KR101536387B1 - High-strength steel sheet having superior cryogenic temperature toughness and low yield ratio property and manufacturing method thereof - Google Patents

Abstract

A steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.02 to 0.12% of C, 0.05 to 0.5% of Si, 0.5 to 2.0% of Mn, 1.0 to 2.0% of Ni, 0.005 to 0.1% of Ti, 0.005 to 0.1% , P: not more than 0.015%, S: not more than 0.015%, the balance Fe and other unavoidable impurities, T _AR = 208 x Al - 25 x Ni> 5 and the microstructure has an average grain size of 10 μm or less Of 10 to 30% of MA (martensite / austenite mixed structure) having an average grain size of 5 占 퐉 or less and excellent low-temperature toughness and resistance-to-brittleness characteristics, and a process for producing the same.
INDUSTRIAL APPLICABILITY According to the present invention, a high strength steel sheet excellent in cryogenic temperature toughness and having a low resistance can be provided at a low cost through a simple manufacturing process.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel sheet excellent in cryogenic temperature toughness,

The present invention relates to a high-strength steel sheet excellent in cryogenic temperature toughness and having a low-resistance characteristic and a method for producing the same.

Due to the increased environmental regulation due to global warming, interest in CO ₂ treatment is increasing, and the industry that stores and transports CO ₂ and is landfilled in offshore oil exploration area is being rescued. Accordingly, a demand for a steel for a tank for liquefying and storing CO ₂ gas is increasing rapidly.

In order to liquefy the CO ₂ gas, pressurization of at least 7 bar is required. For the tank steel, high strength property is required to withstand the pressurization. For the liquefaction of CO ₂ , the gas tank design temperature is -60 ° C or less, Low temperature toughness of 75 ° C or less is warranted.

In addition, stress relieving of welds is an important part of manufacturing tanks by welding. There is a Post Welding Heat Treatment (PWHT) method, and a mechanical stress relief (MSR) method that removes stress from the welded part by hydraulic injection. However, when the stress of the weld is removed by the mechanical stress relief (MSR) method, the yield of the base material is limited to 0.8 or less because deformation by the water pressure is applied to the base material. This is because when the stress is removed by high pressure water, the ratio of the yield strength to the tensile strength is increased when the deformation more than the yield strength is applied to the base material portion, the tensile strength is reached immediately after the yielding, .

However, due to the PWHT method, it is difficult to remove the stress due to the increase of the CO ₂ gas tank. Therefore, most shipbuilders require the mechanical stress relief (MSR) method.

Methods of improving the strength of steel include precipitation strengthening, solid solution strengthening, and martensite strengthening. These strengthening methods have problems of improving strength but deteriorating toughness. However, when the crystal grains are refined and strengthened, high strength can be obtained and the problem of toughness deterioration can be solved by reduction of impact toughness transition temperature.

Patent Document 1 and Patent Document 2 propose a method of limiting the austenite grain size to a certain size by static recrystallization and rolling the pass austenite non-recrystallized region at a pass ratio of 30% or more to realize fine ferrite.

However, in the above-mentioned inventions, there is a problem in that it is necessary to give a great pressure per pass in the rolling process, which is the main process for manufacturing the steel material, and the time between passes is restricted so that the manufacturing conditions are difficult and installation of a very large rolling facility and control system is required . Further, if only the refining of the ferrite grains is carried out simply, the yield strength increases at the same time as the tensile strength is increased, which makes it impossible to realize a low resistance.

Korean Patent Publication No. 2004-0059579 Korean Patent Publication No. 2004-0059581

One aspect of the present invention is to provide a low-cost high-strength steel sheet excellent in cryogenic toughness and resistance to brittleness which can be used for a gas tank, and a method of manufacturing the same.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to accomplish the above object, one aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.02 to 0.12% of C, 0.05 to 0.5% of Si, 0.5 to 2.0% of Mn, 0.1%, Nb: 0.005 ~ 0.1 %, Al: 0.005 ~ 1.0%, P: 0.015% or less, S: include 0.015% or less, the balance Fe and other unavoidable impurities, and _{T AR = 208 × Al - 25} × Ni> 5, and the microstructure includes 10 to 30% of MA (martensite / austenite mixed structure) containing 70% or more of super fine ferrite having an average grain size of 10 탆 or less and an average grain size of 5 탆 or less and having excellent cryogenic toughness And a high strength steel sheet having a low resistance property.

Another aspect of the present invention is to provide a method of manufacturing a semiconductor device, comprising: 0.02 to 0.12% of C, 0.05 to 0.5% of Si, 0.5 to 2.0% of Mn, 1.0% or less of Ni, 0.005 to 0.1% of Ti, 0.005 to 0.1% 0.005 to 1.0% of Al, not more than 0.015% of P, not more than 0.015% of S and the balance of Fe and other unavoidable impurities and satisfying T _AR = 208 x Al - 25 x Ni >Lt; 0 >C; Controlling the average rolled aspect ratio per pass at a temperature of 1200 ° C to Tnr to 0.5 or more, and subjecting the rolled sheet to a cumulative rolling reduction of 30% or more; Completing the finish rolling at a temperature of Ar3 + 30 占 폚 to Ar3 + 70 占 폚 at a rolling reduction of 10% or more per pass and a cumulative rolling reduction of 60% or more; And finishing cooling temperature (FCT) of 300 to 500 ° C at a cooling rate of 10 ° C / s or more. The present invention also provides a method of manufacturing a high strength steel sheet excellent in cryogenic temperature toughness and low resistance to breakage.

According to an aspect of the present invention, a high-strength steel sheet excellent in cryogenic temperature toughness and having a low resistance can be provided at a low cost through a simple manufacturing process.

1 is a graph showing a relationship between a critical strain and an austenite grain size for generation of strain induced dynamic transformation (SIDT).
2 is a stress-strain curve according to the rolling temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. .

The high strength steel sheet according to the present invention contains 0.02 to 0.12% of C, 0.05 to 0.5% of Si, 0.5 to 2.0% of Mn, 1.0% or less of Ni, 0.005 to 0.1% of Ti, 0.005 to 0.1% of Nb, 0.005 to 1.0% of Al, 0.015% or less of P, 0.015% or less of S, and the balance of Fe and other unavoidable impurities. The relationship T _AR = 208 x Al - 25 x Ni> 5 can be satisfied.

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

C: 0.02 to 0.12%

C is an element which needs to contain an appropriate amount for effectively strengthening the steel. In addition, in the present invention, since the MA phase (martensite / austenite mixed structure) is formed and is the most important element for determining the size and fraction of the MA phase formed, It needs to be contained in the steel. However, when the content of C exceeds 0.12%, the low-temperature toughness is lowered and the fraction of the MA phase exceeds 30%. When the content of C is less than 0.02%, the fraction of the MA phase becomes less than 10% Therefore, the range of C is limited to 0.02 to 0.12%.

Si : 0.05 to 0.5%

Si is useful as a deoxidizer in the steelmaking process because it has a strength strengthening effect by the solid solution strengthening effect. However, when it exceeds 0.5%, it lowers the low temperature toughness and deteriorates the weldability at the same time. If it is less than 0.05%, the effect of deoxidation is insufficient and the strength is not improved, so it is limited to 0.05-0.5%. In addition, since the stability of MA (martensite / austenite composite phase) is enhanced, Si can form a large number of MA phases with a small amount of C, which helps to improve the strength and reduce the resistance. However, The more preferred range of Si is 0.1 to 0.4%, which is desirable because it results.

Mn : 0.5 to 2.0%

Mn is a useful element that contributes to ferrite grain refinement and improves strength by solid solution strengthening, so that at least 0.5% needs to be added. However, the addition of more than 2.0% significantly deteriorates the toughness of the welded part due to an increase in the hardenability, so the range of Mn is limited to 0.5 to 2.0%.

Ni : Not more than 1.0%

Ni is an element selectively added in the present invention and can be optimized for upward ferrite transformation temperature. In general, Ni is almost the only element capable of simultaneously improving the strength and toughness of a base material. However, since Ni is an expensive element, addition of more than 1.0% decreases the economical efficiency. In addition, when Ni is added, rolling at low temperature is required to lower the Ar3 temperature to generate SIDT (Strain Induced Dynamic Transformation), thereby increasing the deformation resistance during rolling and causing difficulty in rolling, so that the upper limit is limited to 1.0% .

Ti  : 0.005 to 0.1%

The addition of Ti can improve the low temperature toughness by inhibiting the growth of crystal grains during reheating by forming oxides and nitrides in the steel. Therefore, in order to exhibit the effect, at least 0.005% should be added, and excessive addition of more than 0.1% There is a problem in that the low temperature toughness due to the clogging of the nozzle or the pitting of the center part is reduced, so it is limited to the range of 0.005 to 0.1%.

Nb  : 0.005 to 0.1%

Nb precipitates in the form of NbC or NbCN, which greatly improves the strength of the base material and welds. In addition, Nb dissolved at the time of reheating at a high temperature has the effect of suppressing recrystallization of austenite and suppressing transformation of ferrite or bainite to make the structure finer. Also, it greatly enhances the stability of austenite during cooling after final rolling, thereby promoting the formation of MA phase (martensite / austenite mixed phase) even at low speed cooling. However, when it is excessively added in excess of 0.1%, the possibility of causing a brittle crack at the edge of the steel increases, which is not preferable.

Al  : 0.005 to 1.0%

Al is one of the important factors in the present invention, and is an element capable of inexpensively deoxidizing molten steel, so that it is preferable to add Al in an amount of 0.005% or more. Since the solid solution of Al promotes the formation of the MA phase (martensite / austenite mixed phase), it can form many MA phases even with a small amount of C, which is helpful for improving the strength and reducing the resistance. In addition, when Al is added, the ferrite transformation temperature is increased to increase the rolling temperature for generating SIDT (strain induced dynamic transformation), so that rolling can be facilitated. However, when it is added too much, it causes clogging of the nozzle during continuous casting, so it is limited to 0.005 ~ 1.0%.

P: not more than 0.015%

P causes a problem of brittle steel as an element which causes grain segregation in the base material and welded portion. Therefore, it is necessary to reduce it positively. However, in order to reduce P to the limit, the load of the steelmaking process is intensified. Therefore, the upper limit is set to 0.015% by weight.

S: not more than 0.015%

S is an element which causes a brittle brittleness and MnS or the like is formed to greatly deteriorate the impact toughness. Therefore, it is advantageous to make the S as low as possible, so that the upper limit is preferably 0.015%.

T _AR  = 208 x Al  - 25 × Ni  > 5

Ni and Al affect the ferrite transformation temperature, which is an important factor for SIDT generation. Therefore, in order to increase the ferrite transformation temperature and to realize the optimum manufacturing conditions, the addition amount of Ni and Al is adjusted so as to satisfy T _AR = 208 * There is a need to limit. If the T _AR value is less than 5, the effect of raising the Ar 3 transformation temperature is small, so that the effect of reduction in deformation resistance and shape improvement during rolling can not be sufficiently obtained and productivity can not be improved.

Since the steel sheet having the above composition affects the low resistance and the low temperature toughness depending on the microstructure even if the same composition is used, the microstructure of the steel sheet of the present invention will be described in more detail.

The microstructure of the steel sheet of the present invention preferably contains 10 to 30% of an MA (martensite / austenite mixed structure) having an average grain size of 5 占 퐉 or less in an ultra fine grain ferrite base having an average grain size of 10 占 퐉 or less. That is, the microcrystalline ferrite may be contained in the microstructure in an amount of 70% or more. The MA phase is a phase in which martensite and austenite phase are mixed and refers to a structure in which austenite remains without being decomposed into ferrite and cementite and a part of the retained austenite is transformed into martensite.

When the fine grain ferrite is formed, it is possible to guarantee the cryogenic toughness by lowering the impact transition temperature along with the increase in the strength due to grain refinement. When the fine MA phase is evenly distributed, the continuous breakage due to the movable potential formed at the MA phase and the ferrite structure interface And the work hardening rate is increased, thereby making it possible to realize a low resistance. In addition, since the MA phase contributes to increasing the tensile strength while lowering the yield strength, it is possible to realize a high strength resisting ratio.

In order to realize the above microstructure, the manufacturing conditions are most important, and in particular, optimization of rolling conditions, especially rolling pass schedules and cooling conditions, is important. Hereinafter, a method of manufacturing the high strength steel sheet of the present invention will be described in detail.

The manufacturing process of the steel sheet according to the present invention comprises the steps of reheating the slab, subjecting the steel sheet to rough rolling, finishing rolling and cooling, and detailed conditions of each process are as follows.

First, the steel having the above composition is heated at 1000 to 1200 占 폚.

The heating temperature of the steel is preferably 1000 ° C or higher, in order to sufficiently solidify the Ti carbonitride formed during the casting. When the heating temperature is too low, the deformation resistance during rolling is too high to reduce the reduction rate per pass so that the lower limit is set at 1000 캜. However, when reheating at an excessively high temperature, the austenite grains are excessively coarsened, which may lower the toughness. Therefore, the heating temperature is preferably 1200 ° C or lower.

Then, the average rolled aspect ratio per pass is controlled to be 0.5 or more at a temperature of 1200 ° C to Tnr, and rough rolling is performed at a cumulative reduction ratio of 30% or more. Here, Tnr means a temperature at which recrystallization of austenite stops.

One of the important technical factors in the present invention is to realize the initial austenite grain refinement by optimization of rough rolling conditions. As the austenite grain size becomes smaller, the fraction of austenite grains acting as a nucleation site of ferrite is increased to facilitate ferrite nucleation, thereby lowering the critical strain required for SIDT generation and moving the ferrite transformation temperature to a higher temperature.

As can be seen from FIG. 1, when the AGS is miniaturized in rough rolling, the critical strain necessary for generating SIDT is lowered, which can easily cause SIDT generation and increase the SIDT ferrite fraction. 1 (a) shows that when the AGS becomes fine, the grain size increases and the ferrite nucleation site increases. Therefore, the amount of strain necessary for ferrite formation by SIDT decreases at the same temperature. FIG. 1 (b) The amount of strain decreases and the ferrite fraction due to SIDT increases.

Therefore, in the present invention, the rolling is controlled at an average rolling aspect ratio (SF) of 0.5 or more per pass and a cumulative reduction ratio of 30% or more per pass to control the size of the initial austenite average grain size to 40 탆 or less, The critical strain rate can be minimized. At this time, the rolled aspect ratio SF can be expressed by the following formula.

Rolling aspect ratio (SF) =

After rough rolling, the finish rolling is completed at a temperature of Ar 3 + 30 ° C. to Ar 3 + 70 ° C. at a rolling reduction of 10% or more per pass and a cumulative rolling reduction of 60% or more.

Another important technical element in the present invention is to form super fine ferrite by SIDT (strain induced dynamic transformation) during the finishing rolling. The critical strain for SIDT generation varies with the steel grade, but it can occur when the effective rolling reduction exceeds the threshold value. Therefore, the finishing rolling temperature is limited to Ar3 + 30 ° C to Ar3 + 70 ° C to give the critical deformation. When the rolling temperature is higher than Ar 3 + 70 ° C, super-fine ferrite due to SIDT can not be obtained. When the rolling temperature is lower than Ar 3 + 30 ° C, coarse iron oxide is formed along the austenite grains during rolling, It is possible to cause a decrease in strength and impact toughness, so that the finishing rolling temperature is limited to Ar3 + 30 ° C to Ar3 + 70 ° C. However, when the rolling temperature is less than 850 ℃, the deformation resistance becomes large during rolling, which makes it difficult to achieve a sufficient rolling reduction due to the difficulty in rolling. Thus, the formed steel sheet has a poor shape, Time burden will increase rapidly. Therefore, when the finishing rolling temperature is 850 ° C or higher, the productivity is improved and a low cost steel sheet can be produced.

In addition, FIG. 2 shows a deformation resistance required to give the same deformation amount, which means that the higher the temperature, the smaller the force required to give the same deformation amount, that is, it can easily deform. In the case of the present invention, it is considered that the content of Al and N is optimized to maximize the temperature required for SIDT generation.

In rolling, the reduction rate per rolling pass should be kept at 10% or more while the rolling reduction should be 60% or more. If the reduction rate per rolling pass is less than 10%, it is impossible to obtain a critical strain amount sufficient to generate SIDT, so that the super fine ferrite can not be obtained. If the cumulative reduction ratio is less than 60%, the fraction of super fine ferrite due to SIDT is sufficient So that tissue refinement can not be achieved. By controlling the rolling in this manner, it is possible to obtain an ultra fine grain ferrite having an average grain size of 10 탆 or less.

After the rolling, a cooling process is performed. Cooling is performed at a cooling rate of 10 ° C / s or higher to 300 to 500 ° C as a finishing cooling temperature (FCT).

The cooling condition is one of the main features of the present invention. If the cooling rate is less than 10 ° C / s, coarse pearlite is formed to the second phase to cause the impact toughness to be deteriorated. Since the MA phase can not be obtained, To be implemented. If the cooling end temperature is higher than 500 ° C, the granulated ferrite may be coarsened, which may also deteriorate the impact toughness. Further, the MA phase formed in the second phase has sufficient coarse and MA phase fraction It is impossible to implement a low resistance. When the cooling finish temperature is less than 300 ° C, a martensite phase may be formed in the second phase to lower the toughness of the steel. Therefore, it is preferable to limit the cooling finish temperature to 300 to 500 ° C. When cooled under the above conditions, it is possible to obtain a structure in which 10 to 30% of an MA phase having an average size of 5 탆 or less is distributed as a second phase in an ultra fine grain ferrite matrix.

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

[ Example ]

Steel slabs having the compositions shown in Table 1 below were prepared. The content unit of each component is% by weight.

Steel grade C Si Mn P S Al Ni Ti Nb T _AR Remarks A 0.08 0.25 1.4 0.01 0.003 0.05 0 0.015 0.01 10.4 Invention river B 0.07 0.25 1.3 0.008 0.002 0.07 0 0.012 0.02 14.6 Invention river C 0.05 0.3 1.9 0.01 0.003 0.02 0.5 0.015 0.02 -8.3 Comparative steel D 0.11 0.2 1.4 0.01 0.005 0.01 0.3 0.013 0.01 -5.4 Comparative steel

The slabs were heated at 1150 DEG C and subjected to rough rolling, finishing rolling and cooling in the rolling / cooling conditions shown in Table 2. [ The austenite grains (AGS) were measured on the steel sheets produced under the following conditions and are shown in Table 2 together. At this time, the rough rolling temperature was 950 캜 at a cumulative rolling reduction of 35%.

The austenitic grain size (AGS) was obtained by mirror-polishing the specimen from the 1 / 4t part of the steel sheet, etching it with AGS corrosion solution, observing it 500 times using an optical microscope, measuring the grain size by image analysis, Respectively.

Steel grade division Per pass
Rolling aspect ratio AGS
(탆) Ar3 Finish rolling
Per pass
Reduction rate
(%) Finish rolling
accumulate
Reduction rate
(%) Rolling
Termination temperature
(° C) Cooling
Termination temperature
(° C) A A-1 Invention material 0.61 32 809 20 60 869 450 A-2 Invention material 0.58 36 807 15 65 867 500 A-3 Invention material 0.53 37 805 15 65 865 400 A-4 Comparative material 0.52 37 805 15 65 865 650 A-5 Comparative material 0.61 29 810 15 65 930 500 A-6 Comparative material 0.61 29 809 15 40 869 450 A-7 Comparative material 0.58 36 807 5 65 867 400 A-8 Comparative material 0.25 64 792 15 70 852 500 B B-1 Invention material 0.61 34 810 20 60 870 450 B-2 Invention material 0.58 38 808 15 65 870 500 B-3 Invention material 0.53 39 806 15 65 870 400 B-4 Comparative material 0.52 39 806 15 65 870 650 B-5 Comparative material 0.61 31 811 15 65 930 500 B-6 Comparative material 0.61 31 810 15 40 870 450 B-7 Comparative material 0.58 38 808 5 65 870 400 B-8 Comparative material 0.25 66 793 15 70 870 500 C C-1 Comparative material 0.61 31 719 20 70 789 450 C-2 Comparative material 0.58 35 716 15 65 786 500 C-3 Comparative material 0.53 36 715 11 60 785 400 D D-1 Comparative material 0.6 33 736 20 70 806 450 D-2 Comparative material 0.56 35 734 15 65 804 500 D-3 Comparative material 0.58 34 735 11 60 805 400

The ferrite grain size (FGS) and MA phase (martensite / austenite mixed phase) fractions were measured, and the tensile strength and yield strength of the steel sheet were measured to evaluate the material properties of the steel sheet. (CVN @ -75 [deg.] C) was measured, and the flatness was measured and shown in Table 3.

The ferrite grain size (FGS) was also measured in the same manner as the austenite grain size measurement method.

The MA fraction fraction was measured by taking the specimens from the 1 / 4t part of the steel sheet after mirror polishing, and then observing the specimens 500 times using a light microscope after erosion using the RAPELA corrosion solution. The MA phase fraction was determined by image analysis.

The tensile strength was measured by measuring the tensile strength at room temperature by taking specimen JIS No. 4 in a direction perpendicular to the rolling direction from the 1 / 4t portion of the steel sheet.

The low temperature impact toughness was obtained by sampling the specimens from the 1 / 4t part of the steel sheet in the direction perpendicular to the rolling direction, and then performing the Charpy impact test five times at -75 ° C after the preparation of the V notch test piece.

Flatness measurements were performed as predictions of the occurrence of calibration rates. The flatness tolerance standard in the KS D 3500 standard was less than 15 mm and flatness was judged to be poor in the case of 15 mm or more. In case of the actual flatness bad material, it is acceptable to pass the standard at the room temperature when calibrating the powerful press, but the productivity is lowered due to the increase of the correction rate and the manufacturing cost is increased in the present invention.

Steel grade division Mean FGS
(탆) MA phase fraction
(%) The tensile strength
(MPa) Yield strength
(MPa) Yield ratio CVN @ -75 ° C
(J) flatness
(mm) A A-1 Invention material 7 13 564 441 0.78 330 8 A-2 Invention material 6 12 552 437 0.79 311 11 A-3 Invention material 9 12 578 446 0.77 320 9 A-4 Comparative material 7 0 522 486 0.93 340 5 A-5 Comparative material 37 16 552 416 0.75 52 11 A-6 Comparative material 42 11 534 423 0.79 41 10 A-7 Comparative material 35 12 521 398 0.76 46 9 A-8 Comparative material 45 14 527 392 0.74 32 7 B B-1 Invention material 3 15 559 432 0.77 289 12 B-2 Invention material 6 14 547 431 0.79 281 9 B-3 Invention material 8 14 573 440 0.77 263 6 B-4 Comparative material 9 0 517 485 0.94 305 6 B-5 Comparative material 36 16 547 410 0.75 23 13 B-6 Comparative material 34 14 529 417 0.79 33 8 B-7 Comparative material 41 14 523 392 0.75 46 9 B-8 Comparative material 43 16 522 386 0.74 39 7 C C-1 Comparative material 4 18 575 414 0.72 200 35 C-2 Comparative material 9 17 577 421 0.73 195 34 C-3 Comparative material 10 17 571 405 0.71 177 29 D D-1 Comparative material 7 20 612 422 0.69 223 33 D-2 Comparative material 9 19 598 407 0.68 210 38 D-3 Comparative material 8 19 620 409 0.66 209 41

As can be seen from the results of Table 3, according to the present invention, impact toughness values of 150 J or more at -75 캜 are obtained, high strength of 530 MPa or more in tensile strength and 0.8 or less in resistance are obtained, Thereby obtaining a low-resistance steel material.

However, it was confirmed that the comparative material could not obtain sufficient physical properties for the following reasons.

In the case of A-4, the cooling end temperature exceeded the range of the present invention, the MA fraction was small, and the yield ratio was poor.

In the case of A-5, the rolling finish temperature exceeded the range of the present invention, and the ferrite grain size was large and the low temperature toughness was inferior.

In case of A-6, the cumulative rolled rolling reduction ratio was below the range of the present invention, so that the ferrite grain size was large and the strength and low temperature toughness were inferior.

In the case of A-7, the reduction rate per the finishing rolling pass was below the range of the present invention, and the ferrite grain size was large and the strength and low temperature toughness were poor.

In the case of A-8, the recrystallization reverse rolling aspect ratio was below the range of the present invention, the austenite and ferrite grain size were large, and the strength and low temperature toughness were inferior.

In the case of B-4, the cooling end temperature exceeded the range of the present invention, the MA fraction was small, and the yield ratio was poor.

In case of B-5, the rolling finish temperature exceeded the range of the present invention, and the ferrite grain size was large and the low-temperature toughness was inferior.

In case of B-6, the cumulative rolling reduction ratio was below the range of the present invention, and the ferrite grain size was large and the strength and low temperature toughness were inferior.

In the case of B-7, the reduction rate per the finishing rolling pass was below the range of the present invention, so that the ferrite grain size was large and the strength and low temperature toughness were inferior.

In the case of B-8, the aspect ratio of recrystallization reverse rolling was below the range of the present invention, the austenite and ferrite grain size were large, and the strength and low temperature toughness were poor.

In the case of C-1, C-2, C-3, D-1, D-2 and D-3, when the composition conditions of the steel sheet are out of the range of the present invention, The degree of failure was found to be poor.

Claims (7)

A steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.02 to 0.12% of C, 0.05 to 0.5% of Si, 0.5 to 2.0% of Mn, 1.0 to 2.0% of Ni, 0.005 to 0.1% of Ti, 0.005 to 0.1% , P: not more than 0.015%, S: not more than 0.015%, balance Fe and other unavoidable impurities, T _AR = 208 x Al - 25 x Ni>
The microstructure has excellent low-temperature toughness and excellent low-resistance characteristics, including 10 to 30% of MA (martensite / austenite mixed structure) having an average grain size of 5 占 퐉 or less and an ultra-fine grain size of 10 占 퐉 or less, High strength steel plate. The method according to claim 1,
Wherein the steel sheet has an impact toughness value of 150 J or more at -75 캜, a tensile strength of 530 MPa or more, and a resistance multiple ratio of 0.8 or less. (Excluding 0%) of Ni, 0.005 to 0.1% of Ti, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Nb, 0.5 to 2.0% of Si, 0.05 to 0.5% of Si, : 0.005 to 1.0%, P: 0.015% or less, S: 0.015% or less, the balance Fe and other unavoidable impurities, and satisfying T _AR = 208 x Al - 25 x Ni>Heating;
Controlling the average rolling aspect ratio per pass to a value of 0.5 or more per pass at a temperature of 1200 ° C to Tnr (temperature at which the austenite recrystallization stops), and subjecting the resulting product to a rolling reduction of 30% or more at a cumulative rolling reduction rate;
Completing the finish rolling at a temperature of Ar3 + 30 占 폚 to Ar3 + 70 占 폚 at a rolling reduction of 10% or more per pass and a cumulative rolling reduction of 60% or more; And
A method for producing a high strength steel sheet excellent in cryogenic temperature toughness and having a resistance to brittleness property, comprising cooling the steel sheet at a cooling rate of not less than 10 ° C / s from 300 to 500 ° C as finishing cooling temperature (FCT). The method of claim 3,
And the Ti carbonitride is solidified through the heating step, wherein the Ti carbonitride is solidified through the heating step. The method of claim 3,
Wherein the average grain size of the austenite is controlled to 40 탆 or less through the rough rolling step. The method of claim 3,
Wherein the ultra-fine ferrite having an average grain size of 10 탆 or less is formed through the step of completing the finish rolling. The method of claim 3,
And a step of forming a structure in which 10 to 30% of an MA phase having an average size of 5 占 퐉 or less is distributed in an ultra fine grain ferrite base having an average grain size of 10 占 퐉 or less through the cooling step, in the manufacture of a high strength steel sheet having excellent cryogenic toughness and low- Way.

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