (Al): 0.01~0.1%
[67]
Aluminum (Al), along with Si, is an element that increases the cleanliness of steel by deoxidizing in steelmaking. When the Al content is less than 0.01%, it is difficult to obtain the above effect, and when the content exceeds 0.1%, high-temperature ductility due to excessive AlN formed during the playing process is deteriorated, and there is a problem that slab cracks are easy to occur. Therefore, the content of Al is preferably in the range of 0.01 to 0.1%. The lower limit of the Al content is more preferably 0.015%. The upper limit of the Al content is more preferably 0.08%, even more preferably 0.07%, and most preferably 0.06%.
[68]
[69]
Chromium (Cr): 0.01~0.8%
[70]
Chromium (Cr) is added to secure hardenability of steel like Mn. If the Cr content is less than 0.01%, it may be difficult to secure sufficient hardenability. On the other hand, when the content exceeds 0.8%, the effect of improving the hardenability compared to the added amount is insignificant, and it promotes the formation of coarse iron carbide, thereby making the collision energy absorbing ability inferior, so the upper limit can be limited to 0.8%. Therefore, the content of Cr is preferably in the range of 0.01 to 0.8%. The lower limit of the Cr content is more preferably 0.015%, and even more preferably 0.02%. The upper limit of the Cr content is more preferably 0.75%, and even more preferably 0.7%.
[71]
[72]
Molybdenum (Mo): 0.5% or less (excluding 0%)
[73]
Molybdenum (Mo) not only has the effect of improving the hardenability of steel like Cr and Mn, but also can obtain effects such as increase in bendability by refining grains through the formation of fine precipitates. However, when the Mo content exceeds 0.5%, the upper limit may be limited to 0.5% because it causes an excessive increase in ferroalloy cost compared to the effect. Accordingly, the Mo content is preferably in the range of 0.5% or less (excluding 0%). The Mo content is more preferably 0.45% or less, more preferably 0.4% or less, and most preferably 0.35% or less.
[74]
[75]
Nitrogen (N): 0.02% or less
[76]
The N is included as an impurity in the steel. When the N content exceeds 0.02%, there is a problem in that slab cracks are likely to occur due to AlN formation as in the case of Al. On the other hand, the N as an impurity may not be particularly limited with respect to the lower limit thereof, but may be limited to 0.001% or more because a large manufacturing cost may be required to control the N content to less than 0.001%.
[77]
[78]
On the other hand, the steel material for hot forming according to an aspect of the present invention, in addition to the alloy components described above, selectively Ni: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less of one or more may include more.
[79]
[80]
Nickel (Ni): 0.5% or less
[81]
Nickel (Ni) is an austenite stabilizing element and may improve hardenability of steel through addition of Ni. However, since Ni is an expensive alloying element, it is preferable to set the upper limit to 0.5% in consideration of the increase in manufacturing cost compared to the effect of improving hardenability. Accordingly, the Ni content is preferably in the range of 0.5% or less. On the other hand, in order to sufficiently obtain the effect of curing ability according to the addition of Ni, the lower limit of the Ni content is more preferably 0.01%, more preferably 0.03%, and most preferably 0.05%. The upper limit of the Ni content is more preferably 0.45%, even more preferably 0.4%, and most preferably 0.35%.
[82]
[83]
Niobium (Nb): 0.1% or less
[84]
Niobium (Nb) is an element that can obtain a precipitation strengthening effect through the formation of fine precipitates, and through this, an effect of improving the bendability by increasing the strength and refining the grains can be obtained. In addition, by suppressing excessive grain growth during heating for hot forming, it is possible to achieve robustness against variations in heat treatment conditions. However, if the Nb content exceeds 0.1%, the effect is not only saturated, but also relatively coarse precipitates increase due to an increase in the precipitation temperature, which may reduce cost effectiveness. Therefore, the content of Nb is preferably in the range of 0.1% or less. The lower limit of the Nb content is more preferably 0.005%, even more preferably 0.01%, and most preferably 0.015%. The upper limit of the Nb content is more preferably 0.09%, even more preferably 0.08%, and most preferably 0.07%.
[85]
[86]
Titanium (Ti): 0.1% or less
[87]
Titanium (Ti) is an element that is sometimes added when B is added to secure hardenability by combining with nitrogen remaining as an impurity in steel to form TiN. In addition, precipitation strengthening and grain refinement effects can be expected through the formation of TiC precipitates. However, when the Ti content exceeds 0.1%, rather coarse TiN is formed in a large amount, which makes the collision energy absorbing ability inferior, so the upper limit can be limited to 0.1%. Accordingly, the Ti content is preferably in the range of 0.1% or less. The lower limit of the Ti content is more preferably 0.005%, even more preferably 0.01%, and most preferably 0.015%. The upper limit of the Ti content is more preferably 0.08%, even more preferably 0.06%, and most preferably 0.05%.
[88]
[89]
Boron (B): 0.01% or less
[90]
Boron (B) is an element that can improve hardenability even with a small amount of addition, and can effectively suppress the brittleness of the hot-formed member due to segregation at the grain boundary of P or / and S by segregation at the grain boundary of prior austenite. . However, when the content exceeds 0.01%, the upper limit can be limited to 0.01% because it causes brittleness in hot rolling due to the formation of Fe 23 CB 6 complex compound. Therefore, the content of B is preferably in the range of 0.01% or less. The lower limit of the B content is more preferably 0.0001%, even more preferably 0.0003%, and most preferably 0.0005%. The upper limit of the B content is more preferably 0.009%, even more preferably 0.007%, and most preferably 0.005%.
[91]
[92]
In addition to the above-described steel composition, the remainder may include Fe and unavoidable impurities. Inevitable impurities may be unintentionally mixed in a typical steel manufacturing process, and this cannot be entirely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. In addition, the present invention does not entirely exclude the addition of a composition other than the steel composition mentioned above.
[93]
[94]
The steel material for hot forming according to an aspect of the present invention satisfies the above-described component system, and at the same time, maximizes the collision energy absorption capacity in the hot forming member and minimizes the hardness deviation. The alloy factor expressed by the following relation 1 It is preferable that is controlled to be 7 or more. When the alloy index is less than 7, it is difficult to secure sufficient hardenability, so that a large hardness deviation may occur in the hot-formed member. In addition, grain boundary ferrite is formed in the surface layer portion of the hot-formed member, so that the collision energy absorption ability may be greatly inferior. The alloy index is more preferably 7.5 or more, and more preferably 8 or more. In the present invention, as long as the alloy index is 7 or more, the effect desired by the present invention can be secured, and the upper limit thereof is not particularly limited. However, for example, the alloy index may be 40 or less, more preferably 30 or less in terms of manufacturing cost reduction. On the other hand, the following Relational Equation 1 shows that the inventors used a number of ferroalloys whose contents of main alloying elements were changed based on the same carbon content, heated to an austenite region, and then hardened for each element through a final hardness change test for each cooling rate. It is an expression derived through linear regression analysis on the effect on performance.
[95]
[Relational Expression 1] Alloy index = I(Mn)×I(Si)×I(Cr)ХI(Mo)
[96]
(However, in Relation 1, the I value for each component is I(Mn) = 3.34×Mn+1, I(Si) = 0.7×Si+1, I(Cr) = 2.16×Cr+1, I(Mo ) = 3×Mo+1, and the content for each component is wt%)
[97]
[98]
In the case of hot forming steel used for TWB, energy absorbing capacity is one of the important characteristics, and this energy absorbing capacity is affected by strength and bending characteristics. That is, the higher the strength and the better the bending properties, the better the energy absorption ability. In general, the factor that has the greatest influence on strength after hot forming of steel for hot forming is the martensite fraction, and in particular, when martensite is the main structure, it is greatly affected by the carbon content. In addition, although the bending properties are slightly different depending on the composition of the tissue, it usually exhibits excellent properties when it is composed of a single phase, and when it is composed of two or more phases, the smaller the difference in strength between the phases, the better the characteristic. Considering these characteristics, in the region with a carbon content of less than 0.06%, the strength is not only low, but it is difficult to avoid the formation of soft ferrite during cooling. will fall On the other hand, in the region where the C content exceeds 0.1%, a high-strength martensitic single-phase structure can be easily obtained, but the bending properties decrease according to the increase in strength, and ultimately, the collision energy absorption ability is deteriorated. In addition, even if the C content satisfies the range of the present invention, when the alloy index value is less than 7, it is difficult to avoid the formation of a soft phase such as ferrite during cooling due to insufficient hardenability, so that the bending properties are deteriorated, and thus the collision energy absorption ability is reduced will fall Therefore, both strength and bending characteristics should be considered in order to secure the collision energy absorption capacity at a good level.
[99]
[100]
In the steel material for hot forming according to an aspect of the present invention, the number of carbides having an equivalent circle diameter of 0.5 μm or more is 10 5 pieces/mm 2 or less. The present inventors have concluded that it is important to properly secure the strength and bendability of the material in order to maximize the excellent impact energy absorption capacity of the hot-formed member, and through various experiments, the number density of coarse carbides from the steel sheet is appropriately controlled. It was confirmed that it was possible to secure sex. If the coarse carbide having an equivalent circle diameter of 0.5 μm or more exceeds 10 5 pieces/mm 2 , even if some iron carbide is re-dissolved during heating for hot forming, another part remains in the member after hot forming. Coarse carbides that remain undissolved in this way act as a cracking initiation point during bending deformation, and thus become a factor of lowering the bendability and ultimately inferior in the ability to absorb impact energy. The lower the number density of such coarse carbides, the more advantageous, so the lower limit is not particularly limited in the present invention.
[101]
[102]
The steel material for hot forming according to an aspect of the present invention may include at least one of ferrite: 50 to 90 area%, pearlite: 30 area% or less, bainite: 20 area% or less, and martensite: 20 area% or less have. The ferrite is an effective structure for reducing the blanking process load of the steel sheet when manufacturing the blank as a soft phase, and in order to obtain the above effect, it is preferable to secure 50 area% or more. However, if it exceeds 90 area%, carbon is excessively distributed to the structure other than ferrite during blank manufacturing, and there is a possibility that carbon may be unevenly distributed even after hot forming. Therefore, the ferrite preferably has a range of 50 to 90 area%. If the perlite exceeds 30% by area, cementite may be incompletely dissolved after hot forming to reduce strength or cause material non-uniformity. When the bainite and martensite each exceed 20 area %, the strength of the steel sheet is excessively increased, which may cause problems such as mold wear during blank manufacturing.
[103]
[104]
An aluminum-based plating layer may be formed on at least one surface of the steel material for hot forming according to an aspect of the present invention. In the present invention, the aluminum-based plating layer is not particularly limited, but as a non-limiting embodiment, Si: 6 to 12%, Fe: 1 to 4%, the remainder Al and unavoidable impurities may be included in wt%.
[105]
[106]
Hereinafter, a hot-formed member according to an aspect of the present invention will be described in detail. The hot forming member according to one aspect of the present invention may be manufactured by hot press forming the above-described steel material for hot forming. The hot-formed member according to an aspect of the present invention preferably has the above-described alloy composition and alloy index. In addition, it is preferable that the number of carbides having an equivalent circle diameter of 0.5 µm or more is 10 4 pieces/mm 2 or less. Since carbides existing in the steel sheet before hot forming are dissolved into the steel during the heating step for hot forming, the number density and size of the carbides are reduced compared to the state of the steel sheet before hot forming. However, coarse carbides over a certain size are not completely dissolved in a normal heating step and remain in the hot-formed member. Therefore, when the number of carbides having an equivalent circle diameter of 0.5 μm or more in the hot-formed member exceeds 10 4 /mm 2 , as described above, the coarse carbides act as a cracking initiation point during bending deformation, making the collision energy absorbing ability inferior. have.
[107]
[108]
The hot-formed member according to an aspect of the present invention may have a martensitic single-phase microstructure or a mixed structure including martensite and bainite in an area of ​​40% or less. Since the martensite is an effective structure for securing the strength targeted by the present invention, the microstructure of the present invention may be a martensitic single-phase structure. On the other hand, although bainite has a slightly lower strength than martensite, it does not significantly reduce bendability when formed in a martensite matrix and is an advantageous structure for securing strength. It may have a mixed structure containing bainite of However, when the fraction of bainite is less than 40 area %, it may be difficult to secure the target strength in the present invention.
[109]
[110]
Meanwhile, the microstructure may further include at least one of 10% by area or less of ferrite and 5% or less of retained austenite. The ferrite and retained austenite are structures that may be unavoidably contained in the manufacturing process. When the ferrite structure exceeds 10 area %, strength is reduced as well as bending properties can be greatly inferior, and when the retained austenite structure exceeds 5 area %, the strength is lowered or atmospheric gas during hot forming Hydrogen inflow is increased, which may increase the possibility of hydrogen embrittlement.
[111]
[112]
When the hot-formed member according to an aspect of the present invention is subjected to a three-point bending test according to the VDA standard (VDA238-100), the area from the load-displacement curve obtained in the three-point bending test to the maximum load is reached (CIE: Crack initiation Energy) may be 25000 Nm or more. 1 is a simplified view of the concept of CIE (Crack initiation Energy), which is a criterion for evaluating collision energy absorption capacity in the present invention.
[113]
[114]
The hot-formed member according to an aspect of the present invention measures hardness at arbitrary points of the hot-formed member, and the difference between the maximum value and the minimum value is called the hardness deviation, and the value obtained by dividing the hardness deviation by the average hardness of the hot-formed member When ? is a deviation level, the deviation level may be 0.3 or less. Meanwhile, in the present invention, the number of the arbitrary points is not particularly limited, and, for example, hardness can be measured for any 9 points.
[115]
[116]
The hot-formed member according to an aspect of the present invention may have a yield strength (YS): 800 MPa or more, a tensile strength (TS): 1000 MPa or more, and an elongation (El): 5% or more.
[117]
[118]
Hereinafter, a method for manufacturing a steel material for hot forming according to an aspect of the present invention will be described in detail. However, the manufacturing method described below is only one embodiment among all possible embodiments, and does not mean that the steel for hot forming of the present invention must be manufactured only by the following manufacturing method.
[119]
[120]
First, a steel slab satisfying the above-described alloy composition and alloy index is heated at 1050 ~ 1300 ℃. When the heating temperature of the steel slab is less than 1050° C., it may be difficult to homogenize the structure of the slab, as well as difficult to re-dissolve when using the precipitating element. On the other hand, when the heating temperature exceeds 1300 °C, an excessive oxide layer is formed, which may increase the possibility of causing surface defects after hot rolling. Therefore, the steel slab heating temperature is preferably 1050 ~ 1300 ℃. The lower limit of the heating temperature of the steel slab is more preferably 1070 ℃, even more preferably 1100 ℃. The upper limit of the heating temperature of the steel slab is more preferably 1280 ℃, even more preferably 1250 ℃.
[121]
[122]
Then, finish hot rolling the heated steel slab at 800 ~ 950 ℃ to obtain a hot rolled steel sheet. When the finish hot rolling temperature is less than 800° C., it may be difficult to control the plate shape due to the generation of a mixed structure in the surface layer portion of the steel sheet according to the abnormal rolling. On the other hand, when the finish hot rolling temperature exceeds 950° C., there is a problem in that grain coarsening due to hot rolling easily occurs. Therefore, the finish hot rolling temperature is preferably 800 ~ 950 ℃. The lower limit of the finish hot rolling temperature is more preferably 810°C, and even more preferably 820°C. The upper limit of the finish hot rolling temperature is more preferably 940°C, even more preferably 930°C.
[123]
[124]
Then, the hot-rolled steel sheet is wound at 500 ~ 700 ℃. When the coiling temperature is less than 500° C., martensite is formed in whole or in part of the steel sheet, making it difficult to control the shape of the plate, and also, due to the increase in strength of the hot-rolled steel sheet, there may be a problem of poor rollability in the subsequent cold rolling process. On the other hand, when the coiling temperature exceeds 700 °C, coarse carbide is formed, and the collision energy absorption ability of the hot-formed member may be reduced. Therefore, the coiling temperature is preferably 500 ~ 700 ℃. The lower limit of the coiling temperature is more preferably 520°C, and even more preferably 550°C. The upper limit of the coiling temperature is more preferably 680°C, and even more preferably 650°C.
[125]
[126]
Thereafter, the wound hot-rolled steel sheet is cooled from the coiling temperature to 400°C at a cooling rate of 10°C/Hr or more. When the cooling rate is less than 10° C./Hr, there may be a disadvantage in that a large number of coarse carbides are formed during cooling of the hot-rolled coil due to sufficient time for the carbides to grow. Therefore, it is preferable that the cooling rate is 10° C./Hr or more. The cooling rate is more preferably 12°C/Hr or more, and even more preferably 15°C/Hr or more. On the other hand, in the present invention, as long as the cooling rate is 10° C./Hr or more, the effect desired by the present invention can be obtained. Therefore, the upper limit of the cooling rate is not particularly limited. However, for example, in terms of cost reduction due to facility investment, the cooling rate may be 500° C./Hr or less, more preferably 45° C./Hr or less, and even more preferably 400° C./Hr or less.
[127]
[128]
After the winding and cooling, the process of pickling the cooled hot-rolled steel sheet before cold rolling may be further included. The product surface quality can be improved by removing the scale formed on the surface of the steel sheet through the pickling process.
[129]
[130]
Thereafter, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Although the present invention does not specifically limit the reduction ratio during cold rolling, for example, a reduction ratio of 30 to 80% may be applied to obtain a target thickness of the steel material.
[131]
[132]
Meanwhile, according to an aspect of the present invention, continuous annealing and aluminum-based plating may be performed on the cold-rolled steel sheet, or aluminum-based plating may be performed on the cooled hot-rolled steel sheet immediately after pickling.
[133]
[134]
Thereafter, for annealing the cold-rolled steel sheet, it is preferable to heat the cold-rolled steel sheet in a temperature range from 400° C. to an annealing temperature at a rate of 20° C./s or less. When the heating rate exceeds 20°C/s from 400°C to the annealing temperature, there is not enough time for the carbide precipitated in the hot-rolling step to be re-dissolved, so coarse carbide may remain, and the finally obtained hot-formed member collides Energy absorption may be reduced. Accordingly, the heating rate from 400° C. to the annealing temperature is preferably 20° C./s or less. The heating rate is more preferably 18°C/s or less, and even more preferably 15°C/s or less. On the other hand, in the present invention, as long as the heating rate is 20° C./s or less, the effect desired by the present invention can be obtained, so the lower limit of the heating rate is not particularly limited. However, for example, in consideration of the annealing productivity, the heating rate may be 0.5°C/s or more, more preferably 1°C/s or more, and even more preferably 1.5°C/s or more. On the other hand, in the present invention, the heating rate is not particularly limited in the temperature range from the cold rolling temperature to less than 400° C. This is because the effect on the re-dissolution of carbides is insignificant even if the heating rate is controlled.
[135]
[136]
The heated cold-rolled steel sheet is preferably annealed at 740 ~ 860 ℃. If the annealing temperature is less than 740° C., recrystallization of the cold-rolled structure is not sufficiently performed, resulting in poor plate shape or excessively high strength after plating, which may cause mold wear during the blanking process. On the other hand, when the annealing temperature exceeds 860° C., Si, Mn, etc. may form surface oxides during the annealing process, resulting in a poor plating surface. Therefore, the annealing temperature is preferably 740 ~ 860 ℃. The lower limit of the annealing temperature is more preferably 750°C, and even more preferably 760°C. The upper limit of the annealing temperature is more preferably 850°C, and even more preferably 840°C.
[137]
[138]
On the other hand, the atmosphere during the continuous annealing is preferably a non-oxidizing atmosphere, for example, a hydrogen-nitrogen mixed gas may be used, in which case the dew point temperature of the atmospheric gas may be -70 ~ -30 ℃. . In order for the dew point temperature to be less than -70°C, additional equipment for control is required, and there is a problem in that the manufacturing cost increases. It may cause defects such as plating. Therefore, it is preferable that the dew point temperature (Dew point) of the atmospheric gas during the continuous annealing is -70 ~ -30 ℃. The lower limit of the dew point temperature of the atmospheric gas is more preferably -65°C, and even more preferably -60°C. As for the upper limit of the dew point temperature of the said atmospheric gas, it is more preferable that it is -35 degreeC, and it is still more preferable that it is -40 degreeC.
[139]
[140]
Thereafter, the annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C/s or more. In order to achieve the target number density of carbides with an equivalent circle diameter of 0.5 μm or more in the present invention to be 10 5 pieces/mm 2 or less, the cooling rate from the annealing temperature to 660° C. is preferably 1° C./s or more, and the cooling rate is If it is less than 1° C./s, a large amount of coarse carbides may be formed, so that the finally obtained hot-formed member may have a reduced ability to absorb impact energy. Therefore, it is preferable that the cooling rate is 1° C./s or more. The cooling rate is more preferably 1.5°C/s or more, and even more preferably 2°C/s or more. On the other hand, in the present invention, as long as the cooling rate is 1° C./s or more, the desired effect can be obtained. Therefore, the upper limit of the cooling rate is not particularly limited. However, for example, in terms of suppressing the shape defect of the steel sheet, the cooling rate may be 50° C./s or less, more preferably 45° C./s or less, and still more preferably 40° C./s or less.
[141]
[142]
After cooling the annealed cold-rolled steel sheet, the method may further include forming an aluminum-based plating layer by immersing the cooled cold-rolled steel sheet in an Al-based plating bath. In the present invention, the composition and plating conditions of the Al-based plating bath are not particularly limited. However, as a non-limiting embodiment, the composition of the plating bath may include Si: 6 to 12%, Fe: 1 to 4%, the remainder Al and other unavoidable impurities, and the plating amount is commonly applied in the art. It may be 30~130g/m 2 based on one side being used. When the Si content in the composition of the plating bath is less than 6%, the plating bath temperature is excessively increased to deteriorate the equipment, and when it exceeds 12%, the heating time for hot forming must be prolonged by excessively delaying alloying. There is a downside to If the Fe content is less than 1%, plating adhesion or spot weldability may be inferior, and if it exceeds 4%, dross in the plating bath may be excessively generated and cause poor surface quality. If the plating adhesion amount is less than 30g/m 2 on one side , it may be difficult to secure the desired corrosion resistance of the hot-formed member . It may not be easy to uniformly plated the entire width and length.
[143]
[144]
Hereinafter, a method for manufacturing a hot-formed member according to an aspect of the present invention will be described in detail. However, the manufacturing method described below is only one embodiment among all possible embodiments, and does not mean that the hot-formed member of the present invention must be manufactured only by the following manufacturing method.
[145]
[146]
First, a steel material for hot forming manufactured by the above-described manufacturing method is prepared, and a blank for hot forming is prepared using the steel material for hot forming. And the blank is heated to a temperature range of austenite single-phase region or higher, more specifically, Ac3 temperature or higher and 980°C or lower. If the blank heating temperature is less than the Ac3 temperature, it may be difficult to secure a predetermined strength due to the presence of untransformed ferrite. On the other hand, when the heating temperature exceeds 980 °C, it may be difficult to secure spot weldability due to excessive oxide formation on the member surface. Therefore, the blank heating temperature is preferably Ac3 ~ 980 ℃. The lower limit of the blank heating temperature is more preferably Ac3+5°C, and even more preferably Ac3+10°C. As for the upper limit of the said blank heating temperature, it is more preferable that it is 970 degreeC, and it is still more preferable that it is 960 degreeC.
[147]
[148]
The heated blank is preferably maintained in the temperature range for 1 to 1000 seconds. If the holding time is less than 1 second, the temperature may not be uniform throughout the blank, which may cause material differences for each part. It can be difficult to do. Therefore, the holding time is preferably 1 to 1000 seconds. The lower limit of the holding time is more preferably 30 seconds, and still more preferably 60 seconds. The upper limit of the holding time is more preferably 900 seconds, and still more preferably 800 seconds.
[149]
[150]
Thereafter, the heated and maintained blank is hot-formed and cooled to room temperature to finally prepare a hot-formed member. In the present invention, the specific conditions for the hot forming are not particularly limited, and a hot forming method commonly known in the art to which the present invention belongs may be applied as it is.
[151]
Modes for carrying out the invention
[152]
Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only for exemplifying the present invention and not limiting the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.
[153]
[154]
(Example)
[155]
A steel slab having a thickness of 40 mm having an alloy composition shown in Table 1 was prepared through vacuum melting. The steel slab was heated to 1250°C, and then hot-rolled to a finish hot-rolling temperature of 900°C to obtain a hot-rolled steel sheet. Thereafter, the conditions described in Table 2 below were applied using the simulation heat treatment furnace for the coiling temperature and the cooling rate from the coiling temperature to 400° C. for each steel type, and hot rolling was performed so that the final hot-rolled thickness was all 3 mm. Thereafter, after pickling the hot-rolled steel sheet, cold rolling was performed at a cold rolling reduction ratio of 50% to obtain a cold-rolled steel sheet. Thereafter, the cold-rolled steel sheet is heated by controlling the heating rate from 400° C. to annealing temperature under the conditions shown in Table 2 below, annealed in an atmosphere of 5% hydrogen-95% nitrogen, and then cooled from the annealing temperature to 660° C. A cold-rolled steel sheet was manufactured by controlling the speed. Then, after cooling the cold-rolled steel sheet, Al-based plating was performed. At this time, the Al-based plating bath composition is composed of Al-9%Si-2%Fe and the rest of the unavoidable impurities, and the plating adhesion amount is 80 g/m 2 based on one side.was done with The steel sheet thus prepared was manufactured as a blank, and then hot-formed using a mold for hot forming to prepare a hot-formed member having the form shown in FIG. 2 . At this time, the heating temperature of the blank was 900° C., the holding time was 6 minutes, and the transfer time from the heating furnace to molding was applied in the same way as all 10 seconds. After measuring the number of carbides having an equivalent circle diameter of 0.5 μm or more in the cold-rolled steel sheet prepared as described above, the results are shown in Table 2 below. After measuring the number, microstructure, and mechanical properties of carbides having an equivalent circle diameter of 0.5 μm or more for the hot-formed member prepared as described above, the results are shown in Tables 3 and 4 below.
[156]
[157]
The number of carbides having an equivalent circle diameter of 0.5 μm or more was measured by observing 10 fields of view at a magnification of 10000 times using a transmission electron microscope (TEM) after preparing a thin foil specimen.
[158]
[159]
The microstructure was measured using a scanning transfer microscope after etching the surface of the steel sheet using nital.
[160]
[161]
Yield strength (YS), tensile strength (TS), and elongation (El) were measured by taking ASTM standard specimens in a direction parallel to the rolling direction of the steel sheet, and then performing a tensile test.
[162]
[163]
The impact energy absorption capacity was evaluated by measuring the area until reaching the maximum load (CIE: Crack Initiation Energy) from the load-displacement curve obtained from a three-point bending test according to the VDA standard (VDA238-100). , a case higher than 25000 Nm, which is the CIE value of a conventional 1500 MPa class steel for hot forming, was evaluated as good, and a case lower than that was evaluated as bad.
[164]
[165]
In addition, in order to confirm the uniformity of the properties of the hot-formed member, three specimens were collected from the upper, left, and right sides of the formed member, and then hardness was measured using Vickers hardness, and from this, the average hardness, Hardness deviation (the difference between the maximum and minimum values ​​among the hardness measured at 9 locations) and the level of deviation (hardness deviation/average hardness) were measured, and for the uniformity of physical properties, a deviation level of 0.3 or less was good and an excess of 0.3 was considered bad.
[166]
[167]
[Table 1]
[168]
[169]
[Table 2]
[170]
[171]
[Table 3]
[172]
[173]
[Table 4]
[174]
[175]
As can be seen from Tables 1 to 4, in the case of Inventive Examples 1 to 7, which satisfy all of the alloy composition, alloy index, and manufacturing conditions proposed by the present invention, the collision energy absorption capacity is good and the physical properties in the member are uniform. can be checked
[176]
[177]
In the case of Comparative Examples 1 and 2, the alloy composition and alloy index satisfy the scope of the present invention, but as the manufacturing conditions of the coiling temperature, the cooling rate after winding and the heating rate during annealing are outside the scope of the present invention, the coarse carbide number density It can be seen that the impact energy absorption capacity was poor in the finally obtained hot-formed member by increasing .
[178]
[179]
In Comparative Examples 3 to 5, the alloy composition of the steel sheet is satisfied but the alloy index does not satisfy the conditions of the present invention, and it can be seen that the hardness deviation due to the lack of hardenability occurs excessively, and the physical property uniformity in the member is poor. have.
[180]
[181]
Comparative Examples 6 to 8 are cases in which the C content exceeds the conditions of the present invention, and although there is an effect of increasing the strength according to the increase of the C content, it can be seen that the bendability is greatly decreased compared to the increase in strength, so that the collision energy absorption ability is rather inferior.
[182]
[183]
Comparative Example 9 is a case in which the C content does not meet the conditions of the present invention, and it can be seen that not only the target strength was not secured, but also the collision energy absorption ability was deteriorated due to this.
[184]
[185]
3 is a graph showing the collision energy absorption capacity according to the carbon content and alloy index of Inventive Examples 1 to 7 and Comparative Examples 3 to 9; As shown in Figure 3, the C content and alloy index are directly related to the impact energy absorption capacity of the hot-formed member, and good impact energy absorption capacity can be secured only when the C content and alloy index proposed by the present invention are satisfied. can be known
Claims
[Claim 1]
By weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 0.8% , Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the remainder including Fe and unavoidable impurities, the alloy factor expressed by the following relation 1 is 7 or more, and the equivalent circle diameter is 0.5 A steel material for hot forming in which the carbide content of ㎛ or more is 10 5 pieces/mm 2 or less. [Relational Expression 1] Alloy index = I(Mn) × I(Si) × I(Cr) × I(Mo) (However, the I value for each component in Relation 1 is I(Mn) = 3.34ХMn+1, I(Si) = 0.7ХSi+1, I(Cr) = 2.16ХCr+1, I(Mo) = 3×Mo+1, and the content for each component is wt%)
[Claim 2]
The steel material for hot forming according to claim 1, wherein the steel material further comprises at least one of Ni: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, and B: 0.01% or less.
[Claim 3]
The method according to claim 1, wherein the steel material has a microstructure comprising at least one of ferrite: 50 to 90 area%, pearlite: 30 area% or less, bainite: 20 area% or less, and martensite: 20 area% or less. steel for forming.
[Claim 4]
The steel material for hot forming according to claim 1, wherein the steel material has an aluminum-based plating layer formed on at least one surface thereof.
[Claim 5]
The steel material for hot forming according to claim 4, wherein the aluminum-based plating layer includes, by weight, Si: 6 to 12%, Fe: 1 to 4%, the remainder Al and unavoidable impurities.
[Claim 6]
By weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 0.8% , Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the remainder including Fe and unavoidable impurities, the alloy factor expressed by the following relation 1 is 7 or more, and the equivalent circle diameter is 0.5 A hot-formed member having a carbide content of ㎛ or more of 10 4 pieces/mm 2 or less. [Relational Expression 1] Alloy index = I(Mn) × I(Si) × I(Cr) × I(Mo) (However, the I value for each component in Relation 1 is I(Mn) = 3.34ХMn+1, I(Si) = 0.7ХSi+1, I(Cr) = 2.16ХCr+1, I(Mo) = 3×Mo+1, and the content for each component is wt%)
[Claim 7]
The hot-formed member according to claim 6, wherein the microstructure is a single-phase martensite structure or a mixed structure comprising martensite and bainite in an area of ​​40% or less.
[Claim 8]
The hot-formed member according to claim 7, wherein the microstructure further comprises at least one of 10% by area or less of ferrite and 5% or less of retained austenite.
[Claim 9]
The method according to claim 6, wherein when the member is subjected to a three-point bending test according to the VDA standard (VDA238-100), the area from the load-displacement curve obtained in the three-point bending test to the maximum load (CIE: Crack initiation Energy) is a hot-formed member, characterized in that 25000Nm or more.
[Claim 10]
The hot-formed member according to claim 6, wherein the member has a hardness deviation level of 0.3 or less. (However, the hardness deviation level is the value obtained by dividing the hardness deviation by the average hardness value of the hot-formed member, and the hardness deviation means the difference between the maximum and minimum values ​​obtained by measuring hardness at arbitrary points)
[Claim 11]
The hot-formed member according to claim 6, wherein the member has a yield strength (YS): 800 MPa or more, a tensile strength (TS): 1000 MPa or more, and an elongation (El): 5% or more.
[Claim 12]
By weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 0.8% , Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the balance Fe and unavoidable impurities, including the alloy index (alloy factor) 7 or more expressed by the following relation 1, 1050 ~ 1300 ℃ steel slab heating in; obtaining a hot-rolled steel sheet by hot-rolling the heated steel slab at 800 to 950°C; winding the hot-rolled steel sheet at 500 to 700°C; cooling the wound hot-rolled steel sheet from the coiling temperature to 400°C at a cooling rate of 10°C/Hr or more; obtaining a cold rolled steel sheet by cold rolling the cooled hot rolled steel sheet; heating the cold-rolled steel sheet in a temperature range from 400°C to annealing temperature at a rate of 20°C/s or less; annealing the heated cold-rolled steel sheet at 740 to 860°C; and cooling the annealed cold-rolled steel sheet from an annealing temperature to 660°C at a cooling rate of 1°C/s or more. [Relational Expression 1] Alloy index = I(Mn) × I(Si) × I(Cr) × I(Mo) (However, the I value for each component in Relation 1 is I(Mn) = 3.34 × Mn+1 , I(Si) = 0.7×Si+1, I(Cr) = 2.16×Cr+1, I(Mo) = 3×Mo+1, and the content for each component is wt%)
[Claim 13]
The method of claim 12, further comprising the step of pickling the cooled hot-rolled steel sheet before the cold rolling.
[Claim 14]
The method of claim 12, wherein during the cold rolling, the reduction ratio is 30 to 80%.
[Claim 15]
The method of claim 12, wherein the dew point temperature of the atmospheric gas during annealing is -70 to -30°C.
[Claim 16]
The method of claim 12, further comprising, after cooling the annealed cold-rolled steel sheet, immersing the cooled cold-rolled steel sheet in an Al-based plating bath to form an aluminum-based plating layer.
[Claim 17]
The method of claim 16, wherein the Al-based plating bath comprises, by weight, Si: 6 to 12%, Fe: 1 to 4%, the remainder Al and unavoidable impurities.
[Claim 18]
The method according to claim 16, wherein when the plating layer is formed, the plating adhesion amount is 30 to 130 g/m 2 based on one side of the steel material for hot forming.
[Claim 19]
19. A method comprising: obtaining a blank using a steel material for hot forming prepared according to any one of claims 11 to 18; After heating the blank at Ac3 ~ 980 ℃, maintaining 1 ~ 1000 seconds; and cooling the heated and maintained blank to room temperature after hot forming.