KR102220739B1 - Manufacturing mehtod for ultra thick steel plate having excellent toughness at the center of thickness - Google Patents

Abstract

The ultra-thick steel sheet having excellent toughness at the center of the thickness according to an aspect of the present invention is, by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.3 to 1.8% , Aluminum (Al): 0.01~0.06%, Niobium (Nb): 0.005~0.02%, Copper (Cu): 0.1~0.6%, Nickel (Ni): 0.2~1.2%, Titanium (Ti): 0.005~0.02% , Phosphorus (P): 0.012% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.002 to 0.006%, the remaining Fe and other inevitable impurities, and the steel sheet thickness (t) is 80 mm or more, t The average size of the effective grains measured at the point /2 may be 3㎛ or less.

Description

Manufacturing method for ultra thick steel plate having excellent toughness at the center of thickness}

The present invention relates to an ultra-thick steel sheet and a method for manufacturing the same, and more particularly, to an ultra-thick steel sheet and a method for manufacturing the same, which is particularly suitable for the production of ships, offshore structures, building structures, pressure vessels, etc. due to its excellent thickness central toughness.

Recently, as the size of structures used in the fields of ships, offshore, construction, and civil works is accelerating, the demand for thickening of steel plates applied to such large structures is increasing. In addition, stability is particularly important in such large structures. To support this, not only the properties at the 1/4 point in the thickness direction, which is the average property of the steel plate, but also the 1/2 point, that is, the thickness center property guarantee is required. Is increasing.

However, in general, as the thickness of the steel sheet increases, it becomes very difficult to secure physical properties at the center of the thickness. This is due to the limitations of controlled rolling and accelerated cooling, which are almost indispensably applied in the manufacture of high-performance steel sheets today. Even if controlled rolling and accelerated cooling are performed, the roll load due to the control rolling cannot be sufficiently transmitted to the center of the thickness for extremely thick steel sheets with a thickness of 80 mm or more, and the difference in cooling speed between the surface layer and the center of the thickness is large due to the limit of the heat transfer coefficient of the steel sheet This is because sufficient cooling speed cannot be secured in the central portion of the thickness, and thus the effects of controlled rolling and accelerated cooling cannot be sufficiently secured.

To overcome these limitations, it is one of the methods for securing strength by obtaining sufficient cooling effect even at a low cooling rate. This is an expensive method of so-called hardenability enhancing elements such as copper (Cu), nickel (Ni), and chromium (Cr). A method of introducing a large amount of elements has been previously proposed. However, these elements can impair the weldability of the steel sheet and are very disadvantageous in terms of economy because they are expensive elements.

Recently, in order to solve such a problem, a method of adding boron (B) in place of expensive elements such as copper (Cu), nickel (Ni), and chromium (Cr) has been proposed. Since boron (B) is an element that can greatly improve the hardenability of steel with only a small amount of addition compared to other elements, it is an element that is easy to transform the structure of a steel material into a hard structure even at a low cooling rate. However, even if boron (B) is added to the thick steel plate to form a hard structure in the center of the thickness, there is still a limit to the miniaturization of the structure of the thickness center of the thick steel plate. In other words, even with the addition of boron (B), problems such as roll load transmission due to control rolling and securing a cooling rate in the center of the thickness cannot be solved, so a coarse microstructure is formed in the center of the thickness, and the impact toughness of the steel sheet. This can be greatly degraded.

Patent Document 1 proposes a technique for securing the toughness of the base material by controlling the titanium nitride (TiN) and martensite-austenite structure (M-A structure) in an appropriate form. In the case of Patent Document 1, it is possible to secure economical efficiency by not adding a large amount of elements such as copper (Cu), nickel (Ni), and chromium (Cr), but it causes excessive process load, which is not preferable for field application. This is because in order to prevent coarsening of titanium nitride, the composition of oxides in molten steel must be precisely controlled, and the cooling time must be strictly controlled in the cooling process after casting.

Therefore, it is urgent to develop an ultra-thick steel sheet capable of effectively securing the toughness at the center of the thickness without inducing excessive process load without introducing a large amount of expensive hardenable elements and a method of manufacturing the same.

Republic of Korea Patent Publication No. 10-2009-0034284 (published on April 7, 2009)

According to one aspect of the present invention, an ultra-thick steel sheet having excellent toughness at the center of the thickness and a manufacturing method thereof can be provided.

The subject of the present invention is not limited to the above description. Those skilled in the art will have no difficulty in understanding the additional subject of the present invention from the general contents of the present specification.

The ultra-thick steel sheet having excellent toughness at the center of the thickness according to an aspect of the present invention is, by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.3 to 1.8% , Aluminum (Al): 0.01~0.06%, Niobium (Nb): 0.005~0.02%, Copper (Cu): 0.1~0.6%, Nickel (Ni): 0.2~1.2%, Titanium (Ti): 0.005~0.02% , Phosphorus (P): 0.012% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.002 to 0.006%, the remaining Fe and other inevitable impurities, and the steel sheet thickness (t) is 80 mm or more, t The average size of the effective grains measured at the point /2 may be 2.8㎛ or less.

Here, the average size of the effective grains may mean an average circle equivalent diameter of grains defined by a diagonal grain boundary having a grain orientation difference of 15° or more measured by the Electro Back Scattered Pattern (EBSP) method.

The ultra-thick steel sheet may contain 95 area% or more of ferrite in a microstructure.

The microstructure of the ultra-thick steel sheet may further include at least one of pearlite of 4 area% or less and bainite of 1 area% or less.

The tensile strength of the ultra-thick steel sheet may be 520 MPa or more.

The extremely thick steel sheet may have an impact toughness at a point t/2 of -100°C or less based on a transition temperature of T100J.

Here, the T100J transition temperature may mean a temperature at which the absorbed energy value corresponds to 100J in the curve obtained by converting the absorbed energy values obtained after performing the Charpy impact test under a plurality of temperature conditions into a sigmoid function. .

The ultra-thick steel sheet having excellent toughness at the center of the thickness according to an aspect of the present invention is, by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.3 to 1.8% , Aluminum (Al): 0.01~0.06%, Niobium (Nb): 0.005~0.02%, Copper (Cu): 0.1~0.6%, Nickel (Ni): 0.2~1.2%, Titanium (Ti): 0.005~0.02% , Phosphorus (P): 0.012% or less, Sulfur (S): 0.005% or less, Nitrogen (N): 0.002 to 0.006%, the rest of the slab containing Fe and other inevitable impurities in a temperature range of 1150℃ or more based on the thickness of the core At first heating, the first heated slab is intermediately cooled to a temperature range of 500°C or less based on the thickness center temperature of the slab, and the intermediate cooled slab is Ac3 ~ 1000°C based on the thickness center temperature of the slab Secondly heated to a temperature range of, and finish rolling of the slab in a temperature range of Ar3 to (Ar3+150°C) based on the surface temperature of the second heated slab to provide an extremely thick steel sheet, and the finish rolling It can be manufactured by finally cooling the ultra-thick steel sheet to a temperature range of Bf or less at a cooling rate of 2°C/s or more based on the thickness of the ultra-thick steel sheet.

In the primary heating, the slab may be heated for a period of 1 minute or more per thickness of the slab.

The secondary heated slab may be roughly rolled to provide a rough rolled bar, and the rough rolled bar may be finished rolling after waiting for the surface temperature of the rough rolled bar to reach the temperature range of the finish rolling.

The cumulative reduction ratio of the finish rolling may be 40% or more.

The means for solving the above problems are not all of the features of the present invention, and various features of the present invention and advantages and effects thereof will be understood in more detail with reference to the specific embodiments below.

According to one aspect of the present invention, the tensile strength is 520 MPa or more, the center impact is -100° C. or less based on T100J, and an ultra-thick steel plate having particularly suitable physical properties as a material for structures used in ships, offshore, architecture, and civil engineering fields, and The manufacturing method can be provided.

1 is an optical photograph of a microstructure at a point of 1/2 of the thickness of Specimen 1.
2 is an optical photograph of a microstructure observed at a point of 1/2 of the thickness of the specimen 10.

The present invention relates to an ultra-thick steel sheet having excellent thickness center toughness and a method of manufacturing the same, and hereinafter, preferred embodiments of the present invention will be described. The 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 in order to further detail the present invention to those of ordinary skill in the art to which the present invention pertains.

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise indicated,% indicating the content of each element is based on weight.

The ultra-thick steel sheet having excellent toughness at the center of the thickness according to an aspect of the present invention is, by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.3 to 1.8% , Aluminum (Al): 0.01~0.06%, Niobium (Nb): 0.005~0.02%, Copper (Cu): 0.1~0.6%, Nickel (Ni): 0.2~1.2%, Titanium (Ti): 0.005~0.02% , Phosphorus (P): 0.012% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.002 ~ 0.006%, the remaining Fe and other inevitable impurities may be included.

Carbon (C): 0.02~0.06%

Carbon (C) is the most effective element for securing the strength of steel, so it needs to be contained in the steel within an appropriate range. The present invention may limit the lower limit of the carbon (C) content to 0.02% in order to secure the strength of the ultra-thick steel sheet. However, when carbon (C), which is an element for improving hardenability, is added excessively, the fraction of the low-temperature structure increases, and it is not preferable to secure the impact toughness at the center of the thickness, so the present invention limits the upper limit of the carbon (C) content to 0.06%. can do. Therefore, the carbon (C) content of the present invention may range from 0.02 to 0.06%.

Silicon (Si): 0.1~0.3%

Silicon (Si) is an element that contributes to the deoxidation of steel during steelmaking and to improve the strength of steel through solid solution strengthening. The present invention may limit the lower limit of the silicon (Si) content to 0.1% to achieve this effect. However, if silicon (Si) is excessively added, the strength of the ultra-thick steel sheet is excessively increased, causing a decrease in impact toughness, so the present invention sets the upper limit of the silicon (Si) content to 0.3 in order to secure the desired impact toughness. It can be limited to %. Therefore, the silicon (Si) content of the present invention may range from 0.1 to 0.3%.

Manganese (Mn): 1.3~1.8%

Manganese (Mn) is an element that effectively improves the strength of steel through solid solution strengthening and hardenability improvement, and is an element that can economically improve hardenability due to its low alloy cost compared to other elements contributing to the improvement of hardenability. The present invention may limit the lower limit of the manganese (Mn) content to 1.3% in order to achieve this effect. However, when manganese (Mn) is excessively added, there is a concern about an increase in excessive hardenability and a decrease in impact toughness due to segregation.The present invention sets the upper limit of the manganese (Mn) content to 1.8 in order to secure the desired impact toughness. It can be limited to %. Therefore, the manganese (Mn) content of the present invention may be in the range of 1.3 to 1.8%.

Aluminum (Al): 0.01~0.06%

Since aluminum (Al) is a representative deoxidation element, the present invention may limit the lower limit of the aluminum (Al) content to 0.01% to ensure the cleanliness of the steel. However, if aluminum (Al) is excessively added, there is a concern that the toughness of the welded portion of the ultra-thick steel sheet may be impaired, so the present invention may limit the upper limit of the aluminum (Al) content to 0.06%. Therefore, the aluminum (Al) content of the present invention may range from 0.01 to 0.06%.

Niobium (Nb): 0.005~0.02%

Niobium (Nb) is an element that contributes to improving the strength of the base metal by depositing as carbide or nitride. In addition, niobium (Nb) dissolved during high-temperature reheating is an element that effectively contributes to finer structure since it precipitates very finely in the form of carbides or nitrides during rolling to suppress recrystallization of austenite. In the present invention, 0.005% or more of niobium (Nb) is added to achieve such an effect. However, if niobium (Nb) is excessively added, there is a possibility of causing brittle cracks at the edges of the steel material, and there may be a problem of decrease in toughness due to the formation of excessive precipitates and the formation of a large amount of martensite-austenite structure, The present invention may limit the upper limit of the niobium (Nb) content to 0.02%. Accordingly, the niobium (Nb) content of the present invention may range from 0.005 to 0.02%.

Copper (Cu): 0.1~0.6%

Copper (Cu) is an element that improves hardenability and is an element that effectively contributes to improving the strength of steel. The present invention may limit the lower limit of the copper (Cu) content to 0.1% in order to achieve the effect of improving the strength. However, if copper (Cu) is excessively added, copper (Cu) penetrating into the austenite grain boundaries during the reheating process may cause surface defects in the rolling process, and copper (Cu) is an expensive element, which is desirable from an economic point of view. Therefore, the present invention may limit the upper limit of the copper (Cu) content to 0.6%. Therefore, the copper (Cu) content of the present invention may range from 0.1 to 0.6%.

Nickel (Ni): 0.2~1.2%

Nickel (Ni) is an element that reduces the transformation temperature of austenite to refine ferrite grains, promotes cross-slip at low temperatures, and suppresses a sharp drop in toughness. The present invention may limit the lower limit of the nickel (Ni) content to 0.2% in order to achieve such an effect. However, since nickel (Ni) is also an expensive element and a large amount of addition is not preferable from an economic point of view, the present invention may limit the upper limit of the nickel (Ni) content to 1.2%. Therefore, the nickel (Ni) content of the present invention may be in the range of 0.2 to 1.2%.

Titanium (Ti): 0.005~0.02%

Titanium (Ti) is an element that effectively contributes to improvement of low-temperature toughness because TiN precipitates are formed to suppress the growth of crystal grains in the base metal and the heat-affected zone of welding. The present invention may limit the lower limit of the content of titanium (Ti) to 0.005% in order to achieve the effect of inhibiting grain growth due to formation of TiN precipitates. However, when titanium (Ti) is excessively added, a problem in that low-temperature toughness is rather inferior due to coarse TiN crystallization occurs, so the present invention may limit the upper limit of the titanium (Ti) content to 0.02%. Therefore, the titanium (Ti) content of the present invention may range from 0.005 to 0.02%.

Phosphorus (P): 0.012% or less

Phosphorus (P) is an impurity that is unavoidably incorporated into steel, and is an element that is easily segregated, such as manganese (Mn). Therefore, it is desirable to control the phosphorus (P) content as low as possible, but the present invention may limit the upper limit of the phosphorus (P) content to 0.012% in order to prevent an excessive increase in steelmaking cost.

Sulfur (S): 0.005% or less

Sulfur (S) is an impurity that is unavoidably incorporated into steel, and is an element that forms non-metallic inclusions such as MnS to reduce the physical properties of the center of the thickness. Therefore, it is preferable to control the sulfur (S) content as low as possible, but the present invention may limit the upper limit of the sulfur (S) content to 0.005% in consideration of industrial manufacturing cost.

Nitrogen (N): 0.002~0.006%

Nitrogen (N) is an element that effectively contributes to the microstructure of the tissue by binding with titanium (Ti) and niobium (Nb), so the present invention can limit the lower limit of the nitrogen (N) content to 0.002% to achieve such an effect. have. However, when nitrogen (N) is excessively added, the surface quality of the steel sheet may be deteriorated, so the present invention may limit the upper limit of the nitrogen (N) content to 0.006%. Therefore, the nitrogen (N) content of the present invention may range from 0.002 to 0.006%.

The extremely thick steel sheet of the present invention may contain Fe and unavoidable impurities in addition to the above-described steel composition. Unavoidable impurities may be unintentionally incorporated in a conventional steel manufacturing process, and cannot be completely excluded, and those skilled in the ordinary steel manufacturing field can easily understand their meaning. In addition, the present invention does not entirely exclude addition of a composition other than the aforementioned steel composition.

Hereinafter, the microstructure of the present invention will be described in more detail. Hereinafter, unless otherwise indicated, the% indicating the fraction of the microstructure is based on the area.

The extremely thick steel sheet of the present invention may include ferrite as a main structure, and the area fraction of ferrite may be 95% or more.

The ultra-thick steel sheet of the present invention may include at least one of pearlite and bainite as a second structure, and pearlite and bainite may be included in an area fraction of 4% or less and 1% or less, respectively. This is because when the area fraction of pearlite exceeds 4% or the area fraction of bainite exceeds 1%, it is difficult to secure the impact toughness at the center of the thickness. In addition, the present invention may include the case where the area fraction of pearlite or bainite is 0%.

The ultra-thick steel sheet of the present invention may have an average size of 2.8 μm or less of effective grains measured at the point t/2. Here, t means the thickness (mm) of the steel sheet, and t in the following also means the thickness (mm) of the steel sheet. The average size of the effective grains of the present invention is defined as the average circle equivalent diameter of grains defined as diagonal grain boundaries having a crystal orientation difference of 15° or more when measured by the Electro Back Scattered Pattern (EBSP) method based on the Kikuchi pattern. I can. The ultra-thick steel sheet of the present invention can effectively secure impact toughness in the center of the thickness of the steel sheet because the effective crystal grains in the center of the thickness of the steel sheet are refined.

The ultra-thick steel sheet having excellent thickness center toughness according to an aspect of the present invention may have a tensile strength of 520 MPa or more, and an impact toughness at a point of t/2 of -100° C. or less based on a T100J transition temperature. In the present invention, the T100J transition temperature can be defined as a temperature at which the absorbed energy value corresponds to 100J in the curve obtained by converting the absorbed energy values obtained after conducting a Charpy impact test under a plurality of temperature conditions into a sigmoid function. have. Therefore, the ultra-thick steel sheet of the present invention may have physical properties that are particularly suitable as a material for structures used in ships, offshore, construction, and civil engineering fields.

The thickness of the ultra-thick steel sheet having excellent toughness at the center of the thickness according to an aspect of the present invention may be 80 mm or more. The present invention does not specifically limit the upper limit of the thickness of the ultra-thick steel sheet, but the upper limit of the preferred thickness of the steel sheet may be 200 mm.

Hereinafter, a method of manufacturing an extremely thick steel sheet of the present invention will be described in more detail. The following manufacturing method corresponds to a preferred example of manufacturing the extremely thick steel sheet of the present invention, and the extremely thick steel sheet of the present invention is not necessarily manufactured only by the following manufacturing method.

The ultra-thick steel sheet having excellent toughness at the center of the thickness according to an aspect of the present invention is, by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.3 to 1.8% , Aluminum (Al): 0.01~0.06%, Niobium (Nb): 0.005~0.02%, Copper (Cu): 0.1~0.6%, Nickel (Ni): 0.2~1.2%, Titanium (Ti): 0.005~0.02% , Phosphorus (P): 0.012% or less, Sulfur (S): 0.005% or less, Nitrogen (N): 0.002 to 0.006%, the rest of the slab containing Fe and other inevitable impurities in a temperature range of 1150℃ or more based on the thickness of the core At first heating, the first heated slab is intermediately cooled to a temperature range of 500°C or less based on the thickness center temperature of the slab, and the intermediate cooled slab is Ac3 ~ 1000°C based on the thickness center temperature of the slab Secondly heated to a temperature range of, and finish rolling of the slab in a temperature range of Ar3 to (Ar3+150°C) based on the surface temperature of the second heated slab to provide an extremely thick steel sheet, and the finish rolling It can be manufactured by finally cooling the ultra-thick steel sheet to a temperature range of Bf or less at a cooling rate of 2°C/s or more based on the thickness of the ultra-thick steel sheet.

Steel slab primary heating, intermediate cooling and secondary heating

In an extremely thick steel sheet, the effective grain size at the center of the thickness is a very important factor in securing the impact toughness at the center of the thickness. In the case of a general thin steel sheet, it is possible to refine the central structure of the thickness through controlled rolling and accelerated cooling, but in the case of an ultra-thick steel sheet having a thickness of 80 mm or more as in the present invention, the thickness of the steel sheet is controlled by control rolling and accelerated cooling due to the thick steel sheet thickness. There are technical limitations to sufficiently miniaturizing the core tissue. In addition, if the austenite grain size is reduced in the steel slab reheating step, it is possible to refine the thickness central structure of the final steel plate.However, if the slab reheating temperature is lowered to reduce the austenite grain size, the solution treatment is not sufficient. There may be a problem that the impact toughness in the center of the thickness is deteriorated.

Therefore, the inventors of the present invention conducted an in-depth study on a method of securing the impact toughness of the final steel plate by miniaturizing the thickness center structure of the steel slab before rolling, while sufficiently proceeding the solution treatment for the steel slab, The alloy composition contained in the steel slab is sufficiently solutionized through a two-stage steel slab reheating condition consisting of high-temperature reheating and low-temperature reheating, and a cooling process performed between high-temperature reheating and low-temperature reheating. It was found that it can be sufficiently refined.

Hereinafter, the steel slab primary heating, intermediate cooling and secondary heating conditions of the present invention will be described in more detail.

Since the steel slab of the present invention is provided with an alloy composition corresponding to the above-described ultra-thick steel sheet, the description of the alloy composition of the steel slab of the present invention is replaced with a description of the alloy composition of the aforementioned ultra-thick steel sheet.

The steel slab provided with the above composition may be first reheated under high temperature conditions. Since the steel slab of the present invention contains niobium (Nb), it is possible to limit the lower limit of the primary reheating temperature range in consideration of sufficient solutionization of the niobium (Nb) carbonitride contained in the steel slab. Accordingly, the primary reheating of the present invention may be performed in a temperature range of 1150° C. or higher based on the temperature of the central thickness of the steel slab. This is because when the coarse niobium (Nb) carbonitride formed in the steelmaking process is sufficiently solutionized, it effectively contributes to the improvement of the physical properties of the steel sheet by forming a fine carbonitride in the subsequent rolling and cooling process. In addition, the present invention does not specifically limit the upper limit of the primary reheating temperature range, but may be limited to 1300° C. or less based on the thickness center temperature of the steel slab in consideration of economical aspects.

The present invention does not specifically limit the first reheating time, but the first reheating may proceed for a time during which the alloy composition contained in the steel slab is sufficiently dissolved. In terms of solutionization of the alloy composition included in the steel slab, the preferred first reheating time may be 1 minute or more per thickness (mm) of the steel slab.

The primary reheated steel slab is not immediately provided for rolling, but is intermediately cooled, and the coarse austenite formed by the primary reheating can be transformed into ferrite, pearlite, bainite, and the like. The intermediate cooling may be air cooling or water cooling, and cooling to a temperature range of 500°C or less based on the temperature of the center of the steel slab in order to secure a uniform cooling effect of the steel slab.

In the present invention, the central temperature of the steel slab is the temperature at the point t/2 calculated based on the temperature measured from the surface of the steel slab, the thermal conductivity of the material, and the heating or cooling time. A person skilled in the art can calculate the core temperature of the steel slab based on this information without any special technical difficulties. In addition, the temperature at the center of the steel sheet mentioned below is also the temperature at the point t/2 calculated through this method.

The intermediate cooled steel slab can be reheated secondarily again. Since the heating temperature of the secondary reheating is a very important factor in determining the initial particle size of the austenite, the secondary reheating can be carried out in the range of Ac3~1000℃ based on the temperature at the center of the thickness of the steel slab. Here, Ac3 can be calculated by Equation 1 below.

[Equation 1]

Ac3 = 910-203*[C] ^0.5 + 44.7*[Si]-30*[Mn]-20*[Cu]-15.2*[Ni]-11*[Cr] + 31.5*[Mo] + 700*[ P] + 400*[Al] -400*[Ti] + 104*[V] + 13.1*[W]

(In Equation 1, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [P], [Al], [Ti], [V], [W ] Means the weight% of each alloy composition, and if the corresponding alloy composition is not included, the value means 0.)

If the secondary reheating temperature is less than Ac3, the reverse transformation to austenite is not completed, so austenite and microstructure formed by intermediate cooling are mixed. It is difficult to secure the physical properties of the center of the thickness. This is because the structure formed by primary reheating and intermediate cooling is formed coarse by omitting the rolling process after heating at high temperature, so that the micronization effect of the structure cannot be sufficiently achieved even by the subsequent rolling process. In addition, when the secondary reheating temperature exceeds 1000° C., since the proportion of bainite in the final microstructure increases as austenite grows, it is difficult to secure the impact toughness at the center of the thickness desired in the present invention.

Although the present invention does not specifically limit the secondary reheating time, the preferred secondary reheating time may be 1 minute or more per thickness (mm) of the steel slab.

Rough rolling

The secondary reheated steel slab can be roughly rolled to provide a rough rolled bar. The rough rolling may be performed immediately after the secondary reheating, and a preferable rough rolling reduction amount may be 30% or more in terms of pore compression removal in the center of the thickness of the steel slab.

In addition, the present invention may include a case in which the rough rolling is not performed immediately after the secondary reheating, but the finish rolling is performed immediately. However, in the case of performing finish rolling without performing rough rolling, the time it takes for the steel slab to reach the finish rolling temperature after secondary reheating can be relatively long. It is more preferable.

Finish rolling

After rough rolling, finish rolling may be terminated in a temperature range of Ar3 to (Ar3+150°C) based on the surface temperature of the rough rolling bar. When the finish rolling is performed immediately after the secondary reheating, the finish rolling may be terminated in a temperature range of Ar3 to (Ar3+150°C) based on the surface temperature of the steel slab. Here, Ar3 can be calculated by Equation 2 below.

[Equation 2]

Ar3 = 910-310*[C]-80*[Mn]-20*[Cu]-55*[Ni]-15*[Cr] -80*[Mo]

(In Equation 2, [C], [Mn], [Cu], [Ni], [Cr], and [Mo] mean the weight percent of each alloy composition, and if the alloy composition is not included, the value is 0 Means.)

When the finish rolling end temperature is less than Ar3, a part of austenite is transformed into ferrite during finish rolling, and thus the deformed ferrite remains in the final microstructure, and impact toughness may be greatly reduced. On the other hand, when the finish rolling end temperature exceeds (Ar3+150° C.), it is difficult to secure the target thickness center impact toughness because the micronization of austenite by rolling is not sufficiently achieved.

In finish rolling, the cumulative reduction ratio is preferably 40% or more. When the cumulative reduction ratio is less than 40%, the austenite grain refining effect is insufficient, so it is difficult to secure the target thickness center impact toughness.

Final cooling

Final cooling can be performed on the finish-rolled ultra-thick steel sheet. The cooling method of the final cooling is not particularly limited, but water cooling may be preferable in terms of cooling efficiency. When the cooling rate is less than a certain level, since the desired strength cannot be secured, the final cooling of the present invention may be performed at a cooling rate of 2° C./s or more based on the temperature of the center of the steel sheet. When the cooling end temperature exceeds a certain range, the microstructure in the thickness direction of the steel sheet is formed unevenly, resulting in variations in impact toughness, and thus, it is difficult to secure the target thickness central toughness. In addition, when the cooling end temperature exceeds a certain range, it is difficult to secure the strength of the final steel sheet, and according to the present invention, the final cooling may be performed up to a temperature range of Bf or less based on the central temperature of the steel sheet. Here, Bf can be calculated by Equation 3 below.

[Equation 3]

Bf = 710 -270*[C]-90*[Mn]-37*[Ni]-70*[Cr]-83*[Mo]

(In Equation 3, [C], [Mn], [Ni], [Cr], and [Mo] mean the weight percent of each alloy composition, and if the alloy composition is not included, the value means 0. )

The ultra-thick steel sheet manufactured by the manufacturing method of the present invention may contain 95 area% or more ferrite, 4 area% or less pearlite, and 1 area% or less bainite, and the average of the effective grains measured at the point t/2 It may have a size of 3 μm or less.

In addition, the ultra-thick steel sheet manufactured by the manufacturing method of the present invention has a thickness of 80 mm or more, a tensile strength of 520 MPa or more, and the impact toughness at the point of t/2 is -100° C. or less based on the T100J transition temperature. It is a material for structures used in the fields of marine, architecture, and civil engineering, and can have particularly suitable properties.

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

(Example)

The molten steel was manufactured with the composition shown in Table 1, and a steel slab having a thickness of 300 mm was manufactured through continuous casting. In Table 2, the temperatures of Ac3, Ar3 and Bf calculated based on the respective compositions in Table 1 are described.

Steel grade Alloy composition (wt%) C Si Mn P S Cu Ni Al Ti Nb N A 0.06 0.28 1.78 0.007 0.004 0.14 0.26 0.04 0.015 0.012 0.002 B 0.05 0.26 1.61 0.008 0.003 0.36 0.56 0.02 0.007 0.008 0.003 C 0.04 0.14 1.37 0.006 0.003 0.54 1.14 0.01 0.012 0.017 0.003 D 0.03 0.19 1.58 0.004 0.002 0.32 0.98 0.02 0.01 0.015 0.004 E 0.09 0.29 1.55 0.005 0.002 0.16 0.34 0.03 0.013 0.013 0.005 F 0.06 0.51 1.53 0.003 0.003 0.36 0.76 0.03 0.011 0.018 0.003 G 0.06 0.21 1.98 0.006 0.003 0.25 0.45 0.02 0.012 0.017 0.004

Steel grade Temperature(℃) Ac3 Ar3 Bf A 828 732 524 B 823 728 531 C 810 715 534 D 821 714 523 E 817 736 534 F 828 720 528 G 806 703 499

According to the manufacturing conditions of Table 3, an ultra-thick steel plate specimen having a thickness of 80 mm or 100 mm was prepared. In Table 3, the first heating temperature and the second heating temperature mean the temperature at the center of the thickness of the steel slab. After the first heating, intermediate cooling was performed for all specimens to a temperature range of 500°C or less based on the center temperature of the thickness of the steel slab. In Table 3, the finish rolling end temperature means the surface temperature of the steel slab, and both the final cooling rate and the final cooling end temperature are based on the specimen thickness center temperature.

Psalter
No. Steel grade Steel plate thickness
(mm) Primary heating
Temperature
(℃) Secondary heating
Temperature
(℃) Finish rolling
End temperature
(℃) Finish rolling
Cumulative reduction rate
(%) Final cooling
speed
(℃/s) Final cooling
End temperature
(℃) One A 80 1203 869 812 59 4.1 451 2 B 80 1189 934 834 56 3.8 502 3 C 80 1232 878 786 54 3.5 489 4 D 80 1192 978 793 45 3.9 435 5 A 100 1243 891 788 51 3.1 345 6 B 100 1212 867 818 41 2.9 321 7 C 100 1165 919 809 36 2.4 386 8 D 100 1179 845 765 42 3.8 355 9 A 80 - 894 822 27 3.9 454 10 B 80 1254 1056 790 44 3.3 509 11 C 100 1189 959 889 43 2.8 466 12 D 100 1232 1121 813 47 1.3 410 13 E 80 1209 880 832 51 4.5 506 14 F 80 1181 917 789 49 4.9 558 15 G 80 1232 924 808 54 3.8 398 16 C 100 972 946 803 49 2.9 449 17 D 100 1188 782 769 45 2.3 487

For each specimen, a tensile test piece and an impact test piece were collected at t/2 part (where t means the specimen thickness (mm)) to evaluate physical properties. The tensile test piece was a JIS No. 10 annular test piece, and the impact test piece was a standard test piece having a Charpy V notch. Both the tensile test piece and the impact test piece were processed so that the direction perpendicular to the rolling direction became the length of the test piece, and the physical properties were evaluated. Tensile tests were conducted at room temperature, and impact tests were conducted in various temperature ranges to obtain the T100J transition temperature. The microstructure of each test piece was observed and evaluated using an optical microscope and a scanning electron microscope, and the fraction of each tissue is based on the area. The average size of the effective grains refers to the average value obtained by measuring the equivalent circle diameter of grains defined as diagonal grain boundaries with a crystal orientation difference of 15° or more when measured by the Electro Back Scattered Pattern (EBSP) method, and the T100J transition temperature is In the curve obtained by converting the absorbed energy values obtained after performing the Charpy impact test under a plurality of temperature conditions into a sigmoid function, it means the temperature at which the absorbed energy value corresponds to 100J.

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Psalter
No. Steel grade ferrite
Fraction
(%) Pearlite
Fraction
(%) Bainite
Fraction
(%) Average valid
Grain size
(㎛) Yield strength
(MPa) The tensile strength
(MPa) T100J
Transition temperature
(℃) One A 95 4 One 2.7 398 537 -113 2 B 97 3 0 2.8 387 541 -104 3 C 99 One 0 2.1 412 528 -121 4 D 100 One 0 2.2 426 526 -128 5 A 95 4 One 2.4 388 543 -115 6 B 98 2 0 2.6 394 537 -121 7 C 99 One 0 2.7 409 529 -104 8 D 100 One 0 2.5 422 531 -117 9 A 85 One 14 3.2 389 525 -86 10 B 51 0 49 4.1 367 544 -78 11 C 72 2 26 3.8 394 532 -82 12 D 45 2 53 4.6 375 513 -65 13 E 80 7 13 2.8 385 529 -77 14 F 88 5 7 2.9 411 541 -92 15 G 91 One 8 2.7 421 552 -88 16 C 93 2 5 2,4 451 535 -75 17 D 85 3 12 2.9 439 546 -62

Since the specimens 1 to 8 satisfy all of the alloy composition and manufacturing conditions of the present invention, it can be confirmed that they have the desired microstructure of the present invention, and satisfy both the tensile strength of 520 MPa or more and the T100J transition temperature of -100° C. or less.

Specimen 9 was a case where the second reheating process was immediately performed without going through the first reheating process, and it can be seen that the impact toughness at the center of the thickness is poor because the steel slab is not sufficiently solutionized.

Specimens 10 and 12 are cases in which the secondary reheating temperature exceeds 1000°C, and it can be seen that a large amount of bainite is formed in the microstructure and the impact toughness at the center of the thickness is inferior. In addition, it can be seen that the tensile strength of the steel sheet is heated with the final cooling rate of specimen 12 being less than 2°C/s.

Specimen 11 is a case where the finish rolling end temperature exceeds the range of the present invention, a large amount of bainite is formed in the microstructure, the average effective crystal grains are formed coarse, it can be confirmed that the impact toughness in the center of the thickness is inferior.

Specimen 13 is a case in which the carbon (C) content is out of the scope of the present invention, and it can be seen that the proportion of pearlite and bainite in the microstructure is high, and the impact toughness at the center of the thickness is poor.

Specimen 14 is a case in which the silicon (Si) content is out of the scope of the present invention, and it can be seen that the strength is excellent, but the impact toughness at the center of the thickness is poor.

Specimen 15 is a case where the manganese (Mn) content is out of the scope of the present invention, it can be confirmed that the strength is excellent, but the impact toughness at the center of the thickness is poor.

Specimens 16 and 17 are cases where the primary reheating temperature and the secondary reheating temperature are not within the scope of the present invention, respectively, and it can be seen that the strength is excellent, but the impact toughness at the center of the thickness is poor.

FIG. 1 is an optical picture of observing the microstructure at a point of 1/2 thickness of specimen 1, and FIG. 2 is an optical picture of observing the microstructure at a point of thickness of 1/2 of specimen 10. As shown in FIGS. 1 and 2, it can be seen that the fine core structure of the specimen 1 was effectively refined compared to the specimen 10.

Therefore, it can be confirmed that the ultra-thick steel plate according to an aspect of the present invention has particularly suitable physical properties as a material for a structure used in the fields of ships, offshore, architecture, and civil engineering.

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

Claims (9)

delete delete delete delete delete In% by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.3 to 1.8%, aluminum (Al): 0.01 to 0.06%, niobium (Nb): 0.005 ~0.02%, Copper (Cu): 0.1~0.6%, Nickel (Ni): 0.2~1.2%, Titanium (Ti): 0.005~0.02%, Phosphorus (P): 0.012% or less, Sulfur (S): 0.005% Hereinafter, nitrogen (N): 0.002 ~ 0.006%, the slab containing the remaining Fe and other inevitable impurities are first heated in a temperature range of 1150 ℃ or more based on the thickness center temperature,
Intermediately cooling the first heated slab to a temperature range of 500° C. or less based on the temperature of the central thickness of the slab,
Secondly heating the intermediate cooled slab in a temperature range of Ac3 ~ 1000 °C based on the temperature of the central thickness of the slab,
Finish rolling of the slab in a temperature range of Ar3 to (Ar3+150°C) based on the surface temperature of the secondary heated slab to provide an extremely thick steel sheet,
A method of manufacturing an ultra-thick steel sheet having excellent thickness center toughness by finally cooling the ultra-thick steel sheet to a temperature range of Bf or less at a cooling rate of 2°C/s or more based on the thickness center temperature of the finish-rolled ultra-thick steel sheet. The method of claim 6,
In the first heating, the slab is heated for 1 minute or more per thickness of the slab, a method of manufacturing an ultra-thick steel sheet having excellent thickness central toughness. The method of claim 6,
Rough rolling the secondary heated slab to provide a rough rolled bar,
A method of manufacturing an ultra-thick steel sheet having excellent thickness central toughness after waiting for the surface temperature of the rough rolled bar to reach the temperature range of the finish rolling and then finish rolling the rough rolled bar. The method of claim 6,
The cumulative reduction ratio of the finish rolling is 40% or more.

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