KR100833075B1 - High strength and low yield ratio steel for structure having excellent low temperature toughness and brittle crack arrest property and producing method of the same - Google Patents

Abstract

A high strength steel that satisfies low temperature toughness, brittle crack arrest properties, and low yield ratio is provided, and a manufacturing method of the high strength steel is provided. A high strength and low yield ratio steel for structure having excellent low temperature toughness and brittle crack arrest properties has a steel composition comprising, by weight percent, 0.03 to 0.12% of C, 0.01 to 0.8% of Si, 0.3 to 2.5% of Mn, 0.02% or less of P, 0.01% or less of S, 0.005 to 0.5% of Al, 0.005 to 0.1% of Nb, 3 to 50 ppm of B, 0.005 to 0.1% of Ti, 15 to 150 ppm of N, and the balance of Fe and inevitable impurities, and has a structure comprising martensite-austenite constituent with an average size of 10 mum or less and an area fraction of 3 to 15%, and polygonal ferrite phase with an average size of 5 mum or less and an area fraction of 85 to 97%. The steel composition further comprises one or more selected from the group consisting of 0.05 to 1.0% of Cr, 0.01 to 1.0% of Mo, 0.01 to 2.0% of Ni, 0.01 to 1.0% of Cu, and 0.005 to 0.3% of V.

Description

STRENGTH AND LOW YIELD RATIO STEEL FOR STRUCTURE HAVING EXCELLENT LOW TEMPERATURE TOUGHNESS AND BRITTLE CRACK ARREST PROPERTY AND PRODUCING METHOD OF THE SAME}

1 is a photograph of the tissue of the steel provided by the present invention observed with a scanning electron microscope, and

Figure 2 is a photograph observed by quenching the steel sheet structure after the second reheating during the manufacturing process of the present invention.

The present invention relates to a high-strength resistive ratio structural steel having excellent low temperature toughness and brittle crack propagation stopping characteristics, and a method of manufacturing the same. The main properties required for structural steel using a second phase having high microstructure and high hardness The present invention relates to a high-strength steel and a method for manufacturing the same, which satisfy all of excellent low-temperature toughness, brittle crack propagation stopping properties, and low yield ratio.

Structures such as buildings, bridges, pressure vessels, pipes and the like are often required to have high strength due to the large loads applied to the structures. In addition, since the total weight of the steel is continuously reduced due to the continuous demand for cost reduction input during the manufacture of the structure, the demand for increasing the strength of the steel has become an irresistible trend.

However, as the strength of the steel increases, properties such as low temperature toughness or brittle crack propagation stop characteristic of the steel are generally lowered, and therefore, high strength structural steels tend to have poor low temperature toughness and brittle crack propagation stop characteristics. Low temperature toughness is a measure of how brittle fractures occur at cryogenic temperatures. In general, ductile brittle transition temperature is often used as a measure. If low temperature toughness is weak, brittleness is easily brittle with steel. Because of the destruction, the environment that can be used is inevitably limited. The brittle crack propagation stop characteristic is a measure of how effectively the brittle crack propagates can be stopped.The brittle crack propagation stop characteristic must be excellent to suppress brittle fracture of steel. can do.

In addition, when the strength of the steel is increased, the yield ratio (yield strength / tensile strength), which is a ratio of tensile strength and yield strength, often increases, but when the yield ratio increases, fracture occurs at the point of plastic deformation (yield point). Since the stress difference to the point of view is not large, the building does not have enough room to absorb energy due to deformation and prevent destruction, and it is difficult to ensure safety when a huge external force such as an earthquake is applied.

Therefore, structural steels need to be excellent in both low temperature toughness, brittle crack propagation stopping characteristics, and resistance ratio.

As a invention for improving low-temperature toughness of steel materials, Japanese Unexamined Patent Application Publication No. 58-005967 is mentioned. This document describes a method of improving toughness by appropriately adjusting the steel components and rolling conditions. The low temperature toughness is secured by securing a fine ferrite grain size by adjusting the component system to an appropriate range and rolling in two phases. Was intended.

When the steel is rolled in the two-phase temperature range in which ferrite and austenite coexist, the ferrite produced by transformation becomes processed ferrite, which is refined by processing, so that the toughness of the steel can be improved.

However, when the steel is simply rolled in two phases, it is difficult to obtain a sufficient structure refining effect, and there may be a problem such that rolling resistance is increased because rolling is performed in a low temperature range over the entire thickness of the steel.

In addition, as an invention for improving the brittle crack propagation stop characteristic of steel, Japanese Patent Laid-Open No. 4-141517 discloses a technique for improving the brittle crack propagation stop characteristic by applying an ultrafine grain structure to the surface layer portion. However, the above technique is only for improving the brittle crack propagation stop characteristics, and it is difficult to secure comprehensive resistance to fracture such as toughness, ductility, brittle crack propagation stop characteristics, etc. by forming the super fine grain structure at the surface layer. Do.

In addition, the technique disclosed in Japanese Patent Laid-Open No. 4-141517 is far from a technology that gives a resistance to earthquake ratio having a great effect on the improvement of the seismic resistance.

An object of the present invention is to provide a high-strength steel and a method for manufacturing the same, which satisfy all of the above-described problems of the prior art and satisfy both low temperature toughness, brittle crack propagation stopping characteristics and low yield ratio.

According to an aspect of the present invention for achieving the above object, C: 0.03 ~ 0.12% (hereinafter, the content of each component means weight%), Si: 0.01 ~ 0.8%, Mn: 0.3 ~ 2.5%, P : 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5%, Nb: 0.005 to 0.1%, B: 3 to 50 ppm, Ti: 0.005 to 0.1%, N: 15 to 150 ppm, residual Fe and unavoidable impurities It has a high stress resistance and brittle crack propagation stopping characteristics, characterized in that it has a structure consisting of a polygonal ferrite phase having an average size of 5㎛ or less and an island-like martensite having an average size of 10㎛ or less Steel is provided.

At this time, the steel material is selected from the group consisting of Cr: 0.05 ~ 1.0%, Mo: 0.01 ~ 1.0%, Ni: 0.01 ~ 2.0%, Cu: 0.01 ~ 1.0% and V: 0.005 ~ 0.3% It is preferable to further include.

According to another aspect of the invention, C: 0.03 ~ 0.12% (hereinafter, the content of each component means weight%), Si: 0.01 ~ 0.8%, Mn: 0.3 ~ 2.5%, P: 0.02% or less , S: 0.01% or less, Al: 0.005 ~ 0.5%, Nb: 0.005 ~ 0.1%, B: 3 ~ 50ppm, Ti: 0.005 ~ 0.1%, N: 15 ~ 150ppm, steel Fe containing residual Fe and unavoidable impurities After reheating, rough rolling at a temperature of 1250 ~ 1000 ℃; Cooling the roughly rolled slab below Bf to a temperature of 1 ° C./s or more; Reheating the cooled slab below an Ac1 temperature or above an Ac3 temperature; Mapping the reheated slab to a total reduction of 30% or more to produce a steel sheet; And air-cooling or water-cooling the steel sheet. A method of manufacturing a high-strength resistive ratio structural steel having excellent low temperature toughness and brittle crack propagation stopping characteristics is provided.

In addition, the steel is one or two or more selected from the group consisting of Cr: 0.05 to 1.0%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, and V: 0.005 to 0.3%. It is better to include more.

Hereinafter, the reason for adding each alloy component constituting the steel composition, which is one of the main features of the present invention, and an appropriate content range thereof will be described first.

C: 0.03 to 0.12% (hereinafter, the content of each component means weight%)

In the present invention, since it is the most important element for forming the martensite-Austenite Constituent (MA) and determining the size and fraction of the martensite formed, it needs to be contained in steel within an appropriate range. However, if the C content exceeds 0.12%, the low-temperature toughness decreases and the fraction of island martensite exceeds 15%. If the content of C is less than 0.03%, the fraction of island martensite becomes 3% or less, resulting in a decrease in strength. As a result, the range of C is limited to 0.03 to 0.12%. In the case of a plate used as a steel structure for welding, the range of C is preferably 0.04 to 0.09% for weldability.

Si: 0.01 ~ 0.8%

Si is used as a deoxidizer and is useful because of its strength-improving effect, but when it is more than 0.8%, low-temperature toughness decreases and weldability deteriorates. Moreover, when it becomes 0.01% or less, the deoxidation effect will become inadequate and it will limit to 0.01 to 0.8%. In addition, since Si increases the stability of the phase martensite, it is possible to form more phase martensite with a small C content, thereby improving strength and toughness. More preferable range of Si is 0.1 to 0.4%.

Mn: 0.3 ~ 2.5%

Mn is a useful element that improves strength by solid solution strengthening, so 0.3% or more needs to be added. However, addition of more than 2.5% greatly reduces the toughness of the weld due to excessive increase in hardenability, so that the appropriate Mn content is 0.3-2.5%.

P: 0.02% or less

Although P is an element that is advantageous in improving strength and corrosion resistance, it is advantageous to make it as low as possible because it is an element that greatly impairs impact toughness. Therefore, the upper limit thereof is preferably 0.02%.

S: 0.01% or less

Since S is an element which forms MnS or the like and greatly impairs the impact toughness, S is likely to be as low as possible, so the upper limit thereof is preferably 0.01%.

Al: 0.005 ~ 0.5%

Al is an element that can deoxidize molten steel at low cost, so it is preferable to add 0.005% or more, but addition of 0.5% or more causes nozzle clogging during continuous casting, so it is limited to 0.005 to 0.5%. In addition, since the dissolved Al promotes the formation of phase martensite, it is possible to form a large amount of phase martensite even with a small amount of C, which helps to improve strength and toughness. Therefore, it is preferable to make the range of Al 0.01 to 0.05%.

Nb: 0.005 ~ 0.1%

Nb is the most important element in the production of TMCP steel and precipitates in the form of NbC or NbCN to greatly improve the strength of the base metal and the welded portion. In addition, Nb dissolved in reheating at a high temperature suppresses the recrystallization of austenite and also suppresses the transformation of ferrite or bainite, thereby making the structure finer. In addition, the present invention not only allows bainite to be formed even at low cooling rate when the slab is cooled after rough rolling, but also greatly improves the stability of austenite during cooling after the final rolling, thereby promoting the generation of martensite at low speed. It also plays a role. Therefore, Nb is preferably added at least 0.005%, but if excessively added, the possibility of causing brittle cracks in the corners of the steel is increased, which is not preferable.

B: 3 ~ 50ppm

B is a very inexpensive addition element and is a beneficial element showing strong hardenability. It is preferable to add more than 3 ppm since the strength is greatly improved even with a small amount of addition. When excessively added, Fe 23 (CB) 6 is formed, rather the curing ability is lowered, and the low temperature toughness is also greatly reduced. Therefore, B is limited to 3-50 ppm. In addition, in the present invention, B greatly contributes to the formation of bainite even in low-speed cooling in the cooling after rough rolling, and has the effect of promoting the formation of phase martensite in the final cooling.

Ti: 0.005 ~ 0.1%

The addition of Ti can greatly improve the low-temperature toughness by inhibiting the growth of grains when reheating, so at least 0.005% must be added for the effect to be manifested. Since this problem is reduced, it is limited to the range of 0.005 to 0.1%.

N: 15 ~ 150ppm

Since the addition of N increases the strength while greatly reducing the toughness, it is necessary to limit the content to 150 ppm or less. However, since the N content control of 15 ppm or less increases the steelmaking load, the lower limit of the N content is set to 15 ppm.

The above-described steel having the advantageous emphasis of the present invention can obtain a sufficient effect only by including the alloying elements in the above-described content range, but the characteristics such as the strength and toughness of the steel, the toughness and weldability of the weld heat affected zone, etc. In order to improve, it is preferable to add the following alloying elements within an appropriate range. The following alloying elements may be added in one kind or two or more kinds together.

Cr: 0.05 ~ 1.0%

Since Cr has a great effect on increasing the strength by increasing the hardenability, addition of 0.05% or more is necessary to obtain the effect, and addition of 1.0% or more is preferably limited to 1.0% or less because it greatly reduces the weldability. Moreover, in order to obtain stable phase martensite even at a comparatively low cooling rate, it is more preferable to add in 0.2 to 0.5% of range.

Mo: 0.01 ~ 1.0%

Mo is required to add more than 0.01% because Mo is able to greatly improve the hardenability and suppress the formation of ferrite even with a small amount of addition. Therefore, more than 0.01% adds excessively to the hardness of the weld. Since toughness is inhibited, it is advantageous to add it at 1.0% or less. In the present invention, in order to form the island martensite in an appropriate range for securing the tensile strength, it is more preferably limited to the range of 0.02 to 0.2%.

Ni: 0.01 ~ 2.0%

Ni is almost the only element that can simultaneously improve the strength and toughness of the base material, and in order to show the effect, Ni should be added at least 0.01%. Since Ni is an expensive element, addition of 2.0% or more lowers the economic efficiency. Also, weldability is degraded.

Cu: 0.01 ~ 1.0%

Since Cu is an element that can increase the strength while minimizing the decrease in toughness of the base metal, 0.01% or more must be added to show the effect, and excessive addition greatly inhibits product surface quality, so it is desirable to limit it to 1.0% or less. Do.

V: 0.005 ~ 0.3%

V has a lower solubility temperature than other fine alloys, and it is preferable to add 0.005% or more because it precipitates out the weld heat affected zone and prevents a drop in strength, and excessive addition of 0.3% or less deteriorates toughness. It is limited to 0.3%.

The steel of the present invention having the above-described composition is a steel having improved hardenability than conventional steels, and has a condition capable of forming a desired structure inside the steel even without rapid water cooling or the like. However, when the hardenability of the steel is improved and hard tissue is easily formed therein, the low temperature toughness and brittle crack propagation stopping characteristics are often deteriorated. In the present invention, the preferred structure of the steel is defined as follows. By doing so, even if the hardenability of the steel is improved, the low temperature toughness and brittle crack propagation stop characteristics are not prevented from deteriorating, and the resistance ratio can be easily implemented.

As shown in FIG. 1, the microstructure of the inventive steel is preferably composed of ultrafine polygonal ferrite having an average size of 5 μm or less and a phase martensite having an average size of 10 μm or less. The term consisting of polygonal ferrite and phase martensite here means that the tissue mainly comprises polygonal ferrite and phase martensite and the sum of their fractions is 98% or more. In addition, the icon martensite means a mixture of martensite and austenite (Marteniste-Autenite Constituent). In other words, by forming ultra fine ferrite, it is possible to suppress crack propagation in the entire thickness of the steel to improve the brittle crack propagation stop characteristics, and to lower the Brittle Brittle Transient Temperature (DBTT) of the steel to lower the low temperature of the steel. It plays a role in improving toughness. In addition, the fine island martensite serves to improve the strength of the steel without deteriorating the toughness of the steel due to the fine grains. It is preferable that the island-like martensite of this invention is contained 3 to 15% by the area fraction. When the fraction is insufficient, the tensile strength is low, which is disadvantageous. On the contrary, when the fraction is excessive, the toughness decreases. Even when the size of the island martensite exceeds 10 mu m, the toughness is feared.

Hereinafter will be described a method for producing the advantageous steel of the present invention as described above.

Steel manufacturing process of the present invention consists of the process of slab reheating-rough rolling-after cold rolling steel cooling-reheating-finishing rolling-cooling, detailed conditions for each process are as follows.

Slab reheating temperature: 1000 ~ 1250 ℃

In reheating the steel sheet of the present invention, the heating temperature is preferably set to 1000 ° C or higher, in order to solidify the carbonitride of Ti and / or Nb formed during casting. Moreover, in order to fully solidify the carbonitride of Ti and / or Nb, it is more preferable to heat to 1000 degreeC or more. However, when reheating excessively high temperature, austenite may coarsen, so the reheating temperature is preferably 1250 ° C or lower.

Rough rolling temperature: 1250 ~ 1000 ℃

The reheated steel sheet is subjected to rough rolling after heating to adjust its shape. The rolling temperature is made at 1250 ~ 1000 ° C., and the casting structure such as the dendrite formed during casting is destroyed by rolling. Moreover, the effect of reducing the size of austenite can also be obtained.

Cooling condition after rough rolling: Cooling below Bf at temperature above 1 ℃ / s

The cooling condition is one of the main features of the present invention, in order to obtain the ultrafine grain structure of the present invention, by means of cooling the steel plate after rough rolling at a cooling rate of 1 ° C / s or more, Bf (the temperature at which bainite transformation ends) It is necessary to cool below to form a certain amount of bainite or martensite structure of steel material.

Secondary reheating temperature: Ac1 ~ Ac3

The cold-rolled steel sheet is reheated in the range of Ac1 to Ac3 temperature to have a structure in which the austenite phase is distributed at the bainite or martensite grain boundaries as shown in FIG. 2. Bainite or martensite formed during the cooling is preferably 50% or more in area ratio of the entire structure.

Finish rolling condition: More than 30% of total rolling load

If the total pressure reduction after the reheating is rolled to 30% or more, the bainite or martensite phase is hardened and recrystallized into superfine ferrite during the processing and subsequent cooling to give an ultrafine grain of 5 mu m or less. The austenite present at the grain boundaries is segmented into small sizes by rolling and remains superficially martensite during subsequent cooling due to supersaturated C.

Final cooling: air cooling or water cooling

In order to suppress grain growth of the recrystallized ferrite during cooling after finishing rolling is finished, and to suppress the transformation of residual austenite into pearlite or cementite, it is preferable to water-cool the steel sheet and accelerated cooling, and the hardening ability of the steel is sufficient. Therefore, even if air-cooling is performed, the structure | tissue desired by this invention can be obtained.

In summary, the steel manufacturing method of the present invention, after heating the steel slab having the above-described composition in the temperature range of 1000 ~ 1250 ℃, rough rolling in the temperature range of 1250 ~ 1000 ℃, Bf cooling conditions of 1 ℃ / s or more Bf After cooling to the following temperature and reheated in the range of Ac1 ~ Ac3 temperature again after the total pressure drop 30% or more characterized in that the rolling and cooling.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following examples are only intended to illustrate one embodiment of the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

Steel grade Emphasis Bs B50 Bf C Si Mn P S Al Ti Nb B N Ni Cu Cr Mo V Inventive Steel A 0.04 0.2 1.7 0.013 0.002 0.015 0.01 0.03 6 35 624.2 564.2 504.2 Inventive Steel B 0.11 0.4 1.5 0.013 0.005 0.032 0.014 0.02 15 55 560.3 500.3 440.3 Invention Steel C 0.087 0.2 1.4 0.012 0.002 0.013 0.02 0.08 40 42 575.5 515.5 455.5 Inventive Steel D 0.1 0.3 1.4 0.013 0.002 0.013 0.02 0.04 8 42 0.21 613.2 553.2 493.2 Inventive Steel E 0.072 0.2 1.4 0.013 0.002 0.013 0.02 0.03 15 42 0.32 570.6 510.6 450.6 Inventive Steel F 0.086 0.3 1.4 0.013 0.003 0.013 0.02 0.06 20 42 0.24 559.0 499.0 439.0 Invention Steel G 0.09 0.4 1.6 0.013 0.002 0.013 0.02 0.04 16 42 0.11 547.6 487.6 427.6 Inventive Steel H 0.071 0.3 1.4 0.013 0.002 0.013 0.02 0.01 14 42 0.02 586.8 526.8 466.8 Comparative Steel I 0.01 0.2 1.5 0.014 0.003 0.034 0.012 0.03 20 32 587.3 527.3 467.3 Comparative Steel J 0.19 0.3 0.8 0.013 0.001 0.038 0.013 0.04 24 28 601.7 541.7 481.7 Comparative Steel K 0.09 0.4 1.2 0.013 0.005 0.024 0.01 0.00 14 26 599.7 539.7 479.7 Comparative Steel L 0.079 0.2 1.4 0.015 0.009 0.043 0.09 0.04 One 43 675.7 615.7 555.7 Comparative Steel M 0.1 0.3 1.2 0.015 0.003 0.043 0.012 0.02 60 35 590.0 530.0 470.0

However, in Table 1, the content units of B and N represent ppm, and the content units of the remaining elements represent wt%. In addition, B50 represents the temperature at which 50% or more of the fraction of bainite is produced in the steels corresponding to the respective emphasis properties.

Inventive steel in Table 1 indicates that satisfying the component conditions of the present invention, the comparative steel is to show the component system deviating from the component conditions defined in the present invention, the comparative steel I is C content is less than the range specified in the present invention The comparative steel J is a case where the C content exceeds the range specified in the present invention, the comparative steel K is a case where the Nb content is less than the reference value, and the comparative steel L is a case where the B content is low, the comparative steel M Represents the case where the B content is excessive.

Rough rolling condition Cooling condition Reheat Finish rolling condition Cooling condition after finish rolling Slab thickness Reheat Extraction Temperature Rough rolling end temperature Cooling start material thickness Cooling rate Cooling end temperature Reheat temperature Rolling start temperature Rolling end temperature Finish rolling cumulative reduction Cooling rate Cooling end temperature Inventive Steel A Inventive Example 1 244 1065 988 125 5.0 364 835 815 795 46 0.5 Inventive Example 2 244 1080 1003 75 4.0 429 779 759 739 65 5.0 542 Inventive Example 3 220 115 38 120 6.0 345 856 836 816 58 2.0 664 Comparative Example 1 244 1110 1033 110 3.0 523 775 755 735 35 3.0 413 Comparative Example 2 220 1080 1003 70 4.0 433 890 756 736 56 2.0 643 Comparative Example 3 220 1050 973 80 5.0 409 794 774 754 15 4.0 346 Inventive Steel B Inventive Example 1 244 1060 983 135 7.0 249 830 810 790 46 0.5 Inventive Example 2 244 1100 1023 75 5.0 363 797 777 757 65 6.0 521 Inventive Example 3 220 1110 1033 115 6.0 300 868 848 828 58 2.0 683 Comparative Example 1 244 1105 1028 120 4.0 442 800 780 760 35 4.0 292 Comparative Example 2 220 170 93 70 3.0 396 880 749 729 56 2.0 637 Comparative Example 3 220 1060 983 80 5.0 358 802 782 762 15 3.0 456 Invention Steel C Inventive Example 1 244 1090 1013 120 6.0 312 825 805 785 46 0.5 Inventive Example 2 244 1070 993 60 3.0 420 764 744 724 65 8.0 472 Inventive Example 3 220 1110 1033 125 5.0 331 845 825 805 58 2.0 647 Comparative Example 1 244 1100 1023 100 3.0 496 774 754 734 35 3.0 441 Comparative Example 2 220 1060 983 70 5.0 386 900 770 750 56 2.0 658 Comparative Example 3 220 1075 998 70 5.0 386 790 770 750 15 3.0 483 Inventive Steel D Inventive Example 1 244 1080 1003 120 7.0 323 810 790 770 46 0.5 Inventive Example 2 244 1070 993 60 4.0 443 770 750 730 65 8.0 478 Inventive Example 3 220 1110 1033 125 3.0 416 790 770 750 58 2.0 592 Comparative Example 1 244 1100 1023 100 5.0 491 796 776 756 35 2.0 561 Comparative Example 2 220 1055 978 70 4.0 435 890 756 736 56 2.0 643 Comparative Example 3 220 1060 983 70 5.0 421 786 766 746 15 3.0 478 Inventive Steel E Inventive Example 1 244 1080 1000 120 3.0 379 793 773 753 46 0.5 Inventive Example 2 244 1060 980 60 4.0 403 773 753 733 65 8.0 481 Inventive Example 3 220 1120 1040 125 4.0 351 819 799 779 58 2.0 621 Comparative Example 1 244 1120 1040 100 5.0 451 801 781 761 35 4.0 371 Comparative Example 2 220 1055 975 70 5.0 381 900 771 751 56 2.0 659 Comparative Example 3 220 1050 970 70 6.0 367 803 783 763 20 3.0 511 Inventive Steel F Inventive Example 1 244 1080 1000 120 8.0 247 805 785 765 46 0.5 Inventive Example 2 244 1070 990 60 3.0 403 765 745 725 70 8.0 509 Inventive Example 3 220 1120 1040 125 4.0 339 822 802 782 58 2.0 624 Comparative Example 1 244 1100 1020 100 0.5 529 745 725 705 35 4.0 315 Comparative Example 2 220 1055 975 70 2.0 411 890 739 719 56 2.0 626 Comparative Example 3 220 1070 990 70 6.0 355 806 786 766 15 3.0 498 Invention Steel G Inventive Example 1 244 1090 1010 120 6.0 284 805 785 765 46 0.5 Inventive Example 2 244 1070 990 65 3.0 389 768 748 728 65 8.0 455 Inventive Example 3 220 1110 1030 125 4.0 328 825 805 785 58 2.0 627 Comparative Example 1 244 1120 1040 105 4.0 444 792 772 752 35 5.0 241 Comparative Example 2 220 1055 975 70 2.0 400 930 739 719 56 2.0 627 Comparative Example 3 220 1070 990 65 6.0 350 802 782 762 15 3.0 513 Inventive Steel H Inventive Example 1 244 1080 1000 120 8.0 275 800 780 760 46 0.5 Inventive Example 2 244 1070 990 60 3.0 431 763 743 723 65 8.0 471 Inventive Example 3 220 1120 1040 125 4.0 367 815 795 775 58 2.0 618 Comparative Example 1 244 1100 1020 100 3.0 507 773 753 733 35 4.0 343 Comparative Example 2 220 1055 975 70 2.0 439 920 738 718 56 3.0 579 Comparative Example 3 220 1055 960 90 6.0 359 823 803 783 20 4.0 351

Comparative Steel I Inventive Example 1 244 1080 1000 120 8.0 275 800 780 760 45 0.5 Inventive Example 2 244 1070 990 60 3.0 431 763 743 723 65 5.0 566 Inventive Example 3 220 1160 1080 125 4.0 367 815 795 775 60 2.0 625 Comparative Steel J Inventive Example 1 244 1080 1000 120 8.0 290 800 780 760 45 0.5 Inventive Example 2 244 1070 990 60 3.0 446 762 742 722 65 5.0 565 Inventive Example 3 220 1120 1040 125 4.0 382 813 793 773 60 2.0 623 Comparative Steel K Inventive Example 1 244 1080 1000 120 8.0 288 800 780 760 45 0.5 Inventive Example 2 244 1070 990 60 3.0 444 762 742 722 65 5.0 565 Inventive Example 3 220 1120 1040 125 4.0 380 813 793 773 60 2.0 623 Comparative Steel L Inventive Example 1 244 1080 1000 120 8.0 364 800 780 760 45 0.5 Inventive Example 2 244 1070 990 60 3.0 520 759 739 719 65 5.0 562 Inventive Example 3 220 1170 1090 125 4.0 456 801 781 761 60 2.0 611 Comparative Steel M Inventive Example 1 244 1080 1000 120 8.0 278 800 780 760 45 0.5 Inventive Example 2 244 1070 990 60 3.0 434 763 743 723 65 5.0 565 Inventive Example 3 220 1120 1040 125 4.0 370 815 795 775 55 2.0 606

As can be seen in Table 2, steel sheets were manufactured according to six manufacturing patterns for each steel type. Inventive Examples 1 to 3 show a case of manufacturing according to the pattern specified in the present invention, while Comparative Examples 1 to 3 show a case of manufacturing a pattern deviating from the pattern defined in the present invention. In more detail, Comparative Example 1 shows a case where the cooling end temperature is cooled to a temperature higher than the Bs temperature or higher than the temperature specified in the present invention when the cooling is performed slowly after rough rolling, and Comparative Example 2 is another condition of the present invention. It means that the condition specified in the above is satisfied, but the reheating temperature is too high after cooling, and Comparative Example 3 shows a case where the rolling amount is insufficient.

Table 3 shows the results of testing the physical properties of the rolled steel for each steel type.

Thickness FGS MA fraction YS TS YR DBTT ESSO (Kca> 400 kgf temperature of .m-3 / 2) Invention steel A Inventive Example 1 68 3.1 6 440 630 0.70 -101 -52 Inventive Example 2 26 2.8 3 452 579 0.78 -171 -104 Inventive Example 3 50 3.1 5 436 594 0.73 -119 -64 Comparative Example 1 72 10.5 5 335 472 0.71 -30 -6 Comparative Example 2 31 2.8 5 453 611 0.74 70 41 Comparative Example 3 68 2.9 4 529 616 0.86 45 25 Inventive Steel B Inventive Example 1 73 3.0 12 451 693 0.65 -66 -31 Inventive Example 2 26 2.9 11 458 683 0.67 -87 -44 Inventive Example 3 48 3.2 14 444 725 0.61 -22 -10 Comparative Example 1 78 12.9 13 344 596 0.58 52 7 Comparative Example 2 31 2.8 14 464 745 0.62 64 31 Comparative Example 3 68 2.9 13 533 586 0.91 55 32 Invention Steel C Inventive Example 1 65 3.0 9 433 616 0.70 -124 -67 Inventive Example 2 21 2.8 4 446 580 0.77 -138 -86 Inventive Example 3 53 3.1 8 429 592 0.72 -143 -78 Comparative Example 1 65 12.8 7 324 473 0.69 -25 -4 Comparative Example 2 31 2.9 8 440 603 0.73 -24 -14 Comparative Example 3 60 2.9 7 484 514 0.94 12 8 Inventive Steel D Inventive Example 1 65 3.0 11 444 660 0.67 6 3 Inventive Example 2 21 2.8 6 453 619 0.73 -104 -60 Inventive Example 3 53 2.9 10 448 644 0.70 -118 -63 Comparative Example 1 65 12.9 10 332 528 0.63 7 One Comparative Example 2 31 2.8 10 451 647 0.70 30 16 Comparative Example 3 60 2.9 9 526 562 0.94 24 15 Inventive Steel E Inventive Example 1 65 2.9 7 442 589 0.75 -166 -97 Inventive Example 2 21 2.8 4 447 583 0.77 -134 -82 Inventive Example 3 53 3.0 6 437 562 0.78 -183 -109 Comparative Example 1 65 12.9 5 338 435 0.78 7 One Comparative Example 2 31 2.9 6 443 568 0.78 30 18 Comparative Example 3 56 2.9 6 506 528 0.96 24 15 Inventive Steel F Inventive Example 1 65 2.9 9 445 626 0.71 -129 -70 Inventive Example 2 18 2.8 4 454 585 0.78 -150 -92 Inventive Example 3 53 3.0 8 441 602 0.73 -148 -82 Comparative Example 1 65 12.7 7 355 488 0.73 30 5 Comparative Example 2 31 2.8 8 455 615 0.74 23 14 Comparative Example 3 60 2.9 7 544 573 0.95 15 9 Invention Steel G Inventive Example 1 65 2.9 10 460 651 0.71 -119 -62 Inventive Example 2 23 2.8 4 468 609 0.77 -132 -77 Inventive Example 3 53 3.0 9 456 626 0.73 -138 -73 Comparative Example 1 68 12.9 6 370 499 0.74 30 5 Comparative Example 2 31 2.8 9 470 640 0.73 23 13 Comparative Example 3 55 2.9 8 549 581 0.95 15 9 Inventive Steel H Inventive Example 1 65 2.9 7 446 590 0.76 -168 -97 Inventive Example 2 21 2.8 5 454 608 0.75 -145 -86 Inventive Example 3 53 3.0 6 443 566 0.78 -186 -111 Comparative Example 1 65 11.6 5 364 458 0.79 43 8 Comparative Example 2 31 2.8 5 485 621 0.78 55 32 Comparative Example 3 72 3.0 5 540 559 0.97 24 14

Comparative Steel I Inventive Example 1 66 2.9 One 446 466 0.96 -156 -115 Inventive Example 2 21 2.8 0 454 509 0.89 -132 -93 Inventive Example 3 50 3.0 0 443 446 0.99 -154 -116 Comparative Steel J Inventive Example 1 66 2.9 25 427 928 0.46 99 36 Inventive Example 2 21 2.8 22 435 873 0.50 30 12 Inventive Example 3 50 3.0 24 424 904 0.47 79 30 Comparative Steel K Inventive Example 1 66 7.9 One 321 333 0.97 -114 -43 Inventive Example 2 21 7.8 2 323 363 0.89 -86 -31 Inventive Example 3 50 8.0 One 321 331 0.97 -115 -44 Comparative Steel L Inventive Example 1 66 11.4 One 309 329 0.94 -49 -13 Inventive Example 2 21 11.3 One 310 330 0.94 -50 -13 Inventive Example 3 50 11.4 2 309 349 0.89 -29 -7 Comparative Steel M Inventive Example 1 66 2.9 14 440 578 0.76 31 19 Inventive Example 2 21 2.8 11 448 554 0.81 3 2 Inventive Example 3 56 3.0 13 436 564 0.77 12 7

Here, FGS (Ferrite Grain Size) is the ferrite grain size (㎛), YS is the yield strength (MPa), TS is the tensile strength (MPa), YR is the yield ratio, DBTT is the ductile brittle transition temperature (℃) ). In addition, the ESSO is a measure of the brittle crack propagation stop characteristics, indicating that the lower the temperature can inhibit the brittle crack propagation at low temperatures.

As can be seen from the results of Table 3, the slow cooling was performed after the rough rolling, in the case of Comparative Example 1, which is a case of cooling to a temperature higher than the temperature specified in the present invention, the ultrafine grain structure is not formed to the center of the steel sheet, DBTT and ESSO show bad results, and Comparative Example 2, which is a case where rolling starts with the reheating temperature too high after rough rolling and cooling, is composed of coarse ferrite with a final final microstructure, so that the yield strength and tensile strength are low. The toughness and brittle crack propagation stop characteristics are bad. In addition, in Comparative Example 3, which is a case where the rolling amount is insufficient, coarse ferrite is formed, which makes it difficult to secure toughness and brittle crack propagation stopping characteristics.

In addition, even when rolling under the conditions specified in the present invention, in the case of Comparative Steel I having a low carbon content in stress, the MA fraction is insufficient to obtain sufficient tensile strength, thereby increasing YR. In addition, in the case of comparative steel J having an excessively high carbon content, the MA fraction is excessive and the toughness is poor. Comparative steel K is a case where the Nb content is insufficient, the MA fraction generated during final cooling decreases, the tensile strength decreases, and the YR increases. The comparative strength L is a case where B is insufficient, but when the content of B is insufficient, the hardenability of the steel is insufficient. As in the case where Nb is insufficient, the MA fraction generated during final cooling decreases, the tensile strength decreases, and the YR increases. Comparative steel M is sufficiently secured by B, but excessive B content deteriorates toughness by forming coarse BN or other compounds.

As described above, according to the present invention, it is possible to provide a high-strength steel and a method of manufacturing the same that satisfy both low-temperature toughness, brittle crack propagation stopping characteristics, and low yield ratio.

Claims (4)

C: 0.03 to 0.12% (hereinafter, the content of each component means weight%), Si: 0.01 to 0.8%, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 ~ 0.5%, Nb: 0.005 ~ 0.1%, B: 3 ~ 50ppm, Ti: 0.005 ~ 0.1%, N: 15 ~ 150ppm, balance of Fe and unavoidable impurities, with an average size of 10㎛ or less High strength resistive-strength structural steel having excellent low-temperature toughness and brittle crack propagation stopping characteristics, characterized by having martensite in an area fraction of 3 to 15% and having a structure consisting of polygonal ferrite phase having an average size of 5 µm or less. According to claim 1, wherein the steel material is selected from the group consisting of Cr: 0.05 ~ 1.0%, Mo: 0.01 ~ 1.0%, Ni: 0.01 ~ 2.0%, Cu: 0.01 ~ 1.0% and V: 0.005 ~ 0.3% Or high-strength resistive ratio structural steel having excellent low temperature toughness and brittle crack propagation stopping characteristics, characterized in that it further comprises two or more kinds. C: 0.03 to 0.12% (hereinafter, the content of each component means weight%), Si: 0.01 to 0.8%, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 ~ 0.5%, Nb: 0.005 ~ 0.1%, B: 3 ~ 50ppm, Ti: 0.005 ~ 0.1%, N: 15 ~ 150ppm, temperature of 1250 ~ 1000 ℃ after reheating steel slab containing residual Fe and unavoidable impurities Co-rolling in; Cooling the roughly rolled slab below Bf to a temperature of 1 ° C./s or more; Reheating the cooled slabs back to an Ac1 to Ac3 temperature range; Mapping the reheated slab to a total reduction of 30% or more to produce a steel sheet; And Air-cooling or water-cooling the steel sheet; A method of manufacturing a high strength, high-strength, low-strength structural steel having excellent low temperature toughness and brittle crack propagation characteristics. According to claim 3, wherein the steel is selected from the group consisting of Cr: 0.05 ~ 1.0%, Mo: 0.01 ~ 1.0%, Ni: 0.01 ~ 2.0%, Cu: 0.01 ~ 1.0% and V: 0.005 ~ 0.3% Or at least two or more kinds of low-temperature toughness and brittle crack propagation stopping characteristics.

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