EP4640903A1

EP4640903A1 - Steel plate having high strength and excellent low-temperature impact toughness and method for manufacturing same

Info

Publication number: EP4640903A1
Application number: EP23907501.3A
Authority: EP
Inventors: Tae-Il SO; Sang-Deok Kang
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-19
Filing date: 2023-12-04
Publication date: 2025-10-29
Also published as: EP4640903A4; CN120380187A; WO2024136215A1; KR20240096073A

Abstract

The present invention pertains to a steel plate that can be appropriately used for marine structures such as wind power installation ships and marine plants. More specifically, the present invention pertains to a steel plate having high strength and excellent low-temperature impact toughness and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a steel plate that can be appropriately used for marine structures such as wind power installation ships and marine plants. More specifically, the present disclosure relates to a steel plate having high strength and excellent low-temperature impact toughness and a method for manufacturing the same.

Background Art

Recently, demand for marine structures has been increasing, and there is a trend toward larger marine structures. In addition, as the Jack-up Rig equipment used in marine plants, wind power installation ships, or the like, is aging and the replacement period is approaching, the demand is also increasing.
As the size of these equipment structures increases, an ultra-thick, high-strength steel plate having a thickness of 200 mm or more is required, and as the usage environment expands into extreme condition regions, excellent low-temperature impact toughness is required, and in the case of a steel plate which is subjected to severe processing during the production of a structure, low-temperature strain aging impact toughness is also required.
When manufacturing an ultra-thick steel plate, if a relatively thin slab is used, sufficient rolling force may not be applied to a central portion thereof, and a type and a fraction of a microstructure of a central portion and a surface portion of the manufactured steel plate are different depending on a difference in cooling speeds, resulting in a large difference in physical properties, making it difficult to secure uniform strength in the thickness direction.
Meanwhile, in order to manufacture a high-strength, ultra-thick steel plate, a method of improving the hardenability of steel and increasing the strength is mainly used, by adding an appropriate amount of hardenability-improving elements such as Mn, Cr, and Mo to the steel. In this case, the strength of steel may be improved by generating a large amount of low-temperature structures such as martensite or bainite inside a steel plate through a cooling treatment such as a tempering treatment. However, if such hardenable elements are added excessively, the carbon equivalent increases, which causes problems in which a preheating temperature before welding increases, and cracks occur, so it is necessary to control alloy components.
Meanwhile, Patent Document 1 proposes a technology for securing high strength and low-temperature impact toughness. However, although Patent Document 1 is economically advantageous by reducing an expensive Ni content, it is difficult to secure sufficient strength and low-temperature impact toughness when manufacturing an ultra-thick hot-rolled steel plate other than a medium/thick steel plate, and a high content of Mn relative to Cr can significantly reduce the low-temperature impact toughness thereof.
Accordingly, there is a need for the development of a high-strength, ultra-thick steel plate having excellent low-temperature impact toughness that can be used appropriately for marine structures such as wind power installation ships and marine plants.
(Patent Document 1) Korean Patent Publication No. 10-2020-0075964

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide an ultra-thick steel plate having high strength and excellent low-temperature impact toughness, and a method for manufacturing the same.
An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.

Solution to Problem

According to an aspect of the present disclosure, provided is a steel plate, the steel plate including by weight%: 0.14 to 0.18% of C, 0.2 to 0.5% of Si, 0.1 to 0.7% of Mn, 0.008% or less of P, 0.003% or less of S, 0.015 to 0.045% of Al, 1.0 to 2.0% of Cr, 3.0 to 4.5% of Ni, 0.25% or less of Cu, 0.4 to 0.6% of Mo, 0.002 to 0.01% of N, with a remainder of Fe and inevitable impurities,

wherein the steel plate satisfies the following Relational Expression 1 and Relational Expression 2, and
has a thickness of more than 133 mm and 300 mm or less.

C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95

2.0 < Cr / Mn < 6.5

In Relational Expressions 1 and 2 above, each element refers to a content (weight%).
A microstructure observed at t/4 and t/2 points of the steel plate, where t is a thickness of the steel plate, may include by area fraction, 20 to 70% of tempered martensite, and a remainder of bainite phase.
An average particle size of carbides present within the bainite phase may be 5 µm or less.
The steel plate may have a yield strength of 690 MPa or more, a tensile strength of 800 MPa or more, and an average Charpy impact absorption energy value of 69 J evaluated in a rolling direction at -40°C.
The steel plate may have an average reduction of area of 35% or more after a tensile test evaluated in a thickness direction.
According to another aspect of the present disclosure, provided is a method for manufacturing a steel plate, the method including preparing a steel slab including by weight%, 0.14 to 0.18% of C, 0.2 to 0.5% of Si, 0.1 to 0.7% of Mn, 0.008% or less of P, 0.003% or less of S, 0.015 to 0.045% of Al, 1.0 to 2.0% of Cr, 3.0 to 4.5% of Ni, 0.25% or less of Cu, 0.4 to 0.6% of Mo, 0.002 to 0.01% of N, with a remainder of Fe and inevitable impurities, wherein the following Relational Expression 1 and Relational Expression 2 are satisfied;

heating the steel slab at a temperature within a range of 1050 to 1200°C;
rough rolling the heated slab at a temperature of 1000°C or higher;
finishing rolling the heated slab at a temperature of Ar3 or higher based on a temperature in a central portion thereof, after the rough rolling, to manufacture a hot-rolled steel sheet and then cooling (primary cooling) the hot-rolled steel sheet in the air;
reheating the cooled hot-rolled steel sheet to a temperature between 830 and 930°C, and heat treating the same for 2.3t+30 minutes, where t is a thickness of steel (mm)) or more and then cooling (secondary cooling) the cooled hot-rolled steel sheet at a rate of 0.5°C/s or more to room temperature, and;
performing a tempering heat treatment of the cooled hot-rolled steel sheet at a temperature between 550 to 650°C for 2.3t+30 minutes or more and 3.4t+30 minutes or less and then cooling (tertiary cooling) the cooled hot-rolled steel sheet to room temperature.

C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95

2.0 < Cr / Mn < 6.5

In Relational Expressions 1 and 2 above, each element refers to a content (weight%).
The operation of preparing the steel slab may be an operation of manufacturing a continuous casting slab by forging the same at a temperature of Ac3 or higher to a thickness of 50 to 85% of an initial thickness thereof.
The reheating and cooling operation may be performed two or more times.

Advantageous Effects of Invention

As set forth above, according to an aspect of the present disclosure, an ultra-thick steel material that can secure high strength over the entire thickness and has excellent low-temperature impact toughness is provided.
The various and beneficial advantages and effects of the present disclosure are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present disclosure.

Brief Description of Drawings

FIG. 1 is a photograph illustrating a microstructure at a t/4 point of Inventive Example 1 in an embodiment of the present disclosure.
FIG. 2 is a photograph illustrating a microstructure at a t/4 point of Comparative Example 2 in an embodiment of the present disclosure.

Best Mode for Invention

The terms used in this specification are used to describe the present disclosure and are not intended to limit the present disclosure. In addition, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary.
The meaning of "including" or "comprising" used in the specification specifies a configuration and does not exclude the presence or addition of another configuration.
Unless otherwise defined, all terms, including technical terms and scientific terms used in this specification, have the same meaning as that which could be commonly understood by those skilled in the art in the technical field to which the present disclosure pertains. Terms defined in the dictionary are interpreted as having meanings consistent with related technical literature and the current disclosure.
The inventors of the present disclosure have recognized the need for the development of a technology for securing the properties required for materials due to the enlargement marine structures, or the like, and the application to extreme condition regions. To this end, the present inventors have conducted in-depth research on a method to secure a steel plate with excellent low-temperature impact toughness while ensuring high strength in all thickness directions. As a result, the present inventors have confirmed that an ultra-thick steel plate having target properties could be provided by controlling a composition of components and a relationship between some components in alloy design, and simultaneously, optimizing manufacturing conditions, and thus the present disclosure was provided.
First, an alloy composition of a steel plate according to an aspect of the present disclosure will be described in detail.
Unless otherwise particularly stated in the present disclosure, a content of each element is by weight and a ratio of the structure is by area.

Carbon (C): 0.14 to 0.18 weight % (hereinafter, referred to as %)

Carbon (C) is an effective element for improving the strength of steel, and to obtain the effect described above sufficiently, it is effective that C is included in an amount of 0.14% or more. However, when a content of C exceeds 0.18%, low-temperature impact toughness and strain aging impact toughness are significantly reduced, so the content of C in the present disclosure is effectively 0.14 to 0.18%. The content of C is preferably 0.140 to 0.180%.

Silicon (Si): 0.2 to 0.5%

Silicon (Si) is used as a deoxidizer and is an effective element for improving strength and toughness. However, when a content of Si exceeds 0.5%, low-temperature toughness and weldability may deteriorate. On the other hand, when the content of Si is less than 0.2%, a deoxidation effect may be insufficient. Therefore, it is effective that the content of Si is 0.2 to 0.5%. The content of Si is preferably 0.20 to 0.50%.

Manganese (Mn): 0.1 to 0.7%

Manganese (Mn) is an element which is advantageous in securing strength due to solid solution strengthening, and Mn is preferably added in an amount of at least 0.1% to obtain the effect described above. However, when a content of Mn exceeds 0.7%, Mn combines with S to form MnS, which significantly reduces room temperature elongation, low-temperature impact toughness, and strain aging impact toughness, so it is effective that the content of Mn is 0.1 to 0.7%. The content of Mn is preferably 0.10 to 0.70%.

Phosphorus (P): 0.008% or less

Phosphorus (P) is an element which is advantageous in improving strength and corrosion resistance, but P can significantly impair impact toughness, so it is advantageous to maintain a content of P to be as low as possible. Therefore, it is effective that an upper limit of the content of P is set to be 0.008%, and it is more effective that the upper limit of the content of P is set to be 0.005% or less.

Sulfur (S): 0.003% or less

Sulfur (S) is an element which significantly impairs impact toughness by forming MnS, or the like, so it is advantageous to maintain a content of S to be as low as possible, so it is effective that an upper limit of the content of S is set to be 0.003%.

Aluminum (Al): 0.015 to 0.045%

Aluminum (Al) is an element that can deoxidize molten steel inexpensively, and to exhibit a sufficient effect, it is effective that Al is included in an amount of 0.015% or more. However, when a content of Al exceeds 0.045%, nozzle clogging may occur during continuous casting, and the formation of Al-based oxides may significantly reduce low-temperature impact toughness and strain aging impact toughness. Therefore, it is effective that the content of Al is 0.015 to 0.045%.

Chromium (Cr): 1.0 to 2.0%

Chromium (Cr) is an effective element for increasing hardenability in manufacturing an ultra-thick steel plate, forming bainite or martensite, and securing strength. In order to exhibit a sufficient effect, it is effective that Cr is included in an amount of 1.0% or more. However, Cr not only has a negative effect on welding characteristics by significantly increasing the carbon equivalent, but also can reduce low-temperature impact toughness due to coarsening of carbides, so it is effective that a content of Cr is 2.0% or less. The content of Cr is preferably 1.00 to 2.00%.

Nickel (Ni): 3.0 to 4.5%

Nickel (Ni) is an element that can simultaneously improve strength and low-temperature impact toughness of a base material. To achieve a sufficient effect, it is effective that Ni is included in an amount of 3.0% or more. However, if a content of Ni exceeds 4.5%, there is a problem in that the surface properties due to oxidation may be significantly deteriorated during slab reheating. Therefore, it is effective that the content of Ni is 3.0 to 4.5%. The content of Ni is preferably 3.00 to 4.50%.

Copper (Cu): 0.25% or less (including 0%)

Copper (Cu) is an element that can increase strength while minimizing a decrease in toughness of a base material, so it is an effective element for improving the strength. However, excessive addition of Cu increases the carbon equivalent, which not only hinders weldability but also significantly deteriorates surface quality of a product, so it is effective that a content of Cu is 0.25% or less.

Molybdenum (Mo): 0.4 to 0.6%

Molybdenum (Mo) has the effect of significantly improving hardenability, suppressing ferrite formation, and inducing bainite or martensite formation, and can also significantly improve strength, so it is effective that Mo is included in an amount of 0.4% or more in order to manufacture a high-strength and high-toughness steel plate. However, since Mo is an expensive alloying element and when a large amount of Mn is added, Mo excessively increases hardness, which may reduce toughness, so it is effective that a maximum content of Mo is 0.6%. The content of Mo is preferably 0.40 to 0.60%.

Nitrogen (N): 20 to 100ppm (0.002 to 0.01%)

Nitrogen (N) is an element that suppresses grain growth by forming AlN when added simultaneously with Al, but when a large amount of N is added or not heated at a sufficiently high temperature, N forms coarse AlN, which impairs low-temperature impact toughness, so it is effective that a maximum content of N is 100ppm. However, since controlling a content of N less than 20 mm not only increases a steelmaking load but is also insufficient to suppress grain growth, it is effective that a lower limit of the content of N be 20 ppm.
In addition to the components described above, components such as vanadium (V), titanium (Ti), and boron (B) described below may be additionally included. In this case, a content of each component is as follows.

Vanadium (V): 0.03% or less (including 0%)

Vanadium (V) has a lower temperature at which V is dissolved than other alloying elements, and V forms VC during an air cooling process after hot rolling, which greatly contributes to increasing strength, so that a strength improvement effect may be obtained. However, when a content of V exceeds 0.03%, there is a problem that hardness of a base phase becomes excessively high and a fraction of a hard phase such as MA increases, resulting in a significant decrease in low-temperature impact toughness. Therefore, V may be included in an amount of 0.03% or less.

Titanium (Ti): 0.005% or less (including 0%)

Titanium (Ti) forms TiN when added together with N, thereby reducing the occurrence of surface cracks due to the formation of AlN precipitates. However, when a content of Ti exceeds 0.005%, coarse TiNb (C, N) is formed during reheating of a steel slab, which acts as a factor which inhibits low-temperature impact toughness. Therefore, Ti may be included in an amount of 0.005% or less.

Boron (B): 0.0005% or less (including 0%)

Boron (B) is a low-cost alloying element that exhibits strong hardenability even when added in small amounts, but when a content of B exceeds 0.0005%, B significantly reduces low-temperature impact toughness. Therefore, the content of B is preferably 0.0005% or less.
The remaining component of the present disclosure is iron (Fe). However, since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof.
It is effective that the steel plate of the present disclosure satisfies the following Relational Expressions 1 and 2. In this case, each element represents a content (weight %). $C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95$ $2.0 < Cr / Mn < 6.5$
The present disclosure intends to secure high strength and excellent low-temperature impact toughness by adding a certain amount of elements advantageous for improving strength and hardenability and appropriately controlling the contents thereof in order to secure a target level of strength.
In particular, the present disclosure, C, Mn, Cr, Mo, Cu, Ni, or the like, are added to steel, and when the contents of these elements are excessive, the carbon equivalent (Ceq) may increase, causing problems such as an increase in a preheating temperature before welding or cracks being induced. Therefore, it is effective that the elements are added so that the contents of the elements described above satisfy Relational Expression 1 above.
In addition, the content of Cr relative to the content of Mn should be sufficiently increased to suppress the formation of coarse granular bainite as much as possible and obtain fine tempered bainite or martensite. To this end, when the content of Cr is too high, coarse carbides are likely to be formed during heat treatment, and when the content of Mn is too low, it is difficult to secure strength, so it is effective to satisfy Relational Expression 2 above by considering a ratio of the two components.
The steel plate of the present disclosure, satisfying Relational Expressions 1 and 2 above together with the above-described alloy components, is an ultra-thick steel plate having a thickness of more than 133 mm and 300 mm or less.
The steel plate of the present disclosure may comprise tempered martensite as a microstructure, and the remainder may include tempered bainite.
More specifically, the steel plate includes by area fraction, 20 to 70% of a tempered martensite phase over the entire thickness, and the remainder includes tempered bainite. For example, the steel plate includes by area fraction, 20 to 70% of a tempered martensite phase, and the remainder includes tempered bainite, at t/2 and t/4 points of the steel plate, where t is a thickness (mm) of the steel plate.
When a fraction of the tempered martensite is less than 20%, it is difficult that the strength suggested in the present disclosure is satisfied, and on the contrary, when the fraction of the tempered martensite exceeds 70%, the low-temperature impact toughness may be significantly reduced due to an excessive increase in the strength. The fraction of the tempered martensite may decrease toward the central portion of the thickness.
Meanwhile, it is preferable that fine carbides are present within the tempered bainite, and that an average particle size of the carbides is 5 µm or less. When the average particle size of the carbides within tempered bainite exceeds 5 µm, it can act as a crack initiation point and induce intragranular fracture, which can significantly reduce the low-temperature impact toughness. In this case, the main carbide is Fe-C carbide (e.g., Fe₃C), and may include Mn, Cr, Mo, or the like, therein.
The steel plate of the present disclosure may have a yield strength of 690 MPa or more, a tensile strength of 800 MPa or more, and an average Charpy impact energy (CVN) of 69 J or more at -40°C over the entire thickness of the steel plate, for example, at t/4 and t/2 points in the thickness direction, where t is a thickness of the steel plate (mm), and may have high strength and excellent low-temperature impact toughness.
Meanwhile, the steel plate of the present disclosure has an average impact absorption energy value of 69 J or more when subjected to an impact test at -40°C after 5% strain and aging heat treatment at 250°C for 1 hour, which indicates that the low-temperature impact toughness is not reduced during strain aging.
In addition, the steel plate of the present disclosure has an effect in which an average reduction of area is 35% or more after a tensile test evaluated in the thickness direction.
Hereinafter, a method for manufacturing a steel plate according to another aspect of the present disclosure will be described in detail. An ultra-thick steel plate according to the present disclosure may be manufactured by performing the processes of heating - hot rolling - cooling - reheating - cooling - tempering of a steel slab satisfying both the alloy components and the component Relational Expressions suggested by the present disclosure.
Each process condition is described in detail.

Steel slab heating

It is preferable to perform a process of heating and homogenizing a steel slab before hot rolling, and a heating process may be performed at a temperature within a range of 1050 to 1200°C. When the heating temperature of the steel slab is lower than 1050°C, precipitates (carbides and nitrides) formed within the slab are not sufficiently redissolved, so the formation of precipitates decreases in a process after hot rolling. On the other hand, when the heating temperature exceeds 1200°C, there is a concern that austenite grains may be coarsened, which may deteriorate the properties of the steel.
Meanwhile, the steel slab may be a casting slab obtained through continuous casting, and the casting slab may be heated as is, or the casting slab may be forged prior to heating to obtain a forged slab, and then the heating process may be performed. Specifically, prior to the heating, heating the casting slab to a temperature Ac3 or higher and then forging the casting slab to a thickness of 50 to 85% of the initial thickness of the casting slab may be further included.
In this case, an Ar3 temperature may be derived from the following Relational Expression. Ar3 = 910 - 310 * C - 80 * Mn - 20 * Cu - 55 * Ni - 80 * Mo + 119 * V + 124 * Ti - 18 * Nb + 179 * Al
Each element represents a content (weight %).
The present disclosure ultimately intends to obtain a thick steel plate having a thickness more than 133 mm, and in order to obtain a steel plate having a target thickness within a limited reduction ratio (3:1) during hot rolling, it is necessary to apply a slab having a thickness of 400 mm or more. As described above, in the present disclosure, a casting slab obtained through continuous casting, and when a thickness of the casting slab is approximately 600 to 700 mm, the thickness may be reduced by performing a forging process prior to heating the slab. In particular, by the forging process described above, the thickness may be effectively reduced while minimizing internal voids of the slab, and a sufficient rolling force may be applied to a central portion of the thickness in a subsequent process (hot rolling process).

Cooling after hot rolling (Primary cooling)

A steel slab heated as described above may be hot rolled to manufacture a hot-rolled steel sheet. In this case, the heated steel slab may be rough rolled at a temperature of 1000°C or higher and then finishing hot rolled at Ar3 or higher.
When the temperature during the rough rolling is less than 1000°C, there is a problem that the temperature is lowered during subsequent finishing hot rolling. In addition, when the temperature during the finishing hot rolling is less than Ar3, a rolling load may increase, which may cause quality defects such as surface cracks.
The manufactured hot-rolled steel sheet is cooled (primarily cooled) in the air.

Reheating and Cooling (Secondary Cooling)

The primarily-cooled hot-rolled steel sheet is reheated to a temperature range of 830 to 930°C, heat treated for 2.3t+30 minutes or more, where t means a thickness of steel (mm), and then cooled to room temperature at a cooling rate of 0.5°C/s or more (secondary cooling). In this case, water cooling is used as an example of secondary cooling. Meanwhile, the reheating and cooling (secondary cooling) process can be repeated two or more times.
The formation of fine austenite structures may be promoted through the reheating process, and a low-temperature phase may be formed during subsequent cooling. That is, a hot-rolled steel sheet may be reheated to form an austenite structure, but when a reheating temperature is lower than Ac3, the hot-rolled steel sheet structure may become a dual-phase structure of ferrite and austenite. When the reheating and cooling process are repeated two or more times, an additional austenite refinement effect may be obtained.
Therefore, when reheating the hot-rolled steel sheet, the reheating may be performed at a temperature of Ac3 or higher, preferably at a temperature within a range of 830 to 930°C, and it is preferable to maintain the temperature for 2.3t+30 minutes or more, where t means a thickness of steel (mm), so that 100% of the austenite phase is sufficiently formed to a central portion of the hot-rolled steel sheet.
When the hot-rolled steel sheet manufactured as described above is cooled to room temperature and then reheated at a temperature lower than Ac3, the hot-rolled steel sheet may have a dual phase structure of ferrite and austenite. When the hot-rolled steel sheet is maintained for less than 2.3t+30 minutes, there is not enough time for 100% austenite to be formed. Therefore, it is preferable to maintain the steel sheet at a temperature within a range of 870 to 930°C for 2.3t+30 minutes or more, where t is a thickness of a steel sheet (mm). When a cooling rate is less than 0.5°C/s during secondary cooling to room temperature, a fraction of ferrite or coarse bainite increases, making it difficult to secure the strength. Therefore, the cooling rate is preferably 0.5°C/s or more. However, considering the cooling equipment, cooling may be performed at a maximum cooling rate of 100°C/s. Here, the cooling rate is based on a t/4 region in the thickness direction of the steel sheet.

Tempering heat treatment and Cooling (Tertiary cooling)

After the secondary cooling, a tempering heat treatment process may be performed to form a tempered structure. The secondary cooled-steel sheet may include a low-temperature structure phase, preferably a martensite or bainite phase, as a microstructure. As described above, the steel sheet may include a low-temperature structure, so that the steel sheet may have high strength, but exhibit a brittle property. The steel sheet on which the low-temperature structure is formed may be heated to a certain temperature and then maintained, the strength of steel may be slightly reduced while securing the impact toughness at low temperatures.
Specifically, the secondarily-cooled steel sheet is subjected to a tempering heat treatment at a temperature range within a range of 550 to 650°C for a time of 2.3t+30 minutes or more and 3.4t+30 minutes or less, where t represents a thickness of steel (mm), and then cooled tertiarily cooled in the air.
By performing the tempering heat treatment, tempered martensite and tempered bainite phases may be formed. When the temperature during the tempering heat treatment is lower than 550°C, a heat treatment for a long period of time is required to sufficiently secure the tempering heat treatment effect, which causes a problem of low economic efficiency. On the other hand, when the temperature exceeds 650°C, not only may the strength reduction effect be excessively increased, but there is also a risk that carbides may be coarsened and the impact toughness may also decrease. In addition, when a tempering heat treatment is performed in the temperature range described above, when the time is less than 2.3t+30 minutes, the tempering effect is not sufficient, and when the time exceeds 3.4t+30 minutes, there is a concern that the strength may decrease due to excessive tempering.
The steel plate of the present disclosure, which is air-cooled after quenching and tempering heat treatment, includes 20 to 70% of tempered martensite and a remainder of tempered bainite, and it is effective that an average size of carbides in the tempered bainite is 2 µm or less.

Mode for Invention

Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following examples are only for describing the present disclosure by illustration, and not intended to limit the scope of rights of the present disclosure. The reason is that the scope of rights of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.

(Example)

Molten steel having an alloy composition (weight %, including a remainder of Fe and inevitable impurities) shown in Table 1 below was continuously casted to manufacture a continuous casting slab. As shown in Table 2, the continuous casting slab was manufactured with a thickness of 700 mm. The continuous casting slab was heated to a temperature of Ac3 or higher that can perform a subsequent hot rolling process, and then forged to a thickness of 400 mm to manufacture a forged slab. The forged slab was heated to a temperature within a range of 1100 to 1132°C, rough-rolled, and then finishing rolled in a temperature within a range of 1050 to 1070°C and air cooled to manufacture a steel sheet with a thickness of 210 mm.
Thereafter, the steel sheet was reheated to a temperature of 890°C and maintained for 513 minutes, and then cooled with water to room temperature at a rate of approximately 0.6°C/s based on the central portion. Thereafter, the water-cooled steel sheet was heated to a temperature within a range of 595 to 630°C and maintained for 590 to 774 minutes to perform a tempering heat treatment, and then cooled with air to room temperature to manufacture a final steel plate.
A microstructure of the steel plate manufactured under the conditions in Tables 1 and 2 above was observed, and the results are shown in Table 3. The microstructure was observed using an optical microscope and an electron microscope at t/4 and t/2 points in a thickness direction of the steel plate, and martensite and bainite were distinguished visually and area fractions thereof were measured. In addition, an average diameter of carbides present within tempered bainite was indicated using an image analysis program.
Referring to Table 3 above, in the case of Inventive Examples 1 to 3, at least 20 area % of tempered martensite was secured at t/4 and t/2, and an average size of carbides inside tempered martensite all satisfied 2 µm or less.
On the other hand, in the case of Comparative Example 1, a fraction of tempered martensite was outside of the range suggested by the present disclosure due to a high content of C, and in the case of Comparative Examples 2 and 3, it could be confirmed that the fraction of tempered martensite was outside of the range a value suggested by the present disclosure due to low contents of C and Cr. In the case of Comparative Example 4, it could be confirmed that a size of carbides in tempered bainite is coarse because a tempering heat treatment is maintained for a long period of time at a level outside of the range suggested by the present disclosure. In the case of Comparative Example 5, a tempering temperature was outside of the range suggested by the present disclosure, and it could be confirmed that the size of carbides in tempered bainite was coarse due to a heat treatment at high temperatures.
Meanwhile, FIG. 1 is a photograph obtained by observing a microstructure at a t/4 point of Inventive Example 1 above, and FIG. 2 is a photograph obtained by observing a microstructure at a t/4 point of Comparative Example 2 above. In Inventive Example 1, it could be confirmed that a fraction of tempered martensite was sufficiently high and a size of carbides in tempered martensite was fine, whereas, in Comparative Example 2, it could be confirmed that not only was tempered martensite formed but the size of carbides was also coarse.
Meanwhile, for the manufactured steel plate, tensile properties (yield strength, tensile strength, and elongation) at t/4t and t/2 and Charpy impact energy absorption (CVN) at -40°C were measured, and the results are shown in Table 4. In addition, aging impact absorption energy at -40°C after performing an aging heat treatment at 250°C after 5% strain, and Z-RA values, which are a reduction of area after a tensile test evaluated in a thickness direction, are shown in Table 4.
In the case of Inventive Examples 1 to 3, all of the properties within the range suggested by the present disclosure were satisfied. On the other hand, in the case of Comparative Example 1, it could be confirmed that the tensile properties satisfied the range suggested by the present disclosure but the impact toughness at -40°C did not satisfy a value suggested by the present disclosure due to a high content of C. In the case of Comparative Example 2, tempered martensite was not sufficiently formed due to reduced hardenability due to low contents of Cr and Ni, and a size of carbides in the tempered beanie was coarse, resulting in low strength and impact toughness. In the case of Comparative Example 3, it could also be confirmed that it is difficult to secure strength and impact toughness due to a low fraction of tempered martensite. In the case of Comparative Example 4, it could be confirmed that all the component ranges suggested by the present disclosure were satisfied, but the strength could be sufficiently secured, and it was difficult to secure impact toughness due to coarse carbides because a tempering heat treatment time was too long. In the case of Comparative Example 5, it could be confirmed that the component conditions of the present disclosure were satisfied, but the strength significantly decreased and the toughness also decreased significantly because the tempering heat treatment temperature was too high.
Meanwhile, in the case of Inventive Examples 1 to 3, it could be confirmed that the aging impact absorption energy and Z-RA values satisfied the values suggested by the present disclosure, but in the case of Comparative Examples 1 to 4, the aging impact absorption energy values suggested by the present disclosure were not satisfied.

Claims

A steel plate, comprising by weight%: 0.14 to 0.18% of C, 0.2 to 0.5% of Si, 0.1 to 0.7% of Mn, 0.008% or less of P, 0.003% or less of S, 0.015 to 0.045% of Al, 1.0 to 2.0% of Cr, 3.0 to 4.5% of Ni, 0.25% or less of Cu, 0.4 to 0.6% of Mo, 0.002 to 0.01% of N, with a remainder of Fe and inevitable impurities,
wherein the steel plate satisfies the following Relational Expression 1 and Relational Expression 2, and

has a thickness of more than 133 mm and 300 mm or less, $C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95$ $2.0 < Cr / Mn < 6.5$

in Relational Expression 1 and 2 above, each element refers to a content (weight%).
The steel plate of claim 1, wherein a microstructure observed at t/4 and t/2 points of the steel plate, where t is a thickness of the steel plate, comprises by area fraction, 20 to 70% of tempered martensite, and a remainder of bainite phase.
The steel plate of claim 2, wherein an average particle size of carbides present within the bainite phase is 5 µm or less.
The steel plate of claim 1, wherein the steel plate has a yield strength of 690 MPa or more, a tensile strength of 800 MPa or more, and an average Charpy impact absorption energy value of 69 J or more evaluated at -40°C in a rolling direction.
The steel plate of claim 1, wherein the steel plate has an average impact absorption energy value of 69 J or more when subjected to an impact test at -40°C after aging heat treatment at 250°C for 1 hour after 5% strain and aging heat treatment.
The steel plate of claim 1, wherein the steel plate has an average reduction of area of 35% or more after a tensile test evaluated in a thickness direction.
A method for manufacturing a steel plate comprising:
preparing a steel slab comprising by weight %, 0.14 to 0.18% of C, 0.2 to 0.5% of Si, 0.1 to 0.7% of Mn, 0.008% or less of P, 0.003% or less of S, 0.015 to 0.045% of Al, 1.0 to 2.0% of Cr, 3.0 to 4.5% of Ni, 0.25% or less of Cu, 0.4 to 0.6% of Mo, 0.002 to 0.01% of N, with a remainder of Fe and inevitable impurities, wherein the following Relational Expressions 1 and 2 are satisfied;

heating the steel slab at a temperature within a range of 1050 to 1200°C;

rough rolling the heated slab at a temperature of 1000°C or higher;

finishing rolling the heated slab at a temperature of Ar3 or higher based on a temperature in a central portion thereof, after the rough rolling, to manufacture a hot-rolled steel sheet and then cooling (primary cooling) the hot-rolled steel sheet in the air;

reheating the cooled hot-rolled steel sheet to a temperature between 830 and 930°C, and heat treating the same for 2.3t+30 minutes or more, where t is a thickness of steel (mm) and then cooling (secondary cooling) the cooled hot-rolled steel sheet at a rate of 0.5°C/s or more to room temperature, and;

performing a tempering heat treatment of the cooled hot-rolled steel sheet at a temperature between 550 to 650°C for 2.3t+30 minutes or more and 3.4t+30 minutes or less and then cooling (tertiary cooling) the cooled hot-rolled steel sheet to room temperature, $C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95$ $2.0 < Cr / Mn < 6.5$

in Relational Expressions 1 and 2 above, each element refers to a content (weight%).
The method of manufacturing a steel plate of claim 7, wherein the preparing the steel slab further comprises manufacturing a continuous casting slab by forging the same at a temperature of Ar3 or higher to a thickness of 50 to 85% of an initial thickness thereof.
The method of manufacturing a steel plate of claim 7, wherein the reheating and cooling operation is performed two or more times.