JP2019504199A - Low yield ratio type high strength steel and its manufacturing method - Google Patents

Abstract

In the present invention, carbon (C) is 0.02 wt% to 0.11 wt%, silicon (Si) is 0.1 wt% to 0.5 wt%, and manganese (Mn) is 1.5 wt% to 2 wt%. 0.5 wt%, aluminum (Al) 0.01 wt% to 0.06 wt%, nickel (Ni) 0.1 wt% to 0.6 wt%, titanium (Ti) 0.01 wt% 0.03 wt%, niobium (Nb) 0.005 wt% to 0.08 wt%, chromium (Cr) 0.1 wt% to 0.5 wt%, phosphorus (P) 0.01 wt% Or less (excluding 0 wt%), sulfur (S) 0.01 wt% or less (excluding 0 wt%), boron (B) 5 wtppm to 30 wtppm, and nitrogen (N) 20 wtppm 70 ppm by weight, calcium (Ca) 50 ppm by weight or less (excluding 0 ppm by weight), tin (Sn) 5 ppm by weight It comprises 0 wt ppm, the remainder to low yield ratio high-strength steel consisting of iron (Fe) and other unavoidable impurities.

Description

  The present invention relates to a low yield ratio type high strength steel material and a method for producing the same, and more specifically, a low yield ratio type high strength steel material that has a low yield ratio and high tensile strength and can be suitably used as a construction steel material, and It relates to the manufacturing method.

  Recently, the construction of ultra-thick and high-strength steel materials has been demanded for structures such as buildings and bridges in Japan and overseas as super-high-rise and long-span progress. When high strength steel is used, because it has high allowable stress, it is possible to rationalize and reduce the weight of buildings and bridge structures, and not only allows economic construction, but also can reduce the plate thickness. Machining and welding operations such as cutting and drilling are facilitated.

  On the other hand, when the strength of steel is increased, the yield ratio (yield strength / tensile strength), which is the ratio between tensile strength and yield strength, often increases, but when the yield ratio increases, the point at which plastic deformation occurs (yield point) ) Until the point of failure is not large, it becomes difficult for the building to absorb the energy by deformation and prevent destruction, ensuring safety when a huge external force such as an earthquake acts There is a problem that it is difficult to do. Therefore, the structural steel material must satisfy both high strength and low yield ratio.

  In general, the yield ratio of a steel material is such that a soft phase such as ferrite is the main structure in the metal structure of the steel material, and a hard phase such as bainite and martensite is moderate. It is known to lower by realizing a dispersed structure.

  In order to obtain a structure in which a hard phase is moderately dispersed in such a soft phase-based fine structure, Patent Document 1 discloses appropriate quenching in a dual phase region of ferrite and austenite. ) And tempering to lower the yield ratio. However, since the number of heat treatment steps is added in addition to the rolling manufacturing step, the above method has a problem that it is inevitable to increase the manufacturing unit price as well as the productivity.

  Accordingly, there is a need for the development of a low yield ratio type high strength steel material and a method for manufacturing the same that can solve all the problems such as a decrease in productivity and an increase in the manufacturing unit price, and can ensure an ultra high strength and a low yield ratio.

JP-A-55-97425

  This invention is made | formed in view of the said conventional problem, Comprising: The objective of this invention is providing the low yield ratio type | mold high strength steel material and its manufacturing method. More specifically, an object of the present invention is to provide a low-yield-ratio type high-strength steel material in which ultra-high strength and a low yield ratio are ensured without lowering productivity and increasing manufacturing cost, and a method for manufacturing the same.

  The low yield ratio type high strength steel material according to one aspect of the present invention, which has been made to achieve the above object, comprises 0.02 wt% to 0.11 wt% carbon (C) and 0.1 wt% silicon (Si). % To 0.5% by weight, manganese (Mn) 1.5% to 2.5% by weight, aluminum (Al) 0.01% to 0.06% by weight, nickel (Ni) 0.1% % By weight to 0.6% by weight, 0.01% to 0.03% by weight of titanium (Ti), 0.005% to 0.08% by weight of niobium (Nb), and 0.05% by weight of chromium (Cr). 1% by weight to 0.5% by weight, phosphorus (P) 0.01% by weight or less (excluding 0% by weight), sulfur (S) 0.01% by weight or less (excluding 0% by weight), boron ( B) 5 ppm to 30 ppm, nitrogen (N) 20 ppm to 70 ppm, calcium (Ca) 50 The amount ppm or less (0 wt ppm is excluded), tin (Sn) containing 5 wt ppm~50 ppm by weight, the remainder is characterized by consisting of iron (Fe) and other unavoidable impurities.

  In order to achieve the above object, a method for producing a low yield ratio type high strength steel material according to an embodiment of the present invention is as follows: carbon (C) is 0.02 wt% to 0.11 wt%, and silicon (Si) is 0 0.1 wt% to 0.5 wt%, manganese (Mn) 1.5 wt% to 2.5 wt%, aluminum (Al) 0.01 wt% to 0.06 wt%, nickel (Ni) 0.1% to 0.6% by weight, 0.01% to 0.03% by weight of titanium (Ti), 0.005% to 0.08% by weight of niobium (Nb), chromium (Cr) 0.1 wt% to 0.5 wt%, phosphorus (P) 0.01 wt% or less (excluding 0 wt%), sulfur (S) 0.01 wt% or less (excluding 0 wt%) Boron (B) 5 ppm to 30 ppm, Nitrogen (N) 20 ppm to 70 ppm, Calcium (C ) 50 wt ppm or less (excluding 0 wt ppm), tin (Sn) 5 wt ppm to 50 wt ppm, and the remainder is heated to 1050 ° C. to 1250 ° C. with a slab composed of iron (Fe) and other inevitable impurities A step of roughly rolling the heated slab at 950 ° C. to 1150 ° C. to obtain a bar, and hot rolling the bar (Bar) at a finish rolling temperature of 700 ° C. to 950 ° C. A step of obtaining a rolled steel sheet, and a step of cooling the hot-rolled steel sheet to a cooling end temperature equal to or lower than the Bs temperature at a cooling rate of 25 ° C./s to 50 ° C./s.

  According to the present invention, it is possible to provide a low-yield ratio type high-strength steel material in which an ultra-high strength and a low yield ratio are ensured without lowering productivity and increasing manufacturing cost, and a method for manufacturing the same.

  In the following, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified in various ways, and the technical scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more fully explain the present invention to those having ordinary knowledge in the art.

  Hereinafter, a low yield ratio type high strength steel material according to an embodiment of the present invention will be described in detail.

  The low yield ratio type high strength steel according to an embodiment of the present invention includes carbon (C) 0.02 wt% to 0.11 wt%, silicon (Si) 0.1 wt% to 0.5 wt%, Manganese (Mn) 1.5 wt% to 2.5 wt%, Aluminum (Al) 0.01 wt% to 0.06 wt%, Nickel (Ni) 0.1 wt% to 0.6 wt% , 0.01% to 0.03% by weight of titanium (Ti), 0.005% to 0.08% by weight of niobium (Nb), 0.1% to 0.5% by weight of chromium (Cr) %, Phosphorus (P) 0.01 wt% or less (excluding 0 wt%), sulfur (S) 0.01 wt% or less (excluding 0 wt%), boron (B) 5 wt ppm-30 Weight ppm, nitrogen (N) 20 ppm to 70 ppm, calcium (Ca) 50 ppm or less (0 ppm) Excluded), tin (Sn) containing 5 wt ppm~50 wt ppm, the remainder consisting of iron (Fe) and other unavoidable impurities.

Carbon (C): 0.02% by weight to 0.11% by weight
C is an important element that forms bainite or martensite and determines the size and fraction of the bainite or martensite.

  When the C content exceeds 0.11% by weight, the low-temperature toughness is reduced, and when the C content is less than 0.02% by weight, the formation of bainite or martensite is hindered and the strength is reduced. Therefore, the C content is preferably 0.02% by weight to 0.11% by weight.

  On the other hand, in the case of a plate material used as a steel structure for welding, the upper limit of the C content is preferably 0.08% by weight for better weldability.

Silicon (Si): 0.1 wt% to 0.5 wt%
Si is an element that is used as a deoxidizer and improves strength and toughness.

  If the Si content exceeds 0.5% by weight, not only the low-temperature toughness and weldability will deteriorate, but also the scale will be thickly formed on the surface of the plate material, which may cause poor gas cutting properties and other surface cracks. There is. On the other hand, when the Si content is less than 0.1% by weight, the deoxidation effect is not sufficient. Accordingly, the Si content is 0.1% to 0.5% by weight. More preferably, it is 0.15 weight%-0.35 weight%.

Manganese (Mn): 1.5% to 2.5% by weight
Since Mn is a useful element that improves the strength by solid solution strengthening, it is necessary to add 1.5% by weight or more. However, if the Mn content exceeds 2.5% by weight, the toughness of the welded portion is greatly lowered due to an excessive increase in the hardenability. Therefore, the Mn content is preferably 1.5% by weight to 2.5% by weight.

Aluminum (Al): 0.01% to 0.06% by weight
Al is an element that can deoxidize molten steel at low cost and stabilize ferrite. When the Al content is less than 0.01% by weight, the above effects are not sufficient. On the other hand, when the Al content exceeds 0.06% by weight, nozzle clogging occurs during continuous casting. Therefore, the Al content is preferably 0.01% by weight to 0.06% by weight.

Nickel (Ni): 0.1% to 0.6% by weight
Ni is an element that simultaneously improves the strength and toughness of the base material. In order to sufficiently exhibit the above effects, it is preferable to add 0.1% by weight or more. However, since Ni is an expensive element, if the addition amount exceeds 0.6% by weight, the economic efficiency is lowered and the weldability is also lowered. Therefore, the Ni content is preferably 0.1% by weight to 0.6% by weight.

Titanium (Ti): 0.01% by weight to 0.03% by weight
Ti is preferably added in an amount of 0.01% by weight or more in order to suppress the growth of crystal grains during reheating and greatly improve the low temperature toughness. However, if the Ti content exceeds 0.03% by weight, problems such as clogging of a continuous casting nozzle and a decrease in low-temperature toughness due to crystallization of the central portion occur. Therefore, the Ti content is preferably 0.01% by weight to 0.03% by weight.

Niobium (Nb): 0.005 wt% to 0.08 wt%
Nb is an important element in the production of TMCP steel, and precipitates in the form of NbC or NbCN, greatly improving the strength of the base metal and the weld. In addition, when reheated to a high temperature, the dissolved Nb has the effect of suppressing the recrystallization of austenite and the transformation of ferrite or bainite and making the structure finer. Furthermore, when the slab is cooled after rough rolling, it not only forms bainite even at a low cooling rate, but also enhances austenite stability during cooling after final rolling, and promotes the formation of martensite even at low cooling. Also fulfills.

  In order to sufficiently obtain the above effects, the Nb content is preferably 0.005% by weight or more. However, when the Nb content exceeds 0.08% by weight, brittle cracks occur at the edge of the steel material. Therefore, the Nb content is preferably 0.005 wt% to 0.08 wt%.

Chromium (Cr): 0.1% to 0.5% by weight
Cr is an element added to ensure strength, and also plays a role of increasing hardenability. In order to sufficiently obtain the above effects, it is necessary to add 0.1% by weight or more. However, if the Cr content exceeds 0.5% by weight, the hardness of the welded portion is excessively increased and the toughness is hindered. Therefore, the Cr content is preferably 0.1% by weight to 0.5% by weight.

Phosphorus (P): 0.01% by weight or less P is an element advantageous for strength improvement and corrosion resistance. However, since it significantly impairs impact toughness, P should be kept as low as possible. Therefore, the upper limit is preferably 0.01% by weight.

Sulfur (S): 0.01 wt% or less Since S is an element that forms MnS or the like and greatly impairs impact toughness, it should be kept as low as possible. Therefore, the upper limit is preferably 0.01% by weight.

Boron (B): 5 ppm to 30 ppm by weight
B is a very inexpensive additive element, is a useful element that exhibits a strong hardening ability and contributes greatly to the formation of bainite even during low-speed cooling in cooling after rough rolling.

Since the strength can be greatly improved by adding only a small amount, 5 ppm by weight or more is added. However, when the B content exceeds 30 ppm by weight, Fe ₂₃ (CB) ₆ is formed, conversely, the curability is lowered and the low temperature toughness is also greatly lowered. Therefore, the B content is preferably 5 ppm to 30 ppm by weight.

Nitrogen (N): 20 ppm by weight to 70 ppm by weight
N increases strength but greatly reduces toughness, so it is preferable to control N to 70 ppm by weight or less. However, controlling the N content to less than 20 ppm by weight increases the steelmaking load, so the lower limit of the N content is preferably 20 ppm by weight.

Calcium (Ca): 50 ppm by weight or less (excluding 0 ppm by weight)
Ca is mainly used as an element that suppresses non-metallic inclusions of MnS and improves low-temperature toughness. However, when Ca is added excessively, it reacts with oxygen contained in the steel to produce CaO which is a nonmetallic inclusion, and therefore the upper limit is preferably 50 ppm by weight.

Tin (Sn): 5 ppm to 50 ppm by weight
Sn is an element useful for ensuring corrosion resistance.

  It is preferable to add 5 ppm or more in terms of ensuring corrosion resistance. However, if the Sn content exceeds 50 ppm by weight, a larger amount of defects in the form of swelling or cracking of the surface of the steel material, such as water bubbles, occurs than the effect of contributing to the corrosion resistance improvement. Further, Sn increases the strength of the steel, but the upper limit is preferably 50 ppm by weight because it lowers the stretch ratio and the low temperature impact toughness.

  In the low yield ratio type high strength steel material of the present invention, the remaining component is iron (Fe). However, in a normal manufacturing process, unintended impurities are inevitably mixed from the raw material or the surrounding environment, and this cannot be excluded. These impurities can be easily understood by engineers having ordinary knowledge in the technical field, and therefore, the entire contents thereof are not described in detail in this specification.

  The low yield ratio type high-strength steel material having an advantageous steel composition according to the present invention can provide a sufficient effect even if it contains the alloy elements in the above-described content range, but it is 0.1 wt% to 0.5 wt%. By further including one or more of copper (Cu), 0.15 wt% to 0.3 wt% molybdenum (Mo), and 0.005 wt% to 0.3 wt% vanadium (V) Properties such as the strength, toughness of the steel material, the toughness of the weld heat affected zone, and the weldability can be further improved.

Copper (Cu): 0.1% to 0.5% by weight
Cu is an element that minimizes the toughness reduction of the base material and increases the strength. In order to sufficiently obtain the above effects, it is preferable to add 0.1% by weight or more. However, if the Cu content exceeds 0.5% by weight, the surface quality of the product is greatly impaired. Therefore, the Cu content is preferably 0.1% by weight to 0.5% by weight.

Molybdenum (Mo): 0.15 wt% to 0.3 wt%
Mo has the effect of greatly improving the curability even when added in a small amount, and in order to greatly improve the strength, it is necessary to add 0.15 wt% or more, but if added over 0.3 wt%, welding Increases the hardness of the part excessively and inhibits toughness. Therefore, the Mo content is preferably 0.15 wt% to 0.3 wt%.

Vanadium (V): 0.005% to 0.3% by weight
V has a lower solid solution temperature than other fine alloys, and has an effect of precipitating in the weld heat affected zone and preventing a decrease in strength. In order to sufficiently obtain the above effects, it is preferable to add 0.005% by weight or more. However, if the V content exceeds 0.3% by weight, the toughness is reduced. Therefore, the V content is preferably 0.005 wt% to 0.3 wt%.

  Further, the microstructure of the low yield ratio type high strength steel material of the present invention includes bainitic ferrite and granular bainite as main phases and MA (island martensite) as secondary phases.

  Bainitic ferrite contains many high-angle grain boundaries within the grain while maintaining the initial austenite grain boundary, and is useful for improving strength and impact toughness due to the effect of crystal grain refinement.

  Granular bainite maintains the initial austenite crystal grains like bainitic ferrite, but a secondary phase such as MA exists in the grains or at grain boundaries. There are no high-angle grain boundaries in the grains, which has a somewhat adverse effect on impact toughness, but the strength increases somewhat due to the presence of a large amount of low-angle grain boundaries such as intragranular dislocations.

  By including bainitic ferrite and granular bainite as main phases, a low yield ratio and high strength can be ensured.

  At this time, bainitic ferrite is 80% to 95% in area fraction, granular bainite is 5% to 20%, and M-A is 3% or less (including 0%).

  When the area fraction of bainitic ferrite is less than 80%, it is difficult to ensure high tensile strength, and when it exceeds 95%, the yield ratio increases.

  When the area fraction of granular bainite is less than 5%, not only the tensile strength but also the yield strength increases, and a low yield ratio cannot be secured. If it exceeds 20%, coarse initial austenite grains are effective. Cannot be made fine and the tensile strength is poor.

  The secondary phase such as M-A preferably has an area fraction of 3% or less as a microstructure useful for realizing a low yield ratio. When the area fraction of M-A exceeds 3%, the yield ratio decreases, but since it acts as a starting point for cracks against external stress, it is difficult to ensure high tensile strength.

  On the other hand, the low yield ratio type high strength steel material according to the present invention has PImax. (111) / PImax. (100) is 1.0 or more and 1.8 or less. PImax. (111) is the pole intensity (pole intensity, PImax.) Of the (111) crystal plane obtained by a method such as X-ray diffraction or electron backscattering diffraction. (100) is the pole strength of the (100) crystal plane.

  The pole strength of the crystal plane is determined by the final microstructure of the low yield ratio type high strength steel according to an embodiment of the present invention. When the main phase is bainitic ferrite and granular bainite, the higher the bainitic ferrite fraction, the higher the PImax. As the value of (111) increases and the fraction of granular bainite increases, PImax. The value of (100) increases. The final microstructure of the low-yield ratio type high-strength steel material according to the present embodiment shows that bainitic ferrite has a higher area fraction than granular bainite and PImax. (111) / PImax. When (100) is 1.8 or less, it is possible to produce a low yield ratio type high strength steel material. PImax. (111) / PImax. If (100) exceeds 1.8, the low yield ratio cannot be satisfied, so the upper limit is preferably 1.8 or less. More preferable PImax. (111) / PImax. (100) is 1.6 or less.

  PImax. (111) / PImax. When (100) is less than 1.0, the fraction of granular bainite is as high as more than 20%, and there is a problem that it is difficult to ensure high strength. Therefore, PImax. (111) / PImax. The lower limit of (100) is preferably 1.0 or more, and more preferably 1.2 or more.

  The low-yield ratio type high-strength steel material according to the present invention has a yield ratio of 0.85 or less and can be suitably used as a steel material for construction by ensuring a tensile strength of 800 MPa or more.

  Moreover, the thickness of the steel material by this invention is 60 mm or less.

  The low-yield ratio type high-strength steel material according to the present invention can ensure high strength and low yield ratio, and can reduce the plate thickness to 60 mm or less, so that machining and welding operations such as cutting and drilling are easy. become. Therefore, the thickness of the steel material is preferably 60 mm or less. More preferably, it is 40 mm or less, More preferably, it is 30 mm or less.

  The lower limit is not particularly limited, but may be 15 mm or more in order to be used as a steel material for construction structures.

  Hereinafter, the manufacturing method of the low yield ratio type high strength steel material by one Embodiment of this invention is demonstrated in detail.

  According to an embodiment of the present invention, a method of manufacturing a low yield ratio type high strength steel material includes a step of heating a slab having the above-described alloy composition to 1050 ° C. to 1250 ° C., and a rough heating of the heated slab at 950 ° C. to 1150 ° C. Rolling to obtain a bar, hot rolling the bar at a finish rolling temperature of 700 ° C. to 950 ° C. to obtain a hot rolled steel plate, and heating the hot rolled steel plate to 25 ° C./s to 50 ° C. Cooling to a cooling end temperature below the Bs temperature at a cooling rate of / s.

<Slab heating stage>
A slab having the above alloy composition is heated to 1050 ° C to 1250 ° C.

<Rough rolling stage>
The heated slab is roughly rolled at 950 ° C. to 1050 ° C. to obtain a bar.

  When the rough rolling temperature is less than 950 ° C., the austenite is deformed in a state where recrystallization does not occur. Therefore, the particles are coarsened. When the temperature exceeds 1050 ° C., the recrystallization occurs and the particles grow at the same time. Becomes coarse.

<Hot rolling stage>
A bar is hot-rolled at a finish rolling temperature of 700 to 950 ° C. to obtain a hot-rolled steel sheet.

  When the finish rolling temperature is less than 700 ° C., the temperature of the plate material is low, a load is generated on the rolling mill, and the rolling cannot be performed to the final thickness, and when it exceeds 950 ° C., recrystallization occurs during rolling.

  At this time, the rolling reduction of the hot rolling may be 50% to 80%.

  If the rolling reduction of finish rolling is less than 50%, the load acting on the material increases during rolling, and there is a risk of equipment accidents. If it exceeds 80%, the number of rolling passes increases to the rolling end temperature. The final thickness cannot be ensured.

<Cooling stage>
The hot-rolled steel sheet is cooled to a cooling end temperature below the Bs temperature at a cooling rate of 25 ° C./s to 50 ° C./s.

  When the cooling of the hot-rolled steel sheet is completed at a temperature exceeding the Bs temperature, bainitic ferrite and granular bainite cannot sufficiently undergo phase transformation, and the strength cannot be ensured. In the case of the cooling rate, there are physical restrictions depending on the thickness of the plate material, but at a cooling rate of less than 25 ° C./s, it is difficult to satisfy a tensile strength of 800 MPa or more by generating soft ferrite. Further, at a cooling rate exceeding 50 ° C./s, as the probability that martensite, which is a low temperature transformation structure, is increased, not only the tensile strength but also the yield strength increases, and it is difficult to satisfy the yield ratio of 0.85 or less. It is.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the following examples are merely specific examples of the present invention and do not limit the technical scope of the present invention.

  A slab satisfying the component system shown in Table 1 below was heated to 1160 ° C., roughly rolled at 1000 ° C., and then hot-rolled and cooled to meet the production conditions shown in Table 2 to obtain a steel material. The yield strength, tensile strength, yield ratio, and microstructure of this steel are measured and shown in Table 3.

  Further, the pole strength of the (100) crystal plane and (110) crystal plane of the steel material was measured, and PImax. (111) / PImax. The (100) values are shown in Table 3.

  Yield strength and tensile strength were measured using a universal tensile tester.

  The fine structure was observed with an optical microscope after the steel material was mirror-polished and chemically corroded.

  The pole strength and texture strength were measured using an X-ray diffractometer and an electron backscatter diffractometer.

  In Table 1, the unit of each element content is wt%.

  In Table 3 above, BF means bainitic ferrite, GB means granular bainite, MA means island martensite, AF means acicular ferrite, B means bainite, and the unit is area%.

  It can be seen that Invention Examples 1 to 9 satisfying the alloy composition and production conditions of the present invention ensure a low yield ratio of 0.85 or less and a tensile strength of 800 MPa or more.

  On the other hand, although Comparative Examples 1-3 satisfy | fills the alloy composition of this invention, it does not satisfy | fill manufacturing conditions and it is confirmed that a low yield ratio cannot be ensured or tensile strength is inferior. it can.

  Moreover, although the comparative examples 4, 7, and 8 satisfy | fill the manufacturing conditions of this invention, it can confirm that the alloy composition is not satisfy | filled and a low yield ratio cannot be ensured.

  Although the above has been described with reference to the embodiments, those skilled in the art can make various modifications of the present invention without departing from the technical scope of the present invention.

Claims (10)

Low yield ratio type high strength steel material,
Carbon (C) 0.02 wt% to 0.11 wt%, silicon (Si) 0.1 wt% to 0.5 wt%, manganese (Mn) 1.5 wt% to 2.5 wt% Aluminum (Al) 0.01 wt% to 0.06 wt%, Nickel (Ni) 0.1 wt% to 0.6 wt%, Titanium (Ti) 0.01 wt% to 0.03 wt% %, Niobium (Nb) 0.005 wt% to 0.08 wt%, chromium (Cr) 0.1 wt% to 0.5 wt%, phosphorus (P) 0.01 wt% or less (0 wt%) %), Sulfur (S) 0.01 wt% or less, boron (B) 5 ppm to 30 ppm, nitrogen (N) 20 ppm to 70 ppm, calcium (Ca) 50 wt. ppm or less (excluding 0 ppm by weight), tin (Sn) 5 ppm to 50 ppm by weight, the rest being iron Fe) and low yield ratio high-strength steel material, characterized in that it consists of other unavoidable impurities.   The low-yield ratio type high-strength steel material is 0.1 wt% to 0.5 wt% copper (Cu), 0.15 wt% to 0.3 wt% molybdenum (Mo), and 0.005 wt%. The low yield ratio type high-strength steel material according to claim 1, further comprising at least one of ˜0.3 wt% vanadium (V).   The microstructure of the low yield ratio type high-strength steel material includes bainitic ferrite and granular bainite as main phases and MA (island martensite) as a secondary phase. The low yield ratio type high strength steel described.   In terms of area fraction, the bainitic ferrite is 80% to 95%, the granular bainite is 5% to 20%, and the M-A is 3% or less (including 0%). The low yield ratio type high strength steel material according to claim 3. PImax. Is the ratio of the pole strength (PI intensity) between the (100) crystal face and the (111) crystal face of the low yield ratio type high strength steel material. (111) / PImax. (100) is 1.0 or more and 1.8 or less, The low yield ratio type | mold high-strength steel material of Claim 1 characterized by the above-mentioned.
(Here, the PImax. (111) is the pole intensity of the (111) crystal plane, and the PImax. (100) is the pole intensity of the (100) crystal plane.)   The low yield ratio type high strength steel material according to claim 1, wherein the low yield ratio type high strength steel material has a yield ratio of 0.85 or less and a tensile strength of 800 MPa or more.   The low yield ratio type high strength steel material according to claim 1, wherein a thickness of the low yield ratio type high strength steel material is 60 mm or less. Carbon (C) 0.02 wt% to 0.11 wt%, silicon (Si) 0.1 wt% to 0.5 wt%, manganese (Mn) 1.5 wt% to 2.5 wt% Aluminum (Al) 0.01 wt% to 0.06 wt%, Nickel (Ni) 0.1 wt% to 0.6 wt%, Titanium (Ti) 0.01 wt% to 0.03 wt% %, Niobium (Nb) 0.005 wt% to 0.08 wt%, chromium (Cr) 0.1 wt% to 0.5 wt%, phosphorus (P) 0.01 wt% or less (0 wt%) %), Sulfur (S) 0.01 wt% or less, boron (B) 5 ppm to 30 ppm, nitrogen (N) 20 ppm to 70 ppm, calcium (Ca) 50 wt. ppm or less (excluding 0 ppm by weight), tin (Sn) 5 ppm to 50 ppm by weight, the rest being iron A step of heating Fe) and slabs made of other inevitable impurities 1050 ° C. to 1250 ° C.,
Rough-rolling the heated slab at 950 ° C. to 1050 ° C. to obtain a bar;
Hot rolling the bar at a finish rolling temperature of 700 ° C. to 950 ° C. to obtain a hot rolled steel sheet;
Cooling the hot-rolled steel sheet to a cooling end temperature below the Bs temperature at a cooling rate of 25 ° C./s to 50 ° C./s;
A method for producing a low yield ratio type high strength steel material, characterized by comprising:   The slab comprises 0.1 wt% to 0.5 wt% copper (Cu), 0.15 wt% to 0.3 wt% molybdenum (Mo), and 0.005 wt% to 0.3 wt% The method for producing a low yield ratio type high strength steel material according to claim 8, further comprising at least one of vanadium (V).   The method for producing a low yield ratio type high strength steel material according to claim 8, wherein the hot rolling is performed at a reduction rate of 50% to 80%.