JP3390596B2 - Low yield ratio high strength hot rolled steel sheet excellent in toughness and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to a steel pipe for construction and civil engineering,
A low yield ratio and high strength hot rolled steel sheet (including steel strip, including steel strip) having excellent toughness, which is suitable for use in columns, ERW steel pipes for oil wells, and other general structural materials, and a manufacturing method thereof. is there.

[0002]

2. Description of the Related Art Hot-rolled steel sheets used as materials for steel pipes and columns for construction and civil engineering need properties such as strength and toughness as structural materials, and are used as materials for electric resistance welded steel pipes for oil wells. In addition to the above properties, the hot rolled steel sheet is required to have sour environment resistance (wet hydrogen sulfide environment resistance, hereinafter simply referred to as “sour resistance”). Many proposals have been made so far regarding the manufacturing technology of hot rolled steel sheets used for such purposes. Among them, the method that is generally adopted at present as a technique to achieve both strength and toughness is the fine structure of the structure obtained by subjecting steel mainly composed of ferrite / pearlite structure to a thermomechanical treatment called TMCP. Strengthening treatment by granulation (for example, JP-A-62-62
No. -112722, Japanese Patent Publication No. 62-23056, Japanese Patent Publication No. 62-35452, etc.) and rapid cooling (controlled cooling) treatment after hot rolling are combined.

However, the above-mentioned known technique has the following drawbacks, and there is a problem that it cannot always meet future needs. 1) Extreme grain refinement such as TMCP inevitably raises the yield ratio (yield strength / tensile strength), so it has recently been required to prevent buckling and unstable ductile fracture. It cannot cope with low yield ratio. 2) In TMCP, the deformation due to rolling does not become uniform in the plate thickness direction, so that material nonuniformity occurs in the plate thickness direction. Further, the controlled cooling accompanied by strong cooling is likely to cause a material difference in the longitudinal direction (rolling direction) and is also sensitive to a change in plate thickness, so that material control is difficult. Due to these factors, TMCP
The material in the thickness direction and the material in the longitudinal direction tends to be uneven. 3) In TMCP, it is necessary to perform low-temperature high-pressure reduction in the austenite unrecrystallized temperature range so as to obtain high strength and high toughness, which causes an increase in load of hot rolling equipment and an upper limit of rolling material size. Become. 4) In TMCP, since the dependence on Mn, V, Mo and other strengthening elements is large, the hardenability of these elements increases, the hardness of the weld increases, and the welded part due to island martensite is generated. Degradation of toughness is likely to occur. Therefore, there is a limit to the increase in strength by the TMCP method while maintaining good weldability.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to advantageously solve the above-mentioned problems of the prior art, namely, non-uniformity of material in the thickness direction and length direction, weldability, and resistance to welding. It is an object of the present invention to provide a high-strength hot-rolled steel sheet excellent in toughness and having a low yield ratio, together with its advantageous manufacturing method, without causing deterioration of sourness. A specific object of the present invention is to have a yield strength (YS) of 276 MPa or more, preferably 413 MPa or more, and a yield ratio (YR) of 80% or less.
Preferably, the toughness is 70% or less, the fracture surface transition temperature vTrs is -100 ° C (corresponding to -30 ° C in the DWTT 85% test) or less, and preferably -120 ° C (-4 in the DWTT 85% test).
6 ° C or less), 300 J or more, preferably 310 J or more in Charpy absorbed energy vEo at 0 ° C.,
Strength-toughness balance index 0.3 TS-vTrs is 30
0 or more, preferably 320 or more, the difference in Vickers hardness (ΔHv) between the weld and the base metal is 100 or less, preferably 30.
Below, the toughness of the weld heat affected zone (HAZ) is the fracture surface transition temperature v
Satisfies 0 ° C or lower, preferably -20 ° C or lower in TrS,
Moreover, it is to provide a high-strength hot-rolled steel sheet having excellent sour resistance together with its advantageous manufacturing method.

[0005]

The inventors of the present invention have conducted numerous experiments and studies in order to achieve the above-mentioned object, and as a result, added carbide precipitating element, B to low C steel, and manufactured it under the manufacturing conditions. If the means such as proper control of C are used, 1) Intragranular solid solution C
It is possible to improve the toughness of the ferrite matrix and reduce the YR by optimizing the amount. 2) Precipitated carbide can be effectively used for improving the strength. 3) Grains that have been conventionally observed when the solid solution C is low. Suppresses strength reduction due to coarsening of diameter, and 4) Ferrite (including bainitic ferrite)
We have found that a single-phase structure can improve toughness and sour resistance. The present invention is based on the above findings, and its gist is as follows.

That is, (1) C: 0.005 to less than 0.030 wt%, Si: 1.5 wt% or less,
Mn: 1.5 wt% or less, P: 0.020 wt% or less, S: 0.
015 wt% or less, Al: 0.005-0.10 wt%, N: 0.0100
wt% or less, B: 0.0002 to 0.0100 wt% is included, and T
i: 0.20 wt% or less and Nb: 0.25 wt% or less, and any one or two of them are contained in a relationship of (Ti + Nb / 2) / C ≧ 4, and the balance is Fe and unavoidable. Solid impurities in the grains and the metallic structure of ferrite and / or bainitic ferrite.
A low-yield ratio high-strength hot-rolled steel sheet excellent in toughness, characterized in that the amount is 1.0 to 4.0 ppm.

(2) In addition to the steel composition described in (1) above, Mo: 1.0 wt% or less, Cu: 2.0 wt% or less, Ni: 1.5 wt%
% Or less, Cr: 1.0 wt% or less, and V: 0.10 wt% or less, and a low yield ratio and high strength hot rolled steel sheet excellent in toughness, characterized by containing at least one kind or two or more kinds.

(3) In addition to the steel composition described in (1) or (2) above, Ca: 0.0005 to 0.0050 wt% and REM: 0.001
~ 1 or 2 selected from 0.020 wt%
A high-strength hot-rolled steel sheet having a low yield ratio and excellent in toughness, which is characterized by containing a seed.

(4) C: 0.005 to less than 0.030 wt%, Si: 1.
5 wt% or less, Mn: 1.5 wt% or less, P: 0.020 wt%
Below, S: 0.015 wt% or less, Al: 0.005 to 0.10 wt
%, N: 0.0100 wt% or less, B: 0.0002 to 0.0100 wt%
And at least one of Ti: 0.20 wt% or less and Nb: 0.25 wt% or less (Ti + Nb /
2) After hot-rolling the steel containing and satisfying the relationship of C ≧ 4,
The heat as described in any one of (1) to (3) above, which is characterized by cooling at a rate of 5 ° C / sec or more and 20 ° C / sec or less, and subsequently winding in a temperature range of more than 550 to 700 ° C. Manufacturing method of rolled steel sheet.

[0010]

The present invention will be described in detail below. First, the experimental results on which the present invention is based will be described. C: 0.003 to 0.030 wt%, Si: 0.4 wt%, Mn: 0.6 wt
%, P: 0.010 wt%, S: 0.0020 wt%, Al: 0.035 wt%
%, N: 0.0018 to 0.0043 wt%, B: 0.0008 to 0.0015 wt
%, Ti: 0 to 0.12 wt%, Nb: 0 to 0.25 wt%, (Ti + Nb / 2) / C
= 2 to 10, the steel slab changed, the slab heating temperature (SRT): 1200 ℃, hot rolling end temperature (FDT):
880 ℃, Cooling rate after hot rolling 3 to 30 ℃ / se (cooling rate up to 700 ℃ when coil coiling temperature (CT) is less than 700 ℃), coiling temperature (CT): 500 to 750 ℃ Hot-rolled steel sheet with a thickness of 12 to 20 mm was manufactured.

Regarding the obtained hot-rolled steel sheet, solid solution C in the grains
And the strength (yield strength (YS), tensile strength (TS)), yield ratio (YR = YS / TS), fracture surface transition temperature (vTrs), and values calculated from these values.
3TS (MPa) -vTrs (° C) was determined. here,
YS was determined by the value of 0.5% strain of API standard (usually, it is almost equal to 0.2% proof stress used in non-aged steel or the yield stress used in aged steel). In addition, the aging index AI (Ageing Ind
ex) was used. That is, after applying 7.5% pre-strain, 100 ℃
The curing amount after 30 minutes of heat treatment was measured and taken as the AI value. The AI value is hardly affected by the solid solution C in the grain boundary, and generally has a relation of intragranular solid solution C (ppm) = 0.20 × AI (MPa) with respect to the intragranular solid solution C. It should be noted that the measurement of the solid solution C by the internal friction method is not affected by the grain boundary solid solution C, and is also affected by the grain size and the grain morphology. . Note that 0.3TS (MPa) -vTrs (° C)
Has the following meanings. Due to general strengthening such as precipitation strengthening and solid solution strengthening, the toughness deteriorates and vTrs increases.
Therefore, in order to compare the toughness of steel sheets having different strengths, it is necessary to correct the amount of change in the toughness depending on the strength. The toughness change due to this strengthening is 0.3 TS empirically.
(MPa). Therefore, vTrs-0.3
The lower the value of TS, in other words 0.3TS-vTr
It can be said that the larger the value of s, the better the toughness excluding the effect of strengthening. It can be considered that the toughness value thus obtained represents the toughness obtained by combining the toughness inherent in the matrix of the crystal and the toughness due to grain refinement.

FIG. 1 shows the relationship between the solid solution C in the grain and each of the above characteristics. As is clear from the figure, 1.
It can be seen that excellent toughness and a low yield ratio can be obtained by controlling in the range of 0 to 4.0 ppm. In this way, the mechanism that enables the yield ratio to be lowered by reducing the solid solution C to 4.0 ppm or less is that the upper yield point does not occur, the dislocations fixed to the solid solution C decrease, and It is thought that this is due to an increasing number of people. Further, it is considered that the reason why the toughness is improved is that the same mechanism as that of the low yield ratio facilitates plastic deformation even under impact deformation at low temperatures, and thus the absorbed energy is less likely to decrease. On the other hand, when the solid solution C in the grains is reduced to less than 1.0 ppm, the yield ratio is reduced, but the strength is significantly reduced, and 0.3TS
The value of −vTrs also seems to be due to the coarsening of the crystal grains, but is slightly lowered. As described above, it has been found that it is extremely important to control the solute C in the grain within the range of 1.0 to 4.0 ppm in order to achieve excellent toughness and a low yield ratio.

Next, in the present invention, the reason why the chemical composition, the structure and the manufacturing conditions are limited to the above range will be described. C: 0.005 to less than 0.030 wt% C is an element that improves the strength by precipitation strengthening in the coexistence of Ti and Nb. If the addition amount is less than 0.005 wt%, not only the effect is poor, but also the crystal grain becomes coarse and high strength cannot be achieved without an excessive solid solution strengthening element. In addition, the welded portion is likely to grow grains, which causes breakage due to softening. On the other hand, if 0.030 wt% or more is contained, a large amount of
Even if Nb or Ti is added, it becomes difficult to reduce the solid solution C in the grains to the required amount, and island martensite is generated in the welded portion, which deteriorates the toughness of the welded portion. Therefore, the content of C is 0.005 to less than 0.030 wt%, preferably 0.015 to
0.028 wt%

Si: 1.5 wt% or less Si is an element useful as a strengthening element, and is an element with little adverse effect on toughness in steel having a low solid solution C. However, excessive addition of more than 1.5 wt% has a negative effect on toughness and reduces crack susceptibility of welds. Therefore, the Si content is preferably 1.5 wt% or less, and is preferably 0.8 wt% or less from the viewpoint of the strength improving effect.

Mn: 1.5 wt% or less Mn is an element useful as a strengthening element, but if it is added in excess of 1.5 wt%, it increases the hardness of the weld zone and increases the weld crack susceptibility. Further, there is a concern that island-like martensite is generated and toughness is reduced. Furthermore, excessive Mn addition causes
It is also not preferable from the viewpoint that it has the effect of reducing the diffusion rate of solid solution C and delaying the reduction of solid solution C in the grains due to the precipitation of carbides. Therefore, the Mn content is preferably 1.5 wt% or less, and is preferably 0.8 wt% or less from the viewpoint of the strength improving effect.

P: 0.020 wt% or less P does not adversely affect the toughness as much as the non-aging steel in the steel having the intragranular solid solution C range of the present invention, but if it exceeds 0.020 wt%, the toughness deteriorates. The impact will be greater. Therefore,
The P content is 0.020 wt% or less, preferably 0.012 wt% or less.

S: 0.015 wt% or less S forms sulfides and deteriorates sour resistance.
It is preferable to reduce the amount as much as possible, but it is acceptable in the range of 0.015 wt% or less, preferably 0.005 wt% or less.

Al: 0.005-0.10 wt% Al is an element useful for deoxidizing steel and fixing N. In order to obtain the effect, it is necessary to add at least 0.005 wt%, but the addition of more than 0.10 wt% is disadvantageous in terms of cost, so the content is made 0.005 to 0.10 wt%.

N: 0.0100 wt% or less N causes a decrease in toughness and an increase in YR in a solid solution state, so N is fixed as a nitride such as Ti, Al, or B. However, if the amount of N is large, the cost increases due to the increase in the amount of addition of these elements, so it is preferable to reduce it, but it is permissible within the range of 0.0100 wt% or less. The content is preferably 0.0050 wt% or less.

B: 0.0002 to 0.0100 wt% B is an element required to suppress the excessive growth of crystal grains and secure toughness and strength. Further, due to the lowering of the transformation point during cooling, carbide at high temperature is obtained. Is also an element necessary for suppressing the coarse precipitation of. To obtain these effects, addition of 0.0002 wt% or more is necessary. On the other hand, addition of more than 0.0100 wt% deteriorates toughness due to excessive quenching action. Therefore, B is in the range of 0.0002 to 0.0100 wt%, preferably 0.00
Add in the range of 05-0.0050wt%.

Ti: 0.20 wt% or less, Nb: 0.25 wt% or less, and (Ti + Nb / 2) / C ≧ 4 Ti, Nb are both important elements in the present invention, and solid solution C is precipitated and fixed. Control the intra-grain solid solution C and
It forms C and NbC to bring about high strength by precipitation strengthening. In order to bring about these effects, it is necessary to satisfy (Ti + Nb / 2) / C ≧ 4. However, if the amounts of Ti and Nb are excessive, inclusions increase, which is disadvantageous in terms of toughness of the welded portion, so they are added in the range of 0.20 wt% or less and 0.25 wt% or less, respectively. The preferable (Ti + Nb / 2) / C range is 5-8.

Although the basic components have been described above, in the present invention, Mo, Cu, Ni, Cr, V, Ca and REM can be added appropriately. Mo: 1.0 wt% or less, Cu: 2.0 wt% or less, Ni: 1.5 wt% or less, Cr: 1.0 wt% or less and V: 0.10 wt% or less All of these elements are auxiliary used as strengthening elements. Although it is an element, it is limited to the above range because if added excessively, it causes adverse effects such as reduction in toughness of the welded portion.

Ca: 0.0005 to 0.0050 wt%, REM: 0.001 to
0.020 wt% Ca and REM both spheroidize the sulfide morphology,
It has the effect of improving toughness, sour resistance, weldability, and the like. However, if any of them is added excessively, inclusions increase and the toughness deteriorates, so the content is limited to the above range.

Metal Structure and Amount of Solute C in Grains; The structure of the present invention must be ferrite and / or bainitic ferrite. That is, by controlling to the above structure, macroscopic defects can be reduced, so that deterioration of toughness and sour resistance can be avoided even if strength is increased by precipitation strengthening. The conventional steel had a structure with many macro defects because it utilized the strengthening of the composite structure of ferrite and pearlite. Regarding the influence of the amount of solute C in the grains, as described in FIG. 1, 1.
Controlling in the range of 0 to 4.0 ppm (weight) is an essential requirement for achieving both excellent toughness and low yield ratio. In order to obtain such ferrite and / or bainitic ferrite, the steel having the above-described composition according to the present invention may be manufactured under appropriate conditions described below.

Next, the conditions for producing the hot-rolled steel sheet according to the present invention will be described. Cooling rate after hot rolling: In order to precipitate carbides and adjust the solid solution C in the grains, it is necessary to control the cooling rate after hot rolling, especially in the temperature range up to 700 ° C or higher until winding. There is. If the cooling rate is less than 5 ° C / sec, the crystal grain size becomes coarse and the toughness decreases. On the other hand, when cooled at a rate exceeding 20 ° C./sec, precipitation of carbides tends to be insufficient, and strain tends to remain in ferrite grains, resulting in a decrease in toughness. In addition, if the cooling rate is too high, it becomes difficult to maintain this cooling rate stably over the entire length of the hot-rolled steel strip, and the material becomes uneven in the longitudinal direction of the strip, the surface of the strip and the strip. This causes disadvantages such as non-uniformity of material between the thick center portion and deterioration of steel plate shape. Therefore, the cooling rate of the cooling rate after hot rolling is 5 ° C./sec or more and 20 ° C./sec or less, preferably 5 ° C./s.
ec or more and less than 15 ° C / sec, more preferably 5 ° C / sec
It is necessary to set it to 10 ° C./sec or more.

Winding temperature (CT): The action of adjusting the solid solution C in the grains and precipitation strengthening due to the precipitation of carbides mostly occurs in the slow cooling process after coil winding, so that after hot rolling, The winding temperature is a particularly important requirement. When the winding temperature is 550 ° C. or lower, the amount of solid solution C is not sufficiently reduced, and it is difficult to obtain a uniform material. On the other hand, when the winding temperature exceeds 700 ° C., overaging tends to occur, precipitation strengthening is unlikely to occur, which is disadvantageous in terms of high strength, and solid solution C tends to be too small. Therefore, the winding temperature after hot rolling is more than 550 to 700 ° C., preferably 60.
It is necessary to set the temperature range to 0 ° C or higher.

Although it is in the field of refractory steel, it is not disclosed in
In JP 222484, a high toughness low yield ratio steel in which IF (Interstitial Free) steel is precipitation strengthened is proposed. However, in this proposal, first, IF, that is, solid solution C
Is set to be substantially 0, which is different from the present invention in which the lower limit of solid solution C is required. Next, also in the manufacturing method and the example in the above proposal, rapid cooling-low temperature (550 ° C. or lower) winding is performed in order to secure fire resistance. According to the investigation by the present inventors, under such conditions, it is considered that the solute C actually exists in excess of 4.0 ppm, and the strength-toughness balance as in the present invention cannot be expected.

The above-described cooling rate and coiling temperature after hot rolling are particularly important requirements in the present invention and can be treated under uniform conditions over the entire length and width of the steel strip.
Next, suitable manufacturing conditions other than the above requirements will be described.
Hot rolling of a slab is carried out immediately after continuous casting (so-called C
C-DR) or heating temperature (SRT): 90
0 to 1300 ° C, preferably 120 in terms of energy saving
It is performed after reheating to a range of 0 ° C. or lower. When CC-DR is performed, heat retention or some heating of the end may be performed.

In hot rolling, the rolling end temperature (FDT) is 7
Normal rolling at 50 to 950 ° C may be performed, but if it is lower than the Ar ₃ transformation point of -100 ° C, carbides are precipitated during hot rolling and the precipitation strengthening action is weakened, which is not preferable. In the steel of the present invention, high toughness and high strength are obtained by controlling the amount of solid solution C in the matrix and refining by adding B. Therefore, controlled rolling (strong rolling in the austenite grain unrecrystallized temperature range) ) Does not necessarily have to be applied. When the steel of the present invention is intentionally produced by the controlled rolling method, the recrystallization temperature is low C, so
Since the temperature is lowered to about 00 ° C., it is preferable to ensure that the hot rolling reduction rate is 900% or less and 50% or more (60% or more is more effective). The thickness of the hot-rolled finished plate varies depending on the application, but is usually about 5 to 30 mm.

Although the above manufacturing method is in the hot rolled steel strip manufacturing process, this method can be applied to the thick plate manufacturing process. For example, the method similar to that for hot-rolled steel strip is performed until cooling after hot rolling, and then 1 to 600-700 ° C.
A similar material can be obtained by holding for at least hr and slowly cooling.

[0031]

EXAMPLES Steel slabs having various compositional compositions shown in Tables 1 to 3 were reheated and then hot-rolled under the conditions shown in Table 2 to obtain steel plates having a thickness of 15 mm. The hot-rolled steel sheet thus obtained was subjected to a microstructure investigation and the solid solution C in the grains was measured. Further, as mechanical properties of the steel sheet, properties such as yield strength, tensile strength, yield ratio, fracture transition temperature, absorbed energy at 0 ° C., 0.3TS-vTrs, HIC (sour resistance) were measured. Furthermore, we perform electric resistance welding at the pipe making line,
The Vickers maximum hardness (Hv) of the welded portion, the difference in hardness between this and the base metal portion (ΔHv), and the fracture surface transition temperature of the coarse grain portion of the heat affected zone were measured. Here, the amount of dissolved C in the grain was determined from AI as described above by the amount of dissolved C in the grain (ppm) = 0.20 × AI (MPa). According to JIS Z2201, the tensile test uses JIS No. 5 test piece, and the impact test uses JIS Z2.
It was carried out using a Charpy test piece according to 202. Also,
HIC was performed according to NACE TM-02-84. However, the test solution used was the NACE solution specified in NACE TM0177-90. HIC was evaluated as ○ if there were no cracks due to ultrasonic flaw detection, and the crack size was CSR (Cr
A ck Sensitivity Ratio of less than 1% was evaluated as Δ, and a value of 1% or more was evaluated as x. Of the obtained results, Table 2 shows the structure and intragranular solid solution C, and Table 3 shows the results of various mechanical properties and sour resistance.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

As is clear from Tables 1 to 3, the hot-rolled steel sheets obtained according to the present invention all have the desired characteristics, and the yield strength (YS) is 276 MPa or more in the characteristics of the mother board. Yield ratio (YR) is 80% or less, fracture surface transition temperature vTr
s is −100 ° C. or lower, Charpy absorbed energy vEo at 0 ° C. is 300 J or higher, 0.3TS-vTrs is 300 or higher, and shows excellent sour characteristics. Also, regarding the welded portion, the difference in hardness between the welded portion and the base metal is shown. (ΔHv) is 100
Below, the fracture surface transition temperature vTrS of the welding heat affected zone (HAZ)
Shows 0 ° C. or less, and has a low yield ratio, high strength, and excellent impact properties, sour resistance and weldability. In particular, the symbols 1A, 2A, 3-6 and 8-1
6 is a mother board, YS is 413 MPa or more, YR is 70% or less, vTrs is −120 ° C. or less, vEo is 310 J or more, 0.3TS-vTrs is 320 or more, and ΔHv is 30.
Hereafter, vTrS of HAZ was −20 ° C. or lower, and extremely excellent characteristics were obtained. On the other hand, in the comparative example in which the component composition and the manufacturing conditions were out of the range of the present invention, at least one of the characteristics such as toughness, yield ratio, weld property, sour resistance,
It can be seen that two characteristics are inferior.

[0036]

As described above, according to the present invention, the deterioration of the non-uniformity of the material in the thickness direction and the length direction is prevented,
High-strength hot-rolled steel sheets with excellent toughness, weldability, sour resistance, and low yield ratio can be obtained. Steel pipes for construction, civil engineering, columns, oil-well electric resistance welded steel pipes, etc. that require these characteristics. It has excellent effects when used for.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of solid solution C in a grain, strength, yield ratio, and toughness.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Satoshi Matsuoka, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba, Kawasaki Steel Works, Ltd. Technical Research Institute (56) Reference JP-A-5-222484 (JP, A) ( 58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. C: 0.005 to less than 0.030 wt%, Si: 1.5 wt% or less, Mn: 1.5 wt% or less, P: 0.020 wt% or less, S: 0.015 wt% or less, Al: 0.005 to 0.10 wt%, N: 0.0100 wt% or less, B: 0.0002 to 0.0100 wt%, and Ti: 0.20 wt% or less and Nb: 0.25 wt% or less selected from one or two types (Ti + Nb / 2 ) / C ≧ 4, the balance is Fe and inevitable impurities, the metal structure is ferrite and / or bainitic ferrite, and the amount of solid solution C in the grain is 1.0 to 4.0 ppm. A high yield hot rolled steel sheet with a low yield ratio and excellent toughness. 2. The steel composition according to claim 1, further comprising Mo: 1.0 wt% or less, Cu: 2.0 wt% or less, Ni: 1.5 wt% or less, Cr: 1.0 wt% or less and V: 0.10 wt%. %, A low yield ratio, high strength hot-rolled steel sheet excellent in toughness, characterized in that it contains any one or two or more selected from the following. 3. The steel composition according to claim 1 or 2, further containing any one or two selected from Ca: 0.0005 to 0.0050 wt% and REM: 0.001 to 0.020 wt%. A high yield hot rolled steel sheet with a low yield ratio and excellent in toughness. 4. C: 0.005 to less than 0.030 wt%, Si: 1.5 wt% or less, Mn: 1.5 wt% or less, P: 0.020 wt% or less, S: 0.015 wt% or less, Al: 0.005 to 0.10 wt%, N: 0.0100 wt% or less, B: 0.0002 to 0.0100 wt%, and Ti: 0.20 wt% or less and Nb: 0.25 wt% or less selected from one or two types (Ti + Nb / 2 ) / C ≧ 4 contained steel is hot-rolled, then cooled at a rate of 5 ° C./sec or more and 20 ° C./sec or less, and subsequently wound in a temperature range of more than 550 to 700 ° C. A method for producing a high-strength hot-rolled steel sheet having a low yield ratio and excellent toughness.