KR101568514B1 - High strength structural steel having low yield ratio and preparing method for the same - Google Patents

Abstract

The present invention relates to a chromium (Cr) alloy comprising 0.02 to 0.10% by weight of carbon (C), 2.0 to 4.0% by weight of manganese (Mn), 0.01 to 0.8% ): 1.0 to 2.0 wt%, Cu: 0.01 to 1.0 wt%, Niobium (Nb): 0.005 to 0.10 wt%, V: 0.005 to 0.3 wt%, and Ti: 0.005 to 0.1 wt Sulfur (S): not more than 0.01 wt% (excluding 0 wt%), boron (B): 5 to 40 wt ppm, nitrogen (N) (Al) and a chromium (Cr) component are contained in an amount of 15 to 150 ppm by weight, Ca: 60 ppm by weight or less (excluding 0 ppm by weight), the balance of Fe and other unavoidable impurities. To 1.5 to 3.5 wt%, and a method of manufacturing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-

The present invention relates to a steel material for construction and a method of manufacturing the steel material, and more particularly, to a steel material having a high tensile strength capable of securing a low yield ratio and having a high strength for construction.

Recently, the development of superstructures and high strength steels has been demanded as domestic and overseas buildings, bridges, and other structures are progressing into superstructure and long span. The use of high-strength steel has a high permissible stress, which makes it possible to rationalize and lighten the construction and the bridge structure, making economical construction possible, as well as being able to thin the plate thickness, facilitating machining and welding operations such as cutting and drilling It becomes.

On the other hand, when increasing the strength of the steel, the yield ratio (yield strength / tensile strength), which is the ratio of the tensile strength to the yield strength, often increases. When the yield ratio increases, destruction occurs at the point of plastic deformation (yield point) There is not a large margin for preventing the building from absorbing energy due to deformation due to deformation, and there is a problem in that it is difficult to secure safety when a large external force such as an earthquake acts. Therefore, the structural steel needs to satisfy both high strength and low resistance.

On the other hand, in general, the yield ratio of the steel material is such that the metal structure of the steel is made of a soft phase such as ferrite as a main structure and a hard phase such as bainite or martensite (Hard phase) can be lowered by implementing a suitably dispersed structure.

In order to obtain a structure in which the hard phase is appropriately dispersed in the soft phase-based microstructure described above, Japanese Patent Laid-Open No. 55-97425 discloses a quenching method in a dual phase region of ferrite and austenite, And lowering the yield ratio through tempering. However, since this method adds a number of heat treatment processes in addition to the rolling manufacturing process, it is inevitable that the productivity is lowered as well as the manufacturing cost is increased.

Therefore, with the above-described conventional techniques, there is a limit in manufacturing a steel for construction having an ultra-high strength and a low resistance ratio, while solving problems such as a decrease in productivity and an increase in manufacturing cost.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a construction steel material having a super high strength and a low resistance ratio without increasing the manufacturing cost and a manufacturing method thereof.

In one aspect, the present invention provides a carbon nanotube composite material comprising carbon (C) in an amount of 0.02 wt% or more and less than 0.10 wt%, manganese (Mn) in 2.0 to 4.0 wt%, silicon (Si) (Nb): 0.005 to 0.10 wt.%, Vanadium (V): 0.005 to 0.3 wt.%, Titanium (Cu): 0.1 to 1.5 wt.%, Chromium (Cr): 0.5 to 2.0 wt. S: not more than 0.01% by weight (excluding 0% by weight), boron (B): not more than 0.01% by weight, (Al) is contained in an amount of not more than 40 ppm by weight, nitrogen (N): 15 to 150 ppm by weight, calcium (Ca): not more than 60 ppm by weight (excluding 0 ppm by weight), the balance of Fe and other unavoidable impurities. And Cr (Cr) components is 1.6 wt% or more and less than 3.5 wt%.

The steel material may further include at least one selected from the group consisting of molybdenum (Mo): 0.01 to 1.0 wt% and nickel (Ni): 0.01 to 2.0 wt%.

On the other hand, the microstructure of the steel is preferably a mixed structure in which the ferrite area fraction is 15 to 50% and the remainder is at least one of bainite and martensite.

The steel material preferably has a yield ratio of 0.75 or less, more preferably a yield ratio of 0.70 or less, and a tensile strength of 800 MPa or more.

In another aspect, the present invention provides a method of producing a carbon nanotube, comprising (1) carbon (C): 0.02 wt% to less than 0.10 wt%, manganese (Mn): 2.0 to 4.0 wt%, silicon (Si) : 0.5 to 1.5% by weight, Cr: 0.5 to 2.0%, Cu: 0.01 to 1.0%, Nb: 0.005 to 0.10%, V: 0.005 to 0.3% (Excluding 0% by weight), sulfur (S): 0.01% by weight or less (excluding 0% by weight), boron (B) (Fe) and other unavoidable impurities, wherein the aluminum (Al) contains 5 to 40 wt. Ppm, N (N): 15 to 150 wt ppm, Ca Reheating slabs having a total of Al and Cr components of 1.6 wt% or more and less than 3.5 wt% to 1050 to 1250 캜; (2) _rough rolling the reheated slab at austenite recrystallization temperature (T _nr ) to 1250 ° C; (3) subjecting the rough-rolled slab to finish rolling at a bainite transformation starting temperature (B _s ) +50 ° to T _nr ° C; And (4) cooling the finished slab at a cooling rate of 5 DEG C / s or higher to a temperature not higher than the finishing temperature of the bainite ( _Bf ) DEG C .

The slab may further include at least one selected from the group consisting of 0.01-1.0 wt% of molybdenum (Mo) and 0.01-2.0 wt% of nickel (Ni).

On the other hand, it is preferable that the microstructure of the steel material produced by the above-mentioned method has a ferrite area fraction of 15 to 50% and the remainder is at least one of bainite and martensite.

The steel material produced by the above-described method preferably has a yield ratio of 0.75 or less and a tensile strength of 800 MPa or more.

According to the present invention, it is possible to produce a steel for construction having an ultra high strength and a low resistance ratio by a simple method without increasing the manufacturing cost.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a microstructure photograph of an example of a steel material according to the present invention observed with an optical microscope. FIG.
FIG. 2 is a graph showing the ferrite fraction and the yield ratio change according to the content of aluminum (Al) and chromium (Cr) in the steel of the present invention.
3 is a graph showing changes in ferrite fraction and tensile strength according to the content of aluminum (Al) and chromium (Cr) in the steel material according to the present invention.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

As a result of extensive research, the inventors of the present invention have found that a copper alloy containing at least one element selected from the group consisting of copper, manganese, silicon, aluminum, chromium, copper, niobium, vanadium, titanium, (Al) and chromium (Cr), in particular Ti, P, S, B, N, Ca and F, Of the slab is 1.6 wt% or more and less than 3.5 wt%, it is possible to produce a new steel having a high strength and a low yield ratio in a simple manner without increasing the production cost. Completed.

Specifically, the steel of the present invention has a carbon (C) content of not less than 0.02 wt% and less than 0.10 wt%, a manganese (Mn) content of 2.0 to 4.0 wt%, a silicon (Si) content of 0.01 to 0.8 wt% 0.5 to 1.5 wt% of aluminum (Al), 0.5 to 2.0 wt% of chromium (Cr), 0.01 to 1.0 wt% of copper, 0.005 to 0.10 wt% of niobium, vanadium (V) 0.005-0.3 wt%, titanium (Ti): 0.005-0.1 wt%, phosphorus (P): 0.02 wt% or less (excluding 0 wt%), sulfur (S) 5 to 40 weight ppm of boron (B), 15 to 150 weight ppm of nitrogen (N), 60 weight ppm or less of calcium (Ca is excluded), remaining iron (Fe) and other unavoidable impurities , And the sum of the aluminum (Al) and chromium (Cr) components is 1.6 wt% or more and less than 3.5 wt%.

First, the reason for adding each component constituting the steel composition of the present invention and the appropriate content range thereof will be described in detail.

Carbon (C): 0.02 wt% or more and less than 0.10 wt%

C is included in an appropriate range since it is an important element for forming bainite or martensite and determining the size and fraction of bainite or martensite to be formed. However, when the content of C is 0.10% by weight or more, the low-temperature toughness is lowered. When the content of C is less than 0.02% by weight, the formation of bainite or martensite is hindered and the strength is lowered. Therefore, the content of C is preferably 0.02% . On the other hand, in the case of a plate used as a steel structure for welding, the content of C is more preferably 0.03 to 0.08% by weight for better weldability.

Manganese (Mn): 2.0 to 4.0 wt%

Since Mn is a useful element for improving the strength by solid solution strengthening, it is necessary to add at least 2.0% by weight. However, when the Mn content exceeds 4.0 wt%, the toughness of the welded portion may be significantly lowered due to an increase in the hardenability, and the content of Mn is preferably 2.0 to 4.0 wt%.

Silicon (Si): 0.01 to 0.8 wt%

Si is used as a deoxidizer, which helps improve strength and toughness. However, if it exceeds 0.8% by weight, low temperature toughness and weldability may be deteriorated. On the other hand, if it is less than 0.01% by weight, the effect of deoxidation may be insufficient. Therefore, the content of Si is preferably 0.01 to 0.8% by weight, more preferably 0.1 to 0.4% by weight.

Aluminum (Al): 0.5 to 1.5 wt%

Al is an element capable of deoxidizing molten steel at low cost and stabilizing ferrite, and is preferably contained in an amount of not less than 0.5% by weight in order to exhibit a sufficient effect. However, if it exceeds 1.5% by weight, nozzle clogging may occur during continuous casting.

Cr (Cr): 0.5 to 2.0 wt%

Cr is an element capable of increasing the hardenability and increasing the strength of the steel and also stabilizing the ferrite. In order to exhibit such an effect in the present invention, Cr is preferably contained in an amount of 0.5 wt% or more. However, if it exceeds 2.0% by weight, the weldability is deteriorated.

Aluminum + chromium (Al + Cr): 1.6 wt% or more and less than 3.5 wt%

Al and Cr are elements stabilizing ferrite. It is preferable to add 1.6 wt% or more to ensure a ferrite fraction in an ultra-high strength steel. When 3.5 wt% or more is added, however, the ferrite fraction is excessively formed, have. Therefore, the sum of Al and Cr is preferably 1.6 wt% or more and less than 3.5 wt%, and more preferably 1.6 wt% or more and 3.0 wt% or less when considering the margin of tensile strength.

Copper (Cu): 0.01 to 1.0 wt%

Cu is an element capable of minimizing toughness deterioration of the base material and at the same time increasing the strength thereof. In order to sufficiently obtain the effect, 0.01 wt% or more of Cu should be added. Excessive addition of Cu greatly deteriorates the surface quality of the product. %.

Niobium (Nb): 0.005 to 0.1 wt%

Nb is an important element in the production of TMCP steel and precipitates in the form of NbC or NbCN, which greatly improves the strength of the base material and the weld. In addition, Nb dissolved at the time of reheating at a high temperature suppresses recrystallization of austenite and transformation of ferrite or bainite, thereby exhibiting an effect of making the structure finer. Further, in the present invention, not only bainite is formed at a low cooling rate when the slab is cooled after rough rolling, but also promotes the formation of martensite even at low speed cooling by increasing the stability of austenite even after cooling after final rolling Also. Therefore, Nb should be added in an amount of 0.005 wt% or more, but if it is added in excess of 0.1 wt%, brittle cracks may appear at the edge of the steel, so that the content is preferably 0.005 to 0.1 wt%.

Vanadium (V): 0.005-0.3 wt%

V is low in temperature to be employed as compared with other fine alloys and has an effect of preventing precipitation in the weld heat affected portion and preventing the decrease in strength, so it is added in an amount of 0.005 wt% or more. However, an addition amount exceeding 0.3% by weight may lower the toughness. Therefore, the content of V is preferably 0.005 to 0.3% by weight.

Titanium (Ti): 0.005 to 0.1 wt%

Ti improves the low-temperature toughness by suppressing the growth of crystal grains during reheating. It is added in an amount of 0.005 weight% or more, but excessive addition of more than 0.1 weight% causes problems such as clogging of performance nozzles and reduction in low- It is preferable that the content of Ti is 0.005 to 0.1% by weight.

Phosphorus (P): not more than 0.02% by weight

P is an element favoring strength improvement and corrosion resistance, but it is advantageous to keep it as low as possible since the impact toughness can be greatly inhibited. The upper limit is preferably 0.02 wt%.

Sulfur (S): not more than 0.01% by weight

Since S is an element that significantly inhibits impact toughness by forming MnS or the like, it is advantageous to keep it as low as possible, and the upper limit is preferably 0.01 wt%.

Boron (B): 5 to 40 ppm by weight

B is a very low cost additive element and exhibits a strong curing ability and is a beneficial element contributing greatly to the formation of bainite even in the cooling after the rough rolling and at the low cooling rate. However, if it is added in an excess amount, Fe23 (CB) 6 is formed and the hardenability is lowered and the low-temperature toughness is largely lowered, Ppm by weight.

Nitrogen (N): 15 to 150 ppm by weight

N is preferably less than or equal to 150 ppm by weight because it increases the strength while greatly reducing toughness. However, since the N content control of less than 15 ppm by weight increases the steelmaking load, the lower limit of the N content is preferably 15 ppm by weight.

Calcium (Ca): not more than 60 ppm by weight

Ca is mainly used as an element for suppressing non-metallic inclusions of MnS and improving low-temperature toughness. However, excessive Ca addition reacts with oxygen contained in the steel to produce CaO, which is a nonmetallic inclusion. Therefore, the upper limit of the Ca content is preferably 60 ppm by weight.

The remainder of the present invention is iron (Fe). However, in the ordinary steel 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 steel making.

The steel material having an advantageous steel composition of the present invention as described above can obtain a sufficient effect even if it contains an alloy element in the above-mentioned content range. However, it can improve the properties such as strength, toughness, The following alloying elements can be additionally added in an appropriate range. The following alloying elements may be added singly or in combination of two or more.

Molybdenum (Mo): 0.01 to 1.0 wt%

The addition of a small amount of Mo can greatly improve the hardenability. Therefore, it is necessary to add Mo at a content of 0.01% or more, but if the Mo content exceeds 1.0%, the hardness of the weld portion is excessively increased, Therefore, it is preferable to add 1.0% or less.

Nickel (Ni): 0.01 to 2.0 wt%

Since Ni is an element capable of simultaneously improving the strength and toughness of the base material, it is necessary to add at least 0.01% by weight in order to sufficiently obtain the effect. However, since Ni is an expensive element, addition of an amount exceeding 2.0% And the weldability is lowered, so that the addition amount is preferably 0.01 to 2.0%.

Next, the microstructure of the steel material of the present invention will be described in detail.

As shown in Fig. 1, the steel microstructure of the present invention has a ferrite content of 15 to 50%, more preferably 15 to 40%, in terms of an area fraction, and the remainder is a mixture of bainite and martensite It is preferable to have a structure. The steel according to the present invention having such a microstructure can have both a high tensile strength and a low yield ratio.

More specifically, the steel material of the present invention has an increased area fraction of ferrite when the content of Al and Cr is increased, and the ferrite is a relatively soft structure, so that the steel plays a role of yielding at low strength, (See FIG. 2). However, when the content of Al and Cr increases and the area fraction of ferrite increases, there is a problem that bainite or martensite, which keeps the tensile strength of the steel at a high level, decreases, resulting in a decrease in tensile strength 3). Therefore, in order to have both the high tensile strength and the low yield ratio of the steel material of the present invention, the area fraction of the ferrite must satisfy the appropriate range as described above.

Next, a method of manufacturing the steel material of the present invention will be described in detail.

A method of manufacturing a steel material according to the present invention comprises the steps of (1) carbon (C): 0.02 wt% or more and less than 0.10 wt%, manganese (Mn): 2.0 to 4.0 wt%, silicon (Si) : 0.5 to 1.5% by weight, Cr: 0.5 to 2.0%, Cu: 0.01 to 1.0%, Nb: 0.005 to 0.10%, V: 0.005 to 0.3% (Excluding 0% by weight), sulfur (S): 0.01% by weight or less (excluding 0% by weight), boron (B) (Fe) and other unavoidable impurities, wherein the aluminum (Al) contains 5 to 40 wt. Ppm, N (N): 15 to 150 wt ppm, Ca Reheating slabs having a total of Al and Cr components of 1.6 wt% or more and less than 3.5 wt% to 1050 to 1250 캜; (2) _rough rolling the reheated slab at austenite recrystallization temperature (T _nr ) to 1250 ° C; (3) subjecting the rough-rolled slab to finish rolling at a bainite transformation starting temperature (B _s ) +50 ° to T _nr ° C; And (4) cooling the scrap-rolled slab at a cooling rate of 5 DEG C / s or higher to a finish temperature of the bainite ( _Bf ) DEG C or lower.

As described above, the steel material manufacturing process of the present invention includes a step of reheating the slab, a rough rolling step, a finishing rolling step, and a cooling step, and detailed conditions for each step are as follows.

Slab reheating temperature: 1050 ~ 1250 ℃

In the present invention, the heating temperature in the reheating of the steel sheet is set to 1050 DEG C or more in order to sufficiently solidify the carbonitride of Ti and / or Nb formed during the casting. However, when reheating to an excessively high temperature, austenite may be coarsened, so that the upper limit of the slab reheating temperature is preferably 1250 ° C.

Rough rolling temperature: 1250 to T _nr ° C

The reheated steel sheet is subjected to rough rolling after heating to adjust its shape. The rolling temperature is not lower than the temperature ( _Tnr ) at which recrystallization of the austenite is stopped, and the cast structure such as dendrites formed during the casting is broken by rolling, and the austenite can be made finer.

Finishing rolling conditions: T _nr to B _s + 50 ° C

The austenitic structure of the rough-rolled steel sheet is subjected to finish rolling to introduce uneven microstructure. The finishing rolling temperature is preferably at or above the bainite transformation starting temperature (B _s ) + 50 ° C. In this case, it is possible to suppress the transformation of the ferrite before the start of cooling in the accelerator cooler. On the other hand, when the initiation of finishing rolling is carried out at a high temperature exceeding T _nr , the yield strength may increase and it may be difficult to obtain a resistance ratio of 0.75 or less.

Cooling conditions after rolling: Cooling to below B _f ° C at a cooling rate of 5 ° C / s or higher

When the cooling is terminated at a cooling rate lower than 5 ° C / s or at a temperature above the B _f temperature, which is the Vernite finish temperature, bainite or martensite, which is a hard phase, is not formed and the tensile strength is likely to be less than 800 MPa.

In summary, in the steel material manufacturing method of the present invention, after the slab having the composition described above is reheated in the temperature range of 1000 to 1250 ° C, rolling is terminated at the bainite transformation start temperature Bs + 50 ° C or higher, Or more at a cooling rate equal to or lower than the Bernite finish temperature to form a mixed structure having a ferrite area fraction of 15 to 50% in the microstructure of the steel and at least one of bainite and martensite as the remainder.

Hereinafter, the present invention will be described more specifically by way of examples.

( Example )

The slabs satisfying the composition shown in Table 1 were rolled and cooled to conform to the production conditions shown in Table 2, and then the yield strength, tensile strength, ferrite area fraction, and yield ratio were measured and shown in Table 3 below.

The alloying elements denoted by * in each of the components listed in Table 1 are expressed in terms of ppm by weight, and the units of other alloying elements are wt%.

No. Rough rolling condition Finish rolling condition Cooling conditions Other Steel grade number Slab
thickness Reheating
Extraction temperature Rough rolling
Termination temperature Rolling start
Temperature Rolling finish
Temperature Cooling rate Cooling
Termination temperature Tnr Bs Bf Inventive Steel A A-1 244 1065 985 938 898 15.0 341 950 491 371 A-2 244 1080 1000 918 878 23.0 321 A-3 244 1110 1030 920 880 2.0 301 A-4 220 1050 970 850 810 9.0 451 Invention steel B B-1 244 1070 990 784 744 13.0 219 975 369 249 B-2 244 1075 995 764 724 21.0 199 B-3 244 1105 1025 820 780 2.0 179 B-4 220 1060 980 850 810 11.0 329 Inventive Steel C C-1 244 1080 1000 990 950 10.0 335 960 525 405 C-2 244 1070 990 970 930 25.0 355 C-3 244 1100 1020 820 780 3.0 335 C-4 220 1055 975 850 810 9.0 485 Inventive Steel D D-1 244 1080 1000 919 879 10.0 331 970 521 401 D-2 244 1070 990 899 859 15.0 351 D-3 244 1100 1020 820 780 2.5 331 D-4 220 1055 975 850 810 9.0 481 Invention steel E E-1 244 1080 1000 886 846 10.0 242 960 447 327 E-2 244 1070 990 866 826 19.0 277 E-3 244 1100 1020 820 780 3.0 257 E-4 220 1055 975 850 810 9.0 407 Comparative Steel F F-1 244 1080 1000 875 835 10.0 379 980 537 417 F-2 244 1070 990 855 815 7.0 399 Comparative Steel G G-1 244 1080 1000 963 923 10.0 379 975 608 488 G-2 244 1070 990 943 903 7.0 529 Comparative Steel H H-1 244 1080 1000 754 714 10.0 367 985 528 408 H-2 244 1070 990 734 694 7.0 337 Comparative Steel I I-1 244 1080 1000 955 915 10.0 438 970 632 512 I-2 244 1070 990 935 895 7.0 408

In Table 2, the unit of the thickness of the sleeve is mm, and the units of temperature except for the thickness are all degrees Celsius. 4, C-3, and E-3 are conditions under which the cooling rate is lower than the cooling rate proposed in the present invention, and A-3, B-3, 4, D-4 and E-4 are conditions in which the cooling termination temperature is higher than the temperature proposed in the present invention. All the others meet the conditions proposed by the present invention. Meanwhile, Tnr was measured with a high-temperature torsion tester, and B _s and B _f were measured by a dilator meter capable of measuring volume and expansion at the time of transformation from austenite to bainite in a continuous cooling transformation process. The inflection point at which the volume was changed was measured, and the temperature was measured at that time.

Steel grade number Product thickness (mm) YS (MPa) TS (Mpa) Ferrite fraction (%) YR (%) Inventive Steel A A-1 80 529 813 18.9 65 A-2 25 577 861 18.2 67 A-3 65 494 778 20.1 63 A-4 60 423 708 19.4 60 Invention steel B B-1 80 497 811 29.9 61 B-2 25 541 855 29.2 63 B-3 65 462 776 30.9 60 B-4 60 432 746 30.1 58 Inventive Steel C C-1 80 587 878 26.5 67 C-2 25 643 933 25.2 69 C-3 65 549 790 27.3 69 C-4 60 472 763 26.6 62 Inventive Steel D D-1 80 522 829 35.8 63 D-2 25 532 839 35.4 63 D-3 65 488 795 36.5 61 D-4 60 407 715 35.9 57 Invention steel E E-1 80 582 887 33.9 66 E-2 25 606 911 33.1 67 E-3 65 543 795 34.6 68 E-4 60 480 785 34.0 61 Comparative Steel F F-1 65 331 630 62.1 53 F-2 20 331 630 62.3 53 Comparative Steel G G-1 65 232 509 71.8 46 G-2 20 234 512 72.1 46 Comparative Steel H H-1 65 267 530 77.4 50 H-2 20 267 529 77.6 50 Comparative Steel I I-1 65 725 883 1.8 82 I-2 20 728 886 2.0 82

In Table 3, YS means yield strength, TS means tensile strength, and YR means yield ratio. The yield strength and tensile strength were measured by a uniaxial tensile test using a tensile tester. In the case of high-strength steels, continuous yielding occurs in the tensile test. The yield strength is measured with an elongation of 0.2% off-set. The tensile strength is the tensile strength at which the highest strength is obtained when uniform stretching is terminated and local necking occurs and the strength value begins to fall.

As shown in Table 3, the steel materials A-1, A-2, B-1, B-2, C-1, C-2, D-1, D- , And E-2 exhibited both tensile strengths of 800 MPa or more and yield ratios of 70% or less, satisfying both of low yield ratio and high tensile strength.

On the other hand, in the case of the steel materials F-1, F-2, G-1, G-2, H-1 and H-2 produced using slabs containing not less than 3.5% by weight of Al + Cr, And more than 50%, which means that the tensile strength is extremely low.

Further, in the case of steels I-1 and I-2 produced using slabs containing less than 1.5% by weight of Al + Cr, the ferrite area fraction is less than 15%, and the yield ratio is very high .

On the other hand, A-3, B-3, C-3, D-3, and E-3 produced at a cooling rate of less than 5 ° C / s; A-4, B-4, C-4, D-4 and E-4, which were prepared under the condition that the cooling end temperature was higher than the bainite finish temperature and B _f , It can be seen that ultra high strength of 800 MPa or more can not be obtained.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, It will be obvious to those of ordinary skill in the art.

Claims (8)

(C): 0.02 wt% or more and less than 0.10 wt%, manganese (Mn): 2.0 to 4.0 wt%, silicon (Si): 0.01 to 0.8 wt% 0.005-0.1 wt% of niobium (Nb), 0.005-0.3 wt% of vanadium (V), 0.005-0.1 wt% of titanium (Ti) Sulfur (S): not more than 0.01 wt% (excluding 0 wt%), boron (B): 5 to 40 wt ppm, nitrogen (N) ): 15 to 150 ppm by weight, calcium (Ca): 60 ppm by weight or less (excluding 0 ppm by weight), the balance of iron (Fe) and other unavoidable impurities,
Wherein the sum of the aluminum (Al) and chromium (Cr) components is 1.6 wt% or more and less than 3.5 wt%
The microstructure is a mixed structure in which the ferrite area fraction is 15% to 40% and the remainder is at least one of bainite and martensite,
A yield ratio of 0.75 or less, and a tensile strength of 800 MPa or more.
The method according to claim 1,
Wherein the steel material further comprises at least one selected from the group consisting of molybdenum (Mo): 0.01 to 1.0 wt% and nickel (Ni): 0.01 to 2.0 wt%.
delete delete (C): 0.02 wt% or more and less than 0.10 wt%, manganese (Mn): 2.0 to 4.0 wt%, silicon (Si): 0.01 to 0.8 wt%, aluminum (Al) 0.005-0. 0 wt.% Of niobium (Nb), 0.005-0.3 wt.% Of vanadium (V), 0.005 wt.% Of titanium (Ti) (S): not more than 0.01% by weight (excluding 0% by weight), boron (B): 5 to 40% by weight, (Al) and chromium (Cr) are contained in an amount of 15 to 150 ppm by weight of nitrogen (N), 60 ppm by weight or less (excluding 0 ppm by weight) of calcium (Ca), the balance of Fe and other unavoidable impurities. ) Component is 1.6 wt% or more and less than 3.5 wt% is reheated to 1050 to 1250 캜;
(2) _rough rolling the reheated slab at austenite recrystallization temperature (T _nr ) to 1250 ° C;
(3) subjecting the rough-rolled slab to finish rolling at a bainite transformation starting temperature (B _s ) +50 ° to T _nr ° C; And
(4) cooling the finely-rolled slab at a cooling rate of 5 DEG C / s or more to a finish temperature of the bainite ( _Bf ) DEG C or lower,
The microstructure of the steel material produced by the above-mentioned method is a mixed structure having a ferrite area fraction of 15% to 40% and the remainder being at least one of bainite and martensite,
Wherein the steel material produced by the above production method has a yield ratio of 0.75 or less and a tensile strength of 800 MPa or more.
6. The method of claim 5,
Wherein the slab further comprises at least one selected from the group consisting of molybdenum (Mo): 0.01 to 1.0 wt% and nickel (Ni): 0.01 to 2.0 wt%.
delete delete

Images