KR20120011292A - STEEL PLATE WITH HIGH STRENGTH OF 500MPa GRADE AND LOW TEMPERATURE TOUGHNESS AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

PURPOSE: A high-strength thick plate with improved low-temperature shock toughness and a method of manufacturing the same are intended to provide a complex structure having fine bainite on fine ferrite matrix. CONSTITUTION: A method of manufacturing a thick plate with high-strength and improved low-temperature shock toughness comprises next steps. Slab is re-heated. The re-heated board is first rolled at over a temperature to stop austenite recrystallization. The first rolled board is second rolled at below the temperature to stop austenite recrystallization. The second rolled board is cooled. The slab consists of C of 0.05~0.1 weight%, Si of 0.2~0.4 weight%, Mn of 1.2~1.6 weight%, P of 0.015 weight% or less, S of 0.005 weight% or less, Al of 0.02~0.05 weight%, Ti of 0.005~0.015 weight%, Nb of 0.01~0.04 weight%, Ni of 0.2~0.4 weight%, N of 60ppm or less, Fe and other inevitable impurities.

Description

500MPa grade high strength plate with excellent low temperature impact toughness and its manufacturing method {STEEL PLATE WITH HIGH STRENGTH OF 500MPa GRADE AND LOW TEMPERATURE TOUGHNESS AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high-strength thick plate excellent in low temperature impact toughness and a method for manufacturing the same, and more particularly, to a high-strength thick plate of a tensile strength 500MPa class and its use mainly in the field of structures, plants, shipbuilding, etc. requiring high strength and high toughness It relates to a manufacturing method.

Steel plates are usually manufactured by slab reheating, hot rolling and cooling.

In the slab reheating process, the steel slab, which is semifinished, is reheated.

In the hot rolling process, the reheated slab is rolled using a rolling roll at a specific rolling rate in a specific temperature range.

In the cooling process, the plate finished rolling is cooled.

An object of the present invention is to provide a thick plate excellent in impact toughness at a low temperature, such as high strength and -60 ℃ of 500MPa class of tensile strength for use in the field of heavy industry where high strength and high toughness of structures, shipbuilding and the like is required.

Another object of the present invention by optimizing the trace alloy element such as Ti-Nb-Ni and hot rolling conditions to omit the heat treatment process, to reduce the manufacturing cost, to provide a method for producing a thick plate excellent in high strength and low temperature impact toughness will be.

The thick plate excellent in high strength and low temperature impact toughness according to an embodiment of the present invention for achieving the one object is C: 0.05 ~ 0.1 wt%, Si: 0.2 ~ 0.4 wt%, Mn: 1.2 ~ 1.6 wt%, P : 0.015 wt% or less, S: 0.005 wt% or less, Al: 0.02-0.05 wt%, Ti: 0.005-0.015 wt%, Nb: 0.01-0.04 wt%, Ni: 0.2-0.4 wt%, N: 60 ppm or less and It is composed of the remaining Fe and other unavoidable impurities, characterized in that the tensile strength of 500 MPa or more, the average impact toughness of 200 ~ 350 J at -60 ℃.

According to an embodiment of the present invention for achieving the above object, a high strength and low temperature impact toughness manufacturing method having excellent toughness is C: 0.05 to 0.1% by weight, Si: 0.2 to 0.4% by weight, Mn: 1.2 to 1.6% by weight, P: 0.015 wt% or less, S: 0.005 wt% or less, Al: 0.02 to 0.05 wt%, Ti: 0.005 to 0.015 wt%, Nb: 0.01 to 0.04 wt%, Ni: 0.2 to 0.4 wt%, N: 60 ppm or less And reheating the slab made of the remaining Fe and other unavoidable impurities. Primary rolling the reheated sheet at a temperature above austenite recrystallization stop temperature; Secondary rolling the first rolled sheet at a temperature below the austenite recrystallization stop temperature; Cooling the secondary rolled sheet material; characterized in that it comprises a.

500MPa-grade high strength thick plate and its manufacturing method excellent in low-temperature impact toughness according to the present invention is subjected to the primary rolling so that the residual pressure reduction rate is 40 ~ 80% above the austenite recrystallization stop temperature, 2 below the austenite recrystallization stop temperature By performing the secondary rolling and controlling the speed of the accelerated cooling and the cooling end temperature, a composite structure in which fine bainite is formed on the fine ferrite matrix with uniform final microstructure can be obtained. Therefore, high strength and excellent low temperature impact toughness of 500 MPa or more can be obtained. It provides a secured effect.

1 is a flow chart schematically showing a 500MPa class high strength thick plate manufacturing method excellent in low temperature impact toughness according to an embodiment of the present invention.
Figure 2 is a cross-sectional photograph showing the microstructure of the thick plate prepared according to the Examples and Comparative Examples.
3 is a cross-sectional view showing the microstructure of the thick plate prepared according to the Examples and Comparative Examples.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various different forms, and only the embodiments make the disclosure of the present invention complete, and those skilled in the art to which the present invention pertains. It is provided to fully inform the person having the scope of the invention, which is defined only by the scope of the claims. Like reference numerals refer to like elements throughout.

Hereinafter, a 500MPa-grade high strength thick plate having excellent low temperature impact toughness and a manufacturing method thereof according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Thick plate with high strength and low temperature impact toughness

High strength and low temperature impact toughness of the thick plate according to the present invention is carbon (C): 0.05 ~ 0.1% by weight, silicon (Si): 0.2 ~ 0.4% by weight, manganese (Mn): 1.2 ~ 1.6% by weight, phosphorus (P) : 0.015 wt% or less, Sulfur (S): 0.005 wt% or less, Aluminum (Al): 0.02-0.05 wt%, Titanium (Ti): 0.005-0.015 wt%, Niobium (Nb): 0.01-0.04 wt%, Nickel (Ni): 0.2 to 0.4% by weight, nitrogen (N): 60ppm or less, the rest is composed of Fe and other unavoidable impurities.

Hereinafter, the role and content of each component included in the thick plate excellent in high strength and low temperature impact toughness according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added to secure the strength of the thick plate.

The carbon (C) is preferably added in a content ratio of 0.05 to 0.1% by weight of the total weight of the thick plate. When the carbon (C) is added less than 0.05% by weight, the fraction of the second phase tissue is lowered, and the strength of the thick plate is lowered. When the content of the carbon (C) exceeds 0.1% by weight, the strength of the thick plate is There is a problem that the increase in low-temperature impact toughness and weldability is reduced.

Silicon (Si)

In the present invention, silicon (Si) is added as a deoxidizer for removing oxygen in the steel in the steelmaking process. Silicon (Si) also has a solid solution strengthening effect.

The silicon (Si) is preferably added in an amount of 0.2 to 0.4% by weight of the total weight of the thick plate. If the content of silicon (Si) is less than 0.20% by weight, the effect of the addition of silicon is insignificant. If the content of silicon (Si) is more than 0.50% by weight, oxides are formed on the surface of the plate to inhibit plating properties of the plate, and weldability There is a problem of deterioration.

Manganese (Mn)

Manganese (Mn) is an austenite stabilizing element in the present invention, by lowering the Ar ₃ temperature to enlarge the controlled rolling region, thereby miniaturizing grains by rolling to improve strength and toughness.

The manganese is preferably added in an amount of 1.2 to 1.6% by weight of the total weight of the thick plate. When the manganese (Mn) is added in less than 1.5% by weight is insufficient formation of the second phase tissue does not contribute to the strength improvement. In addition, when the content of the manganese (Mn) exceeds 1.8% by weight, there is a problem that precipitates sulfur dissolved in steel as MnS to lower the low-temperature impact toughness.

Aluminum (Al)

Aluminum (Al) serves as a deoxidizer to remove oxygen in the steel.

The aluminum (Al) is preferably added in an amount ratio of 0.02 to 0.05% by weight of the total weight of the thick plate. When the aluminum (Al) is added in an amount ratio of less than 0.02% by weight, the deoxidation effect is insufficient, and when the content of aluminum (Al) exceeds 0.05% by weight, Al ₂ O ₃ , which is a nonmetallic inclusion, forms a low temperature impact toughness. There is a problem of deterioration.

Titanium (Ti)

In the present invention, titanium (Ti) forms a TiN upon slab reheating and serves to inhibit austenite grain growth.

The titanium (Ti) is preferably added in an amount of 0.005 to 0.015% by weight of the total weight of the thick plate. If the content of titanium is less than 0.005% by weight, the effect of adding titanium is insignificant. If the content of titanium is more than 0.015% by weight, the TiN precipitate is coarsened, thereby reducing the effect of inhibiting grain growth.

Niobium (Nb)

Niobium (Nb) combines with carbon (C) and nitrogen (N) to form carbides or nitrides. This suppresses grain growth during rolling to refine grains, thereby improving strength and low temperature impact toughness.

The niobium (Nb) is preferably added in an amount of 0.01 to 0.04% by weight of the total weight of the thick plate. When the content of niobium (Nb) is less than 0.01% by weight, the niobium addition effect may not be properly exhibited. On the other hand, when the content of niobium (Nb) exceeds 0.05% by weight, the weldability of the thick plate is lowered, and the strength and low temperature impact toughness due to the increase in niobium (Nb) content are no longer improved, but are present in solid solution in the ferrite. Rather, there is a risk of lowering the impact toughness.

Nickel (Ni)

In the present invention, nickel (Ni) is refined to solid crystals and dissolved in austenite and ferrite to strengthen the matrix. In particular, nickel is an effective element for improving low temperature impact toughness.

The nickel is preferably added in an amount of 0.3 to 0.4% by weight of the total weight of the thick plate. When nickel (Ni) is added in an amount ratio of less than 0.3% by weight, the nickel addition effect may not be properly exhibited. In addition, when the content of nickel (Ni) exceeds 0.4% by weight, there is a problem of causing red light brittleness.

Phosphorus (P)

Phosphorus (P) is a representative element for lowering the low temperature impact toughness. The lower the content, the better. Therefore, the content of phosphorus (P) is preferably limited to 0.015% by weight or less.

Sulfur (S)

Sulfur (S) is an element that is inevitably contained in the production of steel together with phosphorus (P), and forms an emulsion-based inclusion (MnS) to lower low-temperature impact toughness. Therefore, the content of sulfur (S) is preferably limited to 0.005% by weight or less.

Nitrogen (N)

Since nitrogen (N) causes inclusions in the steel to degrade the internal quality of the thick plate, it is preferable to manage it at an extremely low content ratio, but for this purpose, the manufacturing cost of the thick plate increases, and there is also difficulty in managing the nitrogen (N). . Therefore, in the present invention, it is preferable to limit the content of nitrogen (N) to 60 ppm or less.

Tensile strength of the thick plate made of the above composition is 500 ~ 600 MPa, has a high strength of 500 MPa or more of the target, satisfies low-temperature impact toughness by showing an average impact toughness of 200 ~ 350 J at -60 ℃. In addition, the thick plate made of the above composition may exhibit a yield strength (YS) of 440 to 550 MPa and an elongation (EL) of 25 to 36%.

This property is controlled by using a microalloy element such as Ti-Nb-Ni included in the composition of the present invention, so that the final microstructure is uniform and fine ferrite as shown in FIG. It can be obtained from the form of a complex tissue in which two phases are bainite.

Plate manufacturing method

1 is a flow chart schematically showing a thick plate manufacturing method excellent in high strength and low temperature impact toughness according to an embodiment of the present invention.

Referring to FIG. 1, the illustrated thick plate manufacturing method includes a slab reheating step (S110), a first rolling step (S120), a second rolling step (S130), and a cooling step. The cooling step is further divided into a first cooling step S140 and a second cooling step S150.

Reheat slab

In the slab reheating step (S110), C: 0.05 to 0.1 wt%, Si: 0.2 to 0.4 wt%, Mn: 1.2 to 1.6 wt%, P: 0.015 wt% or less, S: 0.005 wt% or less, Al: 0.02 to 0.05 Re-heat the slabs made of weight%, Ti: 0.005 to 0.015 weight%, Nb: 0.01 to 0.04 weight%, Ni: 0.2 to 0.4 weight%, N: 60 ppm or less, and the remaining Fe and other unavoidable impurities for control rolling.

It is preferable that slab reheating temperature is 1100-1200 degreeC. If the slab reheating temperature is less than 1100 ° C., the slab temperature is low, resulting in a large rolling load. In addition, when the slab reheating temperature is higher than 1200 ° C, niobium (Nb) and titanium (Ti) precipitates are dissolved to inhibit austenite grain growth, and it is difficult to secure the strength and low temperature impact toughness of the thick plates manufactured by coarsening austenite grains. There is a problem.

Control rolling

After the slab is reheated, control rolling is carried out. In the case of general rolling, a process of performing annealing (Normalizing) at a temperature above Ac3 transformation point after rolling is required. In this case, the overall thick plate manufacturing cost is increased, and as the carbon equivalent is increased in order to secure high strength, the low-temperature toughness may be lowered.

In the present invention, the control rolling is carried out in two stages of primary rolling and secondary rolling.

In the first rolling step (S120), the reheated slab (plate) is first rolled at a temperature above the austenite recrystallization stop temperature, preferably at a temperature of at least 930 ° C.

At this time, the primary rolling is preferably carried out so that the residual reduction ratio of the secondary rolling is 40 to 80%. Here, the thickness or rolling reduction of the plate after the primary rolling can be obtained from the thickness of the plate before the control rolling, the thickness of the plate after the final control rolling, and the residual reduction rate of the secondary rolling. For example, when the thickness of a plate before control rolling is 100 mm, the thickness after completion of control rolling is 10 mm, and the residual reduction rate of secondary rolling is 50%, the thickness of the plate after primary rolling is 20 mm. Therefore, since the primary rolling is to roll a 100 mm thick plate | board material to thickness of 20 mm, the reduction ratio of a primary rolling will be 80%.

In the secondary rolling step (S130), the first rolled sheet is secondary rolled at a temperature below the austenite recrystallization stop temperature.

It is preferable that the residual reduction ratio of the secondary rolling is 40 to 80%. If the residual pressure reduction rate of the secondary rolling made below the austenite recrystallization stop temperature is less than 40%, there is a problem in that the ferrite grain refinement is not sufficiently achieved. In addition, when the residual reduction ratio of the secondary rolling exceeds 80%, there is a problem in that the rolling process takes a long time and the productivity decreases.

Since the end temperature of the secondary rolling is closely related to the strength and low temperature impact toughness of the thick plate produced, it is very important to control it properly.

In the present invention, the completion temperature of the secondary rolling is preferably 800 ~ 850 ℃. When the end temperature of the secondary rolling is less than 800 ° C, abnormal reverse rolling occurs, and thus, stretched ferrite and perlite are present, and a perlite band is formed, thereby greatly reducing ductility and low temperature impact toughness. In addition, when the end temperature of the secondary rolling exceeds 850 ℃ excellent ductility and toughness, but there is a problem that can not sufficiently secure the strength of the thick plate to be manufactured.

When the first and second rolling conditions of the present invention are applied, strain bands are formed in the austenite grains, thereby forming a large amount of ferrite nucleation sites in the austenite grains to form fine grains after the end of rolling.

Cooling

In the cooling step, the secondary rolled plate is cooled, thereby suppressing grain growth through accelerated cooling to form a matrix structure having fine ferrite grains, and forming fine bainite structure as a secondary phase, thereby having high strength and low temperature toughness. Is carried out to secure it.

In the present invention, the cooling step is further divided into a first cooling step (S140) and a second cooling step (S150).

In the first cooling step (S140), the secondary rolled plate is cooled to a predetermined temperature, and may be accelerated by water cooling.

The primary cooling proceeded by the accelerated cooling may be carried out to the cooling rate of 3 ~ 7 ℃ / sec and the cooling end temperature of 500 ~ 650 ℃ to facilitate both high strength and low temperature impact toughness of the thick plate to be manufactured.

If the end temperature of the primary cooling is less than 500 ℃ has a problem that the low-temperature transformation structure is formed a large amount of low temperature impact toughness, and when the end temperature of the primary cooling exceeds 650 ℃ due to the formation of coarse microstructure strength There is a problem that is lowered.

In addition, if the cooling rate of the primary cooling is less than 3 ℃ / sec, it is difficult to secure the strength by promoting the grain formation, and if it exceeds 7 ℃ / sec, the bainite fraction increases, the strength is increased, but low-temperature toughness is lowered There is a problem.

In the second cooling step (S150), the first cooled plate is cooled at a slower speed than the first cooling rate. Secondary cooling may be performed up to room temperature of about 10 ~ 30 ℃ by the air cooling method.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. However, this is presented as a preferred example of the present invention and in no sense can be construed as limiting the present invention.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Manufacture of thick plates

Thick plates according to Examples 1 to 6 and Comparative Examples 1 to 7 were prepared under the compositions shown in Table 1 and the process conditions described in Table 2.

In the case of Examples 1 to 6 and Comparative Examples 1 to 7, each slab was reheated at 1150 ° C. for 2 hours and then subjected to primary rolling and secondary rolling, and cooling was performed by primary cooling, which is water-cooled, and secondary cooling, which is air-cooled. Proceeded. Cooling conditions in Table 2 means the primary cooling conditions. In Comparative Example 3, normal annealing heat treatment was performed without performing control rolling.

 [Table 1]

Unit: weight%

 TABLE 2

2. Characterization

Table 3 shows the mechanical properties and impact toughness of the thick plates prepared according to Examples 1-6 and Comparative Examples 1-7. In Table 3, the average impact toughness was measured three times at -60 ° C, and the average impact toughness was measured.

[Table 3]

Referring to Table 3, in Example 1, the tensile strength was 535 MPa, and the average impact toughness at -60 ° C was 339 J, thereby achieving a target tensile strength of 500 MPa, and having excellent low temperature impact toughness. Able to know.

However, in Comparative Example 1, the tensile strength was very excellent at -60 ° C, but the average impact toughness did not reach the target value of 193 J. In addition, in the case of Comparative Example 2 and Comparative Example 7, the tensile strength reached the target value of 500MPa class, but the average impact toughness did not reach 200 J at -60 ℃ did not satisfy the low temperature impact toughness.

Here, in the case of Comparative Example 1, the content of nickel (Ni) is less than the content according to the present invention, and in the case of Comparative Example 2, the content of nickel (Ni) is in excess of the reference value.

In addition, in the case of Comparative Example 3, the content of titanium (Ti) is less than the reference value, and in the case of Comparative Example 4, the content of titanium (Ti) exceeds the reference value.

In addition, in the case of Comparative Example 5, the content of niobium (Nb) is less than the reference value, and in the case of Comparative Example 6, the content of niobium (Nb) exceeds the reference value.

Thus, in the case of Comparative Examples 1 to 6 as described above did not achieve the reference value of the tensile strength or low temperature impact toughness according to the present invention.

In particular, in Comparative Example 7, to which the conventional annealing heat treatment other than the control rolling was applied, the chemical composition was completely different, and the low temperature impact toughness showed poor results due to the presence of the heat treatment process.

Figure 2 shows a microstructure picture of the thick plate prepared according to Example 1, Figure 3 shows a microstructure picture of the thick plate prepared according to Comparative Example 1.

2 and 3, the microstructure of the thick plate prepared according to Comparative Example 1 is formed of bainite on the coarse ferrite base, the microstructure of the thick plate prepared according to Example 1 is fine and uniform ferrite It can be seen that bainite is finely distributed in the second phase.

The thick plate prepared according to Example 1 is excellent in both tensile strength and low temperature impact toughness because the microstructure is superior to Comparative Example 1 by controlling the chemical composition, rolling and cooling conditions.

As described above, the thick plate manufactured by the manufacturing method according to the present invention satisfies the tensile strength of 500MPa grade, and also has an excellent low-temperature toughness with an average impact toughness of 200 J or more at -60 ° C.

Therefore, the manufactured thick plate can be widely used in plants, structures, shipbuilding fields, etc. where high strength and high toughness are required.

In the above description, the embodiment of the present invention has been described, but various changes and modifications can be made at the level of those skilled in the art. Such changes and modifications may belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention will be determined by the claims described below.

S110: slab reheating stage
S120: first rolling step
S130: secondary rolling step
S140: first cooling stage
S150: 2nd cooling stage

Claims (14)

C: 0.05 to 0.1 wt%, Si: 0.2 to 0.4 wt%, Mn: 1.2 to 1.6 wt%, P: 0.015 wt% or less, S: 0.005 wt% or less, Al: 0.02 to 0.05 wt%, Ti: 0.005 to Reheating the slab of 0.015% by weight, Nb: 0.01-0.04% by weight, Ni: 0.2-0.4% by weight, N: 60 ppm or less and the remaining Fe and other unavoidable impurities;
Primary rolling the reheated sheet at a temperature above austenite recrystallization stop temperature;
Secondary rolling the first rolled sheet at a temperature below the austenite recrystallization stop temperature; And
Cooling the secondary rolled sheet material; excellent plate strength manufacturing method characterized in that it comprises a high strength and low temperature impact toughness.
The method of claim 1,
The slab reheating
A thick plate manufacturing method excellent in high strength and low temperature impact toughness, characterized in that carried out at 1100 ~ 1200 ℃.
The method of claim 1,
The primary rolling is
The high-strength and low-temperature impact toughness excellent steel sheet production method, characterized in that carried out so that the reduction ratio of the secondary rolling is 40 to 80%.
The method of claim 1,
The primary rolling is
A thick plate manufacturing method excellent in high strength and low temperature impact toughness, characterized in that carried out at a temperature of 930 ℃ or more.
The method of claim 1,
The completion temperature of the secondary rolling is
High strength and low temperature impact toughness excellent plate production method characterized in that 800 ~ 850 ℃.
The method of claim 1,
The cooling step
Primary cooling the second rolled sheet; And
And secondly cooling the first cooled plate at a slower speed than the cooling rate of the first cooling. 2.
The method of claim 6,
The primary cooling is carried out by water cooling,
The secondary cooling is a high strength and low temperature impact toughness excellent plate manufacturing method characterized in that carried out by air cooling.
The method of claim 6,
The primary cooling is
A thick plate manufacturing method excellent in high strength and low temperature impact toughness, which is carried out at a temperature of 500 to 650 ° C.
The method of claim 6,
The primary cooling is a high strength and low temperature impact toughness manufacturing method characterized in that the cooling is carried out at a cooling rate of 3 ~ 7 ℃ / sec.
The method of claim 6,
The secondary cooling is a high strength and low temperature impact toughness manufacturing method, characterized in that the end temperature is carried out to a temperature of 10 ~ 30 ℃.
C: 0.05 to 0.1 wt%, Si: 0.2 to 0.4 wt%, Mn: 1.2 to 1.6 wt%, P: 0.015 wt% or less, S: 0.005 wt% or less, Al: 0.02 to 0.05 wt%, Ti: 0.005 to 0.015% by weight, Nb: 0.01-0.04% by weight, Ni: 0.2-0.4% by weight, N: 60 ppm or less, and the remaining Fe and other unavoidable impurities.
High strength and low temperature impact toughness plate, characterized in that the average impact toughness of 200 ~ 350 J at a tensile strength of 500 MPa or more, -60 ℃.
The method of claim 11,
The microstructure of the thick plate has a high strength and low temperature impact toughness, characterized in that it comprises ferrite and bainite.
The method of claim 11,
The thick plate excellent strength and low temperature impact toughness, characterized in that the tensile strength (TS) of 500 ~ 600 MPa.
The method of claim 11,
Yield strength (YS) of the thick plate is 440 ~ 550 MPa, elongation (EL) is 25 ~ 36%, characterized in that the high strength and low temperature impact toughness.

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