KR101799202B1 - High-strength steel sheet having excellent low yield ratio property and low temperature toughness and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a high-strength steel sheet having excellent low yield ratio property and low temperature toughness, capable of ensuring excellent anti-seismic characteristics as well as formability. According to one embodiment of the present invention, the high-strength steel sheet comprises: 0.03-0.08 wt% of C; 0.05-0.3 wt% of Si; 1.0-2.0 wt% of Mn; 0.005-0.04 wt% of Al; 0.005-0.04 wt% of Nb: 0.001-0.02 wt% of Ti; 0.05-0.4 wt% of Cu; 0.6-2.0 wt% of Ni; 0.08-0.3 wt% of Mo; 0.002-0.006 wt% of N; 0.01 wt% or less of P; 0.003 wt% or less of S; and the remainder consisting of Fe and unavoidable impurities. Moreover, a microstructure comprises: 80-92 area% of ferrite; and 8-20 area% of martensite/austenite mixed structure (MA) wherein an average size measured as a circular-equivalent diameter is 3 m or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel sheet excellent in low temperature resistance and low temperature toughness and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

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

In order to be applicable not only to the shipbuilding and marine structural steel products but also to the industrial fields requiring molding and seismic properties, it is necessary to develop a steel material having not only low temperature toughness but also low resistance properties.

The steel having a low resistance has a great difference in yield strength and tensile strength, so that not only the formability is excellent but also the time of plastic deformation until the fracture can be delayed and the energy is absorbed in the process to prevent collapse by external force can do. In addition, even if there is a deformation, it is possible to prevent damage to property and human life due to breakage of the structure by enabling repair before collapse.

Techniques have been developed for two-phase organization of the steel structure to ensure low resistance. Specifically, the first phase is soft ferrite, and the second phase is martensite, pearlite or bainite, thereby realizing a resistance reduction ratio.

However, there is a problem that the impact toughness due to the mild two-phase is decreased and the carbon content is increased for the second phase, which deteriorates the toughness of the welded portion and causes brittle fracture of the structure at low temperatures.

Patent Document 1 has been developed as a technique for securing both low resistance and low temperature impact toughness.

In Patent Document 1, the microstructure contains 2 to 10 vol% of MA (martensite / austenite mixed structure) and 90 vol% or more of acicular ferrite, thereby ensuring low resistance and excellent low temperature toughness.

According to Patent Document 1, it is possible to realize a yield ratio of about 0.8, but it is not possible to realize a sufficient resistance reduction ratio, so that it is insufficient to secure the seismic resistance.

Therefore, there is a demand for development of a high strength steel sheet excellent in resistance to bending characteristic and low temperature toughness which can secure a lower yield ratio and a manufacturing method thereof.

Korean Patent Laid-Open Publication No. 2013-0076577

One aspect of the present invention is to provide a high strength steel sheet excellent in resistance to brittleness and low temperature toughness and a method of manufacturing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

One aspect of the present invention is a steel sheet comprising, by weight%, 0.03 to 0.08% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.005 to 0.04% of Nb, 0.001 to 0.04% of Ti, % Of Cu, 0.05 to 0.4% of Cu, 0.6 to 2.0% of Ni, 0.08 to 0.3% of Mo, 0.002 to 0.006% of N, 0.01% or less of P and 0.003% or less of S and remaining Fe and unavoidable impurities ,

The microstructure includes 80 to 92% of ferrite as an area fraction and 8 to 20% of MA (martensite / austenite mixed structure), wherein the MA has a resistivity reduction characteristic with an average size of 3 탆 or less measured by a circle- To a high strength steel sheet excellent in low temperature toughness.

In another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: C: 0.03 to 0.08%, Si: 0.05 to 0.3%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.04%, Nb: 0.005 to 0.04% 0.001 to 0.02% of Cu, 0.05 to 0.4% of Cu, 0.6 to 2.0% of Ni, 0.08 to 0.3% of Mo, 0.002 to 0.006% of N, 0.01% or less of P and 0.003% or less of S and the balance Fe and unavoidable impurities Heating the slab to 1050 - 1200 캜;

Hot rolling the heated slab to a finish rolling finish temperature of 760 to 850 ° C to obtain a hot rolled steel sheet;

Cooling the hot-rolled steel sheet to 450 캜 or lower at a cooling rate of 5 캜 / s or higher; And

And a normalizing heat treatment step of heating the cooled hot-rolled steel sheet to a temperature range of 850 to 960 ° C and then maintaining it for [1.3t + (10 to 30)] minutes. ≪ / RTI >

(Where t is the thickness of the hot-rolled steel sheet measured in mm)

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to secure an excellent resistance-to-bending characteristic and a low-temperature toughness, and in particular, to secure a low resistance ratio of 0.65 or less, thereby securing not only formability but also excellent seismic resistance.

Accordingly, the present invention can be applied not only to steel materials for shipbuilding and offshore structures but also to industries requiring molding and seismic characteristics.

Fig. 1 is a photograph of the microstructure of Test No. 1 of the invention, taken using an optical microscope (OM).
2 is a photograph of the microstructure of Test Example 1, which is an inventive example, taken using a scanning electron microscope (SEM).
3 is a photograph of the microstructure of Test No. 12, which is a comparative example, taken by using an optical microscope (OM).

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

The inventors of the present invention have been able to obtain a yield ratio of about 0.8 in the prior art and thus can secure moldability to a certain extent. However, the present inventors have found that insufficient resistance can not be achieved and seismic resistance is insufficient. .

As a result, in order to realize a low resistance, it is advantageous that the difference in hardness between the base material and the second phase is larger, and the homogeneous distribution of MA is more advantageous. In the case of Patent Document 1, the hardness difference between MA and the MA is insufficient, Phase is formed at the grain boundaries and the MA size is too large to realize a sufficient resistance reduction ratio.

By using the ferrite as the microstructure of the base material and uniformly distributing the fine MA phase in the ferrite grain boundaries and grain boundaries, it is possible to secure a resistance ratio of 0.65 or less. In order to secure such a structure, the pre- And the present invention has been accomplished.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a high strength steel sheet having excellent resistance to bending characteristic and low temperature toughness according to one aspect of the present invention will be described in detail.

The high strength steel sheet excellent in resistance to breakage characteristics and low temperature toughness according to one aspect of the present invention is characterized by containing 0.03 to 0.08% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.001 to 0.04% of Nb, 0.001 to 0.02% of Ti, 0.05 to 0.4% of Cu, 0.6 to 2.0% of Ni, 0.08 to 0.3% of Mo, 0.002 to 0.006% of N, 0.003% or less, the balance of Fe and unavoidable impurities,

The microstructure includes an area fraction of 80 to 92% of ferrite and 8 to 20% of MA (martensite / austenite mixed structure), and the MA has an average size of 3 탆 or less measured by circle equivalent diameter.

First, the alloy composition of the high-strength steel sheet having excellent resistance to brittle characteristic and low-temperature toughness according to one aspect of the present invention will be described in detail. Hereinafter, the unit of each element content is% by weight.

C: 0.03 to 0.08%

In the present invention, C is an element for causing solid solution strengthening and being present as a carbonitride due to Nb or the like to secure tensile strength.

When the C content is less than 0.03%, the above-mentioned effect is insufficient. On the other hand, when the C content is more than 0.08%, MA is coarsened and pearlite is produced, which may deteriorate impact characteristics at low temperatures, and it is difficult to secure enough bainite.

Si: 0.05 to 0.3%

Si serves to deoxidize molten steel by supporting Al, and is added to secure yield strength and tensile strength.

When the Si content is less than 0.05%, the above-mentioned effect is insufficient. On the other hand, when the Si content is more than 0.3%, the impact characteristics may deteriorate due to the coarsening of the MA, and the welding characteristics may be deteriorated.

Mn: 1.0 to 2.0%

Mn contributes greatly to the strength enhancement effect by the solid solution strengthening and is an element which helps in the formation of bainite.

When the Mn content is less than 1.0%, the above-mentioned effect is insufficient. On the other hand, if it is added excessively, the formation of MnS inclusions and the decrease of toughness due to the segregation of the central portion may cause the upper limit to be 2.0%.

Al: 0.005 to 0.04%

Al must be added in an amount of 0.005% or more as a main deoxidizing agent of the steel. However, when it is added in an amount exceeding 0.04%, the effect is saturated and the low temperature toughness may be lowered due to an increase in the fraction and size of the Al ₂ O ₃ inclusions.

Nb: 0.005 to 0.04%

Nb is an element that suppresses recrystallization during rolling or cooling by precipitation of solid solution or carbonitride to make the structure finer and increase the strength. When the Nb content is less than 0.005%, the above-mentioned effect is insufficient. On the other hand, when the Nb content exceeds 0.04%, there is a problem that the toughness of the base material and the toughness after welding may be lowered.

Ti: 0.001 to 0.02%

Ti forms a precipitate by binding with oxygen or nitrogen, thereby suppressing coarsening of the structure, contributing to micronization and improving toughness.

When the Ti content is less than 0.001%, the above-mentioned effect is insufficient. On the other hand, when the Ti content is more than 0.02%, precipitates are formed to a great extent, which may cause destruction.

Cu: 0.05 to 0.4%

Cu is a component that does not significantly deteriorate impact characteristics, and improves strength by solidification and precipitation. For sufficient strength improvement, it should be contained in an amount of 0.05% or more. However, if the Cu content exceeds 0.4%, surface cracking of the steel sheet due to Cu thermal shock may occur.

Ni: 0.6 to 2.0%

Ni is an element which can improve both strength and toughness at the same time although it does not improve strength with increasing content. It is an element which helps formation of bainite by lowering Ar3 temperature.

When the Ni content is less than 0.6%, the above-mentioned effect is insufficient. On the other hand, if the Ni content exceeds 2.0%, the manufacturing cost may increase and the weldability may deteriorate.

Mo: 0.08 to 0.3%

Mo acts as an austenite stabilizing element to increase the amount of MA and plays a large role in improving the strength. It also prevents the drop in strength during heat treatment and is an element that helps in the formation of bainite.

When the Mo content is less than 0.08%, the above-mentioned effect is insufficient. On the other hand, when the Mo content exceeds 0.3%, there is a problem that the manufacturing cost is increased and the toughness of the base material and the toughness after welding are lowered.

N: 0.002 to 0.006%

N is an element that helps precipitate a precipitate with Ti, Nb, Al and the like to improve the strength and toughness by making the austenite structure finer during the heating of the slab.

When the N content is less than 0.002%, the above-mentioned effect is insufficient. On the other hand, when the N content exceeds 0.006%, surface cracks are generated at a high temperature and precipitates are formed, and the remaining N atoms are present in an atomic state, thereby reducing toughness.

P: not more than 0.01%

P may cause grain boundary segregation as an impurity, which may cause the steel to fall off. Therefore, it is important to control the upper limit, and it is desirable to control it to 0.01% or less.

S: not more than 0.003%

S is an impurity, which mainly binds to Mn to form MnS inclusions, which is a factor that hinders low-temperature toughness. Therefore, it is important to control the upper limit, and it is preferable to control S to 0.003% or less in order to secure low-temperature toughness.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

Hereinafter, the microstructure of a high-strength steel sheet having excellent resistance to brittleness and low-temperature toughness according to one aspect of the present invention will be described in detail.

According to one aspect of the present invention, the microstructure of the high-strength steel sheet excellent in resistance to low-temperature toughness and low-temperature toughness comprises ferrite in an area fraction of 80 to 92% and MA in an amount of 8 to 20% The size is 3 μm or less. Hereinafter, the fraction of microstructure means an area fraction unless otherwise specified.

The ferrite is intended to secure basic toughness and strength, and it is preferably 80% or more. In order to secure a sufficient MA, the upper limit is preferably 92%. Further, it is preferable that the ferrite does not contain an asicula ferrite. This is because the difference in hardness between the eccentric ferrite and the MA is small, and thus sufficient resistance can not be secured.

If the MA is less than 8%, it is difficult to secure a resistance ratio of 0.65 or less. If the MA is more than 20%, the impact toughness may be deteriorated and the elongation rate may be decreased. When the average size measured by the circle equivalent diameter of MA is more than 3 mu m, MA is mainly formed in the crystal grain boundaries, and it is difficult to secure a uniform distribution of MA and a low resistance.

Other inevitable phases other than the above-mentioned ferrite and MA may also be included and are not excluded. For example, pearlite of 1% by area or less can be included.

At this time, in order to secure an excellent resistance-to-brittleness property and low-temperature toughness, it is preferable that not only the MA fraction and the size but also 5 to 13 MAs extending over the straight line are present in the steel sheet of the present invention .

That is, a straight line may be drawn up or down or left and right on a microstructure photograph having a size of 100 μm × 100 μm, and 5 to 13 MAs on each line may exist on the average. MA mainly causing breakdown initiation is MA existing at the grain boundaries, and when the above condition is satisfied, MA is distributed evenly inside the grain boundaries and crystal grains, so that it is advantageous to secure the low resistance.

Also, the ratio of MA existing in the ferrite crystal grains to that existing in the grain boundaries may be 1: 3 to 1:10. The above ratio means the ratio of the number of MA, and it is possible to uniformly distribute the MA in the ferrite grains so that the MA is 0.5 to 5% by area.

The ferrite may have an average size of 20 탆 or less as measured in terms of circle equivalent diameter. If the average ferrite size is more than 20 占 퐉, it may be difficult to secure sufficient toughness and strength.

Meanwhile, the steel sheet according to the present invention is subjected to a normalizing heat treatment, and the microstructure of the steel sheet prior to the normalizing heat treatment may be 50 to 90% by area of bainite.

Since the microstructure of the steel sheet prior to the heat treatment is made bainite in which the carbide is present, it is possible to uniformly distribute the MA in the grain boundaries and inside the crystal grains after the heat treatment, so that the microstructure of the steel sheet before heat treatment is preferably 50 to 90% Do.

The steel sheet according to the present invention may have a yield ratio of 0.5 to 0.65 and a low-temperature impact property at -40 캜 of 100 J or more. By making the yield ratio and the tensile strength difference large by making the yield ratio less than 0.65, not only the formability is excellent but also the time of plastic deformation until the failure can be delayed and the energy is absorbed in this process to prevent the collapse by the external force can do.

Therefore, it can be suitably applied not only to the shipbuilding and marine structural steel products but also to industrial fields requiring molding and seismic characteristics.

At this time, the yield strength of the steel sheet may be 350 to 400 MPa, and the tensile strength may be 600 MPa or more.

Hereinafter, a method of manufacturing a high strength steel sheet having excellent resistance to abrasion and low temperature toughness, which is another aspect of the present invention, will be described in detail.

According to another aspect of the present invention, there is provided a method of manufacturing a high-strength steel sheet excellent in resistance to abrasion characteristics and low-temperature toughness, comprising the steps of: heating a slab having the above- Hot rolling the heated slab to a finish rolling finish temperature of 760 to 850 ° C to obtain a hot rolled steel sheet; Cooling the hot-rolled steel sheet to 450 캜 or lower at a cooling rate of 5 캜 / s or higher; And a normalizing heat treatment step of heating the cooled hot-rolled steel sheet to a temperature range of 850 to 960 ° C, and then maintaining it for [1.3t + (10 to 30)] minutes. And t is a value obtained by measuring the thickness of the hot-rolled steel sheet in mm.

Slab heating step

The slab having the above-described alloy composition is heated to 1050 to 1200 占 폚.

If the heating temperature is higher than 1200 ° C, the austenite grains may be coarsened and toughness may be lowered. If the heating temperature is lower than 1050 ° C, Ti, Nb, etc. may not be sufficiently solidified and the strength may decrease.

Hot rolling step

The hot slab is hot rolled to obtain a finish rolling finish temperature of 760 to 850 ° C to obtain a hot-rolled steel sheet.

The rolling temperature of ordinary heat treated steel is generally about 850 to 1000 ° C. However, in the present invention, it is important to form the initial structure of bainite. Therefore, there is a need for a controlled rolling process to terminate rolling at low temperature instead of conventional rolling which represents ferrite-pearlite structure.

In hot rolling, recrystallization reverse rolling is necessary to miniaturize the austenite grain size, and the reduction in the pass reduction per pass is advantageous in terms of physical properties.

Unrecrystallized reverse rolling should be completed at a temperature above Ar3 of the steel, which means at least about 760 ° C. More specifically, the finishing rolling finishing temperature can be defined at 760 to 850 占 폚. If the finish rolling finish temperature exceeds 850 DEG C, it is difficult to suppress the ferrite-pearlite transformation. If the finish rolling finish temperature is less than 760 DEG C, the microstructure in the thickness direction may be uneven. It may not be possible to form microstructure.

The finish rolling is terminated at a temperature range of 760 to 850 DEG C to suppress the ferrite-pearlite transformation and to realize the bainite structure through cooling. The initial structure of bainite is for uniform MA distribution after heat treatment. In ferrite-pearlite structure, MA is mainly formed in grain boundaries, whereas in bainite structure, MA is formed in both grain boundaries and grain grains.

Cooling step

The hot-rolled steel sheet is cooled to 450 DEG C or lower at a cooling rate of 5 DEG C / s or higher.

Accelerated cooling after hot rolling is very important for the implementation of the target organization of invention steel. Bainite must be implemented for fine and uniform MA formation, and cooling finish temperature and cooling rate are important factors for bainite formation.

If the cooling finish temperature is higher than 450 ° C, the size of the crystal grains may be large, and coarse carbide may cause coarse MA after heat treatment, which may result in deterioration of toughness, It is difficult to secure.

When the cooling rate is less than 5 캜 / s, the microstructure of the needle-like ferrite or the ferrite + pearlite is formed in a large amount to lower the strength, and the coarse ferrite + pearlite structure, which is not the abnormal structure of the ferrite + MA after the heat treatment, There is a problem that it is difficult to secure a bainite of 50 area% or more.

Normalizing  Heat treatment step

The cooled hot-rolled steel sheet is heated to a temperature range of 850 to 960 ° C and held for [1.3t + (10 to 30)] minutes. And t is a value obtained by measuring the thickness of the hot-rolled steel sheet in mm.

When the normalizing temperature is less than 850 DEG C or the holding time is less than (1.3t + 10) minutes, the cementite in the pearlite and bainite and the MA phase are difficult to be reused, The remaining cured image is left to coarse.

On the other hand, when the normalizing temperature exceeds 960 ° C. or the holding time is more than (1.3t + 30) minutes, the carbides existing in the bainite crystal move to the grain boundaries or the carbide coarsening occurs, Can not be formed. In addition, grain growth may occur, resulting in a decrease in strength and deterioration in impact.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

Slabs were prepared using continuous casting after providing molten steel having the composition shown in Table 1 below. The slabs were subjected to rolling, cooling, and normalizing heat treatment under the manufacturing conditions shown in Table 2 below to produce steel sheets.

In Table 3, the bainite fraction and the mechanical properties of the steel sheet before the normalizing heat treatment were measured and described.

In Table 4, the MA fraction of the steel sheet after the normalizing heat treatment, the average MA size, the number of MAs at 100 占 퐉 and the mechanical properties were measured and described. In the case of the inventive example, ferrite was used other than MA, and the average crystal grain size of ferrite was not separately described as 20 탆 or less.

The average MA size is an average size measured by the circle equivalent diameter, and the number of MA in a 100 占 퐉 line is 10 lines of a straight line drawn up or down or right and left in a microstructure photograph of a size of 100 占 퐉 占 100 占 퐉, After counting, the average number is described.

Specifically, the effects of rolling temperature, cooling termination temperature, and heat treatment time were investigated. In Table 3, the MA fraction, yield ratio, and mechanical properties of the steel sheets produced by the components A to H and the manufacturing conditions 1 to 12 were shown.

division Steel grade C Si Mn P S Al Ni Mo Ti Nb Cu N Invention river A 0.045 0.086 1.87 0.005 0.002 0.006 1.19 0.13 0.007 0.008 0.242 0.0037 Invention river B 0.04 0.095 1.92 0.004 0.0017 0.012 1.21 0.15 0.01 0.01 0.235 0.004 Invention river C 0.043 0.105 1.88 0.005 0.0018 0.01 1.18 0.15 0.009 0.011 0.248 0.0038 Invention river D 0.046 0.095 1.91 0.005 0.0018 0.011 1.21 0.16 0.008 0.01 0.251 0.0035 Comparative steel E 0.12 0.12 1.87 0.005 0.0018 0.011 1.21 0.14 0.011 0.01 0.241 0.0035 Comparative steel F 0.037 0.11 1.91 0.005 0.0017 0.013 1.21 0.012 0.012 0.012 0.253 0.0037 Comparative steel G 0.04 0.11 0.85 0.0048 0.0017 0.012 1.17 0.13 0.01 0.012 0.255 0.0035 Comparative steel H 0.042 0.13 1.88 0.0047 0.0018 0.01 0.23 0.12 0.01 0.011 0.239 0.0037

In Table 1, the unit of each element content is% by weight. Invention steels A to D are steel sheets satisfying the component ranges defined in the present invention, and comparative steels E to H do not satisfy the component ranges defined in the present invention. Comparative steel E is over C content, Comparative steel F is below Mo content, Comparative steel G is below Mn content, and Comparative steel H is below Ni content.

division exam
number Steel grade Reheating
Temperature (℃) Finish rolling
Starting temperature (℃) Finish rolling
End temperature (캜) Cooling finish
Temperature (℃) Cooling rate
(° C / s) Normalizing
Temperature (℃) Normalizing
Time (minutes) Honor One A 1151 813 799 329 9.8 910 85 Honor 2 1146 795 781 337 10.5 910 90 Honor 3 B 1138 805 784 332 10.4 910 92 Honor 4 1146 804 787 342 10.8 910 90 Honor 5 C 1143 820 778 384 8.9 875 91 Comparative Example 6 1175 965 923 348 11.2 910 88 Comparative Example 7 1116 802 783 - - 910 95 Honor 8 D 1139 812 791 356 11.1 884 86 Comparative Example 9 1124 809 778 325 10.6 910 240 Comparative Example 10 1135 798 778 652 9.7 910 89 Comparative Example 11 E 1145 812 801 335 9.9 910 85 Comparative Example 12 F 1125 790 773 322 10.3 910 96 Comparative Example 13 G 1135 795 781 365 11.1 910 83 Comparative Example 14 H 1128 786 768 341 10.2 910 98

division exam
number Steel grade Before normalizing heat treatment Bay knight
(area%) Yield strength
(MPa) The tensile strength
(MPa) Yield ratio Elongation
(%) Honor One A 62 541 613 0.88 25.5 Honor 2 72 521 631 0.83 23 Honor 3 B 63 510 637 0.80 22 Honor 4 83 510 636 0.80 21 Honor 5 C 76 524 628 0.83 22.3 Comparative Example 6 11 488 593 0.82 23.4 Comparative Example 7 0 445 556 0.80 26.2 Honor 8 D 77 531 632 0.84 23.1 Comparative Example 9 68 514 647 0.79 27.3 Comparative Example 10 8 568 652 0.87 21.4 Comparative Example 11 E 3 489 584 0.84 23.7 Comparative Example 12 F 28 481 573 0.84 24.8 Comparative Example 13 G 24 487 562 0.86 22.7 Comparative Example 14 H 19 502 604 0.83 21.8

division exam
number Steel grade After normalizing heat treatment MA
Fraction
(area%) Average MA
size
(탆) Number of MAs on 100 μm lines surrender
burglar
(MPa) Seal
burglar
(MPa)  Yield ratio Elongation
(%) Impact toughness
(-40 ° C)
(J) Honor One A 12.5 2.3 7.2 354 617 0.57 30.7 163 Honor 2 11.4 1.8 9.3 357 621 0.57 30.5 181.8 Honor 3 B 9.8 2.6 6.3 355 617 0.58 28.4 167 Honor 4 10.2 1.9 8.2 359 619 0.58 31.6 108.6 Honor 5 C 12.5 2.8 8.1 378 625 0.6 29.6 102.5 Comparative Example 6 2.3 5.2 2.4 471 568 0.83 27.6 67.8 Comparative Example 7 3.4 6.1 0.5 452 548 0.82 28 57.6 Honor 8 D 13.5 2.2 12.4 384 635 0.6 29.4 123.9 Comparative Example 9 1.2 5.3 2.2 482 574 0.84 28.1 42.3 Comparative Example 10 2.6 4.9 1.6 506 624 0.81 24.5 21.5 Comparative Example 11 E 1.2 5.1 1.5 462 571 0.81 25.4 98.4 Comparative Example 12 F 3.5 3.9 2.8 423 538 0.79 26.7 103.4 Comparative Example 13 G 2.3 4.2 3.6 416 557 0.75 25.9 116.3 Comparative Example 14 H 2.6 4.6 2.5 426 574 0.74 26.1 35.6

The inventive examples satisfying all of the alloy composition and the manufacturing conditions proposed in the present invention can secure a yield ratio of 0.65 or less and an impact toughness of -40 DEG C of 100 J or more.

Comparative Examples 6, 7, 9 and 10 satisfied the composition of the alloys proposed in the present invention, but did not satisfy the production conditions and thus failed to secure a sufficient resistance ratio. Also, the impact toughness at -40 ° C was less than 100 J You can see that it is open.

In the case of Test Nos. 11 to 14, which were comparative examples, the production conditions proposed in the present invention were satisfied, but the alloy compositions were not satisfied and sufficient resistance ratio could not be secured. Test Nos. 11 and 14 had impact tensile strengths of less than 100 J As shown in Fig.

As shown in Table 4, the MA fraction is higher than that of the comparative example. As can be seen from Table 3, carbides in the crystal grains of the initial bainite structure and the grain boundaries were transformed into fine MA by securing a high bainite fraction before the normalizing heat treatment.

1 and 2, in which the microstructure of Test Example No. 1 of the invention is photographed, it can be seen that fine and uniform MA is formed.

On the other hand, FIG. 3, in which the microstructure of Test No. 12 of the comparative example is photographed, shows that carbide and pearlite are mainly two phases, and the fraction of MA is low and the MA formed is polygonal and mainly exists in grain boundaries.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (9)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.03 to 0.08% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.005 to 0.04% of Nb, 0.001 to 0.02% 0.4 to 0.4% of Ni, 0.6 to 2.0% of Ni, 0.08 to 0.3% of Mo, 0.002 to 0.006% of N, 0.01% or less of P and 0.003% or less of S and balance of Fe and unavoidable impurities,
The microstructure includes an area fraction of 80 to 92% of ferrite and 8 to 20% of MA (martensite / austenite mixed structure), wherein the MA has an average size of 3 탆 or less,
A high strength steel sheet excellent in resistance to brittleness and low temperature toughness, wherein 5 to 13 MAs are spanned over the straight line when a 100 탆 straight line is drawn on the steel sheet.
delete The method according to claim 1,
Wherein a ratio of MA existing in the ferrite crystal grains to MA existing in the grain boundaries is 1: 3 to 1:10, and excellent in low temperature toughness.
The method according to claim 1,
Wherein said ferrite has an average size measured at a circle equivalent diameter of 20 占 퐉 or less.
The method according to claim 1,
The steel sheet is subjected to a normalizing heat treatment,
Wherein the microstructure of the steel sheet before normalizing heat treatment is 50 to 90% by area of bainite.
The method according to claim 1,
Wherein the steel sheet has a yield ratio of 0.5 to 0.65 and a low-temperature impact property at -40 캜 of 100 J or more, and a high strength steel sheet excellent in low-temperature toughness.
The method according to claim 1,
Wherein the steel sheet has a yield strength of 350 to 400 MPa and a tensile strength of 600 MPa or more.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.03 to 0.08% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.005 to 0.04% of Nb, 0.001 to 0.02% 0.4 to 0.4% of Ni, 0.6 to 2.0% of Ni, 0.08 to 0.3% of Mo, 0.002 to 0.006% of N, 0.01% or less of P and 0.003% or less of S and the remaining Fe and unavoidable impurities at a temperature of 1050 to 1200 ° C ; ≪ / RTI >
Hot rolling the heated slab to a finish rolling finish temperature of 760 to 850 ° C to obtain a hot rolled steel sheet;
Cooling the hot-rolled steel sheet to 450 캜 or lower at a cooling rate of 5 캜 / s or higher; And
And a normalizing heat treatment step of heating the cooled hot-rolled steel sheet to a temperature range of 850 to 960 ° C and then maintaining it for [1.3t + (10 to 30)] minutes. Way.
(Where t is the thickness of the hot-rolled steel sheet measured in mm)
9. The method of claim 8,
Wherein the microstructure of the cooled hot-rolled steel sheet has a bainite content of 50 to 90% by area.

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