KR20210126995A - Steel sheet having excellent low temperature toughness and low yield ratio and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a steel sheet having excellent low-temperature toughness as well as a low yield ratio and a method for manufacturing the same, and more particularly, to a steel sheet having excellent low-temperature DWTT characteristics and a low yield ratio at room temperature and a method for manufacturing the same. The steel sheet according to the present invention comprises: 0.02 to 0.08 wt% of C; 0.1 to 0.5 wt% of Si; 0.8 to 1.8 wt% of Mn; 0.03 wt% or less of P; 0.003 wt% or less of S; 0.06 wt% or less of Al; 0.01 wt% or less of N; 0.005 to 0.08 wt% of Nb; 0.005 to 0.05 wt% of Ti; at least one among 0.05 to 0.3 wt% of Cr, 0.05 to 0.3 wt% of Ni, 0.02 to 0.2 wt% of Mo, and 0.005 to 0.1 wt% of V; and the balance Fe and unavoidable impurities. The microstructure of the steel sheet contains escicular ferrite as a matrix structure, and 30 to 100 bainite bands by Mn segregation in the central 15 mm region of the thickness of the steel sheet.

Description

Low yield ratio steel sheet with excellent low-temperature toughness and its manufacturing method

The present invention relates to a steel sheet having excellent low-temperature toughness and low yield ratio and a manufacturing method thereof, and more particularly, to a steel sheet having excellent low-temperature DWTT characteristics and a resistive yield ratio at room temperature at the same time and a manufacturing method thereof.

Recently, as oil fields have been developed mainly in cold areas, projects are actively underway to transport resources from oil fields to remote consumption areas through line pipes. DWTT (Drop Weight Tear), considering the durability of the steel used in these projects, not only the pressure of the transport gas but also the deformation of the pipe due to cryogenic temperatures and frost heave Test) characteristics and resistance recovery ratio are required. In order for line pipe steel to be used safely at low temperatures, DWTT characteristics showing brittle fracture stopping characteristics must be guaranteed. The steel sheet supplied to these pipes must have a DWTT ductile fracture factor of 85% or more at the operating temperature.

In general, in order to secure the DWTT characteristics, the crystal grains of the steel are refined along with the component control and controlled to have a matrix structure of ecyclic ferrite with excellent low-temperature toughness. Conduct. However, with the microstructure formed through conventional accelerated cooling, it is not easy to secure the DWTT characteristics at a cryogenic temperature of -60° C., and it is difficult to implement a resistive yield ratio.

In general, the resistance yield ratio is closely related to the uniform elongation, and various techniques for steel sheets having excellent elongation and low-temperature fracture toughness have been proposed.

For example, Patent Document 1 discloses a method for manufacturing a steel material having excellent low-temperature fracture toughness and uniform elongation in which the microstructure has a mixed structure of equiaxed ferrite of 30 to 60% and bainite of 40 to 70% as the area fraction as the main phase. In this regard, a steel material containing Ni, Nb and Mo is rolled to a non-recrystallization area rolling reduction of 65% or more, cooled to a Bs temperature at a cooling rate of 15 to 30 ° C / s, and then cooled to a temperature of 30 to 60 ° C / s It is characterized in the process of cooling to 350 ~ 500 ℃ furnace.

Patent Document 2 discloses that the microstructure includes 50 to 65% of ferrite, 30 to 40% of bainite, and 5 to 10% of island martensite by area fraction, and the average effective grain size of the ferrite is 10 μm or less, the It relates to a method for manufacturing a steel material having excellent uniform elongation and low-temperature fracture toughness in which the average effective grain size of bainite is 20 μm or less. After cooling at +50~Ms+110℃, it is characterized in that the steel sheet is air-cooled or allowed to cool to room temperature for self-tempering.

In addition, Patent Document 3 discloses that the microstructure contains 65 to 80% of air-cooled ferrite and 20 to 35% of pearlite by area fraction, and the average effective grain size of the air-cooled ferrite and pearlite is 20 μm or less. It relates to a method, characterized in that it is hot rolled at a cumulative reduction ratio of 73 to 80% at Tnr-190 to Tnr-160° C. and air-cooled to room temperature.

The above patent documents are characterized in that low-temperature rolling is performed for grain refining, but excessive grain refining due to low-temperature rolling causes an increase in yield strength and thus has a disadvantage in that the yield ratio increases as well as DWTT improvement by grain refining has limitations.

Korean Patent Publication No. 10-2013-0073472 Korean Patent Publication No. 10-2014-0083540 Korean Patent Publication No. 10-2014-0084891

According to one aspect of the present invention, it is an object to provide a steel sheet having excellent low-temperature toughness and a low yield ratio.

The subject of the present invention is not limited to the above. A person of ordinary skill in the art will have no difficulty in understanding the further problems of the present invention from the overall content of the present specification.

One aspect of the present invention, by weight, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 0.8 to 1.8% by weight, P: 0.03% or less, S: 0.003% or less, Al: 0.06% or less , N: 0.01% or less, Nb: 0.005~0.08%, Ti: 0.005~0.05%, Cr: 0.05~0.3%, Ni: 0.05~0.3%, Mo: 0.02~0.2%, V: 0.005%~ 0.1% of at least one of them, the balance Fe and unavoidable impurities;

The microstructure contains escicular ferrite as a matrix,

It is possible to provide a low yield ratio steel sheet excellent in low-temperature toughness including 30 to 100 bainite bands by Mn segregation in the thickness direction in a region of 15 mm in the thickness center of the steel sheet.

An average interval of the bainite bands may be 50 μm or more.

The Mn content of the bainite band may be 1.5 times or more compared to the Mn content of the matrix structure.

The hardness of the matrix may be 220 Hv or less, and the hardness of the bainite band may be 250 Hv or more.

The steel sheet may have a DWTT ductile fracture ratio of 85% or more at -60°C, a yield ratio of 88% or less at room temperature, and a yield strength of 450 MPa or more.

Another aspect of the present invention, by weight, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 0.8 to 1.8% by weight, P: 0.03% or less, S: 0.003% or less, Al: 0.06% or less, N: 0.01% or less, Nb: 0.005 to 0.08%, Ti: 0.005 to 0.05%, Cr: 0.05 to 0.3%, Ni: 0.05 to 0.3%, Mo: 0.02 to 0.2%, V: 0.005% During continuous casting with molten steel containing at least one of ~0.1%, the balance Fe and unavoidable impurities, manufacturing a steel slab that flows the molten steel in the center of the thickness of the steel slab;

reheating the manufactured steel slab;

rough rolling the reheated slab;

hot-rolling the rough-rolled slab to a hot-rolling finishing temperature of Ar3~Ar3+200°C; and

Resistance excellent in low-temperature toughness comprising cooling the hot-rolled steel sheet at a temperature range of Ar3-50 to Ar3+100° C. and cooling it at a cooling rate of 5-100° C./s to a temperature range of 300 to 600° C. It is possible to provide a method for manufacturing a double-sided steel sheet.

The step of manufacturing the steel slab may be to flow the molten steel using an electromagnetic force at 30 to 70% of the completion of solidification during continuous casting.

The reheating may be reheating at a temperature range of 1100 to 1300° C. for 2 hours or more.

The hot rolling may be performed at a cumulative reduction ratio of 50% or more.

According to one aspect of the present invention, without excessive grain refinement, it is possible to provide a steel sheet having excellent DWTT ductile fracture factor characteristics at a low temperature of -60°C and having a resistive yield ratio at room temperature at the same time.

1 is a view showing the thickness direction segregation of the central portion of the invention steel according to an embodiment of the present invention.
Figure 2 shows the microstructure and hardness of the invention steel according to an embodiment of the present invention.
3 shows the matrix structure and the components of the segregation part of the central region of the invention steel according to an embodiment of the present invention.
Figure 4 shows the -60 ℃ DWTT experimental fracture surface of the invention steel and the comparative steel according to an embodiment of the present invention.

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. The present embodiments are provided to explain the present invention in more detail to those skilled in the art to which the present invention pertains.

Conventionally, as the low-temperature DWTT guarantee temperature is lowered, the technique of refining the grains by lowering the rolling temperature has been used, but the refinement of grains is the main cause of increasing the yield strength by increasing the yield strength. has been an obstacle to acquisition.

Accordingly, the researchers of the present invention have come up with a technique for reversely using central segregation, which normally adversely affects the quality of steel sheet, while repeating research and experiments, and through this, lower the yield ratio and low temperature without excessive grain refinement by low temperature rolling By developing a technology capable of improving the DWTT characteristics, the present invention has been completed.

The present invention improves low-temperature DWTT characteristics by inducing intermittent fracture by uniformly dispersing fine segregation in the thickness center region of a steel sheet, and at the same time has a resistance yield ratio due to a difference in hardness due to the difference in microstructure between the segregation part and the matrix tissue. characterized.

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, the content of each element is based on weight %.

The steel sheet according to an aspect of the present invention is, by weight, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 0.8 to 1.8% by weight, P: 0.03% or less, S: 0.003% or less, Al: 0.06 % or less, N: 0.01% or less, Nb: 0.005 to 0.08%, Ti: 0.005 to 0.05%, Cr: 0.05 to 0.3%, Ni: 0.05 to 0.3%, Mo: 0.02 to 0.2%, V: 0.005 %~0.1% of at least one of, the balance may include Fe and unavoidable impurities.

Carbon (C): 0.02~0.08%

Carbon (C) is an element closely related to the manufacturing method along with other components, and has the greatest influence on the properties of steel among steel components. If the content of carbon (C) is less than 0.02%, the component control cost during the steelmaking process is excessively incurred, the weld heat-affected zone is softened more than necessary, and it is difficult to secure strength. Low-temperature toughness is reduced and there is a problem of lowering weldability.

Accordingly, the content of carbon (C) may be 0.02 to 0.08%. More preferably, it may be 0.03 to 0.07%.

Silicon (Si): 0.1-0.5%

Silicon (Si) not only acts as a deoxidizer in the steelmaking process, but also serves to increase the strength of steel. When the content of silicon (Si) exceeds 0.5%, low-temperature toughness of the material deteriorates, weldability is impaired, and scale peelability during rolling is reduced, whereas when the content is less than 0.1%, manufacturing cost increases.

Accordingly, the content of silicon (Si) may be 0.1 to 0.5%. More preferably, it may be 0.15 to 0.35%.

Manganese (Mn): 0.8~1.8%

Manganese (Mn) is an element that improves the hardenability of steel without impairing low-temperature toughness. When the content of manganese (Mn) exceeds 1.8%, central segregation occurs excessively, and there is a problem in that low-temperature toughness is lowered as well as hardenability of steel and weldability are lowered. On the other hand, if the content is less than 0.8%, there is a problem in that the cost of alloying elements for securing strength increases.

Accordingly, the content of manganese (Mn) may be 0.8 to 1.8%. In particular, in terms of central segregation, more preferably 0.8 to 1.6%, more preferably 1.0 to 1.5%.

Phosphorus (P): 0.03% or less

Phosphorus (P) is an impurity element, and when its content exceeds 0.03%, weldability is remarkably deteriorated, and there is a problem in that low-temperature toughness is reduced.

Accordingly, the content of phosphorus (P) may be 0.03% or less. In particular, in terms of low-temperature toughness, it may be more preferably 0.012% or less.

Sulfur (S): 0.003% or less

Sulfur (S) is an impurity element, and when its content exceeds 0.003%, there is a problem of reducing ductility, low-temperature toughness and weldability of steel.

Accordingly, the content of sulfur (S) may be 0.003% or less. In particular, since S combines with Mn to form MnS inclusions to reduce the low-temperature toughness of steel, it may be more preferably 0.002% or less.

Aluminum (Al): 0.06% or less

Aluminum (Al) is an element that acts as a deoxidizer to remove oxygen by reacting with oxygen present in molten steel, and aluminum (Al) is generally added enough to have sufficient deoxidation power in steel. However, when the content exceeds 0.06%, a large amount of oxide-based inclusions are formed, which causes a problem that inhibits the low-temperature toughness of the material.

Accordingly, the content of aluminum (Al) may be 0.06% or less. More preferably, it may be 0.04% or less.

Nitrogen (N): 0.01% or less

Since nitrogen (N) is industrially difficult to completely remove from steel, 0.01%, which is an allowable range in the manufacturing process, may be limited as an upper limit. Nitrogen (N) forms a nitride with Al, Ti, Nb, V, etc. to prevent austenite grain growth and helps to improve toughness and strength, but when the content exceeds 0.01%, nitrogen (N) in a solid solution It exists, and nitrogen (N) in a solid solution adversely affects low-temperature toughness.

Accordingly, the content of nitrogen (N) may be 0.01% or less. More preferably, it may be 0.007% or less.

Niobium (Nb): 0.005 to 0.08%

Niobium (Nb) is dissolved during reheating of the slab, inhibits the growth of austenite grains during hot rolling, and then precipitates to improve the strength of the steel. In addition, it serves to increase the strength of the steel while minimizing the increase in the yield ratio by bonding with carbon and precipitation. On the other hand, if the niobium (Nb) is less than 0.005%, there is no effect of increasing the strength due to the addition, and hardenability is reduced, so that it is difficult to secure an escicular ferrite structure. On the other hand, if the content exceeds 0.08%, not only the austenite grains are refined more than necessary, but also the low-temperature toughness due to coarse precipitates is reduced.

Accordingly, the content of niobium (Nb) may be 0.005 to 0.08%. More preferably, it may be 0.02 to 0.06%.

Titanium (Ti): 0.005~0.05%

Titanium (Ti) is an effective element to inhibit the growth of austenite grains in the form of TiN by combining with N during reheating of the slab. When the content of titanium (Ti) is less than 0.005%, the austenite grains are coarsened to reduce the low-temperature toughness, whereas when the content exceeds 0.05%, coarse Ti-based precipitates are formed, thereby reducing the low-temperature toughness. .

Accordingly, the content of titanium (Ti) may be 0.005 to 0.05%. In particular, in terms of low-temperature toughness, it may be more preferably 0.005 to 0.03%.

One or more of Cr, Ni, Mo and V may be included to secure strength and low-temperature toughness. Since the cooling rate decreases as the thickness of the steel sheet increases, the type and amount included may be increased according to the increase in the thickness.

Chromium (Cr): 0.05~0.3%

Chromium (Cr) is an element that is dissolved in austenite during reheating of the slab to increase the hardenability of the steel and contribute to securing the strength of the steel sheet. If the content of chromium (Cr) is less than 0.05%, the above-described effects cannot be secured, whereas if the content exceeds 0.3%, there is a problem in that low-temperature toughness and weldability are deteriorated.

Accordingly, the content of chromium (Cr) may be 0.05 to 0.3%. More preferably, it may be 0.1 to 0.25%.

Nickel (Ni): 0.05-0.3%

Nickel (Ni) is an element that improves the toughness of steel and is added to increase the strength of steel without deterioration of low-temperature toughness. If the content of nickel (Ni) is less than 0.05%, there is no effect of increasing the strength due to the addition, whereas if it exceeds 0.3%, the cost due to the addition increases.

Accordingly, the content of nickel (Ni) may be 0.05 to 0.3%. More preferably, it may be 0.1 to 0.25%.

Molybdenum (Mo): 0.02~0.2%

Molybdenum (Mo) is an element having a similar or more active effect to Cr, and serves to increase the hardenability of steel and increase the strength of steel. If the content of molybdenum (Mo) is less than 0.02%, it is difficult to secure the hardenability of steel, whereas if the content exceeds 0.2%, an upper bainite structure is formed, resulting in a problem in that low-temperature toughness is weakened.

Accordingly, the content of molybdenum (Mo) may be 0.02 to 0.2%. More preferably, it may be 0.05-0.12%.

Vanadium (V): 0.005-0.1%

Vanadium (V) is an element that increases the strength by increasing the hardenability of steel. If the content of vanadium (V) is less than 0.005%, it has no effect on increasing hardenability, whereas if the content exceeds 0.1%, low-temperature toughness decreases due to excessive precipitation and the cost of alloying elements increases.

Accordingly, the content of vanadium (V) may be 0.005 to 0.1%. More preferably, it may be 0.005 to 0.05%.

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

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

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

The steel sheet of the present invention that satisfies the above alloy composition has a matrix structure of escicular ferrite as a microstructure, and 30 or more to less than 100 bands of bainite by Mn segregation in the thickness direction in the thickness center 15 mm region of the steel sheet, The average interval between the bainites may be 50 μm or more, the Mn content of the bainite part may be 1.5 times or more compared to the matrix structure, the hardness of the escicular ferrite may be 220 Hv or less, and the hardness of the bainite part may be 250 Hv or more.

It can have ecyclic ferrite as a matrix structure.

The steel of the present invention is a steel having excellent low-temperature DWTT characteristics, and is preferably manufactured with a microstructure of escicular ferrite excellent in low-temperature toughness. However, since it is difficult to simultaneously satisfy the cryogenic DWTT characteristics of -60°C and the resistive yield ratio with only ecyclic ferrite, it is necessary to disperse a high-hardness secondary phase inside the matrix to satisfy them at the same time.

Bainite bands are formed in escicular ferrite by Mn segregation, and there are 30 to 100 bainite bands in the thickness direction in a region of 15 mm thick at the center of the steel sheet, and the average interval between bainites may be 50 μm or more.

Due to Mn segregation dispersed during the steel slab manufacturing process in the center of the steel sheet thickness, it is characterized in that bainite by Mn segregation is dispersed in a region of 15 mm thick in the center after hot rolling and cooling.

If the number of bands of bainite in the thickness direction of the steel sheet is less than 30, it is not easy to secure intermittent fracture and resistive yield ratio.

The average interval between bainites can be limited to 50㎛ or more to induce intermittent fracture perpendicular to the crack propagation surface during low-temperature fracture. If the gap is less than 50 μm, there is a problem in that the dispersion effect cannot be ensured and cracks occur due to one segregation. The upper limit of the average interval between bainite is not particularly limited, but considering the number of bainite bands in the central region of the thickness, it may be preferably 500 μm or less.

The Mn content of the bainite part may be 1.5 times or more compared to the matrix structure, the hardness of the escicular ferrite may be 220 Hv or less, and the hardness of the bainite part may be 250 Hv or more.

When the hardness of the bainite band is high compared to the matrix structure, the resistance yield ratio can be secured due to the difference in interface hardness with the matrix structure and intermittent fracture can be induced. Mn content of 1.5 times or more must be secured, and the hardness of the matrix structure is limited to 220 Hv or less, and the hardness of the bainite part to 250 Hv or more. The Mn content of the bainite portion is preferably higher than the matrix structure, and since the Mn content of the alloy composition is limited, the upper limit of the Mn content of the bainite portion relative to the matrix structure is not particularly limited, but may be preferably 15 times or less. In addition, although the lower limit of the hardness of the matrix structure and the upper limit of the hardness of the bainite portion are not particularly limited, the hardness of the matrix structure may be preferably 150 Hv or more, and the hardness of the bainite portion may be 350 Hv or less.

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

The steel sheet according to one aspect of the present invention may be manufactured by manufacturing a steel slab satisfying the above-described alloy composition, reheating, hot rolling, and cooling.

Steel Slab Manufacturing

During continuous casting in order to manufacture a steel slab that satisfies the alloy composition described above, the molten steel in the center of the thickness of the steel slab may be flowed by using electromagnetic waves at 30 to 70% of the time of completion of solidification.

The steel slab manufactured by the method for manufacturing a steel slab of the present invention has a difference in segregation compared to the steel slab manufactured by the existing method. In the steel slab manufactured by the conventional method, because of the characteristic that the thickness center solidifies the last, elements such as Mn are concentrated in the thickness center, and a high concentration of Mn segregation occurs in the center of the thickness of the slab.

On the other hand, when a steel slab is manufactured by the method for manufacturing a steel slab of the present invention, fine segregation is dispersed in the center by flowing molten steel in the thickness center before solidification. If the time point at which the molten steel flows exceeds 70% of the completion of solidification, the flow of the molten steel is difficult, whereas when the time point is less than 30%, Mn is dispersed throughout the steel slab, and it is difficult to form fine Mn segregation.

reheat

The manufactured steel slab can be heated in a temperature range of 1100 to 1300° C. for 2 hours or more.

The reheating temperature is a process of heating a steel slab to a high temperature for hot rolling. When the reheating temperature exceeds 1300℃, scale defects not only increase, but also the austenite grains are coarsened to increase the hardenability of the steel, and the matrix structure It becomes upper bainite and the low-temperature toughness is lowered. On the other hand, if the temperature is less than 1100 ℃ or the reheating time is less than 2 hours, there is a problem in that the alloy elements are not sufficiently dissolved.

hot rolled

The reheated steel slab can be hot rolled at a hot-rolling finishing temperature of Ar3~Ar3+200℃, and a cumulative reduction ratio of 50% or more.

When the hot-rolling finishing temperature exceeds Ar3+200℃, the upper bainite structure is formed due to the increase in hardenability due to grain growth to reduce the low-temperature toughness, whereas when the temperature is less than Ar3℃, the cooling start temperature is excessively lowered. There is a problem that the matrix structure is a composite structure of air-cooled ferrite and upper bainite.

If the cumulative reduction ratio during hot rolling is less than 50%, recrystallization by rolling to the center does not occur, so there is a problem in that the grains in the center become coarse and the low-temperature toughness deteriorates.

Cooling

It is possible to start cooling the hot-rolled steel sheet at a temperature range of Ar3-50 to Ar3+100° C. and cool it to a temperature range of 300 to 600° C. at a cooling rate of 5 to 100° C./s.

When the cooling start temperature exceeds Ar3+100°C, it is difficult to secure the cooling end temperature, whereas when the temperature is less than Ar3-50°C, the matrix structure becomes a composite structure of air-cooled ferrite and upper bainite.

If the cooling termination temperature is less than 300 ℃, the MA fraction of the central portion increases, which adversely affects low-temperature toughness and hydrogen embrittlement.

In the present invention, the cooling rate is the cooling rate of the t/4 point (t: thickness of the steel sheet) of the steel sheet. If the cooling rate is less than 5 °C/s, the crystal grains in the center are coarsened, making it difficult to secure the strength and low-temperature toughness of the steel, whereas if the rate exceeds 100 °C/s, the matrix structure becomes upper bainite.

The steel sheet of the present invention prepared as described above has a DWTT ductile fracture factor of 85% or more at -60°C, a yield ratio of 88% or less at room temperature, and a yield strength of 450 MPa or more. characteristics may be provided.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention.

(Example)

A steel slab having the composition shown in Table 1 below was cast, reheated, hot rolled, and cooled under the conditions shown in Table 2 to prepare a steel sheet. It is manufactured by the same process except that it follows the composition of Table 1 and the manufacturing process conditions of Table 2, and specifically, hot rolling is performed under the conditions of Table 2, but after rough rolling is performed under normal conditions, After rolling under the conditions shown in Table 2, cooling was performed. The molten steel flow time in Table 2 below was applied based on 100% of the completion of solidification.

steel grade Alloy composition (wt%) thickness
(mm) Ar3
(℃) C Si Mn P S Al N Ni Cr Mo Nb Ti V A 0.065 0.25 1.32 0.01 0.0007 0.024 0.003 0.21 0.21 0 0.043 0.012 0.02 28.50 777 B 0.055 0.24 1.31 0.008 0.0012 0.023 0.004 0.18 0 0.14 0.041 0.013 0 30.50 775 C 0.057 0.25 1.44 0.008 0.0008 0.031 0.005 0.21 0.12 0.06 0.05 0.011 0.02 20.60 763 D 0.061 0.24 1.32 0.011 0.0009 0.029 0.006 0 0 0 0.035 0.012 0 30.50 793 E 0.07 0.22 1.22 0.006 0.001 0.038 0.004 0.16 0.19 0.08 0 0.013 0 30.50 781

Ar3 = 910-310*[C]-80*[Mn]-20*[Cu]-15*[Cr]- 55*[Ni]-80*[Mo]+0.35*(thickness-8)

(Here, [C], [Mn], [Cu], [Cr], [Ni] and [Mo] are weight % of each element, and the unit of thickness is mm.)

Psalter
number steel grade casting reheat hot rolled Cooling Molten Steel Flow Point
(%) heating
Temperature
(℃) heating
hour
(hr) finish
Temperature (℃) accumulate
reduction ratio (%) Initiate
Temperature
(℃) End
Temperature
(℃) speed
(℃/s) One A 46 1166 4.3 893 80 790 493 46 2 B 42 1158 4 918 77 796 444 51 3 C not done 1149 4.3 850 75 803 468 55 4 D 43 1145 4.2 875 75 789 498 48 5 E 65 1156 3.9 895 77 795 503 38 6 B 19 1143 4.3 873 75 797 434 45 7 B 81 1150 4.2 879 75 801 453 49 8 B 47 1171 4.5 998 75 802 465 47 9 B 37 1119 3.7 706 80 653 421 28 10 B 49 1148 4.3 879 75 893 633 43 11 B 51 1165 3.1 870 75 791 597 4 12 B 64 1147 4.4 865 75 789 412 121

Table 3 shows the results of the microstructure and mechanical properties of Examples. The microstructure and the number of bainite bands were observed using an electron microscope, and the Mn content of each tissue was measured through EDS analysis. Here, the number of bainite bands is a value measured in a total area of 15 mm in the thickness direction centered on the 1/2t point (t: thickness of the steel sheet) of the steel sheet. Then, the hardness of the tissue was measured using a Vickers hardness tester with a load of 0.1 kg, and the DWTT ductile fracture factor was evaluated at -60°C.

Psalter
number steel grade microstructure mechanical properties division base organization center 15mm
Number of bainite strips (pcs) matrix hardness
(Hv(0.1)) bainite band hardness
(Hv(0.1)) Mn content ratio
(Bainite band/base organization)
(ship) surrender
burglar
(MPa) yield ratio
(%) DWTT
ductile fracture factor
(-60℃) One A AF 63 198 269 2.0 523 85 99 Invention lecture 1 2 B AF 55 203 271 1.9 475 83 100 Invention lecture 2 3 C AF One 199 296 2.3 506 89 8 Comparative lecture 1 4 D F 16 183 243 1.8 429 86 89 Comparative lecture 2 5 E AF+F 39 189 256 1.5 444 85 91 Comparative lecture 3 6 B AF 21 195 257 1.9 486 91 59 Comparative lecture 4 7 B AF 17 196 279 2.1 493 90 29 Comparative steel 5 8 B UB 56 255 275 2.0 496 90 73 Comparative lecture 6 9 B F+UB 42 191 259 1.8 412 83 88 Comparative lecture 7 10 B AF+F 13 190 248 1.6 438 86 91 Comparative steel 8 11 B F 9 183 244 1.6 399 86 94 Comparative lecture 9 12 B UB 51 264 295 1.8 527 90 69 Comparative Steel 10

AF: escicular ferrite, F: ferrite, UB: upper bainite

Inventive steels 1 and 2 in Table 3 satisfy the components, casting conditions, reheating, rolling and cooling conditions proposed in the present invention, and form escular ferrite advantageous in low-temperature toughness as a matrix structure, and molten steel flow during casting As a result, the number of bainite bands in the central region with a thickness of 15 mm satisfies 30 to 100 proposed in the present invention, and at the same time, the Mn concentration in the segregation portion compared to the matrix structure is 1.5 times or more, and the hardness of the matrix structure, bainite The hardness of the band also satisfies the range suggested by the present invention. In addition, the inventive steels in Table 3 satisfy the above characteristics and at the same time satisfy the yield strength of 450 MPa or more, the yield ratio of 88% or less, and the DWTT ductile fracture ratio of 85% or more at -60°C.

Comparative Steel 1 is a steel to which the molten steel flow proposed by the present invention is not applied as shown in Table 2, and as shown in Table 3, segregation could not be dispersed in the 15 mm region of the center of the steel plate, so only one segregation band was generated, resulting in yield ratio and DWTT The characteristics did not satisfy the range suggested by the present invention.

Comparative steels 2 and 3 are outside the range of components proposed in the present invention as shown in Table 1, and Comparative Steel 2 does not have sufficient content of alloying elements, and at the same time does not satisfy the hardness of 250Hv or more in the segregation band, Even in the number of , it did not satisfy the range suggested by the present invention. In addition, as shown in Table 3, the yield strength did not satisfy the range suggested by the present invention. Comparative Steel 3 is a case that does not satisfy the lower limit of the Nb content suggested in the present invention as shown in Table 1, and is a case in which ferrite is also formed together with escicular ferrite as a matrix structure. It did not meet the suggested range.

Comparative Steel 4 is a case that does not satisfy the molten steel flow time range suggested in the present invention. In Comparative Steel 4, segregation is dispersed throughout the steel sheet, so that the number of bainite bands in the center did not satisfy the range suggested by the present invention. Due to this, the yield ratio and DWTT characteristics did not meet the ranges suggested by the present invention.

Comparative steel 5 is a case outside the range of the flow point of molten steel proposed in the present invention, and in Comparative Steel 5, segregation is not sufficiently dispersed, so the number of bainite bands does not satisfy the range suggested in the present invention, yield ratio and DWTT characteristics This did not satisfy the range suggested by the present invention.

In Comparative Steel 6, the components and slab manufacturing process satisfy the ranges suggested by the present invention, but as shown in Table 2, the hot rolling finishing temperature exceeds the range suggested by the present invention, and after cooling due to insufficient refinement of austenite Upper bainite was formed as a matrix structure. For this reason, the hardness of the matrix structure exceeded the range suggested by the present invention, the yield ratio exceeded the range suggested in the present invention due to the lack of a difference in hardness with the bainite band, and the DWTT characteristic was also out of the range.

Comparative Steel 7 is a case that does not satisfy the hot-rolling finishing temperature suggested by the present invention as shown in Table 2, and the cooling start temperature also does not satisfy the range suggested by the present invention because the hot-rolling finishing temperature is low. As a result, air-cooled ferrite and upper bainite were formed. The formation of upper bainite is due to the concentration of carbon into austenite due to the formation of air-cooled ferrite, and the high hardenability of austenite upon water cooling. Therefore, as shown in Table 3, the yield strength range suggested by the present invention was not satisfied due to the formation of air-cooled ferrite.

In Comparative Steel 8, as shown in Table 2, the cooling start temperature was outside the range suggested by the present invention. In Comparative Steel 8, due to the high cooling termination temperature as shown in Table 2, the escicular ferrite phase transformation was not sufficient as shown in Table 3 and ferrite was also formed. The number and hardness were out of the range suggested by the present invention, and the yield strength was also out of the range.

Comparative steels 9 and 10 are cases in which the cooling rate is outside the range suggested by the present invention as shown in Table 2, and Comparative Steel 9, which does not meet the scope of the present invention as shown in Table 3, is ferrite, exceeding the scope of the present invention. In Comparative Steel 10, an upper bainite structure was formed, so that the properties of the bainite band and the hardness of the matrix structure were out of the range suggested by the present invention, and the yield strength, yield ratio and DWTT properties were also out of the range.

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 (9)

In wt%, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 0.8 to 1.8 wt%, P: 0.03% or less, S: 0.003% or less, Al: 0.06% or less, N: 0.01% or less, Nb: 0.005~0.08%, Ti: 0.005~0.05%, Cr: 0.05~0.3%, Ni: 0.05~0.3%, Mo: 0.02~0.2%, V: 0.005%~0.1% , the balance Fe and unavoidable impurities,
The microstructure contains escicular ferrite as a matrix,
A low yield ratio steel sheet with excellent low-temperature toughness that contains 30 to 100 bainite bands by Mn segregation in the thickness direction in the region of 15 mm at the center of the thickness of the steel sheet.
According to claim 1,
A low yield ratio steel sheet having an average interval of 50 μm or more of the bainite bands and excellent low-temperature toughness.
According to claim 1,
A steel sheet having excellent low-temperature toughness in which the Mn content of the bainite band is 1.5 times or more compared to the Mn content of the matrix structure.
According to claim 1,
The hardness of the matrix structure is 220Hv or less, and the hardness of the bainite band is 250Hv or more.
According to claim 1,
The steel sheet has a DWTT ductile fracture ratio of 85% or more at -60°C, a yield ratio of 88% or less at room temperature, and a low-temperature toughness with a yield strength of 450 MPa or more.
In wt%, C: 0.02 to 0.08%, Si: 0.1 to 0.5%, Mn: 0.8 to 1.8 wt%, P: 0.03% or less, S: 0.003% or less, Al: 0.06% or less, N: 0.01% or less, Nb: 0.005~0.08%, Ti: 0.005~0.05%, Cr: 0.05~0.3%, Ni: 0.05~0.3%, Mo: 0.02~0.2%, V: 0.005%~0.1% , during continuous casting with molten steel containing the remainder Fe and unavoidable impurities, manufacturing a steel slab for flowing the molten steel in the center of the thickness of the steel slab;
reheating the manufactured steel slab;
rough rolling the reheated slab;
hot-rolling the rough-rolled slab to a hot-rolling finishing temperature of Ar3 to Ar3+200°C; and
Resistance excellent in low-temperature toughness comprising cooling the hot-rolled steel sheet at a temperature range of Ar3-50 to Ar3+100°C and cooling at a cooling rate of 5-100°C/s to a temperature range of 300-600°C A method for manufacturing a biceps steel sheet.
7. The method of claim 6,
The step of manufacturing the steel slab is a method of manufacturing a low yield ratio steel sheet excellent in low-temperature toughness in which the molten steel is flowed using electromagnetic waves at 30 to 70% of the completion of solidification during continuous casting.
7. The method of claim 6,
The reheating step is a method of manufacturing a low yield ratio steel sheet excellent in low-temperature toughness to reheat for 2 hours or more in a temperature range of 1100 ~ 1300 ℃.
7. The method of claim 6,
The step of hot rolling is a method of manufacturing a low yield ratio steel sheet excellent in low temperature toughness is performed at a cumulative reduction ratio of 50% or more.

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