US20210388458A1

US20210388458A1 - High strength steel plate for structure with good seawater corrosion resistant property and method of manufacturing same

Info

Publication number: US20210388458A1
Application number: US17/291,823
Authority: US
Inventors: Jin-ho Park; Ju-Yeon Yi; Seng-Ho YU; Bong-Ju Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2018-11-08
Filing date: 2019-11-08
Publication date: 2021-12-16
Also published as: EP3878996A1; JP2022506661A; CN112969809B; WO2020096398A1; KR20200053342A; JP7332692B2; EP3878996A4; CN112969809A; KR102142774B1

Abstract

The present invention provides a high strength steel plate for a structure and a method of manufacturing same, wherein the high strength steel plate fora structure comprises, by weight %, 0.03% (inclusive) to 0.1% (exclusive) of C, 0.1% (inclusive) to 0.8% (exclusive) of Si, 0.3% (inclusive) to 1.5% (exclusive) of Mn, 0.5% (inclusive) to 1.5% (exclusive) of Cr, 0.1% (inclusive) to 0.5% (exclusive) of Cu, 0.01% (inclusive) to 0.08% (exclusive) of Al, 0.01% (inclusive) to 0.1% (exclusive) of Ti, 0.05% (inclusive) to 0.1% (exclusive) of Ni, 0.002% (inclusive) to 0.07% (exclusive) of Nb, 0.03% or less of P, 0.02% or less of S, and the balance of Fe and unavoidable impurities, and has a microstructure comprising, by area fraction, 20% or more of bainite, less than 80% of polygonal ferrite and acicular ferrite in total, and less than 10% of pearlite and MA as the other phases.

Description

TECHNICAL FIELD

The present disclosure relates to a steel fora structure having excellent corrosion resistance in an environment in which corrosion is accelerated by seawater, such as a steel plate for building structures on the coast, a ballast tank in a ship and related appurtenant equipment, or the like, and a method of manufacturing the steel.

BACKGROUND ART

In general, corrosion of a metal is promoted when there are many inorganic substances in the form of ions dissolving easily in water, such as salt. In particular, in the case of ions having a property of promoting corrosion, such as chlorine ions (Cl⁻), significantly rapid corrosion may occur. Therefore, a metal containing an average of 3.5% NaCl corrodes in a seawater environment at a significantly high rate, so that corrosion is problematic under various conditions such as a structure adjacent to seawater and a ship sailing in a seawater environment, and the like.
Accordingly, a corrosion inhibition technology using various types of anti-corrosion treatment has been proposed. However, since a term of such an anti-corrosion treatment is only 20 to 30 years, maintenance costs may be continuously incurred unless corrosion resistance of a material itself is secured. That is, in order to increase durability of a structure to a long period of 50 years or more and reduce various anti-corrosion costs during a management period of the structure, it is necessary to strengthen the corrosion resistance of the material itself.
Among elements improving seawater resistance of a steel material, chromium (Cr) and copper (Cu) are most effective elements. Chromium and copper may play different roles depending on corrosive environments, and may exhibit an excellent anti-corrosion effect even in an environment, in which corrosion is accelerated by seawater, when added in an appropriate ratio. However, chromium does not have a significant effect in an acidic environment, and copper causes casting cracking to occur in a casting process, so that relatively expensive nickel should be added in a certain level or more. However, in most environments other than a strongly acidic environment, chromium has an effect of improving corrosion resistance, and the minimum amount of nickel added to prevent casting defects of copper-added steel may be reduced due to the recent development in continuous casting technology. Accordingly, the amount of expensive nickel added may be reduced, so that the cost of a product may be reduced.
As the related art concerning a steel material having excellent resistance to seawater, Patent Documents 1, 2, and 3 have been proposed. Patent Document 1 discloses that a composition system and manufacturing conditions are controlled to control a microstructure of a steel sheet, but it is difficult to secure strength when the content of a low-temperature structure is low (less than 20%). In addition, the content of nickel (Ni) is specified as being 0.05% or less, so that many casting defects may occur during casting. In the case of Patent Document 2, 0.1% or more of Al is added to form coarse oxide inclusions in a steelmaking process, and inclusions are crushed and elongated during a rolling process to form elongated inclusions. Accordingly, void formation is promoted to reduce localized corrosion resistance. In addition, when tungsten (W) is added as in the case of Patent Document 3, there are a risk of continuous casting defects and a risk of galvanic corrosion caused by formation of coarse precipitates. In addition, there is a risk that a structure is coarsened by air cooling to decrease strength.
Therefore, it may be difficult to internally secure corrosion resistance to seawater and strength in steel plates for structure according to Patent Documents 1 to 3.

(Patent Document 1) Korean Patent Publication No. 10-2011-0076148
(Patent Document 2) Korean Patent Publication No. 10-2011-0065949
(Patent Document 3) Korean Patent Publication No. 10-2004-0054272

DISCLOSURE

Technical Problem

An aspect of the present disclosure is to provide a steel plate, having excellent corrosion resistance to a seawater environment, in which corrosion characteristics and a microstructure of a surface of the steel plate are controlled through optimization of a composition system and manufacturing conditions to improve strength characteristics of the steel plate and to significantly reduce a corrosion rate.
On the other hand, the feature of the present disclosure is not limited to the above description. It will be understood by those skilled in the art that there would be no difficulty in understanding additional features of the present disclosure.

Technical Solution

According to an aspect of the present disclosure, a high-strength steel (or steel plate) for structure comprising, by weight, carbon (C): 0.03% or more to less than 0.1%, silicon (Si): 0.1% or more to less than 0.8%, manganese (Mn): 0.3% or more to less than 1.5%, chromium (Cr): 0.5% or more to less than 1.5%, copper (Cu): 0.1% or more to less than 0.5%, aluminum (Al): 0.01% or more to less than 0.08%, titanium (Ti): 0.01% or more to 0.1% or less, nickel (Ni): 0.05% or more to less than 0.1%, niobium (Nb): 0.002% or more to less than 0.07%, phosphorus (P): 0.03% or less, sulfur (S): 0.02% or less, and a balance of iron (Fe) and unavoidable impurities. The high-strength steel has a microstructure comprising, by area fraction, 20% or more of bainite, less than 80% of polygonal ferrite and acicular ferrite in total, and less than 10% of pearlite and MA as the other phases.
In the high-strength steel (or steel plate) for a structure, the carbon (C) may be contained in an amount of 0.03% or more to less than 0.09%.
In the high-strength steel (or steel plate) for a structure, the silicon (Si) may be contained in an amount of 0.2% or more to less than 0.8%.
In the high-strength steel (or steel plate) for a structure, the copper (Cu) may be contained in an amount of 0.1% or more to less than 0.45%.
The high-strength steel (or steel plate) for a structure may have yield strength of 500 MPa and tensile strength of 600 MPa.
According to an aspect of the present disclosure, a method of manufacturing a high-strength steel (or steel plate) for a structure includes: reheating a slab to a temperature of 1000° C. or more to 1200° C. or less, the slab comprising, by weight, carbon (C): 0.03% or more to less than 0.1%, silicon (Si): 0.1% or more to less than 0.8%, manganese (Mn): 0.3% or more to less than 1.5%, chromium (Cr): 0.5% or more to less than 1.5%, copper (Cu): 0.1% or more to less than 0.5%, aluminum (Al): 0.01% or more to less than 0.08%, titanium (Ti): 0.01% or more to 0.1% or less, nickel (Ni): 0.05% or more to less than 0.1%, niobium (Nb): 0.002% or more to less than 0.07%, phosphorus (P): 0.03% or less, sulfur (S): 0.02% or less, and a balance of iron (Fe) and unavoidable impurities; hot rolling the reheated slab within a finish rolling temperature of 750° C. or more to 950° C. or less; and cooling a rolled steel plate from a cooling initiation temperature of 750° C. or more to a cooling finish temperature of 400° C. to 700° C. at a cooling rate of 10° C./sec or more.
The technical solutions to the above-mentioned problems do not fully enumerate all features of the present disclosure. Various features of the present disclosure and the resulting advantages and effects will be understood in more detail with reference to the following detailed examples.

Advantageous Effects

As set forth above, according to an example embodiment, a steel (or steel plate) for a structure, in which corrosion resistance of the steel itself is improved in seawater atmosphere, having excellent strength characteristics of yield strength of 500 MPa or more and tensile strength of 600 MPa or more may be provided.
The various and beneficial advantages and effects of the present disclosure are not limited to the above description, and can be more easily understood in the course of describing a specific embodiment of the present disclosure.

DESCRIPTION OF DRAWINGS

FIG. 1 is an image of Inventive Steel 4 observed with a microscope, in which (a) is an image obtained by observing a surface, (b) is an image obtained by observing a ¼t portion in a thickness direction, and (c) is an image obtained by observing a ½t portion in the thickness direction.

BEST MODE FOR INVENTION

Hereinafter, example embodiments of the present disclosure will be described below. Example embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. These embodiments are provided to complete the present disclosure and to allow those skilled in the art to understand the scope of the disclosure.
The present inventors have conducted deep research into a method of improving corrosion resistance of a steel (or steel plate) for a structure itself. As a result, the inventors have found that when the contents of chromium and copper are appropriately controlled and manufacturing conditions such as a reheating temperature, a finish rolling temperature, a cooling end temperature, and the like, are optimized to control a microstructure, excellent seawater-resistant characteristics and strength characteristic may be secured. Based on this knowledge, the inventors have invented the present invention.
Hereinafter, a high-strength steel (or steel plate) for a structure according to an example embodiment will be described in detail.
High-Strength Steel (or Steel Plate) for Structure
First, a composition system of a high-strength steel (or steel plate) fora structure according to an example embodiment will be described. The high-strength steel (or steel plate) for a structure includes, by weight, carbon (C): 0.03% or more to less than 0.1%, silicon (Si): 0.1% or more to less than 0.8%, manganese (Mn): 0.3% or more to less than 1.5%, chromium (Cr): 0.5% or more to less than 1.5%, copper (Cu): 0.1% or more to less than 0.5%, aluminum (Al): 0.01% or more to less than 0.08%, titanium (Ti): 0.01% or more to 0.1% or less, nickel (Ni): 0.05% or more to less than 0.1%, niobium (Nb): 0.002% or more to less than 0.07%, phosphorus (P): 0.03% or less, sulfur (S): 0.02% or less, and a balance of iron (Fe) and unavoidable impurities. Hereinafter, the unit of each alloy element is weight percentage (wt %).
Carbon (C): 0.03% or More to Less than 0.1%
Carbon (C) is an element added to improve strength. When a content of carbon (C) is increased, hardenability may be increased to improve strength. However, as the amount of added carbon is increased, general corrosion resistance is reduced. In addition, since precipitation of carbide or the like is promoted, localized corrosion resistance is also affected. The content of carbon (C) should be decreased to improve the general corrosion resistance and the localized corrosion resistance. However, when the content of carbon (C) is less than 0.03%, it is difficult to secure sufficient strength as a material for a steel (or steel plate) for a structure. When the content of carbon (C) is 0.1% or more, weldability is deteriorated to be inappropriate for the steel (or steel plate) for a structure. Therefore, the content of carbon (C) may be limited to 0.03% or more to less than 0.1%. From the viewpoint of corrosion resistance, the content of carbon (C) may be less than 0.09%. In some cases, the content of carbon (C) may be less than 0.08% to further prevent casting cracking and to reduce carbon equivalent. A lower limit of the content of carbon (C) may be, in detail, 0.035%. An upper limit of the content of carbon (C) may be, in detail, 0.06%. The upper limit of the content of carbon (C) may be, in further detail, 0.054%.
Silicon (Si): 0.1% or More to Less than 0.8%
Silicon (Si) needs to be present in amount of 0.1% or more to serve as a deoxidizer and to serve to increase strength of steel. In addition, since silicon (Si) contributes to improvement in general corrosion resistance, it is advantageous to increase the content of silicon (Si). However, when the content of silicon (Si) is 0.8% or more, toughness and weldability may be deteriorated and it may be difficult to detach a scale during rolling, so that the scale may causes surface defects. Therefore, the content of silicon (Si) may be limited to, in detail, 0.1% or more to less than 0.8%. In some cases, silicon (Si) is added in an amount of 0.2% or more to improve corrosion resistance. A lower limit of the content of silicon (Si) may be, in detail, 0.2%, and, in further detail, 0.27%. An upper limit of the content of silicon (Si) may be, in detail, 0.5% and, in further detail, 0.44%.
Manganese (Mn): 0.3% or More to Less than 1.5%
Manganese (Mn) is an element effect in increasing the strength through solid-solution strengthening without reducing toughness. However, when an excessive amount of manganese (MN) is added, an electrochemical reaction rate of a steel surface may be increased during a corrosion reaction to reduce corrosion resistance. When manganese (Mn) is added in an amount of less than 0.3%, it may be difficult to secure durability of a steel plate for a structure. Meanwhile, when the content of manganese (Mn) is increased, hardenability may be increased to improve strength. However, when manganese (Mn) is added in an amount of 1.5% or more, a segregation zone may b significantly developed in a central portion of thickness during slab casting in a steelmaking process, weldability may be reduced, and corrosion resistance of a surface of a steel plate may be reduced. Therefore, the content of manganese (Mn) may be limited to, in detail, 0.3% or more to less than 1.5%. On the other hand, a lower limit of the content of manganese (Mn) may be, in detail, 0.4% and, in further detail, 0.5%. An upper limit of the content of manganese (Mn) may be, in detail, 1.4% and, in further detail, 0.9%.
Chromium (Cr): 0.5% or More to Less than 1.5%
Chromium (Cr) is an element increasing the corrosion resistance by forming a chrome-containing oxide layer on a surface of the steel in a corrosive environment. Chromium (Cr) should be contained in an amount of 0.5 or more to exhibit a corrosion resistance effect depending on addition of chromium (Cr). However, when chromium (Cr) is contained in an amount of 1.5% or more, toughness and weldability are adversely affected. Therefore, the content of chromium (Cr) may be set to, in detail, be 0.5% or more to less than 1.5%. A lower limit of the content of chrome (Cr) may be, in detail, 0.6% and, in yet further detail, 1.2%. An upper limit of content of chrome (Cr) may be, in detail, 1.4%. That is, in the steel (or steel plate) for a structure according to an example embodiment, the content of chrome (Cr) may be, in detail, 1.2% or more to 1.4% or less (that is, 1.2% to 1.4%).
Copper (Cu): 0.1% or More to Less than 0.5%
When copper (Cu) is contained in an amount of 0.05 wt % or more together with nickel (Ni), exudation of iron (Fe) is delayed, which is effective in improving general corrosion resistance and localized corrosion resistance. However, when the content of copper (Cu) is 0.5% or more, copper (Cu) in a liquid state melts into a grain boundary during production of a slab. Thus, cracking occurs during hot working (“hot shortness”). Therefore, the content of copper (Cu) may be limited to, in detail, 0.1% or more to less than 0.5%. In particular, a lower limit of the content of copper (Cu) may be, in detail, 0.2% and, in yet further detail, 0.28%.
Since surface cracking, occurring during production of the slab, interact with the contents of carbon (C), nickel (Ni), and manganese (Mn), a frequency of occurrence of the surface cracking may vary depending on the content of each element, but the content of copper (Cu) may be set to be, in detail, less than 0.45% and, in yet further detail, 0.43% or less to significantly reduce possibility that surface cracking occurs, irrespective of the content of each element.
Aluminum (AL): 0.01% or More to Less than 0.08%
Aluminum (Al) is an element added for deoxidation, and reacts with nitrogen (N) in the steel in such a manner that an aluminum nitride (AlN) is formed and austenite grains are refined to improve toughness. The content of aluminum (Al) in a dissolved state may be, in detail, 0.01% or more for sufficient deoxidation. A lower limit of the content of aluminum (Al) may be, in detail, 0.02% and, in further detail, 0.022%. When the aluminum (Al) is excessively included in an amount of 0.08% or more, a stretched inclusion, crushed and elongated during rolling, may be formed according to aluminum oxide-based characteristics. Since the formation of such an elongated inclusion promotes formation of a void around the inclusion and such a void may serve as an initiation point of localized corrosion, and the elongated inclusion serves to reduce the localized corrosion resistance. Therefore, the content of aluminum (Al) may be limited to, in detail, less than 0.08%. An upper limit of aluminum (Al) may be, in detail, 0.05% and, in further detail, 0.034%.
Titanium (Ti): 0.01% or More to Less than 0.1%
Titanium (Ti) is bonded to carbon (C) in steel to form TiC when added in an amount of 0.01% or more, serving to improve strength due to a precipitation strengthening effect. Meanwhile, when the content of Ti is added in an amount of 0.1% or more, a strength improvement effect is not large, as compared with the increase in the content thereof. Accordingly, the content of titanium (Ti) may be limited to 0.01% or more to less than 0.1%. A lower limit of the content of titanium (Ti) may be, in detail, 0.015%. In addition, an upper limit of the content of titanium (Ti) may be, in further detail, 0.05%, and, in yet further detail, 0.028%.
Nickel (Ni): 0.05% or More to Less than 0.1%
Similarly to the case of copper (Cu), when nickel (Ni) is contained in an amount of 0.05% or more, it is effective in improving general corrosion resistance and localized corrosion resistance. Meanwhile, a lower limit of the content of Nickel (Ni) may be, in detail, 0.07%. In addition, when nickel (Ni) is added together with copper (Cu), nickel (Ni) reacts with copper (Cu) in such a manner that formation of a copper (Cu) phase is suppressed to prevent hot shortness from occurring. In most Cu-added steels, nickel (Ni) is generally added at one or more times of the content of copper (Cu). However, as in the present disclosure, when the content of an element related to carbon equivalent, such as carbon (C) or manganese (Mn), is low and the content of chromium (Cr) is high, shortness may be sufficiently prevented even nickel (Ni) is added in less than half of the content of copper (Cu). In addition, since nickel (Ni) is an expensive element, an upper limit of the content of nickel (Ni) may be limited to, in detail, 0.1% in consideration of a relative addition effect. In addition, the upper limit of the content of nickel (Ni) may be, in further detail, 0.09%.
Niobium (Nb): 0.002% or More to Less than 0.07%
Niobium (Nb) is an element bonded to carbon in steel to form NbC such as titanium (Ti), serving to strengthen precipitation. When niobium (Nb) is added in an amount of 0.002% or more, Nb may effectively improve strength. However, when Nb is added in an amount of 0.07% or more, a strength improvement effect is not significantly large as compared with an increase in the content of niobium (Nb). Therefore, the content of Nb may be limited to, in detail, 0.002% or more to less than 0.07%. A lower limit of the content of niobium (Nb) may be, in further detail, 0.01% and, in yet further detail, 0.017%. In addition, an upper limit of the content of niobium (Nb) may be, in further detail, 0.05% and, in yet further detail, 0.044%.
Phosphorus (P): 0.03% or Less
Phosphorus (P) is present as an impurity element in steel. When the phosphorous (P) is added in an amount greater than 0.03%, weldability is significantly reduced and toughness is deteriorated. Therefore, the content of phosphorous (P) is limited to, in detail, 0.03% or less. An upper limit of the content of phosphorous (P) may be, in detail, 0.02% and, in further detail, 0.018%. Since phosphorous (P) is an impurity, it is advantageous as the content of phosphorous (P) is reduced. Therefore, a lower limit of the content of phosphorous (P) is not separately limited.
Sulfur (S): 0.02% or Less
Sulfur (S) is present as an impurity in steel. When the content of sulfur (S) is greater than 0.02%, ductility, impact toughness, and weldability of steel are deteriorated. Accordingly, the content of sulfur (S) may be limited to, in detail, 0.02% or less. Sulfur (S) is apt to react with manganese (Mn) to form an elongated inclusion such as manganese sulfide (MnS). And voids, formed on both ends of the elongated inclusion, may be an initiation point of localized corrosion. Therefore, an upper limit of the content of sulfur (S) may be limited to, in further detail, 0.01% and, in yet further detail, 0.008% or less. Since sulfur (S) is an impurity, it is advantageous as the content of sulfur (S) is reduced. Therefore, a lower limit of the content of sulfur (S) is not separately limited. In addition to the above-described alloy elements, a balance may be iron (Fe). However, in a common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or surrounding environments, so that they may not be excluded. Since these impurities are commonly known to those skilled in the art, and all contents thereof are not specifically mentioned in this specification.
The high-strength steel (or steel plate) for a structure according to an example embodiment may have a microstructure including, by area fraction, 20% or more of bainite, less than 80% of polygonal ferrite and acicular ferrite in total, and less than 10% of pearlite and martensite-austenite (MA) as the other phases.
In the microstructure of the high-strength steel (or steel plate) for a structure according to an example embodiment, an area fraction of bainite may be, in detail, 20% or more, in further detail, 30% or more, and, in yet further detail, 51% or more.
In the microstructure of the high-strength steel (or steel plate) for a structure according to an example embodiment, an area fraction of bainite may be 78% or less.
In the microstructure of the high-strength steel (or steel plate) for a structure according to an example embodiment, an area fraction of bainite may be 68% or more to 71% or less.
In the microstructure of the high-strength steel (or steel plate) for a structure according to an example embodiment, an area fraction of polygonal ferrite and acicular ferrite in total may be less than 80% and, in further detail, 45% or less.
In the microstructure of the high-strength steel (or steel plate) for a structure according to an example embodiment, an area fraction of polygonal ferrite and acicular ferrite in total may be 10% or more and, in further detail, 19% or more.
In the microstructure of the high-strength steel (or steel plate) for a structure according to an example embodiment, an area fraction of polygonal ferrite and acicular ferrite in total may be 25% or more to 30% or less and, in further detail, 27% or more to 30% or less.
In the microstructure of the high-strength steel (or steel plate) for a structure according to an example embodiment, an area fraction of pearlite and martensite-austenite (MA) as the other phases may be less than 10%, in detail, 5% or less, in further detail, 4% or less, and, in yet further detail, 2% or less.
In generally, thick steel plate strength of at least 500 MPa, in detail, 600 MPa or more should be secured to be used as a material of a high-strength steel plate for strength. To this end, a microstructure mainly included 20% or more of bainite and other phases of polygonal and/or acicular ferrite. When pearlite and MA, other phases, are contained in an amount of 10% or more, low-temperature toughness and corrosion resistance may be insufficient in an environment in which the steel (or steel plate) for a structure according to the present disclosure is used. Therefore, an upper limit of the area fraction of pearlite and MA may be less than 10%.
The high-strength steel (or steel plate) for a structure according to an example embodiment may satisfy the above-mentioned composition system and microstructure to have yield strength of 500 MPa or more and tensile strength of 600 MPa or more.
Hereinafter, a method of manufacturing a high-strength steel (or steel plate) for a structure according to an example embodiment of the present disclosure will be described.
Method of Manufacturing High-Strength Steel (or Steel Plate) for Structure
A method of manufacturing a high-strength steel (or steel plate) for a structure may include a slab reheating process, a hot rolling process, and a cooling process. Detailed conditions of each of the processes are as follows.
Reheating of Slab
A slab having the above-mentioned composition system is prepared, and then heated within a temperature range of 1000° C. to 1200° C. The reheating temperature may be set to 1000° C. or more to solid-solubilize carbonitride formed during casting. The reheating temperature may be set to, in further detail, 1050° C. or more to fully solid-solubilize the carbonitride. On the other hand, when the slab is reheated at significantly high temperature, austenite may be formed to be coarse. Therefore, the reheating temperature may be, in detail, 1200° C. or less.
Hot Rolling
A hot rolling process, including rough rolling and finish rolling, may be performed on the reheated slab. In this case, the finish rolling may be completed, in detail, at 750° C. or more of finish rolling temperature. When the finish rolling temperature is less than 750° C., it may cause a problem of producing a large amount of ferrite by air-cooling. On the other hand, when the finish rolling temperature is more than 950 C, strength and toughness may be reduced due to structure coarseness. Therefore, the finish rolling temperature may be limited to, in detail, 750° C. to 950° C.
Cooling
The hot-rolled steel material is cooled through water cooling. In the present disclosure, a core technology is to secure high strength of even a thick steel plate through sufficient cooling, and it is necessary to perform a cooling process to a temperature of 700° C. or less at a cooling rate of 10° C. or more. In addition, the cooling process may be started at a cooling initiation temperature of 750° C. or more. However, when the hot-rolled steel material is cooled to a temperature of less than 400° C., micro-cracking may occur in a central portion due to a quenching process to cause deviation of material properties in a surface and a central portion of a product and a deviation of material properties in front/end portions of the product. Therefore, the cooling process may be finished at temperature of, in detail, 400° C. or more. For example, in the cooling process, a steel plate rolled may be cooled, in detail, from the cooling initiation temperature of 750° C. or more to the cooling finish temperature of 400° C. to 700° C. at a cooling rate of 10° C./sec or more. In particular, a range of the cooling finish temperature may be, in further detail, 500° C. to 650° C. and, in yet further detail, 522° C. to 614° C.
An upper limit of the cooling rate is mainly related to equipment capacity. When the cooling rate is 10° C./sec or more, a meaningful change in strength does not occur even with an increase in the cooling rate. Therefore, the upper limit of the cooling rate is not separately limited. On the other hand, a lower limit of the cooling rate may be, in detail, 20° C./sec, in further detail, 25° C./sec, and, in yet further detail, 30° C./sec.

MODE FOR INVENTION

Hereinafter, embodiments of the present disclosure will be described more specifically through examples. However, the examples are for clearly explaining the embodiments of the present disclosure and are not intended to limit the scope of the present disclosure.

Example

A slab was produced by preparing molten steel having a composition system listed in Table 1 below and then performing a continuous casting process. The produced slab was reheated, hot-rolled, and cooled under manufacturing conditions of Table 2 below to manufacture a steel plate.
A microstructure of the manufactured steel plate was observed with optical and electron microscopes to measure an area fraction of each phase, and yield strength and tensile strength were measured through a tensile test and are listed in Table 3. In addition, as an evaluation of seawater-resistant characteristics, a specimen was immersed in a 3.5% NaCl solution, simulating seawater. The specimen was inserted into an ultrasonic cleaner together with a 50% HCl+0.1% hexamethylene tetramine solution to be cleaned, and weight loss was measured and then divided by a surface area of an initial specimen to calculate a corrosion rate. In addition, to compare corrosion rates of comparative steels and inventive steels, a relative corrosion rate was evaluated based on the corrosion rate of Comparative Steel 1 as 100, and the results are listed in Table 3.

TABLE 1

	C	Si	Mn	P	S	Sol.Al	Cu	Ni	Cr	Nb	Ti

IS1	0.041	0.27	0.8	0.008	0.005	0.022	0.43	0.08	1.2	0.044	0.015
IS2	0.035	0.44	0.9	0.018	0.007	0.024	0.28	0.09	1.4	0.032	0.018
IS3	0.054	0.27	0.7	0.012	0.006	0.034	0.32	0.09	0.6	0.041	0.022
IS4	0.052	0.32	0.5	0.011	0.008	0.028	0.29	0.07	1.0	0.017	0.028
CS1	0.068	0.51	1.8	0.008	0.007	0.029	0.12	0.06	0.4	0.009	0.019
CS2	0.092	0.27	2.1	0.009	0.005	0.042	0.04	0.08	0.2	0.026	0.024
CS3	0.049	0.51	2.2	0.018	0.007	0.024	0.07	0.12	0.7	0.047	0.021

IS: Inventive Steel
CS: Comparative Steel

TABLE 2

		Finishing	Cooling	Cooling
	Reheating	Rolling	Initiation	Finish	Cooling
	Temperature	Temperature	Temperature	Temperature	Rate
	(° C.)	(° C.)	(° C.)	(° C.)	(° C./sec)

IS1	1142	870	774	542	36
IS2	1129	901	784	614	42
IS3	1161	840	754	601	39
IS4	1112	904	787	522	44
CS1	1119	875	769	565	15
CS2	1124	891	771	579	23
CS3	1151	880	773	612	14

IS: Inventive Steel
CS: Comparative Steel

TABLE 3

	area fraction of a microstructure (%)

	polygonal	the other
	ferrite +	phases	Yield	Tensile	Relative
	acicular	(pearlite,	Strength	Strength	Corrosion
bainite	ferrite	MA)	(MPa)	(MPa)	Rate

IS1	71	27	2	564	636	66
IS2	68	30	2	512	608	64
IS3	51	45	4	574	678	71
IS4	78	19	3	551	643	77
CS1	64	34	2	574	671	100
CS2	57	39	4	564	659	137
CS3	63	33	4	592	702	143

IS: Inventive Steel
CS: Comparative Steel

As can be seen from Table 1, all of Inventive Steels 1 to 4 satisfied the composition range specified in the present disclosure. Meanwhile, in Comparative Steels 1 to 3, a composition range of Cr, Cu, Ni or Mn was outside the range of the present disclosure.
As a result, Inventive Steels 1 to 4 had a microstructure having a low-temperature structure including 20% or more of bainite based on ferrite, and thus, had high strength of yield strength of 500 MPa or more and tensile strength of 600 MPa or more, so that they had a sufficient material of a steel (or steel plate) for a structure. In addition, it was confirmed that Inventive Steels 1 to 4 satisfied the composition range specified in the present disclosure to exhibit a lower corrosion rate than Comparative Steel 1, and thus, may have sufficient lifespan in a seawater-resistant atmosphere.
Meanwhile, in Comparative Steels 1 to 3, a composition range of Cr, Cu, Ni or Mn was outside the range of the present disclosure. For this reason, although Comparative Steels 1 to 3 were manufactured using a manufacturing method satisfying manufacturing conditions of the present disclosure, they exhibited a high corrosion rate of 100 or more. As a result, Comparative Steels 1 to 3 did not have sufficient lifespan in a sea-resistant atmosphere.
It was confirmed that Inventive Steels 1 and 2, containing 1.2% or more to 1.4% or less of Cr, exhibited a lower corrosion rate than Inventive Steels 3 and 4, not containing 1.2% or more to 1.4% or less of Cr.
From the above, it was confirmed that a steel plate for a structure according to an example embodiment contained 1.2% or more to 1.4% or less of Cr to have most excellent lifespan characteristics in a seawater-resistant atmosphere.
While this disclosure includes specific examples, it will be apparent after gaining an understanding of the disclosure of this application that various changes in forms and details may be made in these examples without departing from the spirit and scope of the claims and their equivalents. The examples described herein are to be considered in a descriptive sense only, and not for purposes of limitation. Descriptions of features or aspects in each example are to be considered as being applicable to similar features or aspects in other examples. Suitable results may be achieved if the described techniques are performed in a different order, and/or if components in a described system, architecture, device, or circuit are combined in a different manner, and/or replaced or supplemented by other components or their equivalents. Therefore, the scope of the disclosure is defined not by the detailed description, but by the claims and their equivalents, and all variations within the scope of the claims and their equivalents are to be construed as being included in the disclosure.

Claims

1. A high-strength steel for a structure comprising, by weight, carbon (C): 0.03% or more to less than 0.1%, silicon (Si): 0.1% or more to less than 0.8%, manganese (Mn): 0.3% or more to less than 1.5%, chromium (Cr): 0.5% or more to less than 1.5%, copper (Cu): 0.1% or more to less than 0.5%, aluminum (Al): 0.01% or more to less than 0.08%, titanium (Ti): 0.01% or more to 0.1% or less, nickel (Ni): 0.05% or more to less than 0.1%, niobium (Nb): 0.002% or more to less than 0.07%, phosphorus (P): 0.03% or less, sulfur (S): 0.02% or less, and a balance of iron (Fe) and unavoidable impurities,

wherein the high-strength steel has a microstructure comprising, by area fraction, 20% or more of bainite, less than 80% of polygonal ferrite and acicular ferrite in total, and less than 10% of pearlite and MA as the other phases.

2. The high-strength steel for a structure of claim 1, wherein the carbon (C) is contained in an amount of 0.03% or more to less than 0.09%.

3. The high-strength steel for a structure of claim 1, wherein the silicon (Si) is contained in an amount of 0.2% or more to less than 0.8%.

4. The high-strength steel for a structure of claim 1, wherein the copper (Cu) is contained in an amount of 0.1% or more to less than 0.45%.

5. The high-strength steel for a structure of claim 1, which has yield strength of 500 MPa and tensile strength of 600 MPa.

6. A method of manufacturing a high-strength steel for a structure, the method comprising:

reheating a slab to a temperature of 1000° C. or more to 1200° C. or less, the slab comprising, by weight, carbon (C): 0.03% or more to less than 0.1%, silicon (Si): 0.1% or more to less than 0.8%, manganese (Mn): 0.3% or more to less than 1.5%, chromium (Cr): 0.5% or more to less than 1.5%, copper (Cu): 0.1% or more to less than 0.5%, aluminum (Al): 0.01% or more to less than 0.08%, titanium (Ti): 0.01% or more to 0.1% or less, nickel (Ni): 0.05% or more to less than 0.1%, niobium (Nb): 0.002% or more to less than 0.07%, phosphorus (P): 0.03% or less, sulfur (S): 0.02% or less, and a balance of iron (Fe) and unavoidable impurities;

hot rolling the reheated slab within a finish rolling temperature of 750° C. or more to 950° C. or less; and

cooling a rolled steel plate from a cooling initiation temperature of 750° C. or more to a cooling finish temperature of 400° C. to 700° C. at a cooling rate of 10° C./sec or more.