CN109328240B

CN109328240B - High-strength steel sheet having low yield ratio characteristics and excellent low-temperature toughness, and method for manufacturing same

Info

Publication number: CN109328240B
Application number: CN201780039727.2A
Authority: CN
Inventors: 金佑谦; 房基铉; 严庆根; 刘承皓
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-07-01
Filing date: 2017-06-30
Publication date: 2022-05-17
Anticipated expiration: 2037-06-30
Also published as: WO2018004297A1; EP3480332B1; EP3480332A1; EP3480332A4; KR101799202B1; JP2019524987A; JP6771047B2; CN109328240A

Abstract

One aspect of the present invention relates to a high strength steel sheet excellent in low yield ratio characteristics and low temperature toughness, comprising, in wt%, 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% 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, 0.01% or less of P, 0.003% or less of S, the balance of Fe, and unavoidable impurities, and a microstructure comprising, in area fraction, 80 to 92% of ferrite, 8 to 20% of MA (martensite/austenite mixed structure), the average size of the MA being 3 μm or less as measured in terms of equivalent circle diameter.

Description

High-strength steel sheet having low yield ratio characteristics and excellent low-temperature toughness, and method for manufacturing same

Technical Field

The present invention relates to a high-strength steel sheet having low yield ratio characteristics and excellent low-temperature toughness, and a method for manufacturing the same.

Background

In order to be applied not only to steel materials for shipbuilding and marine structures but also to industrial fields requiring molding and earthquake resistance, it is necessary to develop steel materials having not only ultra-low temperature toughness but also low yield ratio characteristics.

In the case of a steel material having a low yield ratio, not only can excellent formability be achieved by increasing the difference between the yield strength and the tensile strength, but also the time point of plastic deformation until failure occurs can be delayed, and energy is absorbed in the process to prevent collapse due to external force. In addition, even if there is deformation, the structure can be repaired before collapse, so that life and property loss caused by damage of the structure can be prevented.

In order to ensure a low yield ratio, a technique for imparting a two-phase structure to a steel material has been developed. Specifically, the first phase is composed of soft ferrite, and the remaining second phase is composed of martensite, pearlite, or bainite, thereby achieving a low yield ratio.

However, there are problems in that the light two phases cause a decrease in impact toughness and cause brittle fracture of the structure at low temperatures in order that the toughness of the weld deteriorates due to an increase in the carbon content of the second phase.

Therefore, patent document 1 has been developed as a technique for ensuring a low yield ratio and low-temperature impact toughness.

In patent document 1, a low yield ratio and excellent low-temperature toughness are ensured by including 2 to 10 vol% of MA (martensite/austenite mixed structure) and 90 vol% or more of acicular ferrite in a microstructure.

According to patent document 1, a yield ratio of about 0.8 can be achieved, but a sufficiently low yield ratio cannot be achieved, and it is not sufficient to ensure the shock resistance.

Therefore, it is required to develop a high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness capable of securing a lower yield ratio and a method for manufacturing the same.

(Prior art documents)

(patent document 1): korean patent laid-open publication No. 2013-0076577

Disclosure of Invention

Technical problem

One aspect of the present invention provides a high-strength steel sheet having low yield ratio characteristics and excellent low-temperature toughness, and a method for manufacturing the same.

In addition, the technical problems to be solved by the present invention are not limited to the above-mentioned contents, and the technical problems to be solved by the present invention can be understood based on the entire contents of the present specification, and it will not be difficult for those skilled in the art to understand the additional technical problems of the present invention.

Technical scheme

One aspect of the present invention provides a high-strength steel sheet having excellent low yield ratio characteristics and low-temperature toughness, the steel sheet including, in wt%, 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% 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, 0.01% or less of P, 0.003% or less of S, and the balance of Fe and inevitable impurities, the microstructure including, in area fraction, 80 to 92% of ferrite, and 8 to 20% of MA (martensite/austenite mixed structure), the average size of the MA being 3 μm or less as measured by equivalent circle diameter.

In addition, another aspect of the present invention provides a method for manufacturing a high-strength steel sheet having excellent low yield ratio characteristics and low-temperature toughness, including:

a step of heating a slab to 1050 ℃ to 1200 ℃, the slab comprising, in 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 0.02% 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, 0.01% or less of P, 0.003% or less of S, the balance being Fe and unavoidable impurities;

a step of hot-rolling the heated slab to a finish rolling temperature of 760 ℃ to 850 ℃ to obtain a hot-rolled steel sheet;

a step of cooling the hot rolled steel sheet to 450 ℃ or lower at a cooling rate of 5 ℃/s or higher; and

and a normalizing heat treatment step of heating the cooled hot-rolled steel sheet to a temperature in the range of 850 to 960 ℃ and then maintaining the temperature for [1.3t + (10 to 30) ] minutes.

T is a value measured in mm for the thickness of the hot-rolled steel sheet.

In addition, the above technical solutions do not list all features of the present invention. Various features of the present invention, together with the advantages and effects attendant thereto, may be understood in greater detail with reference to the following detailed description.

Effects of the invention

According to the present invention, excellent low yield ratio characteristics and low-temperature toughness can be ensured, and in particular, a low yield ratio of 0.65 or less can be ensured, so that not only moldability but also excellent shock resistance can be ensured.

Therefore, the method can be applied to the fields of steel materials for shipbuilding and marine structures and also can be applied to the industrial fields requiring forming and earthquake resistance.

Drawings

Fig. 1 is a photograph of a microstructure of test No. 1 as an example of the present invention taken with an Optical Microscope (OM).

Fig. 2 is a photograph of the microstructure of test No. 1, which is an example of the present invention, taken with a Scanning Electron Microscope (SEM).

Fig. 3 is a photograph of the microstructure of the comparative example, test No. 12, taken by an Optical Microscope (OM).

Detailed Description

Preferred embodiments of the present invention are described below. However, the present invention can be modified in various ways, and the scope of the present invention is not limited to the following embodiments. In addition, the following embodiments of the present invention are provided to more fully describe the present invention to those of ordinary skill in the art.

The present inventors have found that formability can be ensured to some extent by ensuring a yield ratio of about 0.8 by the conventional technique, but the low yield ratio is not sufficient to ensure the earthquake-resistant characteristics. The present inventors have conducted intensive studies in order to solve these problems.

As a result of studies, the present inventors found that in order to achieve a low yield ratio, it is more advantageous that the hardness difference between the base material and the second phase is large and the distribution of MA is uniform, and that the base material of patent document 1 is acicular ferrite, and the hardness difference between MA and the base material is insufficient, and the MA phase is formed in the grain boundary and the MA size is large, so that a sufficiently low yield ratio cannot be achieved.

From this, it was confirmed that a low yield ratio of 0.65 or less can be secured by making the microstructure of the base material composed of ferrite and uniformly distributing the fine MA phase in the ferrite grain boundaries and the inside of the crystal grains, and in order to secure such a structure, it is necessary to control the structure before the normalizing heat treatment to include bainite, and the present invention was completed.

A high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness according to an aspect of the present invention is described in detail below.

According to one aspect of the present invention, there is provided a high strength steel sheet excellent in low yield ratio characteristics and low temperature toughness, comprising, in wt%, 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% 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, 0.01% or less of P, 0.003% or less of S, and the balance of Fe and inevitable impurities, and having a microstructure comprising, in area fraction, 80 to 92% of ferrite, 8 to 20% of MA (martensite/austenite mixed structure) having an average size, as measured by circle equivalent diameter, of 3 μm or less.

First, the alloy composition of the high strength steel sheet excellent in low yield ratio characteristics and low temperature toughness according to one aspect of the present invention will be described in detail. The unit of the content of each element below is weight%.

C：0.03％～0.08％

In the present invention, C is an element that causes solid solution strengthening, and is present in the form of carbonitride based on Nb or the like to secure tensile strength.

If the C content is less than 0.03%, the above-mentioned effects are insufficient. Conversely, if the C content is greater than 0.08%, MA becomes coarse and pearlite is formed, which may deteriorate the impact properties at low temperatures and make it difficult to sufficiently secure bainite.

Si：0.05％～0.3％

Si is an element that assists Al in deoxidizing molten steel, and is added in order to secure yield strength and tensile strength.

If the Si content is less than 0.05%, the above-mentioned effects are insufficient. Conversely, if the Si content is more than 0.3%, the impact properties may be deteriorated and the welding properties may be deteriorated due to coarsening of MA.

Mn：1.0％～2.0％

Mn is an element that significantly contributes to the strength increasing effect by solid solution strengthening, and contributes to the formation of bainite.

If the Mn content is less than 1.0%, the above-mentioned effects are insufficient. Conversely, if Mn is excessively added, the upper limit is set to 2.0% because of the formation of MnS inclusions and the segregation at the center, which may cause a decrease in toughness.

Al：0.005％～0.04％

Al is used as a main deoxidizer of steel, and is required to be added by more than or equal to 0.005 percent. However, if the amount is more than 0.04%, the effect is saturated and Al is included₂O₃The fraction and size of inclusions increase, which may cause a decrease in low-temperature toughness.

Nb：0.005％～0.04％

Nb is an element that suppresses recrystallization during rolling or cooling by solid-solution or precipitated carbonitride, refines the structure, and increases the strength. If the Nb content is less than 0.005%, the above-mentioned effects are insufficient. Conversely, if the Nb content is more than 0.04%, there is a problem that the base material toughness and the toughness after welding may be lowered.

Ti：0.001％～0.02％

Ti combines with oxygen or nitrogen to form precipitates to suppress coarsening of the structure, thereby contributing to refinement of the structure and improving toughness.

If the Ti content is less than 0.001%, the above-mentioned effects are insufficient. Conversely, if the Ti content exceeds 0.02%, coarse precipitates are formed, which may cause fracture.

Cu：0.05％～0.4％

Cu is a component that does not cause a significant decrease in impact properties, and improves strength by solid solution and precipitation. In order to sufficiently improve the strength, it is necessary to contain 0.05% or more of Cu, but if the Cu content is more than 0.4%, cracks on the surface of the steel sheet due to thermal shock of Cu may be generated.

Ni：0.6％～2.0％

Ni is an element that can improve both strength and toughness, but the strength does not increase greatly with increasing Ni content, and lowering the Ar3 temperature is an element that contributes to the formation of bainite.

If the Ni content is less than 0.6%, the above-mentioned effects are insufficient. Conversely, if the Ni content is more than 2.0%, the manufacturing cost increases, possibly resulting in deterioration of weldability.

Mo：0.08％～0.3％

Mo is an austenite stabilizing element, affects the increase of the amount of MA, and has a great effect on the improvement of strength. Further, Mo is an element that prevents strength from being decreased during heat treatment and contributes to the formation of bainite.

If the Mo content is less than 0.08%, the above-mentioned effects are insufficient. Conversely, if the Mo content is more than 0.3%, the manufacturing cost increases, and there is a possibility that the base material toughness and the post-welding toughness decrease.

N：0.002％～0.006％

N is an element that contributes to improvement of strength and toughness by forming precipitates together with Ti, Nb, Al, and the like when a slab is heated to refine an austenite structure.

If the N content is less than 0.002%, the above-mentioned effects are insufficient. On the contrary, if the N content is more than 0.006%, surface cracks and precipitates are formed at high temperature, and the remaining N exists in an atomic state, possibly lowering toughness.

P: less than or equal to 0.01 percent

P may cause grain boundary segregation as an impurity, which causes embrittlement of steel. Therefore, it is important to control the upper limit, and it is preferable to control the upper limit to 0.01% or less.

S: less than or equal to 0.003%

S is an impurity, and mainly combines with Mn to form MnS inclusions, which become a factor of impairing low-temperature toughness. Therefore, it is important to control the upper limit, and in order to ensure low-temperature toughness, the S content is preferably controlled to 0.003% or less.

The balance of the present invention is iron (Fe). However, the conventional manufacturing process inevitably involves mixing of unexpected impurities derived from raw materials or the surrounding environment, and thus the mixing of impurities cannot be excluded. These impurities are known to anyone skilled in the art of conventional manufacturing processes and therefore all relevant details are not repeated in this specification.

The microstructure of the high strength steel sheet excellent in low yield ratio characteristics and low temperature toughness according to one aspect of the present invention will be described in detail below.

The microstructure of a high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness according to one aspect of the present invention includes 80% to 92% of ferrite, 8% to 20% of MA having an average size of 3 μm or less as measured by an equivalent circle diameter in terms of area fraction. The fraction of the following microstructure refers to an area fraction, unless otherwise mentioned.

Ferrite is a phase for ensuring basic toughness and strength, and is preferably 80% or more. In order to secure sufficient MA, the upper limit of the ferrite fraction is preferably 92%. Further, the ferrite preferably does not contain acicular ferrite. This is because the hardness difference between acicular ferrite and MA is small, and a sufficiently low yield ratio cannot be ensured.

If MA is less than 8%, it is difficult to secure a low yield ratio of 0.65 or less, and if MA is more than 20%, impact toughness may be reduced and elongation may be reduced. Further, if the average size of MA measured in terms of the equivalent circle diameter is larger than 3 μm, MA is mainly formed at grain boundaries, and it is difficult to ensure uniform distribution of MA and a low yield ratio.

In addition to the ferrite and MA described above, other unavoidable phases may be included, which is not excluded. For example, pearlite may be contained in an amount of 1% by area or less.

In this case, in order to ensure excellent low yield ratio characteristics and low temperature toughness, not only the MA fraction and the size satisfy the above ranges, but also when a 100 μm straight line is drawn on the steel sheet of the present invention, it is preferable that 5 to 13 MAs exist on the straight line.

That is, when several straight lines are drawn up and down or right and left on a microstructure photograph having a size of 100. mu. m.times.100. mu.m, 5 to 13 MA's may be present on average on each straight line. This is because MA mainly causing destruction is MA existing at grain boundaries, and when the condition is satisfied, MA is uniformly distributed at grain boundaries and inside of grains, and thus it is advantageous to ensure a low yield ratio.

Further, the ratio of MA present in ferrite grains to MA present in grain boundaries may be 1:3 to 1: 10. The ratio is a ratio of the number of MAs, and by satisfying the ratio, the MAs existing in the ferrite grains can be uniformly distributed to 0.5 to 5 area%.

Further, the ferrite may have an average size, as measured by an equivalent circle diameter, of less than or equal to 20 μm. This is because, if the average size of ferrite is larger than 20 μm, it is difficult to secure sufficient toughness and strength.

In addition, the steel sheet according to the present invention is a steel sheet that has undergone a normalizing heat treatment, and the bainite in the microstructure of the steel sheet before the normalizing heat treatment may be 50 to 90 area%.

Since MA can be uniformly distributed in grain boundaries and crystal grains after heat treatment by making the microstructure of the steel sheet before heat treatment composed of bainite in which carbide exists inside, the bainite in the microstructure of the steel sheet before heat treatment is preferably 50 to 90 area%.

In addition, the steel sheet according to the present invention may have a yield ratio of 0.5 to 0.65, and low temperature impact characteristics at-40 ℃ of 100J or more. The yield ratio is 0.65 or less, and by increasing the difference between the yield strength and the tensile strength, not only can excellent moldability be achieved, but also the time point of plastic deformation until failure occurs can be delayed, and energy can be absorbed in the process to prevent collapse due to external force.

Therefore, the steel material can be suitably used not only in the field of steel materials for shipbuilding and marine structures but also in the field of industries requiring molding and seismic resistance.

In this case, the steel sheet may have a yield strength of 350 to 400MPa and a tensile strength of 600MPa or more.

A method for manufacturing a high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness according to another aspect of the present invention is described in detail below.

A method for manufacturing a high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness according to another aspect of the present invention includes: a step of heating the slab having the alloy composition to 1050 to 1200 ℃; a step of hot-rolling the heated slab to a finish rolling temperature of 760 ℃ to 850 ℃ to obtain a hot-rolled steel sheet; a step of cooling the hot rolled steel sheet to 450 ℃ or lower at a cooling rate of 5 ℃/s or higher; and a normalizing heat treatment step of heating the cooled hot-rolled steel sheet to a temperature in the range of 850 to 960 ℃ and then maintaining the temperature for [1.3t + (10 to 30) ] minutes. T is a value measured in mm for the thickness of the hot-rolled steel sheet.

Step of heating the slab

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

If the heating temperature is higher than 1200 ℃, austenite grains are coarsened and the toughness may be lowered, and if the heating temperature is lower than 1050 ℃, Ti, Nb, and the like may not be sufficiently dissolved and the strength may be lowered.

Step of Hot Rolling

And hot rolling the heated slab to a finish rolling temperature of 760 to 850 ℃ to obtain a hot-rolled steel sheet.

The rolling temperature of the general heat treatment steel is about 850 ℃ to 1000 ℃, and the conventional rolling is adopted. However, in the present invention, it is important to form the initial structure into bainite. Therefore, a rolling control flow for finishing rolling at a low temperature is required instead of the conventional rolling in which a ferrite-pearlite structure occurs.

In the hot rolling, recrystallization zone rolling is required to refine austenite grain size, and the higher the reduction per pass, the more advantageous the performance.

The unrecrystallized zone rolling should be completed at a temperature of Ar3 or higher of the steel material, which means about 760 ℃ or higher. More specifically, the finish rolling finish temperature may be defined as 760 ℃ to 850 ℃. If the finish rolling temperature is more than 850 ℃, it is difficult to suppress ferrite-pearlite transformation, and if the finish rolling temperature is less than 760 ℃, unevenness of the microstructure in the thickness direction may be caused, and the microstructure to be realized may not be formed because the reduction amount based on the load weight of the roll is reduced.

By finishing the finish rolling at a temperature in the range of 760 ℃ to 850 ℃, ferrite-pearlite transformation can be suppressed and the bainite structure can be realized by cooling. The purpose of the initial structure consisting of bainite is to distribute the MA evenly after the heat treatment. In the ferrite-pearlite structure, MA is mainly formed in grain boundaries, and in the bainite structure, MA is formed in both grain boundaries and inside crystal grains.

Step of Cooling

Cooling the hot rolled steel sheet to 450 ℃ or less at a cooling rate of 5 ℃/s or more.

The rapid cooling after hot rolling is very important to achieve the target structure of the inventive steel. In order to form fine, uniform MA, bainite needs to be formed, and in order to form bainite, the cooling end temperature and the cooling rate are important factors.

If the cooling end temperature is more than 450 ℃, the size of crystal grains may become coarse, coarse MA may be formed after heat treatment due to coarsening of carbides, toughness may be reduced, and it may be difficult to secure bainite at 50 area% or more.

If the cooling rate is less than 5 ℃/s, a large amount of acicular ferrite or ferrite + pearlite microstructure is formed, which may result in a decrease in strength, and after heat treatment, a two-phase structure of ferrite + MA does not appear, but the number of coarse ferrite + pearlite structures or second phases sharply decreases, which may make it difficult to secure bainite of 50 area% or more.

Normalizing heat treatment step

Heating the cooled hot-rolled steel sheet to a temperature ranging from 850 ℃ to 960 ℃, and then keeping the temperature for [1.3t + (10-30) ] minutes. T is a value measured in mm for the thickness of the hot-rolled steel sheet.

If the normalizing temperature is less than 850 ℃ or the holding time is less than (1.3t +10) minutes, re-solid solution of cementite and MA phases in pearlite and bainite is difficult, so that solid-dissolved C is reduced, not only is it difficult to secure strength, but also the hard phase which is finally left is coarse.

In contrast, if the normalizing temperature is more than 960 ℃ or the holding time is more than (1.3t +30) minutes, carbides existing in the bainite grains are all migrated to the grain boundaries or coarsening of the carbides occurs, failing to form the final MA size and uniform distribution. In addition, there is a possibility that the grain growth causes a decrease in strength and deterioration of impact characteristics.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present invention is described in more detail below by way of examples. It should be noted, however, that the following examples are for the purpose of describing specific examples of the present invention only, and are not intended to limit the scope of the claims of the present invention. The scope of the claims of the present invention depends on the contents of the claims and reasonable derivation thereof.

Molten steel having the composition shown in table 1 below was prepared, and then a slab was manufactured by continuous casting. The slab was subjected to rolling, cooling and normalizing heat treatment according to the manufacturing conditions of table 2 below, thereby manufacturing a steel sheet.

The bainite fraction and mechanical properties of the steel sheet before the heat treatment for the fire were measured and are shown in table 3 below.

The MA fraction, average MA size, the number of MAs on the 100 μm line, and mechanical properties of the steel sheet after the heat treatment for a fire were measured and are shown in table 4 below. In the invention examples, ferrite is used in addition to MA, and the average grain size of ferrite is not more than 20 μm, which is not described separately.

The average MA size is the average size measured in terms of equivalent circle diameter. As for the MA number on the 100 μm line, 10 straight lines were drawn up and down or right and left on a microstructure photograph having a size of 100 μm × 100 μm, and the MA number per straight line was measured and the average number was described.

Specifically, the influence on the rolling temperature, the cooling end temperature, and the heat treatment time is intended to be grasped. Further, in Table 3, MA fraction, yield ratio and mechanical properties of steel sheets produced under the production conditions 1 to 12 are shown for the components A to H.

[ TABLE 1 ]

Classification of	Steel grade	C	Si	Mn	P	S	Al	Ni	Mo	Ti	Nb	Cu	N
														Invention steel	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 steel	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 steel	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 steel	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

The unit of the content of each element in table 1 above is weight%. The inventive steels a to D are steel sheets that satisfy the composition ranges specified in the present invention, and the comparative steels E to H are steel sheets that do not satisfy the composition ranges specified in the present invention. Comparative steel E is a steel with an excess of C content, comparative steel F is a steel with an insufficient Mo content, comparative steel G is a steel with an insufficient Mn content, and comparative steel H is a steel with an insufficient Ni content.

[ TABLE 2 ]

[ TABLE 3 ]

[ TABLE 4 ]

The invention examples in which the alloy composition and the production conditions proposed in the present invention are satisfied can ensure that the yield ratio is 0.65 or less and the impact toughness at-40 ℃ is also excellent at 100J or more.

In the case of comparative examples, test nos. 6, 7, 9 and 10, which satisfy the alloy components proposed in the present invention, the manufacturing conditions were not satisfied, and thus a sufficiently low yield ratio could not be secured, and the impact toughness at-40 ℃ was also inferior by less than 100J.

In the case of comparative examples, i.e., test nos. 11 to 14, although the manufacturing conditions proposed in the present invention were satisfied, the alloy components were not satisfied, and thus a sufficiently low yield ratio could not be ensured, and the impact toughness at-40 ℃ of test nos. 11 and 14 was also inferior by less than 100J.

As can be seen from table 4 above, the inventive examples have a higher MA fraction than the comparative examples. From the above table 3, it is confirmed that this means that carbides in the grains of the initial bainite structure and on the grain boundaries are converted into fine MA by securing a high bainite fraction before the normalizing heat treatment.

As is clear from fig. 1 and 2, which are images of the microstructure of test No. 1, which is an inventive example, a fine and uniform MA was formed in the microstructure.

In contrast, as is clear from fig. 3 in which the microstructure of comparative example, test No. 12, was photographed, carbide and pearlite appeared as main two phases, and therefore the fraction of MA was low, and the formed MA was in a polygonal shape and existed mainly at grain boundaries.

The present invention has been described above with reference to the embodiments, but various modifications and changes can be made by those skilled in the art without departing from the technical idea and field of the present invention described in the claims.

Claims

1. A high-strength steel sheet having excellent low yield ratio characteristics and low-temperature toughness, characterized in that:

the steel sheet comprises, in 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 0.02% 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, less than or equal to 0.01% of P, less than or equal to 0.003% of S, and the balance of Fe and unavoidable impurities,

the microstructure comprises, in area fraction, from 80% to 92% of ferrite, from 8% to 20% of MA (martensite/austenite mixed structure) having an average size, measured as the equivalent circle diameter, of less than or equal to 3 μm, and

wherein when a straight virtual line of 100 μm is drawn on the high-strength steel sheet, 5 to 13 MA's exist on the straight line, and

wherein the ferrite has an average size, as measured by equivalent circle diameter, of less than or equal to 20 μm, and

the yield ratio of the high-strength steel plate is 0.5-0.65, the low-temperature impact property at-40 ℃ is greater than or equal to 100J, the yield strength is 350-400 MPa, and the tensile strength is greater than or equal to 600 MPa.

2. The high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness according to claim 1, characterized in that:

the ratio of MA existing in ferrite grains to MA existing in grain boundaries is 1:3 to 1: 10.

3. The high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness according to claim 1, characterized in that:

the steel sheet is a steel sheet subjected to normalizing heat treatment,

the bainite of the microstructure of the steel plate before normalizing heat treatment is 50-90 area%.

4. A method for manufacturing a high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness according to claim 1, comprising:

a normalizing heat treatment step of heating the cooled hot rolled steel sheet to a temperature range of 875 ℃ to 960 ℃ and then maintaining for [1.3t + (10-30) ] minutes,

wherein t is a value measured in mm with respect to the thickness of the hot-rolled steel sheet.

5. The method for manufacturing a high-strength steel sheet excellent in low yield ratio characteristics and low-temperature toughness according to claim 4, characterized in that:

the bainite in the microstructure of the hot-rolled steel sheet after cooling is 50 to 90 area%.