KR101917451B1 - Low-yield ratio steel sheet having excellent low-temperature toughness and method for manufacturing the same - Google Patents

Abstract

An aspect of the present invention is a steel sheet comprising, by weight, 0.05 to 0.1% of C, 0.3 to 0.7% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.04 to 0.07% of Nb, 0.001 to 0.02% % Of Cu, 0.05 to 0.4% of Cu, 0.1 to 0.6% of Ni, 0.01 to 0.08% of Mo, 0.001 to 0.008% of N, 0.015% 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 and 8 to 20% of MA (martensite / austenite mixed structure) as an area fraction, and MA has an excellent resistance to low temperature toughness having an average size of 3 m or less This is related to the steel plate of the double side.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a low-temperature resistant double-coated steel sheet having excellent low temperature toughness,

The present invention relates to a composite steel sheet having excellent 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 the molding and seismic properties, it is necessary to develop a steel having low temperature toughness as well as 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 composite steel sheet having excellent low temperature toughness capable of securing 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 composite steel sheet having excellent 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.

An aspect of the present invention is a steel sheet comprising, by weight, 0.05 to 0.1% of C, 0.3 to 0.7% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.04 to 0.07% of Nb, 0.001 to 0.02% % Of Cu, 0.05 to 0.4% of Cu, 0.1 to 0.6% of Ni, 0.01 to 0.08% of Mo, 0.001 to 0.008% of N, 0.015% 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 and 8 to 20% of MA (martensite / austenite mixed structure) as an area fraction, and MA has an excellent resistance to low temperature toughness having an average size of 3 m or less This is related to the steel plate of the double side.

In another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.05 to 0.1% of C, 0.3 to 0.7% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.04 to 0.07% of Nb, 0.001 to 0.02% of Cu, 0.05 to 0.4% of Cu, 0.1 to 0.6% of Ni, 0.01 to 0.08% of Mo, 0.001 to 0.008% of N, 0.015% or less of P and 0.003% or less of S, 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. will be.

(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 a low resistance and an excellent low-temperature toughness, and in particular, a low resistance ratio of 0.65 or less can be ensured, so that not only formability but also excellent seismic resistance can be ensured. Accordingly, it can be applied to industrial fields such as construction, construction and civil engineering which require seismic characteristics, and it can be applied to steel materials for shipbuilding and offshore structures.

Fig. 1 is a photograph of the microstructure before the normalizing heat treatment of Test No. 1 of Inventive Example.
Fig. 2 is a photograph of a microstructure after the normalizing heat treatment of Test No. 1 of the invention. Fig.
3 is a photograph of the microstructure of the test piece No. 9 after the normalizing heat treatment.
4 is a photograph of the microstructure of the test piece No. 10 after the normalizing heat treatment of the comparative example.

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 obtain a certain degree of formability. However, the present inventors have found that insufficient resistance can not be realized and seismic resistance is insufficient. .

As a result, in order to realize the resistance reduction ratio, 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 difference in hardness between the MA and the MA is insufficient, And the MA size can not be increased sufficiently to realize a sufficient resistance reduction ratio.

By using the ferrite 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 structure before normalizing includes bainite And that the present invention has been accomplished.

Superior low-temperature toughness Low resistance  Steel plate

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a detailed description will be given of a coated steel sheet having excellent low temperature toughness according to one aspect of the present invention.

The steel sheet according to one aspect of the present invention includes 0.05 to 0.1% of C, 0.3 to 0.7% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.04 to 0.04% of Nb 0.001 to 0.07% of Ti, 0.05 to 0.4% of Cu, 0.1 to 0.6% of Ni, 0.01 to 0.08% of Mo, 0.001 to 0.008% of N, 0.015% or less of P and 0.003% or less of S , 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 in circle equivalent diameter.

First, the alloy composition of the present invention will be described in detail. Hereinafter, the unit of each element content is expressed by weight% unless otherwise specified.

C: 0.05 to 0.1%

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 0.05%, the above-mentioned effects are insufficient. On the other hand, when the C content is more than 0.1%, MA is large and pearlite is produced, which can deteriorate the impact property at low temperature and it is difficult to secure sufficient bainite.

Si: 0.3 to 0.7%

Si is added to assist in deoxidizing molten steel by assisting Al and to secure yield strength and tensile strength. It is also an element for controlling the desired MA fraction in the present invention.

When the Si content is less than 0.3%, the above-mentioned effect is insufficient. Therefore, the Si lower limit is preferably 0.3%, and more preferably, the lower limit is 0.35%. On the other hand, when the Si content exceeds 0.7%, the impact characteristics may be deteriorated by 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 decline of toughness due to central knitting 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: 0.04 to 0.07%

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. It is also an element for controlling the desired MA fraction in the present invention.

When the Nb content is less than 0.04%, the above-mentioned effect is insufficient. On the other hand, if it exceeds 0.07%, 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: 0.1 to 0.6%

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 0.1%, the above-mentioned effect is insufficient. On the other hand, if the Ni content exceeds 0.6%, the manufacturing cost may increase and the weldability may deteriorate.

Mo: 0.01 to 0.08%

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 the heat treatment and helps to form bainite.

However, since Mo is an expensive alloying element, there is a problem that the production cost increases when added in large amounts. Therefore, in the present invention, MA is secured by adding a large amount of Si, Nb, and the like. In the alloy composition of the present invention, when Mo is added by 0.01% or more, the above-mentioned effect can be sufficiently secured. On the other hand, when the Mo content exceeds 0.08%, 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: 0.001 to 0.008%

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 0.001%, the above-mentioned effect is insufficient. On the other hand, when the N content exceeds 0.008%, surface cracks are generated at a high temperature and precipitates are formed, and the residual N is present in an atomic state to reduce toughness.

P: not more than 0.015%

P may cause grain boundary segregation as an impurity and cause brittleness of the steel. Therefore, it is important to control the upper limit, and it is desirable to control it to 0.015% 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 the low-temperature resistant double-coated steel sheet 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 FRP steel sheet excellent in low-temperature toughness includes 80 to 92% of ferrite and 8 to 20% of MA (martensite / austenite mixed structure) The average size measured by the circle equivalent diameter is 3 탆 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 acicular ferrite. This is because the hardness difference with the MA is not so small and sufficient resistance can not be secured.

When the MA is less than 8%, it is difficult to secure a resistance ratio of less than 0.65. When the MA is more than 20%, the impact toughness may be lowered and the elongation may be decreased. When the mean size measured by the circle equivalent diameter of MA is more than 3 mu m, MA is mainly formed in the 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 be included and not excluded. For example, pearlite of 1% by area or less can be included.

In this case, in order to secure an excellent resistance-to-brittleness property and a low temperature toughness, it is preferable that not only the MA fraction and the size but also the number of MA spanning the straight line when there is a straight line of 100 mu m on the steel sheet of the present invention is 5-13. That is, a straight line is drawn up or down or left and right on a microstructure photograph having a size of 100 탆 X 100 탆, and MA on each line may exist on an average of 5 ~ 13. 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 existing in the ferrite grains so that the MA is 0.5 to 5% by area.

The ferrite may have an average size of 20 占 퐉 or less as measured by the 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 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 before the heat treatment is made bainite in which the carbide is present, MA can be evenly distributed in the grain boundaries and inside the grain boundary 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. As the yield ratio is less than 0.65, the difference between the yield strength and the tensile strength is increased so that not only the moldability 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 collapse by external force . 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.

Superior low-temperature toughness Low resistance  Method of manufacturing steel sheet

Hereinafter, a method of manufacturing a low-temperature-resisting steel sheet having excellent low temperature toughness, which is another aspect of the present invention, will be described in detail.

A method for manufacturing a composite steel sheet having excellent low-temperature toughness, which is another aspect of the present invention, includes the steps of: heating a slab having the above-described alloy composition to 1050 to 1200 ° C; 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. Where 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 DEG C

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 able to form an organization. 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 realization 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 do.

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.

At this time, the microstructure of the cooled hot-rolled steel sheet may be 50 to 90% by area of bainite. Since the microstructure of the steel sheet before the heat treatment is made bainite in which the carbide is present, MA can be evenly distributed in the grain boundaries and inside the grain boundary after the heat treatment, so that the microstructure of the steel sheet before heat treatment is preferably 50 to 90% Do.

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 ° 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 and the solid solution C is decreased, The 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 present 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.

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 a steel sheet having a thickness of 80 mm.

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 μm, and the mechanical properties were measured and described. In the inventive example, ferrite was used in addition to MA, and the average crystal grain size of ferrite was 20 탆 or less and was not separately described.

The average MA size is an average size measured by the circle equivalent diameter, and the number of MA in a 100 탆 line is 10 micro lines on the microstructure photograph of the size of 100 탆 X 100 탆, and the number of MA And the average number is described.

division Steel grade C Si Mn P S Al Ni Mo Ti Nb Cu N Invention river A 0.081 0.495 1.61 0.01 0.002 0.031 0.15 0.069 0.012 0.047 0.245 0.0037 Invention river B 0.078 0.521 1.78 0.01 0.0018 0.026 0.26 0.054 0.013 0.051 0.239 0.0041 Invention river C 0.084 0.453 1.75 0.009 0.0019 0.027 0.32 0.048 0.011 0.055 0.256 0.0038 Invention river D 0.086 0.535 1.64 0.007 0.0018 0.030 0.25 0.034 0.013 0.049 0.261 0.0034 Comparative steel E 0.046 0.503 1.69 0.009 0.002 0.011 0.147 0.068 0.013 0.042 0.26 0.0039 Comparative steel F 0.085 0.11 1.65 0.012 0.002 0.029 0.15 0.068 0.012 0.045 0.246 0.0042 Comparative steel G 0.084 0.495 1.67 0.009 0.002 0.032 0.147 0.059 0.01 0.021 0.264 0.0036

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 G do not satisfy the component ranges defined in the present invention. The comparative steel E is below the C content, the comparative steel F is below Si, and the comparative steel G is below the Nb 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 1147 798 781 345 10.2 910 121 Honor 2 B 1151 805 798 332 10.3 910 122 Honor 3 C 1142 801 797 362 10.5 910 120 Honor 4 D 1138 788 775 328 10.9 910 122 Comparative Example 5 A 1150 965 938 367 12.3 910 121 Comparative Example 6 B 1134 794 779 - - 910 120 Comparative Example 7 C 1148 803 785 385 10.2 910 395 Comparative Example 8 D 1129 791 780 701 8.3 910 121 Comparative Example 9 E 1136 809 802 365 10.3 910 119 Comparative Example 10 F 1152 810 799 335 10.1 910 124 Comparative Example 11 G 1148 803 786 387 11.6 910 119

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 66 538 617 0.87 25.4 Honor 2 B 71 524 642 0.82 22.5 Honor 3 C 64 498 624 0.80 23.4 Honor 4 D 80 503 621 0.81 21.5 Comparative Example 5 A 8 492 587 0.84 22.5 Comparative Example 6 B 0 452 561 0.81 24.2 Comparative Example 7 C 64 507 644 0.79 25.1 Comparative Example 8 D 7 561 657 0.85 22.4 Comparative Example 9 E One 497 580 0.86 24.3 Comparative Example 10 F 31 487 576 0.85 25.7 Comparative Example 11 G 27 477 553 0.86 21.7

division exam
number Steel grade After normalizing heat treatment MA fraction
(area%) Average MA
size
(탆) Number of MAs on 100 μm lines Yield strength
(MPa) The tensile strength
(MPa)  Yield ratio Elongation
(%) Impact toughness
(-40 < 0 > C, J) Honor One A 11.8 2.4 9 384 653 0.59 31.2 123 Honor 2 B 12.7 2.3 9 375 642 0.58 32.1 154 Honor 3 C 14.2 2.6 9 381 643 0.59 29.8 114 Honor 4 D 16.5 2.1 8 362 639 0.57 30.8 109 Comparative Example 5 A 3.4 6.1 2 371 468 0.79 25.6 58 Comparative Example 6 B 2.8 5.6 One 352 448 0.79 25.4 67 Comparative Example 7 C 3.5 4.5 One 341 474 0.72 24.9 52 Comparative Example 8 D 1.8 3.8 One 365 495 0.74 26.8 34 Comparative Example 9 E 0.4 5.0 0 342 438 0.78 26.7 132 Comparative Example 10 F 0.8 3.1 2 351 458 0.77 25.8 152 Comparative Example 11 G 1.2 5.3 2 348 445 0.78 24.1 117

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 5, 6, 7 and 8 satisfied the composition of the alloys proposed in the present invention, but failed to satisfy the production conditions and thus failed to secure a sufficient resistance ratio, and the impact toughness at -40 ° C was less than 100 J You can see that it is open.

In the case of Test Nos. 9 to 11, which were comparative examples, the manufacturing conditions proposed in the present invention were satisfied, but the alloy compositions were not satisfied and sufficient resistance was not obtained. have. In addition, it can be seen that the strength of C, Si, and Nb is lowered due to insufficient content.

As shown in Table 4, the MA fraction is higher than that of the comparative example. As can be seen from the above Table 3, the initial bainite structure crystal grains and carbides in the crystal grains were transformed into fine MA by securing a high bainite fraction before the normalizing heat treatment. Also, it can be seen that the yield ratio is determined by the formation of these fine MAs.

FIG. 1, which is a photograph of the microstructure before the normalizing heat treatment of Test Example No. 1 of Inventive Example 1, shows that sufficient bainite can be secured, and FIG. 2 which shows the microstructure after the heat treatment shows that fine and uniform MA is formed Able to know.

On the other hand, FIG. 3, which is a photograph of the microstructure of Test No. 9 in the comparative example, shows that the carbon content is lower than that of polygonal ferrite, and the fraction of MA is significantly lowered.

In addition, as shown in FIG. 4 in which the microstructure of Test No. 10 of the comparative example is photographed, it can be seen that the Si content is lowered and the MA fraction is decreased.

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 0.05 to 0.1% of C, 0.3 to 0.7% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.04 to 0.07% of Nb, 0.001 to 0.02% 0.4 to 0.4% of Ni, 0.1 to 0.6% of Ni, 0.01 to 0.08% of Mo, 0.001 to 0.008% of N, 0.015% or less of P and 0.003% or less of S and remaining 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 average size of the MA measured by the circle equivalent diameter is 3 탆 or less,
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet is a normalized heat treated steel sheet and comprises 50 to 90% by area of bainite in a microstructure before the normalizing heat treatment.
The method according to claim 1,
Wherein the steel sheet has 5 to 13 MA spanning the straight line when a straight line of 100 m is formed on the steel sheet.
The method according to claim 1,
Wherein the ratio of MA existing in the ferrite crystal grains to MA existing in the grain boundaries is 1: 3 to 1:10.
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.
delete 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.
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 0.05 to 0.1% of C, 0.3 to 0.7% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.04% of Al, 0.04 to 0.07% of Nb, 0.001 to 0.02% 0.4 to 0.4%, Ni to 0.1 to 0.6%, Mo to 0.01 to 0.08%, N to 0.001 to 0.008%, P to 0.015%, S to 0.003% or less, and the balance Fe and unavoidable impurities, ; ≪ / 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 캜 and holding the cooled hot-rolled steel sheet for [1.3 t + (10 to 30)] minutes,
Wherein the cooled hot-rolled steel sheet comprises 50 to 90% by area of bainite in a microstructure.
(Where t is the thickness of the hot-rolled steel sheet measured in mm)
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