JP2000282175A - Ultra-high strength hot rolled steel sheet excellent in workability and method for producing the same - Google Patents

Abstract

(57) [Summary] (Modified) [Problem] While achieving a tensile strength of 980 MPa or more, both stretch flangeability and strength-ductility balance are excellent.
An ultra-high-strength hot-rolled steel sheet having a low yield ratio and excellent workability and a method for producing the same are proposed. SOLUTION: C, Si, Mn, P, S, Nb, Ti, Cr, Al,
Ni, etc., the remainder is made of Fe.After continuous casting of this steel slab, immediately or once cooled and heated to 1100-1300 ° C, then hot-rolled at the finish rolling end temperature 950-800 ° C. Start cooling within 0.5 seconds after the end of rolling, and
By cooling at a cooling rate of at least 500 g / sec and winding at 500 to 300 ° C., bainite having a volume fraction of less than 60 to 90% is converted into at least one of the main phase, pearlite, ferrite, retained austenite, and martensite. The seed is used as the second phase and the bainite phase has a metal structure with an average particle size of less than 4 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet suitable for use in automobile parts, for example, door impact beams, bumpers, etc., used for the purpose of improving the collision safety of automobiles. The invention relates to an ultra-high strength hot-rolled steel sheet having a workability of not less than 980 MPa and excellent in workability and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, in the field of automobile manufacturing, parts such as bumpers and impact beams have been provided with high strength in order to reduce the weight of a vehicle body and to suppress deformation of a cabin during a collision to improve safety. Steel sheets have been used, and the strength of the steel sheets has been increasing.
These components are manufactured through various forming processes such as press forming, roll forming, hole expanding, and bending. Therefore, the high-strength steel plate requires a material having such workability.

[0003] By the way, the steel strengthening mechanism has conventionally been
Process strengthening, structure strengthening, precipitation strengthening and the like are widely known.
However, if these strengthening mechanisms are used, the strength of the steel sheet can be secured, but on the other hand, the workability is reduced. Specifically, in the conventional high-strength steel sheet, due to the structural nonuniformity, the local mixture of the hard phase and the soft phase, etc., there are many places where cracks start during the hole expansion test. This led to a decrease in hole expandability. Moreover, such workability generally decreases greatly as the strength of the steel increases. Further, even if the structure strengthening which is said to be most advantageous for the low yield ratio is used, if the strength is 980 MPa or more, a sufficiently low yield cannot be obtained, and there is a problem in shape freezing property and the like. For this reason, in the conventional steel plate manufacturing technology, ductility,
In fact, it was necessary to sacrifice processing characteristics such as bending property, stretch flangeability, and low yield ratio. In order to ensure workability, it is conceivable to optimize the structure by heat treatment. However, for this purpose, an extra tempering process such as a tempering process must be added, which inevitably results in cost reduction. However, there is a problem that the yield ratio may be too low depending on the tempering treatment or the like, which is not preferable for securing the strength.

[0004] As a technique for high-strength steel sheets, for example, Japanese Patent Application Laid-Open
JP-A-5-105986, JP-A-60-181231, JP-A-50-15
Japanese Patent Application Laid-Open No. 06622 discloses a steel sheet having a good strength-ductility balance and excellent hole expandability, but the tensile strength in each of the disclosed techniques is at a level less than 980 MPa. Further, JP-A-3-277742 and JP-A-4-236741 disclose a steel sheet having a tensile strength of 980 MPa or more, but the hole expansion rate is low, and it is said that the steel sheet has sufficient workability. It's hard to say.

[0005]

As described above, strength and workability generally show a tendency to contradict each other. At present, good workability such as stretch flangeability is provided, and tensile strength is high. There is no known ultra-high tensile steel having a 980 MPa or higher. In addition, when ultra-high strength steel sheet having such strength is formed into a product of a predetermined shape by roll forming,
There was also a problem that molding defects such as edge waves and wrinkles occurred. The present invention proposes a novel hot-rolled steel sheet to solve such problems of the prior art.
To provide an ultra-high-strength hot-rolled steel sheet excellent in workability that achieves a tensile strength of Pa or more, has excellent balance of stretch flangeability and strength-ductility, and also has a low yield ratio, and a method of manufacturing the same. It is in. The target characteristic aimed at by the present invention is TS × λ as an index indicating stretch flangeability.
(Hole expansion ratio) ≧ 60,000 MPa ·%, TS × El ≧ 16000 MPa ·% as an index of strength-ductility balance, and yield ratio YR: 60-70%. Furthermore, in addition to the above-mentioned respective material properties, it is also an object to provide a property that does not cause forming defects such as edge waves and wrinkles when a thin steel sheet is formed into a product having a predetermined shape by roll forming. Furthermore,
One of the objects of the present invention is to prevent the occurrence of delayed fracture after molding.

[0006]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted intensive experiments from the viewpoints of steel composition, production conditions, metal structure and the like, and have repeated studies. As a result, by controlling the components and manufacturing conditions within the appropriate ranges, the structure is refined, and the bainite-based structure with controlled volume fraction reduces the origin of cracking during processing and reduces the strength level. Without lowering, it is possible to provide an excellent stretch flangeability and a high strength-ductility balance, which have not been achieved before, and it is possible to satisfy the yield ratio YR: 60% to 70%. It has been found that it is also possible to prevent forming defects such as edge waves and wrinkles in roll forming.
At the same time, it has been found that by employing such means, delayed fracture or the like does not occur, and the reliability of the processed part can be improved.

The present invention has been completed based on such findings, and the gist thereof is as follows. (1) C: 0.05 to 0.20 wt%, Si: 0.05 to 0.50 wt%, Mn:
1.0 to 3.5 wt%, P: 0.05 wt% or less, S: 0.01 wt% or less, Nb: 0.005 to 0.30 wt%, Ti: 0.001 to 0.100 wt%,
Cr: 0.01 to 1.0 wt% and Al: 0.1 wt% or less,
The balance is composed of Fe and unavoidable impurities, and the content of each of Si, P, Cr, Ti, Nb, and Mn is represented by the following formula: 0.05 ≦ ((wt%) Si + (wt%) P) / ( (wt%) Cr + (wt
%) Ti + (wt%) Nb + (wt%) Mn) ≦ 0.5, and the metal structure has a volume fraction of 60 to
A structure in which less than 90% of bainite is the main phase and at least one of pearlite, ferrite, retained austenite, and martensite is the second phase, and the average grain size of the bainite phase is less than 4 μm. Ultra-high strength hot rolled steel sheet with excellent workability.

(2) In the steel sheet according to (1), in addition to the above components, Group A: Cu: 0.01 to 1.0 wt%, Ni: 0.01 to 1.0 wt%, M
o: 0.01 to 1.0 wt%, V: 0.01 to 0.3 wt%, Zr: 0.01 to
0.3 wt% and B: 0.0001-0.0050 wt% Group B; Ca: 0.0001-0.0050 wt% and REM: 0.0001-
An ultra-high-strength hot-rolled steel sheet excellent in workability, characterized by containing 0.0050 wt% of one or more selected from one or two groups.

(3) C: 0.05 to 0.20 wt%, Si: 0.05 to 0.5
0 wt%, Mn: 1.0 to 3.5 wt%, P: 0.05 wt% or less, S:
0.01 wt% or less, Nb: 0.005 to 0.30 wt%, Ti: 0.001 to 0.
100 wt%, Cr: 0.01 to 1.0 wt% and Al: 0.1 wt% or less, the balance being composed of Fe and unavoidable impurities, and containing each of the above Si, P, Cr, Ti, Nb and Mn The quantity is given by the following formula: 0.05 ≦ ((wt%) Si + (wt%) P) / ((wt%) Cr + (wt
%) Ti + (wt%) Nb + (wt%) Mn) ≦ 0.5 The steel slab containing the steel satisfying the relationship of 0.5 is continuously or immediately cooled or temporarily cooled, and then heated to 1100 to 1300 ° C., and then subjected to finish rolling. It is characterized by hot rolling at an end temperature of 950 to 800 ° C, cooling within 0.5 seconds after the end of rolling, cooling at a cooling rate of 30 ° C / sec or more, and winding at 500 to 300 ° C. To produce ultra-high strength hot rolled steel sheet with excellent workability.

(4) In the method described in (3) above, in addition to the above-mentioned components, Group A: Cu: 0.01 to 1.0 wt%, Ni: 0.01 to 1.0 wt%, M
o: 0.01 to 1.0 wt%, V: 0.01 to 0.3 wt%, Zr: 0.01 to
0.3 wt% and B: 0.0001-0.0050 wt% Group B; Ca: 0.0001-0.0050 wt% and REM: 0.0001-
A method for producing an ultra-high-strength hot-rolled steel sheet excellent in workability, characterized by containing 0.0050 wt% of one or more selected from any one or two groups.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a description will be given of the reason why the above-mentioned configuration is limited. C: 0.05 to 0.20 wt% C is an indispensable element for strengthening steel by utilizing a low-temperature transformation phase. To obtain tensile strength of 980MPa or more
Although the content of 0.05 wt% or more is necessary, if the content exceeds 0.2 wt%, the weldability deteriorates.
wt% range.

Si: 0.05-0.50 wt% Si is an element that contributes to strength improvement, and its effect is 0.05%.
It is not exhibited at less than wt%. On the other hand, when the content exceeds 0.50 wt%, the ferrite transformation is promoted, the strengthening by the low-temperature transformation phase becomes insufficient, and the yield ratio tends to increase even if the strength is increased. Therefore, the amount of Si is contained in the range of 0.05 to 0.50 wt%.

Mn: 1.0 to 3.5 wt% Mn is an element that plays an important role in obtaining a bainite structure. In the cooling process after the end of hot rolling, the amount of Mn of 1.0 wt% or more is required to suppress the formation of other transformed phases and to stably form a bainite-based structure.
%, The effect is saturated. Therefore, the amount of Mn is added in the range of 1.0 to 3.5 wt%.

P: 0.05 wt% or less P is an element that contributes to improvement in strength and has an effect of preventing hydrogen embrittlement and delayed fracture caused by hydrogen accumulated in steel. However, if it is contained excessively, the structure becomes non-uniform, solidification segregation during casting is remarkable, and internal cracks and workability are deteriorated.

S: 0.01 wt% or less Since S exists as nonmetallic inclusions in steel and becomes a source of stress concentration during stretch flange forming, its content is desired to be low. When the S content is 0.01 wt% or less, even if the strength is very high, it does not significantly affect the hole expandability, so the upper limit may be 0.01 wt%.

Nb: 0.005 to 0.30 wt% Nb is an element which affects the form of precipitates such as NbC and the recrystallization temperature. In particular, in the present invention, Nb effectively acts on fine uniformity of the structure, suppresses the formation of soft ferrite and pearlite phase that cause a decrease in yield strength, and the structure mainly composed of bainite, which is a low-temperature transformation phase. By doing so, it has the effect of providing high elongation and hole expandability despite high strength. Such an effect is 0.005
It appears when it is added in an amount of not less than wt%, but when it is added more than 0.30 wt%, a large amount of hard precipitates are formed in the steel, and the stretch flangeability is reduced. Therefore, the Nb content is in the range of 0.005 to 0.30 wt%. In addition, it is preferably 0.01 to 0.1 wt%.

Ti: 0.001 to 0.100 wt% Ti, like Nb, affects the form of the precipitates and the recrystallization temperature, brings about a fine and uniform structure, yield strength, elongation,
It is an element effective for improving each property of hole expansion. These favorable effects can be obtained by adding 0.001 wt% or more. Also, when this Ti is added in combination with the above-mentioned Nb, the minimum cooling rate at which ferrite transformation does not occur is reduced, and the effect of improving hardenability is brought about.
On the other hand, if more than 0.100 wt% of Ti is contained, a hard carbide or the like is formed, and the stretch flangeability is reduced.
The amount is 0.001 to 0.100 wt%. Incidentally, preferably 0.0
05 to 0.05 wt%.

Cr: 0.01 to 1.0 wt% Cr is an element effective for improving the strength without greatly reducing the elongation or greatly increasing YS, but the effect is less than 0.01 wt% when Cr is less than 0.01 wt%. Less, also 1.0
There is no further effect even if it is contained in a large amount exceeding wt%,
Economically disadvantaged. Therefore, the Cr content is set to 0.01 to 1.0 wt%. Incidentally, the content is preferably 0.05 to 0.5 wt%.

Al: 0.1 wt% or less Al is an effective element for improving the yield of deoxidizing and carbide forming elements. However, even if added in excess of 0.1 wt%, not only does the effect become saturated, Since the surface properties deteriorate as well as the workability, the content should be 0.1 wt% or less.

Cu: 0.01-1.0 wt%, Ni: 0.01-1.0 wt%
%, Mo: 0.01 to 1.0 wt%, V: 0.01 to 0.3 wt%, Zr: 0.
01 to 0.3 wt% and B: 0.0001 to 0.0050 wt% These elements are effective elements for increasing the strength of the steel sheet, and the effect can be obtained at 0.01 wt% or more with any of the elements. V and Zr are elements that improve local elongation in addition to the above-mentioned effects. However, among these elements, adding more than 1.0 wt% for Cu, Ni, and Mo, and adding more than 0.3 wt% for V and Zr, respectively, does not provide any further effect and increases costs. Will be invited. Therefore, each element of Cu, Ni, and Mo is 0.01-1.0 wt%, V
And Zr are added in the range of 0.01 to 0.3 wt%. It should be noted that any of these elements shows the same behavior when used alone or in combination. B is also an element effective for improving the strength. By adding B, the formation of a soft phase can be suppressed and a bainite phase can be easily formed during rapid cooling after the end of hot rolling. In order to obtain such an effect, it is necessary to add 0.0001% by weight or more. However, even if it exceeds 0.0050% by weight, no further effect can be obtained, so 0.0050% by weight is added as an upper limit.

Ca: 0.0001 to 0.0050 wt%, REM: 0.0001 to 0.0050 wt% Ca and REM are formed by spheroidizing precipitates such as sulfides, reducing sharp precipitates, and reducing stress concentration. And has an effect of suppressing a decrease in stretch flangeability. If the addition amount of each of these elements is less than 0.0001 wt%, the effect of addition is not exhibited, while if the addition exceeds 0.0050 wt%, the effect is saturated. Therefore, both Ca and REM are added in the range of 0.0001 to 0.0050 wt%.

0.05 ≦ ((wt%) Si + (wt%) P) / ((wt
%) Cr + (wt%) Ti + (wt%) Nb + (wt%) Mn) ≦ 0.5 In order to obtain a desired YR, it is necessary to control the structure described later. If a large amount of ferrite formation promoting elements such as Si and P are contained, the amount of ferrite phase, pearlite phase, etc. generated during continuous cooling and rolling after hot rolling increases, and the volume fraction of bainite phase decreases, This leads to a decrease in elongation and a decrease in YR. On the other hand, carbide-forming elements such as Cr, Ti, and Nb that suppress the production of pearlite also play an important role, and when the content of these elements is small, the production of pearlite is promoted, and the volume fraction of the bainite phase is also reduced. It causes a decrease in elongation and a decrease in YR. In addition, Mn is an austenite stabilizing element, which suppresses the formation of ferrite during continuous cooling and promotes the formation of a bainite phase. On the other hand, when it is excessive, the volume fraction of the bainite phase increases and the YR is increased. Has an action. Investigations have been made in consideration of the effects of each of these elements. As a result, to obtain the target YR, ((wt%) S
i + (wt%) P) / ((wt%) Cr + (wt%) Ti + (wt%) N
It is necessary to control the value of (b + (wt%) Mn) in the range of 0.05 to 0.5. That is, by adjusting the component balance to such a range and rolling under appropriate conditions, at least one of bainite (main phase) having a volume fraction of less than 60 to 90%, pearlite, ferrite, and retained austenite is obtained. It becomes possible to have a second phase of tissue.

Metal structure: If the volume fraction of the bainite phase is too large, YS tends to be high. In order to satisfy properties such as strength and YR, the bainite phase volume fraction needs to be 60 to less than 90%. This is because if the bainite phase volume fraction is less than 60%, the local deformability and the hole expandability deteriorate, whereas if this fraction exceeds 90%, the YR increases and the press formability decreases, and the rollability decreases. This is because poor molding (generation of edge waves and wrinkles) is caused in molding. It is also necessary to make the average particle size of the bainite phase less than 4 μm in order to satisfy the target material properties. When the crystal grain size is reduced and the structure is made uniform, it is possible to obtain a superior strength-to-hole expansion ratio balance which has not been achieved in the past. The average grain size of the bainite structure was calculated according to the method for determining the average grain size of ferrite (JIS G0552), and was a value measured over the entire thickness of each section in the rolling direction and the direction perpendicular to the rolling direction. Shall be determined from the average of

The bainite phase volume fraction is set to 60 to 90.
%, And by satisfying the bainite grain size,
Ultra high strength of 980 MPa or more, excellent stretch flangeability,
A comprehensive material balance with excellent ductility can be achieved, and good press forming (hole expanding) and roll formability can be exhibited. In the present invention, TS ×
λ (hole expansion rate) ≧ 60000 MPa ·%, TS × El ≧ 1600
0 MPa ·% and a yield ratio YR of 60 to 70% can be achieved. If YR is less than 60, an edge wave is generated at the time of roll forming, resulting in poor forming. If it exceeds 70%, wrinkles occur during bending after roll forming.

Next, the manufacturing conditions will be described. The slab is desirably manufactured by a continuous casting method in order to prevent macroscopic segregation of components, but it is also possible to manufacture the slab by an ingot making method or a thin slab casting method. After production, the slabs are cooled to room temperature and then heated again. Energy saving process steps such as direct rolling can be applied without any problem. However, from the viewpoint of uniformity and fineness of the initial structure, even when performing direct rolling, etc., once,
It is more desirable to reheat after completing the γ → α transformation.

Slab heating temperature (SRT): 1100 to 1300 ° C. The slab heating temperature is set so that crystal grains can be uniformly refined.
It is preferable to keep the temperature as low as possible below 1300 ° C, but it is also necessary to secure the finish rolling temperature,
It should be in the range of 1100-1300 ° C. Note that the temperature is preferably set to 1200 ° C. or less.・ Finishing finish temperature: 800-950 ° C If the finish rolling end temperature is less than 800 ° C, the deformation resistance during rolling is large and the structure becomes non-uniform. On the other hand, if the temperature is higher than 950 ° C., a fine and uniform structure cannot be obtained. Therefore,
The finish rolling finish temperature is in the range of 800 to 950 ° C.

Cooling after completion of hot rolling After completion of hot rolling, cooling is started within 0.5 seconds, and forced cooling is performed at a cooling rate of 30 ° C./sec or more. When left to cool after completion of hot rolling, the crystal grain size of the finally obtained hot rolled sheet becomes coarse.
In order to reduce the average grain size of bainite to less than 4 μm, it is necessary to forcibly cool within 0.5 seconds after the end of hot rolling. The cooling rate at this time is extremely important because it determines the structure after the completion of hot rolling. If the cooling rate is lower than 30 ° C./sec, a phase other than the bainite phase such as a soft ferrite phase is formed. By cooling at a rate of 30 ° C./sec or more, it is possible to obtain a uniform and fine structure having bainite as a main phase. -Winding temperature: 300-500 ° C If the winding temperature is too low, a hard martensite phase is formed, and a bainite-based uniform structure is not obtained, so that sufficient workability cannot be obtained. On the other hand, if the winding temperature is too high, the low-temperature transformation phase softens and the crystal grains become coarse, so that properties such as strength become insufficient.

FIG. 1 shows the relationship between the stretch flangeability (hole expansion ratio) and the tensile strength of the present invention and the conventional one. The invention steel plate is 0.08-0.15wt% C-0.20
wt% Si-3.0 wt% Mn-0.04 wt% Al-0.04 wt% Nb-0.020
wt% Ti-0.10wt% Cr-0.020wt% P-0.0020wt% S steel slab, slab heating temperature is 1150 ℃, finish rolling end temperature is 900 ℃, cooling start after hot rolling is completed within 0.3 seconds, The cooling rate was 85 ° C./sec and the winding temperature was 350 to 400 ° C. The comparative steel sheet differs from the invention mainly in the conditions of the heating temperature, cooling rate, winding temperature and the like. From FIG. 1, it can be seen that the hot-rolled steel sheet according to the present invention has not only a tensile strength of 980 MPa or more, but also has a much higher hole expansion ratio than the comparative steel sheet.

[0029]

EXAMPLE 1 A continuous cast slab (slab thickness 260 mm) having a composition shown in Table 1 and a balance of substantially Fe was cooled to room temperature, heated to 1048 ° C. to 1116 ° C., and finished. Rolling end temperature 87
Hot rolling is performed at 0 to 900 ° C, and within 0.4 seconds after the completion of hot rolling, cooling is performed at a cooling rate of 60 to 70 ° C / sec.
It was wound at 310 ° C. to obtain a 2.0 mm hot-rolled steel sheet. The following material characteristics were implemented for the obtained hot-rolled steel sheet.・ Hole expansion rate: punching a hole with initial diameter d ₀ = 10 mm, 60 °
When the hole was expanded by raising the conical punch, the punch was stopped from rising when the crack penetrated the plate, and the punched hole diameter d after the crack was penetrated was measured. The hole expansion ratio (%) = ((d−d ₀₎ ) / D
₀ ) × 100.・ Presence or absence of edge wave: Hat shape (molding height 50mm, punch shoulder
After roll-forming into 10 mmφ and a die shoulder of 10 mmφ), the end face shape was visually determined. -Bending: The steel sheet is formed into a pipe with a square cross-section (corner R10 mmφ, cross-section size 100 mm x 60 mm) and bent at a radius of curvature of 500 mm, and the presence or absence of wrinkles is visually determined. did. Delayed fracture test: Deep drawing was performed on a cylinder having a drawing ratio of 1.80 with a punch having a diameter of 50 mmφ, and this was immersed in pure water and evaluated for the occurrence of cracks after about one week. Table 2 shows the test results.

Example 2 Next, using the steel slab H in Table 1, a hot-rolled steel sheet was manufactured under the conditions shown in Table 3. Similar tests were performed on the obtained hot-rolled steel sheets to investigate the material properties. Table 3 also shows the results.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

According to the present invention, it can be seen that an ultra-high strength hot rolled steel sheet having a tensile strength of 980 MPa or more, and having an excellent balance between ductility and hole expandability, which has not been achieved conventionally, can be obtained.

[0035]

As described above, according to the present invention,
It is possible to provide an ultra-high-strength hot-rolled steel sheet excellent in workability, having both excellent stretch flange formability and strength-ductility balance and having a low yield ratio while having a tensile strength of 980 MPa or more. Further, according to the present invention, it is possible to prevent molding defects such as edge waves and wrinkles which are likely to be generated by roll molding. Therefore, the present invention greatly contributes to further weight reduction and cost reduction in manufacturing a strength member of an automobile, an impact beam, and the like.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a hole expansion ratio and a tensile strength.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term in the Technical Research Laboratory, Kawasaki Steel Co., Ltd. 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA19 EA20 EA23 EA25 EA27 EA31 EA32 EA35 EA36 EB05 EB07 EB08 EB11 FA02 FA03 FC03 FC04 FD04 FE01 FE06 JA06

Claims (4)

[Claims] 1. C: 0.05 to 0.20 wt%, Si: 0.05 to 0.50 wt%, Mn: 1.0 to 3.5 wt%, P: 0.05 wt% or less, S: 0.01 wt% or less, Nb: 0.005 to 0.30 wt% , Ti: 0.001 to 0.100 wt%, Cr: 0.01 to 1.0 wt% and Al: 0.1 wt% or less, with the balance being Fe and inevitable impurities, and the above Si, P, Cr, Ti, Nb Each content of Mn and Mn is represented by the following formula: 0.05 ≦ ((wt%) Si + (wt%) P) / ((wt%) Cr + (wt
%) Ti + (wt%) Nb + (wt%) Mn) ≦ 0.5, and the metal structure has a volume fraction of 60 to
A structure in which less than 90% of bainite is the main phase and at least one of pearlite, ferrite, retained austenite, and martensite is the second phase, and the average grain size of the bainite phase is less than 4 μm. Ultra-high strength hot rolled steel sheet with excellent workability. 2. The steel sheet according to claim 1, further comprising: An ultra-high-strength hot-rolled steel sheet excellent in workability, characterized by containing one or more selected from any one group or two groups. 3. C: 0.05 to 0.20 wt%, Si: 0.05 to 0.50 wt%, Mn: 1.0 to 3.5 wt%, P: 0.05 wt% or less, S: 0.01 wt% or less, Nb: 0.005 to 0.30 wt% , Ti: 0.001 to 0.100 wt%, Cr: 0.01 to 1.0 wt% and Al: 0.1 wt% or less, with the balance being Fe and inevitable impurities, and the above Si, P, Cr, Ti, Nb Each content of Mn and Mn is represented by the following formula: 0.05 ≦ ((wt%) Si + (wt%) P) / ((wt%) Cr + (wt
%) Ti + (wt%) Nb + (wt%) Mn) ≦ 0.5 After casting the steel slab containing it, satisfying the relation, immediately or once cooled, heated to 1100-1300 ° C, and finished rolling. Hot-rolled at a temperature of 950 to 800 ° C, after rolling
A method for manufacturing an ultra-high-strength hot-rolled steel sheet excellent in workability, characterized by starting cooling within 0.5 seconds, cooling at a cooling rate of 30 ° C / sec or more, and winding at 500 to 300 ° C. . 4. The method according to claim 3, further comprising: A method for producing an ultra-high-strength hot-rolled steel sheet excellent in workability, characterized by containing one or more selected from any one group or two groups.