KR101271792B1 - Steel having excellent strength and impact toughness and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel having excellent strength and impact toughness and a method of manufacturing the same, and more particularly, to a steel having excellent strength and impact toughness and a method of manufacturing the same that can be used for welding structures such as offshore wind towers and foundation structures. will be.
In the present invention, by weight%, C: 0.03-0.12%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Al: 0.005-0.5%, P: 0.02% or less, B: 5-40 ppm, N: 15 ~ 150ppm, S: 100ppm or less, Nb: 0.005 ~ 0.10%, balance Fe and other unavoidable impurities, microstructure consists of 5 ~ 10 area% MA (Martensite / Austenitic mixed tissue) tissue and balance granular bay It is made of a nitrite and bainitic ferrite, and provides a steel and excellent manufacturing method of the impact and toughness including 5 × 10 ⁵ ~ 10 × 10 ⁵ carbide per 1 mm ² and its manufacturing method.
According to one aspect of the invention, according to one aspect of the invention, it is possible to provide a steel having a tensile strength of 500MPa or more and Charpy impact energy of 150J or more at -50 ℃.

Description

Steel material with excellent strength and impact toughness and its manufacturing method {STEEL HAVING EXCELLENT STRENGTH AND IMPACT TOUGHNESS AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel having excellent strength and impact toughness and a method of manufacturing the same, and more particularly, to a steel having excellent strength and impact toughness and a method of manufacturing the same that can be used for welding structures such as offshore wind towers and foundation structures. will be.

Recently, development of alternative energy such as solar power and wind power is being actively conducted as an environmentally friendly energy resource. In the case of wind power generation, wind power generators, which were mainly built on land, have been recently constructed on the sea, which is advantageous for high efficiency and reduction of pollution such as noise. In particular, the expansion of installation in cryogenic offshore areas such as North America and Northern Europe is expected. Accordingly, the development of normalized heat-treated steel for low temperature impact toughness is required.

In the case of steel materials used in wind power generators and the like, they are mainly produced by a method called normalizing heat treatment. However, coarse austenite is formed by growing and transforming austenite mainly at the grain boundary during the conventional normalizing heat treatment, and ferrite is formed from the coarse austenite during cooling.

In other words, such normalizing heat treatment has many problems in forming coarse ferrite to impart important steel properties such as lowering strength and guaranteeing low temperature impact toughness.

One aspect of the present invention is to provide a steel and a method for producing the same by improving the tensile strength and low-temperature impact toughness by appropriately controlling the composition components, and evenly distributed carbide in the steel by performing controlled rolling and controlled cooling.

In the present invention, by weight%, C: 0.03-0.12%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Al: 0.005-0.5%, P: 0.02% or less, B: 5-40 ppm, N: 15 ~ 150ppm, S: 100ppm or less, Nb: 0.005 ~ 0.10%, balance Fe and other unavoidable impurities, contains 5 × 10 ⁵ to 10 × 10 ⁵ carbides per 1 mm ² , with 5-10 area of microstructure It provides steel with excellent strength and impact toughness consisting of% MA (martensite / austenite mixed tissue) tissue and residual granular bainite and bainitic ferrite.

At this time, the steel material may further include Ca: 0.0005 to 0.0060%, Cu: 0.010 to 1.0% or Ni: 0.01 to 2.0% of one or more may be further included, V: 0.005 At least 0.3%, Mo: 0.01-1.0%, Cr: 0.05-1.0%, Ti: 0.005-0.1%, Co: 0.005-0.2%, W: 0.005-0.2% can do. The average particle size of the MA structure is preferably 5 μm or less (excluding 0). The steel material has a tensile strength of 500 MPa or more, and Charpy impact energy at -50 ° C is preferably 150 J or more.

In the present invention, by weight%, C: 0.03-0.12%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Al: 0.005-0.5%, P: 0.02% or less, B: 5-40 ppm, N: 15 Reheating step of reheating steel material which consists of -150 ppm, S: 100 ppm or less, Nb: 0.005-0.10%, remainder Fe and other unavoidable impurities at 1050-1250 degreeC; Crude rolling step of roughly rolling the reheated steel at Tnr ~ 1250 ℃; A finishing rolling step of finishing the roughly rolled steel at Bs ~ Tnr ° C .; A cooling step of cooling the filament-rolled steel at a rate of 2 to 10 ° C./s to stop cooling at 600 to 700 ° C .; Tempering treatment step of maintaining the cooled steel for 1 to 2 hours per 10mm thickness at 500 ℃ ~ Ac1; A heating maintenance step of heating the tempered steel at Ac 3 to 1000 ° C. (1.3 × t + 10 to 30 minutes), where t denotes the thickness of the steel (mm); And it provides a method of producing a steel having excellent strength and impact toughness comprising the step of air-cooling the heated and maintained steel.

At this time, the steel material may further include Ca: 0.0005 to 0.0060%, Cu: 0.010 to 1.0% or Ni: 0.01 to 2.0% of one or more may be further included, V: 0.005 At least 0.3%, Mo: 0.01-1.0%, Cr: 0.05-1.0%, Ti: 0.005-0.1%, Co: 0.005-0.2%, W: 0.005-0.2% can do.

According to one aspect of the present invention, it is possible to provide a steel having a tensile strength of 500MPa or more and Charpy impact energy of 150J or more at -50 ° C.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows that reverse transformation austenite is formed at the time of normalizing heat processing from carbide.
2 is a microstructure photograph of an example of a steel material observed under an optical microscope after controlled rolling-controlled cooling according to the present invention.
3 is a microstructure photograph of an example of a steel material observed under an optical microscope after controlled rolling-controlled cooling-tempering according to the present invention.
Figure 4 is a graph showing the change in the MA tissue fraction and carbide density according to the cooling end temperature of the invention examples in accordance with the present invention.

Hereinafter, the present invention will be described.

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

C is the most important element that not only forms martensite Austenite Constituent (MA) but also determines the size and fraction of the formed martensite tissue. When C is less than 0.03%, the fraction of phase martensite is less than or equal to 1%, and it may be difficult to secure sufficient strength, and when C exceeds 0.12%, the pearlite fraction is increased so that phase martensite formation is degraded. There is this. Therefore, the content of C is preferably in the range of 0.03 to 0.12%. On the other hand, when used as a welded steel structure, it is more preferable to make the range of C 0.03 to 0.09% for weldability.

Mn: 0.3 ~ 2.5%

Mn is a useful element that improves strength by solid solution strengthening, so it is necessary to add 0.3% or more. However, when the content exceeds 2.5%, the toughness of the weld portion is greatly reduced due to excessive increase in hardenability, so that the Mn content is preferably 0.3 to 2.5%.

Si: 0.01 ~ 0.6%

Si is used as a deoxidizer and is useful because of its strength-improving effect, but when added in excess of 0.6%, Si lowers toughness at the same time and deteriorates weldability. When added at less than 0.01%, the deoxidation effect is insufficient, so the content of Si is preferably 0.01 to 0.6%. On the other hand, since the Si increases the stability of the phase martensite, it can form a large amount of martensite even with a small C content, which is helpful in improving the strength, but results in a decrease in toughness, so that the range of 0.1 to 0.4% It is more preferable to add.

Al: 0.005-0.5%

Since Al is an element that can deoxidize molten steel at low cost, it is preferable to add 0.005% or more, but when it exceeds 0.5%, it causes nozzle clogging during continuous casting. Therefore, the content of Al is in the range of 0.005 to 0.5%. It is preferable. On the other hand, since the solid solution Al promotes the formation of phase martensite, a large amount of phase martensite can be formed even with a small amount of C, which is helpful in improving strength and toughness, so that it is added in the range of 0.01 to 0.05%. More preferred.

P: 0.02% or less

Although P is an element that is advantageous in improving the strength and corrosion resistance, it is preferable to be contained as low as possible because it is an element that greatly impairs the impact toughness.

S: 0.01% or less

Since S is an element which forms MnS or the like and greatly impairs impact toughness, S is preferably contained as low as possible. However, since S is an unavoidable impurity in the manufacturing process, S is preferably managed with an upper limit of 0.01%.

B: 5-40 ppm

B is an inexpensive element and is an advantageous element showing strong hardenability. It is preferable to add 5ppm or more because the strength is greatly improved even with a small amount of addition, but when it exceeds 40ppm, Fe ₂₃ (CB) ₆ is formed, rather the curing ability is lowered, and the low temperature toughness is also greatly reduced. Therefore, the content of B is preferably in the range of 5 to 40 ppm. On the other hand, B contributes greatly to the formation of bainite even in low-speed cooling in the cooling after rough rolling, and also has the effect of promoting the formation of phase martensite in the final cooling.

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 preferably 15 ppm.

Nb: 0.005 to 0.10%

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, when reheated at a high temperature, the dissolved Nb has an effect of suppressing recrystallization of austenite and suppressing transformation of ferrite or bainite to refine the structure. 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. For the above effect, Nb is preferably added at 0.005% or more, but when it exceeds 0.10%, the possibility of causing brittle cracks in the corners of the steel increases, which is not preferable. Accordingly, the content of Nb is preferably in the range of 0.005 to 0.10%.

Cu: 0.010 ~ 1.0%

Since Cu is an element capable of increasing the strength while minimizing the toughness of the base metal, at least 0.01% must be added in order for the effect to appear. On the other hand, since the surface quality of the product is greatly inhibited when it exceeds 1.0%, the content of Cu is preferably set to 0.010% to 1.0 %%.

Ni: 0.01 to 2.0%

Ni is an element that can improve the strength and toughness of the base material at the same time, and 0.01% or more must be added for the effect to appear. However, since Ni is an expensive element, when it exceeds 2.0%, the economic efficiency is lowered, and there is also a problem of deterioration in weldability. Therefore, the content of Ni is preferably in the range of 0.01 to 2.0%.

V: 0.005-0.3%

The temperature of V is lower than that of other fine alloys, and it is preferable to add 0.005% or more because it has an effect of preventing the drop in strength by depositing in the weld heat affected zone. However, when exceeding 0.3%, since it can reduce, it is preferable to make said V into 0.005 to 0.3%.

Mo: 0.01 ~ 1.0%

Mo needs to be added at 0.01% or more because Mo can greatly improve the hardenability and suppress the formation of ferrite, and the strength can be greatly improved even with a small amount of addition. However, if it exceeds 1.0%, the hardness of the weld is excessive. May increase and inhibit toughness. Therefore, the Mo content is preferably in the range of 0.01 to 1.0%, and in the present invention, in order to form the island martensite in an appropriate range for securing the tensile strength, it is more preferably in the range of 0.02 to 0.2%. .

Cr: 0.05 to 1.0%

Since Cr has a great effect on increasing the strength by increasing the hardenability, it is necessary to add 0.05% or more in order to obtain such effects, but when it exceeds 1.0%, the weldability can be greatly reduced, so the range of 0.05 to 1.0% It is preferred to be added. 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.

Ti: 0.005 to 0.1%

Addition of Ti can greatly improve low-temperature toughness by inhibiting grain growth upon reheating, so 0.005% or more must be added to express the effect. However, if the content exceeds 0.1%, there is a problem that the low temperature toughness due to clogging of the playing nozzle or crystallization of the center part is reduced, so that the Ti content is preferably in the range of 0.005 to 0.1%.

Co: 0.005 ~ 0.2%

Co is an element advantageous to prevent softening of the matrix structure and to form carbides, but is expensive, so it is preferably added in the range of 0.005 to 0.2%.

W: 0.005-0.2%

W is an effective element that prevents softening of the matrix structure and precipitates carbide, but is expensive, so it is preferably added in the range of 0.005 to 0.2%.

Ca: 0.0005-0.0060%

Ca may be added to form CaS to suppress non-metallic inclusions of MnS, for which 0.0005% or more may be added. However, if the content is more than 0.0060%, and reacts with oxygen contained in the steel to produce CaO, a nonmetallic substitute, the content of Ca is preferably 0.0005 to 0.0060%.

The steel proposed by the present invention preferably contains 5 × 10 ⁵ to 10 × 10 ⁵ carbides per mm ² . BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows that reverse transformation austenite is formed at the time of normalizing heat treatment from carbide, (a) is an existing material, (b) shows an invention material. As can be seen in Figure 1, the carbide acts as a reverse transformation austenite nucleation site during normalizing heat treatment, unlike the existing material, by producing a large amount of carbide, to form fine austenite grains to fine during air cooling By transforming to ferrite, the strength and low temperature impact properties of the steel can be improved. In order to express the effect, it is necessary to form 5 × 10 ⁵ or more carbides per 1 mm ^{2 in the} steel, but when it exceeds 5 × 10 ⁵ coarse carbides are formed to reduce toughness.

2 is a microstructure photograph of an example of a steel material observed under an optical microscope after controlled rolling-controlled cooling according to the present invention. As shown in Figure 2, the microstructure of the steel of the present invention is preferably composed of 5 to 10% area of MA (martensite / austenite mixed tissue) tissue and the balance granule bainite and bainitic ferrite. MA structure condenses carbon and exists as a mixed structure of martensite and austenite at room temperature. At the time of tempering, the carbon condensed in the MA diffuses to the matrix and carbide forming elements are bonded to distribute fine carbide evenly to the matrix. When the MA structure is less than 5%, it may cause ductility deterioration, and when it exceeds 10%, there is a risk that the carbides formed on the base at the time of tempering are coarse to inhibit toughness. On the other hand, in order to obtain such a MA tissue, the remainder tissue is effective to be made of granular bainite or bainitic ferrite.

At this time, the average particle size of the MA structure is preferably 5 μm or less. When the average particle size of the MA structure exceeds 5 μm, it is difficult to uniformly distribute carbides in the substrate during tempering because the MA is formed evenly in the tissue rather than evenly distributed inside the substrate. The smaller the average particle size of the tissue of the MA, the better. The lower limit thereof is not particularly limited.

As described above, the steel material satisfying the component system, the number of carbides, and the tissue fraction proposed by the present invention will have a tensile strength of 500 MPa or more, and the Charpy impact energy at -50 ° C will also be 150 J or more, thereby providing excellent strength and low temperature impact. Toughness is secured.

Hereinafter, the production method of the present invention will be described.

Reheat the steel satisfying the above composition at 1050 ~ 1250 ℃. In order to solidify the Ti, Nb carbides and their composite carbides formed during casting, it is preferable to reheat at a temperature of 1050 ° C or higher. However, when reheating excessively high temperature, austenite may coarsen, so the upper limit of the reheating temperature is preferably 1250 ° C.

Then, to adjust the shape, the reheated steel is roughly rolled at Tnr ~ 1250 ℃. The rough rolling is preferably at or above the temperature (Tnr) at which recrystallization of austenite stops, and as the rough rolling is performed in the rolling temperature range, casting structures such as dendrite formed during casting are destroyed or the size of austenite is reduced. Can be expressed.

Thereafter, in order to introduce a non-uniform microstructure into the austenite structure of the steel, the roughly rolled steel is subjected to filamentous rolling at Bs ~ Tnr ° C.

The finishing steel is completed is cooled at a rate of 2 ~ 10 ℃ / s, the cooling is stopped at 600 ~ 700 ℃. As described above, by limiting the cooling end temperature to 600 ~ 700 ℃ it can form a MA structure of 5 area% or more in the microstructure of the steel. When the cooling stop temperature is less than 600 ℃ it is difficult to effectively secure the MA structure, when the temperature exceeds 700 ℃ ferrite tissue may occur to reduce the strength, it may be difficult to form the MA structure. If the cooling rate is less than 2 ° C / s, the base structure is not polygonal ferrite is formed can not form the MA structure, if it exceeds 10 ° C / s the fraction of the MA structure by forming the bainitic ferrite on the base This is less than 5%.

3 is a microstructure photograph of an example of a steel material observed under an optical microscope after controlled rolling-controlled cooling-tempering according to the present invention. As can be seen in Figure 3, by performing a tempering treatment to maintain the cooled steel for 1 to 2 hours per 10mm thickness at 500 ℃ ~ Ac1, C concentrated in the MA structure during the tempering heat treatment is a carbide forming element in the substrate And a large amount of carbide to distribute inside the substrate. When the tempering temperature is less than 500 ° C., the carbon condensed in the MA structure diffuses to the matrix to form a carbide, and when it exceeds Ac1, the matrix is decomposed to form an austenite phase.

In addition, if the holding time is less than 1 hour, the carbon condensed in the MA structure is insufficient to diffuse to the base, so that it is difficult to form an appropriate amount of carbide, and if it exceeds 2 hours, the carbide grows coarse and the carbide density is too high. Will be less. Ac1 temperature is as follows.

Ac1 = 723-10.7 × Mn-16.9 × Ni + 29.1 × Si + 16.9 × Cr + 290 × As + 6.38 × W

Thereafter, the tempered steel is subjected to a normalizing heat treatment, and the steel is heated at Ac 3 to 1000 ° C., and then maintained for (1.3 × t + 10 to 30 minutes) (where t is the thickness of the steel (mm)). It is preferable to mean). The holding time 1.3 × t is a time required for the steel to be uniformly heated to the heat treatment temperature, and the steel is uniformly heated when it is held for 10 to 30 minutes after reaching the heat treatment temperature. For example, the time required for normalizing heat treatment of a steel sheet having a thickness of about 100 mm is about 150 minutes, and when it exceeds this, the heat treatment time is too long and is not effective for industrial use. Ac3 temperature is as follows.

Ac3 = 910-203 × C ¹ /2-15.2 × Ni + 44.7 × Si + 104 × V + 31.5 × Mo + 13.1 × W

Subsequently, the steel proposed by the present invention can be secured by air-cooling the heated and maintained steel.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are merely examples for describing the present invention in more detail, and do not limit the scope of the present invention.

(Example)

Steel slabs having the component systems described in Tables 1 and 2 below were made of steel under the conditions of Table 3 below.

division Chemical composition (% by weight) C Si Mn P S Al Ni Cu Cr Inventive Steel 1 0.04 0.25 1.53 0.012 0.002 0.014 0.28 0.13 0.32 Invention river 2 0.08 0.45 1.69 0.013 0.004 0.035 0.17 0 0.24 Invention steel 3 0.045 0.25 1.57 0.012 0.002 0.026 0 0 0.22 Inventive Steel 4 0.11 0.35 1.42 0.013 0.002 0.015 0.27 0.23 0.1 Invention steel 5 0.085 0.25 1.52 0.013 0.004 0.017 0 0.23 0.32 Inventive Steel 6 0.080 0.35 1.43 0.012 0.003 0.015 0 0 0.25 Invention steel 7 0.095 0.45 1.51 0.013 0.002 0.025 0 0 0.19 Comparative River 1 0.003 0.25 1.52 0.014 0.004 0.035 0 0 0 Comparative River 2 0.18 0.35 0.83 0.012 0.001 0.028 0 0 0 Comparative Steel 3 0.06 0.45 1.22 0.013 0.005 0.027 0 0 0 Comparative Steel 4 0.055 0.25 1.42 0.015 0.009 0.044 0 0 0

division Chemical composition (% by weight) Mo Ti Nb V Co W B
(ppm) N
(ppm) Ca Inventive Steel 1 0.07 0.017 0.04 0.03 0.04 0 6 35 0.0015 Invention river 2 0.13 0.015 0.03 0 0 0.02 15 55 - Invention steel 3 0.2 0.023 0.05 0 0 0.1 4 42 - Inventive Steel 4 0.32 0.014 0.04 0.05 0 0 8 42 - Invention steel 5 0.04 0.02 0.04 0 0.1 0 15 42 - Inventive Steel 6 0.2 0.022 0.06 0.04 0 0.15 20 42 0.0030 Invention steel 7 0.15 0.031 0.05 0 0 0 16 42 - Comparative River 1 0 0.013 0.02 0 0 0 20 32 - Comparative River 2 0 0.015 0.03 0 0 0 24 28 - Comparative Steel 3 0 0.011 0 0 0 0 14 26 - Comparative Steel 4 0 0.008 0.03 0 0 0 One 43 -

division Rough rolling condition Cooling conditions Steel grade Specimen No. Slab thickness
(mm) Reheat Extraction Temperature
(℃) Rough rolling end temperature
(℃) Cooling rate
(° C / s) Cooling end temperature
(℃) Inventive Steel 1 Inventory 1 244 1065 914 7 640 Inventory 2 244 1080 894 5 680 Invention 3 220 1120 884 4 667 Comparison 1 220 1050 850 5 530 Invention river 2 Invention 4 244 1070 990 6 655 Invention Article 5 244 1075 995 5 665 Inventions 6 220 1110 1030 4 690 Comparative material 2 220 1060 850 5 505 Invention steel 3 Invention 7 244 1080 1040 6 635 Invention 8 244 1070 1040 5 655 Invention 9 220 1120 1030 4 678 Comparative material 3 220 1055 850 5 525 Inventive Steel 4 Inventions 10 244 1080 1060 6 645 Invention invention 11 244 1070 1040 5 678 Invention 12 220 1120 1030 4 699 Comparison 4 220 1055 975 5 535 Invention steel 5 Invention invention 13 244 1080 1000 6 658 Invention Article 14 244 1070 990 5 643 Invention material 15 220 1120 1040 4 657 Comparative material 5 220 1055 975 5 545 Inventive Steel 6 Invention material 16 244 1080 1000 6 632 Invention 17 244 1070 990 5 678 Invention 18 220 1120 1040 4 688 Comparative material 6 220 1055 975 5 515 Invention steel 7 Invention Material 19 244 1080 1000 6 677 Invention 20 244 1070 990 5 656 Inventive Materials21 220 1120 1040 4 688 Comparison 7 220 1055 975 5 522 Comparative River 1 COMPARISON 8 244 1080 1000 6 645 Comparative material 9 244 1070 990 5 633 Comparative material 10 220 1120 1040 4 655 Comparative River 2 Comparative material 11 244 1080 1000 6 679 Comparative material 12 244 1070 990 5 655 Comparative material 13 220 1120 1040 4 636 Comparative Steel 3 Comparative material 14 244 1080 1000 6 638 Comparative material 15 244 1070 990 5 669 Comparative material 16 220 1120 1040 4 679 Comparative Steel 4 Comparative Material17 244 1080 1000 6 676 Comparative Material 18 244 1070 990 5 697 Comparative Material 19 220 1120 1040 4 679

division Thickness
(mm) Tempering condition Normalizing condition Steel grade Specimen No. Tempering temperature
(℃) Tempering time
(minute) Normalizing
Temperature (℃) Retention time
(minute) Inventive Steel 1 Inventory 1 75 663 450 907 113 Inventory 2 30 653 180 915 54 Invention 3 55 633 330 901 87 Comparison 1 60 550 360 895 89 Invention river 2 Invention 4 70 650 421 910 106 Invention Article 5 30 640 180 918 54 Inventions 6 50 550 302 904 81 Comparative material 2 40 700 240 913 67 Invention steel 3 Invention 7 80 680 480 913 119 Invention 8 20 570 120 921 41 Invention 9 58 690 348 907 90 Comparative material 3 30 60 180 916 54 Inventive Steel 4 Inventions 10 70 550 420 916 106 Invention invention 11 35 630 210 924 61 Invention 12 50 570 300 910 80 Comparison 4 20 610 120 919 41 Invention steel 5 Invention invention 13 55 565 330 919 87 Invention Article 14 60 655 360 927 93 Invention material 15 70 645 420 913 106 Comparative material 5 40 575 240 922 67 Inventive Steel 6 Invention material 16 60 655 360 922 93 Invention 17 35 700 210 930 61 Invention 18 40 590 240 916 67 Comparative material 6 30 635 180 925 54 Invention steel 7 Invention Material 19 55 630 330 925 87 Invention 20 45 620 270 933 74 Inventive Materials21 35 630 210 919 61 Comparison 7 30 555 180 928 54 Comparative River 1 COMPARISON 8 20 560 120 928 41 Comparative material 9 25 570 150 936 48 Comparative material 10 50 630 300 922 80 Comparative River 2 Comparative material 11 60 570 360 916 93 Comparative material 12 25 600 150 931 48 Comparative material 13 50 635 300 931 80 Comparative Steel 3 Comparative material 14 55 670 330 939 87 Comparative material 15 20 680 120 925 41 Comparative material 16 40 635 240 919 67 Comparative Steel 4 Comparative Material17 50 615 300 934 80 Comparative Material 18 60 610 360 934 93 Comparative Material 19 30 550 180 942 54

After measuring the microstructure, carbide density and mechanical properties of the steel produced as described above, the results are shown in Table 5 below.

division MA fraction
(area%)
Carbide density
(× 10 ⁵ pieces / mm ² ) The tensile strength
(MPa) vE _{-50 ℃}
(J) Steel grade Specimen No. Inventive Steel 1 Inventory 1 6.19 6.37 532 166 Inventory 2 6.94 7.76 565 188 Invention 3 6.72 7.31 554 181 Comparison 1 2.8 2.43 439 102 Invention river 2 Invention 4 6.5 6.89 544 174 Invention Article 5 6.69 7.24 553 180 Inventions 6 7.09 8.11 573 194 Comparative material 2 1.76 1.52 418 87 Invention steel 3 Invention 7 6.08 6.19 528 163 Invention 8 6.5 6.89 544 174 Invention 9 6.91 7.69 563 187 Comparative material 3 2.6 2.25 435 99 Inventive Steel 4 Inventions 10 6.3 6.54 536 168 Invention invention 11 6.91 7.69 563 187 Invention 12 7.21 8.42 580 199 Comparison 4 3 2.61 443 105 Invention steel 5 Invention invention 13 6.56 7 547 176 Invention Article 14 6.25 6.47 534 167 Invention material 15 6.54 6.96 546 175 Comparative material 5 3.38 2.97 452 110 Inventive Steel 6 Invention material 16 6.01 6.08 525 161 Invention 17 6.91 7.69 563 187 Invention 18 7.06 8.04 571 193 Comparative material 6 2.19 1.88 426 93 Invention steel 7 Invention Material 19 6.89 7.66 562 186 Invention 20 6.52 6.93 545 175 Inventive Materials21 7.06 8.04 571 193 Comparison 7 2.48 2.14 432 97 Comparative River 1 COMPARISON 8 2.1 1.75 423 91 Comparative material 9 2.4 1.95 428 94 Comparative material 10 2.3 1.88 426 93 Comparative River 2 Comparative material 11 1.7 1.51 417 87 Comparative material 12 2.1 1.75 423 91 Comparative material 13 1.9 1.62 420 88 Comparative Steel 3 Comparative material 14 2.3 1.88 426 93 Comparative material 15 1.7 1.51 417 87 Comparative material 16 1.5 1.41 415 85 Comparative Steel 4 Comparative Material17 2.7 2.18 433 98 Comparative Material 18 3.3 2.7 445 106 Comparative Material 19 3.4 2.8 448 108

As can be seen in Tables 1 to 5, Inventive Materials 1 to 21 in accordance with the present invention has a MA structure of more than 5%, and also achieved a high level of carbide density, excellent tensile strength and low temperature impact toughness It is confirmed that it has a value.

However, Comparative Materials 1 to 7 satisfying the composition and the range proposed by the present invention did not satisfy the manufacturing conditions of the present invention, and thus could not secure a certain level of MA structure or carbide density, and thus tensile strength. And low temperature impact toughness also have a low range.

Although Comparative Materials 8 to 19 were manufactured to meet the manufacturing conditions of the present invention, the present invention did not satisfy the composition and range proposed by the present invention, and thus high tensile strength and low temperature impact toughness could not be obtained.

Figure 4 is a graph showing the change in MA structure fraction and carbide density according to the cooling end temperature of the invention materials 1 to 21. As can be seen in Figure 4, it can be seen that the MA tissue fraction and the carbide density increases linearly as the cooling end temperature increases. This indicates that the influence of the MA structure fraction is increased as the cooling end temperature is increased, and the carbide density in the steel is increased during tempering as the MA tissue fraction is increased.

Claims (11)

By weight%, C: 0.03-0.12%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Al: 0.005-0.5%, P: 0.02% or less, B: 5-40 ppm, N: 15-150 ppm, S: 100 ppm or less, Nb: 0.005 to 0.10%, Mo: 0.01 to 1.0%, Ti: 0.005 to 0.1%, balance Fe and other unavoidable impurities,
Contains 5 × 10 ⁵ to 10 × 10 ⁵ carbides per 1 mm ² ,
Steel with excellent strength and impact toughness, consisting of MA (Martensitic / Austenitic mixed tissue) tissue with 5-10 area% of microstructure and residual granular bainite and bainitic ferrite. The steel material of claim 1, wherein the steel further comprises Ca: 0.0005 to 0.0060%.
The steel material according to claim 1, wherein the steel further comprises at least one of Cu: 0.010 to 1.0% or Ni: 0.01 to 2.0%.
The strength of claim 1, wherein the steel further comprises at least one member selected from the group consisting of V: 0.005 to 0.3%, Cr: 0.05 to 1.0%, Co: 0.005 to 0.2%, and W: 0.005 to 0.2%. And steel with excellent impact toughness.
The steel of claim 1, wherein the MA grains have an average particle size of 5 µm or less (excluding 0).
According to claim 1, wherein the steel is excellent in strength and impact toughness, characterized in that the tensile strength of 500MPa or more.
The steel according to claim 1, wherein the steel has excellent strength and impact toughness with a Charpy impact energy of 150 J or more at -50 ° C.
By weight%, C: 0.03-0.12%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Al: 0.005-0.5%, P: 0.02% or less, B: 5-40 ppm, N: 15-150 ppm, S: 100 ppm or less, Nb: 0.005 to 0.10%, Mo: 0.01 to 1.0%, Ti: 0.005 to 0.1%, Reheating step of reheating the steel material consisting of residual Fe and other unavoidable impurities at 1050 to 1250 ℃;
Crude rolling step of roughly rolling the reheated steel at Tnr ~ 1250 ℃;
A finishing rolling step of finishing the roughly rolled steel at Bs ~ Tnr ° C .;
A cooling step of cooling the filament-rolled steel at a rate of 2 to 10 ° C./s to stop cooling at 600 to 700 ° C .;
Tempering treatment step of maintaining the cooled steel for 1 to 2 hours per 10mm thickness at 500 ℃ ~ Ac1;
A heating maintenance step of heating the tempered steel at Ac 3 to 1000 ° C. (1.3 × t + 10 to 30 minutes), where t denotes the thickness of the steel (mm); And
Method of producing a steel having excellent strength and impact toughness comprising the step of air-cooling the heated and maintained steel.
(However, the Tnr is austenite recrystallization temperature, Bs is the bainite transformation start temperature.)
The method of claim 8, wherein the steel further comprises Ca: 0.0005 to 0.0060%. 10.
The method of claim 8, wherein the steel further comprises at least one of Cu: 0.010 to 1.0% or Ni: 0.01 to 2.0%.
The strength of claim 8, wherein the steel further comprises at least one selected from the group consisting of V: 0.005 to 0.3%, Cr: 0.05 to 1.0%, Co: 0.005 to 0.2%, and W: 0.005 to 0.2%. And a method for producing steel having excellent impact toughness.

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