KR101126019B1 - Producing method of weather resistable steel having excellent toughness and high strength for using at the seaside atmosphere - Google Patents

Abstract

The present invention relates to a weather-resistant steel for the beach and a method of manufacturing the same, which exhibits excellent toughness and strength, and more particularly, does not cause brittle fracture even at low temperatures, and has excellent corrosion resistance and corrosion resistance in a corrosive environment having a high concentration of chlorine ions. It relates to a high strength weather resistant steel and a method of manufacturing the same. In the present invention, by weight%, C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, Cu: 0.2-1.0%, Ni: 0.2-5.0%, Al: 0.001-0.1%, P: 0.03 % Or less, S: 0.002 to 0.03%, Ca: 0.001 to 0.01%, Nb: 0.02 to 0.07%, V: 0.1% or less and B: 0.003% or less, remaining Fe and other unavoidable impurities, , S, Al, Si is the following relation 1,

[Relationship 1] Ca (%) / S (%)> [1.5Al (%) + 2Si (%)]

After satisfying the requirements, hot-rolled steel with excellent weathering resistance, containing 30% or more of water-soluble CaS inclusions in the Ca-based nonmetallic inclusions in the steel, and having a cumulative reduction ratio of 30% or more under the austenitic uncrystallized region, It is characterized by cooling to a cooling stop temperature satisfying the following relational formula 4 at a satisfactory cooling rate.

[Relationship 3] Cooling Rate (℃ / sec) ≥ 9.7 / {[% Mn] + [% Cu] + [% Ni] + 20 × ([% Si] / 30 + [% Mo] / 10)}

[Relationship 4] Cooling stop temperature (℃) ≤ 865-272 x [% C]-74 x [% Mn]-9 x [% Mo]-5 x [% Cu]

High strength, high toughness, weather resistance, rolling reduction, austenite recrystallization temperature

Description

PRODUCING METHOD OF WEATHER RESISTABLE STEEL HAVING EXCELLENT TOUGHNESS AND HIGH STRENGTH FOR USING AT THE SEASIDE ATMOSPHERE}

The present invention relates to a weather-resistant steel for the beach and a method of manufacturing the same, which exhibits excellent toughness and strength, and more particularly, does not cause brittle fracture even at low temperatures, and has excellent corrosion resistance and corrosion resistance in a corrosive environment having a high concentration of chlorine ions. It relates to a high strength weather resistant steel and a method of manufacturing the same.

Conventional weathering steels contain a small amount of elements such as Cu, Cr, and P, and are known to have atmospheric corrosion resistance of 4 to 8 times that of general steel. The reason why the weathering steel has better corrosion resistance than that of general steel is that during the initial period when the weathering steel is exposed to the atmosphere, rust is generated and released or peeled off, but as the period of time passes, a part of the rust is gradually deposited on the base metal. This is because they form a stable stable rust, and the rust layer acts as a protective film against the corrosive environment. Such stable rust that can act as a protective film against corrosion environment is known as iron hydroxide (FeOOH) or α-iron hydroxide (α-FeOOH) having an amorphous structure.                         

However, the conventional weathering steel as described above is known to exhibit excellent corrosion resistance after a long period of three years or more in a general atmospheric corrosion environment, that is, a power plant zone or a plant zone containing SO ₂ , but relatively chlorine ion concentration In high coastal areas or in areas where sodium chloride is applied as a cryoprotectant, there is no significant difference over ordinary rivers over time. The reason is that the chlorine ions attached to the steel surface promote acidification of the water film formed on the steel surface, which increases the overall corrosion rate of the steel and prevents the formation of stable rust, and also forms defective corrosion products such as β-FeOOH. Because it is. Therefore, in order to improve weather resistance in coastal areas where the concentration of chlorine ions is high or in areas where sodium chloride is applied, it is necessary to form β-FeOOH and to form α-FeOOH having a dense structure to form a complete passivation film.

The structure of the rust layer formed during the exposure to the atmosphere is affected by the corrosive environment, and the higher the pH of the water film formed on the corrosive environment, that is, the surface of the steel, the formation of β-FeOOH is suppressed and the formation of α-FeOOH is promoted. However, in practice, since it is very difficult to control the pH by controlling the atmospheric environment, attempts have been made to increase the pH of the water film formed on the steel surface by adding basic elements to the steel.

Examples of the prior art of increasing weather resistance by adding a basic element such as Ca or Mg to increase the pH of the steel surface include Japanese Patent Application Laid-Open Nos. Hei 2-125839 and Hei 5-51668. Japanese Unexamined Patent Application Publication No. 2-125839 discloses a method of improving weather resistance by adding Ca to steel to form a (Al, Ca) O composite oxide. A method of increasing the pH of a steel surface by adding a pretreated powder obtained by kneading CaO particles having a particle size of 1 μm or less and iron powder having an average particle size of 200 μm or less is proposed.

In the prior art, the inclusions produced by adding Ca in Japanese Patent Application Laid-Open No. 2-125839 are composed of Ca-based composite oxides instead of CaO single oxides, even if directly added in CaO form as in Pyeong 5-51668. The injected CaO reacts easily with other oxides such as Al ₂ O ₃ and SiO ₂ present in molten steel, and thus compounds such as liquid inclusions (Al, Si, Ca) O, CaO-2Al ₂ O ₃ , CaO-6Al ₂ O ₃ Will exist. As a result of investigating these prior arts, the present inventors have found that Ca-based composite oxides do not dissolve in water and thus do not contribute significantly to the increase of the pH of the steel surface, and thus the effect of improving weather resistance is insignificant.

In addition, the present inventors also added Ca-Si wire in molten steel to form a Ca-based complex emulsion and Ca-based composite oxide in the steel to increase the pH of the steel surface to improve the weatherability of the Republic of Korea Patent Publication No. 2002-25571 The issue was presented. However, in the prior art, it is possible to form the inclusions in the steel composed of Ca-based composite oxide and Ca-based complex emulsion by Ca-Si addition, but when checking the fraction of each of these inclusions, about 95% is Ca-based composite Due to the presence of oxides, the effect on the increase of pH of steel surface was insignificant, which did not contribute to the improvement of weather resistance.

In addition, when the weathering steel is used at a low temperature, the weather resistance and low temperature toughness that does not cause brittle fracture at the low temperature is required at the same time, and since the main use of the weathering steel is a bridge, a structure, etc., the strength is gradually increased. In other words, when the river is used in the cold or winter, a large amount of cryoprotectant, such as calcium chloride, is used and may be exposed to a corrosive environment, and the impact transition temperature (DBTT) at which the steel starts to cause brittle fracture at low temperatures should be reduced as much as possible. It is necessary to, and due to the demand for light weight of the structure it is necessary to achieve high strength.

Accordingly, an object of the present invention is to provide a weather-resistant steel for seashore that satisfies both high strength and high toughness with a tensile strength of 50kgf / mm ² or more and an impact transition temperature of -50 ° C or less.

In order to achieve the above object, the present invention, in weight%, C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, Cu: 0.2-1.0%, Ni: 0.2-5.0%, Al: 0.001 0.1% or less, P: 0.03% or less, S: 0.002 to 0.03%, Ca: 0.001 to 0.01%, Nb: 0.02 to 0.07%, V: 0.1% or less and B: 0.003% or less, with the remaining Fe and other unavoidable impurities In the composition, Ca, S, Al, and Si are represented by the following relationship 1,

[Relationship 1]                     

Ca (%) / S (%)> [1.5Al (%) + 2Si (%)]

After satisfying the requirements, hot-rolled steel with excellent weathering resistance, containing 30% or more of water-soluble CaS inclusions in the Ca-based nonmetallic inclusions in the steel, and having a cumulative reduction ratio of 30% or more under the austenitic uncrystallized region, It is characterized by cooling to a cooling stop temperature satisfying the following relational formula 4 at a satisfactory cooling rate.

[Relationship 3]

Cooling rate (℃ / sec) ≥ 9.7 / {[% Mn] + [% Cu] + [% Ni] + 20 × ([% Si] / 30 + [% Mo] / 10)}

[Relationship 4]

Cooling stop temperature (℃) ≤ 865-272 × [% C]-74 × [% Mn]-9 × [% Mo]-5 × [% Cu]

At this time, the steel material, Ti: 0.005 ~ 0.1%, Mo: 0.01 ~ 1.0% and W: preferably comprises at least one member selected from the group consisting of.

In addition, the reduction ratio below the austenite uncrystallized region of the steel is preferably 90% or less.

Hereinafter, a characteristic configuration of the present invention will be described in detail.

The present invention has the emphasis and characteristics described in Korean Patent Application No. 10-2004-0040356 filed by the applicant of the present invention with the emphasis on the weather resistance of the beach used in the environment of high chlorine ions such as cold or cold beach where a large amount of antifreeze agent is sprayed Use it as is.

First, the specific mechanism (mechanism) for securing the weather resistance for the beach of the steel in the present invention is as follows.

In general, the weathering steel is initially formed with a rust layer, such as γ-FeOOH or Fe ₃ O ₄ , these rust layer is leaked and peeled off not only deteriorate weather resistance but also poor initial landscape. In particular, in coastal areas where the proportion of salinity is high, β-FeOOH, which is worse in weatherability, is generated in addition to these rust layers, thereby harming weatherability and initial landscape. However, when the pH of the steel surface is increased, the formation of a rust layer such as β-FeOOH, γ-FeOOH, and Fe ₃ O ₄ is suppressed and the formation of α-FeOOH, which is a stable rust that can contribute to improving weather resistance, is encouraged.

For this reason, conventionally, Ca was added to steel materials to raise the pH of the initial steel surface. However, when Ca is added to molten steel as described above, the added Ca reacts with Si, Al, and O present in the steel rather than forming a water-soluble CaO or CaS that raises the pH of the aqueous solution (Al, Si, Ca (O) complex oxide (90% or more) is formed. In addition, these inclusions are larger than 3 µm in size, and their distribution does not appear uniformly. Moreover, the composite oxide thus formed is not soluble in water and therefore does not contribute to raising the pH of the steel surface.

Therefore, the present inventors have repeated the research and experiment to overcome the problems of the prior art, as a result of optimizing the components of Ca, S, Al, Si, etc. affecting the formation of inclusions in the steel, as well as Ca input method By optimizing and separating the Ca-based composite oxides from the inclusions during the steelmaking process, weather-resistant steels with excellent weatherability with a fraction of 30% or more of the water-soluble CaS inclusions in the Ca-based nonmetallic inclusions in the final product were produced.

Hereinafter, the reason for setting the composition of the weathering steel of the present invention will be described.

C: 0.15 wt% or less

The C is an element added to increase the strength to increase the hardenability by increasing the content, but if the content exceeds 0.15% by weight, the weldability is impaired, so that the content is limited to 0.15% by weight or less. It is preferable.

Si: 1.0 wt% or less

The Si is a component that not only acts as a deoxidizer but also increases the strength of the steel. If the content of Si exceeds 1.0% by weight, there is a problem of inhibiting toughness and weldability and forming a complex oxide that does not dissolve in water by reacting with Ca, thereby reducing the effect of increasing weather resistance due to the addition of Ca. It is preferable to limit it to% or less.                     

Mn: 2.0 wt% or less

The Mn is an effective ingredient to increase the strength without decreasing the toughness, and as the content is increased, the hardenability increases to increase the strength, but when it is added in excess of 2.0% by weight, the weldability decreases, so the content thereof is 2.0. It is preferable to limit it to weight% or less.

Cu: 0.2-1.0 wt%

The Cu is an effective component to induce refinement and densification of rust layer particles and to improve weather resistance of steel. When the content of Cu is less than 0.2% by weight, it is difficult to expect the weathering resistance improving effect. When the content of Cu is more than 1.0% by weight, the weathering resistance improving effect is saturated, and when reheating the slab for rolling, Cu having low melting point penetrates into the grain boundary of steel and is hot. Cracking may occur during processing, so the content is preferably limited to 0.2 to 1.0% by weight.

Ni: 0.2-5.0 wt%

Ni is one of the important elements contributing to the improvement of weather resistance, and when Ni is added, the amorphous rust layer or α-FeOOH rust layer is refined and densified to suppress the permeation of the material through the rust layer, thereby contributing to the improvement of weather resistance, and especially high proportional salt content. Effective for improving weather resistance in coastal areas. If the content of Ni is less than 0.2% by weight, the effect of the addition cannot be obtained. If the content of Ni is more than 5.0%, the effect of improving weather resistance is not only saturated but also the manufacturing cost is increased by adding a large amount of expensive Ni. Therefore, it is preferable to limit the content to 0.2 to 5.0% by weight.

Al: 0.001-0.1 wt%

Al is an element that is essentially added for deoxidation during steelmaking, and improves shock absorption energy, but, like Si, causes Al to inhibit the effect of improving weather resistance of Ca because it forms a complex oxide that does not dissolve in water. . If the content of Al is less than 0.001% by weight, deoxidation is not made sufficiently. If the content of Al exceeds 0.1% by weight, there is a problem of impairing impact toughness. Therefore, the content is preferably limited to 0.001 to 0.1% by weight.

P: 0.03 wt% or less

P is an effective component for improving the weather resistance by inhibiting the rust permeation of the rust layer by increasing the cation selective permeability of the rust layer by forming PO ₄ ^3- ions in aqueous solution when present in the steel. However, if the content is added in excess of 0.03% by weight, not only the weldability is significantly lowered but also the toughness deteriorates, so it is preferable to limit the content to 0.03% by weight or less.

S: 0.002-0.03 wt%                     

In general, S acts as a starting point of corrosion by forming MnS by reacting with Mn. However, as shown in the present invention, S reacts with Ca to form CaS that can be dissolved in an aqueous solution to increase pH, thereby improving weather resistance. Will contribute. In order to obtain a pH synergistic effect by CaS, at least 0.002% by weight or more should be added, but if the content exceeds 0.03% by weight, there is a problem of deteriorating impact toughness and weldability. Therefore, it is preferable to limit the content to 0.002 to 0.03% by weight.

Ca: 0.001-0.01 wt%

The Ca reacts with Al, Si, and O in molten steel to form a complex oxide of (Al, Si, Ca) O form, and then reacts with S to form CaS. The coarse complex oxide not only contributes to the increase in pH at all, but also has no effect on improving weather resistance, but rather impairs impact toughness. However, fine CaS inclusions are dissolved in water to raise the pH of the steel surface to promote stable rust formation and contribute to improving weather resistance. The Ca should be added at least 0.001% by weight or more in order to anticipate the effect of improving the weather resistance due to CaS formation, but if it exceeds 0.01% by weight, there is a problem that causes melting of the refractory during the steelmaking process, the content is 0.001 to 0.01 weight It is desirable to limit to%.

In the present invention, at least one element selected from the group consisting of Nb, V and V is further added to the above components.                     

Nb: 0.02-0.07 wt%

The Nb is a very useful component to refine the grains and at the same time serves to greatly enhance the strength. If the content of Nb is less than 0.02% by weight, the effect of the addition may not be obtained. If the content of Nb is more than 0.07% by weight, coarse Nb precipitates are formed, which may cause cracking in the steel and also impair impact characteristics. do. Therefore, the content of Nb is preferably limited to 0.02 ~ 0.07% by weight.

V: 0.1 wt% or less

V is useful for improving strength by forming carbonitride, like Nb, but when the amount exceeds 0.1% by weight, the strength increase effect is saturated, but rather, the impact property of steel is impaired. Therefore, the content of V is preferably limited to 0.1% by weight or less.

B: 0.003 wt%

B is a component effective for improving the strength of the steel by remarkably increasing the hardenability of the steel even with a small amount of addition. When the content of B exceeds 0.003% by weight, Fe ₃ B is formed to cause redembrittlement and also inhibit weldability. Therefore, the content is preferably limited to 0.003% by weight or less.

In addition to the above components, the remainder comprises Fe and other unavoidable impurities.

Further, in the present invention, it is possible to ensure more excellent weather resistance by further adding at least one element selected from the group consisting of Ti, Mo and W to the above-mentioned components.

Ti: 0.005 to 0.1 wt%

The Ti forms fine oxides and nitrides in steel, and since they have a high melting point, they do not dissolve even at reheating temperatures or higher, thereby inhibiting austenite grain growth due to a pinning effect, thereby contributing to the improvement of impact toughness. In addition, the fine Ti oxide acts as a starting point for reacting with Ca to induce a fine distribution of inclusions. If the content of Ti is less than 0.005% by weight, the effect of the addition cannot be obtained. If the content of Ti is more than 0.1% by weight, the amount of Ti dissolved increases, which impairs the toughness of the base material and the welded part. It is preferable to limit it to 0.1 weight%.

Mo: 0.01-1.0 wt%

Mo is a component that is effective in improving strength as well as improving weather resistance by forming MoO ₄ ^2- ions in aqueous solution to suppress permeation of chlorine ions. When the content of Mo is less than 0.01% by weight, the effect of the addition cannot be obtained, and when the content is more than 1.0% by weight, the effect is saturated, so it is preferable to limit the content to 0.01 to 1.0% by weight.

W: 0.01-1.0 wt%

W, like Mo, is an effective component for improving beach weatherability by forming WO ₄ ^2- ions in aqueous solution to inhibit the permeation of chlorine ions through the rust layer. If the content of W is less than 0.01% by weight, the effect of the addition is not obtained, and if the content exceeds 1.0% by weight, the effect is saturated, so the content is preferably limited to 0.01 to 1.0% by weight.

In addition to the above appropriate emphasis, it is necessary to form inclusions of an appropriate composition for the formation of α-FeOOH, which is a stable rust. The inclusion to be formed in the present invention is a CaS-based inclusion as described above, Ca not only forms CaS contributing to the improvement of weather resistance, but also forms a composite oxide that does not help to improve weather resistance. Therefore, in order to suppress the formation of the composite oxide and increase the formation of CaS inclusions, it is necessary to appropriately control the addition amount of Ca, S, Al, and Si. Equation 1 below is an empirical formula derived from the inventor's repeated experiments as a relationship for inhibiting the composite oxide and increasing CaS inclusion formation.                     

[Relationship 1]

Ca (%) / S (%)> [1.5Al (%) + 2Si (%)]

Even if the component range of the present invention is satisfied, if the above relation 1 is not satisfied, the fraction of CaS inclusions in the Ca-based nonmetallic inclusions in the final product cannot be controlled to a desired value. That is, by controlling the Ca, S, Al, and Si content to satisfy the above relationship 1, the fraction of CaS inclusions in the Ca-based non-metallic inclusions can be appropriately controlled, thereby improving the weather resistance of the present invention.

Next, the internal structure of the weather resistant steel for beach excellent in the said strength and toughness is demonstrated.

The steel having the weathering steel composition of the present invention, which satisfies both high strength and high toughness, preferably has a grain size of 20 µm or less. The reason is that in order to increase the low temperature toughness of the steel, the grain boundary area that interferes with the propagation during crack propagation should be wide. For this purpose, fine grains must be formed inside the steel. This is because the strength increase effect due to the effect can be obtained.

Next, the method of manufacturing the steel material excellent in the said weather resistance is demonstrated.                     

In order to increase the toughness and strength of steel having excellent weather resistance, the size of austenite grains should be finely formed after rolling. The reason is that, as described above, in order to increase the toughness and strength of the steel, fine grains must be formed in the steel, but in order to refine the grains as described above, the grains after rolling are preferably refined. That is, since the crystal grains formed in the steel after cooling are formed using the austenite grain boundaries before cooling as nucleation sites, the number of nuclei of the crystal grains is increased as the number of nucleation sites increases. As is increased, the grain size can be refined.

The inventors of the present invention have studied diligently to this end, and by sufficiently rolling the steel in the region below the austenite uncrystallized region, it is possible to make the austenitic pancake (pancake) to finer the structure formed at a lower temperature afterwards It was found to be effective in improving low temperature toughness and strength. Based on this, using the austenite pancake effect, the reduction ratio of the region below the austenite uncrystallized zone to obtain a fine final structure of 20 µm or less as defined above was found. It was found that the larger the reduction ratio of the region, the more favorable.

In addition, the strength of the steel can be obtained due to the energy introduced into the steel during processing and the residual stress due to tissue transformation during cooling, in addition to the above grain refining effect, the potential accumulation is higher as the grain is smaller as described above High strength can be achieved, and the amount of energy introduced into the steel should be minimized by the recovery in the subsequent cooling process. When the reduction ratio of the region below the austenite uncrystallized region becomes 30% or less, the rolling end temperature is high. The cumulative reduction ratio should be 30% or more because the cooling process after rolling is prolonged and energy is easily dissipated by dislocation recovery.

For the above reason, the cumulative reduction ratio below the austenite uncrystallized region should be more than 30%. In addition, as described above, the higher the cumulative reduction ratio below the austenite unrecrystallized region, the more effective. However, when the cumulative reduction ratio is 90% or more, the upper limit is preferably 90% due to the increase in facility load and economic burden.

In addition, the rolled steel is preferably water-cooled after rolling. The reason is that water cooling to steel increases the number of nucleation of ferrite inside austenite grains, and as a result, the internal grain refining effect is increased, and thus the low temperature toughness can be further improved. This is because the high strength can be achieved.

At this time, the lower limit of the water cooling rate of the steel can be adjusted in consideration of the hardenability of the steel. In other words, steel materials may be advantageous to develop low-temperature tissues by composition, and may be disadvantageous. In the case where the low-temperature tissues are advantageous to develop, high-strength steels may be obtained even when low-temperature cooling is performed by low water cooling rate. This is because in the following disadvantages, high strength steels may be obtained by performing stronger cooling.

The inventors of the present invention have studied the ease of development of the low-temperature tissues, and as a result, the extent to which such low-temperature tissues are easy to be produced tends to be determined by the content of manganese, which is a representative quenching enhancement element, and thus the Representing the characteristics as a representative value, it was found that the reduction of the other elements to the manganese equivalent value (Mn_eq) of the following relation 1, which plays the same role as manganese, can define the ease of low temperature tissue development by steel composition.

[Relationship 2]

Mn_eq = [% Mn] + [% Cu] + [% Ni] + 20 × ([% Si] / 30 + [% Mo] / 10)

As described above, since the manganese equivalent value (Mn_eq) is an index indicating the hardenability of steel, a comparison between the manganese equivalent value and the cooling rate for satisfying the strength, toughness, and yield ratio desired in the present invention, It was found that the cooling rate of the steel should be cooled above the required cooling rate expressed by the following Equation 3.

[Relationship 3]

Required cooling rate (℃ / sec) = 9.7 / Mn_eq                     

In addition to the above cooling rate, the cooling stop temperature should also be below a certain temperature in order for the low temperature structure to be sufficiently developed. Since the upper limit of the cooling stop temperature is determined according to the degree of quenching of the steel in the same manner as the reason for defining the cooling rate, the cooling stop temperature may be represented by the following Equation 4 according to the composition of the steel.

[Relationship 4]

Upper limit of cooling stop temperature (℃) = 865-272 × [% C]-74 × [% Mn]-9 × [% Mo]-5 × [% Cu]

Hereinafter, with reference to the accompanying drawings and preferred embodiments of the present invention will be described, but the following examples are not intended to limit the scope of the present invention but only one preferred embodiment of the present invention.

(Example)

Slabs were manufactured through a steelmaking-continuous casting process to produce weather resistant steel for the beach having the composition of the present invention.

In particular, in the steelmaking process, in order to satisfy the relationship of Ca (%) / S (%)> [1.5Al (%) + 2Si (%)], a wire containing Ca-Si powder is divided into two portions and In addition, Ca initially injected reacts with the oxygen remaining in the molten steel to form a composite oxide, and Ca injected second time forms an emulsion to finally form an acid emulsion.

Subsequently, the slab was manufactured through continuous casting, and in order to eliminate component deviations, the first slab (super cast) and the last slab (end cast) produced during continuous casting from the steel supplied from one ladle of the steelmaking process. A total of seven casts were prepared. These slabs may be regarded as having the same slab internal quality or composition, thereby minimizing analytical deviations generated by slabs in the steelmaking and continuous casting stages.

The composition of the slab prepared by the above process and used in the experiment of the present invention is shown in Table 1 below.                     

[Table 1]

Steel grade Ingredient Content (wt%) Tnr ① ② ③ Stop temperature
(℃) C Si Mn Cu Ni Al P S Ca Nb V B Ti Mo W A 0.08 0.2 0.7 0.3 0.9 0.015 0.013 0.002 0.0047 0.03 - - - 998.0 2.35 0.42 4.8 789.9 B 0.095 0.27 0.73 0.32 0.95 0.01 0.014 0.003 0.0058 0.03 0.01 - - 938.8 1.93 0.56 4.4 783.5 C 0.0
87 0.3
4 0.9 0.3
5 0.9
8 0.0
13 0.0
13 0.0
02 0.0
042 0.02 0.01 0.02 - 0.2 979.2 2.1 0.70 3.9 773.0 D 0.0
85 0.2
7 0.8
7 0.3
8 1.0
One 0.0
05 0.0
12 0.0
05 0.0
036 0.02 0.001 - - 0.
2 931
.3 0.72 0.55 4.0 775.6 E 0.092 0.31 0.8 0.38 1.0 0.01 0.013 0.003 0.0049 0.03 0.001 0.01 0.2 - 1003.4 1.63 0.64 3.5 777.1 F 0.079 0.25 1.0 0.35 1.02 0.011 0.013 0.003 0.0041 0.02 0.02 - 0.2 - 978.6 1.37 0.52 2.9 766.0 G 0.081 0.31 0.8 0.41 0.98 0.009 0.012 0.006 0.0042 0.001 - 0.4 - 923.5 0.7 0.63 3.2 778.1 ① = Ca / S, ② = 1.5Al + 2Si, ③ = 9.7 / Mn_eq (Cooling rate lower limit, ℃ / sec)
Tnr: Austenitic recrystallization temperature (℃)
Stopping temperature: It means upper limit of cooling stop temperature in relation (3)
However, 7 slabs are collected for each steel grade.

The composition of each steel grade shown in Table 1 means the average composition of the total 7 slabs collected by each steel grade, and the compositional deviation of the composition taken from the 7 slabs is less than 5% for each component, and there is almost no composition deviation. It was used after confirming. In addition, Tnr used in the above table represents the theoretical temperature at which the austenite recrystallization temperature was calculated for each steel composition.

However, since the strength tends to vary greatly depending on the Mo component content, in the following results, A, B, C, and D, which do not include Mo, and E, F, and G, which include Mo, will be separately compared.

The slabs of each composition were heated in a heating furnace, followed by rough rolling followed by air-cooling. Specific finish rolling conditions are shown in Table 2 below.

TABLE 2

Processing pattern Rolling start temperature
(℃) Rolling end temperature
(℃) Cooling rate
(℃ / sec) Cooling stop temperature
(℃) Rolling amount at finish rolling
(%) One 1133 1008 Air cooling - 63.1 2 1056 931 Air cooling - 46.8 3 935 855 8.1 625 65.3 4 817 722 8.6 572 58.7 5 881 819 8.0 626 56.6 6 889 811 5.2 632 55.1 7 873 812 2.2 655 56.2 8 931 858 4.7 797 63.7

As shown in Table 2, there is a slight difference depending on the composition of each steel species, and thus the austenite recrystallization temperature and Ar3 temperature, but the treatment pattern 1 is mainly rolled in the austenite recrystallization region, the treatment pattern 2 is austenite Rolling is performed in the region between the nitrate recrystallization zone and the non-recrystallization zone, and the austenitic recrystallization zone rolling or the unrecrystallization zone rolling is mainly performed according to the austenitic recrystallization temperature for each steel type. Rolling was performed mainly in the unrecrystallized region, and the processing pattern 4 was unrecrystallized region rolling + abnormal region (γ + α region) rolling, and the processing patterns 5, 6, 7 and 8 were mainly subjected to unrecrystallized region rolling. An example is shown. Also, in the cooling method, the treatment patterns 1 and 2 perform air cooling to confirm the case where the cooling pattern of the present invention is out of the range of the present invention, and the processing patterns 7 and 8 respectively show that the cooling rate and the cooling stop temperature are outside the range of the present invention. It represents the case. The remaining processing patterns 3 to 5 show processing patterns according to the present invention.

The results after rolling and cooling the slabs having the composition of Table 1 under the conditions of Table 2 under the respective conditions are shown in Table 3 below.                     

[Table 3]

Steel grade Processing pattern Tnr or less
Rolling reduction (%) YS (kgf / mm ² ) TS (kgf / mm ² ) DBTT (℃) Remarks A One 0.0 29.8 43.7 -38 Comparative example A 2 31.0 32.8 44.9 -43 Comparative example A 3 65.3 41.5 51.6 -77 Inventive Example A 4 58.7 48.9 54.5 -113 Inventive Example A 5 56.6 45.9 52.0 -100 Inventive Example A 6 55.1 43.7 51.3 -90 Inventive Example A 7 56.2 40.0 48.6 -83 Comparative example A 8 63.7 38.6 47.8 -64 Comparative example B One 0.0 28.2 41.5 -29 Comparative example B 2 3.6 31.1 42.8 -34 Comparative example B 3 65.3 39.7 50.4 -68 Inventive Example B 4 58.7 47.2 52.3 -104 Inventive Example B 5 56.6 44.2 50.9 -92 Inventive Example B 6 55.1 42.0 50.1 -82 Inventive Example B 7 56.2 38.3 46.4 -74 Comparative example B 8 63.7 36.9 45.6 -56 Comparative example C One 0.0 31.0 44.5 -30 Comparative example C 2 22.3 33.9 45.7 -35 Comparative example C 3 65.3 42.5 52.4 -69 Inventive Example C 4 58.7 50.0 55.3 -105 Inventive Example C 5 56.6 46.9 52.8 -92 Inventive Example C 6 55.1 44.8 52.1 -82 Inventive Example C 7 56.2 41.0 49.4 -75 Comparative example C 8 63.7 39.6 48.6 -56 Comparative example D One 0.0 28.5 41.5 -29 Comparative example D 2 0.1 31.2 42.8 -34 Comparative example D 3 62.2 39.9 50.4 -68 Inventive Example D 4 58.7 47.3 52.3 -104 Inventive Example D 5 56.6 44.3 50.9 -91 Inventive Example D 6 55.1 42.1 50.1 -81 Inventive Example D 7 56.2 38.4 46.4 -74 Comparative example D 8 63.7 37.0 45.6 -55 Comparative example

Table 3-continued

Steel grade Processing pattern Tnr or less
Rolling reduction (%) YS (kgf / mm ² ) TS (kgf / mm ² ) DBTT (℃) Remarks E One 0.0 31.4 46.5 -33 Comparative example E 2 33.5 34.1 47.7 -38 Comparative example E 3 65.3 42.7 54.3 -72 Inventive Example E 4 58.7 50.2 57.3 -108 Inventive Example E 5 56.6 47.1 54.8 -95 Inventive Example E 6 55.1 45.0 54.1 -85 Inventive Example E 7 56.2 41.2 51.4 -78 Comparative example E 8 63.7 39.8 50.6 -59 Comparative example F One 0.0 31.0 46.0 -34 Comparative example F 2 22.0 33.8 47.2 -39 Comparative example F 3 65.3 42.4 53.8 -73 Inventive Example F 4 58.7 49.9 56.8 -109 Inventive Example F 5 56.6 46.8 54.3 -96 Inventive Example F 6 55.1 44.7 53.6 -86 Inventive Example F 7 56.2 40.9 50.9 -79 Comparative example F 8 63.7 39.5 50.1 -60 Comparative example G One 0.0 28.0 44.0 -27 Comparative example G 2 0.0 30.6 45.3 -32 Comparative example G 3 55.9 39.3 51.9 -66 Inventive Example G 4 58.7 46.7 54.8 -102 Inventive Example G 5 56.6 43.7 52.4 -89 Inventive Example G 6 55.1 41.5 51.6 -79 Inventive Example G 7 56.2 37.8 48.9 -72 Comparative example G 8 57.1 36.4 48.1 -53 Comparative example

The impact transition temperature (DBTT) described in Table 3 represents the temperature at which the steel destruction mechanism is moved from ductile fracture to brittle fracture, which is an important measure of the toughness of the material. In other words, the lower the impact transition temperature, the better the toughness of the material.

As described above, when looking at the steel grades A, B, C and D first, there is a slight difference depending on the steel grade, but in the case of the treatment patterns 1,2,7 and 8, the tensile strength is higher than that of the invention examples 3,4,5 and 6 It can be seen that the strength and yield strength is as low as 1.5 ~ 11kgf / mm ² . In addition, it can be seen that the processing patterns 1 and 2 are inferior in low temperature toughness as they are higher than 30 ° C. compared with the case where the impact transition temperature is also different.

As a result of analyzing the causes and effects of each pattern, the following conclusions were reached. First, in the case of pattern 1, all the rolling was performed in the austenite recrystallization zone regardless of the steel type. In this case, the austenite shape is recrystallized without pancakeization during rolling to obtain coarse austenite. This is because a coarse ferrite and pearlite mixed structure is obtained.

Next, in the case of pattern 2, the rolling was terminated at a temperature lower than the austenite recrystallization temperature. However, in the overall rolling process, the reduction ratio in the unrecrystallized region, which is lower than the austenite recrystallization temperature, is too low to sufficiently austenite grains. It was judged that results similar to those of pattern 1 could be obtained because the shape control was not performed or the subsequent cooling process was performed by the air cooling process, so that hard structures were not sufficiently formed.

The patterns 3, 4, 5, and 6 are all cases in which the cumulative reduction ratio of the austenite uncrystallized region or the ideal region is 30% or more, and both the cooling rate and the cooling stop temperature satisfy the range according to the present invention. It was a pattern in which sufficient grain refinement effect, dislocation integration effect, and hard phase formation effect by this invention can be obtained. As a result, good physical properties with tensile strength of 50kgf / mm ² or more and impact transition temperature of -50 ℃ were obtained in all steel grades.

Microscopic examination of the tissue of the hot-rolled thick steel sheets prepared in Examples 3, 4, 5 and 6 showed that they had a fine ferrite grain size (FGS) in the range of 10 to 20 μm in all cases.

In the case of pattern 7, the reduction ratio and the cooling stop temperature below the austenite uncrystallized temperature satisfy the scope of the present invention, but the cooling rate is not smaller than the value defined in the above Equation 2, which does not satisfy the conditions of the present invention. Strength and tensile strength are lower than the value desired in the present invention.

In the case of pattern 8, the reduction ratio and cooling rate below the austenite unrecrystallization temperature satisfy the scope of the present invention, but the cooling stop temperature is less than the value defined in the above Equation 3, which does not meet the conditions of the present invention. Cooling is terminated before sufficiently transforming into this hard phase, and thus yields a value lower than the value desired in the present invention.

The same tendency is observed in steel grades E, F, and G added with Mo, but in the case of the steel grades, tensile strength and yield strength are about 1 to 2 kgf / mm than other steel grades due to the strong hardening effect of Mo. ² shows the result of upward revision.

From the above results, the following conclusions were obtained.

The factor which has the greatest influence on the low temperature toughness of the steel composition according to the present invention is the cumulative reduction ratio in the temperature range below the austenite recrystallization temperature, which has a great influence on the strength of the steel and also in order to further improve the strength of the steel. It is to satisfy the cooling rate of the relation 2 and the upper limit of the cooling stop temperature of the relation 3.

From the above experimental results it can be seen the effect of the manufacturing method of the weather-resistant steel for seashore according to the present invention. However, the above experimental results are only one preferred embodiment for showing the effects of the present invention and are not intended to limit the present invention. In other words, the scope of the present invention extends as long as the matters described in the claims and their equivalents.

According to the present invention, it has excellent weather resistance in a corrosive environment such as cold weather or winter season where a large amount of beach or cryoprotectant is sprayed, and has a high strength of 50kgf / mm ² or more and a low temperature toughness of DBTT -50 ° C. Weatherable steel can be manufactured by a simple method.

Claims (3)

By weight, C: 0.15% or less (excluding 0%), Si: 1.0% or less (excluding 0%), Mn: 2.0% or less (excluding 0%), Cu: 0.2-1.0%, Ni: 0.2 5.0%, Al: 0.001-0.1%, P: 0.03% or less (excluding 0%), S: 0.002-0.03%, Ca: 0.001-0.01%, Nb: 0.02-0.07%, V: 0.1 % Or less, and B: 0.003% or less selected from the group consisting of the remaining Fe and other inevitable impurities, Ca, S, Al, Si in the above components 1 [Relationship 1] Ca (%) / S (%)> [1.5Al (%) + 2Si (%)] After satisfying the requirements, hot-rolled steel with excellent weathering resistance, containing 30% or more of water-soluble CaS inclusions in the Ca-based nonmetallic inclusions in the steel, and having a cumulative reduction ratio of 30% or more in the austenitic unrecrystallized region, A method of manufacturing high strength, weather resistant steel for high strength beaches, characterized by cooling to a cooling stop temperature satisfying the following relational formula 4 at a satisfactory cooling rate. [Relationship 3] Cooling rate (℃ / sec) ≥ 9.7 / {[% Mn] + [% Cu] + [% Ni] + 20 × ([% Si] / 30 + [% Mo] / 10)} [Relationship 4] Cooling stop temperature (℃) ≤ 865-272 × [% C]-74 × [% Mn]-9 × [% Mo]-5 × [% Cu] The high-strength beach having excellent toughness according to claim 1, wherein the steel further comprises at least one member selected from the group consisting of Ti: 0.005 to 0.1%, Mo: 0.01 to 1.0%, and W: 0.01 to 1.0%. Method for manufacturing weathering steel for use. The method according to claim 1 or 2, wherein the rolling reduction below the austenite uncrystallized region of the steel is 90% or less.