KR102142774B1 - High strength steel plate for structure with a good seawater corrosion resistive property and method of manufacturing thereof - Google Patents

Abstract

The present invention is by weight, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and less than 1.5%, Cr: 0.5% or more and less than 1.5%, Cu: 0.1% or more and 0.5% Less than, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and less than 0.1%, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, High-strength structure containing residual Fe and unavoidable impurities, microstructure, area fraction, bainite 20% or more, polygonal ferrite and acicular ferrite in total less than 80%, pearlite and MA as other phases less than 10% Provided is a molten steel and its manufacturing method.

Description

High-strength structural steel with excellent seawater resistance and its manufacturing method{HIGH STRENGTH STEEL PLATE FOR STRUCTURE WITH A GOOD SEAWATER CORROSION RESISTIVE PROPERTY AND METHOD OF MANUFACTURING THEREOF}

The present invention relates to structural steel having excellent corrosion resistance in a corrosion-accelerated environment caused by seawater, such as a structural structural steel on the shore or a ballast tank and related accessories inside a ship, and a manufacturing method thereof.

Corrosion of metals is generally facilitated if there are many soluble ions in the form of an inorganic substance in water, such as salt, in particular chloride ions ^- very rapid erosion takes place if there are ions which have properties that promote corrosion, such as (Cl) . Therefore, in a seawater environment containing an average of 3.5% NaCl, corrosion occurs at a very fast rate in metal, and thus, corrosion is problematic in various conditions such as structures adjacent to seawater and ships operating in the seawater environment.

Accordingly, a technique for suppressing corrosion by various types of anticorrosive treatments has been proposed. However, since the anti-corrosion period of such anti-corrosion treatment is only 20 to 30 years, maintenance costs are constantly incurred if the corrosion resistance of the material itself is not secured. That is, in order to increase the durability of the structure for a long period of 50 years or more and to reduce various anti-corrosion costs during the operation period of the structure, it is essential to strengthen the corrosion resistance of the material itself.

Among the elements that improve the water resistance of steel materials, chromium (Cr) and copper (Cu) are the most effective elements. Chromium and copper play different roles depending on the corrosive environment, and adding an appropriate ratio can exert excellent corrosion resistance even in an environment in which corrosion is accelerated by seawater. However, since chromium does not exert a great effect in an acidic environment and copper causes casting cracking during the casting process, there is a problem in that expensive nickel must be added to a certain level or more. However, in most environments other than strong acid, chromium has an effect of improving corrosion resistance, and the recent development of continuous casting technology has reduced the minimum amount of nickel added to prevent casting defects in copper-added steel, thereby reducing the cost of products by reducing the amount of expensive nickel added. It became possible to reduce.

On the other hand, with respect to steel materials having excellent seawater resistance, Patent Documents 1, 2 and 3 have been proposed as prior art. Patent Document 1 proposes to control the microstructure of the steel sheet by controlling the component system and manufacturing conditions, but it is difficult to secure the strength when the low-temperature tissue content is low (less than 20%), and the Ni content is regulated to be 0.05% or less. When casting, there is a possibility that casting defects may occur. In the case of patent document 2, a 0.1% or more Al is added to form a coarse oxidative inclusion in the steelmaking process, and an elongated stretch inclusion is generated when the inclusion is broken during rolling, thereby causing formation of voids, thereby inhibiting local corrosion resistance. have. In addition, as in the case of Patent Document 3, when W is added, there is a fear of galvanic corrosion due to generation of coarse precipitates along with the fear of occurrence of performance defects, and there is a fear of strength drop due to coarsening of the tissue due to air cooling.

Therefore, in the structural steels according to Patent Documents 1 to 3, it is difficult to secure seawater corrosion resistance and strength by itself.

Korean Patent Publication No. 10-2011-0076148 Korean Patent Publication No. 10-2011-0065949 Korean Patent Publication No. 10-2004-0054272

The present invention improves the strength properties of the steel sheet and minimizes the corrosion rate by controlling the corrosion properties and microstructure of the steel sheet surface through optimization of the component system and manufacturing conditions, thereby producing steel sheets having excellent corrosion resistance properties in the seawater environment of the steel sheets and manufacturing them. I want to provide a method.

In addition, the subject of this invention is not limited to the above-mentioned content. The subject matter of the present invention will be understood from the entire contents of the present specification, and those skilled in the art to which the present invention pertains will have no difficulty in understanding the additional subject matter of the present invention.

High-strength structural steel according to an aspect of the present invention, by weight, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and 0.9% or less, Cr: 1.2% or more and less than 1.5% , Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and less than 0.1%, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% Hereinafter, S: 0.02% or less, including residual Fe and unavoidable impurities, the microstructure is an area fraction, 20% or more of bainite, polygonal ferrite and acicular ferrite are less than 80% in total, pearlite as other phases And MA is less than 10%.

In the high-strength structural steel, C may be included in an amount of 0.03% or more and less than 0.09%.

In the high-strength structural steel, Si may be included in 0.2% or more and less than 0.8%.

In the high-strength structural steel, Cu may be included in an amount of 0.1% or more and less than 0.45%.

The high-strength structural steel may have a yield strength of 500 MPa or more, and a tensile strength of 600 MPa or more.

Method of manufacturing a high-strength structural steel according to another aspect of the present invention is by weight, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and 0.9% or less, Cr: 1.2% or more Less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and less than 0.1%, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P : 0.03% or less, S: 0.02% or less, re-heating the slab containing the residual Fe and unavoidable impurities at a temperature of 1000°C or more and 1200°C or less; Hot rolling the reheated slab to a finishing rolling temperature of 750°C or higher and 950°C or lower; And cooling the rolled steel sheet at a cooling rate of 10° C./sec or more from a cooling start temperature of 750° C. or higher to a cooling end temperature of 400 to 700° C.; It includes.

According to the present invention, the corrosion resistance of the steel sheet itself is improved in a seawater atmosphere, and a structural steel sheet having excellent strength characteristics having a yield strength of 500 MPa or more and a tensile strength of 600 MPa or more can be provided.

The various and beneficial advantages and effects of the present invention are not limited to the above, and will be more readily understood in the course of describing specific embodiments of the present invention.

1 is a photograph of the invention steel observed by a microscope, (a) is a surface, (b) is a thickness direction 1/4t portion, (c) is a thickness direction 1/2t portion observation.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

The present inventors have studied in depth the method for improving the corrosion resistance of the structural steel itself, and as a result, appropriately control the content of chromium and copper, and manufacture conditions such as reheating temperature, finish rolling temperature, cooling end temperature, cooling rate, etc. By controlling the microstructure by optimizing, it was confirmed that excellent seawater resistance and strength properties could be secured, and the present invention was completed.

Hereinafter, a high-strength structural steel according to an aspect of the present invention will be described in detail.

[High strength structural steel]

First, a component system of high strength structural steel according to an aspect of the present invention will be described. High-strength structural steel according to an aspect of the present invention, by weight, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and 0.9% or less, Cr: 1.2% or more and less than 1.5% , Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and less than 0.1%, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% Hereinafter, S: 0.02% or less, the balance Fe and unavoidable impurities are included. Hereinafter, the unit of each alloy element is% by weight.

Carbon (C): 0.03% or more but less than 0.1%

C is an element added to improve the strength, and if its content is increased, the quenching property can be improved to improve the strength, but as the amount added increases, the corrosion resistance of the front surface is inhibited and precipitation of carbides and the like is promoted. Resistance also has some effect. In order to improve the resistance to front corrosion and local corrosion, the C content must be reduced, but if the content is less than 0.03%, it is difficult to secure sufficient strength as a structural steel use material, and when it is 0.1% or more, weldability is deteriorated, making it undesirable as a steel for welding structures. Therefore, in the present invention, the C content may be limited to 0.03% or more and less than 0.1%. Meanwhile, in terms of corrosion resistance, the C content may be less than 0.09%, and in some cases, the C content may be less than 0.08% in order to further improve casting cracking and reduce carbon equivalent.

Silicon (Si): 0.1% or more but less than 0.8%

Si is an element that not only acts as a deoxidizing agent but also exerts a role of increasing the strength of steel, and is required at least 0.1% in order to exhibit its effect. In addition, Si is advantageous to increase the content because it contributes to the improvement of the corrosion resistance of the front surface, but when the Si content is more than 0.8%, toughness and weldability are inhibited and surface peeling due to scale is difficult due to difficulty in peeling of the scale during rolling. Causes Therefore, in the present invention, the Si content may be limited to 0.1% or more and less than 0.8%. Meanwhile, in some cases, in order to improve corrosion resistance, the Si content may be limited to 0.2% or more.

Manganese (Mn): 0.3% or more and 0.9% or less

Mn is an effective component to increase strength through solid solution strengthening without lowering toughness. However, when added in excess, corrosion resistance may be reduced by increasing the electrochemical reaction rate of the steel surface during corrosion reaction. When the Mn is added to less than 0.3%, there is a problem that it is difficult to secure the durability of the structural steel. On the other hand, if the Mn content increases, the quenching property increases and the strength increases, but if it is added more than 0.9%, the segregation part in the center of thickness is greatly developed during slab casting in the steelmaking process, and the weldability decreases. There is a problem of deteriorating corrosion resistance. Therefore, in the present invention, it is preferable to limit the Mn content to 0.3% or more and 0.9% or less.

Chromium (Cr): 1.2% or more but less than 1.5%

Cr is an element that increases corrosion resistance by forming an oxide film containing Cr on the surface of a steel in a corrosive environment. In the seawater environment, it should be contained at least 1.2% in order to sufficiently exhibit the corrosion resistance effect by adding Cr. However, if the Cr is excessively contained in 1.5% or more, it adversely affects toughness and weldability, so it is preferable to limit the content to 1.2% or more and less than 1.5%.

Copper (Cu): 0.1% or more but less than 0.5%

When Cu is contained in an amount of 0.1% or more together with Ni, the elution of Fe is delayed, and thus it is effective in improving the resistance to front corrosion and local corrosion. However, when the Cu content is added more than 0.5%, when the slab is manufactured, liquid Cu melts into the grain boundary and causes a hot shortness phenomenon that causes cracks during hot working, so the Cu content in the present invention is 0.1% or more. It is preferred to limit it to less than 0.5%. On the other hand, since the surface cracks generated during slab production interact with each other with the C, Ni, and Mn contents, the frequency of surface cracking may vary depending on the content of each element, but the possibility of surface cracking is generated regardless of the content of the elements. In order to minimize, it is more preferable to make the Cu content less than 0.45%.

Aluminum (Al): 0.01% or more but less than 0.08%

Al is an element added for deoxidation, and is an element that reacts with N in steel to form AlN to refine austenite grains to improve toughness. The Al is preferably contained at least 0.01% in the dissolved state for sufficient deoxidation. On the other hand, if Al is excessively contained in an amount of 0.08% or more, an inclusion is formed in a coarse oxide in the steelmaking process, and a stretched inclusion that is broken and elongated during rolling is formed according to the characteristics of the aluminum oxide system. The formation of such stretched inclusions promotes the formation of cavities around the inclusions, and these cavities act as a starting point for local corrosion, thereby inhibiting local corrosion resistance. Therefore, in the present invention, it is preferable to limit the Al content to 0.01% or more and less than 0.08%.

Titanium (Ti): 0.01% or more but less than 0.1%

When Ti is added at 0.01% or more, TiC is formed by bonding with carbon in steel to improve strength by precipitation strengthening. On the other hand, when the Ti content is added at 0.1% or more, the effect of improving strength compared to the increase of the content is not significant. Therefore, in the present invention, the Ti content may be limited to 0.01% or more and less than 0.1%.

Nickel (Ni): 0.05% or more but less than 0.1%

Ni is effective in improving the corrosion resistance of the entire surface and the local corrosion when 0.05% or more is contained like Cu. In addition, when added together with Cu, it reacts with Cu to suppress the formation of a Cu phase with a low melting point, thereby suppressing hot shortness. In most Cu-added steels, it is common to add Ni at least 1 times the Cu content. As in the present invention, when the content of elements related to carbon equivalents such as C and Mn is low and the Cr content is large, even shorter than half of the Cu content can sufficiently prevent shortness, and since Ni is an expensive element, considering the relative input effect It is desirable to limit the upper limit of the Ni content to less than 0.1%.

Niobium (Nb): 0.002% or more but less than 0.07%

Nb is an element that plays a role of precipitation strengthening by forming NbC in combination with carbon in steel, such as Ti, and effectively improves strength when added at 0.002% or more. However, when the content is added more than 0.07%, the effect of improving the strength compared to the increase of the content is not so great. Therefore, in the present invention, the Nb content is preferably limited to 0.002% or more and less than 0.07%.

Phosphorus (P): 0.03% or less

P is present as an impurity in the steel, and when its content is added in excess of 0.03%, the weldability is remarkably lowered and toughness deteriorates. Therefore, it is desirable to limit the P content to 0.03% or less. On the other hand, since P is an impurity, the lower the content, the more advantageous, so the lower limit may not be limited.

Sulfur (S): 0.02% or less

S is present as an impurity in the steel, and when its content exceeds 0.02%, there is a problem of deteriorating the ductility, impact toughness and weldability of the steel. Therefore, in the present invention, it is preferable to limit the S content to 0.02% or less. Particularly, it is more preferable to limit the content to 0.01% or less, because S reacts with Mn to easily form a stretched inclusion, such as MnS, and the pores present at both ends of the stretched inclusion can be local corrosion starting points. On the other hand, since S is an impurity, the lower the content, the more advantageous, so the lower limit may not be limited.

The high-strength structural steel of the present invention is an iron (Fe) component other than the above-mentioned alloying elements. However, in the normal manufacturing process, impurities that are not intended from the raw material or the surrounding environment can be inevitably mixed, and therefore cannot be excluded. Since these impurities are known to anyone skilled in the art, they are not described in detail.

On the other hand, the high-strength structural steel according to one aspect of the present invention is a microstructure with an area fraction of 20% or more of bainite, less than 80% of polygonal ferrite and acicular ferrite in total, and pearlite and MA as other phases. Site) may be less than 10%.

In order to use it as a high-strength structural steel material, it is necessary to secure a strength of at least 500Mpa, typically 600Mpa or more, and for this purpose, 20% or more of bainite as a microstructure and other polygonal and/or acicular ferrite-oriented structures were constructed. In addition, the upper limit was limited to less than 10% because other merchants, pearlite and MA, may have low temperature toughness and corrosion resistance in the environment in which the structural steel according to the present invention is used when 10% or more is included.

High-strength structural steel according to an aspect of the present invention can have a yield strength of 500 MPa or more and a tensile strength of 600 MPa or more by satisfying the above-described component system and microstructure.

Next, a method of manufacturing high strength structural steel according to another aspect of the present invention will be described in detail.

[Method of manufacturing high strength structural steel]

The method for manufacturing high-strength structural steel according to another aspect of the present invention consists of slab reheating-hot rolling-cooling, and detailed conditions for each manufacturing step are as follows.

Slab reheating stage

First, a slab made of the above-described component system is prepared, and the slab is reheated to a temperature range of 1000 to 1200°C. It is more preferable to heat the reheating temperature to 1000°C or higher to solidify the carbonitride formed during casting and to 1050°C or higher to sufficiently solidify the carbonitride. On the other hand, when reheating to an excessively high temperature, there is a fear that austenite may be coarse, so the reheating temperature is preferably 1200°C or less.

Hot rolling stage

Hot rolling including rough rolling and finishing rolling may be performed on the reheated slab. At this time, the finishing rolling is preferably completed at a finishing rolling temperature of 750°C or higher. When the finish rolling temperature is less than 750°C, a problem that a large amount of air-cooled ferrite is generated may occur. On the other hand, when the finish rolling temperature exceeds 950°C, strength and toughness may be reduced due to coarsening of the tissue. Therefore, in the present invention, it is preferable to limit the finish rolling temperature to 750 to 950°C.

Cooling stage

Forced cooling is carried out through water cooling for the hot-rolled steel. In the present invention, it is a core technology to secure high strength even in a thick material through sufficient cooling, and cooling to a temperature of 700° C. or less at a cooling rate of 10° C./s or more is required to prevent tissue coarsening. Further, the cooling may be started at a cooling start temperature of 750°C or higher. However, when cooling to a temperature below 400℃, micro-cracks may be generated in the center by the rapid cooling process, and it may cause product surface and core material deviation and product front/rear material material deviation. It is desirable to end. On the other hand, the upper limit of the cooling rate is mainly related to the capacity of the facility, and if it is 10°C/s or more, there is no significant change in strength even if the cooling rate increases, so the upper limit of the cooling rate may not be limited.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the items described in the claims and the items reasonably inferred therefrom.

(Example)

First, a molten steel having a component system shown in Table 1 was prepared, and then slabs were prepared using continuous casting. Subsequently, the slabs were reheated, hot rolled, and cooled under the manufacturing conditions in Table 2 to prepare steel sheets.

Table 3 shows the area fraction of each phase by observing the microstructure with an optical and electron microscope for the prepared steel sheet, and measuring the yield strength and tensile strength through a tensile test. In addition, as an evaluation of the characteristics of seawater, after immersing it in a 3.5% NaCl solution simulating seawater for a day, put it in an ultrasonic cleaner with 50% HCl+0.1% Hexamethylene tetramine solution, wash the specimen, measure the weight loss, and divide it into the initial specimen surface area The corrosion rate was calculated, and to compare the corrosion rate of the comparative steel and the invention steel, the relative corrosion rate was comparatively evaluated based on the corrosion rate of the comparative steel 1 as 100, and the results are shown in Table 3.

division C Si Mn P S Sol.Al Cu Ni Cr Nb Ti Invention Steel 1 0.041 0.27 0.8 0.008 0.005 0.022 0.43 0.08 1.2 0.044 0.015 Invention Steel 2 0.035 0.44 0.9 0.018 0.007 0.024 0.28 0.09 1.4 0.032 0.018 Comparative Steel 1 0.068 0.51 1.8 0.008 0.007 0.029 0.12 0.06 0.4 0.009 0.019 Comparative Steel 2 0.092 0.27 2.1 0.009 0.005 0.042 0.04 0.08 0.2 0.026 0.024 Comparative Steel 3 0.049 0.51 2.2 0.018 0.007 0.024 0.07 0.12 0.7 0.047 0.021

division Reheating temperature
(℃) Finish rolling temperature
(℃) Cooling start temperature
(℃) Cooling end temperature
(℃) Cooling rate
(℃/sec) Invention Steel 1 1142 870 774 542 36 Invention Steel 2 1129 901 784 614 42 Comparative Steel 1 1119 875 769 565 15 Comparative Steel 2 1124 891 771 579 23 Comparative Steel 3 1151 880 773 612 14

division Area fraction of microstructure (%) Yield strength
(MPa) The tensile strength
(MPa) Relative corrosion
speed Bainite Polygonal ferrite + needle ferrite Other awards
(Perlite, MA) Invention Steel 1 71 27 2 564 636 66 Invention Steel 2 68 30 2 512 608 64 Comparative Steel 1 64 34 2 574 671 100 Comparative Steel 2 57 39 4 564 659 137 Comparative Steel 3 63 33 4 592 702 143

As can be seen from Table 1, the invention steels 1 and 2 both satisfy the component ranges defined in the present invention, whereas comparative steels 1 to 3 have the component ranges of Cr, Cu, Ni or Mn of the present invention. Out of range

As a result, the invention steels 1 and 2 have a microstructure having a low-temperature structure of 20% or more of bainite on a ferrite base, and have a high strength of 500 MPa or more in yield strength and 600 Mpa or more in tensile strength, thereby providing sufficient materials for structural steel. In addition, it was confirmed that by satisfying the component range specified in the present invention, it exhibits a lower corrosion rate compared to Comparative Steel 1, and thus has a sufficient life in an atmosphere of seawater.

On the other hand, the comparative steels 1 to 3 are Cr, Cu, Ni or Mn as the component range is out of the scope of the present invention, despite being prepared by a manufacturing method that satisfies the manufacturing conditions of the present invention, can be seen in Table 3 above. As a result, it exhibited a high corrosion rate of 100 or more relative corrosion rate, and as a result, it did not have a sufficient life in the seawater atmosphere.

Although described with reference to the above embodiments, those skilled in the art understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below. Will be able to.

Claims (6)

In weight percent, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and less than 0.9%, Cr: 1.2% or more and less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al : 0.01% or more and less than 0.08%, Ti: 0.01% or more and less than 0.1%, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, balance Fe and Contains inevitable impurities,
High-strength structural steel with microstructure having an area fraction of 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 other phases.
According to claim 1,
The C is a high-strength structural steel, characterized in that contained more than 0.03% and less than 0.09%.
According to claim 1,
The Si is a high-strength structural steel, characterized in that contained 0.2% or more and less than 0.8%.
According to claim 1,
The Cu is a high-strength structural steel, characterized in that contained more than 0.1% and less than 0.45%.
According to claim 1,
The high strength structural steel is a high strength structural steel, characterized in that it has a yield strength of 500MPa or more and a tensile strength of 600MPa or more.
In weight percent, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and less than 0.9%, Cr: 1.2% or more and less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al : 0.01% or more and less than 0.08%, Ti: 0.01% or more and less than 0.1%, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, balance Fe and Reheating the slab containing inevitable impurities at a temperature of 1000°C or higher and 1200°C or lower;
Hot rolling the reheated slab to a finishing rolling temperature of 750°C or higher and 950°C or lower; And
Cooling the rolled steel sheet at a cooling rate of 10° C./sec or more from a cooling start temperature of 750° C. or higher to a cooling end temperature of 400-700° C.;
Method of manufacturing a high-strength structural steel comprising a.

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