KR101304822B1 - Ultra high strength steel plate having excellent fatigue crack arrestability and manufacturing method the same - Google Patents

Abstract

The present invention is in weight%, carbon (C): 0.02 to 0.10%, silicon (Si): 0.01 to 0.6%, manganese (Mn): 1.8 to 2.5%, aluminum (Al): 0.005 to 0.5%, niobium (Nb) ): 0.005 to 0.10%, boron (B): 5 to 40 ppm, titanium (Ti): 0.005 to 0.1%, nitrogen (N): 15 to 150 ppm, phosphorus (P): 0.02% or less, sulfur (S): 0.01 % Or less, containing residual iron (Fe) and other unavoidable impurities, and the microstructure includes, as a columnar, bainitic ferrite having a grain boundary fraction of 20 to 40% having a grain boundary orientation difference of 55 to 65 degrees between bainite laths, and yielding. The present invention relates to an ultra-high strength steel sheet having excellent fatigue crack propagation suppression property of 690 MPa or more.

Fatigue Crack, Ultra High Strength, Bainitic Ferrite, Σ3

Description

ULTRA HIGH STRENGTH STEEL PLATE HAVING EXCELLENT FATIGUE CRACK ARRESTABILITY AND MANUFACTURING METHOD THE SAME

The present invention relates to a steel sheet used in a super large welded structure and a method of manufacturing the same, and more particularly, when fatigue cracking occurs under repeated stresses, a high strength steel sheet having excellent fatigue crack suppression characteristics by lowering the growth rate of such fatigue cracks. And to a method of manufacturing the same.

In recent years, steel structures are becoming increasingly large in size, and the use of high-strength steels is increasing according to this trend. As the strength of the steel increases, the stress concentrated around the crack also increases, so the crack propagation speed of the steel also increases.

In the fatigue process of steel, cracks are generated at stress concentration and cracks are propagated. Especially, in the case of welded structures, there are many defects inside the weld metal, so it is practical to completely prevent the occurrence of fatigue cracks. It is impossible. Therefore, in order to improve the fatigue life of the welded structure, it is important to slow the crack growth rate from the existing state rather than to prevent the crack generation itself.

Japanese Patent Application Laid-Open No. 2000-17379 describes such a technique. When this technique is referred to as ND as the vertical direction of the steel plate surface, crystal grains in which the (100) plane of the ferrite crystal has a direction parallel to the ND direction, The boundary between grains whose (111) planes have an orientation parallel to ND is equal to the ferrite (111) plane fraction and (100) in the measurement plane that crosses at least one place at least 30 μm along the crack propagation direction or is parallel to the steel plate surface. The ratio of the ratio of the surface fraction to 1.25 to 2.0 relates to a steel sheet having improved fatigue crack propagation suppression characteristics.

An object of the present invention is to provide an ultra high strength steel sheet having excellent fatigue crack suppression characteristics and a method of manufacturing the same, which slow down the growth rate of fatigue cracks generated by repeated stress applied to the weld structure.

In one embodiment, the present invention provides, in weight percent, carbon (C): 0.02 to 0.10%, silicon (Si): 0.01 to 0.6%, manganese (Mn): 1.8 to 2.5%, aluminum (Al): 0.005 to 0.5 %, Niobium (Nb): 0.005 to 0.10%, boron (B): 5 to 40 ppm, titanium (Ti): 0.005 to 0.1%, nitrogen (N): 15 to 150 ppm, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, containing residual iron (Fe) and other unavoidable impurities, and the microstructure is a columnar, bainitic ferrite having a grain boundary fraction of 20 to 40% having a grain boundary orientation difference of 55 to 65 degrees between bainite laths. It includes, and provides an ultra-high strength steel sheet excellent in fatigue crack propagation suppression properties of yield strength of 690MPa or more.

The steel sheet is chromium (Cr): 0.05 ~ 1.0%, molybdenum (Mo): 0.01 ~ 1.0%, nickel (Ni): 0.01 ~ 2.0%, copper (Cu): 0.01 ~ 1.0% and vanadium (V): 0.005 ~ It may further comprise one or two or more of 0.3%.

The steel sheet may include 1% or less of island-like martensite of 1 μm or less.

The fatigue crack growth rate of the steel sheet is preferably 2.5 * 10 ^-5 mm / cycle or less.

As another embodiment of the present invention, in weight%, carbon (C): 0.02 to 0.10%, silicon (Si): 0.01 to 0.6%, manganese (Mn): 1.8 to 2.5%, aluminum (Al): 0.005 to 0.5 %, Niobium (Nb): 0.005 to 0.10%, boron (B): 5 to 40 ppm, titanium (Ti): 0.005 to 0.1%, nitrogen (N): 15 to 150 ppm, phosphorus (P): 0.02% or less, sulfur (S): reheating the slab containing 0.01% or less, balance iron (Fe) and other unavoidable impurities at 1050-1250 ° C .; Roughly rolling the reheated slab at an austenite recrystallization temperature (Tnr) ˜1250 ° C .; Finishing the roughly rolled steel sheet in the range of bainite transformation start temperature (Bs) to Tnr; And a cooling step of cooling the finishing rolled steel sheet at a cooling rate of 250 ° C. to bainite transformation temperature (Bf) at a cooling rate of 5 ° C./s or more, to provide an ultra-high strength steel sheet having excellent fatigue crack growth suppression characteristics. do.

The present invention may provide a steel sheet having a fatigue crack growth rate of 2 * 10 ^-5 mm / cycle or less, a yield strength of 690 MPa or more, and a tensile strength of 800 MPa or more.

The present invention controls the component system of the steel sheet and controls the cooling end temperature after rolling to form bainitic ferrite having a Σ3 grain boundary fraction of 55 to 65 degrees having a grain boundary orientation difference between bainite laths as a main phase, Ultra-high strength steel sheet with reduced fatigue crack propagation speed.

The grain boundary orientation difference between the bainite laths at around 60 degrees is a very stable grain boundary because the interfacial energy at the grain boundary is high and the grain boundary is low. Fatigue Cracks Advance When encountering the grain boundaries, low interfacial energy acts as a resistance. Therefore, in order for fatigue crack to progress, high breakdown energy is required, and it becomes difficult to break the grain boundary and progress. In the case of the Σ3 grain boundary, the tissue plays a role of suppressing the propagation of fatigue cracks caused by repeated stress.

As shown in FIG. 1, (a) is an optical micrograph of a bainite of a lath type manufactured by a conventional manufacturing method, and has an orientation difference of less than 5 degrees, and a charpy at low temperature (-40 ° C). When the impact test is carried out, brittle fracture is shown as in (b). On the contrary, (c) is an optical micrograph of the bainitic ferrite structure prepared according to the present invention, which is composed of Σ3 grain boundaries having a grain boundary orientation difference of 55 to 65 degrees between bainite laths, and a Charpy impact test at low temperature (-40 ° C). When (d) is carried out, it has the property of exhibiting soft fracture. Therefore, the bainitic ferrite structure of the present invention not only has excellent fatigue cracking but also low temperature toughness as compared with the conventional bainite structure.

Hereinafter, the component system of the steel sheet of the present invention will be described.

Carbon (C): 0.02 to 0.10 wt%

Carbon is an element that forms bainitic ferrite as a matrix structure in the present invention and has a great influence on grain size, fraction, and the like of the martensite phase. If the content of carbon is less than 0.02% by weight, the formation of acicular bainitic ferrite is prevented and the strength of the steel is lowered. However, when the content of carbon exceeds 0.10% by weight, low-temperature toughness is lowered, and the fraction of the island martensite exceeds 1%.

Silicon (Si): 0.01-0.6 wt%

Silicone is used as a deoxidizer and is a useful element for improving strength. In addition, silicon increases the stability of the phase martensite and can form the phase martensite even in a low carbon steel, thereby improving strength but causing toughness. This effect is insignificant when the content of silicon is less than 0.01% by weight. However, when it exceeds 0.6 weight%, low-temperature toughness and weldability will fall.

Manganese (Mn): 1.8-2.5 wt%

Manganese is an element capable of improving the strength of steel by solid solution strengthening, and in order to exhibit such an effect in the present invention, it is preferable to be included in an amount of 1.8 wt% or more. However, when it exceeds 2.5 weight%, hardenability may become high too much and the toughness of a weld part may fall significantly.

Aluminum (Al): 0.005 to 0.5 wt%

Aluminum is an element used as a deoxidizer of molten steel, and in order to exhibit such an effect in the present invention, it is preferably included at least 0.005% by weight. However, if it exceeds 0.5% by weight, nozzle clogging may occur during continuous casting. However, since the solid solution of aluminum helps to form island martensite, the lower limit of the aluminum content is more preferably limited to 0.01% by weight.

Niobium (Nb): 0.005 to 0.1 wt%

Niobium is the most important element in the production of TMCP steel, and is an element that can be precipitated in the form of NbC or NbCN to greatly improve the strength of the base metal and the welded portion. In addition, niobium dissolved in reheating at a high temperature can suppress the recrystallization of austenite and suppress the transformation of ferrite or bainite to finely control the tissue. In addition, when slab is cooled after rough rolling, bainite may be formed even at a low cooling rate, and it greatly increases the stability of austenite during cooling after the final rolling, and may play a role in promoting the martensite planet even at a low cooling rate. Can be. This effect is insignificant when the content of niobium is less than 0.005% by weight. On the other hand, when it exceeds 0.1% by weight, brittle cracks are caused at the corners of the steel.

Boron (B): 5-40 ppm

Boron is an element for improving hardenability and is an inexpensive element, and preferably 5 ppm or more in order to improve hardenability in the present invention. However, when it exceeds 40 ppm, yes forms Fe ₂₃ (CB) ₆ and rather curability falls, and low-temperature toughness also falls large.

Titanium (Ti): 0.005 to 0.1 wt%

Titanium is an element that suppresses the growth of crystal grains upon reheating the slab and greatly improves low temperature toughness. In order to exhibit such an effect in the present invention, titanium is preferably included by 0.005% by weight or more. However, when it exceeds 0.1 weight%, nozzle clogging occurs at the time of performance, and low-temperature toughness falls.

Nitrogen (N): 15-150ppm

Nitrogen is an element that increases the strength of steel but decreases its toughness. When the nitrogen content is less than 15 ppm, the steelmaking load is increased. When the nitrogen content is more than 150 ppm, the toughness of the steel sheet is lowered.

The remainder of the present invention is iron (Fe). However, in the normal steel manufacturing process, impurities that are not intended from raw materials or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.

However, since phosphorus and sulfur are generally mentioned impurities, the following briefly describes them.

Phosphorus (P): 0.02 wt% or less

Phosphorus is an impurity contained inevitably and is contained in steel to lower toughness. Therefore, it is preferable to control phosphorus as low as possible. In theory, the phosphorus content is advantageously limited to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is preferably limited to 0.02% by weight.

Sulfur (S): 0.01 wt% or less

Sulfur is inevitable impurity, and reacts with manganese to form MnS, which lowers the low temperature toughness. In theory, the sulfur content is advantageously limited to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, the upper limit of the sulfur content in the present invention is preferably limited to 0.01% by weight.

In addition, the steel material of the present invention is added when one or two or more elements of chromium (Cr), molybdenum (Mo), nickel (Ni), copper (Cu) and vanadium (V) to be described below additionally The effect can be further improved.

Chromium (Cr): 0.05-1.0 wt%

Chromium is an element that can increase the strength of steel by increasing the hardenability, and in order to exhibit such an effect in the present invention, it is preferably included 0.05% by weight or more. However, when it exceeds 1.0 weight%, weldability will fall.

Molybdenum (Mo): 0.01-1.0 wt%

Molybdenum is an element capable of greatly improving the hardenability, suppressing the formation of ferrite, and greatly improving the strength. In order to exhibit such an effect in the present invention, it is preferable to include 0.01 wt% or more. However, when it exceeds 1.0 weight%, the hardness of a weld part will be excessively increased and toughness will be inhibited.

Nickel (Ni): 0.01-2.0 wt%

Nickel is an element capable of improving the strength and toughness of the base material at the same time. In order to exhibit such an effect in the present invention, it is preferably included 0.01 wt% or more. However, when it exceeds 2.0 weight%, economic efficiency and weldability will fall.

Copper (cu): 0.01 ~ 1.0 wt%

Copper is an element capable of minimizing toughness reduction of the base metal and at the same time improving strength, and in order to exhibit such an effect in the present invention, it is preferably included 0.01 wt% or more. However, when it exceeds 1.0 weight%, the quality of a product surface can be greatly reduced.

Vanadium (V): 0.005-0.3 wt%

Vanadium is a low solubility temperature compared to other alloying elements, is an element that can be deposited in the heat affected zone to prevent a drop in strength, in order to exhibit such an effect in the present invention is preferably included at least 0.005% by weight. Do. However, when it exceeds 0.3 weight%, the toughness of steel materials will fall.

As a steel having the above-described component system, it is necessary to limit the microstructure of the steel to preferable conditions for becoming a steel having excellent fatigue crack growth suppression characteristics. The structure of the steel of the present invention may be a bainitic ferrite single phase or may contain some phase martensite. The grain boundary orientation difference between the bainitic laths of bainitic ferrite is preferably from 55% to 65 °, and the Σ3 grain boundary fraction is preferably 20% or more. The Σ3 grain boundary theoretically exhibits an orientation difference around 60 degrees, and the Σ3 grain boundary within the range of 55 to 65 degrees exhibits the same grain boundary characteristics and thus may exhibit similar crack growth suppression characteristics in the present invention. Therefore, in the present invention, the azimuth difference between Σ3 grain boundaries is limited to 55 to 65 degrees. In the case of including bainitic ferrite having a Σ3 grain boundary fraction of 55 to 65 degrees of 20% or more, the fatigue crack propagation rate can be controlled to be 2.5 * 10 ^-5 mm / cycle or less even under repeated stress. In addition, in the manufacturing process of this invention, the upper limit of (Sigma) 3 grain fraction is limited to 40%. When the phase martensite is included, the fatigue crack is easily passed through the Σ3 grain boundary, so that the smaller the phase martensite is included, the more preferably the phase martensite is included in an area fraction of 1% or less. In addition, when the grain size exceeds 1 µm, the grains become excessively coarse, and thus, a boundary that is vulnerable to impact toughness is formed. Therefore, the grain size is preferably limited to 1 µm or less.

In addition, as shown in Figure 2, depending on the cooling end temperature of the present invention can obtain a Σ3 grain boundary fraction of 20 to 40% of 55 ~ 65 degrees, fatigue crack growth rate is controlled to 2.5 * 10 ^-5 mm / cycle or less can do. In addition, the steel sheet of the present invention can be controlled to yield strength of 690MPa or more, tensile strength of 800MPa or more by the above structure characteristics.

The most preferred method elicited by the present inventors for the production of steels meeting the objects of the present invention as described above is described below.

In the manufacturing method of the present invention, after heating a slab having a steel composition of the present invention, the heated slab is subjected to one or two or more steps of multi-stage rolling in an austenite recrystallization zone, and then lower than the austenite recrystallization temperature. After finishing rolling in one or two or more multi-stages at a temperature higher than the temperature at which the nit transforms into ferrite (Ar3), it is cooled at a cooling rate of 5 ° C / s or more and cooled at 250 ° C to bainite transformation end temperature (Bf). Quit.

Hereinafter, detailed conditions of each step will be described.

Slab reheating temperature: 1050 ~ 1250 ℃

The reheating temperature of the steel of the present invention is preferably at least 1050 ° C, in order to solidify the carbonitrides of titanium and niobium formed during casting. However, when reheating excessively high temperature, austenite may coarsen, so the reheating temperature is preferably 1250 ° C or lower.

Rough rolling: Tnr ~ 1250 ℃

The slab reheated as described above is subjected to rough rolling after heating to adjust its shape. The rolling temperature is at least the temperature Tnr at which recrystallization of austenite stops. By rolling, the casting structure such as dendrites formed during casting can be destroyed and the size of austenite can be reduced.

Finish rolling: Bs ~ Tnr

Finish rolling is performed by a method for introducing a nonuniform microstructure into an austenite structure of a roughly rolled steel sheet. The rolling temperature is preferably limited to the bainite transformation start temperature (Bs) or more than the austenite recrystallization temperature (Tnr). When the recrystallization temperature is exceeded, the size of the austenite becomes coarse.

Cooling condition after rolling: Cooling finish at 250 ℃ ~ Bf temperature with cooling speed over 5 ℃ / s

The cooling condition of the present invention is one of the main features of the present invention, and the cooling is terminated at a temperature of 250 ° C to Bf at a cooling rate of 5 ° C / s or more. According to these cooling conditions, the microstructure of the steel material can be produced as bainitic ferrite in the columnar phase. When cooling is complete | finished beyond Bf temperature, bainitic ferrite cannot be obtained as a main phase, and the Σ3 grain boundary fraction which is 55-65 degree | times becomes less than 20%. It is preferable to limit the lower limit of the cooling end temperature to 250 ° C on site conditions. Further, even when the cooling rate is less than 5 ° C / s, bainitic ferrite cannot be obtained as the main phase, and the strength is lowered. However, the upper limit of the cooling rate is not necessarily limited, but the upper limit may be limited due to the field operating conditions.

A graph that can be compared is shown in FIG. 3, and the cooling conditions of the present invention are shown in FIG. 3 as a cooling curve 3, and it can be seen that the main phase can obtain bainitic ferrite. On the other hand, the cooling curve 2 is the case where the cooling end temperature is Bf or more, granular bainite is the main phase, the cooling curve 1 is less than 5 ℃ / s, it can be seen that the granular bainite is columnar.

Hereinafter, the present invention will be described in detail by way of examples.

(Example)

Steel slabs having a component system satisfying Table 1 below were rolled and cooled according to the manufacturing conditions of Table 2 below, and the yield strength, Σ3 grain boundary fraction and crack propagation rate of 55 to 65 degrees were measured and shown in Table 3 below. In addition, a microstructure photograph of Inventive Example 1 is shown in FIG. 4, a microstructure photograph of Inventive Example 2 is shown in FIG. 5 (a), and a graph of azimuth difference between? 3 grain boundaries is shown in FIG. 5 (b).

division C Si Mn P S Al Ni Cu Cr Mo Ti Nb V B N Grade 1 0.053 0.17 2.1 0.013 0.002 0.015 0.42 0.22 0.32 0.16 0.016 0.04 0.04 0.0016 0.0042 Gangjong 2 0.062 0.38 1.92 0.013 0.005 0.032 0.82 - - 0.38 0.013 0.02 0.04 0.0014 0.0053 Gangjong 3 0.055 0.16 2.18 0.012 0.002 0.013 0.32 0.17 - - 0.018 0.05 0.04 0.0038 0.0041 Steel grade 4 0.082 0.26 2.08 0.013 0.002 0.013 0.23 - 0.5 - 0.019 0.04 0.0007 0.0043 Steel grade 5 0.033 0.27 2.25 0.013 0.002 0.013 - 0.21 - - 0.017 0.03 - 0.0014 0.0045 Steel grade 6 0.082 0.32 2.12 0.013 0.003 0.013 - - 0.22 - 0.019 0.06 - 0.0021 0.0044 Steel grade 7 0.012 0.21 1.53 0.014 0.003 0.034 - - - - 0.013 0.03 - 0.0021 0.0035 Steel grade 8 0.18 0.32 0.85 0.013 0.001 0.038 - 0.31 - - 0.014 0.04 - 0.0023 0.0029 Steel grade 9 0.095 0.42 1.22 0.013 0.005 0.024 - - 0.48 - 0.01 - - 0.0013 0.0028 Grade 10 0.079 0.23 1.42 0.015 0.009 0.43 - - - 0.07 0.008 0.04 - 0.0001 0.0043

In Table 1, the content unit of each element is weight percent.

Steel grades 1 to 6 are steel grades that satisfy all the component systems of the present invention. In contrast, the carbon content of steel grade 7 is less than the carbon content limited by the present invention, and the steel grade 8 has a carbon content exceeding the carbon content limited by the present invention. Steel grade 9 does not contain niobium, and steel grade 10 has a boron content less than the limit of the present invention.

Psalter
number Steel grade
Reheating
extraction
Temperature
(℃) Tnr
(℃) Crude rolling
End
Temperature
(℃) Bs
(℃) Filamentary pressure
Initiation
Temperature
(℃) Filamentary pressure
End of year
Temperature
(℃) Cooling
speed
(℃) Cooling
End
Temperature
(℃) Bf
(℃) Remarks
(Control Conditions of the Invention
Satisfaction)
(○: satisfaction, ×: dissatisfaction) Inventory 1 One 1065 988 985 533 925 878 14.5 370 413 ○ Inventive Example 2 One 1080 988 1000 533 908 868 22.5 350 413 ○ Inventory 3 One 1120 988 1040 533 898 858 14.9 320 413 ○ Comparative Example 1 One 1110 988 1030 533 920 880 2.5 330 413 × (cooling speed not) Comparative Example 2 One 1050 988 970 533 1058 1018 7.5 385 413 × (Notional rolling temperature exceeded) Comparative Example 3 One 1050 988 970 533 850 810 9.5 480 413 × (cooling end temperature exceeded) Honorable 4 2 1070 834 990 525 775 735 12.5 381 405 ○ Inventory 5 2 1075 834 995 525 755 715 21.5 361 405 ○ Inventory 6 2 1110 834 1030 525 745 705 13.0 331 405 ○ Comparative Example 4 2 1105 834 1025 525 820 780 2.5 341 405 × (cooling speed not) Comparative Example 5 2 1060 834 980 525 905 865 8.5 396 405 × (Notional rolling temperature exceeded) Comparative Example 6 2 1060 834 980 525 850 810 10.5 491 405 × (cooling end temperature exceeded) Honorable 7 3 1080 1040 1000 552 989 949 11.0 362 432 ○ Inventive Example 8 3 1070 1040 990 552 969 929 25.5 382 432 ○ Proposition 9 3 1120 1040 1040 552 959 919 15.5 352 432 ○ Comparative Example 7 3 1100 1040 1020 552 820 780 3.5 362 432 × (cooling speed not) Comparative Example 8 3 1055 1040 975 552 1119 1079 7.5 377 432 × (Notional rolling temperature exceeded) Comparative Example 9 3 1055 1040 975 552 850 810 8.0 512 432 × (cooling end temperature exceeded) Inventory 10 4 1080 987 1000 530 938 898 10.5 332 410 ○ Exhibit 11 4 1070 987 990 530 918 878 15.5 352 410 ○ Inventory 12 4 1120 987 1040 530 908 868 15.5 322 410 ○ Comparative Example 10 4 1100 987 1020 530 820 780 3.0 332 410 × (cooling speed not) Comparative Example 11 4 1155 987 1105 530 1048 1008 7.5 367 410 × (Notional rolling temperature exceeded) Comparative Example 12 4 1055 987 975 530 850 810 9.5 482 410 × (cooling end temperature exceeded) Inventory 13 5 1080 936 1000 558 867 827 10.5 359 438 ○ Inventory 14 5 1070 936 990 558 847 807 18.5 384 438 ○ Honorable Mention 15 5 1120 936 1040 558 837 797 14.5 364 438 ○ Comparative Example 13 5 1100 936 1020 558 820 780 3.5 374 438 × (cooling speed not) Comparative Example 14 5 1055 936 1035 558 977 937 7.5 389 438 × (Notional rolling temperature exceeded) Comparative Example 15 5 1055 936 975 558 850 810 9.5 524 438 × (cooling end temperature exceeded) Inventory 16 6 1080 1071 1000 546 1027 987 10.5 357 426 ○ Inventory 17 6 1070 1071 990 546 977 937 17.5 377 426 ○ Inventory 18 6 1120 1071 1040 546 997 957 15.5 347 426 ○ Comparative Example 16 6 1100 1071 1020 546 820 780 2.5 357 426 × (cooling speed not) Comparative Example 17 6 1055 1071 1035 546 1107 1067 7.5 372 426 × (Notional rolling temperature exceeded) Comparative Example 18 6 1055 1071 975 546 850 810 9.5 507 426 × (cooling end temperature exceeded) Comparative Example 19 7 1080 925 1000 637 883 843 10.5 484 517 × (carbon content exceeded) Comparative Example 20 7 1070 925 990 637 863 823 7.5 464 517 × (carbon content exceeded) Comparative Example 21 7 1120 925 1040 637 853 813 15.0 434 517 × (carbon content exceeded) Comparative Example 22 8 1080 1013 1000 657 971 931 10.0 500 537 × (less than carbon content) Comparative Example 23 8 1070 1013 990 657 951 911 7.5 480 537 × (less than carbon content) Comparative Example 24 8 1120 1013 1040 657 941 901 15.5 450 537 × (less than carbon content) Comparative Example 25 9 1080 804 1000 712 749 709 10.0 561 592 × (not including niobium) Comparative Example 26 9 1070 804 990 712 729 689 7.5 541 592 × (not including niobium) Comparative Example 27 9 1120 804 1040 712 719 679 15.0 511 592 × (not including niobium) Comparative Example 28 10 1080 1005 1000 619 948 908 10.5 470 499 × (boron content under) Comparative Example 29 10 1070 1005 990 619 928 888 7.0 450 499 × (boron content under) Comparative Example 30 10 1120 1005 1040 619 918 878 15.5 420 499 × (boron content under)

Psalter YS (MPa) Σ3 fraction (%) which is 55-65 degrees Crack Growth Rate Inventory 1 719 29 1.98 Inventive Example 2 769 32 1.8 Inventory 3 751 37 1.53 Comparative Example 1 686 35 1.62 Comparative Example 2 673 27 2.12 Comparative Example 3 608 18 2.97 Honorable 4 704 28 2.08 Inventory 5 753 30 1.9 Inventory 6 740 35 1.63 Comparative Example 4 680 33 1.72 Comparative Example 5 670 26 2.21 Comparative Example 6 609 17 3.07 Honorable 7 702 30 1.91 Inventive Example 8 757 27 2.09 Proposition 9 731 32 1.82 Comparative Example 7 671 30 1.91 Comparative Example 8 679 28 2.04 Comparative Example 9 580 16 3.26 Inventory 10 721 35 1.64 Exhibit 11 731 32 1.82 Inventory 12 750 36 1.55 Comparative Example 10 687 35 1.64 Comparative Example 11 686 29 1.95 Comparative Example 12 607 18 2.99 Inventory 13 705 31 1.88 Inventory 14 722 26 2.19 Honorable Mention 15 724 30 1.92 Comparative Example 13 663 29 2.01 Comparative Example 14 671 27 2.15 Comparative Example 15 570 15 3.36 Inventory 16 706 31 1.86 Inventory 17 724 28 2.04 Inventory 18 735 32 1.77 Comparative Example 16 670 31 1.86 Comparative Example 17 682 29 1.99 Comparative Example 18 585 16 3.21 Comparative Example 19 610 17 3.01 Comparative Example 20 613 19 2.83 Comparative Example 21 673 19 2.66 Comparative Example 22 596 16 3.15 Comparative Example 23 600 18 2.97 Comparative Example 24 661 19 2.7 Comparative Example 25 538 14 3.7 Comparative Example 26 544 15 3.52 Comparative Example 27 609 16 3.25 Comparative Example 28 622 18 2.88 Comparative Example 29 625 19 2.7 Comparative Example 30 684 19 2.65

In Table 3, the unit of crack propagation rate is 10 ⁻⁵ mm / cycle.

As shown in Table 2 and Table 3, Comparative Examples 1, 4, 7, 10, 13 and 16, the cooling rate is less than 5 ℃ / s, the main tissue is granular bainite yield strength is more than 690MPa I didn't get it. In Comparative Examples 2, 5, 8, 11, 14, and 17, the finishing rolling temperature exceeded Tnr, and thus yield strength was not more than 690 MPa. In Comparative Examples 3, 6, 9, 12, 15, and 18, the cooling end temperature exceeded Bf, and the Σ3 grain boundary fraction of 55 to 65 degrees was less than 20%, and the crack growth rate was 2.5 * 10 ^-5 mm / cycle. Exceeded. In Comparative Examples 19 to 30, the manufacturing conditions were within the limits of the present invention, but did not satisfy the component system. The yield strength was less than 690 MPa, the Σ3 grain boundary fraction of 55 to 65 degrees was less than 20%, and the crack growth rate was 2.5 * 10- ^. Exceeded ⁵ mm / cycle

On the other hand, Inventive Examples 1 to 18 have a yield strength of 690 MPa or more, a tensile strength of 800 MPa or more, a Σ3 grain boundary of 20 to 40% having a 55 to 65 degree, and a crack propagation rate of 2.5 * 10 ^-5 mm / cycle or less. Thus, it can be confirmed that excellent fatigue crack suppression characteristics and strength can be secured. In addition, as shown in Figure 4, the base tissue was a bainitic ferritic, as shown in Figure 5, it can be confirmed that the bearing difference is around 60 degrees.

Figure 1 (a) is an optical micrograph of the microstructure (bainite) of the steel sheet produced by the prior art.

FIG. 1 (b) is a photograph of the steel sheet shown in FIG.

Figure 1 (c) is an optical micrograph of the microstructure (bainite) of the steel sheet produced by the present invention.

Figure 1 (d) is a photograph of the steel plate shown in Figure 1 (c) after the Charpy impact test (-40 ℃) observed the fracture surface through the scanning transfer microscope.

2 is a graph showing the correlation between the fraction of Σ3 and the crack growth rate according to the cooling end temperature.

3 is a view that can compare the cooling conditions of the present invention and the conventional cooling conditions.

4 is a microstructure photograph of Inventive Example 1. FIG.

FIG. 5 is a graph showing the orientation difference between the microstructured photograph of Example 2 and the lath of bainite structure. FIG.

Claims (6)

By weight, carbon (C): 0.02 to 0.10%, silicon (Si): 0.01 to 0.6%, manganese (Mn): 1.8 to 2.5%, aluminum (Al): 0.005 to 0.5%, niobium (Nb): 0.005 0.10%, boron (B): 5-40 ppm, titanium (Ti): 0.005-0.1%, nitrogen (N): 15-150 ppm, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, It contains residual iron (Fe) and other unavoidable impurities, and the microstructure is a columnar phase and contains bainitic ferrite having a Σ3 grain boundary fraction of 20 to 40% having a grain boundary orientation difference of 55 to 65 degrees between bainite laths, and a yield strength of 690 MPa. Ultra-high strength steel sheet with excellent fatigue crack growth suppression characteristics. The method of claim 1, wherein the steel sheet is chromium (Cr): 0.05 ~ 1.0%, molybdenum (Mo): 0.01 ~ 1.0%, nickel (Ni): 0.01 ~ 2.0%, copper (Cu): 0.01 ~ 1.0% and vanadium (V): Ultra high strength steel sheet excellent in fatigue crack propagation suppression properties further comprising one or two or more of 0.005 ~ 0.3%. The ultra-high strength steel sheet according to claim 1, wherein the steel sheet is excellent in fatigue crack propagation suppression property including 1% or less of phase martensite of 1 µm or less. The ultra-high strength steel sheet having excellent fatigue crack growth suppression characteristics according to claim 1, wherein the fatigue crack growth rate of the steel sheet is 2.5 * 10 ^-5 mm / cycle or less. By weight, carbon (C): 0.02 to 0.10%, silicon (Si): 0.01 to 0.6%, manganese (Mn): 1.8 to 2.5%, aluminum (Al): 0.005 to 0.5%, niobium (Nb): 0.005 0.10%, boron (B): 5-40 ppm, titanium (Ti): 0.005-0.1%, nitrogen (N): 15-150 ppm, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, Reheating the slab comprising residual iron (Fe) and other unavoidable impurities at 1050-1250 ° C .; Roughly rolling the reheated slab at an austenite recrystallization temperature (Tnr) ˜1250 ° C .; Finishing the roughly rolled steel sheet in the range of bainite transformation start temperature (Bs) to Tnr; And A method of manufacturing an ultra-high strength steel sheet having excellent fatigue crack propagation suppression characteristics, including a cooling step of cooling the finishing rolled steel sheet at a cooling rate of 5 ° C./s or more at 250 ° C. to bainite transformation end temperature (Bf). The steel sheet is chromium (Cr): 0.05 ~ 1.0%, molybdenum (Mo): 0.01 ~ 1.0%, nickel (Ni): 0.01 ~ 2.0%, copper (Cu): 0.01 ~ 1.0% and vanadium (V): A method for producing an ultra high strength steel sheet having excellent fatigue crack propagation suppression characteristics, including one or two or more of 0.005 to 0.3%.

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