KR20150075306A - Ultra-high strength hot-rolled steel sheet with excellent in bending workability, and method for producing the same - Google Patents

Abstract

The present invention relates to an ultra-high strength hot-rolled steel sheet utilized for a component that high strength and excellent bendability are required such as a bumper reinforcing material and a door impact beam which are mainly vehicle body reinforcing materials. The present invention provides ultra-high strength hot-rolled steel sheet with excellent bendability capable of obtaining an excellent weight-lightening effect by being formed to be thin due to high strength and capable of easily securing the uniformity of a component shape due to excellent bendability and a manufacturing method of the ultra-high strength hot-rolled steel sheet. The ultra-high strength hot-rolled steel sheet comprises 0.1-0.25 wt% of C, 0.01-0.2 wt% of Si, 0.5-2.0 wt% of Mn, 0.005-0.02 wt% of P, 0.001-0.01 wt% of S, 0.001-0.35 wt% of at least one selected from a group composed of Ti, Nb, Mo, Cr, and B, and the remainder consisting of Fe and inevitable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to an ultra-high strength hot-rolled steel sheet having excellent bending workability and a method of manufacturing the same. [0002]

The present invention relates to an ultra-high strength hot-rolled steel sheet used for components requiring high strength and excellent bending workability, such as a bumper reinforcement which is an automobile body reinforcing material and a door impact beam, and a method of manufacturing the same. High strength hot-rolled steel sheet which is excellent in the effect of weight reduction due to high strength due to high strength and excellent in bending workability and easy to ensure component shape crystallinity through roll forming, and a method for producing the same.

In order to obtain high strength, a conventional high-strength hot-rolled steel sheet is generally manufactured by adding C, Si, Mn, Ti, Nb, Mo and V to a high purity steel in which impurities in the steel are minimized.

In order to manufacture the high-strength hot-rolled steel sheet, conventionally, a hot-rolled steel sheet is manufactured by adding precipitation strengthening of these elements by adding Ti, Nb, V, Mo or the like (Patent Document 1, Patent Document 2) (Patent Literature 3, Patent Literature 4), or a method of strengthening impact strength and tensile properties by annealing Mn and Cr-added steels (patent document 5) are known.

The ultra-high strength hot-rolled steel sheets used for bumper reinforcement and door impact beams, which are most automobile body reinforcing materials, require high strength and excellent bending workability for roll forming.

However, the solid solution strengthening by alloying components such as C, Si, Mn, Cr, Mo and W and the precipitation hardening by alloy components such as Ti, Nb and Mo, which are used in the conventional high strength hot- There is a problem that the bending workability is lowered and the price competitiveness is deteriorated due to the decrease in the productivity in the temper annealing treatment.

Patent Document 1: Japanese Patent Application No. 2010-279711 Patent Document 2: Japanese Patent Application No. 2003-156473 Patent Document 3: European Patent Application No. 2003-396059 Patent Document 4: Korean Patent Application No. 1996-7005330 Patent Document 5: PCT Patent Application No. IB2011-01436

The present invention provides an ultra-high strength hot-rolled steel sheet having high strength and excellent bending workability.

The present invention also provides a method for producing an ultra-high strength hot-rolled steel sheet having high strength and excellent bending workability.

The present invention relates to a steel sheet comprising 0.1 to 0.25 weight% of C, 0.01 to 0.2 weight% of Si, 0.5 to 2.0 weight% of Mn, 0.005 to 0.02 weight% of P and 0.001 to 0.01 weight% of S, , Nb, Mo, Cr and B in an amount of 0.001 to 0.35% by weight, the balance comprising iron (Fe) and other unavoidable impurities, and has a bending workability satisfying the following relational expression 1 Is achieved by an excellent ultra high strength hot-rolled steel sheet.

[Relation 1]

[Cr] -2.6 [Ni] -6.6 [Ti] -660.6 [B] -39 [P]? 0

(Wherein, [C], [Si], [Mn], [Cr], [Ni], [Ti], [B] and [P]

Preferably, the ultrahigh-strength hot-rolled steel sheet has a tensile strength of 1 GPa or more and a tensile strength x elongation (TSxT-EL) of 10,000 or more.

Preferably, the bending workability (R / t) of the ultrahigh-strength hot-rolled steel sheet satisfies the following relational expression (2).

[Relation 2]

Bending workability (R / t)? (Tensile strength x 0.00517 - 2.60345)

(Where R is the minimum bending radius at which cracks do not occur after the 90 ° bend test, and t is the thickness of the steel sheet)

Preferably, the microstructure of the ultra-high strength hot-rolled steel sheet has an area fraction of 95% or more of martensite and less than 5% of the second phase.

The present invention also relates to a steel sheet comprising 0.1-0.25 wt% of C, 0.01-0.2 wt% of Si, 0.5-2.0 wt% of Mn, 0.005-0.02 wt% of P and 0.001-0.01 wt% of S, A slab containing 0.001-0.35 wt% of at least one component selected from the group consisting of Ti, Nb, Mo, Cr, and B and containing the balance iron (Fe) and other unavoidable impurities, ; Reheating the slab at a temperature of 1100 to 1300 占 폚; Hot-rolling the reheated slab to a finish rolling temperature of 850 to 1000 占 폚 to obtain a hot-rolled steel sheet; Cooling the hot-rolled steel sheet at a cooling rate of 100 to 300 DEG C / s and satisfying the following expression (3); And a step of winding the cooled steel sheet at a coiling temperature of 350 DEG C or less. The method for manufacturing an ultra-high strength hot-rolled steel sheet having excellent bending workability.

[Relation 1]

[Cr] -2.6 [Ni] -6.6 [Ti] -660.6 [B] -39 [P]? 0

(Wherein, [C], [Si], [Mn], [Cr], [Ni], [Ti], [B] and [P]

[Relation 3]

[Cooling Rate] > 0 < / RTI > < RTI ID = 0.0 >

(C), [Si], [Mn], [Cr], [Ni], [Ti], [B] and [P] Lt; / RTI > and the cooling rate from the finish rolling temperature to the coiling temperature)

Preferably, the method further comprises a step of reheating the rolled hot-rolled steel sheet at a temperature of 450 to 480 캜 after the pickling treatment to form a zinc-plated layer on the surface by performing hot-dip galvanizing.

It is possible to provide an ultrahigh-strength hot-rolled steel sheet excellent in strength and excellent in bending workability by being manufactured according to the component range, relation and manufacturing conditions according to the present invention.

Fig. 1 is a graph showing the values derived from the relational expression 1 showing the TSxT-EL and the bending workability in the inventive and comparative examples.

The present invention relates to an ultra-high strength hot-rolled steel sheet excellent in lightweighting effect due to high strength due to high strength and excellent in bending workability and easy to ensure component shape crystallinity through roll forming, and a method for manufacturing the same.

The present inventors have derived a relational expression indicating bending workability through measurement of bending test values of steels having various components. Based on this relational expression, the present inventors have found that a bending workability (tensile strength x elongation (TSxT-EL) It is possible to provide an excellent super high strength hot-rolled steel sheet.

Hereinafter, the ultrahigh-strength hot-rolled steel sheet excellent in bending workability of the present invention will be described in detail.

The composition range of C: 0.1 to 0.25% by weight, Si: 0.01 to 0.2% by weight, Mn: 0.5 to 2.0% by weight, P: 0.005 to 0.02% by weight, and S: 0.001 to 0.01 wt%, balance iron (Fe) and other unavoidable impurities, and 0.001 to 0.35 wt% of at least one component selected from the group consisting of Ti, Nb, Mo, Cr and B.

Hereinafter, reasons for limiting the range of the alloy component of the present invention will be described.

Carbon (C): 0.1 to 0.25 wt%

C is the most economical and effective element to fortify the river. When the carbon content is less than 0.1% by weight, it is difficult to secure a desired strength. On the other hand, when the carbon content exceeds 0.25% by weight, the bending workability is deteriorated due to an excessive increase in strength. Therefore, it is preferable that the carbon content is 0.1 to 0.25 wt%.

Silicon (Si): 0.01 to 0.2 wt%

Si deoxidizes molten steel and has solid solution strengthening effect. When the content of silicon is less than 0.01% by weight, the deoxidation effect and the strength improving effect are insufficient. On the other hand, when the content of silicon exceeds 0.2% by weight, a red color scale due to Si is formed on the surface of the steel sheet during hot rolling, thereby deteriorating the surface quality of the steel sheet and deteriorating the weldability. Therefore, the content of the silicon is preferably 0.01 to 0.2% by weight.

Manganese (Mn): 0.5 to 2.0 wt%

Mn, like Si, is an effective element for strengthening the steel. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.5 wt% or more. However, when the content of manganese exceeds 2.0% by weight, the segregation portion is significantly developed at the center of the thickness of the slab during the casting process, thereby deteriorating the weldability and formability of the final product. Therefore, the content of Mn is preferably 0.5 to 2.0% by weight.

Phosphorus (P): 0.005 to 0.02 wt%

P, like Si, has the effect of strengthening the solution and promoting ferrite transformation. When the content of phosphorus is less than 0.005% by weight, the strength of the present invention is insufficient to obtain the desired strength. On the other hand, when the content of phosphorus exceeds 0.02% by weight, the bending workability is lowered due to the band structure due to micro segregation. Accordingly, it is preferable that P is contained in an amount of 0.005 to 0.02% by weight.

Sulfur (S): 0.001 to 0.01%

The sulfur is inevitably contained as an impurity, which is combined with Mn or the like to form a nonmetallic inclusion, thereby significantly reducing the toughness of the steel. Therefore, the content of the sulfur is preferably suppressed as much as possible. The theoretical sulfur content is advantageous to be limited to 0 wt%, but it is inevitably contained in the manufacturing process inevitably. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is preferably limited to 0.01 wt%.

0.001 to 0.35% by weight of at least one element selected from the group consisting of titanium (Ti), niobium (Nb), molybdenum (Mo), chrome (Cr) and boron (B) . By the addition of the above elements, high tensile strength and excellent bending workability can be obtained, and the effect of the present invention can be further improved.

Ti exists as TiN in the steel and has an effect of inhibiting the growth of the crystal grains in the heating process for hot rolling. In addition, Ti, which reacts with nitrogen, is a useful component for improving the strength of steel by solid solution strengthening among steels.

Nb forms a niobium-based precipitate such as Nb (CN) as a precipitate-forming element. When it is solidified in a heating furnace at about 1200 DEG C, fine precipitates are formed in hot rolling, effectively increasing the strength of the steel.

Mo is a useful component for enhancing yield strength through strengthening of solid solution and strengthening impact toughness and bending workability by strengthening grain boundaries.

Cr enhances the solidification of the steel and serves to retard martensite formation by delaying the bainite phase transformation during cooling.

B is also contained as a substitute element of Si, and it improves the hardenability by strengthening the grain boundary and the strength by an extremely small amount.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

The super high strength steel excellent in bending workability of the present invention can be obtained by satisfying the following relational expression 1 obtained by evaluating the bending workability in various component systems while satisfying the alloy composition range as described above.

[Relation 1]

[Cooling Rate] > 0 < / RTI >< RTI ID = 0.0 >

Here, [C], [Si], [Mn], [Cr], [Ni], [Ti], [B] and [P] refer to weight percent of each component content.

The above relational expression 1 is a relational expression obtained from values obtained by measuring the bending workability of steels having various components. By satisfying the above-mentioned relational expression 1, sufficient martensite microstructure can be secured.

In the super high strength steel having a tensile strength of 1 Gpa or more, when the value of the relational expression 1 is less than 0, the room temperature R / t of the relational expression 2 exceeds (tensile strength x0.00517-2.60345), and the bending workability becomes poor.

[Relation 2]

Bending workability (R / t)? (Tensile strength x 0.00517 - 2.60345)

Preferably, by satisfying the above-mentioned relational expression (2), it is possible to form a smooth part of an ultra-high strength steel having a tensile strength of 1Gpa or more. That is, as the value of the bending workability (R / t) is smaller, it is possible to form a smooth part, and if the value is less than the above value (tensile strength x 0.00517 - 2.60345), the part can be formed through smooth roll forming.

The hot-rolled steel sheet provided by the present invention satisfies the above-described composition conditions and has a microstructure containing ferrite of 95% or more by area and at least one member selected from the group consisting of carbides such as bainite, martensite and cementite 2 phase is preferably 5% or less, and it is possible to secure sufficient ductility by securing the microstructure as described above. When the fraction of the second phase is more than 5%, bainite and coarse carbonitride are formed around the ferrite grain boundaries, so that desired strength can not be obtained, or there may be a difference in hardness between phases, so that it may be difficult to secure the bending workability .

The tensile strength of the ultrahigh-strength hot-rolled steel sheet of the present invention is preferably 1 GPa or more. If the tensile strength is less than 1 GPa, there is a limit to the thinness due to the lack of strength, and the effect of lighter parts is poor.

The ultra-high strength hot-rolled steel sheet of the present invention preferably has a tensile strength x elongation (TSxT-EL) of 10000 or more. When the value is less than 10,000, moldability or shape dynamism is poor to be.

Hereinafter, a method for producing an ultra-high strength hot-rolled steel sheet excellent in bending workability of the present invention will be described in detail.

As described above, in order to produce the ultra-high strength hot-rolled steel sheet of the present invention having excellent strength and excellent bending workability, first, a slab having a composition satisfying the relational expression 1 and the alloy component range of the present invention is prepared. After the slab was heated at a temperature of 1100 to 1300 ° C, the slab was hot-rolled at a finishing rolling temperature of 850 to 1000 ° C, cooled, cooled at 350 ° C or lower, High-strength hot-rolled steel sheet excellent in workability is completed.

Hereinafter, detailed conditions for each step will be described.

Slab reheating temperature: 1100 ~ 1300 ℃

The reheating temperature of the slab of the present invention is desirably 1100 DEG C or higher, and the rolling load is reduced by securing the temperature of the slab plate. However, if the reheating is carried out at an excessively high temperature, the austenite may be coarsened. Therefore, the reheating temperature is preferably 1300 ° C or lower.

Rolling finish temperature: 850 to 1000 ° C

As described above, the hot-rolled slab can be subjected to hot rolling. At this time, the finish rolling is preferably performed at 850 to 1000 캜. If the hot finish rolling temperature is lower than 850 占 폚, the rolling load greatly increases. On the other hand, when the hot finish rolling temperature exceeds 1000 캜, the structure of the steel sheet becomes coarse, the steel becomes weak, the scale becomes thick, and the surface quality deteriorates such as a high-temperature rolling-ability scale defect. Therefore, the hot finish rolling is preferably limited to 850 to 1000 ° C.

Cooling speed: 100 to 300 ° C / s

It is preferable to cool the hot-rolled steel sheet as described above. Further, it is preferable that the steel sheet is cooled at a cooling rate of 100 to 300 DEG C / s from the finish hot rolling temperature of the hot-rolled steel sheet until the cooling end temperature is reached, and then wound. When the cooling rate is less than 100 ° C / s, the fraction of the second phase other than martensite exceeds 5%, making it difficult to secure the strength to be secured by the present invention. On the other hand, if it exceeds 300 DEG C / s, there is a problem that elongation and toughness are lowered.

The cooling of the hot-rolled steel sheet is carried out in a cooling rate range obtained by the following formula (3).

[Relation 3]

[Cooling Rate] > 0 < / RTI >< RTI ID = 0.0 >

Here, [C], [Si], [Mn], [Cr], [Ni], [Ti], [B] and [P] Is the cooling rate in ° C / s from the finish rolling temperature to the coiling temperature.

The above-mentioned relational expression (3) can be obtained by adding the factor of the cooling rate capable of securing sufficient martensite to the relational expression 1 obtained from the values obtained by measuring the bending workability of the steels having various components, And can be applied to a manufacturing method.

Coiling temperature : 350 ° C or less

It is preferable that the hot rolled steel sheet is cooled and cooled at a cooling rate of 100 to 300 DEG C / s until the temperature reaches 350 DEG C or lower from the finish hot rolling temperature of the hot rolled steel sheet. When the temperature at which the cooling is terminated exceeds 350 DEG C, the microstructure of the steel mostly contains bainite, so that the microstructure to be secured by the present invention can not be secured. The coiling temperature is a temperature at which cooling is terminated. If the coiling temperature is 350 DEG C or lower, cooling at any temperature may be terminated and the coiling may be performed. However, in order to cool the temperature at which the cooling is terminated to 20 ° C or lower, which is the normal temperature, a separate apparatus is required. Therefore, it is preferable to terminate the cooling at a temperature of 20 ° C and wind up.

The picked hot-rolled steel sheet may be naturally cooled at room temperature, pickled, removed from the surface layer scale, and further roughened to produce a pickled steel sheet.

After the winding or pickling, the steel sheet is reheated at 450 to 480 캜 and hot-dip galvanized to produce a hot-dip galvanized steel sheet. If the reheating temperature is 450 ° C, the plating adhesion is lowered and the hot dip galvanizing may not be performed. If the reheating temperature is higher than 480 ° C, the precipitate is coarsened due to the heat treatment effect, There is a risk of drop in strength, and there may also arise problems of environmental problems due to vaporization of molten zinc and deterioration of plating quality.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

[ Example ]

The steel slabs satisfying the component systems described in Table 1 were heated to 1150 占 폚 and subjected to hot rolling at the temperature (FDT) shown in Table 2 below. Thereafter, cooling was carried out at a cooling rate of 214 占 폚 / s up to the coiling temperature (CT) shown in Table 2, and then rewound at the temperature (CT) shown in Table 2 below.

Examples 1 to 6 of the following Table 1 show the composition of the slab satisfying the composition range of the present invention, and Comparative Examples 1 to 9 show the composition of the slab having the component composition out of the range of the present invention in terms of weight%. Further, a material test was conducted on the hot-rolled steel sheet produced as described above, and the results are shown in Table 2 below.

division C Mn Si P S Cr Ti Nb B Comparative Example 1 0.14 1.6 0.1 0.015 0.003 0.5 0.015 0.015 0.002 Comparative Example 2 0.12 1.7 0.1 0.015 0.003 0 0.015 0.015 0.002 Comparative Example 3 0.15 1.5 0.1 0.015 0.003 0 0.015 0.015 0.002 Comparative Example 4 0.18 One 0.1 0.015 0.003 0 0.015 0 0.002 Comparative Example 5 0.19 1.2 0.1 0.015 0.003 0 0.015 0 0.002 Comparative Example 6 0.2 1.2 0.1 0.015 0.003 0.2 0.015 0 0.002 Comparative Example 7 0.19 1.4 0.1 0.015 0.003 0.5 0.015 0 0.002 Comparative Example 8 0.2 1.2 0.1 0.015 0.003 0.4 0.015 0 0.002 Comparative Example 9 0.21 1.1 0.1 0.015 0.003 0.3 0.015 0 0.002 Inventory 1 0.13 1.4 0.1 0.015 0.003 0 0.015 0.015 0.002 Inventory 2 0.15 One 0.1 0.015 0.003 0 0.015 0.015 0.002 Inventory 3 0.14 1.4 0.1 0.015 0.003 0 0.015 0.015 0.002 Honorable 4 0.19 1.2 0.1 0.015 0.003 0.2 0.015 0 0.002 Inventory 5 0.19 One 0.1 0.015 0.003 0.2 0.015 0 0.002 Inventory 6 0.2 0.7 0.1 0.015 0.003 0.2 0.015 0 0.002

(Unit: wt%)

In the following Table 2, FDT and CT mean the hot rolling finishing temperature and the coiling temperature, respectively. YS, TS, T-El and TSxT-EL mean yield strength, tensile strength, elongation, tensile strength and elongation. In addition, YS means 0.2% off-set yield strength or lower yield point, and the yield ratio is the ratio of yield strength to tensile strength. The tensile test was carried out on specimens taken in accordance with JIS No. 5 standard with respect to the rolling direction of the rolled sheet material in the direction of 90 °.

R / t (actual) in Table 2 is the value obtained by taking the specimen with reference to the direction of 90 ° with respect to the rolling direction of the rolled plate, and dividing the minimum bending radius R without cracking after 90 ° bending test by the thickness t of the material , And R / t (limit) represents the value calculated by (tensile strength x 0.00517-2.60345). If R / t (actual) exceeds R / t (limit), the bending workability is evaluated as heat and expressed as X, otherwise it is indicated as O.

Psalter FDT CT Relation 3 Martensite area fraction (%) YS TS T-EL TSxT-EL R / t
(Actual) R / t
(Limit) Bending workability Comparative Example 1 947 172 14.5 96 1195 1343 6.0 8058 3.5 4.3 o Comparative Example 2 949 170 23.0 99 968 1257 7.0 8799 3.3 3.9 o Comparative Example 3 968 402 14.5 53 730 784 11.4 8938 1.3 1.4 o Comparative Example 4 952 367 7.1 64 877 964 9.0 8676 2.3 2.4 o Comparative Example 5 954 378 3.2 78 916 985 6.2 6107 2.2 2.5 o Comparative Example 6 955 163 -1.0 96 1264 1542 6.6 10174 5.8 5.4 x Comparative Example 7 961 174 -0.2 97 1215 1519 6.6 10027 5.6 5.2 x Comparative Example 8 958 198 -2.0 98 1207 1548 6.5 10061 6.0 5.4 x Comparative Example 9 965 155 -4.2 99 1232 1579 6.4 10103 6.3 5.6 x Inventory 1 971 188 21.1 96 1017 1287 8.6 11065 3.3 4.1 o Inventory 2 849 195 16.5 97 1094 1350 7.6 10261 3.5 4.4 o Inventory 3 905 222 18.0 97 1032 1323 7.9 10450 3.4 4.2 o Honorable 4 909 204 2.2 96 1204 1505 7.1 10688 4.1 5.2 o Inventory 5 912 221 3.0 96 1186 1501 6.7 10057 4.1 5.2 o Inventory 6 915 212 1.0 98 1225 1531 6.7 10255 4.3 5.3 o

In Comparative Examples 1 and 2, the bending workability was good, but the TSxT-EL value deviated from the range of the present invention due to the elongation rate due to the Mn segregation band.

In Comparative Examples 3, 4 and 5, the CT temperature was out of the range of the present invention, and the tensile strength was less than 1 Gpa because the bainite structure was not a martensite structure of 95% or more.

Comparative Examples 6, 7, 8 and 9 did not satisfy the relational expression 3, and the bending workability evaluation result was also inferior.

FIG. 1 is a graph showing values derived from the TSxT-EL and the relational expression 1 or 3 in the comparative example and the inventive example. When the cooling rate is set to 214 占 폚 / s as in the above embodiment, the values of relational expression 1 and relational expression 3 are the same. The portions indicated by the square dots are comparative examples, and the portions indicated by the circular dots are examples. It can be seen that all of the circular points corresponding to the inventive aspects of the present invention are located in the gray portion.

All the inventive examples 1 to 6 satisfied all of the criteria of bending workability and confirmed that not only tensile strength and elongation but also yield strength were excellent.

While the illustrative embodiments of the present invention have been described with reference to the drawings, various modifications and alternative embodiments may be made by those skilled in the art. Such variations and other embodiments will be considered and included in the appended claims, all without departing from the true spirit and scope of the invention.

Claims (6)

Wherein the steel sheet contains 0.1 to 0.25 weight% of C, 0.01 to 0.2 weight% of Si, 0.5 to 2.0 weight% of Mn, 0.005 to 0.02 weight% of P and 0.001 to 0.01 weight% of S, , Cr and B in an amount of 0.001 to 0.35% by weight, the balance comprising iron (Fe) and other unavoidable impurities, and satisfying the following relational expression (1) and having excellent bending workability, Steel plate.
[Relation 1]
[Cr] -2.6 [Ni] -6.6 [Ti] -660.6 [B] -39 [P]? 0
(Wherein, [C], [Si], [Mn], [Cr], [Ni], [Ti], [B] and [P]
The ultrahigh-strength hot-rolled steel sheet according to claim 1, wherein the ultrahigh-strength hot-rolled steel sheet has a tensile strength of 1 GPa or more and a tensile strength x elongation (TSxT-EL) of 10000 or more.
The ultra-high strength hot-rolled steel sheet according to claim 1, wherein the bending workability (R / t) of the super high strength hot-rolled steel sheet satisfies the following relational expression (2).
[Relation 2]
Bending workability (R / t)? (Tensile strength x 0.00517 - 2.60345)
(Where R is the minimum bending radius at which cracks do not occur after the 90 ° bend test, and t is the thickness of the steel sheet)
The ultra-high strength hot-rolled steel sheet according to claim 1, wherein the microstructure of the ultra-high strength hot-rolled steel sheet has an area fraction of martensite of 95% or more and a second phase of less than 5%.
Wherein the steel sheet contains 0.1 to 0.25 weight% of C, 0.01 to 0.2 weight% of Si, 0.5 to 2.0 weight% of Mn, 0.005 to 0.02 weight% of P and 0.001 to 0.01 weight% of S, , Cr and B in an amount of 0.001 to 0.35% by weight, the balance comprising iron (Fe) and other unavoidable impurities, and satisfying the following relational expression (1):
Reheating the slab at a temperature of 1100 to 1300 占 폚;
Hot-rolling the reheated slab to a finish rolling temperature of 850 to 1000 占 폚 to obtain a hot-rolled steel sheet;
Cooling the hot-rolled steel sheet at a cooling rate of 100 to 300 DEG C / s and satisfying the following formula 3; And
And winding the cooled steel sheet at a coiling temperature of 350 占 폚 or less to obtain a super high strength hot rolled steel sheet having excellent bendability.
[Relation 1]
[Cr] -2.6 [Ni] -6.6 [Ti] -660.6 [B] -39 [P]? 0
(Wherein, [C], [Si], [Mn], [Cr], [Ni], [Ti], [B] and [P]
[Relation 3]
[Cooling Rate] > 0 < / RTI >< RTI ID = 0.0 >
(C), [Si], [Mn], [Cr], [Ni], [Ti], [B] and [P] Lt; / RTI > and the cooling rate from the finish rolling temperature to the coiling temperature)
The method of manufacturing a hot-dip galvanized steel sheet according to claim 5, further comprising a step of reheating the rolled hot-rolled steel sheet at a temperature of 450 to 480 캜 after the pickling treatment to form a zinc plated layer on the surface by hot- Method of manufacturing high strength hot - rolled steel sheet.

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