KR102560057B1 - High yield ratio and high strength steel sheet having excellent bendability and the method for manufacturing the same - Google Patents

Abstract

The present invention relates to a high-strength steel sheet and a manufacturing method thereof, and more particularly, to provide a high-yield ratio high-strength steel sheet excellent in bending workability and a manufacturing method thereof.

Description

High yield ratio high-strength steel sheet with excellent bending workability and manufacturing method thereof

The present invention relates to a high-strength steel sheet and a manufacturing method thereof, and more particularly, to a high-yield-ratio high-strength steel sheet excellent in bending workability and a manufacturing method thereof.

In order to improve formability, conventional high-strength hot-rolled steel sheets for boom arms are made of ferrite-bainite or ferrite-martensite dual-phase composite structure steel, or high-strength high-burring steel having ferrite or bainite as a base structure. Techniques have been proposed. In addition, a technique for manufacturing high-strength steel having martensite as a base structure by cooling to room temperature by applying a high cooling rate has been proposed.

For example, Patent Document 1 discloses that a tensile strength of 950 MPa or more can be secured by dispersing and precipitating precipitates including Ti and Mo, but addition of expensive alloy components not only increases manufacturing cost, but also has a problem of not securing the impact resistance properties required for thick hot-rolled steel sheets.

Patent Document 2 discloses a technique for manufacturing a high-strength hot-rolled steel sheet using a dual phase (DP) steel of ferrite and martensite. However, when using a step-cooling technology, it is difficult to apply to hot-rolled steel, which is also a thick product, and there is also a problem in that manufacturing cost increases as expensive alloy components are added. In addition, since the yield ratio is low due to the nature of the composite structure steel, excessively high tensile strength and a large amount of alloying elements are required to satisfy the desired yield strength.

Patent Document 3 discloses a technique of controlling the cooling rate to 150 to 350° C./s after completion of hot rolling. However, when martensite is produced by cooling at an excessively fast cooling rate, it is difficult to secure high yield strength due to a low yield ratio, and high tensile strength is required to meet the yield strength standard, resulting in poor impact properties and bending workability.

In addition, Patent Document 4 discloses a technique of controlling the winding temperature to 300 to 550°C. As such, when winding at a temperature of 300 ° C. or higher, the microstructure approaches an equiaxed crystal with a low aspect ratio due to the formation of bainite, which is advantageous in bending workability, but the impact resistance is lowered, and it is difficult to accurately control the winding temperature. When the coiling temperature is increased to facilitate control, there is a problem in that manufacturing cost increases because a large amount of alloying elements are required to secure strength.

Japanese Unexamined Patent Publication No. 2003-089848 Japanese Unexamined Patent Publication No. 2003-321737 Japanese Unexamined Patent Publication No. 2003-105446 Japanese Unexamined Patent Publication No. 2000-109951

According to one aspect of the present invention, it is intended to provide a high-yield ratio high-strength steel sheet excellent in bending workability and a manufacturing method thereof.

The object of the present invention is not limited to the above. A person skilled in the art will have no difficulty understanding the further subject matter of the present invention from the general content of this specification.

One aspect of the present invention, in weight%, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 0.8 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 1.2%, Mo: 0.005 to 0.5%, P: 0.001 to 0.01%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.05%, B: 0.001 to 0.003%, the balance including Fe and unavoidable impurities,

The T value defined in the following relational expression 1 is 1.0 to 5.0,

The microstructure includes 30% or more and less than 50% of bainite, 50% or more and less than 70% of tempered martensite and less than 15% of other residual structures, by area%,

The average size of epsilon carbide in the tempered martensite lath is 0.3 μm or less,

A steel sheet having a bending formability R/t of 2.5 or less can be provided.

[Relationship 1]

T = (4[C]+[Mn])/(0.7[Cr]+1.5[Mo])

(Where [C], [Mn], [Cr] and [Mo] are the weight percent of the corresponding alloy element.)

The other remaining structures may include tempered bainite, retained austenite, ferrite, and martensite.

Among the other remaining structures, ferrite may be less than 5%.

The steel sheet may have a yield strength of 900 MPa or more, a tensile strength of 980 MPa or more, a yield ratio of 0.90 or more, and a work brittleness temperature of -30 °C or less.

Another aspect of the present invention, in weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 0.8 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 1.2%, Mo: 0.005 to 0.5%, P: 0.001 to 0.01%, S: 0.001 to 0.01% , N: 0.001 to 0.01%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.05%, B: 0.001 to 0.003%, the balance including Fe and unavoidable impurities, Reheating a steel slab having a T value of 1.0 to 5.0 defined in relational expression 1 below in a temperature range of 1150 to 1350 ° C;

Hot rolling the reheated steel slab at a finish rolling temperature in the range of Tn-50 to Tn defined by the following relational expression 2;

Primary cooling of the hot-rolled steel sheet to a temperature range of 450 to 550 ° C. at a cooling rate (CR1) of CCRmin to CCRmax defined in the following relational expression 3; and

It is possible to provide a method for manufacturing a steel sheet comprising the step of winding the primary cooled steel sheet after secondary cooling at a cooling rate (CR2) of CCRmin to CCRmax-15 defined in the relational expression 3 to a coiling temperature at which R satisfies 60 to 70 defined in the following relational expression 4.

[Relationship 1]

T = (4[C]+[Mn])/(0.7[Cr]+1.5[Mo])

(Where [C], [Mn], [Cr] and [Mo] are the weight percent of the corresponding alloy element.)

[Relationship 2]

Tn = 780+171[C]-35[Si]+50[Mn]+21.4[Cr]+24.1[Mo]+389[Ti]+980[Nb]+385[B]

(Where [C], [Si], [Mn], [Cr], [Mo], [Ti], [Nb] and [B] are the weight percent of the corresponding alloying element.)

[Relationship 3]

Log(CCRmax) = 4.7 - (0.1[C]-0.4[Si]+1.1[Mn]+0.6[Cr]+1[Mo]+450[B])

Log(CCRmin) = 4.7 - (0.1[C]-0.3[Si]+1.1[Mn]+0.6[Cr]+1[Mo]+550[B])

(Where [C], [Si], [Mn], [Cr], [Mo] and [B] are the weight percent of the corresponding alloy element.)

[Relationship 4]

R = 246EXP(-730/((CT+M)/2+273))

M = 543-414[C]-15.5[Si]-41.2[Mn]-19.8[Cr]-97[Al]-25.0[Mo]+17[Ti]-283[Nb]-1786[B]

(Where CT is the winding temperature (℃), and [C], [Si], [Mn], [Cr], [Al], [Mo], [Ti], [Nb] and [B] are the weight percent of the corresponding alloy element.)

After the winding, a step of cooling the coil to room temperature at 0.001 to 10 °C/s may be further included.

According to one aspect of the present invention, it is possible to provide a high-yield ratio high-strength steel sheet with excellent bending workability and excellent bending brittleness in a low temperature region and a manufacturing method thereof.

Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

In order to solve the above-mentioned conventional problems, the present inventors investigated changes in strength, bending workability, and bending brittleness in a low temperature range according to the characteristics of the components and microstructures for steels having various components and microstructures.

From the results, the inventors derived Relational Equation 1 for optimizing the component content of the alloy components, and also derived Relational Equations 2 to 4 for optimizing the manufacturing process conditions to optimize the microstructure, thereby providing excellent bending workability. It was confirmed that high yield ratio and high strength could be secured, and the present invention was completed.

Hereinafter, the present invention will be described in detail.

Hereinafter, the steel composition of the present invention will be described in detail.

In the present invention, unless otherwise specified, % indicating the content of each element is based on weight.

Steel according to one aspect of the present invention, in weight percent, C: 0.05-0.15%, Si: 0.01-0.5%, Mn: 0.8-2.0%, Al: 0.01-0.1%, Cr: 0.005-1.2%, Mo: 0.005-0.5%, P: 0.001-0.01%, S: 0.001-0.01 %, N: 0.001-0.01%, Nb: 0.01-0.05%, Ti: 0.005-0.05%, B: 0.001-0.003%, the balance Fe and unavoidable impurities may be included.

Carbon (C): 0.05 to 0.15%

Carbon (C) is the most economical and effective element for reinforcing steel, and when the added amount increases, the tensile strength increases due to the increase in martensite or bainite fraction. If the content of carbon (C) is less than 0.05%, it is difficult to sufficiently obtain the above-mentioned effects, and if the content exceeds 0.15%, the formation of coarse carbides and precipitates is excessive, resulting in lower formability and low-temperature impact resistance, and poor weldability. There is a problem.

Therefore, the content of carbon (C) may be 0.05 to 0.15%, more preferably 0.06 to 0.12%.

Silicon (Si): 0.01 to 0.5%

Silicon (Si) is an element advantageous for deoxidizing molten steel, having a solid solution strengthening effect, and delaying the formation of coarse carbides to improve formability. If the content of silicon (Si) is less than 0.01%, it is difficult to obtain the above-mentioned effect, while if the content exceeds 0.5%, a red scale due to silicon (Si) is formed on the surface of the steel sheet during hot rolling, so that the surface quality of the steel sheet is very poor, and weldability is also deteriorated.

Accordingly, the content of silicon (Si) may be 0.01 to 0.5%, more preferably 0.1 to 0.3%.

Manganese (Mn): 0.8 to 2.0

Manganese (Mn), like Si, is an element effective for solid solution strengthening of steel, and may facilitate the formation of martensite and bainite during cooling after heat treatment by increasing hardenability of steel. If the content of manganese (Mn) is less than 0.8%, the effect of addition cannot be obtained, and if the content exceeds 2.0%, the segregation portion is greatly developed at the center of the thickness when casting the slab in the casting process, and the formation of MnS is facilitated. Bending workability may be inferior.

Therefore, the content of manganese (Mn) may be 0.8 to 2.0%, more preferably 1.0 to 1.8%.

Chromium (Cr): 0.005 to 1.2%

Chromium (Cr) strengthens steel by solid solution and, upon cooling, delays ferrite transformation to help the formation of martensite and bainite. However, if the content of chromium (Cr) is less than 0.005%, the above effect cannot be obtained, and if the content exceeds 1.2%, the segregation portion at the center of the thickness is greatly developed, similar to Mn, and the microstructure in the thickness direction is non-uniform.

Accordingly, the content of chromium (Cr) may be 0.005 to 1.2%, more preferably 0.4 to 1.0%.

Molybdenum (Mo): 0.005 to 0.5%

Molybdenum (Mo) increases the hardenability of steel and facilitates the formation of martensite and bainite. However, if the content of molybdenum (Mo) is less than 0.005%, the above effects due to addition cannot be obtained, and if the content exceeds 0.5%, it may be economically disadvantageous and weldability may be deteriorated.

Accordingly, the content of molybdenum (Mo) may be 0.001 to 0.5%, more preferably 0.1 to 0.3%.

Phosphorus (P): 0.001 to 0.01%

Phosphorus (P) has a solid solution strengthening effect, but brittleness may occur due to grain boundary segregation. In order to manufacture a phosphorus (P) content of less than 0.001%, a lot of manufacturing costs are required, which is economically unfavorable. On the other hand, if the content exceeds 0.01%, as described above, brittleness due to grain boundary segregation may occur.

Accordingly, the content of phosphorus (P) may be 0.001 to 0.01%.

Sulfur (S): 0.001 to 0.01%

Sulfur (S) is an impurity present in steel, and when its content exceeds 0.01%, it combines with Mn to form non-metallic inclusions. Accordingly, when cutting steel, fine cracks easily occur and impact resistance is greatly reduced. There is a problem. On the other hand, in order to manufacture the content of sulfur (S) to less than 0.001%, it takes a lot of time during steelmaking operation, and productivity may decrease.

Accordingly, the content of sulfur (S) may be 0.001 to 0.01%.

Aluminum (Al): 0.01 to 0.1%

Aluminum (Al) is mainly added for deoxidation, and if the content of aluminum (Al) is less than 0.01%, the effect of the addition is insufficient, and if the content exceeds 0.1%, AlN is formed by combining with N. During casting, corner cracks are likely to occur in the slab, and defects due to inclusion formation may occur.

Therefore, the content of aluminum (Al) may be 0.01 to 0.1%, more preferably 0.01 to 0.05%.

Nitrogen (N): 0.001 to 0.01%

Nitrogen (N) is a representative solid-solution strengthening element together with C, and forms coarse precipitates with Ti and Al. In general, the solid solution strengthening effect of nitrogen (N) is superior to that of C, but there is a problem in that toughness is greatly reduced as the amount of nitrogen (N) in steel increases, so the upper limit is limited to 0.01%. On the other hand, in order to manufacture the content to less than 0.001%, excessive time is required during steelmaking operation, resulting in a decrease in productivity.

Therefore, the content of nitrogen (N) may be 0.001 to 0.01%.

Titanium (Ti): 0.005 to 0.05%

Titanium (Ti) is a representative precipitation strengthening element along with Nb and V, and forms coarse TiN with a strong affinity with N. Such TiN has an effect of suppressing grain growth during the heating process for hot rolling, and it is advantageous to utilize B added to improve hardenability because dissolved N is stabilized. In addition, Ti remaining after reacting with N is dissolved in the steel and combined with C to form TiC precipitates, which is useful for improving the strength of the steel. However, if the content of titanium (Ti) is less than 0.005%, it is difficult to obtain the above-mentioned effect, and if the content exceeds 0.05%, there may be a problem of inferior impact resistance at low temperature due to generation of coarse TiN and coarsening of precipitates during heat treatment.

Therefore, the content of titanium (Ti) may be 0.005 to 0.05%, more preferably 0.01 to 0.03%.

Niobium (Nb): 0.01 to 0.05%

Niobium (Nb) is a representative precipitation hardening element along with Ti and V, and is effective in improving the strength and impact toughness of steel due to the effect of refining crystal grains by delaying recrystallization by precipitating during hot rolling. If the content of niobium (Nb) is less than 0.01%, the above effect cannot be obtained, and if the content exceeds 0.05%, recrystallization is excessively delayed due to precipitates formed during rolling, and the precipitates grow to reduce low-temperature working brittleness. There may be a problem.

Therefore, the content of niobium (Nb) may be 0.001 to 0.05%, more preferably 0.001 to 0.03%.

Boron (B): 0.001 to 0.003%

Boron (B), when present in a solid solution state in steel, has an effect of improving the brittleness of steel in a low temperature region by stabilizing grain boundaries. If the content of boron (B) is less than 0.001%, it is difficult to obtain the above effect, and if the content exceeds 0.003%, recrystallization behavior during hot rolling is delayed and hardenability is greatly increased, resulting in poor formability.

Accordingly, the content of boron (B) may be 0.001 to 0.003%.

The steel of the present invention may include remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing field, not all of them are specifically mentioned in this specification.

The steel of the present invention may have a T value of 1.0 to 5.0 defined in the following relational expression 1.

Relational Equation 1 is a component formula for securing brittleness of steel, and in the present invention, yield strength and bending brittleness can be secured.

If the T value defined in relational expression 1 is less than 1.0, the yield ratio may be lowered, and if the value exceeds 5.0, bending brittleness may be inferior.

[Relationship 1]

T = (4[C]+[Mn])/(0.7[Cr]+1.5[Mo])

(Where [C], [Mn], [Cr] and [Mo] are the weight percent of the corresponding alloy element.)

Hereinafter, the steel microstructure of the present invention will be described in detail.

In the present invention, % representing the fraction of the microstructure is based on the area unless otherwise specified.

The microstructure of the steel according to one aspect of the present invention may include bainite of 30% or more and less than 50%, tempered martensite of 50% or more and less than 70%, and other residual structures of less than 15% by area%. Other residual structures may include tempered bainite, retained austenite, ferrite, and martensite. Among them, the fraction of ferrite may be less than 5%.

If the bainite is less than 30%, excessive martensite is formed in the early stage, the yield ratio is lowered, and the bending properties are deteriorated due to high strength.

If the tempered martensite is less than 50%, the increase in yield strength by auto-tempering is not sufficiently achieved, resulting in a low yield ratio and inferior bending properties.

Other residual structures may contain less than 15% of tempered bainite, retained austenite, ferrite, and martensite. Among other remaining structures, if the ferrite phase is 5% or more, it is unfavorable for securing strength, and it is disadvantageous for brittleness because phases containing a large amount of carbon are created by delaying the transformation of other low-temperature transformation phases.

The average size of epsilon carbide formed in the tempered martensite lath may be 0.3 μm or less.

In the present invention, by using the auto-tempering effect, epsilon carbide is formed in the tempered martensite lath of the initial steel sheet, which has the effect of increasing the yield ratio and increasing the ductility to improve the bending properties. However, there is a problem that brittleness is inferior due to epsilon carbide when the average size exceeds 0.3 μm due to excessive effect.

Hereinafter, the steel manufacturing method of the present invention will be described in detail.

Steel according to one aspect of the present invention can be produced by reheating, hot rolling, primary cooling, secondary cooling and winding a steel slab satisfying the above-described alloy composition.

reheat

A steel slab satisfying the above-described alloy composition may be reheated in a temperature range of 1150 to 1350 ° C.

If the reheating temperature is less than 1150 ° C., the precipitate is not sufficiently re-dissolved, so the formation of the precipitate is reduced and coarse TiN remains in the process after hot rolling, and it may be difficult to solve the segregation generated during casting by diffusion. On the other hand, when the temperature exceeds 1350 ° C., austenite crystal grains may deteriorate in strength and cause non-uniform structure due to abnormal grain growth.

hot rolled

The reheated steel slab may be hot rolled at a finish rolling temperature in the range of Tn-50 to Tn defined by the following relational expression 2.

In the present invention, recrystallization delay can be controlled by performing hot rolling through the following relational expression 2 determined by alloy components. When the finish rolling temperature is less than Tn-50, the recrystallization delay becomes excessive, and the grains are elongated, resulting in severe anisotropy and poor bending properties. On the other hand, if the temperature exceeds Tn, the temperature of the hot-rolled steel sheet increases, resulting in a coarse grain size and poor surface quality of the hot-rolled steel sheet.

In the present invention, the continuous casting and hot rolling process may be directly connected processes.

[Relationship 2]

Tn = 780+171[C]-35[Si]+50[Mn]+21.4[Cr]+24.1[Mo]+389[Ti]+980[Nb]+385[B]

(Where [C], [Si], [Mn], [Cr], [Mo], [Ti], [Nb] and [B] are the weight percent of the corresponding alloying element.)

1st cooling

The hot-rolled steel sheet may be primarily cooled to a temperature range of 450 to 550 ° C. at a cooling rate of CCRmin to CCRmax defined in the following relational expression 3.

During the primary cooling, if the cooling rate exceeds CCRmax, it is difficult to produce bainite in the steel, resulting in inferior bending properties, and if the cooling rate is less than CCRmin, ferrite is generated, which may be disadvantageous in securing desired strength.

At the time of primary cooling, if the cooling end temperature is less than 450 ° C, even if the cooling rate is satisfied, martensite formation is excessive (bainite formation is difficult) and the bending characteristics are deteriorated, and if the temperature exceeds 550 ° C, excessive ferrite may be formed.

[Relationship 3]

Log(CCRmax) = 4.7 - (0.1[C]-0.4[Si]+1.1[Mn]+0.6[Cr]+1[Mo]+450[B])

Log(CCRmin) = 4.7 - (0.1[C]-0.3[Si]+1.1[Mn]+0.6[Cr]+1[Mo]+550[B])

(Where [C], [Si], [Mn], [Cr], [Mo] and [B] are the weight percent of the corresponding alloy element.)

Secondary cooling and winding

The primary cooled steel sheet may be coiled after secondary cooling at a cooling rate of CCRmin to CCRmax-15 defined in the relational expression 3 to a coiling temperature at which R satisfies 60 to 70 defined in the following relational expression 4.

During the secondary cooling, if the cooling rate exceeds CCRmax-15, tempering of martensite does not sufficiently occur, making it difficult to form tempered martensite, resulting in poor bendability.

If the R value defined in the following relational expression 4 is less than 60, the temperature of the coil is too low, and it is difficult to wind in the process, and a large amount of martensite for brittleness may be generated due to excessive hardness. In addition, the yield ratio and bendability may be inferior due to insufficient auto-tempering effect. On the other hand, if the value exceeds 70, epsilon carbide may grow due to excessive tempering, and brittleness may be inferior.

[Relationship 4]

R = 246EXP(-730/((CT+M)/2+273))

M = 543-414[C]-15.5[Si]-41.2[Mn]-19.8[Cr]-97[Al]-25.0[Mo]+17[Ti]-283[Nb]-1786[B]

(Where CT is the winding temperature (℃), and [C], [Si], [Mn], [Cr], [Al], [Mo], [Ti], [Nb] and [B] are the weight percent of the corresponding alloy element.)

After winding, the coil can be cooled to room temperature at 0.001 to 10°C/s or kept warm. At this time, the coil may be air-cooled or forced-cooled. Since there is no change in the microstructure and size of the epsilon carbide in the lath of tempered martensite according to the cooling rate, the cooling rate is not particularly limited, but it is preferable to cool or heat it at 0.001 to 10 ° C / s in consideration of productivity.

The steel of the present invention prepared as described above has a yield strength of 900 MPa or more, a tensile strength of 980 MPa or more, a yield ratio of 0.90 or more, a bending formability R / t of 2.5 or less, and a workability brittleness temperature of -30 ° C or less.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the present invention.

(Example)

Table 1 below shows alloy components according to steel types and values calculated by relational expression 1 through them. In addition, Table 2 below shows manufacturing process conditions for each steel type, and according to the alloy composition of each steel type, the Tn value, average cooling rate (CR1, CR2), coiling temperature (CT ), M value and R value of relational expressions 2 to 4 are calculated and shown. At this time, the average cooling rate means the average cooling rate of the steel surface. The reheating temperature not shown in Table 2 was 1250° C., and the thickness of the hot-rolled steel sheet immediately after hot rolling was 3 mm, which was equally applied.

steel grade Alloy component (% by weight) Relation 1 (T) C Si Mn Cr Al P S N Mo Ti Nb B A 0.070 0.100 2.050 0.100 0.030 0.010 0.003 0.005 0.100 0.020 0.030 0.0010 10.6 B 0.080 0.010 1.200 0.500 0.030 0.010 0.003 0.005 0.500 0.030 0.070 0.0015 1.4 C 0.080 0.100 1.300 0.700 0.030 0.010 0.003 0.004 0.100 0.015 0.030 0.0020 2.5 D 0.050 0.010 1.500 0.600 0.020 0.010 0.003 0.004 0.300 0.020 0.040 0.0007 2.0 E 0.130 0.500 1.100 0.300 0.030 0.010 0.003 0.005 0.200 0.020 0.020 0.0030 3.2 F 0.060 0.500 1.900 0.700 0.030 0.010 0.003 0.005 0.070 0.020 0.030 0.0010 3.6 G 0.100 0.300 1.300 0.100 0.030 0.010 0.003 0.005 0.250 0.020 0.015 0.0025 3.8 H 0.140 0.200 0.850 1.000 0.020 0.010 0.004 0.004 0.100 0.010 0.030 0.0025 1.7 I 0.140 0.015 1.100 0.300 0.020 0.010 0.003 0.005 0.220 0.020 0.040 0.0025 3.1 J 0.060 0.100 1.200 0.900 0.020 0.005 0.002 0.005 0.100 0.025 0.020 0.0020 1.8 K 0.080 0.200 1.500 0.100 0.020 0.010 0.003 0.007 0.500 0.030 0.030 0.0015 2.2 L 0.060 0.400 1.700 0.200 0.020 0.010 0.003 0.004 0.500 0.020 0.020 0.0010 2.2 M 0.070 0.080 1.800 0.400 0.040 0.005 0.005 0.008 0.200 0.030 0.010 0.0010 3.6 N 0.080 0.070 1.300 0.800 0.025 0.009 0.002 0.004 0.001 0.015 0.040 0.0020 2.9

[Relationship 1]

T = (4[C]+[Mn])/(0.7[Cr]+1.5[Mo])

(Where [C], [Mn], [Cr] and [Mo] are the weight percent of the corresponding alloy element.)

steel grade hot rolled
(Relationship 2) Cooling (Relation 3, Relation 4) Tn Tn-50 Finish rolling temperature
(℃) Primary
Cooling down
temperature
(℃) CCRmin CCRmax Primary
cooling rate
(CR1)
(℃/s) Secondary
Cooling end temperature
(winding temperature (CT)) CCRmax-15 CCRmin Secondary
cooling rate
(CR2)
(℃/s) M
(℃) R A 933 883 897 490 57 74 69 175 59 57 57 411 68 B 957 907 902 510 56 80 80 124 65 56 54 413 64 C 909 859 885 485 47 76 91 103 61 47 58 424 63 D 931 881 926 509 101 119 71 108 104 101 64 427 64 E 880 830 872 498 40 89 74 111 74 40 75 412 63 F 922 872 906 511 52 73 71 141 58 52 42 404 65 G 883 833 882 485 46 88 81 199 73 46 47 424 71 H 898 848 877 504 54 101 75 54 86 54 64 410 58 I 938 888 912 497 51 91 82 107 76 51 54 405 62 J 899 849 884 504 46 75 67 135 60 46 59 436 67 K 918 868 871 496 52 77 74 180 62 52 55 418 69 L 905 855 895 521 59 82 79 114 67 59 61 416 63 M 914 864 903 495 56 72 71 167 57 56 57 418 68 N 919 869 879 503 50 81 76 121 66 50 54 422 64

[Relationship 2]

Tn = 780+171[C]-35[Si]+50[Mn]+21.4[Cr]+24.1[Mo]+389[Ti]+980[Nb]+385[B]

(Where [C], [Si], [Mn], [Cr], [Mo], [Ti], [Nb] and [B] are the weight percent of the corresponding alloying element.)

[Relationship 3]

Log(CCRmax) = 4.7 - (0.1[C]-0.4[Si]+1.1[Mn]+0.6[Cr]+1[Mo]+450[B])

Log(CCRmin) = 4.7 - (0.1[C]-0.3[Si]+1.1[Mn]+0.6[Cr]+1[Mo]+550[B])

(Where [C], [Si], [Mn], [Cr], [Mo] and [B] are the weight percent of the corresponding alloy element.)

[Relationship 4]

R = 246EXP(-730/((CT+M)/2+273))

M = 543-414[C]-15.5[Si]-41.2[Mn]-19.8[Cr]-97[Al]-25.0[Mo]+17[Ti]-283[Nb]-1786[B]

(Where CT is the winding temperature (℃), and [C], [Si], [Mn], [Cr], [Al], [Mo], [Ti], [Nb] and [B] are the weight percent of the corresponding alloy element.)

Table 3 below shows microstructural characteristics and mechanical properties according to each type of steel. The microstructure was observed with an optical microscope, and the average size of epsilon carbide was observed with a transmission electron microscope for specimens extracted by the carbon replica method. Bending workability (R/t) and bending brittleness were shown as mechanical property values, and yield strength, tensile strength, and yield ratio were tested by taking a JIS5 standard test piece in a direction parallel to the rolling direction. Bending workability R / t is the value obtained by dividing the mold curvature (R) by the thickness (t) after a 90-degree bending test, and for bending brittleness, the specimen subjected to the bending test at R / t = 3 was laid down and fixed, and then a cylindrical weight (4.44kg) was free-falled from a height of 1m and fractured. A drop weight test was performed. At this time, the temperature of the specimen was tested at 10 ° C intervals in the range of -40 to 0 ° C.

steel grade microstructure (% area) mechanical properties division martensite Tempered martensite ferrite retained austenite bainite Tempered Bainite Epsilon Carbide Average Size
(μm) yield strength
(MPa) tensile strength
(MPa) yield ratio Bending workability (R/t) Machining brittleness
temperature
(℃) A 0 57 0 0 43 0 0.28 958 1029 0.93 3.6 -10 Comparative Example 1 B 0 65 0 0 35 0 0.13 941 1037 0.91 4 -40 Comparative Example 2 C 6 75 0 0 19 0 0.05 918 1052 0.87 3 -40 Comparative Example 3 D 11 22 11 4 62 0 0.11 829 935 0.89 2.4 -40 Comparative Example 4 E 0 79 0 0 21 0 0.11 948 1086 0.87 2.6 -40 Comparative Example 5 F 0 42 0 2 56 0 0.16 889 967 0.92 2.2 -40 Comparative Example 6 G 0 53 0 0 47 0 0.38 978 1011 0.97 3.8 -20 Comparative Example 7 H 34 29 0 0 37 0 0.02 891 1071 0.83 2.8 -40 Comparative Example 8 I 5 51 0 0 44 0 0.08 912 1016 0.90 2.4 -40 Invention example 1 J 0 61 0 0 39 0 0.16 941 1033 0.91 2.2 -40 Invention Example 2 K 0 67 0 0 32 One 0.25 971 1034 0.94 2.4 -40 Invention Example 3 L 3 63 0 0 34 0 0.10 939 1042 0.90 2.4 -40 Invention Example 4 M 0 69 0 0 31 0 0.21 963 1049 0.92 2.2 -40 Invention Example 5 N 0 59 0 0 41 0 0.11 935 1031 0.91 2.2 -40 Example 6

As shown in Table 3, the inventive examples satisfying the alloy composition and manufacturing method proposed in the present invention secured all the mechanical properties targeted in the present invention.

It can be seen that comparative examples that do not satisfy the alloy composition or manufacturing method of the present invention do not secure the mechanical properties desired by the present invention.

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (6)

In weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 0.8 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 1.2%, Mo: 0.005 to 0.5%, P: 0.001 to 0.01%, S: 0.001 to 0.01%, N: 0.00 1 to 0.01%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.05%, B: 0.001 to 0.003%, the balance including Fe and unavoidable impurities,
The T value defined in the following relational expression 1 is 1.0 to 5.0,
The microstructure includes 30% or more and less than 50% of bainite, 50% or more and less than 70% of tempered martensite and less than 15% of other residual structures, by area%,
The average size of epsilon carbide in the tempered martensite lath is 0.3 μm or less,
Bending formability R / t is 2.5 or less,
Steel sheet with a processing brittleness temperature of -30℃ or less.
[Relationship 1]
T = (4[C]+[Mn])/(0.7[Cr]+1.5[Mo])
(Where [C], [Mn], [Cr] and [Mo] are the weight percent of the corresponding alloy element.)
According to claim 1,
Steel sheet containing tempered bainite, retained austenite, ferrite, and martensite as the other remaining structures.
According to claim 2,
A steel sheet containing less than 5% of ferrite among the other remaining structures.
According to claim 1,
The steel sheet has a yield strength of 900 MPa or more, a tensile strength of 980 MPa or more, and a yield ratio of 0.90 or more.
In weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 0.8 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 1.2%, Mo: 0.005 to 0.5%, P: 0.001 to 0.01%, S: 0.001 to 0.01%, N: 0.00 Reheating a steel slab containing 1 to 0.01%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.05%, B: 0.001 to 0.003%, the balance Fe and unavoidable impurities, and having a T value of 1.0 to 5.0 defined in the following relational expression 1 in the temperature range of 1150 to 1350 ° C;
Hot rolling the reheated steel slab at a finish rolling temperature in the range of Tn-50 to Tn defined by the following relational expression 2;
Primary cooling of the hot-rolled steel sheet to a temperature range of 450 to 550 ° C. at a cooling rate (CR1) of CCRmin to CCRmax defined in the following relational expression 3; and
The steel sheet that has been cooled for the first time at a cooling rate (CR2) of CCRmin to CCRmax-15 defined in the above relational expression 3 to a coiling temperature at which R satisfies 60 to 70 defined in the following relational expression 4, followed by secondary cooling and then winding.
[Relationship 1]
T = (4[C]+[Mn])/(0.7[Cr]+1.5[Mo])
(Where [C], [Mn], [Cr] and [Mo] are the weight percent of the corresponding alloy element.)
[Relationship 2]
Tn = 780+171[C]-35[Si]+50[Mn]+21.4[Cr]+24.1[Mo]+389[Ti]+980[Nb]+385[B]
(Where [C], [Si], [Mn], [Cr], [Mo], [Ti], [Nb] and [B] are the weight percent of the corresponding alloying element.)
[Relationship 3]
Log(CCRmax) = 4.7 - (0.1[C]-0.4[Si]+1.1[Mn]+0.6[Cr]+1[Mo]+450[B])
Log(CCRmin) = 4.7 - (0.1[C]-0.3[Si]+1.1[Mn]+0.6[Cr]+1[Mo]+550[B])
(Where [C], [Si], [Mn], [Cr], [Mo] and [B] are the weight percent of the corresponding alloy element.)
[Relationship 4]
R = 246EXP(-730/((CT+M)/2+273))
M = 543-414[C]-15.5[Si]-41.2[Mn]-19.8[Cr]-97[Al]-25.0[Mo]+17[Ti]-283[Nb]-1786[B]
(Where CT is the winding temperature (℃), and [C], [Si], [Mn], [Cr], [Al], [Mo], [Ti], [Nb] and [B] are the weight percent of the corresponding alloy element.)
According to claim 5,
After the winding, the method of manufacturing a steel sheet further comprising the step of cooling the coil to room temperature at 0.001 ~ 10 ℃ / s.