KR20200032866A - High strength steel having excellent low-temperature fracture toughness and method for manufacturing the same - Google Patents

Abstract

A method for manufacturing high-strength steel comprises the steps of: reheating, at a temperature of 1,150 to 1,200°C, a steel slab containing 0.01 to 0.14 wt% of carbon (C), 0.30 to 0.40 wt% or less of silicon (Si), 0.45 to 1.55 wt% of manganese (Mn), more than 0 wt% and equal to or less than 0.012 wt% of phosphorus (P), more than 0 wt% and equal to or less than 0.003 wt% of sulfur (S), 0.001 to 0.005 wt% of cerium (Ce), 0.015 to 0.06 wt% of soluble aluminum (S_Al), 0.10 to 0.20 wt% of chromium (Cr), 0.25 to 0.35 wt% of nickel (Ni), more than 0 and equal to or less than 0.01 wt% of titanium (Ti), 0.05 to 0.15 wt% of copper (Cu), 0.01 to 0.02 wt% of niobium (Nb), 0.04 to 0.08 wt% of molybdenum (Mo), 0.015 to 0.025 wt% of vanadium (V), and the remainder of iron (Fe) and other inevitable impurities; primarily rolling the heated steel slab over an austenite recrystallization region; secondarily rolling the primarily rolled plate into a final steel plate product shape to be ended at a temperature of 900°C to 1,000°C; and cooling the steel plate.

Description

High-strength steel with excellent low-temperature impact toughness and its manufacturing method {HIGH STRENGTH STEEL HAVING EXCELLENT LOW-TEMPERATURE FRACTURE TOUGHNESS AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel material and a manufacturing method thereof, and more particularly, to a high-strength steel material for guaranteeing low-temperature impact toughness and a manufacturing method thereof.

Structural steel materials for ships, offshore structures, etc., and heavy-duty steel plates for multi-purpose tanks that mix various liquefied gases such as carbon dioxide, ammonia, LNG, etc. have very harsh environments. Therefore, the strength is very important, the strength of the marine structural steel or tank steel is not only strength, but also toughness at low temperatures. The marine structural steel is a cold region, such as the North Pole, which is rich in marine petroleum resources due to resource depletion in warm regions, and its use environment is gradually shifting, so it can withstand the harsh cryogenic environment as described above only with high-strength steel sheets with excellent low-temperature toughness. Is getting harder.

In order to secure the low-temperature toughness of the steel sheet for the tank, a method of controlling microstructure through the addition of 6 to 12% by weight of Ni or the treatment of quenching and tempering was used.

A related technology is the Republic of Korea Patent Publication No. 2016-0079166 (2016.07.06 published, low-temperature impact toughness excellent resistance to high-strength steel and its manufacturing method).

The problem to be solved by the present invention is to provide a steel material having improved low-temperature impact toughness through control of an alloy composition and hot rolling conditions, and a manufacturing method thereof.

High-strength steel according to an aspect of the present invention, by weight, carbon (C): 0.01% ~ 0.14%, silicon (Si): 0.30% ~ 0.40% or less, manganese (Mn): 0.45% ~ 1.55%, phosphorus (P): more than 0 and less than 0.012%, sulfur (S): more than 0 and less than 0.003%, cerium (Ce): 0.001% to 0.005%, soluble aluminum (S_Al): 0.015% to 0.06%, chromium (Cr): 0.10 % To 0.20%, nickel (Ni): 0.25% to 0.35%, titanium (Ti): more than 0 to 0.01% or less, copper (Cu): 0.05% to 0.15%, niobium (Nb): 0.01% to 0.02%, molybdenum (Mo): 0.04% to 0.08%, Vanadium (V): 0.015% to 0.025%, contains residual iron (Fe) and other inevitable impurities, tensile strength (TS): 520MPa to 550MPa, yield strength (YS): Charpy impact absorption energy at 380 MPa to 450 Pa, and -40 ° C satisfies 140 J or more.

Method of manufacturing a high-strength steel according to another aspect of the present invention, by weight, carbon (C): 0.01% ~ 0.14%, silicon (Si): 0.30% ~ 0.40% or less, manganese (Mn): 0.45% ~ 1.55 %, Phosphorus (P): more than 0 and less than 0.012%, sulfur (S): more than 0 and less than 0.003%, cerium (Ce): 0.001% to 0.005%, soluble aluminum (S_Al): 0.015% to 0.06%, chromium (Cr ): 0.10% to 0.20%, nickel (Ni): 0.25% to 0.35%, titanium (Ti): more than 0 and less than 0.01%, copper (Cu): 0.05% to 0.15%, niobium (Nb): 0.01% to 0.02 %, Molybdenum (Mo): 0.04% to 0.08%, vanadium (V): 0.015% to 0.025%, re-heating the steel slab containing residual iron (Fe) and other unavoidable impurities at a temperature of 1,150 ° C to 1,200; First rolling the heated steel slab above the austenite recrystallization region; Secondly rolling the first rolled sheet material to a final steel plate product shape to end at 900 ° C to 1,000 ° C; And cooling the steel sheet.

The step of primary rolling the steel slab ends at 1,050 ° C to 1,150 ° C, and the step of secondary rolling preferably has a reduction ratio of 20% or more per pass.

After the cooling step, the steel sheet may satisfy tensile strength (TS): 520 MPa to 550 MPa, yield strength (YS): 380 MPa to 450 Pa, and Charpy impact absorption energy at -40 ° C or greater than 140 J.

According to the present invention, by adding the rare earth element cerium (Ce), it is possible to increase the toughness resistance by accelerating the production of ferrite, which is the cause of improving toughness, and changing the size of the crystal grains. In addition, by increasing the rolling amount, the phase transformation nucleation site is maximized to compensate for the strength due to ferrite refinement and to suppress the formation of precipitates due to the addition of cerium (Ce) and to improve the toughness by increasing the ferrite fraction. Can be secured.

1 is a process flow chart schematically showing a method of manufacturing a high-strength steel according to an embodiment of the present invention.
2 and 3 show the results of observing the wavefront shape after the Charpy impact test (CVN) on the steel of the comparative example and the steel of the example with an optical microscope.
4 is a graph showing the change in the ferrite fraction according to the amount of addition and rolling amount of cerium (Ce).

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art to which the present invention pertains can easily practice. The present invention can be implemented in many different forms, and is not limited to the embodiments described herein. Throughout this specification, the same reference numerals are assigned to the same or similar components. In addition, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the subject matter of the present invention are omitted.

The present invention proposes a high-strength steel with improved low-temperature impact toughness through control of the alloy composition and hot rolling conditions of the steel and a method for manufacturing the same. The inventors of the present invention recognize that the change of the physical properties of the steel material for pressure vessels of 55Kg high-strength high-temperature and low-temperature toughness is closely related to the phase transformation of microstructures and the formation of precipitates, and the composition of the steel alloy to improve the strength of the steel through solid solution strengthening. It was controlled appropriately. In particular, by adding the rare earth element cerium (Ce), ferrite, which is a cause of improving toughness, was accelerated and the grain size was changed to increase toughness resistance. The addition of an appropriate amount of cerium (Ce) increased the ferrite fraction as a softened phase and suppressed the formation of fine precipitates, thereby increasing the low-temperature impact increase. In addition, by increasing the amount of rolling, one of the process variables, the phase transformation nucleation site was maximized.

Hereinafter, a high-strength steel material having excellent low-temperature impact toughness, which is an aspect of the present invention, will be described.

High-strength steel with excellent low-temperature impact toughness

One aspect of the present invention is a high strength steel material having excellent low-temperature impact toughness in weight%, carbon (C): 0.01% to 0.14%, silicon (Si): 0.30% to 0.40% or less, manganese (Mn): 0.45% to 1.55 %, Phosphorus (P): more than 0 and less than 0.012%, sulfur (S): more than 0 and less than 0.003%, cerium (Ce): 0.001% to 0.005%, soluble aluminum (S_Al): 0.015% to 0.06%, chromium (Cr ): 0.10% to 0.20%, nickel (Ni): 0.25% to 0.35%, titanium (Ti): more than 0 and less than 0.01%, copper (Cu): 0.05% to 0.15%, niobium (Nb): 0.01% to 0.02 %, Molybdenum (Mo): 0.04% to 0.08%, vanadium (V): 0.015% to 0.025%.

The rest of the above components are made of iron (Fe) and impurities inevitably contained in the steelmaking process.

Hereinafter, the role and content of each component included in the high-strength steel sheet according to the present invention will be described. At this time, the content of the component elements all means weight percent.

Carbon (C)

Carbon (C) is an indispensable element for imparting high strength to the steel sheet, and is a major element that increases the hardenability of the steel sheet and determines the strength after quenching. Depending on the content of the carbon (C) and the manufacturing method, it may become solid carbon inside the material structure, and combine with elements with very high properties to combine with carbon (C) to form carbide. In order to obtain such an effect, it is preferable to contain 0.10% by weight or more of the total steel material weight. However, when the content of carbon (C) is excessive, weldability, elongation, and corrosion resistance are lowered, so it is preferable to control the content to 0.14% by weight.

Silicon (Si)

Silicon (Si) is an element added as a deoxidizer, and it is preferable to add 0.30% by weight or more for deoxidation, but when its content exceeds 0.40% by weight, oxides can be formed on the surface of the steel to deteriorate the weldability of the steel. have.

Manganese (Mn)

Manganese (Mn) is an element that increases the strength and toughness of steel and increases the quenching properties of steel. In order to effectively exhibit this effect, manganese (Mn) is added at 1.45% by weight to 1.55% by weight of the total steel weight. When the content of manganese (Mn) is less than 1.45% by weight, it may be difficult to secure the strength. Conversely, when the content of manganese (Mn) exceeds 1.55% by weight, the strength increases, but segregation occurs and tissue unevenness occurs. I can do it.

Phosphorus (P)

Phosphorus (P) is a representative element that contributes to the improvement of strength, but lowers the toughness of the weld, the lower the content, the better. Therefore, in the present invention, the content of phosphorus (P) was limited to more than 0 to 0.012% by weight of the total weight of the steel.

Sulfur (S)

Sulfur (S) is an element that is inevitably contained in the production of steel together with phosphorus (P), and forms MnS to degrade the toughness of the base material and the weld. Therefore, in the present invention, the content of sulfur (S) was limited to more than 0 to 0.003% by weight of the total weight of the steel.

CE (Ce)

Cerium (Ce) is one of the rare earth metals that acts as a nucleation for dendrite (dendrite) structures when solidification of molten steel, thereby minimizing the size of dendrite and inhibiting columnar tissue growth and promoting the formation of equiaxed crystal structures. Therefore, it reduces the size and amount of columnar crystals, which are a problem of grain boundary embrittlement, and increases the amount of equiaxed crystals with excellent high temperature ductility, thereby improving the hot workability of the casting structure. In addition, the solid solution of niobium (Nb) in austenite is increased to suppress the formation of NbC precipitates. In addition, it is segregated at the grain boundaries to make compounds such as phosphorus (P), sulfur (S), etc., which lower the breaking strength of grain boundaries, thereby reducing the adverse effects of P and S. However, when the content of cerium (Ce) is less than 0.001% by weight, this effect is insignificant, and when it exceeds 0.005% by weight, the effect is saturated. Therefore, it is preferable to limit the addition amount to 0.001% by weight to 0.005% by weight.

Soluble aluminum (S_Al)

Soluble aluminum (S_Al) is used as a deoxidizer, and at the same time, it suppresses the precipitation of cementite like silicon (Si), stabilizes austenite, and serves to improve strength.

Soluble aluminum (S_Al) is preferably added in a content ratio of 0.015% by weight to 0.06% by weight of the total weight of the steel according to the present invention. When the content of soluble aluminum (S_Al) is less than 0.015% by weight, it is difficult to expect an austenite stabilizing effect. Conversely, when the content of soluble aluminum (S_Al) exceeds 0.06% by weight, a nozzle clogging problem may occur during steelmaking, and hot brittleness occurs due to Al oxide during casting, resulting in crack generation and ductility reduction.

Chrome (Cr)

Chromium (Cr) is an element added to increase hardenability and secure strength. In addition, chromium (Cr) serves to increase the hardenability, but as the content increases, local corrosion occurs by Cr-O ₂ generated by the combination of chromium (Cr) and oxygen, and the toughness decreases. Therefore, the content of chromium (Cr) is controlled to 0.10% by weight to 0.20% by weight of the total weight of the steel. When the content of chromium (Cr) is less than 0.10% by weight, strength and hardenability may not be sufficient. Conversely, when the content of chromium (Cr) exceeds 0.2% by weight, the crack resistance may be reduced.

Nickel (Ni)

In the present invention, nickel (Ni) is refined into crystal grains and solidified in austenite and ferrite to strengthen the matrix. In particular, nickel (Ni) is an effective element for improving low-temperature impact toughness. The nickel (Ni) is added in an amount of 0.25% to 0.35% by weight of the total weight of the steel. When the content of nickel (Ni) is less than 0.25% by weight, the effect of adding nickel cannot be properly exhibited. On the contrary, when the content of nickel (Ni) exceeds 0.35% by weight, there is a problem in that the weldability is inhibited and the red brittleness is caused.

Titanium (Ti)

Titanium (Ti) can improve the toughness of the steel sheet by preventing the coarsening of the austenite grains in the slab heating step by generating a high temperature stability Ti (C, N) precipitate. Titanium (Ti) is added in an amount of 0.005% to 0.01% by weight of the total weight of the steel. When the content of titanium (Ti) is less than 0.005% by weight, the effect of improving toughness is negligible. On the contrary, when the content of titanium (Ti) exceeds 0.01% by weight, the toughness of the steel sheet after extremes is reduced by generating coarse precipitates, and there is a problem of increasing the manufacturing cost without further effect.

Copper (Cu)

Copper (Cu) together with nickel (Ni) serves to improve the hardenability and low temperature and impact toughness of steel materials. Copper (Cu) is added in an amount of 0.05% to 0.15% by weight of the total weight of the steel sheet according to the present invention. When the content of copper (Cu) is less than 0.05% by weight, the effect of adding copper cannot be properly exhibited. Conversely, when the content of copper (Cu) exceeds 0.15% by weight, it may cause red brittleness.

Niobium (Nb)

Niobium (Nb) combines with carbon (C) and nitrogen (N) contained in the steel at high temperatures to form carbides or nitrides, and these niobium-based carbides or nitrides inhibit recrystallization and grain growth during rolling to refine grains. It improves both the strength and toughness of the steel sheet. Niobium (Nb) is added in an amount of 0.01% to 0.02% by weight of the total weight of the steel sheet. When the content of niobium (Nb) is less than 0.01% by weight, the effect of adding niobium (Nb) is negligible. Conversely, when the content of niobium (Nb) exceeds 0.02% by weight, toughness of the steel sheet after extremes may be reduced.

Molybdenum (Mo)

Molybdenum (Mo) increases the hardenability of the steel, and serves to improve both yield strength and tensile strength. Molybdenum (Mo) is added in an amount of 0.04% to 0.08% by weight of the total weight of the steel sheet according to the present invention. If the content of molybdenum (Mo) is less than 0.04% by weight, yield strength and tensile strength may not be sufficient. Conversely, when the content of molybdenum (Mo) exceeds 0.08% by weight, toughness and ductility of the steel sheet may be reduced.

Vanadium (V)

Vanadium (V) is an element that combines with carbon (C) or nitrogen (N) to form V (C, N) precipitates, contributes to strength improvement, and improves hardenability. The vanadium (V) is contained in an amount of 0.015% to 0.025% by weight based on the total weight of the steel. When the vanadium (V) is contained in less than 0.015% by weight, the effect of its addition is minimal, and when the vanadium (V) exceeds 0.025% by weight, further effects are difficult to expect and only the manufacturing cost rises.

The remaining component of the invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may be inevitably mixed from the raw material or the surrounding environment, and thus cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.

By controlling the specific components of the above-described alloy composition and their content range, and the present invention through the manufacturing method of the steel sheet to be described later, while showing the tensile strength (TS): 520MPa ~ 550MPa and yield strength (YS): 380MPa ~ 450Pa,- Charpy impact absorption energy at 40 ° C. is 140 J or more, and a ductile-brittle transition temperature (DBTT) is reduced.

The alloy composition steel material is excellent in strength and toughness, and has excellent low-temperature impact properties, and thus may be suitable for use as a steel material for a pressure vessel for medium and high temperature.

Another aspect of the present invention comprises the steps of reheating the slab made of the alloy composition described above; Primary hot rolling the heated slab at 1050 ° C to 1150 ° C; The primary rolled steel sheet is a secondary rolling at a temperature of 900 ℃ ~ 1,000 ℃; And cooling the second rolled steel sheet at a cooling end temperature of 450 ° C to 550 ° C and a cooling rate of 10 ° C / s to 15 ° C / s.

Hereinafter, a method of manufacturing a high-strength steel according to the present invention will be described with reference to the accompanying drawings.

Method of manufacturing steel

1 is a process flow chart schematically showing a method of manufacturing a high-strength steel according to an embodiment of the present invention.

Referring to Figure 1, the method of manufacturing a steel according to an embodiment of the present invention is a slab reheating step (S110), hot rolling step (S120), primary rolling step (S120), secondary rolling step (S130) and cooling Step S140 is included.

In the steel material manufacturing method according to the present invention, the slab plate material in the semi-finished product state that is the subject of the hot rolling process can be secured through a continuous casting process after obtaining molten steel having a predetermined composition through the steelmaking process. The slab plate material, by weight, carbon (C): 0.01% ~ 0.14%, silicon (Si): 0.30 ~ 0.40% or less, manganese (Mn): 0.45% ~ 1.55%, phosphorus (P): more than 0 0.012 % Or less, sulfur (S): more than 0 and less than 0.003%, cerium (Ce): 0.001% to 0.005%, soluble aluminum (S_Al): 0.015% to 0.06%, chromium (Cr): 0.10% to 0.20%, nickel ( Ni): 0.25% to 0.35%, titanium (Ti): more than 0 and less than 0.01%, copper (Cu): 0.05% to 0.15%, niobium (Nb): 0.01% to 0.02%, molybdenum (Mo): 0.04% to 0.08%, vanadium (V): 0.015% to 0.025%, and the rest include iron (Fe) and other inevitable impurities.

Slab reheating step (S110)

In the slab reheating step (S110), the slab plate material having the above composition is reheated at a slat reheating temperature (SRT) of 1,150 ° C to 1,200 ° C. Through such reheating, re-use of segregated components and re-use of precipitates may occur during casting. At this time, the slab may be a slab plate material manufactured by a continuous casting process performed before the slab reheating step (S110).

When the slab reheating temperature is less than 1,150 ° C, there is a problem in that the heating load is insufficient and the rolling load increases. In addition, there is a problem in that the austenite grains are rapidly coarsened because the Nb-based precipitates do not reach the solid solution temperature and cannot be reprecipitated as fine precipitates during hot rolling, thereby preventing the growth of austenite grains. In addition, on the contrary, when the reheating temperature exceeds 1,200 ° C, there is a problem in that it is difficult to secure the strength and low-temperature toughness of the steel produced by the coarsening of austenite grains or decarburization.

After heating the slab, the slab is rolled. In order for the steel sheet to have low-temperature toughness, austenitic grains must be present in a fine size, which is possible by controlling the rolling temperature and rolling reduction. The rolling step of the present invention is carried out in two temperature ranges. In addition, the recrystallization behavior in each temperature region is different from each other, so the conditions are set differently.

1st rolling step (S120)

The heated slab is subjected to primary rolling after heating to adjust its shape first. The first rolling step (S120) is preferably performed at a temperature (Tnr) or more at which austenite recrystallization stops. It has the effect of destroying the casting structure, such as dendrites formed during casting by rolling, and reducing the grain size through austenite recrystallization. Such grain refinement has an important effect on improving the strength and toughness of the steel sheet. The primary rolling step (S120) is preferably terminated at 1,050 ℃ ~ 1,150 ℃.

Second rolling step (S130)

Secondary rolling is performed to introduce a non-uniform microstructure into the austenite structure of the steel sheet that is first rolled. The second rolling step is to further crush the grains and develop a dislocation by deformation inside the grains to facilitate transformation into acicular ferrite upon cooling. In order to exhibit this effect, it is preferable to control the reduction ratio per pass to 20% or more in the second rolling step. The second rolling step (S130) may be performed in the austenite unrecrystallized region. In this case, the austenite grains can be further refined, and superior impact toughness can be realized as compared to the case where the finish rolling is performed in the austenite recrystallization zone. The second rolling step is to end at 900 ℃ ~ 1,000 ℃. When the secondary rolling temperature exceeds 1,000 ° C, the austenite grains are coarsened, and after transformation, the ferrite grains are not sufficiently refined, and thus strength may be difficult to secure. Conversely, when the secondary rolling temperature is performed at less than 900 ° C, a rolling load may be caused to decrease productivity and reduce heat treatment effects.

Cooling step (S140)

In the cooling step (S140), when the thickness of the steel sheet is t, it is cooled to 450 ° C to 550 ° C at a cooling rate of 10 ° C / s to 15 ° C / sec based on the point t / 4. Cooling conditions are factors affecting the microstructure, and when cooled at a cooling rate of 10 ° C / sec, the amount of austenite / martensitic (M & A) on the island may increase excessively, inhibiting strength and toughness, and the cooling rate If it exceeds 15 ℃ / sec, shape control may be poor due to the warping of the steel sheet due to excessive cooling. In addition, it is preferable to control the cooling temperature to less than 550 ° C so that no M & A structure is formed. However, if the cooling temperature is too low, the effect is not only saturated, but also may cause warping of the steel sheet due to excessive cooling, and also excessive strength. Since the impact toughness may decrease due to the rise, the lower limit is preferably limited to 450 ° C.

The steel materials produced by the above process (S110 to S140) show tensile strength (TS): 520 MPa to 550 MPa and yield strength (YS): 380 MPa to 450 Pa due to proper control of rolling conditions, and Charpy impact absorption energy at -40 ° C. Can be improved to a steel grade that satisfies 140J or more.

Hereinafter, the present invention will be described in detail through examples, but this is only a preferred embodiment of the present invention, and the scope of the present invention is not limited by the scope of the examples. The contents not described here will be sufficiently technically inferred by those skilled in the art, and thus the description thereof will be omitted.

Example

First, steel slabs of Comparative Examples and Examples having the compositions shown in Tables 1 and 2 below were prepared.

division Ingredient (% by weight) C Si Mn P S Ce S_Al Comparative Example 1 0.123 0.32 1.49 0.007 0.001 - 0.020 Comparative Example 2 0.123 0.35 1.50 0.007 0.001 - 0.015 Comparative Example 3 0.121 0.35 1.51 0.007 0.001 - 0.013 Comparative Example 4 0.120 0.35 1.51 0.008 0.001 - 0.011 Example 1 0.122 0.32 1.51 0.008 0.001 0.042 0.016 Example 2 0.123 0.35 1.51 0.007 0.001 0.047 0.016

division Ingredient (% by weight) Cr Ni Ti Cu Nb Mo V Comparative Example 1 0.15 0.29 0.001 0.12 0.017 0.051 0.017 Comparative Example 2 0.12 0.28 0.001 0.12 0.018 0.056 0.018 Comparative Example 3 0.15 0.28 0.001 0.12 0.018 0.054 0.016 Comparative Example 4 0.14 0.26 0.001 0.12 0.017 0.054 0.016 Example 1 0.16 0.26 0.001 0.12 0.016 0.055 0.018 Example 2 0.16 0.29 0.001 0.12 0.019 0.052 0.017

The prepared steel slabs were reheated under the conditions listed in Table 2, and subjected to primary and secondary rolling, followed by cooling.

division Reheating temperature (℃) Cooling rate (℃ / s) Cooling end temperature (℃) Comparative Example 1 1200 - - Comparative Example 2 1200 - - Comparative Example 3 1191 - - Comparative Example 4 1194 - - Example 1 1200 14 505 Example 2 1193 15 507

For the prepared steel sheet, yield strength (YS), tensile strength (TS), elongation (EL), and Charpy impact absorption energy (CVN) at 18 ° C, 0 ° C, -20 ° C, -40 ° C, -60 ° C ), Respectively, and the results are shown in Table 4.

division YP (MPa) TS (MPa) EL (%) CVN (J) @ 18 ℃ 0 ℃ -20 ℃ -40 ℃ -60 ℃ Comparative Example 1 404 546 32 267 161 143 87 22 Comparative Example 2 399 542 32 272 154 141 83 18 Comparative Example 3 405 551 29 Comparative Example 4 403 550 33 Example 1 389 545 36 386 375 265 150 30 Example 2 402 534 32 379 369 257 144 45

According to the tensile test and low-temperature impact test result table in Table 4, the tensile strength, yield strength, and elongation are satisfactory for the steels of Comparative Examples 1 to 4 without addition of cerium (Ce), but showed low results in the impact test.

On the other hand, in the case of the steels of Examples 1 and 2 to which cerium (Ce) was added, tensile strength, yield strength, and elongation can be satisfied, and the impact test showed excellent results compared to the comparative steel. In particular, it can be seen that the low-temperature impact test performed at -40 ° C and -60 ° C shows superior low-temperature impact toughness compared to the comparative steel.

Figure 2 shows the results of observing the wavefront shape after performing a Charpy impact test (CVN) at -40 ° C for the steel of the comparative example and the steel of the example.

The left side shows the wavefront shape of the steel of the comparative example, and the right side shows the wavefront formation of the steel of the example. As shown, in the case of the steel of the example, it can be seen that the wavefront is not smooth compared to the steel of the comparative example, which can absorb more energy in the impact test and can exhibit excellent low-temperature impact toughness compared to the comparative steel. This indicates that the addition of cerium (Ce), which has low affinity with carbon (C), suppresses the formation of cementite nuclei, thereby suppressing the production of pearlite and improving the toughness due to the increase in the fraction of ferrite by ferrite refinement.

3 is a photograph showing the observation of the structure of the steel of the comparative example and the steel of the example after performing the second rolling at 900 ° C with an optical microscope.

In the case of the steel (right side) of the example, the fraction of the ferrite in the soft phase is increased and the crystal grains are uniformly formed compared to the steel (left side) of the comparative example, and it can be seen that the low temperature impact toughness is improved.

4 is a graph showing the change in the ferrite fraction according to the amount of addition and rolling amount of cerium (Ce).

Referring to FIG. 4, when the content of cerium (Ce) increases from 0.0012% by weight to 0.0047% by weight, the ferrite fraction also increases, but when it is 0.018% by weight, it can be seen that the ferrite fraction decreases. Therefore, as suggested in the present invention, adding the content of cerium (Ce) at 0.001% by weight to 0.005% by weight of the steel sheet increases the fraction of ferrite to secure excellent low-temperature impact toughness.

In addition, it can be seen that the ferrite fraction can be increased compared to the case of 20% when the rolling amount is 40% in the proper temperature section of the hot rolling step.

As described above, according to the present invention, by adding the rare earth element cerium (Ce), ferrite, which is a cause of improving toughness, can be accelerated and the grain size can be changed to increase toughness resistance. In addition, by increasing the amount of rolling, one of the process variables, the phase transformation nucleation site was maximized. Thus, it is possible to improve the toughness by compensating the strength due to the refinement of ferrite and suppressing the formation of precipitates due to the addition of cerium (Ce) and increasing the ferrite fraction, thereby ensuring excellent low-temperature impact toughness even at low temperatures of -40 or -60 ° C. .

In the above, the description has been mainly focused on the embodiment of the present invention, but various changes or modifications can be made at the level of those skilled in the art. It can be said that such modifications and variations belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention should be judged by the claims set forth below.

Claims (4)

In weight percent, carbon (C): 0.01% to 0.14%, silicon (Si): 0.30% to 0.40% or less, manganese (Mn): 0.45% to 1.55%, phosphorus (P): more than 0 and 0.012% or less, sulfur (S): more than 0 and less than 0.003%, cerium (Ce): 0.001% to 0.005%, soluble aluminum (S_Al): 0.015% to 0.06%, chromium (Cr): 0.10% to 0.20%, nickel (Ni): 0.25 % To 0.35%, titanium (Ti): more than 0 and less than 0.01%, copper (Cu): 0.05% to 0.15%, niobium (Nb): 0.01% to 0.02%, molybdenum (Mo): 0.04% to 0.08%, vanadium (V): 0.015% to 0.025%, contains residual iron (Fe) and other inevitable impurities,
Tensile strength (TS): 520MPa ~ 550MPa, yield strength (YS): 380MPa ~ 450Pa, and Charpy impact absorption energy at -40 ℃ satisfies 140J or more,
High strength steel.
In weight percent, carbon (C): 0.01% to 0.14%, silicon (Si): 0.30% to 0.40% or less, manganese (Mn): 0.45% to 1.55%, phosphorus (P): more than 0 and 0.012% or less, sulfur (S): more than 0 and less than 0.003%, cerium (Ce): 0.001% to 0.005%, soluble aluminum (S_Al): 0.015% to 0.06%, chromium (Cr): 0.10% to 0.20%, nickel (Ni): 0.25 % To 0.35%, titanium (Ti): more than 0 and less than 0.01%, copper (Cu): 0.05% to 0.15%, niobium (Nb): 0.01% to 0.02%, molybdenum (Mo): 0.04% to 0.08%, vanadium (V): reheating the steel slab containing 0.015% to 0.025%, residual iron (Fe), and other inevitable impurities at a temperature of 1,150 ° C to 1,200;
First rolling the heated steel slab above the austenite recrystallization region;
Secondly rolling the first rolled sheet material to a final steel plate product shape to end at 900 ° C to 1,000 ° C; And
Cooling the steel sheet,
Method for manufacturing high-strength steel.
According to claim 2,
The step of first rolling the steel slab ends at 1,050 ° C to 1,150 ° C,
In the second rolling step, the rolling reduction per pass is 20% or more,
Method for manufacturing high-strength steel.
According to claim 2,
After the cooling step, the steel sheet has tensile strength (TS): 520 MPa to 550 MPa, yield strength (YS): 380 MPa to 450 Pa, and Charpy impact absorption energy at -40 ° C satisfies 140 J or more,
Method for manufacturing high-strength steel.

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